PhD Part 1
-
Upload
chris-blake -
Category
Documents
-
view
380 -
download
3
Transcript of PhD Part 1
THE MINERALOGICAL CHARACTERISATION AND INTERPRETATION OF A PRECIOUS METAL-BEARING FOSSIL GOSSAN, LAS
CRUCES, SPAIN
Volume 1
Text and References
A thesis submitted to the University of Cardiff in Candidature for the degree of Doctor of Philosophy
Christopher Blake
Department of Earth, Ocean and Planetary SciencesCardiff University
2008
Declaration and Statements
DECLARATION
This work has not previously been accepted in substance for any degree and is
not concurrently submitted in candidature for any degree.
Signed …………………………………………………………. (candidate)
Date …………………………
STATEMENT 1
This thesis is being submitted in partial fulfilment of the requirements for the
degree of PhD
Signed …………………………………………………………. (candidate)
Date …………………………
STATEMENT 2
This thesis is the result of my own independent work/investigation, except where
otherwise stated.
Other sources are acknowledged by explicit references.
Signed …………………………………………………………. (candidate)
Date …………………………
STATEMENT 3
I hereby give consent for my thesis, if accepted, to be available for photocopying
and for inter-library loan, and for the title and summary to be made available to
outside organisations.
Signed …………………………………………………………. (candidate)
Date …………………………
ii
Abstract
Abstract
The Las Cruces VMS deposit lies on the southern margins of the Iberian Pyrite Belt, Spain. The primary base metal massive sulphide is overlain by a supergene enriched zone and precious metal gossan that remains well preserved under approximately 150 metres of Tertiary marl.
The mineralogy, mineral textures and associations of five boreholes containing precious metal gossan mineralisation were characterised using a combination of optical microscopy, SEM and XRD techniques.
The mineralogy and geochemical profile of the gossan suggests that it was formed under near-surface weathering conditions, resulting in the development of the supergene zone and a mature gossan profile characterised by elevated levels of Au and Ag. The Au and Ag probably remobilised as chloride complexes under strongly acid, oxidising conditions, precipitating as high fineness Au and discrete Ag-bearing phases lower in the gossan profile.
The original Fe-oxyhydroxide dominated gossan mineral assemblage has subsequently been extensively replaced by later stages of siderite, greigite, galena and high fineness Au mineralisation that reflect marked changes in the depositional environment relative to the original gossan mineral assemblage. Fluctuating oxidising and reducing conditions, coupled with biogenic processes within the Niebla Posadas aquifer, situated directly above the present day Las Cruces gossan, provide a suitable mechanism for the formation of the extensive siderite and greigite mineralisation as well as precious metal remobilisation as a thiosulphate complex under near-neutral to alkaline conditions. Strongly negative δ13C stable isotope values for the siderite are consistent with biogenic processes involving Fe3+ and/or sulphate reducing bacteria as well as a significant influence from the oxidation of methane.
iii
Acknowledgement
Acknowledgement
I would never have started my thesis if it were not for the encouragement of my
mentor, Dr. Ivan Reynolds. Many thanks for your time and support during the
past 17 years of my career as a mineralogist with Rio Tinto.
A great deal of support has also been received from Dr. Hazel Prichard, helping
me through the maze of preparing a thesis and posting me the occasional
photocopy to save me the long trek into Cardiff. Many thanks for spending
endless hours reading through my thesis.
Finally, to my sister and parents. Thanks for providing me the love and support
of a great family.
About the Author
The author graduated with a B.Sc. honours degree in Geology and Geography
from the Cheltenham and Gloucester College of Higher Education in July 1991.
Following a vacation job with Rio Tinto's Anamet Services in the summer of 1990,
the author returned to Anamet Services as a technician/trainee mineralogist
under the guidance of Dr. Ivan Reynolds in November 1991. Anamet Services
closed in late 1997 and the mineralogy department was relocated to Clevedon
where, after a few years, the authors PhD project was approved and supported
by Rio Tinto. The author continued to work full time with Rio Tinto as Senior
Mineralogist in the Clevedon laboratory, working on his PhD during evenings,
weekends and vacation time. The Clevedon laboratory closed in December of
2008. The author now works as a consultant mineralogist.
v
Dedication
Dedication
To my mum and dad
vi
List of Contents
List of Contents
VOLUME 1: TEXT AND REFERENCES
Declaration and Statements iiAbstract iiiAcknowledgement vAbout the Author vDedication viList of Contents viiList of Figures xvList of Tables xix
1 INTRODUCTION 1
1.1 Introduction 1
1.2 Aims of Study 1
1.3 Thesis Outline 3
2 GEOLOGY 4
2.1 Regional Geological Setting - The Iberian Peninsula 4
2.2 The Iberian Pyrite Belt 7
2.3 The Guadalquivir Basin 11
2.4 Las Cruces - Exploration History 13
2.5 Las Cruces - Geology and Mineralogy 15
2.5.1 Introduction 152.5.2 Gossan 172.5.3 Secondary Massive Sulphide 182.5.4 Primary Massive Sulphide 19
2.6 Las Cruces - Evolutionary History 20
2.7 Sample Suite 30
3 GOSSANS 34
3.1 Introduction 34
3.2 The Gossan Forming Process 36
3.3 Influences On Gossan Formation 41
3.3.1 Introduction 413.3.2 Effect of Primary Geology on pH 423.3.3 Effects of Climate on Gossan Formation 433.3.4 Effects of Geomorphology on Gossan Formation 45
vii
List of Contents
3.4 Element Mobility and Gossan Profiles 46
3.4.1 Introduction 463.4.2 Fe 473.4.3 Au and Ag Element Mobility 503.4.4 Au and Ag Mineralogy and Geochemical Profiles 553.4.5 Cu 593.4.6 Pb 613.4.7 As and Sb 643.4.8 Si, Sn and Ti 653.4.9 Other metals 69
3.5 Ancient Seafloor Weathering 71
3.5.1 Introduction 713.5.2 Ochres 713.5.3 Umbers 73
3.6 Modern Seafloor Weathering 76
3.6.1 Introduction 763.6.2 Modern Seafloor Fe-Oxide and Oxyhydroxide Deposits of Secondary Origin 763.6.3 Modern Seafloor Fe-Mn-Si Oxide and Oxyhydroxide Deposits of Primary Origin 79
3.7 Comparing Modern and Ancient Deposits 81
4 METHODS OF INVESTIGATION 83
4.1 Introduction 83
4.2 Sample Preparation 83
4.3 Microscopy 85
4.3.1 Transmitted Light 854.3.2 Reflected Light 85
4.4 Scanning Electron Microscopy 86
4.4.1 Qualitative Methods 864.4.2 SEM Image Collection and Enhancement 864.4.3 Image Analysis Techniques 894.4.4 Quantitative Methods 92
4.5 X-Ray Powder Diffraction 93
4.6 Fluid Inclusion Analyses 94
4.7 Isotope Analyses 95
4.8 Geochemical Whole Rock Analyses 96
5 BOREHOLE CR194 – SAMPLE DESCRIPTIONS 97
5.1 Introduction 97
5.2 Borehole CR194 - Chemistry 98
5.2.1 Introduction 985.2.2 Geochemical Profile 100
5.3 Borehole CR194 – Gossan 103
viii
List of Contents
5.3.1 Introduction 1035.3.2 Quartz 1035.3.3 Siderite 1055.3.4 Limonite 1075.3.5 Fe-Clay 1085.3.6 Accessory Transparent Gangue Minerals 1095.3.7 Fe-Sulphides 1095.3.8 Galena and Pb-Bearing Sulphides 1105.3.9 Secondary Pb-bearing Phases 1115.3.10 Amalgam and Hg-Bearing Phases 1125.3.11 Precious Metal Mineralisation 1135.3.12 Accessory Minerals 114
5.4 Borehole CR194 – Gossan Contact with Massive Sulphide 115
5.4.1 Introduction 1155.4.2 163.75 to 164.60m Sample Interval - Upper Portion 1155.4.3 163.75 to 164.60m Sample Interval - Middle Portion 1165.4.4 163.75 to 164.60m Sample Interval - Lower Portion 117
5.5 Borehole CR194 – Massive Sulphide Contact with Gossan 121
5.5.1 Introduction 1215.5.2 Clay-Rich Layer 1225.5.3 Galena-Rich Layer 1225.5.4 Leached Pyrite-Rich Layer 1285.5.5 Lower Core 129
5.6 Borehole CR194 – Massive Sulphide 130
5.6.1 Introduction 1305.6.2 Massive Sulphide 1305.6.3 Massive Sulphide/Shale 132
5.7 Borehole CR194 – Shale 134
5.7.1 Introduction 1345.7.2 Mineralogy 134
5.8 Borehole CR194 – Summary Diagram 135
6 BOREHOLE CR149 – SAMPLE DESCRIPTIONS 136
6.1 Introduction 136
6.2 Borehole CR149 - Chemistry 138
6.2.1 Introduction 1386.2.2 Geochemical Profile 140
6.3 Borehole CR149 - Tertiary Sand 143
6.3.1 Introduction 1436.3.2 General Mineralogy 143
6.4 Borehole CR149 - Gossan 145
6.4.1 Introduction 1456.4.2 Quartz 1466.4.3 Siderite 1466.4.4 Limonite 147
ix
List of Contents
6.4.5 Accessory Transparent Gangue Minerals 1486.4.6 Fe-Sulphides 1486.4.7 Galena and Pb-Bearing Sulphides 1496.4.8 Accessory Minerals 1496.4.9 Precious Metal Mineralisation 150
6.5 Borehole CR149 - Gossan/Massive Sulphide Contact 151
6.5.1 Introduction 1516.5.2 Transparent Gangue 1516.5.3 Pyrite and other Fe-Sulphides 1526.5.4 Galena 1526.5.5 Accessory Minerals 1526.5.6 Precious Metal Mineralisation 152
6.6 Borehole CR149 - Massive Sulphide 154
6.6.1 Introduction 1546.6.2 General Mineralogy 154
6.7 Borehole CR149 – Summary Diagram 155
7 BOREHOLE CR038 – SAMPLE DESCRIPTIONS 156
7.1 Introduction 156
7.2 Borehole CR038 - Chemistry 158
7.2.1 Introduction 1587.2.2 Geochemical Profile 159
7.3 Borehole CR038 - Quartz Replaced Tuffs 161
7.3.1 Introduction 1617.3.2 Transparent Gangue Mineralogy 1617.3.3 Ore Mineralogy 1637.3.4 Precious Metal Mineralisation 164
7.4 Borehole CR038 - Quartz Replaced Tuff/Partial Massive Sulphide Contact165
7.4.1 Introduction 1657.4.2 Transparent Gangue Mineralogy 1657.4.3 Ore Mineralogy 1657.4.4 Precious Metal Mineralisation 166
7.5 Borehole CR038 - Partial Massive Sulphide 167
7.5.1 Introduction 1677.5.2 Transparent Gangue Mineralogy 1677.5.3 Ore Mineralogy 167
7.6 Borehole CR038 – Summary Diagram 168
8 BOREHOLE CR191 – SAMPLE DESCRIPTIONS 169
8.1 Introduction 169
8.2 Borehole CR191 - Chemistry 171
8.2.1 Introduction 1718.2.2 Geochemical Profile 172
8.3 Borehole CR191- Tertiary Polymict Conglomerate/ Gossan Contact 174
x
List of Contents
8.3.1 Introduction 1748.3.2 General Mineralogy 174
8.4 Borehole CR191 - Upper Gossan 176
8.4.1 Introduction 1768.4.2 Gangue Mineralogy 1768.4.3 Ore Mineralogy 1778.4.4 Precious Metal Mineralisation 178
8.5 Borehole CR191 - Middle Gossan 179
8.5.1 Introduction 1798.5.2 Gangue Mineralogy 1798.5.3 Ore Mineralogy 1808.5.4 Precious Metal Mineralisation 181
8.6 Borehole CR191 - Lower Gossan 182
8.6.1 Introduction 1828.6.2 Gangue Mineralogy 1828.6.3 Ore Mineralogy 1838.6.4 Precious Metal Mineralisation 184
8.7 Borehole CR191- Partial Massive Sulphide 185
8.7.1 Introduction 1858.7.2 General Mineralogy 185
8.8 Borehole CR191 – Summary Diagram 187
9 BOREHOLE CR123 – SAMPLE DESCRIPTIONS 188
9.1 Introduction 188
9.2 Borehole CR123 - Chemistry 189
9.2.1 Geochemical Profile 1919.3 Borehole CR123- Tertiary Polymict Conglomerate 192
9.3.1 Introduction 1929.3.2 Gangue Mineralogy 1929.3.3 Ore Mineralogy 1939.3.4 Accessory Mineralogy 193
9.4 Borehole CR123 - Upper Siderite Gossan 194
9.4.1 Introduction 1949.4.2 General Mineralogy 1949.4.3 Precious Metal Mineralisation 195
9.5 Borehole CR123 - Middle Calcite Gossan 196
9.5.1 Introduction 1969.5.2 Gangue Mineralogy 1969.5.3 Ore Mineralogy 1979.5.4 Precious Metal Mineralisation 197
9.6 Borehole CR123 - Lower Siderite Gossan 198
9.6.1 Introduction 1989.6.2 Gangue Mineralogy 1989.6.3 Ore Mineralogy 1999.6.4 Precious Metal Mineralisation 200
xi
List of Contents
9.7 Borehole CR123- Gossan/Shale Conglomerate Contact 201
9.7.1 Introduction 2019.7.2 General Mineralogy 201
9.8 Borehole CR123 – Shale Conglomerate/Gossan Contact 203
9.8.1 Introduction 2039.8.2 Transparent Gangue 2039.8.3 Pyrite 2049.8.4 Cinnabar and Sulphosalt Minerals 2049.8.5 Precious Metal Mineralisation 204
9.9 Borehole CR123 – Partial Massive Sulphide/Shale 206
9.9.1 Introduction 2069.9.2 General Mineralogy 206
9.10 Borehole CR123 – Summary Diagram 207
10 ENVIRONMENT AND FORMATIONAL MECHANISMS 208
10.1 Introduction 208
10.2 Siderite Formational Environment 209
10.2.1 Introduction 20910.2.2 Oxic Zone (Berner, 1981) 21210.2.3 Sulphate Reduction Zone (Curtis et al., 1986; Irwin et al., 1977) 21310.2.4 'Methanic' or methanogenic zone (e.g. Berner 1981, Curtis et al., 1986) 21410.2.5 Methane Oxidation 21610.2.6 Fe3+ Reduction 21810.2.7 Nitrate Reduction 22110.2.8 Abiotic reactions - Thermally induced decarboxylation 222
10.3 Formation of Fe-sulphides 223
10.3.1 Introduction 22310.3.2 Formation Mechanisms 22310.3.3 The Role of Biological Processes 227
10.4 Mineral stability fields 229
10.4.1 Siderite 22910.4.2 Fe-sulphides 23010.4.3 Siderite/Fe-Sulphide Relationships 232
11 MINERALOGY: KEY FEATURES AND PARAGENESIS 234
11.1 Introduction 234
11.2 Quartz 235
11.2.1 Relative Abundance 23511.2.2 Grain Size, Shape and Texture 23511.2.3 Fluid Inclusion and Isotope Analysis 240
11.3 Siderite 242
11.3.1 Relative Abundance 24211.3.2 Grain Size, Shape and Textures 242
xii
List of Contents
11.3.3 Associations 24811.3.4 Mineral Chemistry 25011.3.5 Isotope Analysis 25211.3.6 Fluid Inclusion Analysis 253
11.4 Galena 254
11.4.1 Relative Abundance 25411.4.2 Grain Size and Shape 25411.4.3 Associations 256
11.5 Fe-Sulphide Phases 259
11.5.1 Introduction 25911.5.2 Relative Abundance 26011.5.3 Reflected Light Characterisation 26111.5.4 Optical Properties and Occurrences of the Fe-sulphides 268
11.6 Au-Bearing Phases 271
11.6.1 Relative Abundance 27111.6.2 Grain Size and Shape 27211.6.3 Associations 27411.6.4 Chemistry 275
11.7 Gossan Paragenesis 276
11.7.1 Introduction 276
12 DISCUSSION AND CONCLUSIONS 281
12.1 Introduction 281
12.2 Seafloor Gossan Formation 282
12.3 Sub-Aerial Gossan Formation 284
12.4 Gossan reworking 288
12.5 Marine incursion and seawater alteration 290
12.6 Deep burial by Tertiary sediments 291
12.7 Modern day gossan and aquifer 293
12.7.1 Introduction 29312.7.2 Siderite and Greigite 29312.7.3 Pb-bearing sulphides 29812.7.4 Precious metals 299
12.8 Conclusions 303
12.9 Future Investigations 306
REFERENCES 307
xiii
List of Contents
VOLUME 2: APPENDICES
Appendix 1: List of Mineral Formulae A1Appendix 2: Sample List A3Appendix 3: Assay Data A6Appendix 4: XRD Data A15Appendix 5: SEM Analyses A28Appendix 6: Borehole CR194 Illustrations A49Appendix 7: Borehole CR149 Illustrations A108Appendix 8: Borehole CR038 Illustrations A133Appendix 9: Borehole CR191 Illustrations A152Appendix 10: Borehole CR123 Illustrations A175
xiv
List of Figures
List of Figures
Figure 2.1 - a) A geological map of Spain and Portugal showing the relative positions of the Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL) and the Southern Portuguese Zone (SPZ). Figure 2.1b is a more detailed map of the area marked by a red rectangle on Figure 2.1a....................................................................................................................................6Figure 2.2 - The main lithostratigraphic units in the Iberian Pyrite Belt.............................9Figure 2.3 - A general map of the Betic Cordillera showing the position of the Guadalquivir Basin and Las Cruces.................................................................................11Figure 2.4 - Summary of the main lithostratigraphic units at Las Cruces (Knight, 2000). .........................................................................................................................................16Figure 2.5 – An idealised, simplified N-S cross-section through the Las Cruces orebody that is based on the interpretation of drill core data and block modelling information performed by Rio Tinto consultants..................................................................................17Figure 2.6 - Stage 1 - formation of the Las Cruces primary massive sulphide deposit during a primary hydrothermal event with waxing and waning thermal history................21Figure 2.7 - Stage 2 - Sub-marine oxidation and secondary Cu-sulphide enrichment during the waning stages of hydrothermal activity...........................................................23Figure 2.8 - Stage 3 - Sustained volcanism and sedimentation leading to the burial of the massive sulphide beneath ~1000m Palaeozoic Culm sediments..............................24Figure 2.9 - Stage 4 - Tilting of the primary massive sulphide occurred during the Hercynian, with uplift and erosion being followed by sub-aerial weathering and the development of the gossan, silica cap and supergene Cu-sulphides..............................25Figure 2.10 - Stage 5 - Reworking of the gossan and silica cap possibly prior to and following the onset of the marine incursion during the Miocene......................................26Figure 2.11 - Stage 6 - Burial and preservation of the Las Cruces deposit under up to 1000 metres Tertiary sediments.......................................................................................27Figure 2.12 – a) A map of the Las Cruces deposit illustrating the extent of the Au mineralisation (solid yellow line), supergene Cu-sulphide mineralisation (solid blue line) and the positions of the boreholes selected for examination during this investigation.....31Figure 3.1 – Diagram illustrating the zones of weathering in terms of Eh and pH according to Sato (1960)..................................................................................................38Figure 3.2 – Eh/pH diagram at 25oC and 1 atmosphere total pressure, illustrating the relationships between groundwater position and mineral stability ranges.......................39Figure 3.3 - Idealised zones in the weathering profile of a VHMS Zn-Pb-Cu deposit that has been weathered to produce a mature gossan profile................................................47Figure 3.4 – Eh/pH diagram illustrating the stability relations between iron oxides and iron sulphides in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-6................................................................................................................................49Figure 3.5 – Eh/pH diagram illustrating the stability relations of some Au compounds in water at 25oC and 1 atmosphere total pressure at total dissolved chloride species of 100
and sulphur activity of 10-1................................................................................................51
xv
List of Figures
Figure 3.6 – Eh/pH diagram illustrating the stability relations of some Cu minerals in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10 -1, CO3
activity of 10-3....................................................................................................................60Figure 3.7 – Eh/pH diagram illustrating the stability relations of Pb compounds in water at 25oC and 1 atmosphere total pressure. Total dissolved sulphur of 10-1, pCO2 of 10-4..62Figure 3.8 - An illustration of the field relationships of a typical small umber hollow related to seafloor faulting, Troodos Massif, Cyprus........................................................75Figure 4.1 - A monochrome backscattered electron image illustrating a rather complex Fe-oxide-rich sample with fine intergrowths of galena.....................................................88Figure 4.2 - The monochrome backscattered electron image has been false coloured and permits the reader to readily distinguish the mineral species. Galena (white) occurs as fine skeletal aggregates. Limonite fragments (yellow-brown shades) exhibit a wide range in brightness that reflects degrees of hydration. Darker browns represent more hydrated Fe-oxides (e.g. goethite). The darkest brown/black portions of the image represent areas of high porosity.......................................................................................88Figure 4.3 - A typical backscattered electron image as captured by the image analysis system..............................................................................................................................90Figure 4.4 - The system recognises the range of grey shades of interest (red areas), depending on criteria set by the operator.........................................................................90Figure 4.5 - Each grain of interest is recognised by the electron microscope and selected for analysis/measurement..................................................................................91Figure 4.6 - An example of an EDX spectrum captured using a very rapid (typically 200msec) EDX analysis of each grain.............................................................................91Figure 5.1 - Illustrating the chemistry variation in borehole CR194.................................99Figure 5.61 - Diagram illustrating the key mineralogical features for the 'Gossan', 'Gossan/Massive Sulphide Contact', 'Massive Sulphide/Gossan Contact', 'Massive Sulphide', 'Massive Sulphide/Shale' and 'Shale'............................................................135Figure 6.1 - Illustrating the chemistry variations in borehole CR149............................139Figure 6.27 - Diagram illustrating the key mineralogical features for the 'Tertiary Sand', 'Gossan', 'Gossan /Massive Sulphide Contact' and 'Massive Sulphide'.........................155Figure 7.1 - Diagram illustrating chemistry variations in borehole CR038...................158Figure 7.21 - Diagram illustrating the key mineralogical features for the 'Quartz Replaced Tuffs', 'Quartz Replaced Tuff/Partial Massive Sulphide Contact' and 'Partial Massive Sulphide'.........................................................................................................................168Figure 8.1 - Diagram illustrating chemistry variations in borehole CR191....................171Figure 8.25 - Diagram illustrating the key mineralogical features for the 'Tertiary Conglomerate/Gossan Contact', ‘Upper Gossan', 'Middle Gossan', 'Lower Gossan’ and ‘Partial Massive Sulphide’..............................................................................................187Figure 9.1 - Diagram illustrating chemistry variations in borehole CR123....................190Figure 9.35 - Diagram illustrating the key mineralogical features for the ‘Tertiary Polymict Conglomerate’, ‘Upper Siderite Gossan', 'Middle Calcite Gossan', 'Lower Siderite Gossan’, ‘Gossan/Shale Conglomerate Contact’, ‘Shale Conglomerate/Gossan Contact’ and ‘Partial Massive Sulphide/Shale’.............................................................................207Figure 10.1 – A diagram illustrating the three distinct biogeochemical environments that mark the boundaries between regimes of aerobic and anaerobic metabolism..............211
xvi
List of Figures
Figure 10.2 – Eh/pH diagram illustrating the stability of hematite, magnetite and siderite at 25oC and 1 atmosphere total pressure and pCO2 = 10-2 atmosphere with total activity of dissolved species = 10-6.............................................................................................229Figure 10.3 - Pe/pH diagrams illustrating the stability relations for iron sulphides in seawater at 25oC, 1 atmosphere total pressure. A) Iron activity 10−6, sulphur activity 10−2.551, C(IV) activity 10−3.001, troilite and pyrrhotite suppressed. B) Same as A with pyrite also suppressed. C) Same as B with marcasite suppressed. D) Same as C with greigite and mackinawite suppressed. E) Same as C but solution changed to world average river water with iron activity 10−6, sulphur activity 10−3.902, C(IV) activity 10−3.06. F) Same as C but iron activity 10−3 and C(IV) activity 10−2.5..................................................................231Figure 10.4 – Eh/pH diagram illustrating the stability relations between iron oxides, sulphides and carbonates in water at 25oC and 1 atmosphere total pressure at ΣCO2 of 100 and ΣS of 10-6...........................................................................................................232Figure 11.1 - Borehole CR123 - A colour transmitted light photomicrograph of fibrous quartz (white and grey shades) developed around the margins of pyrite crystals (black)........................................................................................................................................238Figure 11.2 - Borehole CR038 - A colour transmitted light photomicrograph illustrating more coarsely crystalline quartz fragments that are cemented by fine-grained, partially recrystallised chalcedony...............................................................................................239Figure 11.3 - Borehole CR149 - A colour, crossed polarised transmitted light photomicrograph from the gossan/massive sulphide contact illustrating a cavity that has been filled by fibrous chalcedony...................................................................................240Figure 11.4 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of angular siderite ‘fragments’ in a matrix of quartz and oxidised siderite..............................................................................................................243Figure 11.5 - Borehole CR194 – False colour backscattered electron images illustrating a) a siderite ‘fragment’ that actually represents a cavity filling. b) Compositionally zoned siderite filling a euhedral cavity in quartz. c) Siderite that appears to have extensively replaced barite. d) Siderite filling cavities in botryoidal limonite.....................................244Figure 11.6 - Borehole CR194 – a digitised photograph showing apparent ‘fragments' of siderite............................................................................................................................245Figure 11.7 - Borehole CR194 - False coloured backscattered electron image illustrating the presence of galena replacing siderite along grain boundaries and highlighting different generations of siderite mineralisation...............................................................246Figure 11.8 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of late-stage, unoxidised siderite filling a cavity in an oxidised, opaque siderite matrix.....................................................................................247Figure 11.9 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of early-formed siderite crystals that have formed in a cavity..............................................................................................................................247Figure 11.10 - Borehole CR194 – False colour backscattered electron image illustrating the presence of siderite and galena filling and partially filling cavities in hematite.........249Figure 11.11 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of skeletal galena in euhedral Fe-sulphide crystals..................................250Figure 11.12 - Borehole CR191 – False colour backscattered electron image illustrating the selective leaching of compositional zones within siderite crystals...........................251
xvii
List of Figures
Figure 11.13 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of a fine-grained galena aggregate that is being progressively replaced from the upper left to lower right by siderite...................................................................256Figure 11.14 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of associations between galena and other minerals..........................................................................................................................258Figure 11.15 - An X-ray diffractogram clearly illustrating the presence of greigite........260Figure 11.16 – Colour, reflected light photomicrograph illustrating Type 1 Fe-sulphide, consisting of feathery, colloidal radiating aggregates of Fe-sulphide.............................261Figure 11.17 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals with marcasite/pyrite inclusions forming overgrowths on a colloidal Fe-sulphide aggregate.......................................................................................................................262Figure 11.18 – Colour, reflected light photomicrograph illustrating colloidal radiating aggregates of Fe-sulphide and finely disseminated euhedral Fe-sulphide crystals in siderite............................................................................................................................263Figure 11.19 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals in siderite...........................................................................................................264Figure 11.20 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals with marcasite/pyrite inclusions in siderite........................................................264Figure 11.21 - Colour, reflected light photomicrograph illustrating feathery, strongly anisotropic Fe-sulphide with cubic overgrowths of Fe-sulphide crystals in siderite.......265Figure 11.22 - Colour, reflected light photomicrograph illustrating platelets of an anisotropic Fe-sulphide phase with minor pyrite/marcasite in a matrix of siderite ........266Figure 11.23 - Platy textures in pyrite that has pseudomorphously replaced Type 4 Fe-sulphide in siderite..........................................................................................................267Figure 11.24 - Marcasite extensively replaces the strongly anisotropic Fe-sulphide phase that is partially filling a resin-filled euhedral cavity in quartz ...............................267Figure 11.25 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of morphologies and associations of the Au and Au-bearing grains...........................................................................................................273Figure 11.26 – Montage of false colour backscattered electron images illustrating partial and complete paragenetic sequences observed during this investigation.....................277Figure 12.1 - Diagram illustrating a) Primary massive sulphide and seafloor gossan preserved under culm sediments produced by continued volcanic activity. b) Tilting of the deposit during the Hercynian would have resulted in a steeply dipping primary massive sulphide and preserved seafloor gossan quite distinct from the sub-aerially derived, horizontal gossan, silica cap and supergene mineralisation..........................................283Figure 12.2 – A schematic illustrating the distinct biogeochemical and abiotic environments that mark the boundaries between regimes of aerobic and anaerobic metabolism and subsequent carbonate and/or sulphide mineral precipitation..............295Figure 12.3 – Diagram illustrating an idealised cross section through the Las Cruces deposit............................................................................................................................303
xviii
List of Tables
List of Tables
Table 11.1 - Siderite 13C and 18O Ratios.....................................................................252Table 12.1 - Comparison of Mature Gossans and Las Cruces Gossan Mineralogy......287
xix
Chapter 1 Introduction
1 INTRODUCTION
1.1 Introduction
The Las Cruces Volcanogenic Massive Sulphide (VMS) deposit is situated within
the Iberian Pyrite Belt (IPB), one of the world’s largest massive sulphide
provinces. The IPB is approximately 250Km long and up to 70Km wide and
hosts more than 80 known mines including Aznalcollar-Los Frailes and Rio Tinto
in Spain and Neves Corvo in Portugal (Leistel et al., 1998). Las Cruces was
discovered by Rio Tinto in 1994.
The Las Cruces primary massive sulphide is essentially similar to other VMS
deposits within the IPB. However, unlike other VMS deposits within the region,
the gossan and supergene mineralisation at Las Cruces is undisturbed by
historical mining activity or erosion being extremely well preserved under
approximately 150 metres of Tertiary deposits.
Early mineralogy reports on the Las Cruces gossan conducted by Rio Tinto
Limited at their Anamet Services laboratory (R2643, 1996; R2644, 1996; R2696,
1997) confirmed that the mineralogy is markedly different from other VMS derived
gossans described in the literature. The Las Cruces gossan consists
predominantly of siderite, galena and subordinate amounts of Fe-sulphides
whereas most sub-aerially derived gossans typically consist of Fe-oxyhydroxide
and metal sulphates.
1.2 Aims of Study
The main focus of this investigation is the Las Cruces gossan. A thorough
characterisation of the mineralogy, the mineral associations and textures through
five sections of precious metal gossan provide the basis of this study. The
mineralogy is used to identify sequences of events in the history of the gossan to
help understand the processes that have resulted in the mineralogical
assemblage. Particular attention is given to the nature and mode of occurrence
of siderite, greigite and galena, the dominant gossan minerals, and to Au and Ag,
the only elements likely to be worthy of economic interest within the gossan.
Page 1
Chapter 1 Introduction
The mineralogy and geochemical profiles developed in the gossan are compared
and contrasted with the mineralogy and geochemical profiles developed in
gossans described in the literature, with the aim of interpreting, as far as
possible, the formational history of the gossan.
Reflected and transmitted light microscopy, X-ray powder diffraction (XRD) and
scanning electron microscopy (SEM) are used to identify and illustrate the
different mineral species present in Las Cruces. Modern SEM-based image
analysis techniques are also used to locate large numbers of precious metal-
bearing grains, with a large number of backscattered electron images being
prepared to illustrate textural information and mineral associations.
The Las Cruces deposit remains buried and as yet unexploited for its mineral
wealth. Removal of the overburden has commenced by the deposits’ current
owners, MK Resources Company, and it is expected that mining of the
supergene Cu ore will begin in 2008. The Las Cruces VMS deposit is considered
to be one of the highest grade Cu deposits in the world.
With only limited information currently available on the Las Cruces gossan, this
investigation provides significant detail on the mineralogy, geochemistry and
styles of mineralisation that may be used as a basis for future investigations.
The final data collection for this thesis took place on 28th September 2007.
Page 2
Chapter 1 Introduction
1.3 Thesis Outline
Chapter 2 describes the local and regional geology of Las Cruces and the Iberian
Pyrite Belt and exploration history of the Las Cruces orebody. The locations of
the samples selected for examination during this investigation are also discussed
in Chapter 2. Chapter 3 includes a literature review on gossans, the processes
involved in gossan formation and predominant influences on gossan formation.
The geochemical profiles, mineralogy and element mobility are discussed. Sub-
marine gossan formation in modern and ancient deposits is also discussed.
The methodologies employed during this investigation, including sample
preparation techniques, reflected and transmitted light microscopy, scanning
electron microscopy and X-ray powder diffraction are given in Chapter 4.
Chapters 5 through to 9 include the major and minor element geochemistry and
geochemical profiles of the Las Cruces gossan, together with detailed
descriptions of each of the boreholes selected for examination. The illustrations
are provided in Appendices 6 to 10.
Chapter 10 describes the environment and formational mechanisms for siderite
and greigite. Chapter 11 summarises the key mineralogical features of the Las
Cruces gossan.
Chapter 12 discusses the evidence presented in the previous chapters and
compares how the mineralogy and geochemistry of the Las Cruces gossan fit
with the model of formation described by Knight (2000), the only other significant
academic work on this deposit to date. Conclusions are drawn from the evidence
presented in the previous chapters. This chapter concludes with a brief
discussion on how future investigations may be focussed.
Page 3
Chapter 2 Geology
2 GEOLOGY
2.1 Regional Geological Setting - The Iberian Peninsula
The Iberian Peninsula is largely underlain by a Hercynian belt of approximately
750 km in length, extending in a NW-SE direction (Figure 2.1a, blue). The
Hercynian belt consists of a number of discrete zones or terranes that were
progressively accreted during the Pan-African/Cadomian and Hercynian
Orogenies. These zones are the Cantabrian Zone (CZ), West Asturian-Leonese
Zone (WALZ), Galicia Tras-os-Montes Zone (GTZ), Central Iberian Zone (CIZ),
the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the
Pulo do Lobo (PL), the Southern Portuguese Zone (SPZ) and the Porto Tomar
Shear Zone (Figure 2.1a). Precambrian and Palaeozoic sequences in Alpine
belts are shown in red (Leistel et al., 1998).
The Central Iberian Zone belongs to the Iberian Autochthon onto which the other
zones were accreted (Ribeiro et al., 1990; Quesada, 1991). The Badajoz-
Cordoba Shear Zone is a major suture formed during the Pan-African and
Hercynian Orogenies (Quesada, 1991). The CIZ and accreted OMZ underwent a
passive margin type evolution in the northern margin of Gondwana until the onset
of the Hercynian orogeny in early to mid-Devonian (Leistel et al., 1998). The
Pulo do Lobo Zone is a complex ophiolite sequence formed as a result of the
subduction of oceanic lithosphere at the outer margin and underneath the OMZ
(Leistel et al., 1998).
The collision of the Southern Portuguese Zone with the Ossa-Morena Zone
resulted in the lateral escape of units that coincided with bimodal magmatism,
hydrothermal circulation and ore deposition (Leistel et al., 1998; Oliveira, 1990
and Quesada et al., 1991). These marginal units represent what is known today
as the Iberian Pyrite Belt. The tectonic setting was extensional and epicontinental
and developed during the Hercynian plate convergence, culminating in thin-
skinned deformation and accretion of the South Portuguese terrane to the Iberian
Palaeozoic continental block (Leistel et al., 1998) (Figure 2.1a).
Page 4
Chapter 2 Geology
Additional divisions of the Southern Portuguese Zone include the Baixo Alentejo
Flysch Domian and the SW Portugal Domain. These sub divisions are related to
the breakdown of a Devonian platform resulting from the continent-continent
collision that occurred throughout the early Carboniferous (Saez et al., 1996).
The northern sector of the Southern Portuguese Zone consists of siltstones of
Precambrian to upper Palaeozoic age that have been metamorphosed to
greenschist facies. These rocks are thought to be the source of the clastic
materials of the Iberian Pyrite Belt sediments (Strauss and Madel, 1974). The
central sector of the Southern Portuguese Zone makes up the Iberian Pyrite Belt.
The southern sector comprises marine sandstones, shales and limestones of
Devonian to Carboniferous age (Oliveira, 1983).
Page 5
Chapter 2 Geology
Figure 2.1 - a) A geological map of Spain and Portugal showing the relative positions of the Central Iberian Zone (CIZ), the Badajoz-Cordoba Shear Zone (BCSZ), the Ossa-Morena Zone (OMZ), the Pulo do Lobo (PL) and the Southern Portuguese Zone (SPZ). The Hercynian orogenic belt is shown in blue. Precambrian and Palaeozoic sequences in Alpine belts are shown in red. Figure 2.1b is a more detailed map of the area marked by a red rectangle on Figure 2.1a. This map shows the locations of the main Volcanogenic Massive Sulphide (VMS) deposits, including the Las Cruces deposit, positioned toward the southeast of the region under the post Palaeozoic cover. (Modified from Quesada, 1991)
Page 6
Chapter 2 Geology
2.2 The Iberian Pyrite Belt
The Las Cruces deposit lies within the Iberian Pyrite Belt (IPB), in the south west
part of the Iberian Peninsula (Figure 2.1b). Situated within late Devonian to
middle Carboniferous rocks, covered in places by Tertiary-Quaternary terrace
and alluvial deposits, the IPB is 250Km long and 25-70Km wide.
The IPB hosts a huge quantity of volcanic-hosted massive sulphide (VMS)
mineralisation with more than 80 known mines totalling 1700Mt of sulphides and
containing 14.6Mt Cu, 13.0Mt Pb, 34.9Mt Zn, 46100t Ag and 880t Au (Leistel
et al., 1998). Mining of the outcropping deposits in the IPB dates back to the
Chalcolithic era (5000–3000BC) with Tartassians, Phoenicians and Romans
extracting Cu, Au and Ag from oxide and supergene zones overlying the massive
sulphide orebodies (Strauss et al., 1990).
With recent cessation of the use of pyrite for sulphuric acid production, large
scale mining in the IPB belt is now limited. Only five mines remain active in the
belt today, namely Soteil-Migollas, Aznalcollar-Los Frailes, Rio Tinto and Tharsis
in Spain and Neves Corvo in Portugal (Leistel et al., 1998). The locations of
these deposits are shown in Figure 2.1b. With the discovery of the Neves-Corvo
Cu-Sn deposit in 1977, renewed interest in exploration for deep, 'blind' VMS
deposits resulted in a number of new discoveries, Las Cruces being one of the
more recent and significant additions.
Throughout the Phanerozoic, Europe has been subjected to three continuous
compressional and extensional periods of geotectonic activity, namely the
Caledonian Orogeny (circa 600–350Ma), Variscan Orogeny (circa 550-250Ma)
and Alpine Orogeny (circa 250-0Ma) (Rickard, 1999). The Iberian Pyrite Belt was
formed during the Variscan Orogeny, during the development of pull-apart basins
alongside continental margins. The Variscan Orogeny resulted from the closure
of the pre-Mediterranean Tethys Ocean, climaxing at around 300Ma and is partly
synonymous with the Hercynian Orogeny of Northern Europe (Rickard, 1999). All
the sequences of the Pyrite Belt were deformed during the Hercynian orogeny,
which was accompanied by low-grade regional metamorphism, ranging from
zeolite to lower greenschist facies (Munha, 1983).
Page 7
Chapter 2 Geology
The massive sulphide deposits of the IPB exhibit specific features that aid in their
identification and characterisation, including mineralogy and geochemistry, Pb
isotope data, hydrothermal alteration and structure. The geochemistry of a large
part of the basic lavas associated with the IPB are comparable to those of
mantle-derived basalts emplaced in extensional tectonic settings and the
associated acidic rocks were produced by melting of a basic crustal protolith at
low to medium pressures and a steep geothermal gradient (Leistel et al., 1998).
The IPB consists of an extremely complex succession of Late Devonian to Middle
Carboniferous rocks resulting from several facies variations and intense tectonic
deformation overlain by Tertiary to Quaternary sediments (Oliveira, 1990).
The stratigraphy of the IPB has classically been sub-divided into three principle
units, the Phyllitic Quartzite (PQ) formation, the Volcano-Siliceous (VS,
Devonian-Carboniferous) complex and the Culm (or Flysch, Upper
Carboniferous) group (Schermerhorn, 1971). The main lithostratigraphic units in
the Iberian Pyrite Belt are illustrated in Figure 2.2.
The PQ formation, estimated to be greater than 1000m in thickness (Strauss,
1970), consists of Late Devonian shale, quartz sandstone and rare conglomerate
that essentially form the base of the IPB. The depositional environment is
thought to be a shallow epicontinental sea (Leistel et al., 1998). Dating of the
upper 30m thick sequence of carbonates and bioclastic lenses indicate late
Famennian age (late Devonian circa. 367–362Ma) (Van den Boogard et al.,
1980). Towards the top of the unit, the uniform nature of the PQ formation
changes and is marked by high energy sedimentary deposits registering the
tectonic evolution of the IPB basins (Moreno et al., 1996).
Page 8
Chapter 2 Geology
Figure 2.2 - The main lithostratigraphic units in the Iberian Pyrite Belt. 1. Shales and greywackes 2. Black shales, siliceous shales and tuffites 3. Exhalites (mostly jaspers) 4. Shales, greywackes, quartzwackes and quartzites 5. Polymetallic massive sulphides and stockworks 6. Felsic volcanic rocks, mostly tuffs 7. Mafic rocks (spilites and dolerites) 8. Phyllites and quartzites (modified from Carvalho 1999).
The VS complex dates from late Famennian to middle Visean (circa 342–339Ma)
(Oliveira, 1990) and varies in thickness between 100 and 600m (Leistel et al.,
1998). Exposure of the VS complex is restricted to the IPB. Although somewhat
variable between zones in the IPB, the VS complex essentially consists of
alternating felsic and mafic, sub-aerial to sub-marine volcanics within detrital and
Page 9
Chapter 2 Geology
chemically derived sediments (Saez et al., 1996). The most complete VS
sequence, evident in some units of the southern branch of the belt are (Leistel et
al., 1998):-
1. A lowermost rhyolitic sequence (VA1), with fine to coarse-grained
pyroclastics and lavas
2. A second rhyolitic sequence (VA2), with pyroclastics and lavas
3. A third rhyolitic sequence (VA3), mainly reworked tuffs and siliceous shale.
4. Basic lava, locally pillowed, intercalated between VA1 and VA3; basic
dykes and sills injected into the lower part of the complex (possibly feeder
zones).
5. A purple shale situated directly below VA3.
6. A pelite-black shale and sandstone containing beds of jasper and rare
limestone, interstratified with VA1 to VA2 volcanics.
The Culm facies or Baixo Alentejo flysch group is a thick turbidite formation
forming a south-westward prograding detrital cover that is diachronous over the
underlying VS complex. The thickness of this facies is estimated to be up to
3000m (Strauss and Madel, 1974).
Moreno (1993) describes three stratigraphic units for the Culm facies:-
1. The Basal Shaly Formation (BSF), consisting of volcanic and non-volcanic
sediments, marking the end of volcanic activity in the region and the
beginning of autochthonous sedimentation of pelagic clay.
2. The Culm Facies Turbidite Formation (CFTF), consisting of turbidite
sequences of sandstones, shales and minor conglomerates.
3. The Shallow-Platform Sandy Unit (SPSU) consisting of shales and
sandstones reworked and redeposited following the erosion of volcanic
uplands.
Page 10
Chapter 2 Geology
2.3 The Guadalquivir Basin
The Guadalquivir Basin is situated along the eastern end of the IPB. Las Cruces
lies along the western margin of the Guadalquivir Basin and is buried under
approximately 150m of Tertiary sediments. This relatively flat lying area ranges
from 15 to 50 metres above sea level. The Guadalquivir Basin lies between the
Iberian Foreland to the north and the Betic Cordillera to the south (Figure 2.3).
The Betic Cordillera is the northern segment of an arcuate orogen that extends
over 600Km westward across the Gibraltar Arc into the Rif Chain. The inner part
of this orogen is occupied by the extensional basin of the Alboran Sea. The
cordillera contains numerous Neogene basins, including the Sado Basin to the
NW and the Guadalquivir Basin in the SE (Sanz de Galdeano and Vera, 1992).
Figure 2.3 - A general map of the Betic Cordillera showing the position of the Guadalquivir Basin and Las Cruces (modified from Gomez et al., 2003).
The Guadalquivir Basin was formed during the Alpine orogeny (Miocene to
Recent) as the African Plate continued to collide with the Eurasian Plate. Dewey
et al. (1989) determined that this area experienced in the order of 200Km of N-S
convergence between the mid-Oligocene (circa 30Ma) and late Miocene (circa
6Ma), followed by approximately 50Km of WNW-directed oblique convergence in
the late Miocene to recent times. The evolution of the Guadalquivir Basin ended
in the Messinian (circa 6Ma) when the basin was partially filled by Miocene
sediments, consisting predominantly of marine marls (Fernandez et al., 1998).
Page 11
Chapter 2 Geology
The basement of the Guadalquivir Basin consists predominantly of PQ, VS and
Culm Palaeozoic sediments that dip gently in a SSE direction. The Miocene
sediments increase in thickness towards the south, reaching a maximum
thickness of ~15Km. Fernandez et al. (1998) suggest that the Iberian Massif to
the north of the Guadalquivir Basin provided clastic infill to the basin. These
clastics were subsequently redistributed along the ENE-WSW axis of the
basement by turbiditic currents.
The external zones of the Betic Cordillera provide a gravitational infill to the
south, producing sedimentary deposits known as 'olistostromes', consisting
predominantly of chaotic mixtures of Triassic evaporites, clays, limestones and
Upper Cretaceous to Palaeozoic limestones (Fernandez et al., 1998). There has
been significant erosion of the Miocene sediments in recent times and the Las
Cruces deposit may at one time have been buried by as much as 1000m of
sediment (Knight, 2000).
Page 12
Chapter 2 Geology
2.4 Las Cruces - Exploration History
The Las Cruces volcanogenic massive sulphide deposit is situated on the
western margin of the Guadalquivir basin, in the southern region of the Iberian
Pyrite Belt in the Seville Province of Andalucia approximately 15km northwest of
the city of Seville (Figures 2.1 and 2.3). After an absence from the Pyrite Belt of
several years, in 1990, Rio Tinto began the exploration programme that led to the
Las Cruces discovery. The block of ground that contains Las Cruces
(Faralaes II) is covered almost entirely by recent Tertiary sediments and was
applied for in 1991 on the supposition that Palaeozoic rocks prospective for base
metals were concealed below the Tertiary sediments. The presence of Boliden's
Aznalcóllar orebody in exposed Palaeozoic rocks 12.5km to the west gave weight
to this supposition.
The initial exploration method used by Riomin Exploraciones (Rio Tinto Mining
and Exploration) was gravity surveying. This is a costly technique and in 1992
Rio Tinto departed from traditional practice by reducing the density of the survey
points from 50 - 100/km² to 11 - 15/km² thus allowing more extensive and faster
survey coverage of all available prospective ground. This was largely
responsible for the discovery of Las Cruces. The key steps in the discovery of
the Las Cruces deposit were:-
Early 1994, a gravity anomaly of exceptional size and strength was
detected at Las Cruces.
The first scout hole to investigate the anomaly was drilled in May 1994,
showing that the cause was a large concealed sulphide deposit (the first
hole returned insignificant amounts of base metal, the sulphide consisting
almost entirely of pyrite).
September 1994, the first promising secondary Cu mineralisation was
intersected grading 3.77 per cent Cu over 42 metres.
Page 13
Chapter 2 Geology
June 1995, very high-grade secondary Cu mineralisation was intersected
close to the central part of the main orebody, grading 19.49 per cent Cu
over 17 metres.
June 1995 to October 1996, progressive delineation of secondary Cu-rich
mineralisation at shallow depth continued, together with the confirmation of
thick zones of primary Zn-Cu mineralisation.
October 1996, Las Cruces moved to project status, with a first phase
feasibility study being completed in September 1998.
The Las Cruces deposit was sold to MK Gold Company (now MK
Resources Company), a subsidiary of US-based Leucadia National
Corporation, in 1999. Inmet Mining Corporation is now the majority owner
of the project after it acquired 70 percent from MK Resources in August
2005.
Page 14
Chapter 2 Geology
2.5 Las Cruces - Geology and Mineralogy
2.5.1 Introduction
The mineralisation occurs in volcano-sedimentary rocks of Devonian to
Carboniferous age. Similar associated deposits include Neves Corvo in Portugal
and the Rio Tinto, Soteil and Aznalcollar mines in Spain. The Las Cruces deposit
is covered by a thick layer of Tertiary sediments of Miocene age, dating from
circa 6Ma. These sediments have prevented the erosion of the Las Cruces
orebody and have resulted in a high degree of preservation of the supergene
massive sulphide mineralisation and associated gossan.
Although Las Cruces lies outside of the confines of the IPB, the basement rocks
are the same as those described for the IPB and consist of the Phyllitic Quartz
formation, the Volcano-Sedimentary sequence and the Culm facies. The host
rock lithology for Las Cruces is typical for the IPB. The footwall is dominated by
highly deformed acid volcaniclastics, with interbedded shales becoming
increasingly important to the west (Knight, 2000). The main lithostratigraphic
units at Las Cruces are illustrated in Figure 2.4.
Page 15
Chapter 2 Geology
Figure 2.4 - Summary of the main lithostratigraphic units at Las Cruces (Knight, 2000). The massive sulphides lie within an approximately 80 metre thick sequence of black shales and consist of gossan, secondary Cu, primary Cu/Zn and stockwork zones.
The volcaniclastics include high level intrusive lavas and tuffs which, in part, have
been altered due to seawater interaction (Knight, 2000). Hydrothermal alteration
is prevalent in the footwall sequence, with some zoning around the stockwork
and chloritic and sericitic alteration throughout. Kaolinitic alteration is evident in
the centre of the footwall, with some late-stage carbonate replacement and
silicification also being evident (Knight, 2000).
The massive sulphide deposit is hosted within an approximately 80 metre thick
sequence of black shales with some volcanic material. The black shale host is
no more pyritic than other shale units in this sequence (Knight, 2000). The
hangingwall consists predominantly of shales with subordinate interbedded
volcaniclastics which, in places, are almost indistinguishable from the footwall
volcaniclastics, although some extensive zones of brecciation are present
(Knight, 2000).
Page 16
Chapter 2 Geology
The Las Cruces orebody consists of a gossan cap that overlies secondary and
primary massive sulphide mineralisation. The massive sulphide orebody consists
of a number of discrete primary and secondary sub-lenses comprising HC (HCH
and HCL), CZ (primary Zn and primary Cu), C4 and CB (Figures 2.4 and 2.5).
These lenses are described in greater detail in the following section.
Figure 2.5 – An idealised, simplified N-S cross-section through the Las Cruces orebody that is based on the interpretation of drill core data and block modelling information performed by Rio Tinto consultants (R2795, 1998). CB = Cu lens Barren, C4 = covellite zone, CZ = primary Cu/Zn, HCF = High Cu Footwall, HC = High Cu.
2.5.2 Gossan
The gossan is situated within the Carboniferous hangingwall, directly above the
HC secondary massive sulphide orebody and is developed in the pre-Tertiary
oxidising zone. The gossan ranges from between 0 and 20 metres in thickness
Page 17
Chapter 2 Geology
(R2795, 1998). In 1997, the gold bearing gossan resource was estimated to be
133,000 tonnes averaging 6.7ppm Au (R2703, 1998).
The mineralogy of the gossan is dominated by the presence of siderite and
quartz together with the more typical assemblage of Fe-oxides and Fe-
hydroxides. Sulphide minerals are also abundant throughout the gossan and
include galena and Fe-sulphides. Textural evidence suggests that the gossan
has been subjected to some degree of reworking and mechanical transportation.
The gossan is markedly enriched in Au and Ag relative to the underlying massive
sulphide mineralisation. The gossan is the main focus of this thesis and is
described in greater detail in Chapters 5 to 9.
2.5.3 Secondary Massive Sulphide
The HC (High Cu) sub-lens represents the secondary supergene enriched zone
and contains the bulk of the economic mineralisation. It is a flat-lying tabular unit
with a strong undulating footwall and flatter hangingwall. The bulk of the
secondary mineralisation occurs in the central and eastern part of the deposit.
Two thicker areas of secondary massive sulphide in the SW and NE are linked by
a thinner region of secondary massive sulphide forming a dumbbell shaped lens
(Knight, 2000).
The HC lens is divided into HCH (High Cu, High density) and HCL (High Cu, Low
density). The HCH lens is interpreted as the supergene replacement of massive
sulphides and the HCL is interpreted as the supergene replacement of partial
massive sulphides and associated wallrocks. The HCF (High Copper Footwall) is
a low tonnage, discontinuous, disseminated sulphide lens occurring just below
the HC footwall (R2795, 1998).
Localised E-W trending faults have produced permeable zones within the
orebody, increasing the depth of penetration of the supergene fluids, resulting in
an increase in thickness of the supergene enrichment (R2795, 1998). The
mineralogy and textures observed in the secondary massive sulphide reflect the
nature of the primary mineralisation and the degree of supergene alteration. The
HC zone consists of pyrite and digenite together with subordinate amounts of
Page 18
Chapter 2 Geology
chalcocite, covellite, chalcopyrite, bornite, tetrahedrite-tennantite, enargite and
galena (Knight, 2000). Gangue minerals include quartz, barite, calcite and
alunite. The C4 sub-lens is a small secondary massive sulphide lens at the base
of the primary sulphide orebody dominated by pyrite and covellite.
By 2005, the present owners of the deposit, Inmet Mining estimated that at a 1.0
percent Cu cutoff, the measured plus indicated resource for the combined HCH,
HCL and C4 lenses is 15.6 million tonnes averaging 6.89 percent copper, with an
additional inferred resource of 0.360 million tonnes averaging 8.66 percent
copper.
2.5.4 Primary Massive Sulphide
The CZ (Cu-Zn) sub-lens represents the main primary massive sulphide orebody
and consists of a tabular structure dipping to the north at an angle of
approximately 35o, flattening towards the west of the deposit. The upper portion
of this zone is typically Zn-rich relative to the lower Cu-rich portion (R2795, 1998).
The relative proportions of the dominant ore, gangue and accessory minerals
vary significantly, largely reflecting the primary depositional processes. The ore
is typically fine-grained and exhibits a range of textural features reflecting
variations in the degree of recrystallisation (Knight, 2000). In 1997, the CZ
resource was estimated to be 13.9 million tonnes at 2.2 per cent Cu, 0.9 per cent
Pb and 2.5 per cent Zn (R2703, 1998).
Pyrite is the dominant mineral together with subordinate amounts of chalcopyrite,
sphalerite and galena. Accessory minerals include tennantite-tetrahedrite,
arsenopyrite, enargite, cassiterite and Bi-bearing sulphosalts. Gangue minerals
include quartz, barite, clays, dolomite and calcite (Knight, 2000).
The CB (Cu lens, Barren) is a barren sub-lens within the primary massive
sulphide orebody. The footwall rocks are volcano-sedimentary silicate-rich rocks
containing some mineralised stockwork structures. The hangingwall rocks
consist predominantly of unmineralised Carboniferous volcano-sedimentary
silicate-rich rocks that are overlain by a thick sequence of Tertiary sand and marl
sediments (R2795, 1998).
Page 19
Chapter 2 Geology
2.6 Las Cruces - Evolutionary History
The emplacement of the Las Cruces orebody has many similarities to other
massive sulphide deposits of the IPB. This similarity ends with the events that
post date the emplacement of the primary massive sulphide and resulted in the
preserved supergene mineralisation and gossan that is observed today.
The only significant works to date on the formation of the massive sulphide
orebody is by Knight (2000), who produced a model for the development of the
primary and secondary mineralisation based on the mineralogy, stable isotopes,
fluid inclusions and noble gas data. Knight (2000) concludes that the paragenetic
sequence that resulted in the formation of the present day deposit at Las Cruces
included seven distinct events (Figures 2.6 to 2.10):-
Stage 1 - A primary hydrothermal event with waxing and waning thermal
history resulting in temporal and spatial mineralogical zoning (Figure 2.6).
Knight (2000) proposes that a suite of primary ore facies developed under
characteristic hydrothermal conditions whereby cycles of volcanic activity and
episodes of diffuse flow lead to focussed fluid discharge and the formation of
massive sulphide deposits over time. At Las Cruces, this initially resulted in the
primary precipitation within, and replacement of, the host black shales. This was
followed by diffuse flow of mixed hydrothermal and seawater fluids, leading to the
replacement and overgrowth of different generations of pyrite.
Saez et al. (1999) also suggest that interaction between the black shales and
hydrothermal fluids highlights one of the main differences between southern IPB
massive sulphides and other VMS deposits.
Page 20
Chapter 2 Geology
Figure 2.6 - Stage 1 - formation of the Las Cruces primary massive sulphide deposit during a primary hydrothermal event with waxing and waning thermal history (modified from Knight, 2000).
Saez et al. (1999) note that Pb isotope data suggest a single (or homogenised)
metal source derived from both the volcanic piles and the underlying Devonian
rocks. The authors also consider that the IPB deposits had magmatic activity as
the heat source, but the environment was not strictly volcanogenic, with many of
the evolutionary stages possibly occurring in conditions similar to those of
sediment hosted massive sulphides.
Saez et al. (1999) suggest that dispersion of hydrothermal fluids may have been
restricted and therefore focussed by the black shales, with massive sulphides
subsequently forming by deposition and replacement processes (citing
Almodovar et al, 1998).
The mineralogy and chemistry of the massive sulphide mound was modified over
a period of hundreds or thousands of years by cycles of hydrothermal diagenesis,
with each hydrothermal cycle involving a waxing stage, in which prograde
Page 21
Chapter 2 Geology
diagenesis occurs, a period of peak hydrothermal conditions and a waning stage,
in which retrograde diagenesis occurs (Knight 2000, citing Knott, 1994).
At Las Cruces, early diagenetic conditions are represented by a prograde
assemblage, which developed as a result of hydrothermal insulation within the
mound. Increased massive sulphide thickness provided both thermal and
chemical insulation of the hydrothermal fluids from the surrounding seawater.
Increased intensity of the hydrothermal system resulted in the development of the
Zn-Fe-Pb-(Cu) sulphides (Knight, 2000).
The peak hydrothermal stage is associated with pervasive, focussed, high
temperature mineralisation, with hydrothermal fluid temperatures >300oC,
resulting in the development of a high temperature, chalcopyrite-rich core with a
cooler, outer margin rich in sphalerite (Figure 2.7). These conditions are
analogous to the conditions in the central conduit of a black smoker chimney
(Knight, 2000).
Stage 2 - Oxidation during the waning stages of the hydrothermal system
resulting in the formation of secondary Fe oxides/hydroxides and
secondary Cu sulphides (Figure 2.7).
During the waning stages of hydrothermal activity, long term, low temperature
(~100oC–300oC) fluid circulation and diffuse venting of white smoker chimneys
replaced those of the focussed high temperature activity. These changes
resulted in the late overgrowth of silica, minor sphalerite, galena, barite and
covellite (Knight, 2000).
Page 22
Chapter 2 Geology
Figure 2.7 - Stage 2 - Sub-marine oxidation and secondary Cu-sulphide enrichment during the waning stages of hydrothermal activity (modified from Knight, 2000).
The retrograde hydrothermal conditions also lead to increasing conductive
cooling and seawater mixing, generating a low pH and oxidation. It is likely that
some oxidation of the massive sulphide orebody occurred during the waning
stages of sub-marine hydrothermal activity, similar to that described for modern
seafloor sulphide deposits. Knight (2000) provides evidence of Fe-oxide dustings
in silica samples suggesting the oxidation may have taken place at a similar time
to the late-stage silicification event that is also strongly correlated to a phase of
secondary Cu-sulphide mineralisation. This event, which took place at
temperatures of <200oC, produced characteristic isotope and fluid inclusion
signatures.
The secondary Cu-sulphides exhibit a slightly enriched 34S isotope signature most
likely caused by the addition of reduced seawater sulphate. Fluid inclusion and
oxygen isotope data for the associated quartz also confirm modified seawater
type solutions (Knight, 2000). This evidence supports the theory of oxidation and
supergene enrichment during the waning stages of hydrothermal activity. Due to
Page 23
Chapter 2 Geology
successive secondary events, however, evidence of the original seafloor
enrichment may be overprinted by later enrichment (Knight, 2000).
Stage 3 - Burial by a thick sequence of Culm sediments (up to c. 1500m)
during the late Carboniferous resulting in recrystallisation of the primary
massive sulphides (Figure 2.8).
Figure 2.8 - Stage 3 - Sustained volcanism and sedimentation leading to the burial of the massive sulphide beneath ~1000m Palaeozoic Culm sediments (modified from Knight, 2000).
Sustained sedimentation and volcanism led to the burial of the massive sulphide
beneath a thick succession of Palaeozoic sediments. Burial at this depth would
have resulted in an increase in the geothermal gradient and recrystallisation of
the massive sulphide deposit. Alteration of the secondary Cu-sulphides formed
during seafloor oxidation may also have occurred (Knight, 2000).
Stage 4 - Sub-aerial supergene enrichment following uplift and erosion.
This extensive period of sub-aerial weathering resulted in the development
of gossan sequences, a silica cap and pervasive supergene Cu sulphide
enrichment (Figure 2.9).
Page 24
Chapter 2 Geology
Figure 2.9 - Stage 4 - Tilting of the primary massive sulphide occurred during the Hercynian, with uplift and erosion being followed by sub-aerial weathering and the development of the gossan, silica cap and supergene Cu-sulphides (modified from Knight, 2000).
Palaeozoic sedimentation eventually ceased and tectonic uplift resulted in the
erosion of much of the Culm sequence. No further deposition occurring until the
Miocene. Regional tectonics deformed, folded and faulted the mineralised
sequence. This resulted in a hinge zone that effectively separates the steeply
dipping primary mineralisation from the largely horizontal secondary
mineralisation. The essentially horizontal secondary mineralisation reflects the
development of the weathering profile below a palaeo-surface that is similarly
oriented to the present day surface (Knight, 2000).
Partial exposure of the palaeo-surface before and during the Tertiary resulted in
the development of a mature gossan profile. The climate was warm with high
rainfall (Knight, 2000, citing Sanz de Galdeano and Vera, 1992; Moreno, 1993),
creating ideal conditions for oxidation and the development of a deep weathering
profile. Downward percolating groundwater caused oxidation and leaching of the
more mobile elements above the water table, with metal ions, notably Cu being
precipitated as sulphides in the reducing environment below (Knight, 2000).
Page 25
Chapter 2 Geology
The secondary sulphide mineralisation resulting from sub-aerial enrichment is
more pervasive than seafloor secondary mineralisation with extensive enrichment
and replacement of primary Cu-sulphides, notably chalcopyrite, by secondary Cu-
sulphides, (largely digenite). The sulphur isotope and fluid inclusion data support
the theory that the formation of the secondary sulphide mineralisation occurred
from modified meteoric fluids at ambient temperature, with 34S enrichment of the
Cu-sulphides resulting in response to dissolution of the primary sulphates,
notably anhydrite (Knight, 2000).
Stage 5 - Reworking of the sub-aerial gossan by seawater during the
Miocene (Figure 2.10).
Figure 2.10 - Stage 5 - Reworking of the gossan and silica cap possibly prior to and following the onset of the marine incursion during the Miocene (modified from Knight, 2000).
The gossan, enriched in less mobile elements, including Fe, Si and Au, contains
both sub-aerial and sub-aqueous derived gossanous materials in addition to
materials reworked during and after the marine incursion that followed. The
variability of the gossan types may reflect the topography of the palaeo-surface
with erosion evident on palaeo-topographic highs and accumulation of gossanous
Page 26
Chapter 2 Geology
materials in palaeo-lows (Knight, 2000). Knight (2000) and Doyle et al. (2003)
also note the presence of gossanous clasts in the Tertiary conglomerate,
suggesting that reworking occurred during the Tertiary marine incursion.
Reworking of the gossan may also have occurred prior to the marine incursion.
Similar reworked textures are observed in the Rio Tinto gossan, possibly
resulting from the activities of flash floods during periods of high rainfall
(Kosakevich et al., 1993).
Stage 6 - Burial by a thick sequence of Miocene sediments resulting in an
increase in geothermal gradient and the development of retrograde
mineralogical sequences and a high degree of preservation of the
supergene ore (Figure 2.11).
Figure 2.11 - Stage 6 - Burial and preservation of the Las Cruces deposit under up to 1000 metres Tertiary sediments (modified from Knight, 2000).
The marine incursion during the Miocene resulted in the Las Cruces deposit
being buried by as much as 1000 metres or more of Tertiary sediments. This not
only protected the gossan and supergene zones from further weathering and
erosion, but also had an additional impact resulting from interactions with
Page 27
Chapter 2 Geology
seawater and subsequent retrograde reactions resulting from burial. These
retrograde, re-sulphidation reactions are evident in the supergene Cu-sulphide
mineralogy in which digenite is replaced by bornite which, in turn is replaced by
chalcopyrite (Knight, 2000). Knight (2000) also notes that re-sulphidation of the
gossan may have occurred as a result of a rise in the water table immediately
prior to inundation and burial by the Tertiary sediments.
Stage 7?? - In addition, although not discussed in detail, Knight (2000)
also suggested that the present day water table may also have had some
effect on, in particular, the secondary sulphide zone.
The Las Cruces deposit is sandwiched between the top of the Palaeozoic
basement rocks and an overlying sequence of Miocene sands, conglomerates
and limestones which vary from less than 1m thick in the vicinity of the deposit to
over 50m thick further to the east under the Huelva River. The sands are in turn
overlain by a 140m thick sequence of marls (R2795, 1998).
Groundwater movement within the Miocene sand unit occurs by porous medium
flow in the lesser consolidated sand layers and by fracture flow in the
conglomerates and limestones. There is vertical hydraulic connection between
the Palaeozoic rocks and the overlying conglomerates, limestones and sands.
The sand sequence is termed locally the Niebla Posadas aquifer and provides
groundwater for local agricultural and industrial use and also for emergency back-
up water supplies for Seville (R2795, 1998).
The Niebla Posadas aquifer therefore lies directly above the gossan at Las
Cruces. Significant scope exists for marked changes in pH and Eh in and around
the gossan due to the influence of the aquifer. The aquifer is likely to carry
dissolved CO2, oxygen and chlorine (particularly during dryer periods) and could
also act as a transporting medium for dissolved metal ions. Although
groundwater quality is good in the recharge area, water quality deteriorates as it
moves downdip and the groundwater in the deeper parts of the basin is more
typically a sodium chloride type. Low level occurrences of trace metals and
Page 28
Chapter 2 Geology
increased concentrations of sulphate occur in some areas of the Palaeozoic
rocks, particularly in the Las Cruces area (R2795, 1998).
The Las Cruces gossan exhibits a high degree of porosity and the presence of an
aquifer is therefore likely to have a significant impact on the mineralogy of the
gossan and may go some way to explaining the distinctive mineralogy. The
impact of the aquifer is considered later in this thesis.
Page 29
Chapter 2 Geology
2.7 Sample Suite
The initial selection of core samples was based on whole rock geochemical
assay data provided by the Rio Tinto exploration geologists. In excess of 220
individual boreholes had been drilled into the precious metal and massive
sulphide mineralisation at the time of sample selection, with core material being
stored in warehouse facilities close to the Iberian exploration offices in Seville.
Access to these materials was limited. The list of samples used in this thesis
together with Rio Tinto whole rock assay data are provided in Appendices 2 and
3 respectively.
Five boreholes were selected for detailed mineralogical characterisation. The
relative positions of the boreholes are provided in Figure 2.12. Previous
mineralogical reports on the precious metal mineralogy of the Las Cruces gossan
(R2643, 1996; R2644, 1996; R2696, 1997) revealed that a significant proportion
of the Au mineralisation is sub-microscopic or extremely fine-grained in nature.
Therefore, selection of boreholes for examination was based on samples that
contained significant Au contents (>5ppm) over several metres of core, thereby
improving the chances of locating and identifying any Au-bearing phases.
Some consideration was given to the spatial distribution of the boreholes.
Boreholes CR149, CR194 and CR123 are situated due south of the main
supergene enriched massive sulphide mineralisation. The gossan is
mechanically and chemically reworked and may also occur some distance from
the original source. Boreholes CR038 and CR191 provided information on the
nature of the precious metal mineralisation away from the main supergene
orebody and it was predicted that these might differ somewhat from those in
direct contact with the underlying massive sulphide orebody. The Las Cruces site
is situated approximately 30 to 35 metres above sea level.
Page 30
Chapter 2 Geology
Figure 2.12 – a) A map of the Las Cruces deposit illustrating the extent of the Au mineralisation (solid yellow line), supergene Cu-sulphide mineralisation (solid blue line) and the positions of the boreholes selected for examination during this investigation. The contours represent gravity survey data. The red and purple contours represent areas of high gravity (relative to the surrounding areas shown in yellow, green and blue, scale unknown). The region of high gravity in the central left hand portion of the map represents the supergene enriched massive sulphide deposit and the central upper region of high gravity represents the primary massive sulphide orebody. Boreholes CR194, CR123 and CR038 are vertical holes and boreholes CR149 and CR191 are inclined holes. The grid spacing is in units of 60 metres. (Modified diagram courtesy of Rio Tinto Limited.)
Borehole CR194 exhibits extensive Au mineralisation with grades in excess of
14ppm Au in the gossan and in excess of 13ppm Au within the supergene
enriched massive sulphide. The Ag mineralisation is extensive, particularly
towards the base of the gossan, with grades exceeding 1100ppm. The gossan in
borehole CR194 lies directly above the supergene enriched massive sulphide
mineralisation. The supergene massive sulphide contains elevated Cu values in
addition to deleterious elements (from a mining perspective), including As, Bi, Hg
and Sb. The supergene massive sulphide mineralisation lies above a Cu-
Page 31
Chapter 2 Geology
enriched shale. This borehole was selected for examination due to the extensive
precious metal mineralisation and the central position relative to the underlying
massive sulphide and supergene Cu sulphide mineralisation.
The gossan in borehole CR149 also lies directly above the supergene enriched
massive sulphide mineralisation with Au contents ranging between 0.67 and
48.54ppm Au between 170.20 and 190.00 metres down hole. This borehole is an
inclined hole, the angle of dip being approximately 60 degrees. Therefore, the
depths are not representative of the vertical extent of the mineralisation. The Au
mineralisation is confined to the gossan with elevated Cu values occurring in the
underlying massive sulphides. The Ag content of this borehole is relatively low
with a significant increase in the Ag content (~730ppm) occurring at the contact
between the gossan and massive sulphide. Relatively high levels of As, Bi, Hg,
Sb and Sn occur throughout the gossan and massive sulphide. This borehole
was selected for examination because of the extensive precious metal
mineralisation and the central position relative to the underlying massive sulphide
and supergene Cu sulphide mineralisation.
The Au lens/gossan zone in borehole CR038 occurs between 150.80 and 157.25
metres and exhibits extensive precious metal mineralisation (1.33–11.31ppm Au,
3.8–1240ppm Ag). This borehole lies towards the margins of the precious metal
mineralisation for the Las Cruces orebody and away from the main massive
sulphide zone. The underlying geology is that of partial massive sulphide that
largely represents pyrite-rich shales and wall rocks that exhibit some degree of
supergene enrichment. This borehole was selected for examination because of
the extensive precious metal mineralisation and the marginal location relative to
the massive sulphide mineralisation.
Borehole CR191 is also extensively mineralised with respect to Au (0.61–
12.04ppm) with the Ag content (5.3–58.6ppm) being less significant than
previous boreholes. The Au zone occurs between 137.95 and 153.85 metres.
However, this borehole is an inclined hole, the angle of dip being approximately
70 degrees. Therefore, the depths are not representative of the vertical extent of
the mineralisation. Borehole CR191 was selected for examination because of the
Page 32
Chapter 2 Geology
extensive Au mineralisation and the marginal position relative to the main
supergene massive sulphide mineralisation.
The gossan zone of boreholes CR123 occurs between 152.40 and 172.85
metres. The Au content of the core is relatively high (1.47–56.55ppm), with
moderate amounts of Ag (13.6–175.3ppm) also being present. This borehole lies
on the margin of the Au and secondary Cu mineralisation, towards the southern
most region of the Las Cruces orebody. The gossan lies above partially
supergene enriched pyritised shales and wall rocks. Borehole CR123 was
selected for examination because of the extensive Au mineralisation and the
marginal position relative to the main supergene massive sulphide mineralisation.
Of the five boreholes selected for the current study, only borehole CR038 had
been examined previously (R2644, 1996). This earlier investigation revealed that
the bulk of the precious metal mineralisation occurred in the form of relatively
coarse native Au grains, with discrete grains commonly exceeding 25µm in
maximum dimension. Reports R2643 (1996), R2644 (1996) and R2696 (1997)
also provided some initial mineralogical information on the nature and mode of
occurrence of the precious metal mineralisation in other boreholes from the Las
Cruces gossan. However, only limited information was available on the textures
and association of the precious metal mineralisation, with the bulk of the
investigation being based on crushed reject materials from assay sampling.
The five boreholes consist of several hundred metres of core, with the upper 100
metres typically consisting of marl and unmineralised overburden. The field
geologists often discarded this material, as it had no commercial value with only
mineralised intersections (with respect to Au and/or Cu), and material
immediately above or below the mineralisation being retained for examination. It
was therefore not possible to examine material from all sample intervals within
each borehole. Subsequently, the samples selected for investigation consist
predominantly of Au-bearing gossan and material directly above and directly
below the Au lens. However, borehole CR194 contains significant Au values
within the massive sulphide zone and the sample suite therefore also included
this Au-bearing material.
Page 33
Chapter 3 Gossans
3 GOSSANS
3.1 Introduction
Gossans have been a source of great interest since ancient times, with the
earliest prospectors recognising gossans as the surface expressions of base and
precious metal-bearing orebodies. Recently, gossan evaluation has been
focussed on characterising the mineralogy and geochemistry of these oxidised
outcrops, with the aim of differentiating between barren and fertile gossans and
ironstones. This has become particularly important in the field of exploration
geology, because in many mineralised terrains, gossans provide the only visible
indication of potentially economic ore hidden at depth.
Some of the more notable work has been by Blain and Andrew (1977) and
Andrew (1978, 1984), with reviews on gossan typology, mineralogy and
geochemistry for both base and precious metal-bearing orebodies.
Recent literature on gossans is somewhat limited relative to those produced on
the underlying orebodies. This may be related to the degree of economic interest
in gossans. Although many gossans contain economic quantities of metals, their
value is often less than that in the underlying orebody. The bulk of detailed
papers on gossans have focussed on pathfinder geochemistry, identifying
economic sub-surface mineralisation.
Many gossans, particularly those in the Iberian Pyrite Belt, in which this study is
focussed, have been mined since before Roman times and much of the gossan
has long been removed. English language publications of Iberian Pyrite Belt
massive sulphide deposits are common, but information on the nature of the
respective gossans is scarce, being confined largely to Spanish and Portuguese
research papers held in university and research departments.
Limited information is available on gossans and massive sulphides in the Rio
Tinto mine, Spain (Kosakevitch et al., 1993 and Williams, 1933-34 and 1950).
Page 34
Chapter 3 Gossans
Nickel (1984), Taylor and Sylvester (1982), Taylor and Appleyard (1983) and
Scott et al. (2001) have produced detailed accounts of gossan profiles associated
with both barren (base/precious metal-poor) and fertile (base/precious metal-rich)
orebodies.
Boyle (1995) examined the gossan of the Murray Brook Deposit, New Brunswick.
Hannington et al. (1986, 1988) and Herzig et al. (1991) provide examples of the
weathering and formation of gossans in present day seafloor sulphides.
The geochemistry of gossan forming processes is reviewed by Blain and Andrew
(1977) and Andrew (1978, 1984). Thornber (1975, 1976) and Thornber and
Wildman (1984) provide experimental data on the chemical and electrochemical
processes of gossan formation, and associated formation of carbonates,
sulphates and oxide minerals. Mann (1984) and Webster and Mann (1984)
studied the mechanisms of precious metal mobilisation effects of climate and
geomorphology on gossan formation.
The following section consists of a literature review of gossan forming processes,
gossan geochemistry and geochemical profiles, element mobility, gossan
typology and mineralogy, especially those developed above polymetallic, pyrite
hosted Cu-Pb-Zn massive sulphide deposits and/or Au-bearing, sulphide-rich
orebodies, similar to the Las Cruces massive sulphide.
Page 35
Chapter 3 Gossans
3.2 The Gossan Forming Process
Most hypogene sulphide minerals are unstable under near-surface weathering
conditions, particularly in the presence of weathering agents such as water-
dissolved oxygen, carbon dioxide and ionic species. These cause the sulphide
body to re-equilibrate electrochemically (Blain and Andrew, 1977) and the
sulphide minerals oxidise to form sulphates and the metal-sulphur bonds are
broken, releasing metal cations that are either dissolved in the co-existing
groundwaters or precipitated as insoluble oxidate minerals. This gives give rise
to more stable secondary sulphide and oxide mineral assemblages.
The residues of Fe-bearing minerals and varying amounts of introduced silica,
are commonly the most abundant constituents of a gossan above massive
sulphides (Blain and Andrew, 1977). As sulphide minerals corrode to stable
oxide, carbonate and sulphate phases near the water table, they become
disconnected from the main sulphide ore zone, are poor conductors and no
longer contribute to the major electrochemical corrosion processes acting on the
orebody (Blain and Andrew, 1977).
At the water table, a dramatic increase in Eh results in decomposition of
Fe-sulphides, producing goethite and a low pH environment (Taylor and
Sylvester, 1982):-
4FeS2 + 10H2O + 15O2 → 4FeOOH + 8SO42– + 16H+
Thornber and Wildman (1984) compare the results of reacting different ore types
under varying conditions over a wide pH range. They highlight high and low pH
processes of Fe hydrolysis, where Fe is a major metal being released from a
sulphide (e.g. pyrite and/or Fe-S-hosted orebodies).
1. The high pH process (pH>7). Base metals, including ferrous Fe will be
hydrolysed and mixed Fe-Cu hydroxycarbonates and hydroxysulphates
form for Cu, and mixed Fe-Pb hydrocarbonates form for Pb. The Fe is
located in these initial compounds as a green rust where it is effectively
Page 36
Chapter 3 Gossans
bound as ferric hydroxide. Subsequent oxidation of this hydroxide
produces no further acid:-
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3
2. The low pH process (pH<7). Even though some of the ferrous Fe will have
precipitated as an equivalent to Fe(OH)2, the solubility at low pH is such
that sufficient Fe2+ will remain in solution:-
4Fe2+ + O2 + 10H2O → Fe(OH)3 + 8H+
The dissociation of the water molecule during oxidation of Fe2+ and subsequent
H+ production is known as ferrolysis. During this reaction, the pH will fall even
further, so that the gossan forming environment will be at a pH less than 5 and
may be as low as 3. At these low pH values, the base metals are soluble and
only elements such as Se, As, Mo and Sb, are likely to be bound into gossans
(Thornber and Wildman, 1984).
The Eh and pH ranges in the sulphide oxidation zones were first measured by
Sato (1960), who deduced the limiting conditions of natural environments in
terms of Eh and pH functions (Figure 3.1). Sato (1960) attempted to define the
ranges of Eh and pH by direct measurement of mine waters in the vicinity of
oxidising orebodies, deducing the limiting conditions of the environment based on
geological observations.
The results of these investigations define a flattened, wedge-shaped area on the
Eh-pH diagram (Figure 3.1), portraying the chemical conditions under which
sulphide orebodies commonly oxidise. However, Sato’s measurements were
made on waters exposed to atmosphere and it is likely that contamination
occurred, resulting in higher Eh values than were present before mining
commenced.
Page 37
Chapter 3 Gossans
Figure 3.1 – Diagram illustrating the zones of weathering in terms of Eh and pH according to Sato (1960).
Anderson (1990) illustrates the Eh/pH environment in terms of the position of
groundwater and describes, in general terms, the associated mineralogy (Figure
3.2). Anderson’s illustration follows a similar trend to that described by Sato, with
the oxidation and enrichment of sulphides being dependant on the oxidation
potential (pO2) of the environment. Anderson (1990) notes that the highest pO2
obtained is that of air, with pO2 decreasing with increasing depth towards the
groundwater table. Within the aerated and recharge zones, sulphide minerals
oxidise to form metal oxides and sulphates. Below the groundwater table marks
the transition between oxidising and reducing conditions where metal sulphides
remain stable.
Page 38
Chapter 3 Gossans
Figure 3.2 – Eh/pH diagram at 25oC and 1 atmosphere total pressure, illustrating the relationships between groundwater position and mineral stability ranges (Anderson, 1990).
As well as the oxidation of Fe-sulphides in the massive sulphide deposit,
oxidation of associated transparent gangue minerals and the subsequent
weathering of surrounding wall rocks will also have an impact on the local
mineralogy. Trescases (1992) describes how interstitial fluids produce secondary
minerals and classifies as follows:-
Page 39
Chapter 3 Gossans
1. Dissolution – particular affects minerals with a high solubility, including
salts, sulphates and carbonates.
2. Oxidation - an increase in oxidation state, typically through the introduction
of oxygen through percolating groundwaters.
3. Hydrolysis - the reaction of minerals with water.
4. Transformation - solid-state reaction in which the organisation of oxygen
and most silicon ions is retained.
5. Alkalinolysis - weathering in very alkaline environments.
6. Acidolysis - weathering in very acid environments.
These processes may be linked in the weathering environment. Morris and
Fetcher (1987) note that the solubility of quartz increases following the ferrous-
ferric Fe reaction (oxidation). Trescases (1992) remarks that oxidation is a
process that accompanies or follows reaction with water (hydrolysis). The
hydrolysis of common silicate minerals may result in the formation of distinctive
solid residues including gibbsite (allitization), kaolinite (monosiallitization) and
smectitic (bisiallitization) (Trescases, 1992).
Page 40
Chapter 3 Gossans
3.3 Influences On Gossan Formation
3.3.1 Introduction
The pH of the environment is the most important parameter in determining the
initial minerals that form in the gossan. A high pH may result from buffering by
wall-rock minerals, a low Fe content and a high metal to sulphur ratio in the
sulphide.
Blain and Andrew (1977) note that the weathering environment is complex, and is
governed by climate, geology, geomorphology and groundwater movement.
Depth of sulphide oxidation, depth and stability of the water table, degree of
profile erosion and groundwater salinity also influence the composition of the
base metal gossans (Andrew, 1984).
Studies by Mann (1984) on pH and the effect of acid buffering on gossans within
the Yilgarn Block, Western Australia suggest that the ferrolysis reaction produces
very acid pallid zone profiles. Ground water seepages with pH values less than
2.5 are common. If the bicarbonate ion (e.g. from weathering of basic rocks) is
present this acid production may be neutralised (Mann, 1984). The Eh-pH
environment in the oxide zone and activities of radicals such as carbonate,
sulphate and silicate dictate the stability of secondary oxidate minerals in the
gossans (Andrew, 1984).
The effects of climate, geomorphology and groundwater movement on the
mobility of Au and Ag through the weathering profile of the Upper Ridges mine
Papua New Guinea, and at Westonia, Western Australia are discussed in detail
by Webster and Mann (1984).
Boyle (1995) describes the weathering profile of the Murray Brook deposit, New
Brunswick and lists factors affecting the rate of oxidation of the orebody and Au
and Ag mobility, including:-
Position of the sulphide body in the hydrologic regime
Diffusion rate of atmospheric O2 into the sulphide body
Flow characteristics of O2-CO2 bearing groundwater through sulphides
Page 41
Chapter 3 Gossans
Strength of electrochemical processes through the sulphide body
Primary and secondary porosity and permeability
Type and abundance of sulphides
Type and abundance of gangue minerals
Grain size distributions (reactive surface area)
Degree and types of microbiological activity
Climate
Ambient and internal temperatures
Nature and composition of surrounding wall rocks
Geometry and structure of the sulphide body
Microbiological activity aside, these factors may be attributed to one of three
criteria affecting pH, namely primary geology, climate and geomorphology.
3.3.2 Effect of Primary Geology on pH
Primary ore composition affects the ability to generate acid during near-surface
weathering conditions, particularly the Fe content, metal to sulphur ratio and the
grain size or reactive surface area of the sulphide minerals. Also, the gangue
minerals, particularly carbonates, may buffer the acid solutions and their grain
size and composition will determine the degree of acid buffering that will occur.
For example, calcite will react more rapidly as an acid buffer than siderite.
Low pH and the redistribution of Au and Ag in lateritic weathering profiles appear
to be more common over granitic and gneissic basement and may be inhibited by
carbonate in the weathering zone of basic rock sequences (Mann, 1984). The
leaching effect of pyrite-rich ore during oxidation is inhibited by reactive gangue
such as carbonate or mafic silicates (Andrew, 1984).
At Sierra de Cartagena, SE Spain, the presence of pyrite and marcasite produces
low pH conditions (~3, as indicated by the presence of jarosite), resulting in a
greater development of Fe- and Mn-oxides. Conversely, in ores where most of
the Fe is in the form of magnetite and siderite metal leaching is less pronounced
(Lopez Garcia et al., 1988).
Page 42
Chapter 3 Gossans
The low Fe-sulphide content of a Zn-Pb lode at Dugald River, North-West
Queensland limits the acidic solutions formed by oxidation reactions. Any acid is
rapidly buffered by reaction with dolomite and calcite in the gangue and wall
rocks. Oxidation of sphalerite, galena and the Fe sulphides has produced
sulphate-rich, mildly acidic solutions. Elements that are normally leached are
only partially leached, and occur in secondary sulphate minerals (Taylor and
Appleyard, 1983).
The geometry of the orebody and the presence of faulting, folding and other
structural features in the primary ore will also have a significant influence on the
diffusion rates of oxygenating groundwaters and atmospheric O2 as will the
primary and secondary porosity developed during early leaching of reactive
phases such as Fe-sulphides and carbonates. Removal of reactive minerals at
the early stage of oxidation may result in a significant increase in porosity and
rapidly accelerate the gossan forming process.
3.3.3 Effects of Climate on Gossan Formation
The most important climatic factor in the formation of gossans is the volume of
moisture derived from rain, mist and less commonly snow (Butt and Zeegers,
1992). Rainfall affects the position/depth of the water table, flow characteristics
of O2- and CO2-bearing groundwater, groundwater salinity, acidity (pH) of metal-
bearing solutions and overall groundwater composition (Boyle, 1995). Moderate
intensity rainfall is more effective at entering the soil than intense rainfall. High
intensity rainfall has a tendency to impact the surface of the soil and may runoff
rather than infiltrate, particularly if vegetation is sparse or a surface crust has
developed. Conversely, light rainfall may be trapped by vegetation and not reach
the weathering profile (Butt and Zeegers, 1992).
The Rio Tinto gossan and associated secondary mineralisation formed during the
late Miocene in a near sub-tropical climate with short periods of seasonal high
rainfall (Leistel et al., 1994) producing a gossan and supergene zone of between
80-120 metres thick (Almodovar et al., 1997). Here, Kosakevitch et al. (1993)
observed gossans 'transportado' formed by flash floods during periods of high
rainfall.
Page 43
Chapter 3 Gossans
Today, mine waters at Broken Hill are carbonate-sulphate waters of low salinity
and near-neutral pH. However, the region has recently emerged from a period of
aridity when the water table was at least 1000 feet lower than it is today. The
effect of aridity on groundwater is normally to greatly increase the concentration
of dissolved salts, especially chlorides (Taylor, 1958). The presence of chlorides
substantially increases the mobility of Pb, Au and Ag. The oxidation of sulphide
ores in arid regions is known to occur as deep as 800m below the present
surface (Blain and Andrew, 1977).
At the Upper Ridges mine in Papua New Guinea, heavy rainfall in a region of
rugged relief dilutes groundwater and prohibits the development of acid
conditions on a regional scale. At Westonia, Western Australia an arid climate
combined with low relief has brought about the formation of acidic, saline
groundwaters in the Fe-rich laterite profiles, creating ideal conditions for Au
transportation as AuCl4– (Webster and Mann, 1984).
Temperature influences the rate of chemical reactions and moisture has
significance as a reagent and transport medium for other reagents and reaction
products. The rate of reaction increases by a factor of 2 for every 10oC (Butt and
Zeegers, 1992).
Temperature is greatest in tropical areas. Cloud and vegetation cover reduce the
effectiveness of solar radiant energy, but act as insulators during the night, so
temperature extremes are less likely. Deserts and less well-vegetated areas
often experience extremes of temperature. Temperature declines with altitude.
Temperature will also have an impact on groundwater movement through
evaporation, including laterite and evaporite formation (Butt and Zeegers, 1992).
3.3.4 Effects of Geomorphology on Gossan Formation
Geomorphology impacts on gossan formation by controlling the drainage. By
affecting the movement of groundwater, it has a direct impact on pH and
therefore the gossan forming process.
Page 44
Chapter 3 Gossans
The geomorphology may affect the diffusion of atmospheric oxygen into the
orebody and subsequently affect the depth of oxidation. Highly erratic and
undulating oxide/sulphide interfaces may reflect irregular surface topography
prior and during oxidation. In stable areas, the depth of zonation commonly
relates to an even land surface and a present-day water table (Blain and Andrew,
1977).
Geomorphology may provide an indication of previous climatic events, such as
glaciation. Pleistocene glaciation, dominant in the higher latitudes, may have
destroyed near-surface gossans. Conversely, gossans in lower latitudes may
reflect sub-aerial weathering over tens or hundreds of millions of years (Butt and
Zeegers, 1992).
Page 45
Chapter 3 Gossans
3.4 Element Mobility and Gossan Profiles
3.4.1 Introduction
Hydromorphic dispersion is the mobilisation of elements by groundwaters during
weathering and is affected by the presence of other dissolved species, the
interactions between the solutions and mineral surfaces and the oxidation state of
the elements being taken into solution (Thornber, 1992).
During gossan formation, metals which are more mobile than Fe are significantly
leached from the system. A proportion of the original metal content is commonly
fixed in the oxide zone as stable oxidate minerals, or as adsorbed or co-
precipitated metal on finely crystalline goethite, metal-rich colloids or even on gel
silica (Blain and Andrew, 1977).
Some elements behave comparably during near surface weathering conditions
and distinct geochemical profiles may occur, resulting in a characteristic
mineralogy. These are discussed by Andrew (1984), Leistel et al. (1998), Garcia
Palomero et al. (1986), Nunez et al. (1987) and Taylor and Sylvester (1982).
Sulphide deposits may have been subjected to complex weathering histories.
Generally, cations are far more mobile at low pH and anions at high pH.
Therefore, the resultant geochemical and mineralogical profiles may differ
considerably (Blain and Andrew, 1977). Scott et al. (2001) refer to this complex
history as gossan maturity and compare and contrast several orebodies from the
Lachlan Fold Belt, NSW, Australia.
The ideal mature gossan profile above weathered sulphides consists of a zone of
supergene sulphides overlain by zones of secondary sulphate minerals,
carbonate minerals and phosphate minerals, before moving into the Fe-oxide
dominated surficial material (Scott et al., 2001) (Figure 3.3).
Page 46
Chapter 3 Gossans
Figure 3.3 - Idealised zones in the weathering profile of a VHMS Zn-Pb-Cu deposit that has been weathered to produce a mature gossan profile (Scott et al., 2001).
If these zones are not present, then the profile is considered to be immature.
These are typically enriched in Cu, Pb and Zn but depleted in As and Sn (Scott et
al., 2001). Immature gossans are discussed by Nickel (1984) and Thornber and
Wildman (1984) who conclude that pH plays a key role in metal dissolution,
mobilisation and reprecipitation.
Gangue minerals contribute trace elements as a result of acid-buffering reactions
or residual enrichment. Silica, supplied by the hydrolysis of silicate gangue, plays
an important role in the formation of gossans (Andrew, 1984). During supergene
oxidation of the sulphide minerals, metals such as Cu, Ni, Zn, Co and Pb are
initially released into solution at high concentrations (Thornber and Wildman,
1984). Patterns in element behaviour during oxidation, together with the
associated mineralogy are discussed in greater detail in this section.
3.4.2 Fe
Fe is the most abundant and probably most important element associated with
gossans formed during weathering of pyrite-rich orebodies and because of its
particular hydrolysis properties, is the least mobile of base metals in oxygenating
waters (Thornber and Wildman, 1984).
Page 47
Chapter 3 Gossans
At extremely low pH, the retardation in the oxidation of ferrous to ferric ion and
the low activity of hydroxyl prevent the in situ precipitation of goethite, and the
ferrous ion may be transported varying distances before it precipitates as ferric
oxides (Blain and Andrew, 1977).
Figure 3.4 illustrates the stability relations for some of the common Fe minerals at
25oC and the conditions under which Fe2+ and Fe3+ are stable in solution.
Pyrrhotite is stable under strongly reducing conditions at near neutral pH. Below
pH 7, the Fe-sulphides are relatively unstable, with the area of solubility
increasing markedly at lower pH. Fe2+ is soluble under a wide range of Eh
conditions at low pH. Fe3+ is only soluble under the most extreme oxidising, acid
conditions. Under strongly reducing conditions and high pH, hematite may be
reduced to magnetite. Hematite is the stable phase over a wide range of Eh
conditions, particularly at moderate to high pH.
Goethite (ideally -Fe3+O(OH)) and hematite (ideally Fe2O3) are the dominant Fe-
bearing minerals in all gossans examined during the literature review, including
Rio Tinto, Spain (Williams, 1950; Vinals et al., 1995), Lagoa Salgada, Portugal
(Oliveira et al., 1998), Teutonic Bore, Australia (Nickel, 1984), Mt Lyell, Tasmania
(Solomon, 1967), Murray Brook, Canada (Boyle, 1995), 18 gossans of the
Lachlan Fold Belt, Australia (Scott et al., 2001), Mugga Mugga, Australia (Taylor
and Sylvester, 1982) and lateritised gossans from Brasil (Angelica et al., 1996).
Jarosite (ideally KFe3(SO4)2(OH)6) is a common accessory mineral at Rio Tinto,
Murray Brook and the Lachlan Fold Belt.
Goethite and hematite typically occur as massive botryoidal aggregates
deposited in cavities (Kosakevitch et al., 1993; Williams, 1950; Vinals et al., 1995;
Nickel, 1984 and Solomon, 1967). Boxwork textures in hematite and goethite are
rare or absent at Rio Tinto (Williams, 1950), Lagoa Salgada (Oliveira et al., 1998)
and Flambeau (Ross, 1997). Boxwork textures are more common in the less
mature (medium-high pH) gossan profiles of Teutonic Bore (Nickel, 1984) and
Murray Brook (Boyle, 1995). In mature (low pH) gossan profiles, boxwork
textures are often destroyed as the Fe is taken into solution and reprecipitated as
botryoidal aggregates (Williams, 1950; Scott et al., 2001).
Page 48
Chapter 3 Gossans
Figure 3.4 – Eh/pH diagram illustrating the stability relations between iron oxides and iron sulphides in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-6. Boundaries of solids are for total ionic activity of 10-6 (Garrels and Christ, 1965).
Fe may be mechanically and/or chemically transported out of the sites of the
original sulphide grains. Breccias commonly form by the mechanical transport of
clasts and are subsequently recemented by a chemically transported Fe-oxide
matrix (Blain and Andrew, 1977). The Rio Tinto gossan ‘transportado’ consists of
a jumbled aggregate of silicified rock fragments cemented by goethite (Williams,
1950). The gossan is composed of layered precipitates of colloidal Fe-oxides with
clastic material and indications of an old pedogenesis (soil development) at the
Page 49
Chapter 3 Gossans
top of the gossan (Kosakevitch et al., 1993). Pebbles of chert and botryoidal
masses of hematitic occur within a gossan conglomerate at Mt Lyell (Solomon,
1967).
3.4.3 Au and Ag Element Mobility
Native Au is almost inert under the physio-chemical conditions prevailing in most
weathering environments and Au only becomes geochemically mobile where
complexing anions are present in groundwaters (Gray et al., 1992) (see Figure
3.5). Garrels and Christ (1965) note that the Eh/pH diagram for Au in oxygenated
water is simple, in that only native Au appears, with no ions exceeding 10-6.
The mobility of Au and Ag in the weathering environment has been well
documented and a number of distinct mechanisms for Au and Ag dissolution,
mobilisation and reprecipitation are recognised. The mobility of Au and Ag is of
particular interest in this study as these precious metals form the main focus of
economic interest in the Las Cruces gossan.
Several ligands have been proposed and investigated for complexing with Au in
low-temperature, oxidising groundwaters (Ross, 1997). Although many ligands
have the ability to bond with Au+, only a few occur in natural groundwater
systems in sufficient quantities to induce Au mobilisation (Vassopoulos and
Wood, 1990).
Acidic, oxidising, saline groundwaters in an Fe-rich environment are the ideal
conditions for Au transportation as AuCl4- (Webster and Mann, 1984; Koshman
and Yugay, 1973; Williams, 1933-34; Mann, 1984). Au solubility as a chloride
complex has been demonstrated to occur in a restricted pH-Eh environment;
pH<5.5; Eh >0.9V; aCl->10-3.2 (Cl- activity) (Webster and Mann, 1984). Figure 3.5
illustrates that in the presence of high chloride, Au is soluble in certain acid,
oxidising solutions.
Page 50
Chapter 3 Gossans
Figure 3.5 – Eh/pH diagram illustrating the stability relations of some Au compounds in water at 25oC and 1 atmosphere total pressure at total dissolved chloride species of 100 and sulphur activity of 10-1. Boundaries of solids are for total ionic activity of 10-6 (Garrels and Christ, 1965).
Experimental evidence suggests that very acid chloride solutions generated by
ferrolysis are responsible for the dissolution of Au and Ag (Mann, 1984). Natural
waters in near-surface conditions will be oxidised by the atmosphere and can
contain abundant chlorine from the dissolution of salts (Ross, 1997). The
dissolution of Au to form a Au-chloride complex is expressed in the chemical
reaction below.
Page 51
Chapter 3 Gossans
4Au0 + 16Cl- + 3O2 + 12H+ → 4AuCl4- + 6H2O
The reaction requires the presence of oxygen, acid (H+), and a notably large
concentration of chloride ions (Mann, 1984). Such chlorine-rich groundwaters
have been sampled at Kalgoorlie, Western Australia, where Cl concentrations
ranged between 21,000 to 107,000mg/L (Grey et. al., 1992).
Further examples of Au and Ag believed to have been remobilised as chloride
complexes under oxidising conditions include Westonia and Yilgarn Block,
Western Australia (Webster and Mann, 1984, Mann, 1984) where dissolution of
Ag and/or Au is a relatively frequent occurrence along the rim of nuggets,
especially when there are adjoining Fe-oxides.
Secondary Au precipitated by reduction at the site of Fe oxidation is often of
higher fineness (lower Ag content) than the primary Au (Webster and Mann,
1984; Mann, 1984 and Saunders, 1991). This feature can be readily explained
by the nature of Au- and Ag-chloride complexes and their behaviour under near
surface weathering conditions. Following the release of Au and Ag from primary
Au and electrum grains as chloride complexes, the supergene solutions migrate
downward through the weathering profile and reducing conditions are
encountered. The Au-chloride is subsequently re-precipitated by reduction of the
AuCl4- ion with Fe2+ (Mann, 1984).
4AuCl4- + 3Fe2+ + 6H2O → Au0 + 3FeOOH + 4Cl- + 9H+
This reaction is thought to occur near the water table, where Fe2+ would be
present in the weathering profile (Mann, 1984).
Ag-chloride complexes are not initially affected by the encounter with reducing
conditions, because of the relative redox potentials of Fe2+/Fe3+ and Ag/AgCl0
(Mann 1984) and will remain in solution, migrating downward (Saunders, 1991).
The solubility of Ag is comparatively high as a chloride complex with respect to
Page 52
Chapter 3 Gossans
Au (Webster, 1986) and therefore the refinement of Au in supergene process is
the product of the different stabilities of Au- and Ag-chloride complexes
(Saunders, 1991).
The proposed mechanism of Au and Ag transportation as chloride complexes has
also been suggested for gossans associated with modern day seafloor sulphides.
Hannington et al. (1988) have documented precious metal-bearing grains from
supergene zones from the Mid Atlantic Ridge and note the relatively high purity of
the native Au grains.
Au and Ag mobilisation as chloride complexes is not the only viable mechanism
and a number of other possibilities are discussed in the literature. Garrels and
Christ (1965) suggest that under neutral to alkaline reducing conditions, Au is
soluble as a AuS- complex (Figure 3.5). More recent experimental works by
Vlassopoulos and Wood (1990) show that in groundwaters circulating through
oxidising orebodies, Au(S2O3)23- (thiosulphate), AuHS0 and Au(HS)2
- are the
stable solution species. Webser and Mann (1984) also suggest that thiosulphate
complexes are the stable species under alkaline oxidising conditions, citing
examples including the Upper Ridges Mine, Papua New Guinea, where, under
neutral to basic, moderately oxidising conditions, found in the vicinity of the
weathering carbonate veins, Au and Ag may be complexed by thiosulphate to
form Au(S2O3)23- and Ag(S2O3)2
3- or a mixed complex. Au of low fineness (high Ag
content) is re-precipitated by reduction at the water table, as, unlike the chloride
complexes, both the Au and Ag thiosulphate complexes destabilise under similar
pH and Eh conditions (Webser and Mann, 1984).
Thornber (1992) comments, however, that although sulphate complexes are
more stable than chloride complexes, because most natural waters have higher
activities of chloride than sulphate, chloride complexes are more important for
geochemical mobility.
Boyle (1995), on the Murray Brook precious metal-bearing gossans, notes that
during progressive oxidation and physico-chemical erosion of the gossan zone,
Au was transported downward in the groundwaters, probably as an Au0 colloid
Page 53
Chapter 3 Gossans
complex, to be concentrated in the lower horizons of the gossan profile. Boyle's
hypothesis was based on leaching experiments and microprobe analyses, which
indicate that Au is present in the gossan as sub micron composite sols of Au-Ag-
silica.
A number of other mechanisms for Au and Ag mobility are mentioned in the
literature. These include organic ligands, such as humic acid, cyanide complexes
CN- or SCN-, which can form locally from biogenic processes (Webster and
Mann, 1984). New thermodynamic data and theoretical calculations for gold
hydrolysis demonstrate that in conditions prevailing for most supergene waters
the complex that should control the solubility is AuOH(H2O)0 rather than AuCl4-
(Vlassopoulos and Wood, 1990).
Although it is generally accepted that only one complexing agent is active in a
deposit (Mann, 1984), Angelica et al. (1996) suggest that more than one
complexing agent may have been active at different stages of gossans
development. Angelica et al. (1996) describe a lateritised gossan in Brasil and
suggest the most accepted model for the dissolution of Au during the gossan
formation in this case is through thiosulphate complexes, in oxidising, neutral to
alkaline environments. In a second stage, however, the authors suggest that
during the laterisation of pre-existing gossan, other physiochemical conditions
may have prevailed in a more oxygenated environment, resulting in a new
remobilisation of Au through the combination of humates, thiocyanates and also
H2O-OH complexes.
Recent studies (Lengke and Southam, 2005; Reith and McPhail, 2006) have
shown that bacteria in the natural environment may play an important role in both
the mobilisation and reprecipitation of Au and other metals. Experimental studies
by Reith and McPhail (2006) have shown that aerobic and anaerobic microbiota
in auriferous soils from the Tomakin Park Gold Mine, New South Wales, Australia
are capable of dissolving finely disseminated Au bound within the soil fractions.
In the anoxic experiment, the maximum concentrations of solubilised Au were
lower than that of the oxic experiment. The authors show that Au can be
solubilised in in vitro studies with heterotrophic bacteria and found that Au amino
Page 54
Chapter 3 Gossans
acid complexes dominated. When the amino acids are utilized more rapidly than
they are produced, the Au-ions, if present in solution, are left without complexing
ligands and become unstable in solution, precipitating and/or re-adsorbing to the
solid soil phases.
Southam and Beveridge (1996) have shown that octahedral gold was formed
through indirect bacteria involvement when organic acids were released from
dead bacteria, which then formed complexes with gold in solution and finally
transformed to crystalline octahedral gold.
In carbon limited system such as quartz/Au veins, the resident microbiota also
released Au, but the Au release appears to be linked to a different microbially
mediated Au solubilisation process, probably Fe or sulphide oxidation. Fe- and
sulphur-oxidising bacteria such as strains of Acidithiobacillus sp. and
Leptospirillum sp. have been observed to mediate the release Au by breaking
down the sulphides in sulphidic Au ore (Reith and McPhail, 2006). The organisms
use Fe2+ and sulphide as electron donors in their metabolisms and oxidise them
to Fe3+, thiosulphate, and sulphate respectively.
Reith and McPhail (2006) note that despite the studies undertaken to date little is
known about the mobility of Au and its interactions with microorganisms in a
complex natural environment. The species or groups of bacteria and other
microorganisms that are important in affecting Au mobility need to be identified
more specifically and the speciation of Au needs to be identified.
3.4.4 Au and Ag Mineralogy and Geochemical Profiles
The review of selected gossans in the literature reveals that similarities occur in
both the precious metal mineralogy and the resultant profiles developed within
the gossan. The mineralogy and profiles are therefore discussed in greater detail
in this section.
Williams (1933-34) describes the Rio Tinto gossan in detail, and, although limited
analytical techniques were available at the time, significant detail on the nature of
the precious metal mineralogy and geochemical profile was obtained. One of the
Page 55
Chapter 3 Gossans
key features noted by Williams was the development of a precious metal layer at
the base of the gossan.
Williams describes this enrichment as being due to a concentration of the traces
of Au and Ag that were originally present in the sulphide deposits. Williams
comments that jarosite is the dominant mineral in this earthy, precious metal
bearing layer and as well as Au and Ag, this layer is also marked by enrichment
in Pb, Sb, Bi and Se.
Ag has been identified as cerargyrite and is also probably present as acanthite
(Williams, 1933-34). Much of William's work has been verified by Vinals et al.
(1995) who confirm that Ag is present in a number of forms, including members
of the beudantite-jarosite group of minerals, cerargyrite (plus or minus some
bromide and iodide), acanthite and Hg/Ag sulpho-halides. Vinals et al. (1995)
also confirm the presence of micrometre-sized native Au grains and note that the
majority of the Au contained in the ore is probably submicroscopic.
In the Salomon-Cerro Colorado area of Rio Tinto, Spain, the base of the oxide
zone is 10 to 40m deep and the contact is generally sharp. An earthy precious
metal layer (1 to 2m vertical interval) below the oxide zone, overlies a thin horizon
of leached pyrite. A well developed zone of secondary sulphide enrichment (30
to 40m vertical interval) grades into the hypogene ore (Blain and Andrew, 1977).
At Lagoa Salgada an increase in precious metals (Au and Ag) is evident in its
supergene enrichment zone, with Au contents reaching a maximum value of
2.38ppm. Ag occurs, at least in part, as relatively coarse grains of amalgam (Ag-
Hg alloy) that may be visible in hand specimen (Oliveira et al., 1998).
Lopez Garcia et al. (1988) on Sierra de Cartagena, southeastern Spain note that
in horizon 1, derived from the oxidation of a magnetite and siderite primary
assemblage, Ag occurs mainly as cerargyrite and native metal. In horizon 2,
derived from the oxidation of a pyrite and marcasite primary assemblage, it
occurs principally in jarosite and as native Ag. These differences in Ag
mineralogy are largely controlled by acid generation during oxidation of different
Page 56
Chapter 3 Gossans
primary geologies. A contributing factor to the formation of cerargyrite was the
proximity of this area to the sea, which resulted in an important source of wind-
borne chlorine (Lopez Garcia et al., 1988).
In the Eastern Lachlan Fold Belt, NSW, Australia, Scott et al. (2001) describe the
Au and Ag distribution and associations for a number of deposits within the
region. The authors note that for Woodlawn and Currawang, Ag may be severely
depleted in the gossanous outcrop relative to the original ore but substantial
enrichment may occur in the supergene sulphide, sulphate and carbonate zones.
Au is also significantly enriched in the carbonate zone of the gossan, relative to
the primary sulphide.
Scott et al. (2001) also comment that Ag is retained in high concentrations in the
gossans of Kangiara, Lewis Ponds, Peelwood and Mt Costigan and note
correlations between Ag-Pb and Ag-Sb contents. This may relate to gossan
maturity and different weathering susceptibilities of the primary Ag-bearing
phases, as Ag may be present in more than one phase within the primary
orebody. Ag initially concentrates in Cu-rich secondary sulphides, although with
continued weathering, they are concentrated in the Pb-bearing alunite-jarosite
minerals and to a lesser extent the Fe-oxides (Scott et al., 2001).
Au contents typically increase with Ag content in the gossans of the Lachlan Fold
Belt, except in four Ag-rich deposits. Au contents also increase with As except in
the five high As gossans. The Au grades of the gossans commonly represent a
significant enrichment relative to the primary ores, although the immature
gossans are less likely to show the extreme enrichment of some mature gossans
(Scott et al. 2001).
At the Murray Brook deposit, New Brunswick, Boyle (1995) notes a strong
correlation between Au, Sn and Si. However, much of Boyle's microscopic
interpretations appear to be based on a single occurrence of Au in the gossan as
a small grain of Au-Ag-silica gel-like material. Because halide minerals are not
present in the Murray Brook gossan or other gossans in the Bathurst Camp area,
it is unlikely that Au was transported as a halide complex (Boyle, 1995).
Page 57
Chapter 3 Gossans
Boyle suggests that the precipitation of silica may have had a controlling effect on
Au concentration and cites Fujii and Haramura (1976) and Fujii et al. (1977) who
have shown that colloidal silica is a good precipitating agent of Au sols, and that
acid silica solutions act as a reducing media for Au3+. As Si solubility decreases
with decreasing pH, groundwaters moving down into the oxidising pyrite-rich
zones would precipitate silica during the oxidation of pyrite (Boyle, 1995).
Boyle's hypothesis for the close association between Au, Sn and Si are to some
degree corroborated by microscopic interpretation of the primary ore. Boyle
remarks that in the primary ore, Sn is concentrated mainly in the pyrite-rich zones
and, because cassiterite has been shown to be very resistant to weathering
processes, the Au-rich zones in the gossan represent the former positions of
primary pyrite-rich zones. Boyle (1995) observed that native Ag in these ores
have a physical appearance similar to physically re-worked grains (e.g. from a
placer), however, much of the Ag occurs in jarosite group minerals with some Ag
in the pyrite-quartz sand occurring as acanthite.
Costa et al. (1999) and Angelica et al. (1996) have studied a number of lateritised
gossans from South America and conclude that Au mineralisation is closely
associated with the Fe oxyhydroxides in the gossans, with a great range of Au
compositions in the different parts of the profile. The higher Au values coincide
with the respectively greater goethite and hematite contents of the profile
(Angelica et al 1996). Ag was detected only in the upper part of the profile with
Cu, Mo, Sn and As also present in high values and exhibiting a good correlation
with Au (Angelica et al. 1996).
Costa et al. (1999) reveal that the gossan elements (Au, As, B, Cu, Mn, Mo, Ni,
Pb, Sn, W, Y and Zn) display good correlations and these persist in laterite,
latosols and colluvium. The authors suggest that this behaviour reinforces the
primary nature of these materials, controlled mainly by minerals that are still
preserved as resistates in the supergene materials. The most important are
dravite (B), wolframite (W), cassiterite (Sn), and Au (Costa et al. 1999).
Page 58
Chapter 3 Gossans
3.4.5 Cu
Cu, Pb, As and Sb often exhibit a close correlation in weathering profiles and
their presence and/or absence may also be indicative of the degree of profile
maturity and subsequently reflect conditions under which the gossan has formed.
This is evident in many of the gossan profiles studied in the literature.
In mature gossan profiles, Cu is typically depleted in the uppermost portions of
the gossan, but may be concentrated in the lower gossan within the supergene-
enriched zone in the form of secondary Cu sulphides. These secondary Cu
sulphide minerals may also host a significant proportion of As and Sb as well as
Ag and include chalcocite, enargite/luzonite and chalcanthite (Scott et al., 2001).
Cu is largely absent from the upper part of most mature gossan profiles, including
those of Rio Tinto, Lagoa Salgada and Murray Brook. Angelica et al. (1996)
revealed the presence of bornite, cuprite, malachite, chalcocite, native Cu, azurite
and chrysocolla in the secondary sulphide enrichment zone of a lateritised
gossan in the Amazon region.
The absence of Cu in mature gossan profiles is largely a result of the relatively
high solubility of Cu under the acid, oxidising conditions that often prevail during
the weathering of massive sulphide orebodies (Figure 3.6). Under oxidising,
near-neutral pH conditions, Cu-sulphates may form (e.g. chalcanthite), but their
high solubility often results in rapid redissolution and reprecipitation with Fe
oxides and hydroxides (Anderson, 1990). At high Eh and pH, Cu-oxides are the
stable species. The Cu-sulphides dominate under strongly reducing conditions
(Figure 3.6)
Page 59
Chapter 3 Gossans
Figure 3.6 – Eh/pH diagram illustrating the stability relations of some Cu minerals in water at 25oC and 1 atmosphere total pressure at total sulphur activity of 10-1, CO3 activity of 10-3
(Anderson, 1990).
Less mature gossans may contain elevated levels of Cu. Cu is particularly
abundant in the surficial gossan of the immature profile at Currawang where it is
present, partly, as malachite, although substantial amounts are also retained in
the Fe-oxides and plumbojarosite (Scott et al., 2001). At Mugga Mugga, Cu is
depleted at the base of the weathered zone, with levels increasing up the profile.
The Cu is incorporated into the hematite structure as well as being adsorbed by
Page 60
Chapter 3 Gossans
goethite (Taylor and Sylvester, 1982). The authors also note a similar trend for
Ag in this deposit.
Similarly, the Teutonic Bore contains between 500 and 1000ppm Cu in the upper
part of the gossan, the bulk of which has co-precipitated with Fe, presumably in
the form of Fe-oxyhydroxides. Nickel (1984) refers to Thornber and Wildman
(1984) noting that the coprecipitation of cations is favoured by a high pH, in the
case of the Teutonic Bore, probably resulting from the high level of carbonates in
the groundwater and partly dissolved carbonate species from the ore (Nickel,
1984). At the Dulgald River Lode, Taylor and Appleyard (1983) note that Cu
appears in relatively high concentrations within the bulk of the gossan profile,
indicative of an immature gossan profile developed during near neutral to alkaline
conditions.
3.4.6 Pb
Pb is one of the least mobile metals and is commonly observed throughout a
large number of the gossans in a variety of forms. Figure 3.7 illustrates the
stability fields for Pb compounds under conditions that resemble near-surface
weathering conditions. This illustration serves to confirm that Pb is soluble only
under the most extreme acid or alkaline conditions. Galena is the stable phase
under most reducing conditions, with anglesite and cerussite dominating under
acid and alkaline oxidising conditions respectively (Garrels and Christ, 1965).
Scott et al. (2001) comment that Pb is strongly retained in both mature and
immature gossans of Woodlawn and Currawang respectively. Despite its
immobility, the mineral hosts for Pb change significantly during weathering. The
great bulk of the Pb in the primary ores reviewed here occurs in the form of
galena. However, the respective gossans typically exhibit a wide variety of Pb-
rich species. Nickel (1984) notes that for the Teutonic Bore, Pb has been found
as a major component in twelve secondary minerals, the chief ones being
cerussite and plumbogummite.
Page 61
Chapter 3 Gossans
Figure 3.7 – Eh/pH diagram illustrating the stability relations of Pb compounds in water at 25oC and 1 atmosphere total pressure. Total dissolved sulphur of 10-1, pCO2 of 10-4. Boundaries of solids shown are for total ionic activity of 10-6 (Garrels and Christ, 1965).
Scott et al. (2001) note that high Pb gossans (Pb >4%) are typically immature,
probably due to the lesser abundance of pyrite and hence less acid conditions
during weathering. The authors identify a close association between Pb-As and
Pb-Sb in some gossans but not in the primary ore suggesting that As and Sb are
distributed between several phases in the ore but become associated with Pb in
alunite-jarosite during prolonged weathering. However, in many immature
Page 62
Chapter 3 Gossans
gossans, the Pb-As and Pb-Sb associations have not had time to develop and
the Pb is mainly present in oxidate phases like cerussite that typically contain
very low contents of As and Sb (Scott et al., 2001).
At Woodlawn, the carbonate zone contains acicular crystals of cerussite and the
sulphate zone anglesite. The supergene sulphide zone contains anglesite and
relict galena. At Currawang gossans retain boxwork textures, indicating that the
profile is immature, and contains cerussite. The sulphate zone material consists
of dark Fe-oxides with a basic Pb sulphate. Alunite-jarosite minerals are also
present, intergrown with the Fe-oxides (Scott et al., 2001).
At the Dulgald River Lode gossan, Pb minerals include plumbian jarosite,
plumbogummite and anglesite (Taylor and Appleyard, 1983). At the immature
Mugga Mugga gossan, Pb is retained and even concentrated in the lower part of
the profile, where it occurs as secondary sulphate, arsenate and phosphate
minerals of the alunite-jarosite series. Pb is, however, depleted in the upper part
of the profile (Taylor and Sylvester 1982). Pb has co-precipitated with Fe-oxides
in the immature gossans of the Teutonic Bore (Nickel, 1984).
Oliveira et al. (1998) note that the Lagoa Salgada gossan contains high Pb and
As values in the form of mimetite crystals. Williams (1933-34) notes a marked
enrichment in Pb at the base of the Rio Tinto gossan associated with the
precious metal layer and Vinals et al. (1995), revealed that Pb occurs as solid
solutions of beudantite-plumbojarosite-potassium jarosite. Pb was also detected,
but only occasionally, as anglesite associated with gangue species (Vinals et al.,
1995). Cerussite may also be present in minor amounts (Williams, 1933-34).
Lopez Garcia et al. (1988) on Sierra de Cartagena, south-eastern Spain,
comment that Pb was leached from the primary ores and precipitated as Pb- and
Ag-bearing jarosites, anglesite, cerussite, Pb-bearing coronadite and goethite.
The Pb bearing minerals of the Sierra de Cartagena gossan differ depending on
the composition of the primary ore. Anglesite, cerussite, Mn-oxides and goethite
occur in horizon 1, a gossan formed under weakly acid conditions resulting from
a low Fe-sulphide content in the primary ore and acid buffering from associated
Page 63
Chapter 3 Gossans
carbonates. Pb occurs in Mn-oxides and jarosite in horizon 2, a primary geology
rich in pyrite, forming strongly acidic conditions during weathering. In addition,
the ore textures also differ from one horizon to the other with pseudomorphic
textures frequently observed in horizon 1, but in horizon 2 primary textural
features have largely been obliterated (Lopez Garcia et al. 1988).
Supergene galena occurs throughout the transition zone of Broken Hill, Northern
Zimbabwe. Taylor (1958) relates this considerable migration of Pb in the zone of
weathering to a former period of aridity and increased salinity of the groundwater.
Cerussite is abundant in the oxide ore. Taylor highlights the abundance of
pyromorphite as an indication that the chloride ion is present. Blain and Andrew
(1977) also conclude that solutions enriched in chloride and bicarbonate ions
favour the dissolution of galena, thus enhancing the Pb content of the solutions
from which secondary sulphides may subsequently precipitate.
3.4.7 As and Sb
Arsenic and Sb are often closely associated in ores and gossans. In the
Currawang and Woodlawn deposits, the As and Sb occurs predominantly in
tetrahedrite-tennantite and enargite-luzonite solid solution series in the primary
ore and in alunite-jarosite minerals in the gossan, whereas in more As-rich
primary ores of the Lachlan fold belt, As is largely present as arsenopyrite,
occurring as scorodite in the profiles of immature gossans (Scott et al., 2001).
The gossan of the Dulgald River Lode contains elevated levels of As and Sb
possibly introduced as a result of leaching of the surface gossan (Taylor and
Appleyard, 1983). Similarly, in the Mugga Mugga massive sulphide deposit of
the Yilgarn Block, Taylor and Sylvester (1982) note that the anomalous
concentrations of As and Sb in the surface gossan result from the precipitation of
secondary Pb-bearing minerals of the alunite-jarosite series. The authors note
that the concentration of As immediately above the water table in secondary Pb
minerals is followed by a trend of slightly increasing As content up the profile.
This distribution reflects the low mobility of As in weakly acidic solutions and its
ready co-precipitation with Fe-oxides (citing Boyle and Jonasson, 1973).
Page 64
Chapter 3 Gossans
Similarly, at Murray Brook, Boyle (1995) notes that Pb, As, Sb and Bi correlate
strongly and occur within specific horizons. As with Dulgald River, Currawang
and Woodlawn, these elements are typically associated with the precipitation of
jarosite-group minerals within the gossan. Boyle also notes that borehole
sections rich in these metals contain lower than average Au contents, indicating
that precipitation of these hydroxyl-sulphate-oxide minerals has had little control
on the localisation of Au.
Williams (1933-34) notes enrichment in Sb associated with the precious metal
layer at the gossan/sulphide contact of the Rio Tinto deposit. Vinals et al., (1995)
comment that As was detected in members of the beudantite-plumbojarosite-
potassium jarosite solid solutions, appearing as powdery aggregates of zoned
and skeletal crystals, which could suggest a formation through successive
crystallisation re-dissolution processes. Sb was observed as fine-grained oxides
of the stibiconite-bindheimite group (Vinals et al., 1995).
Vink (1996) predicts that under both acid and alkaline oxidising conditions, Sb is
highly mobile as SbO3-(aq) and as Sb2O4
2-(aq) under strongly reducing alkaline
conditions. In the absence of sulphur, As is highly mobile under almost all
conditions, with native As only occurring under very strongly reducing conditions.
The high mobility of As means that arsenate and arsenite ionic species are widely
available for forming precipitates with many types and combinations of cations,
hence the wide variety of As-bearing species often observed in gossans (Vink,
1996).
3.4.8 Si, Sn and Ti
The breakdown of silicate minerals during the gossan forming process may result
in the supersaturation of SiO2 in the mineralising solutions. Below pH 9, silica is
in solution as the uncharged molecule Si(OH)4 and above pH 9 as Si(OH)4-
(Thornber, 1985).
Si is a common constituent of the gossans reviewed during this investigation,
occurring predominantly as quartz. Quartz is essentially a resistate phase and
exhibits a close correlation with other resistate phases, including cassiterite (Sn)
Page 65
Chapter 3 Gossans
and TiO2 in a number of the gossans. A significant proportion of the Si content of
these gossans is, however, present in the form of remobilised Si that appears to
have formed as a result of the dissolution of wall rocks and associated Si-rich
gangue minerals. This is typically followed by the subsequent reprecipitation of
the Si, largely as chert/jaspers in specific zones in the gossan. Blain and Andrew
(1977) note that it is quite likely that the acid buffering, hydrolysis reactions of
silicate wall rocks account for the release of silica.
The dissolution and mobilisation of Si is evident in the sub-rounded nature of
quartz grains in the gossan of the Flambeau mine, Wisconsin, U.S.A. Ross
(1997) suggests that rounding the quartz grains occurs during dissolution of
quartz grain edges by acidic supergene alteration fluids. Citing Morris and
Fletcher (1987), Ross (1997) also proposes a hypothesis that a reaction between
ferrous Fe in solution and quartz may have formed a thin layer of ferrous silicate
that would subsequently oxidise to form a hydrous Fe oxide (goethite), while
rapidly releasing silica into solution. Thus, the presence of ferrous Fe would
greatly increase the solubility of quartz, as opposed to the solubility of quartz in a
solution devoid of ferrous Fe (Ross, 1997). May (1977) describes a 5 metre thick
gossan and siliceous cap that is in sharp contact with the massive sulphide of
Flambeau.
Boyle (1995) describes the dissolution, reprecipitation and subsequent
accumulation of silica in the Murray Brook gossan. The bulk of the silica is
present as euhedral quartz and in lesser amounts, as amorphous silica. The
author notes that most of the quartz exhibits a chalky white appearance due to
attack by acidic solutions. During oxidation, the silicate minerals, and to a much
lesser extent, primary quartz, are dissolved by acidic solutions to form cation
complexes, silicic acid, and colloidal silica. During changes in pH and electrolyte
composition with depth, the colloidal silica becomes unstable in solution and
precipitates lower in the oxidising profile as amorphous silica (Boyle, 1995).
In Mugga Mugga, Taylor and Sylvester (1982) note that there has been an
absolute accumulation of silica immediately above the sulphide zone, occurring in
the form of a massive and slightly ferruginous chert derived from rock weathering
Page 66
Chapter 3 Gossans
and deposition from groundwaters. The authors also note that generally SiO2
and Fe2O3 contents vary antipathetically with an overall decrease of SiO2 and
increase in Fe2O3 up the profile.
Solomon (1967) describes cherts associated with the fossil gossan of Mt. Lyell,
Tasmania and compares them to the goethite-veined cherts overlying pyritic ore
on the Cerro Colorado at Rio Tinto, Spain (citing Williams, 1933-34). Oliveira et
al. (1998) note the gossan associated with Lagoa Salgada is more siliceous near
to the base. Taylor and Appleyard (1983) indicate that Si and Sn are essentially
immobile in the weathered zone of the Dulgald River Lode, illustrating the
resistate nature of the primary minerals within which these elements occur.
The presence of remobilised and reprecipitated cherts is also recognised at
Skouriotissa, Cyprus, where Constantinou and Govett (1972) note that the cherts
are common but are restricted to the ochre (oxide) sulphide contact. The authors
also confirm that their deposition was probably controlled by pH conditions, both
directly in their effect on the precipitation of colloidal silica (optimum pH 4.5),
(citing Okamoto et al., 1957) and indirectly as they affected the precipitation of Fe
and aluminium hydroxides (Constantinou and Govett, 1972).
An alternative mechanism for the formation of siliceous materials within gossans
is discussed by Hannington et al. (1986, 1988, 1991a, b, c and d) where
hydrothermal activity has continued intermittently during weathering of modern
seafloor sulphide mounds, resulting in localised silicification.
Sn2+ and Sn4+ are important in the aqueous gossan environment. As a cation,
Sn2+ is quite soluble below pH 5, occurring as the anion Sn(OH)3- above pH 9,
with some solubility of Sn(OH)20 at intermediate pH. Sn4+ is essentially insoluble.
In the primary ore, Sn4+, usually in the form of cassiterite, is highly resistate in
nature and will usually remain in the gossan. Sn2+, often occurring in the sulphide
minerals, may coprecipitate with the Fe-oxides, but will eventually oxidise to Sn4+
and form cassiterite (Thornber, 1985).
Page 67
Chapter 3 Gossans
Sn is a common accessory component of several of the massive sulphide
associated gossans described in the literature. Scott et al. (2001), referring to
Woodlawn and Currawang, note that in the primary ore, Sn is present as
cassiterite and stannite (ideally Cu2FeSnS4), with the latter breaking down to
additional cassiterite during weathering. Sn is residually concentrated in mature
gossans (Scott et al., 2001). Oliveira et al. (1998) on Lagoa Salgada note that
the values of Sn are relatively high, especially in the gossans with the bulk of the
Sn occurring as cassiterite (Oliveira et al., 1998).
At Murray Brook, Sn, as cassiterite, is conservative and correlates strongly with
Au, and Si (Boyle, 1995). At Mugga Mugga there is some concentration of Sn
above the water table. There is some suggestion of slight concentration of Sn in
the kaolinite zones indicating possible derivation from the amphibolite rather than
the sulphide mineralisation (Taylor and Sylvester, 1982). At Rio Tinto, Sn was
detected as anhedral grains (5-100um) of cassiterite commonly associated with
Sb oxides (Vinals et al., 1995).
Titanium has very low mobility under almost all environmental conditions, mainly
due to the high stability of TiO2 under all but the most acidic of conditions
(Brookins, 1988). Rutile, brookite and anatase are the naturally occurring
polymorphs of TiO2, with rutile being the most common, particularly in the primary
massive sulphide ores. These forms of TiO2 are highly resistate phases that are
largely retained and often concentrated during the gossans forming process. TiO2
therefore often exhibits a close correlation with other resistate phases, notably
quartz (Si) and cassiterite (Sn). Taylor and Sylvester (1982) comment that at
Mugga Mugga, Ti appears to be concentrated low in the weathering profile,
partially as a result of residual concentration.
Ti also occurs as a minor constituent of other less resistate phases, notably
amphibole and biotite, phases that may be predominantly leached from
surrounding wall rocks. Dimanche and Bartholome (1976) suggest that Ti is not
entirely immobile during weathering. Skrabal (1995) notes that Ti may exist in a
fully hydrated form, TiO(OH)2, in water above pH 2, being transported in a
Page 68
Chapter 3 Gossans
colloidal state rather than as a dissolved ion. Hutton et al. (1972) suggest that
Ti4+ is mobilised more readily in the presence of organic acids at pH <4.5.
3.4.9 Other metals
A number of other elements of interest are mentioned briefly in the literature and
are worthy of note. Hg is present as an accessory element in a number of the
gossans studied, probably occurring in solid solution in pyrite and sulphosalt
minerals within the primary ores. Boyle (1995), notes that in the Murray Brook
deposit, Hg is concentrated in cinnabar, the only stable secondary sulphide in the
massive sulphide gossan, and correlates strongly with Au, Sn and Si. At Mugga
Mugga, there is an enrichment of Hg close to the water table but throughout the
remainder of the weathered zone, Hg levels are approximately an order of
magnitude lower than in the fresh sulphide rock (Taylor and Sylvester, 1982).
Scott et al. (2001) on various deposits of the Lachlan fold belt comment that in
the primary ore, Bi can be hosted by pyrite, galena and/or native Bi and
bismuthinite and that Bi in different phases appears to behave differently during
weathering. However, according to the authors, Bi as well as Sn and Au may be
significantly enriched in the carbonate zone of the gossan, relative to the primary
sulphide zone
Because of their extreme solubility in acid solutions, Zn and Cd are generally
highly mobile during gossan formation and are therefore largely absent from even
the immature gossans. Both the mature and immature gossans of Woodlawn
and Currawang are severely depleted in Zn and Cd (Scott et al., 2001). Taylor
and Sylvester (1982) note that for Mugga Mugga, because of its extreme
solubility in acid solutions, Zn is depleted in the weathering zone although some
Zn may be fixed in goethite. Similarly, for Dulgald River, Zn and Cd have been
depleted by between 85 and 90 per cent relative to the primary ore, although
some high Zn is associated with secondary Pb minerals (Taylor and Appleyard,
1983).
Scott et al. (2001) on the eastern Lachlan fold belt note that Ba is mainly present
as barite and is highly variable within the profiles. In addition, secondary barite
Page 69
Chapter 3 Gossans
may occur as a result of remobilisation during weathering (from both primary
barite and/or Ba-bearing feldspars). Such secondary barite at Lewis Ponds
contains 2.1% Pb. Other elements and compounds described as highly mobile
under acidic, gossan forming conditions include MgO, CaO, K2O, MnO, S, CO2,
Cu, Co, Ni, Tl, Na and Sr (Taylor and Sylvester 1982; Taylor and Appleyard.
1983).
Page 70
Chapter 3 Gossans
3.5 Ancient Seafloor Weathering
3.5.1 Introduction
Thick gossanous Fe-oxides, overlain by Cretaceous pillow lavas and sediments
were found to cap the sulphide deposits on Cyprus (Robertson and Boyle, 1983).
This relationship indicates that some gossans formed on the ancient seafloor,
where massive sulphide deposits were exposed to oxidising seawater.
Fe-Mn-oxide sediments have been recognised in association with the massive
sulphide deposits of the Troodos Massif, Cyprus. Constantinou and Govett
(1972) describe the mineralogy and geochemistry of 'ochres' and 'umbers'
associated with the three major sulphide ores at Skouriotissa, Mousoulos and
Mathiati. The authors suggest that the primary ore, consisting of pyrite with trace
quantities of chalcopyrite and sphalerite contemporaneously or subsequently
underwent considerable oxidative leaching and secondary enrichment under sub-
marine conditions on the Cretaceous ocean floor. Ravizza et al. (2001) suggest
that the Skouriotissa mound may have been exposed to the seafloor for millions
of years.
Robertson and Boyle (1983) describe the Fe- and Mn-rich oxide sediments of the
Troodos Massif in the context of seafloor spreading in a small ocean basin during
the Upper Cretaceous. This produced major cupriferous sulphides and both Fe-
rich (ochres) and Fe-Mn-rich (umbers) oxide sediments. The authors conclude
that field, isotope and fluid inclusion data show that the sulphide ores were
derived by deep leaching of mafic ophiolitic rocks with an important component
derived from seawater. The sulphides formed from solutions released along
major faults located close to sites of lava extrusions near the spreading axis.
Robertson and Boyle examine several ancient deposits from the Mesozoic
Tethys Ocean, including the Troodos Massif (Cyprus), Semail Nappe (UAE) and
Baer-Bassit (Syria).
3.5.2 Ochres
Constantinou and Govett (1972) describe the ochre as a Mn-poor, Fe-bearing
sediment commonly containing sulphides as bands and fragments, enriched in
Cu and Zn, with varying proportions of interbedded chert, tuffaceous material and
Page 71
Chapter 3 Gossans
limestone. The ochre, which in some places is separated from the
stratigraphically higher umber by pillow lavas, is restricted to the immediate
surface of the sulphide bodies, and owes its origin directly to oxidation of the
sulphide ore.
The pyrite in the Skouriotissa ochre is detrital, intensely corroded and not in
equilibrium with the enclosing rocks. Pyrite in the ochre is unzoned, whereas
zoned pyrite predominates in the underlying ore. The graded bedding of the
ochres is presumably a detrital feature (Constantinou and Govett, 1972).
Constantinou and Govett (1972) conclude a marine environment of ochre
deposition, postulated on the basis of the interlayering of ochre and limestone at
Mathiati, where the limestone contains no terrestrial microfossils but has algal
filaments and liquid hydrocarbons and the presence of pillow lavas of presumed
submarine origin. The ochres at Mousoulos and Mathiati appear to have been
deposited in a quiet environment, probably in deep water at Mousoulos and in
relatively shallow water at Mathiati. The ochre at Skouriotissa was probably
deposited in a high-energy environment, as testified by graded bedding, fine
banding, pyrite washouts and the dispersal of Fe oxides for up to half a mile from
the orebody (Constantinou and Govett, 1972).
The ores of the Troodos Massif, Cyprus have also been described by Robertson
and Boyle (1983) and placed in context not only with other ancient deposits
formed in the Mesozoic Tethys Ocean, but also with the modern day seafloor
sulphides. The authors describe the ochres as brightly coloured ferruginous, Mn
poor metalliferous oxide-sediments restricted to immediately around and above
the massive sulphide orebodies. The ochres are distinct from sub-aerially formed
gossans. Robertson and Boyle (1983) recognise four distinct types of ochre:-
1. Massive and pseudo-conglomeratic ochre (Mathiati and Mousoulos)
formed by sedimentary transport and collapse upon oxidation, up to
several metres thick above the massive sulphide.
Page 72
Chapter 3 Gossans
2. Brown, grey or orange ochreous metalliferous siltstones, mudstones, clays
and volcaniclastics, with nodules of chalcedonic chert (Skouriotissa),
containing detrital sulphides and sedimentary structure indicative of
transport over surface of mineralised lavas and sulphides. Consists of
goethite, minor quartz, feldspar (probably authigenic), mixed layer clays
and some opaline silica. Enriched in Fe, Cu and Zn.
3. Finely laminated brown or grey oxide sediments, often with veins of Se-
bearing gypsum, found either above massive sulphides (Mathiati) or
interbedded with mineralised lavas (Skouriotissa). Consists of goethite,
gypsum, minor quartz and smectite. The Mn content is higher relative to
types 1 and 2.
4. Orange or red ferruginous veins and interstitial oxide sediments within
mineralised pillow lavas, consisting of goethite and hematite, with low Mn
and trace metal contents.
Robertson and Boyle conclude that the Cyprus ochres formed by seafloor
oxidation of sulphides, by erosion of ores and adjacent lavas, and by precipitation
of Mn-depleted ferruginous sediments from hydrothermal solutions released
during and soon after sulphide precipitation. Any Mn precipitated at this time was
remobilised during later stages of volcanism and hydrothermal activity.
More recent studies by Herzig et al. (1991) conclude that the abundance of
jarosite in the Skouriotissa ochres is a clear indication that they were derived
from sulphide oxidation, and their occurrence within a sequence of Cretaceous
pillow lavas and sediments strongly suggest that the oxidation took place on the
sea floor.
3.5.3 Umbers
Constantinou and Govett (1972) describe umbers as Mn- and Fe-rich sediment
essentially devoid of sulphides. The authors conclude that the umber owes little
of its character to the sulphide orebodies, being essentially a degradation product
of the pillow lavas.
Page 73
Chapter 3 Gossans
This interpretation of the origins of umbers by Constantinou and Govett predates
our current understanding of black smokers in modern seafloor sulphide deposits.
It is now understood that the umbers are formed by precipitation of Mn from
seawater. The Mn remains in solution in seawater as it exits the black smokers,
often being transported some distance before being precipitated in seafloor
hollows as Mn-rich umbers.
Robertson and Boyle (1983) describe the typical Fe-Mn umber as a finely
laminated oxide-sediment that is distributed in small fault-bound hollows in the
pillow lava surfaces (Figure 3.8). The lavas immediately below the umbers often
comprise of fault talus and are generally highly decomposed, chemically altered
and impregnated with Fe-oxide sediment.
Basal umbers, up to several centimetres thick, are generally bright orange and
overlain by finely laminated darker umber. Higher in the succession, the umber,
often several metres thick, may contain siliceous mudstone intercalations, small
black manganiferous concretions and nodules of chalcedonic quartz. The
umbers then typically pass upward into radiolarian cherts or argillaceous
sediments, then into bentonitic clays (Robertson and Boyle, 1983).
At Skouriotissa, the umbers directly overlie sulphides and lavas and pass
conformably upwards into manganiferous umber. The umbers typically consist of
poorly crystalline goethite, quartz, mixed layer clays and opaline silica. The
umbers are typically FeMn-rich and are particularly enriched in Ba, Co, Cu, Ni.
Pb, V and Zr relative to pelagic clays (Robertson and Boyle, 1983).
Robertson and Boyle (1983) conclude that the umbers precipitated within and
above pillow lavas that were erupted on the flanks of the spreading axis. Fe
precipitated first to form the basal Fe-rich umbers, with Fe-Mn umbers rapidly
accumulating as Mn oxidised. Mn was oxidised more slowly than Fe and
remained in solution, eventually precipitating in the overlying umbers after
volcanism ended.
Page 74
Chapter 3 Gossans
Figure 3.8 - An illustration of the field relationships of a typical small umber hollow related to seafloor faulting, Troodos Massif, Cyprus (after Robertson and Boyle, 1983).
Page 75
Chapter 3 Gossans
3.6 Modern Seafloor Weathering
3.6.1 Introduction
The presence of Fe- and/or Mn-rich oxide deposits have been recognised in
association with modern seafloor sulphides and are described in the literature by
Hannington et al. (1986, 1988, 1991a, 1991b), Herzig et al. (1991), Knott et al.
(1995), Rona et al. (1993), Hekinian et al. (1993) and Binns et al. (1993),
amongst others. These oxide sediments have been attributed to both primary
and secondary processes, with Fe-rich, Mn-poor deposits resulting from the
alteration and weathering of the seafloor sulphides and Fe- and Mn-rich deposits
resulting from primary deposition from hydrothermal vents.
3.6.2 Modern Seafloor Fe-Oxide and Oxyhydroxide Deposits of Secondary Origin
These deposits relate most closely with the 'ochres' described by Constantinou
and Govett (1972) and Roberston and Boyle (1983) for the Troodos Massif,
Cyprus and consist predominantly of Fe-rich oxide deposits that are present
either in direct contact with, or adjacent to the associated massive sulphides.
Seafloor gossans display a long history of submarine weathering (up to 40,000
years) which is recognized in a complex suite of Fe-oxide assemblages
(Hannington et al., 1991a). These include Fe-oxides with secondary sulphides,
Fe-oxide-atacamite assemblages, Fe-oxide-jarosite assemblages, in situ Fe-
oxide crusts, resedimented Fe-oxide debris and manganiferous Fe-oxide umbers.
Hannington et al. (1991b) describes the major components of the gossanous
sediments at the TAG hydrothermal mount as consisting of coarse sulphide and
Fe-oxide fragments up to several centimetres across, sand-sized detrital Fe-
oxides and pyrite and red or red-brown to yellow Fe-oxide muds. The Fe-oxide
mineralogy consists predominantly of amorphous oxyhydroxides and goethite,
with minor lepidocrocite, akaganeite, and hematite with late-stage Mn seeps
locally replacing pre-existing Fe-oxide gossan, producing ferromanganiferous
umbers. The sand fraction consists of atacamite, siliceous fragments
(amorphous silica, +/- quartz, +/- cristobalite), detrital Mn-oxides, basaltic glass,
and carbonate forams. The silt and mud fractions consist predominantly of Fe-
oxides, jarosite and fine-grained sulphides. Hydrothermal activity has continued
Page 76
Chapter 3 Gossans
intermittently during weathering of the mounds, resulting in local silicification of
gossanous material and the formation of ferruginous chert (Hannington et al.,
1991b).
Hannington et al. (1991a) note that mass wasting of the gossans has produced
abundant clastic Fe-oxide debris that is weakly lithified and cemented. The
authors note that coarse debris at the top of the sequence grade downward into
mixed sulphide-oxide sand with fine-grained oxidized material occurring at the
bottom of the cores. The authors conclude that the detrital Fe-oxides are derived
largely from weathered sulphide chimneys with the pyrite being derived from
mechanically weathered chimneys and from large blocks of chemically weathered
anhydrite.
Knott et al. (1995) note that at the Galapagos Rift, massive sulphides occur as
upstanding edifices up to 3m high, projecting through a cover of gossanous
sediments consisting of Fe oxyhydroxides, sulphides and silica. Chalcopyrite was
altered on grain edges fractures to covellite, with trace amount of bornite and
idaite. Amorphous Fe-oxyhydroxides and hydrous Fe-sulphates precipitate
around the sulphides and are preserved as a dusting within silica. Pervasive
oxidation appears to predate silica precipitation.
Knott et al. (1995) associate the oxidation with the retrograde stage as
hydrothermal activity declines, during which, minor oxidation of sulphides by
mixed seawater-hydrothermal solutions generate a low pH. At temperatures
below 250oC, metals, including Cu and Au, become increasingly mobile. Herzig
at al. (1991) recognises this initial stage of weathering by an abundance of
secondary Cu-sulphides and the production of jarosite.
Rona et al. (1993) describe localised extensive weathering of pyrite-rich talus that
has formed along the margins of the TAG hydrothermal mount. This has resulted
in oxidation of the pyrite and the formation of gossans composed of Fe-oxides,
secondary sulphides, atacamite and jarosite with minor covellite and digenite.
The authors note that low temperature hydrothermal Mn-oxide replacement of
existing Fe-oxide gossan has also occurred. Mass wasting of partially oxidised
Page 77
Chapter 3 Gossans
sulphides and Fe gossans has also produced abundant metalliferous sulphide-
oxide sediment which is deposited at the base of the talus slope. The main
components of the sediment are coarse sulphide and Fe-oxide fragments, detrital
pyrite and bright red to yellow Fe-oxide mud, with thin layers of Fe-oxides often
protecting the interior of the sulphides against rapid oxidation.
Hekinian et al. (1993) describe the Fe and Si oxyhydroxide deposits of the South
Pacific intraplate volcanoes and East Pacific Rise and note four distinct types of
deposit. Types 1, 3 and 4 are primary in origin and are described in Section 4.6.3.
The ochreous, type 2 oxyhydroxide deposits are of secondary origin and are
associated with sulphides, occurring either coating the exterior or plugging the
interior of sulphide chimneys. They also form powdery ochreous products
resulting from the alteration of massive Fe sulphides, form the cement of sulphide
breccias or occur as loose powdery deposits containing traces of sulphides. This
type of deposits is more commonly associated with dead, off-axis vents and is
commonly coated with a Fe-Mn crust.
Rona et al. (1993) suggest that supergene reactions of older sulphides with
seawater have produced secondary Au enrichment at the TAG hydrothermal
mount. Hannington et al. (1988) revealed the presence of supergene Au grains
up to 15µm in size within the seafloor gossans. The Au grains contained little or
no detectable Ag, although some Cu was present. The authors suggest that Au
may be dissolved from the primary sulphides during oxidation of sulphide
minerals and subsequently transported as Au-chloride complexes (AuCl4-), with
re-precipitation of the Au occurring as a result of increasing pH or reduction by
ferrous Fe.
The supergene enrichment zones of modern and ancient seafloor sulphides and
sub-aerially weathered sulphide bodies are essentially similar, with digenite,
covellite and bornite replacing primary chalcopyrite. Amorphous Si may also be
abundant within the supergene zones. The Fe-oxides may, however, differ from
those described in gossans derived from aerial weathering, with Fe-oxides
commonly found coating primary and secondary sulphide grains. Boxwork
textures have not been observed (Hannington et al., 1988).
Page 78
Chapter 3 Gossans
3.6.3 Modern Seafloor Fe-Mn-Si Oxide and Oxyhydroxide Deposits of Primary Origin
Although it is clear that oxidation of seafloor sulphides produce Fe-bearing
oxyhydroxide deposits, research by Hannington et al. (1986, 1988, 1991(a),
1991(b)), Herzig et al. (1991), Knott et al. (1995), Rona et al. (1993), Hekinian et
al. (1993) and Binns et al. (1993) describe Fe-bearing oxyhydroxide deposits of
primary origin that may well represent the dominant Fe-oxyhydroxide deposits in
modern seafloor sulphide environments. These typically MnFe-oxyhydroxide
deposits are akin to the 'umbers' described by Constantinou and Govett (1972)
and Robertson and Boyle (1983).
Hekinian et al. (1993) describe types 1, 3 and 4 Fe and Si oxyhydroxides that are
primary low temperature (<70oC) hydrothermal precipitates, forming edifices,
mounds and flat lying deposits. Type 1 or ‘purple-red’ Fe oxyhydroxide
precipitates form mounds up to 15m in height and consist predominantly of
amorphous Fe oxyhydroxides with subordinate amounts of disseminated
hexagonal globules or close packed lamellae of goethite. The clay-rich type 3
Fe-Si oxyhydroxides are characterised by alternating lamellae and layers of light
greenish yellow nontronite and purple-red limonite-goethite, coated by a dark
brown Fe-Mn crust. Type 3 deposits form concentric concretions of hydrothermal
edifices, flat lying powdery precipitates and irregular slabs. These Fe-Si
oxyhydroxides are coated with Mn-oxide and are associated with inactive
hydrothermal mounds. Type 4 deposits consist of soft, lightweight, gel-like
precipitates of amorphous silica, quartz and amorphous Fe oxide. These opaline
Si-Fe oxyhydroxides form edifices similar to type 1 Fe oxyhydroxides.
Rona et al. (1993) note that presence of low temperature venting and
precipitation of Fe and Mn oxide deposits at TAG hydrothermal mount. Knott et
al. (1995) note that at the Galapagos Rift, present day low temperature
hydrothermal activity also forms Fe and Mn oxide and silica deposits. The
authors recognise that venting of low temperature (<17oC) fluids is precipitating
Fe-Si-Mn oxyhydroxides. Within these deposits, amorphous silica is abundant,
occurring as late, 20µm coatings of globular habit, lining all cavities. Elsewhere,
Page 79
Chapter 3 Gossans
the silica may completely coat Fe-Cu sulphides. The authors suggest that
microbial action may play an important role in the precipitation of metals, with
filamentous silica in cavities probably representing coatings on filamentous
bacteria.
Binns et al. (1993) describe Fe-Si-Mn-oxyhydroxide/nontronite deposits of
hydrothermal exhalative origins at the Franklin Seamount, noting that the first
material to form is filamentous silica, likely nucleated on or within microbes. Fe
oxyhydroxide platelets form next, overgrowing or partly replacing the filamentous
silica. Simultaneously or later, Mn-oxides form where hydrothermal fluids and
seawater interact, with much of the Mn-oxide escaping to the deposit surface,
forming crusts as it is oxidised by seawater.
On metal mobility within these primary deposits, Binns et al. (1993), citing
Hannington (1991c and d), note the repeated association of Au and Ag with As,
Sb, Pb and Zn, ascribed to the fact that aqueous sulphur complexes rather than
chlorocomplexes are the transport mechanism in moderate temperature (~250oC)
hydrothermal fluids. The nature and behaviour of Au in the primary deposits of
modern seafloor sulphides is described by Hannington et al. (1986), noting that
the most substantial Au enrichment occurs within the late, low temperature
precipitates associated with the more mature massive sulphide deposits. The
authors describe the close association between Pb, As, Sb and Ag in Au-rich
samples, attributing this to the precipitation of Au with the sulphosalts of these
elements.
Hannington et al. (1986) comment that the bulk composition of the 'Devil's Mud'
gossanous cap found in the ophiolite-hosted massive sulphide deposits of Cyprus
is similar to that of the late, hydrothermal caps associated with the modern
seafloor sulphides. The Devil's Mud is described as containing up to 1 weight
percent Pb together with sulphates of Cu, Fe and Zn as well as up to 50 volume
percent of white friable silica. Prichard and Maliotis (1998) note a strong
correlation between Au and Si in low temperature, off-axis fluids associated with
the ancient Troodos ophiolite.
Page 80
Chapter 3 Gossans
3.7 Comparing Modern and Ancient Deposits
Since the onset of detailed mapping and seafloor exploration in the late 1970's, a
greater understanding of the relationships between modern seafloor sulphides
and their ancient equivalents has been achieved. Published works by
Hannington et al. (1986, 1988, 1991(a), 1991(b)) and Herzig et al. (1991)
describe gossan development and associated mineralogy of the modern day
deposits and compare these to the ancient massive sulphide ores of the Troodos
complex.
Constantinou and Govett's paper on the Cyprus ores was written out of context
with modern seafloor sulphides and Fe-bearing oxyhydroxides, as it predated the
discovery of the seafloor sulphides during the late 1970's. Robertson and Boyle
(1983) compared the ancient metalliferous sediments of the Mesozoic Tethys
Ocean with modern seafloor sulphide deposits, noting similarities in mineralogy
and geochemistry of the Fe-Mn-oxide sediments associated with the massive
sulphides and volcanics.
Robertson and Boyle (1983) note that the manganiferous ores in the highly
faulted Ligurian Apennine ophiolite can be compared closely with the TAG
hydrothermal field in the mid-Atlantic Ridge at 26oN, exhibiting similar Mn-rich
crusts and comparable chemistry. The Troodos is similar to the East Pacific Rise
at Juan de Fuca, where discoveries of sulphide- and oxide-sediments have been
made. Similarly at 00o45'N, 86o07'W on the Galapagos spreading axis, Fe-oxide
vents are reported as well as large volumes of dispersed Fe- and Mn-oxide
precipitates. The Fe-oxide 'vents' are similar to the dispersed Fe-oxide ochre in
the Troodos ophiolite (Robertson and Boyle, 1983).
Discovery of both black and white 'smokers' at the East Pacific Rise and the
associated sulphide chimneys are compared with similar structures observed at
the Troodos Mavravouni orebody. Robertson and Boyle (1983) suggest that
ochreous conglomerates associated with sulphides in the Semail lavas could be
collapsed oxidised chimneys.
Page 81
Chapter 3 Gossans
Robertson and Boyle (1983) conclude that comparisons of modern oceanic
sediments indicate that high temperature discharge from less rifted, fast
spreading axes produced major stratiform cupriferous sulphide orebodies and Fe-
Mn oxide sediments (umbers). Rifting and slower spreading allowed greater
seawater penetration and favoured formation of small stratiform cupriferous
sulphides and Fe-poor, Mn-oxide sediments. The metals of the Mesozoic
Tethyan rifts and passive margin precipitated from more dilute lower temperature
thermal springs, with varying degrees of trace element scavenging from
seawater. The end product was the condensed Fe-Mn nodules and crusts which
slowly accumulated on sediment-starved seamounts and subsiding platforms.
More recent studies by Hannington et al. (1986, 1988, 1991a, 1991b), Herzig et
al. (1991), Knott et al. (1995), Rona et al. (1993), Hekinian et al. (1993) and Binns
et al. (1993) indicate that although secondary Fe-bearing oxyhydroxides of
secondary origin are a common feature of modern seafloor sulphide deposits,
primary, low temperature Fe-bearing oxyhydroxides from hydrothermal vents are
also abundant and should be considered when characterising ancient massive
sulphides and the weathering processes.
Page 82
Chapter 4 Methods of Investigation
4 METHODS OF INVESTIGATION
4.1 Introduction
Characterisation of the Las Cruces mineralogy was achieved using a combination
of transmitted and reflected light microscopy, scanning electron microscopy and
X-ray powder diffraction. The extremely fine-grained nature of the mineralogy
and often the poor polish taken by the fine-grained and porous samples
hampered mineral identification and some of the more exotic mineral
assemblages could not be positively identified.
A small number of reflected and transmitted light photomicrographs were
captured using a Buehler Omnimet ‘Enterprise’ image analysis system and JVC
digital camera. By far the bulk of the illustrations used in this thesis are
backscattered electron images taken on a Leo 440 SEM. The illustrations form
the basis of the borehole descriptions (Chapters 5 to 9) and are provided in
Appendices 6 to 10.
4.2 Sample Preparation
The sections of drill core were examined macroscopically after which one or more
thin slices were cut parallel to the length of the core using a diamond saw. The
resultant slices were examined using a binocular microscope and representative
mineralised areas of each were selected for the preparation of polished sections.
The selected areas were cut out using a diamond saw then mounted in epoxy
resin and polished prior to examination using reflected light microscopy and
SEM-based techniques. A number of slices were also used for the preparation of
thin sections for microscopic examination using transmitted light methods where
appropriate.
Due to the highly friable nature of much of the core it was not always possible to
cut slices using the diamond saw. In these cases, the rubble-like material was
washed and wet screened. A selection of sized materials and larger fragments
were subsequently mounted in epoxy resin and polished in preparation for
examination using reflected light microscopy and SEM-based techniques.
Page 83
Chapter 4 Methods of Investigation
Due to the high degree of porosity of a number of the drill core samples, the more
friable materials were mounted in a low viscosity resin and then placed in a
vacuum impregnation unit to aid resin penetration into the sample material. A
total of 434 polished sections were prepared from the five boreholes selected for
examination. Each of the polished sections was initially ground both on a
diamond cup wheel and on several grades of silicon carbide paper prior to
automated polishing using 14, 6, 3 and 1µm diamond suspensions with a hand
finish on 1/4um. The author carried out all polished section sample preparation.
Camborne School of Mines Associates prepared the thin sections from slices
selected by the author.
Page 84
Chapter 4 Methods of Investigation
4.3 Microscopy
4.3.1 Transmitted Light
Each of the thin sections was systematically examined using a Zeiss Axioskop
transmitted light microscope, providing information on the textural relationships
between the transparent gangue and ore minerals as well as deformation,
recrystallisation and secondary alteration phenomena. A total of 20 thin sections
were prepared from the five boreholes.
4.3.2 Reflected Light
Each of the 434 polished sections was systematically examined using a Zeiss
Axioplan reflected light microscope. Each polished section from the Au-bearing
sample intervals was also systematically searched for the presence of discrete
Au-bearing phases using relatively high power magnification (typically 20x air
objective) to ensure the observation of any tiny Au grains (typically <5µm) in
addition to the larger grains that are readily observed at lower power
magnification. Due to the extremely fine-grained and complex nature of much of
the gossan mineralisation, a 100x oil immersion objective was also used for
mineral identification purposes.
Page 85
Chapter 4 Methods of Investigation
4.4 Scanning Electron Microscopy
4.4.1 Qualitative Methods
All qualitative and quantitative Scanning Electron Microscope (SEM) analyses
were performed using a LEO 440 scanning electron microscope, fitted with a
high-resolution Oxford Instruments Germanium Energy Dispersive X-ray (EDX)
detector and a Microspec 400 Wavelength Dispersive X-ray (WDX) detector.
4.4.2 SEM Image Collection and Enhancement
After initial examination of the polished sections by optical microscope-based
techniques, a representative selection of the sections was carbon coated and
placed in the scanning electron microscope in preparation for examination and
characterisation using backscattered electron (BSE) imaging techniques.
Carbon coating is required for non-conductive samples so as to provide an
effective 'earth' for the electron beam. The Oxford Instruments 'Tetra' BSE
detector allows the operator to distinguish between discrete mineral phases
based on compositional variations, which result in mineral species appearing in
different shades of grey on the SEM monitor. The BSE images are particularly
efficient at distinguishing between gangue and ore minerals, based on the
differences in the mean atomic number of each phase. The lower limits of
resolution are also particularly good, with discrete grains of 1µm or less being
readily resolved using this technique.
The BSE detector captures electrons that are backscattered from the surface of
the polished section and an image of the section surface is created from these
electrons. Contrast and brightness adjustments allow for the differentiation of
discrete mineral phases. Minerals that exhibit a high mean atomic number (e.g.
galena, Au) will backscatter more electrons than those minerals that exhibit a low
mean atomic number (e.g. quartz, calcite). The brightness of the BSE image is a
factor of the number of electrons that are backscattered from the polished section
surface and, as a result, Au and galena may appear 'bright' on the image, relative
to quartz and calcite. This allows for the operator of the SEM to readily
differentiate mineral species and, for example, mineral zoning, based on subtle
Page 86
Chapter 4 Methods of Investigation
variations in brightness of the image. This was the main technique used for
documentation of the five borehole samples, with 621 images being collected in
total, the bulk of which were BSE images. Not all images were used in the final
thesis. The selection of the images used was based on providing a broad
understanding of the nature of the Las Cruces gossan mineralisation.
Borehole CR194 was the first borehole to be examined and the images collected
documented both 'typical' features and more unusual textures and associations
observed during the investigation. The selection of polished sections for
examination by SEM was based on providing a thorough range of materials that
were seen as typical of each sample interval based on the optical microscope
examination. Because of the time consuming nature of SEM operation and the
large number of polished sections prepared for this study (434 sections), not all
sections were examined.
Due to the often fine-grained and complex nature of the Las Cruces mineralogy,
the grey-scale BSE images were enhanced and false coloured using CorelDraw
and Corel Photopaint prior to incorporation into the main body of the text. These
bitmap and vector graphics packages allowed for the colouring of the grey-scale
images, with a key being added to distinguish between the mineral phases. The
relative brightness of the colours used typically reflects the relative brightness of
the grey shade in the original image.
Consistency of colours for the same minerals was also used throughout the
thesis where possible. The use of colour not only permits the rapid differentiation
of minerals, but other characteristic features may be highlighted. These include
porosity, compositional zoning, fine-scale intergrowths, oxidation and topography
(reflecting different polishing hardness of the minerals). These features are
described, where relevant, in the figure caption of the illustrations. An example of
how colour is used in this way is provided in Figures 4.1 and 4.2.
Page 87
Chapter 4 Methods of Investigation
Figure 4.1 - A monochrome backscattered electron image illustrating a rather complex Fe-oxide-rich sample with fine intergrowths of galena. Differences in brightness reflect different mineral species, variations in mineral chemistry, including oxidation, hydration and compositional zoning, porosity and variations in polishing hardness.
Figure 4.2 - The monochrome backscattered electron image has been false coloured and permits the reader to readily distinguish the mineral species. Galena (white) occurs as fine skeletal aggregates. Limonite fragments (yellow-brown shades) exhibit a wide range in brightness that reflects degrees of hydration. Darker browns represent more hydrated Fe-oxides (e.g. goethite). The darkest brown/black portions of the image represent areas of high porosity.
Page 88
Chapter 4 Methods of Investigation
4.4.3 Image Analysis Techniques
Due to the complex and fine-grained nature of the precious metal mineralogy
contained in the Las Cruces samples, modern computing methods were
combined with the backscatter electron imaging and EDX analysis to provide a
rapid, automated method of locating and identifying Au-bearing grains. This
technique is referred to as image analysis. Native Au grains and Au-bearing
amalgam were successfully located using an Oxford Instruments IMQuant–X
image analysis system incorporated into the SEM analytical software.
Many of the Las Cruces gossan samples contained abundant fine-grained galena
and other Pb-bearing phases that resulted in the backscatter electron images
commonly containing many hundreds or thousands of very bright grains in each
field of view, even at relatively high magnifications (~500x). As a result, the
presence of native Au and/or other Au-bearing phases was masked by the
presence of large numbers of other high mean atomic number phases.
Despite these complications, the image analysis system successfully located
many hundreds of precious metal grains. The polished sections subjected to this
automated searching were typically scanned at 500x magnification. This allowed
for the identification of precious metal-bearing grains to an effective lower size
limit of ~0.5µm. Using an analysis time of 200 milliseconds, five grains per
second could be analysed. Nonetheless, each polished section could typically
contain in excess of 10,000 high mean atomic number grains at the magnification
selected for the scans and each polished section could take in excess of 24
hours to search.
Once a grain of interest was located, further information, including chemical
composition, grain size and coordinates within the polished section, was
recorded. Grains of interest were subsequently examined and documented using
backscattered electron imaging where appropriate. The basic steps involved in
the image analysis process are illustrated in Figures 4.3 to 4.6.
Page 89
Chapter 4 Methods of Investigation
Figure 4.3 - A typical backscattered electron image as captured by the image analysis system. Differences in brightness of the mineral phases in the image reflect variations in mineral chemistry. The white areas consist of high mean atomic number phases and may include native Au or Au-bearing grains. The light grey areas are Fe-sulphides and the darker grey background is siderite. Pore spaces are black.
Figure 4.4 - The system recognises the range of grey shades of interest (red areas), depending on criteria set by the operator. Each bright phase (or phase of interest) is automatically analysed by the electron microscope using the EDX analyser.
Page 90
Chapter 4 Methods of Investigation
Figure 4.5 - Each grain of interest is recognised by the electron microscope and selected for analysis/measurement. Each grain is assigned a random colour.
Figure 4.6 - An example of an EDX spectrum captured using a very rapid (typically 200msec) EDX analysis of each grain. This is adequate to recognise the presence or absence of Au. In this example, the grain is a Sb-bearing Pb(Sb)-sulphide, recognised by the presence of Pb, S and minor Sb.
Page 91
Chapter 4 Methods of Investigation
4.4.4 Quantitative Methods
Quantitative microbeam analyses were performed on native Au and Au-bearing
amalgam grains, siderite and various sulphides to provide information on their
compositional ranges and to aid in their characterisation. Mineral stoichiometry
was also calculated to confirm the mineral identifications and to provide an
additional check on the quality of the analyses. Due to the presence of a carbon
coat on the polished sections and the inability of the SEM to accurately determine
the C content of the siderite, CO2 was calculated by difference.
Where suitable standards were available, the SEM EDX system was calibrated
using certified reference materials provided by Micro Analysis Consultants.
Where suitable standards were not available, the 'virtual standards' (standardless
data) supplied with the EDX system were utilised. In all cases, beam current
variations were corrected prior to analysis using a cobalt standard. Detection
limits for EDX analysis are in the order of 0.5 weight percent. Calibrations and
analyses were performed for 30 seconds using a detector dead time of ~30 per
cent, a beam current of ~5nA and 20Kv accelerating voltage. The results of the
EDX analyses and the mineral recalculations are provided in Appendix 5.
Page 92
Chapter 4 Methods of Investigation
4.5 X-Ray Powder Diffraction
A selection of handpicked mineral grains, magnetic separates and rock fragments
were subjected to X-ray powder diffraction (XRD) analysis. Suitably prepared
samples were irradiated using monochromated Cu K-α radiation and appropriate
instrumental settings to ensure the optimum resolution of reflections. The
resultant data were captured automatically using dedicated Panalytical
‘Highscore Plus’ software that also allowed for the measurement of peak
positions and the calculation of both d-spacings and intensities. The resultant
diffractograms and peak measurements were checked visually and the major and
minor phases were identified using a computer-based search and identify
program.
The identities of the phases were confirmed by comparison of the measured d-
spacings with the standard ASTM data sets using methods recommended by the
Joint Committee on Powder Diffraction Standards (JCPDS). The detection limits
for the identification of crystalline materials using x-ray powder diffraction
techniques is typically between 5 and 10 per cent by volume. All XRD analyses
were performed using a Panalytical PW3040/60 X’Pert diffractometer and
X’celerator detector.
XRD analysis was extensively utilised for the positive identification of both major
and minor phases. The results of the analyses are provided in Appendix 4. A
single sample of poorly crystalline clay was submitted to the British Geological
Survey, Keyworth, for XRD analysis. The result of this analysis is provided in
Appendix 4.
Page 93
Chapter 4 Methods of Investigation
4.6 Fluid Inclusion Analyses
Two samples of siderite-bearing material were submitted to Dr. Jamie Wilkinson
of the Royal School of Mines, Imperial College, London for fluid inclusion
analysis. Inclusions that were of sufficient size were subjected to conventional
microthermometry using a Linkam MDS600 motorised heating-freezing stage
mounted on a Nikon Eclipse 600 binocular microscope equipped with a x50 long-
working distance LCD objective, digital camera and image-grabbing software.
Stage calibration was carried out at –56.6, 10.0, 30.6 and 294°C using an in-
house synthetic fluid inclusion standard. Accuracy and precision of temperature
measurements is ±0.1°C at sub-ambient temperature and ±1°C at elevated
temperature.
Page 94
Chapter 4 Methods of Investigation
4.7 Isotope Analyses
Three samples of siderite were submitted to Dr. Steve Crowley of the University
of Liverpool for isotope analysis. Carbon dioxide was prepared for isotope ratio
measurement by reacting 6-7mg of finely powdered sample with anhydrous
phosphoric acid at 50oC following the method of McCrea (1950). The CO2 and
H2O released by the reaction were separated cryogenically, and H2S generated
by acid decomposition of galena was removed by reaction with AgPO4. The
resultant clean CO2 was subsequently analysed by conventional stable isotope
ratio mass spectrometry using a VG SIRA10 mass spectrometer. Isotope ratios
were corrected for 17O effects following the procedures of Craig (1957) and
oxygen isotope data were adjusted for isotopic fractionation between siderite and
H3PO4 using a fractionation factor () of 1.01046 (Rosenbaum and Sheppard,
1986). Isotopic ratios are reported in conventional delta () notation in per mille
(o/oo) relative to the VPDB (Vienna Pee Dee Belemnite) international standard,
e.g.
18Osiderite(o/oo) = [(18O/16Osiderite – 18O/16Ostd)/(18O/16Ostd)] x 1000
Analytical precision for carbon and oxygen isotope ratios is better than 0.2o/oo (1)
and 0.3o/oo (1) for 13C and 18O respectively (Crowley, pers. comms.).
Page 95
Chapter 4 Methods of Investigation
4.8 Geochemical Whole Rock Analyses
All geochemical whole rock analyses were carried out by the former Rio Tinto
laboratory Anamet Services, located in Avonmouth, Bristol, UK. Au analyses
were determined using fire assay with an AAS or ICP finish. Cu, Pb, Zn, Fe and
Ag were determined using Atomic Absorption Spectrometry (AAS). Sulphur was
determined using a Leco sulphur analyser and As, Sb and Sn were determined
using X-Ray Fluorescence (XRF). Whole rock assay data, together with details
of the methods and equipment used and information on the precision and
accuracy of the results are included, courtesy of Rio Tinto, in Appendix 3.
Page 96
Chapter 5 Borehole CR194 - Sample Descriptions
5 BOREHOLE CR194 – SAMPLE DESCRIPTIONS
5.1 Introduction
Chapter 5 provides a detailed description of the chemistry and mineralogy of
borehole CR194. Details of the field geologists' core log lithocodes and lens
descriptions are provided in the Appendix 2. Section 5.2 describes the major and
minor element chemistry of the borehole with particular emphasis given to their
relative abundance and degree of correlation.
Many of the sample intervals exhibit similar bulk mineralogy. Therefore the
mineralogical description is provided in five main sections, describing the
mineralogy of the ‘gossan’ (Section 5.3), ‘gossan contact with massive sulphide’,
(Section 5.4) ‘massive sulphide contact with gossan’, (Section 5.5), the ‘massive
sulphide’ (Section 5.6) and 'shale' (Section 5.7). A summary diagram of the
mineralogy is provided in Section 5.8. Each section is extensively illustrated.
The illustrations are provided in Appendix 6 in the order that they are described in
the main body of the text and are therefore not necessarily in depth order.
Tertiary conglomerate was unavailable for characterisation. The mineralogical
characterisation is focussed on the Au and/or Ag-rich samples gossan samples.
The gossan and massive sulphide core exhibits a high degree of preservation of
the sample intervals and lithologies. Sample selection extended into the first of
the underlying shale samples, below which precious metal, base metal and
deleterious metal content were significantly depleted.
Borehole CR194 intersects the fossil gossan at a depth of 149.80 metres. A thin
cap of Tertiary polymict conglomerate overlies the gossan, above which is
predominantly Tertiary marl. This borehole is relatively central to the supergene
massive sulphide ore and intersects the sulphide at a depth of 164.60 metres.
The supergene massive sulphide continues for approximately 15 metres at which
point the borehole intersects the underlying shales. The characterisation of
borehole CR194 is based on the preparation and examination of 100 polished
sections and 3 thin sections.
Page 97
Chapter 5 Borehole CR194 – Sample Descriptions
5.2 Borehole CR194 - Chemistry
5.2.1 Introduction
The major and minor element chemistry data are provided in Appendix 3. The
major element chemistry exhibits significant variation reflecting major changes in
the mineralogy, marking the prominent boundaries between the gossan, massive
sulphide and underlying shales.
Borehole CR194 is characterised by the presence of variable, but significant
minor elements including precious metals (Au and Ag) and those considered as
deleterious (As, Bi, Hg, Sb) from a mining perspective. The minor/trace element
chemistry also exhibits significant variation with some correlation between
elements also being recognised. In order to facilitate ease of interpretation, these
were plotted on several graphs and combined in a diagram showing the position
of each sample interval (Figure 5.1). The diagram representing the borehole has
been colour coded to show the position of the Tertiary conglomerate, gossan,
sulphide and underlying shales. The borehole depths represent depth down hole
and are approximately equivalent to depth from surface, with CR194 being a
vertical hole.
Page 98
Chapter 5 Borehole CR194 – Sample Descriptions
Figure 5.1 - Illustrating the chemistry variation in borehole CR194. Each sample interval is displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The Tertiary conglomerate was not available for examination. The variation in chemistry with increasing depth is displayed on four graphs in the centre of the illustration. The major, precious and deleterious element chemistry clearly exhibits a significant degree of variation that reflects an equally wide variation in the mineralogy of each interval. TCP - Tertiary Polymict Conglomerate, GHS - Strong Hematitic Gossan, GBM - Moderate Hematite Magnetic, MMP - Massive Sulphide, QXM - Massive Quartz/Shale, SXM - Massive Shale.
Page 99
Chapter 5 Borehole CR194 – Sample Descriptions
5.2.2 Geochemical Profile
The Cu content of the core is relatively low within the gossan but exhibits a
marked increase within the supergene massive sulphide. The increase in Cu
content of the sulphide ore is associated with a similar marked increase in the S
content within the sulphide zone. The Cu content of the massive sulphide
remains relatively consistent with no prominent peaks or troughs occurring.
Similar consistencies are also evident in the Fe, S and As contents in the
massive sulphide, possibly indicating some close associations between these
elements and possible consistencies in the nature of the mineralogy throughout
the massive sulphide.
The Pb content of this borehole is highly variable, with a number of prominent
peaks occurring, notable at or near the contact between the gossan and massive
sulphide and less significantly within the massive sulphide in a mixed
sulphide/shale zone (lithocoded MMP/QXM). The Pb content of the upper
gossan region is relatively low, and increases significantly, peaking to a
maximum (7.9%) at the contact between the gossan and sulphide. The Pb
content of the bulk of the sulphide and shale zones is, however, relatively low.
The increase in Pb at the gossan/massive sulphide contact is associated with a
dramatic increase in the Au, Sn, Ag, Bi, Sb and As contents and to a lesser
extent the Hg content. Within the mixed massive sulphide/shale zone, the less
significant increase in Pb content is again associated with a marked increase in
the Au, Sn, Ag, Bi and Sb contents although the As content appears to be
relatively unaffected in this region of the core.
The Fe content of the gossan is relatively high and exhibits a marginal but
progressive increase towards the lower portion of the gossan. The Fe content
exhibits a moderate decrease at the gossan/sulphide contact zone, which is
associated with a marked increase in the S and Cu contents. The Fe content of
the sulphide zone is relatively consistent and exhibits a marked decrease at the
contact with the underlying shales. The Fe content of the shales is consistently
low.
Page 100
Chapter 5 Borehole CR194 – Sample Descriptions
The S content is relatively low but variable within the gossan reflecting the
presence of minor amounts of Fe-sulphide, galena and other Pb-bearing
sulphosalts. The S content of the massive sulphide is high, occurring
predominantly in pyrite and supergene copper sulphide mineral. The S content
exhibits a marked decrease at the contact with the underlying shales.
The Ag content is typically low within the bulk of the core, but exhibits a marked
increase at the contact between the gossan and the massive sulphide. The
marked increase in the Ag content at this contact zone is also accompanied by a
marked increase in the Pb, Au, Bi, Hg, Sb and Sn content. The As content peaks
slightly higher in the gossan profile. The Ag content also exhibits a less
pronounced increase in the mixed sulphide/shale zone (lithocoded MMP/QXM).
This is again accompanied by an increase in the Pb, Au, Bi, Hg, Sb and Sn
content and a marginal decrease in the Fe and S content.
The geochemical profile for Au follows a very similar pattern to that of Ag and
also exhibits similar associations with other minor elements, including Pb, Bi, Hg,
Sb and Sn. The most prominent increases in the Au content of the core occur at
the gossan/sulphide interface and in the mixed sulphide/shale zone. The
distribution of Au within the gossan is, however, somewhat more erratic, with a
marginal increase in the Au content being observed in the 155.75 to 156.70
metre sample interval. This peak in the Au content is associated with a slight
increase in the Pb and S content of the core. The increase in the Au content of
the gossan/sulphide interface also occurs slightly higher in the gossan profile
than that observed for Ag and Hg and may be more closely aligned with that of
Sn and Bi.
The As content of the upper gossan is low, but increases significantly towards the
gossan/sulphide contact zone, reaching a maximum slightly higher in the gossan
profile than that observed for Pb, Au, Bi, Ag, Sn, Hg and Sb. The As content of
the massive sulphide remains consistent with increasing depth and decreases
significantly in the underlying shales.
Page 101
Chapter 5 Borehole CR194 – Sample Descriptions
The geochemical profiles for Bi, Sb and Sn are essentially similar, occurring in
relatively minor amounts in the upper gossan, but increasing significantly with
increasing depth in the lower gossan and peaking at the contact with the massive
sulphide. The mixed sulphide/shale zone also exhibits elevated levels of Bi, Sb
and Sn.
The Hg profile is similar to that described for Pb, Au, Sn, Bi and Sb, peaking at
the gossan/sulphide interface and in the mixed sulphide/shale zone. However,
the Hg profile is more similar to that of Ag, as the increase in Hg levels are not
observed as high in the gossan profile as those observed for Pb, Au, Sn, Bi and
Sb.
Page 102
Chapter 5 Borehole CR194 – Sample Descriptions
5.3 Borehole CR194 – Gossan
5.3.1 Introduction
The 14 sample intervals from 149.80 metres to 163.75 metres are described
collectively as 'gossan'. The gossan is characterised by the presence of variable
amounts of Fe (27.06–62.74%), S (0.34–7.90%), Cu (0.01–0.34%) and Pb (0.95–
7.92%). The gossan also contains significant but highly variable amounts of Ag
(1.9–178.5ppm) and Au (0.07–7.39ppm) and an abundance of deleterious
elements including As (329–17897ppm), Bi (25–1388ppm), Hg (0.2–69.0ppm),
Sb (252–3375ppm) and Sn (32–450ppm).
The bulk of the gossan consists of fragmented quartz-rich rock fragments that
have been partially and/or extensively replaced by siderite (Figure 5.2). The
presence of siderite gives the core a characteristic reddish-brown colour that is
clearly observed in hand specimen. The gossan is highly variable in nature,
reflecting variations in the relative proportions of siderite, quartz, limonite and
sulphide minerals. The degree of porosity of the core also has a marked effect
on its physical properties with the more porous sections of core often being highly
friable in nature.
X-ray powder diffraction analysis confirms the presence of quartz, siderite,
hematite, goethite, anglesite, barite and anatase.
5.3.2 Quartz
Quartz is the dominant Si-bearing phase present in the gossan and, apart from
subordinate amounts of an Fe-rich clay, no other significant Si-bearing phases
were located. Unfortunately, SiO2 analyses of the core are not available, as
these would have provided a quantitative indication of the quartz distribution.
The proportion of quartz varies considerably within and between sample intervals
and ranges in abundance from <1 percent of the sample to >60 percent by
volume. Quartz is most abundant in the upper portion of the gossan (Figure 5.2),
but gradually decreasing with increasing depth towards the contact with the
underlying massive sulphide.
Page 103
Chapter 5 Borehole CR194 – Sample Descriptions
The quartz is typically present as fragments that range in size from a few
micrometres to several millimetres (Figure 5.2). Examination of the quartz-rich
rock fragments in thin section confirms that they are polycrystalline in nature and
the size of the crystallites ranges from a few micrometres to tens or hundreds of
micrometres. The fine and coarse crystallites may occur in close association
within the same quartz fragments. Conversely, many fragments consist almost
entirely of either coarse or fine-grained crystallites. The majority of the quartz
fragments contain crystallites that are extremely fine-grained and polycrystalline,
possibly reflecting partially recrystallised chalcedony.
The bulk of the quartz fragments within the gossan matrix exhibits highly irregular
morphologies (Figures 5.3a, 5.6a and 5.7b), probably indicative of some degree
of dissolution. The larger quartz fragments may also exhibit fracturing, resulting
in a very angular morphology (Figures 5.3a and 5.7a). These fractures are often
filled with siderite. The finer quartz groundmass may exhibit more rounded
features that are also possibly a result of chemical dissolution. A subordinate
portion of the quartz occurs as elongated fragments, possibly representing
fragments of former quartz veinlets. Euhedral cavities are also locally present in
the quartz, reflecting the presence of former minerals that have been removed
during oxidation and/or dissolution.
The quartz crystallites often exhibit a 'grain-flattening' fabric that simply reflects
growth into an open cavity or fracture. These textures have clearly not formed in
situ and therefore cannot be interpreted in context with the surrounding
mineralogy. A subordinate but significant portion of the quartz crystallites exhibit
a fibrous texture that is commonly associated with euhedral pore spaces in the
quartz fragments. The euhedral pore spaces have often been filled by siderite.
The wide variety of textures exhibited by the quartz crystallites suggests multiple
sources for the quartz fragments.
The quartz is largely free from intergrowths and inclusions. Rare and often
micrometre-sized inclusions of pyrite are locally present. There appears to be no
direct association between the presence of quartz and the relative abundance of
other phases in the gossan. Quartz also occurs as fine-grained and porous
Page 104
Chapter 5 Borehole CR194 – Sample Descriptions
shale-like rock fragments. These ferruginised fragments are clearly visible in
hand specimen (Figure 5.2). The shale fragments appear to have been
extensively leached of their primary components, including phyllosilicate minerals
with only the chemically resistate components of the original shale host
remaining, including quartz, some TiO2 (largely anatase) and carbonaceous
materials. Siderite may partially fill some of the pore spaces present in the shale
fragments.
5.3.3 Siderite
Siderite is the dominant mineral in the gossan and often exceeds 50 percent of
the sample by volume. The siderite commonly occurs as millimetre-sized
‘fragments’ (Figures 5.2 and 5.7a) and as a cement that replaces the fine-
grained, quartz-rich matrix (Figures 5.3b, 5.5b and to a lesser extent 5.4a and
5.6a). The gossan is essentially a conglomerate consisting of quartz-rich rock
fragments in a fine-grained, quartz-rich matrix with extensive replacement by
siderite (Figure 5.2).
The siderite ‘fragments’ actually consist of cavity fillings and late-stage
pseudomorphous replacements (Figures 5.2, 5.3b, 5.7a, 5.8 and 5.12b). The
cavity-filling nature of these ‘fragments’ is further evidenced by the presence of
euhedral crystals that develop into the open cavities (Figures 5.7a, 5.13a and
5.14a). Euhedral siderite crystals are a relatively common feature throughout the
gossan and they often exhibit less oxidation than the surrounding siderite matrix
(Figure 5.12a) due to their late-stage nature.
The siderite matrix is a late-stage cement that may replace the quartz (Figures
5.2 and 5.7a). Late-stage, unoxidised siderite also occurs as veinlets that
traverse the gossan (Figure 5.23a) or cements earlier stages of siderite
mineralisation (Figure 5.22b).
Dissolution of siderite is evident in the gossan, with rhomb-shaped voids often
being leached of their original carbonate content (Figure 5.28b), leaving empty
cavities or skeletal galena.
Page 105
Chapter 5 Borehole CR194 – Sample Descriptions
Examination of the late-stage, unoxidised siderite in thin section reveals that it
consists of granular aggregates of siderite that are typically coarse grained
compared to the surrounding matrix, with discrete siderite crystallites exceeding
tens or even hundreds of micrometres in size. The coarse grained siderite is less
reactive than the fine-grained siderite and is therefore more resistant to oxidation
and subsequent replacement by limonite. The late-stage siderite veinlets also
consist of relatively coarse-grained crystallites.
Cavity-filling siderite (Figures 5.3b, 5.5b and 5.27b) clearly post-dates the
reworking of the quartz-rich rock fragments, as it infills spaces in the reworked
conglomerate. These later stages of siderite mineralisation are commonly
associated with sulphide minerals, including an Fe-sulphide assemblage (Figure
5.5b) and galena (Figures 5.3a, 5.27b and 5.28b).
A discrete, needle-like Pb(SbAs)-bearing sulphide is also present in some of the
late siderite (Figure 5.5b). This phase was too fine-grained for a quantitative
SEM analysis, however, semi-quantitative analysis confirmed that it contains in
excess of 5 weight percent Sb and similar levels of As. The presence of
significant As, which does not typically occur in such high quantities in galena,
and the acicular morphology, which again is uncommon in cubic minerals,
suggest that this phase probably represents a discrete sulphosalt mineral as
opposed to Sb- or As-bearing galena.
Galena commonly lines the margins of siderite veinlets and cavities (Figures
5.3b, 5.22b and 5.23a) and skeletal galena may partially fill cavities associated
with the siderite (Figures 5.4a and 5.6a). Skeletal galena also forms complex
textures within the late siderite (Figures 5.12b and 5.13b). Although the siderite
appears to be replaced by the galena in this case, SEM examination confirms
that compositional zoning in the siderite overprints/cross-cuts the galena
mineralisation and therefore probably represents a cavity filling.
As well as replacing the quartz-rich rock fragments and fine quartz matrix, the
late-stage siderite also extensively replaces relict barite (Figures 5.3a, 5.8 and
5.11b), fine-grained and porous Fe-clay (Figures 5.4b and 5.28a) and the
Page 106
Chapter 5 Borehole CR194 – Sample Descriptions
limonite-rich matrix (Figures 5.13a and 5.26a). There is some compositional
variation in the siderite. The extensive oxidation of much of the early-formed
siderite has, however, significantly masked the composition of earlier stages of
mineralisation. A number of quantitative SEM analyses were performed on the
less oxidised siderite, the results of which are provided in Appendix 5.
The compositional variations largely reflect changes in the relative proportions of
Mg and Ca at the expense of Fe. Other elements are below the detection limits
for this technique (~0.5%). The MgO and CaO contents of the siderite ranges
between an effective lower limit of less than 0.5 per cent to the highest values
that may exceed 1.9 per cent and 5.6 per cent respectively. These variations
reflect compositional zoning within the late-stage siderite rather than discrete
stages of siderite mineralisation.
The siderite exhibits varying degrees of oxidation and replacement by limonite
that often highlight different stages of mineralisation, with the latest stages of
siderite typically exhibiting little or no oxidation.
5.3.4 Limonite
The term ‘limonite’ is used here to describe the Fe-oxide and Fe-hydroxide
assemblage that is intimately associated with the gossan. XRD analysis confirms
that the bulk of the ‘limonite’ consists of hematite, with goethite being present in
relatively minor amounts. The presence of hematite gives the core a distinctive
blood-red appearance in hand specimen (Figure 5.2).
Limonite is most abundant in the fine-grained matrix (Figures 5.10b and 5.12b),
probably reflecting the more reactive nature of the fine-grained siderite and, to
some degree, the partial oxidation of associated Fe-sulphide. The oxidation of
the siderite results in an increase in the porosity of the matrix due to volume
changes that occur during the oxidation/hydration process. This porosity is
clearly illustrated in Figure 5.5a.
The scale of the replacement textures between siderite and limonite ranges from
relatively coarse caries texture (Figures 5.8a and 5.8b) to the development of
Page 107
Chapter 5 Borehole CR194 – Sample Descriptions
microscopic and sub-microscopic needles and botryoidal aggregates of limonite
on a scale that is on the limit of resolution for the electron microscope (Figure
5.7b).
Locally, limonite is abundant and occurs as delicately banded botryoidal
aggregates (Figures 5.25b, 5.26a and 5.26b). The botryoidal limonite may
exhibit fracturing (Figure 5.27a) and is often extensively replaced by late-stage,
unoxidised siderite. Late stage siderite and galena may also fill or partially fill the
cavities within the botryoidal limonite aggregates (Figures 5.26, 5.27 and 5.28b).
The fine-grained and more reactive siderite-rich matrix typically exhibits a higher
degree of oxidation relative to the less reactive, coarser-grained siderite (Figures
5.10b and 5.12b).
5.3.5 Fe-Clay
The gossan is characterised by the presence of variable amounts of a poorly
crystalline Fe-clay. XRD analysis confirmed this mineral is nontronite (ideally
Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O, see Appendix 4).
This clay-like phase is typically fine-grained and porous in nature (Figures 5.4b
and 5.28a) but also forms radiating and concretionary textures (Figures 5.18a
and 5.23a). Characteristic dehydration cracks are also typically present (Figures
5.21b and 5.30a)
This phase occurs throughout the gossan in relatively minor amounts and is often
associated siderite (Figures 5.4b and 5.28a), occurring as a cavity filling.
Nontronite is also occasionally developed along the margins of the siderite
veinlets (Figure 5.23a). Rarely, the Fe-clay is replaced by a AgSb(As)-sulphide
(possibly pyrargyrite, ideally Ag3SbS3) (Figure 5.23b). Qualitative SEM analysis
of the clay confirms that it consists predominantly of Fe, Si and O together with
subordinate but variable amounts of Na, Mg, Al, P, Ca, K and S, some of which
are likely adsorbed species.
Page 108
Chapter 5 Borehole CR194 – Sample Descriptions
5.3.6 Accessory Transparent Gangue Minerals
A number of gangue minerals are present in the gossan in minor to moderate
amounts. Barite is a common accessory phase and occurs in localised patches
throughout the gossan (Figures 5.10b and to a lesser extent 5.3a and 5.4a). The
barite is typically extensively replaced by siderite (Figure 5.11b).
Another excellent example of replacement of barite by siderite is illustrated in
Figures 5.8a and 5.8b. In this association, the tabular, euhedral barite crystals
have been pseudomorphously replaced by siderite, with only minor amounts of
the original barite crystals remaining. Note the highly irregular morphology and
corroded appearance of the barite in Figure 5.8b. The siderite has been
subsequently oxidised and partially replaced by limonite. Apatite was also
observed in the gossan in minor amounts, occurring as small rounded grains in
the siderite-rich matrix (Figure 5.6b).
5.3.7 Fe-Sulphides
Fe-sulphides are a common accessory and account for the magnetic nature of
the core. These phases are described in greater detail in Chapter 10. Their very
fine-grained nature made optical identification difficult. However, they likely
consist of one or more of amorphous FeS, mackinawite, greigite, pyrrhotite and
marcasite/pyrite. For simplicity, they are simply described as Fe-sulphide in this
chapter.
Fe-sulphide is most abundant in the fine-grained matrix of the fragmented gossan
(Figures 5.3b and 5.13a) and within pore spaces (Figures 5.4b and 5.12a)
associated with late-stage siderite. The Fe-sulphides often occur as aggregates
of platelets that may exceed several hundred micrometres in size (Figures 5.4b
and to a lesser extent 5.3b). Discrete Fe-sulphide platelets may exceed 50µm in
length.
Fe-sulphide also occurs as fine-grained and porous granular aggregates (Figures
5.13a and 5.14a) with discrete grains rarely exceeding 20µm in size (Figure
5.6b). The granular Fe-sulphides may also be finely disseminated throughout the
siderite- and limonite-rich matrix. Euhedral crystals of Fe-sulphide feature within
Page 109
Chapter 5 Borehole CR194 – Sample Descriptions
some of the late-stage siderite-rich veinlets where they appear to line the margins
of former cavities (Figure 5.5b) or are disseminated throughout the unoxidised
siderite (Figure 5.17b). Detailed examination of the Fe-sulphides at high power
magnification revealed the presence of minor amounts of intergrown Pb(SbAs)-
bearing sulphides (Figures 5.5b and 5.17b). Fine-grained galena is also
commonly present in the cores of the Fe-sulphide aggregates (Figures 5.5a and
5.12a).
5.3.8 Galena and Pb-Bearing Sulphides
Galena hosts the bulk of the Pb content of the gossan, typically ranging between
1 and 5 per cent by volume. The galena typically occurs as euhedral crystals
(Figures 5.6b, 5.23a and 5.23b) and skeletal aggregates (Figures 5.4a, 5.25b,
5.28a and 5.29a) with discrete crystals rarely exceeding a few micrometres in
size.
Well-defined skeletal textures may be observed in the galena locally (Figures
5.20a and 5.25b). These skeletal aggregates commonly occur within cavities in
the siderite- and limonite-rich core (Figures 5.25b and 5.20a). Several stages of
galena and galena + siderite mineralisation are often evident (Figures 5.27b,
5.28a, 5.28b, 5.4a and 5.6a). These textures are extremely common features of
the gossan.
The skeletal galena may also fill or partially fill rhombohedral voids (Figure
5.28b). These voids probably reflect the presence of former siderite, which has
subsequently been leached, leaving behind the skeletal galena that was present
as a component of the siderite + galena mineralisation.
Tiny euhedral galena crystals may also line the margins of former cavities (Figure
5.3b) or fractures (Figure 5.23a) that have subsequently been filled by siderite.
The galena may also be more randomly disseminated throughout the siderite-
and limonite-rich core (Figure 5.6b) or form thin veneer-like aggregates along the
margins of the late-stage siderite (Figure 5.10b).
Page 110
Chapter 5 Borehole CR194 – Sample Descriptions
Galena may form complex, fine-grained and porous aggregates associated with
one or more of Fe-sulphides (Figures 5.13a and 5.13b), cerussite (Figure 5.17a)
and anglesite and may replace mimetite (ideally Pb5(AsO4)3Cl) (Figures 5.18b,
5.19a and 5.19b) and a CuFe-sulphide phase (Figures 5.11a, 5.14a and 5.16b).
Minor amounts of As and Sb are also often present in the galena aggregates,
possibly indicating the presence of discrete PbSbAs-sulphides or possibly some
Sb in solid solution with the galena. Due to the extremely fine-grained nature of
these aggregates, it was not possible to positively identify any discrete PbSbAs-
sulphides phases.
The galena-rich aggregates typically occur within the most porous regions of the
core, along grain boundaries and within former cavities (Figures 5.12b, 5.13a and
5.13b). The Pb-rich aggregates therefore often occur within the fine-grained,
porous matrix of the gossan (Figures 5.12b, 5.13a and 5.13b). Complex galena
textures are observed in a number of the siderite aggregates (Figures 5.12b and
5.13b). These complex textures represent the partial replacement of galena by
late-stage siderite leaving relict, skeletal galena.
A discrete PbAs-bearing sulphosalt phase is also present in the gossan,
occurring with Fe-sulphide and siderite (Figures 5.5b and 5.17b). This phase
typically occurs as finely disseminated, micrometre-sized needle-like grains and
was too fine-grained for a positive identification. XRD analysis confirms that the
galena also exhibits some degree of oxidation and replacement by anglesite.
5.3.9 Secondary Pb-bearing Phases
The gossan is characterised the presence of a number of secondary Pb-bearing
phases that are typically present in relatively minor amounts. These secondary
Pb-bearing phases are often present as components of the fine-grained galena-
rich aggregates (Figures 5.12b to 5.17a). Pyromorphite (ideally Pb5(PO4)3Cl) and
to a lesser extent mimetite (ideally Pb5(AsO4)3Cl) are minor accessory phases in
the gossan. Pyromorphite may occur as discrete euhedral crystals and granular
aggregates that exceed 100m in size (Figure 5.15b). Pyromorphite may also
form pseudo-hexagonal skeletal crystals (Figures 5.8a and 5.9a).
Page 111
Chapter 5 Borehole CR194 – Sample Descriptions
Cerussite may be locally abundant where it typically occurs as fine-grained
aggregates and acicular crystals that are intimately associated with galena and,
to a lesser extent, anglesite (Figure 5.17a). Discrete cerussite crystals may
exceed 50m in maximum dimension.
5.3.10 Amalgam and Hg-Bearing Phases
Amalgam is absent throughout the bulk of the gossan, but becomes increasingly
abundant towards the contact with the massive sulphide. Although only present
in minor amounts, amalgam hosts the bulk of the Hg in the gossan. The
amalgam typically occurs within the siderite- and limonite-rich matrix.
Although amalgam aggregates may exceed 100µm in size (Figures 5.24a and
5.24b), the bulk of the amalgam occurs as finely disseminated grains that are
significantly finer. The amalgam aggregates commonly exhibit a highly irregular
morphology (Figures 5.24a and 5.24b) that may reflect some degree of
dissolution (Figure 5.24b). The amalgam exhibits some degree of replacement
along the margins by cinnabar (ideally HgS) (Figure 5.24a) and possibly limonite
(Figure 5.24b). It is possible that the amalgam may have initially formed under
conditions where the Ag-Hg alloy was stable, but subsequent changes in the
environment have resulted in the partial dissolution and/or replacement of the
amalgam by cinnabar. Quantitative SEM analyses of the amalgam are provided
in Appendix 5 (analyses #1 to #3). The analyses of the amalgam are relatively
consistent and confirm that it consists predominantly of Ag (54.2-56.4%) and Hg
(44.2-46.7%).
Aggregates consisting of one or more of acanthite, amalgam, cinnabar, (Figures
5.26b and 5.29b), iodargyrite (ideally AgI), a AgSe-sulphide (possibly aguilarite,
ideally Ag4SSe) and a Ag-selenide (possibly naumanite, ideally Ag2Se) were also
recognised and typically occur within botryoidal limonite aggregates.
Fine-grained and often granular native arsenic is also occasionally present within
the amalgam.
Page 112
Chapter 5 Borehole CR194 – Sample Descriptions
5.3.11 Precious Metal Mineralisation
A small number of native Au grains were observed in the gossan and typically
occur as micrometre-sized grains that are present within the nontronite and
limonite-rich matrix (Figures 5.20b, 5.21a, 5.21b, 5.22a, 5.25a, 5.30a and 5.30b).
None of the grains exceeded 15µm in maximum dimensions. The morphology of
the grains varied from euhedral (Figure 5.30a) to subhedral (Figures 5.30b and
5.25a) to more irregularly shaped grains (Figures 5.20b, 5.21a, 5.21b and 5.22a).
The native Au grains may be intergrown or closely associated with one or more of
the CuFe-sulphide phase, native Bi, Fe-sulphides, bismuthinite and Pb-bearing
sulphides (Figures 5.20b, 5.21a, 5.21b and 5.22a). Other grains were associated
with siderite and limonite (Figures 5.25a and 5.30b). A single, euhedral native Au
grain was observed within a cavity in botryoidal limonite (Figure 5.30a). The
cavity was filled by nontronite. The native Au grain is approximately 5m in
maximum dimension and is compositionally zoned, with a fine rim of electrum
developed on the margins.
All of the native Au grains were too fine-grained for a quantitative SEM analysis.
Qualitative SEM analysis of the native Au grains confirms that they consist
predominantly of Au together with minor (close to detection limits, ~0.5 wt.%) to
moderate (typically ~5-15 wt.% percent) amounts of Ag being detected in a
number of grains.
The lower portion of the gossan is characterised by elevated levels of Ag.
Amalgam is by far the most common of the Ag-bearing phases. A AgSb(As)-
sulphide, possibly a member of the proustite-pyrargyrite solid solution series
(ideally Ag3AsS3–Ag3SbS3) (Figure 5.23b) occurs locally within the nontronite.
Other discrete Ag-bearing phases observed in very minor to trace amounts
include a Bi-Ag-sulphide, Ag-bearing chalcopyrite and a Ag-sulphide (possibly
acanthite) (Figures 5.26a and 5.29b). The AgSb(As)-sulphide was too fine-
grained for a positive identification by SEM analysis.
The systematic searching of the polished sections prepared from the gossan
samples failed to identify the presence of a significant number of native Au or Au-
bearing phases. The bulk of the native Au grains that were observed were
Page 113
Chapter 5 Borehole CR194 – Sample Descriptions
extremely fine-grained in nature and it envisaged that a significant proportion of
the Au in the gossan is present in the form of sub-microscopic grains.
5.3.12 Accessory Minerals
The gossan contains a wide range of accessory minerals that are present in the
samples in relatively minor amounts. The accessory minerals include zircon,
TiO2 and cassiterite that are typically present as sub-rounded grains and angular
fragments that rarely exceed 10m in size. Qualitative SEM analysis confirmed
the presence of minor amounts of vanadium within a number of the cassiterite
grains.
A discrete CuFe-sulphide phase may be intimately associated with the galena-
rich aggregates (Figures 5.14a, 5.14b and 5.15a). Chalcopyrite was confirmed
by optical examination, although other CuFe-sulphides may be present.
A small number of tiny, micrometre-sized native bismuth grains were recognised
in the gossan and typically occur within the galena-rich aggregates and the fine-
grained nontronite and siderite/limonite matrix. The native bismuth grains are
also commonly associated with a discrete Bi-sulphide phase that probably
represents the mineral bismuthinite (ideally Bi2S3) (Figure 5.21a). Other phases
identified include a discrete BiAg-sulphide and a AgFe-sulphide (possibly
sternbergite, ideally AgFe2S3).
Page 114
Chapter 5 Borehole CR194 – Sample Descriptions
5.4 Borehole CR194 – Gossan Contact with MassiveSulphide
5.4.1 Introduction
The lowest portion of the gossan occurs from 163.75 to 164.60 metres and is
lithocoded as Strong Hematitic Gossan. This final interval of gossan marks the
contact with the underlying massive sulphide zone and is extremely complex. It
is therefore described separately from the other gossan samples. In particular, it
is the last 10 to 15 centimetres of the gossan in this sample interval that exhibits
the most distinctive mineralogy and chemistry. The gossan and massive sulphide
components of the contact zone are illustrated in Figure 5.31. Figure 5.31a
shows an approximately 1:1 scale digitised image of a section of core taken from
the lower 10 centimetres of gossan from the 163.75 to 164.60 metre sample
interval.
The 163.75 to 164.60 metres sample interval contains significant amounts of Fe
(58.87%) together with moderate amounts of Pb (7.31%), minor S (0.88%) and
traces of Cu (0.04%). This sample is characterised by the presence of significant
amounts of Ag (1114.4ppm) and Au (14.42ppm). This sample also contains
significant amounts of As (4550ppm), Sb (5135ppm), Bi (1629ppm), Sn (626ppm)
and Hg (645.9ppm). X-ray powder diffraction analysis confirms the presence of
quartz, siderite, hematite, goethite, anglesite and galena.
Macroscopic examination of the 163.75 to 164.60 metre interval confirms that it
consists of three distinct zones, exhibiting a different bulk and trace mineralogy.
These are described separately as 'upper', 'middle' and 'lower' in the following
section of the thesis.
5.4.2 163.75 to 164.60m Sample Interval - Upper Portion
This portion of the core is essentially similar to the previous gossan samples and
consists of extensively oxidised siderite and fine-grained nontronite. The limonite
(oxidised siderite) and nontronite are intimately associated and typically occur as
fine-grained and porous aggregates. The limonite may occur as delicately
banded, botryoidal aggregates.
Page 115
Chapter 5 Borehole CR194 – Sample Descriptions
Galena is abundant and occurs as finely disseminated, micrometre-sized grains,
skeletal crystals and as fine, granular aggregates. The galena may also exhibit
some degree of oxidation with the development of cerussite rims. Pb(AsSb)-
bearing sulphides are also typically finely intergrown with the galena.
Accessory phases include quartz, cassiterite, TiO2 and barite. A small number of
AuAgHg grains were also identified. These grains are typically <2m in size and
are intimately intergrown with galena, a Pb-arsenate and a CuSbAg-sulphide
(possibly Ag-bearing tetrahedrite, ideally Cu12Sb4S13).
5.4.3 163.75 to 164.60m Sample Interval - Middle Portion
The middle portion of this sample interval exhibits a distinctive yellow colour and
no fragmented gossan textures survive. X-ray powder diffraction analysis
confirms the presence of goethite (ideally -Fe3+O(OH)) and hematite. The
goethite and hematite are fine-grained and porous in nature and are intimately
associated with siderite. Galena is also abundant and typically occurs as finely
disseminated grains, and as fine, granular aggregates that form discrete, narrow
veinlets. Qualitative SEM analysis of a number of the galena grains confirmed
the presence of Pb and S together with minor amounts of Ag.
Detailed examination of the fine-grained galena-rich aggregates also revealed the
presence of a number of discrete, Au, Hg, Ag, Bi and S-bearing phases. These
phases are typically <5m in size and include a AuHgAg alloy (Au-bearing
amalgam), Ag-sulphide, amalgam, cinnabar and native bismuth (Figures 5.32a,
b, c and d). Pyromorphite is also present locally within these aggregates. Minor
amounts of a secondary Cu-sulphide (possibly covellite) were also recognised
within a siderite veinlet. Quantitative SEM analyses of the Au-bearing amalgam
are provided in Appendix 5 (Au-amalgam analyses #1 to #4) . The Au-bearing
amalgam grains exhibit some degree of compositional variation and typically
contain minor amounts of Fe (2.0-2.8%) that probably reflects the presence of
associated limonite. The Au content is low and somewhat variable (5.4–11.6%)
and appears to reflect similar variations in the Hg content (38.0–42.6%). The Ag
content is relatively consistent (48.5–50.7%).
Page 116
Chapter 5 Borehole CR194 – Sample Descriptions
5.4.4 163.75 to 164.60m Sample Interval - Lower Portion
This portion of the sample interval is complex and exhibits a marked change in
the bulk and trace mineralogy and chemistry relative to the overlying gossan
samples. This sample represents the last ~10 centimetres of core prior to the
contact with the underlying massive sulphide. Figure 5.31a illustrates a typical
section through this portion of the core. The core exhibits a distinctive reddish
brown colour that reflects the presence of siderite and limonite. Numerous black
bands of nontronite also traverse the core. Within the clay and siderite-rich
areas, millimetre-sized amalgam grains are evident in hand specimen.
Extensively oxidised siderite is dominant (Figure 5.33a). Less extensively
oxidised siderite often exhibits replacement by hematite along grain boundaries
and fractures (Figures 5.33a and 5.35a). The siderite is notably fine-grained, with
large fragments of coarsely crystalline siderite, similar to those observed in the
overlying gossan samples, being essentially absent.
The oxidised and porous siderite/limonite aggregates are traversed by unoxidised
siderite veinlets that represent a later stage of mineralisation. The unoxidised
siderite veinlets significantly reduce the porosity of the sample locally (Figure
5.33b). These siderite veinlets may contain micrometre-sized euhedral crystals
of galena (Figure 5.34b) and less commonly, granular aggregates of anglesite
(Figure 5.33b). Radiating crystals of Fe-sulphide may also occur within the
siderite (Figure 5.34a). The siderite veinlets may be fractured, particularly within
the clay-rich areas, probably as a result of the dehydration and shrinkage
associated with the nontronite (Figure 5.34b). Quantitative SEM analysis of the
late-stage siderite confirms that it consists predominantly of Fe, C and O with Ca
and Mg typically being below detection limits for this technique (see Appendix 5).
Nontronite is particularly abundant in the final few centimetres of gossan before it
intersects with the underlying massive sulphide. The bulk of the nontronite
occurs within discrete bands within the extensively oxidised siderite (Figures
5.34b and 5.36a) and exhibits a delicately banded texture and marked
dehydration cracking (Figures 5.34b, 5.36a and 5.36b).
Page 117
Chapter 5 Borehole CR194 – Sample Descriptions
Fine-grained galena (Figures 5.34b, 5.36b and 5.37a) and pyromorphite are
finely disseminated throughout the nontronite. The pyromorphite and galena
often exhibit a preferred orientation that reflects the marked lamination within the
clay (Figures 5.34b and 5.36b). Granular aggregates of amalgam and Au-
bearing amalgam are typically present within the clay layers (Figures 5.36a,
5.36b and 5.37a).
Galena is abundant and occurs as finely disseminated grains (Figures 5.33a and
5.36b), as discrete euhedral crystals in siderite veinlets (Figure 5.34b) and as
complex, fine-grained and often porous aggregates (Figures 5.33b, 5.35b, and
5.37a). The galena aggregates commonly occur along the margins of the
relatively unoxidised siderite linings of former cavities that have been
subsequently filled by the siderite.
Pyromorphite and galena are particularly abundant within the clay (Figures 5.34b
and 5.36b). Pyromorphite is a mineral of secondary origin that forms as a result
of the interaction between phosphoric acid and galena or cerussite. Galena
cores are commonly present in the larger pyromorphite aggregates (Figure
5.35a) and it is presumed that at least part of the pyromorphite has formed as a
result of the oxidation of galena. Pyromorphite forms a solid solution series with
mimetite.
Qualitative SEM analysis of the galena confirms that it consists predominantly of
Pb and S but also consistently contains minor amounts of Sb. Detailed
examination of the galena aggregates revealed the presence of a discrete PbSb-
sulphide (Figure 5.35b). However, due to the extensive replacement of the
PbSb-sulphide by galena, it was not possible to obtain a quantitative analysis of
this mineral for identification. Optical examination of this mineral confirms that it
is strongly anisotropic and exhibits grey-green colours in reflected light,
properties exhibited by a number of sulphosalt minerals, further inhibiting a
positive identification.
Aggregates of amalgam are present within the fine-grained, extensively oxidised
limonite/siderite and also within discrete clay layers. The amalgam grains exhibit
Page 118
Chapter 5 Borehole CR194 – Sample Descriptions
a wide range in grain size with discrete grains commonly exceeding 1mm. The
amalgam grains typically exhibit a highly irregular morphology (Figures 5.36a,
5.36b and 5.37b), with rare subhedral to euhedral crystals also being observed
(Figure 5.37a).
A narrow zone of high porosity is also commonly developed along the margins of
the amalgam possibly indicative of some degree of dissolution (Figures 5.36b
and 5.37a). The amalgam aggregates commonly exhibit complex textural
relationships with one or more of Ag-sulphide and cinnabar. These phases are
typically developed along the margins of the amalgam grains and may represent
alteration products. Quantitative SEM analyses of the amalgam are provided in
Appendix 5.
The amalgam analyses are relatively consistent. Analyses #4 to #6 (52.9 -
54.6% Ag; 45.1 - 46.7% Hg) were performed on amalgam grains within a veinlet
several millimetres above the amalgam grains represented by analyses #7 to #10
(58.1 - 59.9% Ag; 39.7 - 42.9% Hg). The variation in Ag and Hg is therefore
relatively consistent within discrete horizons but suggests that some degree of
variation may occur within the sample as a whole. It is interesting to note that the
composition of the amalgam within the upper part of this sample (analyses #4 to
#6) is similar in composition to those grains described in Section 5.3 (analyses #1
to #3).
This portion of the gossan also contains Au-bearing amalgam that appears to be
confined to the clay bands that are traversed by siderite veinlets (Figure 5.38a, b,
c and d). The Au-bearing amalgam is typically present as highly irregular grains
that are rimmed by fine-grained aggregates of galena (Figures 5.38a, b, c and d).
The Au-bearing amalgam grains rarely exceed 20m in size. Au-bearing
amalgam hosts the bulk of the Au in this portion of the gossan. Quantitative SEM
analyses of the Au-bearing amalgam are provided in Appendix 5 (analyses #5 to
#11).
The composition of the Au-bearing amalgam grains is highly variable and exhibit
significantly higher Au contents (27.7-56.5%) than the amalgam grains observed
Page 119
Chapter 5 Borehole CR194 – Sample Descriptions
in the middle portion of the 163.75 to 164.60 metre sample interval (5.4-11.6 %
Au).
Accessory minerals include a AgFe-sulphide (possibly sternbergite), native
bismuth, cassiterite (Figure 5.37b) and native arsenic. The native bismuth may
be intimately associated with the amalgam with discrete grains exceeding 20m
in size (Figure 5.37b).
Cassiterite typically occurs within fine-grained aggregates along the margins of
the amalgam (Figure 5.37b). These aggregates may also contain significant
amounts of Bi, Pb, As and Fe and possibly represent fine-grained intergrowths
between several discrete Pb, As, Bi and Fe-bearing phases. Cassiterite grains
may exceed 30m in size. Qualitative SEM analysis confirmed the presence of
minor amounts of vanadium within a number of the cassiterite grains.
Fe-sulphides are present in this sample in minor amounts and typically occur as
radiating tabular crystals and granular aggregates (Figure 5.34a). The Fe-
sulphide is often intergrown with minor amounts of magnetite and sphalerite.
Qualitative SEM analysis confirmed the presence of minor amounts of Hg within
a number of the sphalerite grains. Zircon was also observed during this
investigation.
Page 120
Chapter 5 Borehole CR194 – Sample Descriptions
5.5 Borehole CR194 – Massive Sulphide Contact withGossan
5.5.1 Introduction
The massive sulphide contact zone occurs within the 164.60 to 165.80 metre
interval and is lithocoded as Massive Sulphide. This contact zone between the
sulphides and overlying gossan is extremely complex. It is therefore described
separately to the other massive sulphide samples. The first 10 to 15 centimetres
of the 164.60 to 165.80 metres sample interval exhibits the most complex and
distinctive mineralogy and chemistry.
The massive sulphide contact with the gossan is illustrated in Figure 5.31b.
Figure 5.31b shows an approximately 1:1 scale digitised image of a section of
core taken from the upper 10-15 centimetres of massive sulphide from the 164.60
to 165.80 metres sample interval.
The 164.60 to 165.80 metres sample interval contains significant amounts of Fe
(37.74%) and S (45.89%) together with moderate amounts of Pb (5.75%) and Cu
(7.42%). This portion of core is characterised by the presence of significant
amounts of Ag (546.4ppm) and Au (5.43ppm) and also contains significant
amounts of As (4892ppm), Sb (927ppm), Bi (517ppm), Sn (203ppm) and Hg
(69.4ppm). X-ray powder diffraction analysis confirms the presence of quartz,
siderite, hematite, goethite, pyrite, tennantite, melanterite, chalcopyrite, anglesite
and galena.
The upper 10-15cm of this sample interval consists of a narrow (~1cm) layer of
nontronite and finely disseminated galena (Figures 5.31b and 5.39a). This
extends into a more galena-rich layer of approximately 2cm in depth that exhibits
some delicate banding (Figures 5.31b and 5.39a). Within this layer, euhedral
crystals and angular fragments of quartz are often present (Figures 5.39a and
5.39b).
Below the galena-rich layer is an extensively leached pyrite zone (Figure 5.31b).
Galena and siderite are also common in the leached pyrite zone (Figures 5.47a
and 5.47b). Due to their complexity, the upper 10-15cm of core was examined in
Page 121
Chapter 5 Borehole CR194 – Sample Descriptions
greatest detail. The mineralogy of the 164.60 to 165.80 metres sample interval is
described in four parts, describing the mineralogy of the clay-rich layer, galena-
rich layer and leached pyrite-rich layer. The remaining (~1m) of core consists
predominantly of pyrite and is essentially similar to the underlying massive
sulphide. This portion of core is therefore discussed briefly as Lower Core in
Section 5.5.5.
5.5.2 Clay-Rich Layer
This layer is essentially similar in appearance to the clay-rich layers described in
the lower portion of the previous sample interval. It consists predominantly of
poorly crystalline nontronite together with subordinate amounts of fine-grained
galena and pyromorphite (Figure 5.39a). The nontronite exhibits a marked
lamination and is traversed by numerous fractures that probably represent
dehydration cracks. Sb-bearing galena is finely disseminated throughout the clay
and may exhibit a preferred orientation that is developed parallel to the lamination
of the clay. Narrow, galena-rich veinlets are also present within this layer and
appear to traverse the clay-rich layer.
5.5.3 Galena-Rich Layer
This highly complex layer consists of fine-grained, delicately banded and
deformed aggregates that consist predominantly of one or more of galena,
amalgam, nontronite, chalcopyrite, tennantite, tetrahedrite, PbSb-sulphide, quartz
and siderite (Figures 5.39a, 5.39b, 5.41a, 5.41b, 5.44a and 5.44b).
Quartz is common and occurs as angular fragments and euhedral crystals
(Figure 5.39a). The smaller fragments typically exhibit highly irregular margins,
often with concave faces, indicative of dissolution. The overall grain size of the
quartz fragments is finer than that observed in the gossan, typically ranging
between a few micrometres and a few hundred micrometres in size.
Examination of the quartz in thin section confirms that it exhibit a variety of
textures and crystallite sizes. The polycrystalline quartz aggregates are typically
fine to medium grained. Some fibrous quartz is also present. A small number of
narrow quartz veinlets also traverse the sample. These veinlets consist of
Page 122
Chapter 5 Borehole CR194 – Sample Descriptions
extremely fine-grained quartz crystallites that probably represent partially
recrystallised chalcedony.
Siderite is common and becomes increasingly abundant towards the contact with
the leached pyrite zone (Figure 5.39b). The increase in abundance is marked by
a similar decrease in the presence of quartz. The siderite fills former cavities that
are lined by euhedral sulphosalt minerals (Figures 5.39b, 5.40a, 5.40b, 5.43a and
5.43b).
Quantitative SEM analysis of the late-stage siderite confirms that it consists
predominantly of Fe, C and O with Ca and Mg typically being below detection
limits for this technique (Appendix 5). Minor amounts of nontronite are also
typically present within the galena-rich matrix.
Galena occurs as fine-grained, granular aggregates that are intimately associated
with one or more of amalgam, Au-bearing amalgam, nontronite, TiO2, cassiterite,
native bismuth, chalcopyrite, tennantite, tetrahedrite, Cu-arsenides, PbSb-
sulphide, quartz, native arsenic and siderite (Figures 5.39a, 5.39b, 5.40b and
5.43b). The galena also forms rims on other minerals (Figures 5.40b) including
tetrahedrite–tennantite and chalcopyrite, reflecting partial and extensive
replacement relationships. Qualitative SEM analysis also revealed the presence
of minor amounts of Se that are occasionally present in the galena. This
probably represents the solid solution between galena and the Pb-selenide
clausthalite (ideally PbSe). XRD confirms the localised oxidation of galena to
anglesite.
Tetrahedrite and tennantite are common accessory phases and are particularly
abundant within this galena-rich layer. The tennantite (As-rich end member) is
confined largely to the upper portion of the galena-rich layer, adjacent to the Fe-
rich clay. Tetrahedrite (Sb-rich end member) appears to be more abundant
within the lower portion of the galena layer. Tetrahedrite and tennantite may also
occur within the fine-grained, galena-rich matrix (Figure 5.41b). The tennantite
and tetrahedrite may be intimately intergrown with chalcopyrite, galena and
amalgam (Figures 5.40a, 5.40b and 5.41b).
Page 123
Chapter 5 Borehole CR194 – Sample Descriptions
The tennantite and tetrahedrite also occurs along the margins of former cavities
that are subsequently filled by siderite (Figures 5.39a, 5.40a and 5.40b). The
tennantite and tetrahedrite may exhibit euhedral morphologies, indicative of
growth into open pores (Figures 5.40a and 5.40b). Galena and amalgam exhibit
replacement relationships with the tetrahedrite and tennantite (Figures 5.40a and
5.40b).
Quantitative analyses of the tennantite and tetrahedrite grains are provided in
Appendix 5. The analyses confirm that the tennantite and tetrahedrite are close
to the theoretical end member compositions. The tennantite exhibits a small
range in Cu (40.2 - 45.8%), Fe (5.8 - 7.9%), Sb (0.0 - 1.4%), As (20.6 - 22.2%)
and S (27.8 - 29.8%) contents, with Zn being below detection limits (~0.5%). The
tetrahedrite exhibits a small range in Cu (35.7 - 36.8%), Fe (4.8 - 5.8%), Zn (1.5 -
2.5%), Sb (29.0 - 30.5%), As (0.4 - 1.6%) and S (25.2 - 26.0%) contents.
The galena-rich layer is characterised by the presence of a number of discrete
Cu-rich arsenides. The Cu-arsenides occur as narrow veinlets (Figure 5.42a)
and as radiating tabular or prismatic crystals that are present within siderite-filled
cavities (Figures 5.43a and 5.43b). The euhedral crystals probably reflect the
pseudomorphous replacement of arsenopyrite (pers. comms. Rob Ixer). Galena
and amalgam exhibit replacement relationships with the Cu-arsenide crystals.
Three discrete Cu-arsenides are evident. Quantitative analyses are provided in
Appendix 5. One of the Cu-arsenides (Cu 59.1 - 59.7%; As 34.9 - 36.7%
analyses #1 to #3) occurs as narrow veinlets (Figure 5.42a, lower of the two
veinlets). The veinlet commonly contains cores of amalgam. The amalgam cores
exhibit highly irregular and corroded morphologies. The amalgam may represent
a relict phase that has been largely replaced by the Cu-arsenide. The Cu-
arsenide phase exhibits a moderate reflectivity in reflected light and appears
steel-grey in colour. Minor amounts of Ag (4.3 - 4.5%) are also typically present.
The semi-quantitative analyses and optical properties are consistent with that of
the mineral novakite (ideally (Cu,Ag)21As10).
Page 124
Chapter 5 Borehole CR194 – Sample Descriptions
The novakite may also be intimately associated with a Cu-arsenide phase that
exhibits a distinctive crimson red or cerise colour in reflected light (Figure 5.42b).
This phase is present in relatively minor amounts and typically occurs along the
margins of amalgam grains. These grains rarely exceed a few micrometres in
size. The SEM analytical totals (analyses #4 to #6) are relatively low due to the
very fine grain size of the Cu-arsenide grains. The results are, however,
consistent and suggest that this phase may represent a Ag-poor variety of the
mineral novakite (Cu 59.4 - 60.1%; As 35.0 - 37.8%). The distinctive crimson
colour of this phase is not consistent with that of novakite, however, rapid
tarnishing and the iridescence sometimes observed in novakite may, at least in
part, explain the anomalous colours.
The third Cu-arsenide phase also commonly occurs as veinlets that appear to
partially replace the amalgam (Figure 5.42a, upper of the two veinlets). This Cu-
arsenide phase is distinguished from the previous Cu-arsenides by its bluish grey
colour in reflected light. The quantitative SEM analyses and the appearance of
this phase in reflected light suggest that it represents the mineral koutekite
(ideally Cu5As2) (Cu 64.9%; As 33.3 - 33.4%). Novakite and koutekite are
relatively common accessory minerals within this sample interval and may also
occur within the fine-grained galena and amalgam-rich matrix.
Chalcopyrite is abundant in the galena-rich layer and typically occurs as granular
aggregates that are complexly intergrown with the fine-grained galena and
amalgam (Figures 5.41a and 5.41b). Chalcopyrite may occur within the
tetrahedrite-tennantite-rich aggregates, particularly within the siderite-filled
cavities (Figure 5.40a). The chalcopyrite aggregates typically exhibit highly
irregular morphologies and may be rimmed and replaced by galena (Figure
5.41a). Amalgam is also often intimately associated with the chalcopyrite (Figure
5.44b). Discrete chalcopyrite aggregates may exceed 150µm in size (Figure
5.41a).
Amalgam is abundant and hosts the bulk of the Ag and Hg content of this
sample. The amalgam grains exhibit a wide range in grain size with discrete
grains commonly exceeding several hundred micrometres (Figures 5.39b, 5.44a,
Page 125
Chapter 5 Borehole CR194 – Sample Descriptions
5.44b and 5.45a). The amalgam grains commonly exhibit highly irregular grain
boundaries, possibly indicative of some degree of dissolution (Figures 5.44b and
5.45a). Some of the amalgam aggregates exhibit more rounded morphologies
(Figure 5.44a). The fine-grained galena-rich matrix typically exhibits some
degree of deformation around the margins of the larger amalgam grains (Figures
5.39b and 5.44a).
The larger amalgam aggregates are typically elongated and exhibit a preferred
orientation parallel to the lamination observed in hand specimen. The larger,
elongated amalgam aggregates also commonly occur within specific horizons
within the galena-rich zone and may have originally occurred in the form of
narrow veinlets that have been fragmented during dissolution or replacement
(Figures 5.39b, 5.44a and 5.45a). Finely disseminated amalgam also typically
exhibits highly irregular morphologies that may represent the remnants of larger
grains that have been partially replaced by other phases, notably galena (Figures
5.42b and 5.44b). Quantitative SEM analyses of the amalgam are provided in
Appendix 5 (analyses #4 to #10). The amalgam exhibits minor variations in Ag
(52.9 - 59.9%) and Hg (39.7 - 46.7%) contents.
A small number of Au-bearing amalgam grains were also observed during the
detailed examination of this sample (Figures 5.46a, b, c and d). The Au-bearing
amalgam is also typically present as highly irregular (Figures 5.46a, b and c) and
subhedral grains (Figures 5.46b and d). Quantitative SEM analyses of the Au
amalgam are provided in Appendix 5 (analyses #12 to #18). The analyses of the
amalgam are relatively consistent and exhibit only minor variations in Ag (45.7 -
48.1%), Au (16.4 - 21.0%) and Hg (34.1 - 36.8%) contents.
Examination of this sample at relatively low power magnification confirms that the
Au-bearing amalgam is present within a narrow (~1mm wide) band that also
contains significant amounts of angular quartz fragments. This quartz-rich band
is evident in Figure 5.39a, although the Au-bearing amalgam grains are not
resolved in this illustration. Examination of the fine-grained galena-rich matrix
both above and below this quartz-rich band failed to reveal the presence of any
other Au-bearing grains.
Page 126
Chapter 5 Borehole CR194 – Sample Descriptions
A single, relatively large Au-bearing amalgam grain was also observed (Figure
5.46d). This Au-bearing grain was the largest observed in this suite of samples
and exceeded 100m in length. This amalgam grain is also compositionally
zoned and exhibits a wide range in compositional variation that largely reflects
the Au-rich nature towards the margins of the grain.
Quantitative SEM analyses were performed on the compositionally zoned grain to
illustrate the degree of variability (Appendix 5, analyses #19 to #24). The core of
the compositionally zoned grain is typically Au-poor (8.3 - 11.3%) and Ag-rich
(49.9 - 50.7%). Conversely, the margin of the grain is typically Au-rich (15.4 -
31.9%) and Ag-poor (31.6 - 41.9%). This suggests that the margins of the grain
may have been subjected to some degree of leaching of the Ag or that the Au-
rich amalgam was precipitated at a later date.
Cassiterite is a common accessory phase that typically occurs as small, angular
fragments that rarely exceed 30m in size (Figure 5.45b). Zircon is also present
in very minor amounts and also typically occurs as small, angular fragments
within the fine-grained, galena-rich matrix. Quartz, cassiterite and zircon are
extremely resilient to chemical and physical weathering and it is therefore
possible that these represent resistate phases that have originated from
elsewhere in the orebody. A discrete PbSb-sulphide and PbSbCl-oxide phase
(probably nadorite, ideally PbSbO2Cl) were also observed in very minor amounts.
Native bismuth is also moderately common and occurs within the fine-grained
galena-rich matrix, often associated with amalgam.
Native arsenic may be intimately associated with amalgam (Figure 5.43a). This
often forms rims on the amalgam grains and may also occur within the fine-
grained galena-rich matrix. It appears to have been subjected to some degree of
oxidation to form a discrete As-oxide phase (possibly arsenolite or claudetite,
both ideally As2O3). In addition, the fine-grained matrix may also contain minor
amounts of a discrete Pb(Fe)-arsenate phase. This phase is fine-grained and
porous in nature and could therefore not be positively identified. The Pb-rich
Page 127
Chapter 5 Borehole CR194 – Sample Descriptions
arsenate becomes increasingly abundant towards the lower part of the galena-
rich layer and may contain fine-grained Ag-sulphide (Figure 5.45b).
5.5.4 Leached Pyrite-Rich Layer
Directly below the galena-rich layer is a layer of extensively leached pyrite
(Figures 5.47a, 5.47b, 5.48a and 5.48b) that extends for a depth of a few
centimetres, after which the degree of leaching is less prominent. The underlying
massive sulphide typically exhibits a low degree of porosity and the pyrite is
granular in nature (Figure 5.51).
The pyrite in the leached layer was probably subjected to oxidation. The
resultant goethite and/or jarosite oxidation products that would normally be
formed during this process have, however, been completely leached resulting in
a highly porous, pyrite-rich aggregate.
The extensive leaching of the pyrite was superseded by the precipitation of
galena, which lines the margins of the bulk of the pyrite grains (Figures 5.47a,
5.47b, 5.48a and 5.48b). The galena also appears to partially replace the pyrite,
evident by the progressive penetration of galena along narrow fractures in many
of the pyrite aggregates (Figure 5.47b). Detailed examination of the galena rims
confirms that they consist of tiny, euhedral galena crystals and botryoidal
aggregates.
The bulk of the porosity created during the leaching of the pyrite has been filled
by siderite (Figures 5.47a and 5.47b). The siderite may exhibit euhedral
morphologies, indicative of growth in open space (Figure 5.48b). Euhedral
crystals of pyromorphite (Figure 5.48b) are also a common feature. Discrete
pyromorphite crystals may exceed 100m in size.
Minor amounts of tetrahedrite (Figures 5.48a and 5.48b), chalcopyrite, Ag-
sulphide, Ag(As,Sb)-sulphide (probably proustite-pyrargyrite), a PbBa-sulphate
and a fine-grained and porous Pb-arsenate (Figure 5.48b) are also typically
developed within the open pore spaces and appear to post-date the deposition of
Page 128
Chapter 5 Borehole CR194 – Sample Descriptions
the fine galena rims. The Pb-arsenate is extremely porous and variable in nature
and may be intimately intergrown with minor amounts of nontronite.
Amalgam is notably absent from this layer. Minor amounts of native As were
observed. Chalcopyrite (Figures 5.48a and 5.47a) replaces pyrite along
crystallographic planes and probably forms a component of the secondary Cu-
sulphide mineralogy.
5.5.5 Lower Core
This portion of the core consists of extensively fractured pyrite together with
subordinate amounts of a fine-grained, Al and Si-rich clay (probably kaolinite,
ideally Al2Si2O5(OH)4) (Figures 5.49a, 5.49b and 5.50a), chalcopyrite (Figure
5.49b) and enargite (Figures 5.49a and 5.49b). The kaolinite typically occurs
along grain boundaries and fractures developed within the pyrite (Figures 5.49a,
5.49b and 5.50a).
The kaolinite clay may be intimately associated with chalcopyrite, enargite
(Figures 5.49a and 5.49b) and an unidentified Ba-Al-silicate (Figure 5.50a).
Enargite typically forms discrete narrow veinlets within the fractured pyrite grains
(Figure 5.49a). Quantitative SEM analyses of the enargite are provided in
Appendix 5 (analyses #1 to #3) and confirm that the enargite is close to the
theoretical end member composition, consisting predominantly of Cu (46.1 -
46.5%), As (18.5 - 19.9%) and S (32.8 - 33.7%) and only minor Fe (0.9 - 1.3%).
The Sb content (0.2 - 0.4%) is below detection limits for this technique. Minor
amounts of supergene Cu-sulphide (Figure 5.50b), compositionally zoned
tetrahedrite and Se-bearing galena are also present locally within the pyrite-rich
core.
Page 129
Chapter 5 Borehole CR194 – Sample Descriptions
5.6 Borehole CR194 – Massive Sulphide
5.6.1 Introduction
The sample intervals from 165.80 to 178.50 metres consist predominantly of
massive sulphide ore. The massive sulphide ore is intersected by a shale-rich
horizon that is characterised by its highly friable, quartz-rich nature and a marked
increase in the precious metal content. This shale-rich interval occurs at a depth
of between 172.50 and 174.50 metres. The 165.80 to 178.50 metre sample
intervals are described as two discrete components entitled ‘massive sulphide’
and ‘massive sulphide/shale’.
5.6.2 Massive Sulphide
The 165.80 to 172.50 metres and 174.50 to 178.50 metres sample intervals are
essentially similar in bulk mineralogy and are logged by the field geologists as
‘massive sulphide’. Pyrite is the dominant mineral and typically occurs as
granular aggregates that have been subjected to a significant degree of fracturing
(Figures 5.51a, 5.51b, 5.52b and 5.53b).
Detailed SEM examination of the pyrite confirms that it exhibits complex
compositional zoning that reflects minor variations the presence of As in solid
solution in the pyrite crystal structure (Figures 5.51a and 5.51b). XRD analysis
confirms the presence of pyrite and djurleite (ideally Cu1.9S). Supergene Cu-
sulphides are common in the massive sulphide and may consist of several
discrete phases, including djurleite, chalcocite and covellite, although only
djurleite was confirmed by XRD analysis. These phases are therefore broadly
described as ‘Cu-sulphides’, unless optical properties, XRD or chemical
composition indicate otherwise.
Supergene Cu-sulphide occurs along grain boundaries and in fractures within the
pyrite (Figures 5.50b). The Cu-sulphide often exhibits a prominent cleavage and
may contain abundant inclusions of bornite (Figure 5.52a). The Cu-sulphide may
occur as discrete euhedral crystals locally, indicative of growth in open space.
Galena is abundant and occurs as fine-grained aggregates and narrow veinlets
that may line the margins of small cavities or be complexly and intimately
Page 130
Chapter 5 Borehole CR194 – Sample Descriptions
intergrown with the Cu-sulphide and other Cu-bearing sulphide minerals (Figures
5.52a, 5.52b and 5.53a). Minor amounts of a BaAl-silicate (Figure 5.52b) occur
along the grain boundaries of the granular pyrite aggregates and in fractures.
Insufficient material was available for identification by XRD.
Chalcopyrite (Figure 5.53b), enargite (Figure 5.54a) and members of the
tetrahedrite–tennantite solid solution series (Figures 5.54a, 5.54b, 5.55a and
5.55b) are also present in subordinate amounts along the pyrite grain boundaries
and in fractures. Relatively simple supergene enrichment textures are evident in
the chalcopyrite and bornite associations observed in the massive sulphide
sample (Figures 5.53a and 5.53b). These associations typically consist of relict
chalcopyrite that has been partially replaced by bornite, Cu-sulphide and galena.
The chalcopyrite probably represents a component of the primary Cu-sulphide
mineralisation.
The associations observed between Cu-sulphide, enargite and tetrahedrite are
complex and represent the partial replacement of tetrahedrite/tennantite and
enargite by later stages of supergene Cu-sulphide. Figure 5.54a, 5.54b and 5.55b
illustrate the presence of relict enargite crystals that are fractured and partly
replaced by tetrahedrite. The tetrahedrite is subsequently replaced by later
stages of Cu-sulphide mineralisation. The tetrahedrite/tennantite associated with
the enargite contains variable levels of Hg.
The tetrahedrite-tennantite also exhibits complex compositional zoning that
largely reflects variations in the As-Sb ratios (Figure 5.55a). The Hg content of
the tetrahedrite and tennantite may also exhibit some degree of variation.
Quantitative SEM analyses were performed on the Hg-bearing tetrahedrite-
tennantite grains and the results are provided in Appendix 5 (analyses #1 to #5).
The analyses confirm that the tetrahedrite-tennantite exhibits a wide variation in
composition, with the bulk of the analyses consisting of intermediate members of
the solid solution series. The Hg content (5.5 to 12.8%) is variable and
associated with equally variable levels of Cu (34.5 - 41.4%), Zn (1.8 - 4.0%), Sb
(9.2 - 22.2%) and As (3.2 - 12.8%). The S content (23.8 - 25.8%) is relatively
consistent and the Fe content is low (0.8 - 1.5%).
Page 131
Chapter 5 Borehole CR194 – Sample Descriptions
5.6.3 Massive Sulphide/Shale
The massive sulphide/shale sample intervals occur at a depth of between 172.50
and 174.50 metres. This highly porous and friable layer consists predominantly
of quartz-rich rock fragments and massive sulphide fragments in a fine-grained,
quartz- and pyrite-rich matrix (Figure 5.56a and 5.56b). The relative proportions
of sulphide and transparent gangue vary considerably over relatively short
distances. XRD analysis confirms the presence of pyrite, djurleite, quartz and
galena.
The quartz in the massive sulphide/shale sample intervals consists of millimetre-
sized fragments in a matrix of micrometre-sized grains and every range of grain
size in between these extremes (Figure 5.56a). The quartz-rich rock fragments
are often porous and the quartz typically exhibits a fabric or preferred orientation
that is typical of the leached shale fragments observed in the gossan samples.
The shale-like rock fragments in the massive sulphide/shale sample intervals also
appear to have been leached of the phyllosilicate minerals that were presumably
present as a component of the original shale. Minor amounts of TiO2 (Figure
5.56b) and carbon are also present in the shale fragments and also in the fine-
grained quartz-rich matrix.
More compact quartz fragments are also present in these sample intervals,
possibly representing fragments of vein quartz (Figure 5.56a). The quartz grains
within the matrix are typically fine-grained in nature and exhibit highly irregular
morphologies (Figures 5.57a, 5.57b and 5.59a) possibly indicating dissolution.
Fibrous quartz is also developed in places along the margins of the pyrite and
within fractures.
Pyrite is the dominant sulphide mineral and the textures are similar to those
described for the massive sulphide sample interval. Pyrite is present as granular
aggregates (Figure 5.56a) and as euhedral crystals within the fine-grained matrix
(Figures 5.56b and 5.57b). The pyrite crystals do not exhibit signs of reworking,
fracturing, alteration or rounding. Locally, a later-stage, porous pyrite overgrowth
Page 132
Chapter 5 Borehole CR194 – Sample Descriptions
is present on the earlier formed pyrite (Figure 5.58a). The pyrite exhibits
collomorphic textures and contains detectable (>0.5wt.%) Cu and Co.
The euhedral pyrite crystals and granular aggregates may exhibit complex
compositional zoning that reflects the presence of variable amounts of As (Figure
5.58b). Quantitative SEM analyses were performed on the pyrite (Appendix 5)
and confirm that the As content may exceed 2 per cent within discrete zones.
Complex enargite and tetrahedrite aggregates are a common feature and are
often replaced by Cu-sulphide (Figures 5.57a and 5.58b). Very minor amounts of
native Bi were also observed within a number of the tetrahedrite. The tetrahedrite
is Hg-rich and a single quantitative SEM analyses is provided in Appendix 5
(analyses #6). The analysis confirms that it is close to the theoretical end
member composition for tetrahedrite. The tetrahedrite contains extremely high
levels of Hg (21.7%).
Enargite is extensively fractured and replaced by the tetrahedrite, reflecting a
relative enrichment of Sb in the supergene ore. Quantitative SEM analyses are
provided in Appendix 5 (analyses #4 to #7) and confirm that the enargite is close
to the theoretical end member composition, consisting predominantly of Cu (47.5
- 50.0%), As (17.7 - 20.9%) and S (31.5 - 32.3%).
Djurleite is the dominant Cu-bearing phase as confirmed by XRD. The Cu-
sulphide occurs within the massive sulphide fragments (Figures 5.57a, 5.58b and
5.59b) and as euhedral crystals (pseudomorphous after arsenopyrite) and
granular aggregates within the fine-grained and porous quartz-rich matrix (Figure
5.57b). The Cu-sulphide commonly forms rims on the pyrite crystals and
aggregates (Figures 5.57b, 5.58a and 5.59a). Galena is also commonly
associated with the Cu-sulphide, occurring as fine-grained, skeletal intergrowths
and as rims (Figure 5.57b). Minor amounts of arsenopyrite (Figure 5.59a) are
also present within the fine-grained matrix, occurring as euhedral crystals that
exhibit replacement by Hg-tetrahedrite and Cu-sulphide.
Page 133
Chapter 5 Borehole CR194 – Sample Descriptions
5.7 Borehole CR194 – Shale
5.7.1 Introduction
Borehole CR194 sample intervals that are classified as ‘massive sulphide’ cease
at a depth of 178.50 metres. Below this level, the core is classified by the field
geologists as ‘massive shale’ and is characterised by decreasing levels of base
and precious metals with increasing depth. A single sample from the 178.50 to
180.00 metres sample interval was collected for examination.
5.7.2 Mineralogy
The mineralogy of this sample is relatively simple. Although this sample is
classified as massive shale, the phyllosilicate minerals that would be expected in
a shale-like rock have been extensively leached. The quartz-rich rock is highly
porous and the quartz is largely fine-grained in nature (Figure 5.60b). The quartz
also exhibits a preferred orientation that would be consistent with shale (Figure
5.60b). The preferred orientation is also evident in hand specimen. Locally the
quartz appears more robust in nature and porosity is significantly reduced. This
is particularly apparent around the margins of pyrite aggregates (Figures 5.60a
and 5.60b).
Accessory minerals observed in minor amounts include TiO2 (Figure 5.60a) and
carbon. The TiO2 and carbon are finely disseminated throughout the quartz-rich
matrix of the shale.
Pyrite occurs as framboids typically ranging between 10 µm and 50µm in size
(Figures 5.60a and 5.60b). Framboidal pyrite is a common feature of shales and
other sedimentary rocks. The framboids may be partially replaced by Cu-
sulphide (probably djurleite) and to a lesser extent galena (Figure 5.60a). Pyrite
is also present as euhedral crystals that may exceed 200µm in size (Figure
5.60b). The framboidal and euhedral pyrite crystals are typically associated with
a region of low porosity quartz (Figure 5.60). The pyrite framboids and euhedral
crystals exhibit a preferred orientation parallel to the lamination developed within
the porous, quartz-rich matrix.
Page 134
Chapter 5 Borehole CR194 – Sample Descriptions
5.8 Borehole CR194 – Summary Diagram
Section 5.3: The Gossan is characterised by a high Fe and low S content with moderate amounts of Pb. Base and precious metal content is low. Siderite, limonite and quartz are the dominant minerals together with subordinate amounts of Fe-clay, galena and Fe-sulphide. Accessory minerals include PbSbAs-sulphides, TiO2, zircon, cassiterite, apatite, Ag(Sb,As)-sulphides, barite, Ag-sulphide, cinnabar, bismuthinite, pyromorphite and amalgam. Microscopic native Au grains are rare.
Section 5.4: The Gossan/Massive Sulphide contact is characterised by a high Fe and low S content and elevated levels of Pb, Au, Sn, Ag, Bi, Hg, Sb and As. Siderite, limonite and Fe-clay are the dominant phases. Marked by an increase in the abundance of galena, pyromorphite, Ag-bearing sulphides, Au-bearing amalgam and amalgam. Accessory minerals include quartz, sternbergite, cassiterite, native Bi, Fe-sulphide, zircon, anglesite and PbSb-sulphides.
Section 5.5: The Massive Sulphide/Gossan contact is characterised by high Fe and S contents and elevated levels of Cu, Pb, Au, Sn, Ag, Bi, Hg, Sb and As. Dominated by the presence of galena, quartz, Fe-clay and siderite, with subordinate amounts of amalgam, tetrahedrite/tennantite, Cu-arsenides and chalcopyrite. Accessory minerals include Au-amalgam, cassiterite, zircon, pyromorphite and AlSi-clay. Pyrite becomes increasingly abundant with increasing depth.
Section 5.6: The Massive Sulphide contains high Fe, S and Cu contents and is dominated by pyrite and secondary Cu-sulphide with subordinate amounts of chalcopyrite, enargite, tetrahedrite, bornite, BaAl-silicate and galena.
Section 5.6.2: The Massive Sulphide/Shale is a highly porous zone that exhibits a slight decrease in the Fe and S content relative to the massive sulphide and elevated levels of Au, Ag, Pb, Sn, Bi, Hg and Sb. Pyrite and fine-grained quartz are dominant, together with subordinate amounts of Cu-sulphide, tetrahedrite, enargite and galena and minor TiO2, carbon, calcite and arsenopyrite.
Section 5.6: Massive Sulphide.
Section 5.7: The Shale exhibits a marked decrease in the precious and base metal content. Dominated by the presence of quartz and subordinate amounts of pyrite. Accessory minerals include Cu-sulphide, galena, TiO2 and carbon.
Figure 5.61 - Diagram illustrating the key mineralogical features for the 'Gossan', 'Gossan/Massive Sulphide Contact', 'Massive Sulphide/Gossan Contact', 'Massive Sulphide', 'Massive Sulphide/Shale' and 'Shale'.
Page 135
Chapter 6 Borehole CR149 - Sample Descriptions
6 BOREHOLE CR149 – SAMPLE DESCRIPTIONS
6.1 Introduction
Chapter 6 describes the chemistry and mineralogy of borehole CR194. The
sample list is provided in Appendix 2. Section 6.2 describes the major and minor
element chemistry of the borehole including their relative abundance and degree
of correlation.
The mineralogical description is grouped into four separate sections,
summarising the mineralogy of the ‘Tertiary Sand' (Section 6.3)’, ‘gossan’
(Section 6.4), ‘gossan/massive sulphide contact’ (Section 6.5) and the ‘massive
sulphide’ (Section 6.6) respectively. A summary diagram of the geochemistry
and mineralogy is provided in Section 6.7.
This borehole was selected for examination as a result of the extensive precious
metal mineralisation and the relatively central position relative to the underlying
massive sulphide and supergene copper sulphide mineralisation. The location of
borehole CR149 is illustrated and described in detail in Chapter 3. This borehole
is an inclined hole, the angle of dip being approximately 60 degrees.
The detailed mineralogical characterisation of this borehole is focussed on the
gossan samples, and in particular on the Au and/or Ag-rich intersections. The
sample intervals are extensively illustrated. These illustrations are provided in
Appendix 7. The nature of the core is highly variable (Figures 6.2 and 6.3).
In places, the core is competent in nature and exhibits high recoveries during
drilling. However, a significant proportion of the core is friable in nature and only
poor core recoveries were achieved.
The contact between the gossan and massive sulphide was not well preserved
due to the highly friable nature of the gossanous material directly above the
massive sulphide. However, marked changes in the mineralogy of the contact
zone are evident in the rubble-like material that was selected from this zone.
Page 136
Chapter 6 Borehole CR149 - Sample Descriptions
Sample selection starts in the Tertiary sand and extends into the first two
intersections of the partial massive sulphide, below which precious metal content
was significantly depleted. Borehole CR149 intersects the fossil gossan at a
depth of 170.90 metres. Tertiary sand overlies the gossan. The massive
sulphide ore is intersected at a depth of 190.00 metres.
The characterisation of borehole CR149 was based on the preparation and
examination of 105 polished sections and 5 thin sections.
Page 137
Chapter 6 Borehole CR149 - Sample Descriptions
6.1 Borehole CR149 - Chemistry
6.1.1 Introduction
Borehole CR149 exhibits a wide range in chemistry that largely reflects distinct
changes in the mineralogy of the core and marks the prominent boundary
between the gossan and massive sulphide. The minor/trace element chemistry is
also variable with some correlation between elements. The major and minor
element chemistry data are provided in Appendix 3. These were plotted on
several graphs and combined with a diagram showing the position of each
sample interval (Figure 6.1).
Page 138
Chapter 6 Borehole CR149 - Sample Descriptions
Figure 6.1 - Illustrating the chemistry variations in borehole CR149. Each sample interval is displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The sample intervals examined from borehole CR149 consist of the Tertiary sand, gossan, gossan/massive sulphide contact and massive sulphide mineralisation. The variation in chemistry with increasing depth is displayed to the right of the borehole schematic. The major, precious and deleterious element chemistry clearly exhibits a significant degree of variation that reflects an equally wide variation in the mineralogy of each sample interval. The borehole depths represent depth down hole and are therefore not equivalent to depth from surface, with CR149 being an inclined hole. TSA - Tertiary Sand, GHS - Strong Hematitic Gossan, GMS - Strong Magnetic Gossan, GEM - Moderately Leached Gossan, GLM - Moderate Limonitic Gossan, GEW - Weakly Leached Gossan, GHM - Moderate Hematitic Gossan, GLS - Strong Limonitic Gossan, GLW - Weak Limonitic Gossan, MMP - Massive Sulphide.
Page 139
Chapter 6 Borehole CR149 - Sample Descriptions
6.1.2 Geochemical Profile
The Cu content is low within the Tertiary sand and gossan exhibiting a slight
increase within the partial massive sulphide. Due to the relatively low Cu content
of the core, no correlations are particularly evident with other elements discussed
in this section.
The Pb content of this borehole is also relatively low, with a marginal increase in
the Pb content of the core occurring in the lower portion of the gossan, close to
the contact with the massive sulphide. The increase in Pb at the gossan/massive
sulphide contact is associated with a more marked increase in the Au and As
content.
The Fe content of the Tertiary sand is moderate. The Fe content of the gossan is
highly variable, but remains relatively high throughout the gossan intersections.
The contact between the Tertiary sand and the gossan contains significant Fe,
but with increasing depth the Fe content reduces markedly. The central gossan
region then exhibits a marked increase in the Fe content. The relatively high Fe
content of the Tertiary sand contact and central portion of the gossan is
associated with a similar pattern of abundance to that observed for Au, Bi, As and
Sb.
This pattern is not, however, repeated for the gossan/massive sulphide contact,
where a marked increase in the Au, Bi and As content is associated with a
decrease in the Fe content. The Fe content of the massive sulphide is high and
remains constant with increasing depth. The Fe content of the massive sulphide
exhibits a strong correlation with that of the S content.
The S content is relatively low throughput the Tertiary sand and gossan, but
slightly increases towards the base of the gossan, close to the contact with the
massive sulphide. The S content of the massive sulphide is high and exhibits a
strong correlation with the Fe content.
Page 140
Chapter 6 Borehole CR149 - Sample Descriptions
The Ag content is typically low throughout the core, but exhibit a marginal
increase at the contact between the gossan and the massive sulphide. The
increase in Ag at the gossan/massive sulphide contact is associated with a more
marked increase in the Au and As content and a slight increase in the Bi and Hg
content.
The variation in Au content down hole is highly variable, with numerous peaks
and troughs occurring throughout the gossan. The Tertiary sand is essentially
devoid of significant Au with the upper portion of the gossan containing significant
Au values.
The elevated Au content of the upper gossan region is associated with a similar
increase in the Fe, Bi, As and Sb content and very slight increases in the Ag and
Hg content. The high Au content in the upper gossan is characterised by a rapid
decrease and then significant increase in Au levels. This second 'spike' in the Au
content, which occurs in the fourth gossan sample interval (lithocoded
GLM/GEW) is somewhat spurious, and does not correlate with any other of the
elements described during this investigation.
The central gossan region is also characterised by the presence of another
significant increase in Au. This 'spike' in the Au content is associated with very
strongly correlated increases in the Fe, Bi, As and Sb content. The Au continues
to vary somewhat down hole and then increases markedly at the contact with the
massive sulphide. The increase in Au content at the gossan/sulphide contact is
associated with only relatively small increases in the Ag, Bi and Hg content and a
more marked increase in the As content. The Au content of the massive sulphide
is consistently low.
The As content of the upper gossan is moderate, but variable and exhibits a
similar trend to that observed for Fe, Pb, Bi, Sn and Sb. The As content
increases significantly in the central portion of the gossan and again towards the
gossan/sulphide contact zone, reaching a maximum slightly higher in the gossan
profile than that observed for Au. The As content of the massive sulphide
appears to increase with increasing depth.
Page 141
Chapter 6 Borehole CR149 - Sample Descriptions
The geochemical profiles for Bi and Sb are essentially similar, exhibiting a
cyclical increase then decrease in the upper gossan, peaking in the middle
portion of the gossan. This profile is also recognised in Fe, Au and As, and to a
lesser extent, Sn. The Bi and Sb contents also exhibit marginally elevated
abundance at the gossan/massive sulphide contact, which is associated with
more pronounced increases in the Au and As content and a decrease in the Fe
content. The Bi content of the massive sulphide is low. The Sb content of the
massive sulphide exhibits a pronounced peak or spike several metres below the
gossan contact that is strongly associated with a similar spike in the Sn content.
The Hg content of this borehole is consistently low, but exhibits marginal
increases in the upper portion of the gossan and at the contact between the
gossan and massive sulphide. The Sn content is somewhat more variable,
exhibiting a similar profile to that of Fe, Bi, As and Sb in the upper gossan, but
then peaking very markedly, several metres above the gossan/sulphide contact.
This marked increase in the Sn content is associated with a small, and
significantly less pronounced increase in the S, Au and As content and a
decrease in the Fe content. The Sn profile also peaks several metres below the
gossan/sulphide interface, similar to that observed for Sb.
Page 142
Chapter 6 Borehole CR149 - Sample Descriptions
6.2 Borehole CR149 - Tertiary Sand
6.2.1 Introduction
The Tertiary sand is characterised by the presence of moderate amounts of Fe
(10.95%) together with minor to trace amounts of S (0.75%), Cu (0.03%) and Pb
(0.16%). The Ag (3.6ppm) and Au (1.57ppm) contents of this sample are
relatively low. Minor to trace amounts of deleterious elements are also present
including As (113ppm), Bi (55ppm), Hg (0.3ppm), Sb (261ppm) and Sn (77ppm).
The Tertiary sand was taken at a depth of between 170.20 and 170.90 metres
close to the contact with the underlying gossan. XRD analysis confirms the
presence of quartz, plagioclase feldspar, glauconite, siderite, anatase, galena
and chlorite.
6.2.2 General Mineralogy
The Tertiary sand is dominated by the presence of rounded, millimetre-sized
aggregates of glauconite (ideally (K,Na)(Fe3+,Al,Mg)2(Si,Al)4(OH)2). The bulk of
the quartz fragments in the Tertiary sand range from between 10µm and 100µm
in size and consist predominantly of monocrystalline aggregates with fine-grained
polycrystalline aggregates occurring in lesser amounts. The crystallite size
typically ranges between 10µm and 100µm.
The quartz fragments are typically angular to sub-angular and do not exhibit the
highly irregular morphologies often seen in the gossans. No fibrous or
deformation textures were observed. The quartz is largely free from inclusions
and intergrowths and presumably forms a component of the original sediment.
Angular fragments of plagioclase and K-feldspar are also present (Figure 6.4b).
Siderite is common and typically occurs as euhedral crystals that exhibit
compositional zoning (Figure 6.4a). The compositional zoning reflects variations
in the Fe, Ca and Mg content. Quantitative SEM analyses are provided in
Appendix 5. The cores are typically FeO-poor (43.0-45.5%) and CaO and MgO-
rich (5.3-7.6% and 4.9-6.2% respectively) relative to the rims that are more FeO-
rich (49.7-51.1%) and CaO and MgO poor (3.3-3.6% and 2.7-3.4% respectively).
Page 143
Chapter 6 Borehole CR149 - Sample Descriptions
The euhedral nature of the siderite suggests that it has grown in situ and does
not represent a component of the original sediment.
Subordinate amounts of fine-grained glauconite and chlorite are present along
the margins of the glauconite spheroids. This fine-grained glauconite and chlorite
matrix also hosts a number of accessory minerals including galena, pyrite,
sphalerite, ilmenite and chromite. The pyrite typically occurs as fine-grained
framboidal aggregates (Figure 6.4a). Qualitative SEM analysis of the ilmenite
confirms that it also contains minor amounts of Mn. The ilmenite may exhibit
some degree of replacement by sphene. No discrete precious metal-bearing
grains were observed in this sample.
Page 144
Chapter 6 Borehole CR149 - Sample Descriptions
6.3 Borehole CR149 - Gossan
6.3.1 Introduction
The gossan occurs at a depth of between 170.90 and 190.00 metres and is
highly variable in both its nature (Figures 6.2 and 6.3) and composition. The
188.90-190.00 metre interval marks the contact between the overlying gossan
and massive sulphide and exhibits a markedly different mineralogy compared to
the bulk of the gossan and is therefore described separately in Section 6.5.
The gossan is characterised by the presence of highly variable amounts of Fe
(4.34–44.15%), S (0.57–8.87%), Cu (0.01–0.24%) and Pb (0.31–3.70%). The
sample intervals also contain significant but highly variable amounts of Ag (3.3–
71.7ppm) and Au (0.67–48.54ppm) and an abundance of minor elements
including As (63–4892ppm), Bi (178–3497ppm), Hg (0.6–14.5ppm), Sb (1112–
7556ppm) and Sn (68–7326ppm). The great bulk of the gossan consists of
extensively fragmented quartz-rich rock fragments that have been partially and/or
extensively replaced by reddish-brown siderite that is clearly observed in hand
specimen (Figures 6.2a and 6.3c). XRD analysis confirms the presence of
quartz, siderite, hematite, goethite, anglesite, rutile, anatase, lepidocrocite, native
sulphur, greigite, pyrite, marcasite, cassiterite and calcite.
Core recovery is particularly poor in many parts of the gossan, largely due to the
friable nature of much of the material. This is due partly to the extensive
oxidation of the siderite (Figure 6.3b), which results in an increase in porosity and
less competent core. The oxidation of the siderite to form limonite also results in
characteristic colour changes in the core (Figure 6.3b). A subordinate, but
significant portion of the gossan consists of fine-grained and porous quartz-rich
sediments that exhibit little evidence of replacement by siderite (Figures 6.2b,
6.2c and 6.3a). These fine-grained, quartz-rich sediments may exhibit some
degree of fracturing and/or brecciation and appear to consist predominantly of
extensively leached, fine-grained, quartz-rich fragments.
Page 145
Chapter 6 Borehole CR149 - Sample Descriptions
6.3.2 Quartz
The quartz content of the gossan varies significantly and often occurs in close
association with siderite. The relative proportions of quartz and siderite vary
considerably throughout the core, ranging from samples that consist almost
entirely of quartz (Figures 6.2b, 6.2c and 6.3a), to more siderite-rich sample
intervals that contain relatively minor amounts of quartz (Figures 6.2a and 6.3c).
Quartz is the dominant phase in the upper portion of the gossan and decreases
in abundance towards the middle portion of the gossan and this is reflected by
the relative increase in Fe content (siderite). The quartz content increases again
in the lower portion of the gossan.
The quartz largely represents the relicts of extensively leached rock fragments
(Figures 6.5a, 6.5b, 6.6a, 6.6b, 6.7a and 6.7b). The gossan commonly exhibits a
fragmental nature with large, millimetre-sized quartz fragments occurring within a
fine-grained quartz-rich matrix (Figures 6.5b and 6.6b). The large quartz-rich rock
fragments and fine-grained matrix are occasionally cemented by later stages of
chalcedony that often form narrow veinlets (Figure 6.7a). Elsewhere in the core,
the fragmental nature is less evident, with the great bulk of the quartz occurring
as grains that rarely exceed 100m in size (Figures 6.5a and 6.6a).
In the siderite-rich portions of the core, the quartz typically occurs as angular
fragments that may exceed several millimetres in size. Fine-grained quartz is
also often disseminated throughout the siderite-rich portions of the core. The
discrete quartz grains commonly exhibit highly irregular morphologies that reflect
dissolution (Figure 6.5b). Fibrous textures were not observed in the quartz from
the gossan.
6.3.3 Siderite
Siderite is locally abundant and accounts for the characteristic red and reddish
brown colour observed in certain portions of the core (Figures 6.2a and 6.3c).
The proportions of siderite relative to quartz are more clearly reflected in the Fe
content of the gossan samples, as illustrated in Figure 6.1.
Page 146
Chapter 6 Borehole CR149 - Sample Descriptions
Siderite commonly replaces and cements the porous, quartz-rich sample intervals
(Figures 6.5a, 6.5b, 6.6a, 6.6b, 6.7a and 6.7b). The great bulk of the siderite
occurs as granular aggregates that may also be associated with one or more Pb-,
and Fe-sulphides (Figures 6.5a and 6.6a). A subordinate portion of the siderite
occurs as discrete euhedral crystals that may exceed several hundred
micrometres in size (Figure 6.10a). These euhedral crystals may be extensively
oxidised and/or replaced by limonite.
Several stages of siderite mineralisation are clearly evident in these ores, with
early-formed siderite often highlighted by partial and/or extensive oxidation and
replacement by limonite (Figures 6.8a and 6.10a). Figure 6.10a illustrates the
presence of partially oxidised euhedral siderite crystals that are surrounded by a
later stage of unoxidised siderite.
Quantitative SEM analyses (Appendix 5) confirm that the siderite exhibits a
relatively uniform composition and consist predominantly of FeO (49.0-51.3%)
together with subordinate amounts of CaO (3.5-4.9%) and MgO (3.3-4.2%).
Compositional zoning is evident locally (Figure 6.21).
6.3.4 Limonite
XRD analysis confirms that the bulk of the ‘limonite’ consists of hematite, with
goethite being present in relatively minor amounts. Limonite typically occurs as
an oxidation product of siderite (Figures 6.8a and 6.9b). The oxidation of the
siderite to limonite results in a volume change that may increase the porosity and
friability of the core (Figure 6.3b).
Tiny, micrometre-sized limonite platelets (specular hematite) may also occur
within the more porous quartz-rich aggregates (Figure 6.7b). Limonite typically
replaces siderite along the margins and within growth zones or grain boundaries
(Figure 6.15b). Goethite occurs largely as a result of the hydration of hematite.
Lepidocrocite (ideally γ-Fe3+O(OH)) occurs locally as an oxidation product of the
Fe-sulphide assemblage and is often associated with native sulphur.
Page 147
Chapter 6 Borehole CR149 - Sample Descriptions
6.3.5 Accessory Transparent Gangue Minerals
A number of transparent gangue minerals are present in the gossan in minor,
occurring largely in rock fragments. These include K-feldspar, plagioclase and
microcline. Barite is also locally abundant and may occur as inclusions in quartz-
rich rock fragments, or as fractured grains and crystals that have been
extensively replaced by siderite (Figure 6.10b).
TiO2 (rutile and anatase) is a common accessory and typically occurs as angular
grains and fragments within the fine-grained quartz-rich matrix. TiO2 grains rarely
exceed 30µm in size and probably represent resistate phases. Minor amounts of
fine-grained nontronite clay may also occur locally in the gossan (Figures 6.18a
and 6.18b) and is often intimately associated with siderite. The clay is particularly
abundant within the Au-rich horizons. Apatite was also recognised in the gossan
in very minor amounts.
6.3.6 Fe-Sulphides
Fe-sulphides are a common accessory and account for the magnetic nature of
the core. Greigite, marcasite and pyrite were positively identified by XRD,
although other Fe-sulphides are possibly present. These phases are described
in greater detail in Chapter 10. The Fe-sulphides typically occur as granular
aggregates and euhedral crystals (Figures 6.8b, 6.9b and 6.12b) with discrete
Fe-sulphide crystals rarely exceeding 50µm in size.
Fe-sulphides may be locally abundant, with granular aggregates exceeding
several hundred micrometres in size (Figures 6.9b, 6.11a and 6.11b). The Fe-
sulphides may also exhibit concretionary textures (Figures 6.11a and 6.11b) that
typically exhibit less well-developed morphologies than the discrete Fe-sulphide
crystals. These aggregates are often porous in nature and may exhibit complex
intergrowths with siderite and galena. Plate-like crystals of Fe-sulphide are also
present in the gossan, with discrete crystals rarely exceeding 50µm in length.
Fe-sulphides are often intimately associated with siderite, limonite (Figures 6.8b,
6.9b and 6.11b) and galena-rich aggregates (Figures 6.11a and 6.12a). The Fe-
sulphides occur predominantly along fractures and within the fine-grained quartz-
Page 148
Chapter 6 Borehole CR149 - Sample Descriptions
rich matrix along mineral grain boundaries (Figures 6.7b and 6.8b). Oxidation
and replacement of the Fe-sulphides by lepidocrocite is also evident locally.
6.3.7 Galena and Pb-Bearing Sulphides
The bulk of the galena occurs within fine-grained aggregates that typically occur
along grain boundaries and within cavities in the quartz (Figures 6.6b, 6.10a,
6.11a and 6.12b). The textures observed in the galena are similar to those
described for borehole CR194, with discrete euhedral crystals (Figure 6.13a) and
skeletal aggregates (Figure 6.13b) being recognised. Minor amounts of As and
Sb are typically present in the galena aggregates, probably reflecting intimately
intergrown sulphosalts and/or some degree of Sb solid solution. Galena often
forms overgrowths on native Au grains with some of the best examples illustrated
in Figures 6.19b, 6.19c and 6.19d. XRD confirms the localised oxidation of
galena to anglesite.
Locally, euhedral crystals of discrete PbSb-sulphides are intimately associated
with siderite and galena (Figures 6.5a, 6.6a, 6.14a and 6.14b). These phases
are anisotropic and therefore differ from the PbSb-sulphide recognised in
borehole CR194. These phases were not positively identified.
6.3.8 Accessory Minerals
Cassiterite is present in the gossan in variable amounts. Fine-grained,
micrometre-sized cassiterite is finely disseminated throughout the quartz and
siderite aggregates. Discrete cassiterite grains rarely exceed 10µm in size.
Cassiterite may be locally abundant where it occurs as fine-grained granular
aggregates within the quartz-rich core (Figure 6.15a). The cassiterite grains
typically exhibit a rounded morphology (Figure 6.15b) and may represent a
resistate phase.
Native bismuth (Figure 6.14b) and bismuthinite are present in minor amounts and
typically occur within the Pb-rich aggregates and/or in association with native Au
(Figure 6.22). Mimetite was also recognised during this investigation, occurring
close to the base of the gossan (Figure 6.13a), forming botryoidal aggregates
associated with galena and cerussite. The mimetite may partly represent the
Page 149
Chapter 6 Borehole CR149 - Sample Descriptions
oxidation products of galena and/or other Pb-bearing sulphides. Barite is a
common accessory mineral that occurs throughout the gossan (Figure 6.10b) and
typically exhibits highly irregular morphologies indicative of extensive
replacement by siderite (Figure 6.10b).
6.3.9 Precious Metal Mineralisation
The gossan is characterised by the presence of variable but significant amounts
of Au (0.67–48.54ppm). The systematic examination of the polished sections
prepared from this borehole revealed the presence of a large number of precious
metal-bearing grains (in excess of 100). The great bulk of the Au content of the
gossan appears to be present in the form of extremely fine-grained native Au
grains that rarely exceed a few micrometres in size (Figures 6.15b to 6.23b). The
morphology of the Au grains is highly variable and ranges from subhedral
(Figures 6.17b, 6.17c and 6.20a, for examples) to irregular (Figures 6.17a and
6.19a).
The distribution of the native Au grains is variable. The Au grains may be locally
abundant (Figures 6.18a and 6.18b), or occur as relatively isolated grains within
the porous, fine-grained quartz-rich matrix (Figures 6.19a, 6.19b, 6.19c, 6.20a,
6.20b and 6.20d) and within siderite (Figures 6.16 and 6.17). Galena is often
present as rims on the native Au grains (Figures 6.19b, 6.19c, 6.19d and 6.23).
A single occurrence of a bismuthinite rim on native Au was also observed (Figure
6.22). Rarely, the Au occurs as inclusions in Fe-sulphide (Figure 6.15b).
Qualitative SEM examination of the native Au grains confirms that they consist
predominantly of Au with the Ag content typically being below detection limits
(~0.5%). A small number of native Au grains contain detectable amounts of Ag.
The Ag content of the gossan is relatively low (3.3–71.7ppm). The Ag exhibits no
direct association with Au and this is confirmed by the high fineness of the native
Au grains. No discrete Ag-bearing phases were recognised in the gossan and it
is envisaged that the bulk of the Ag may be present in solid solution within the
more common sulphide minerals, notably the Pb-bearing sulphides.
Page 150
Chapter 6 Borehole CR149 - Sample Descriptions
6.4 Borehole CR149 - Gossan/Massive Sulphide Contact
6.4.1 Introduction
The 188.90-190.00 metre interval marks the contact between the gossan and
massive sulphide and exhibits a markedly different mineralogy compared to the
bulk of the gossan. The gossan/massive sulphide contact is characterised by a
marked increase in the Au (42.75ppm), Ag (735.8ppm) and Hg (83.5ppm)
content. Macroscopic examination of the core also revealed a distinctive change
in the colour relative to the overlying gossan, exhibiting a mottled white and dark
grey colour. Siderite is notably absent in this portion of the core. The contact is
relatively poorly preserved due to the highly friable nature of the core. XRD
analysis confirms the presence of pyrite, quartz, galena, anglesite, greigite,
calcite and native sulphur.
6.4.2 Transparent Gangue
The transparent gangue mineralogy of the contact between the gossan and
massive sulphide is dominated by the presence of quartz and calcite (Figure
6.24a). The relative proportions of quartz and calcite are highly variable, with
quartz typically occurring in subordinate amounts. Quartz may be locally
abundant, where it appears to contain numerous cavities that have subsequently
been filled by pyrite and calcite. The quartz may also occur as angular fragments
that have been extensively fractured. Examination of the quartz in thin section
confirms that it consists of fragments and cryptocrystalline chalcedony that
typically infills former cavities (Figure 6.25b). These patches of chalcedony rarely
exceed 200µm in size. A single occurrence of native Au in a chalcedony-filled
cavity was observed (Figure 6.25b).
Calcite is abundant and gives the core a distinctive milky white appearance in
hand specimen. This is contrasted by the dark grey appearance of pyrite and
galena that typically fill fractures in the calcite. Calcite is locally abundant, where
it exhibits extensive fracturing and replacement by one or more of pyrite, galena
and a AgFe-sulphide mineral that is confirmed by optical properties as being
sternbergite (ideally AgFe2S3) (Figure 6.24a).
Page 151
Chapter 6 Borehole CR149 - Sample Descriptions
6.4.3 Pyrite and other Fe-Sulphides
Pyrite is the dominant sulphide mineral and occurs as granular and porous
aggregates that occur along fractures in calcite (Figure 6.24a). Pyrite may also
occur as fine needle-like aggregates that probably represent the replacement
products of former marcasite or pyrrhotite aggregates. The pyrite aggregates are
often complexly intergrown with one or more of sternbergite, galena, native Au
and Hg (Figures 6.24a, 6.24b and 6.25a). Other Fe-sulphides (probably largely
greigite) are present within the pyrite-rich aggregates. These Fe-sulphides are
typically extensively replaced by the pyrite.
6.4.4 Galena
Galena is a common accessory mineral and appears to be present almost
exclusively as complex skeletal aggregates and euhedral crystals within the
pyrite and sternbergite (Figures 6.24b and 6.25a). The galena may be intimately
associated with native Au (Figure 6.25a), Hg and Se. XRD confirms the localised
oxidation of galena to anglesite.
6.4.5 Accessory Minerals
A number of accessory minerals were observed including a Bi-bearing sulphosalt,
bismuthinite and a AgSb-sulphide (probably pyrargyrite). These phases were
intimately associated with the sternbergite and precious metal-bearing
aggregates.
6.4.6 Precious Metal Mineralisation
Sternbergite is the dominant precious metal-bearing mineral and is intimately
associated with the pyrite (Figures 6.24a, 6.24b and 6.25a). The sternbergite
may exhibit complex textural relationships with skeletal galena and native Au
(Figure 6.25a). The sternbergite may exhibit some degree of oxidation locally,
with the development of a FeAg-sulphate phase.
Discrete native Au grains are rare, with the great bulk of the Au occurring within
the fine-grained Pb- and Hg-rich aggregates that are associated with the
sternbergite. The Au content of this sample is relatively high. As only a few
occurrences of microscopic native Au were recognised, it is envisaged that the
Page 152
Chapter 6 Borehole CR149 - Sample Descriptions
bulk of the Au may be present in a sub-microscopic form, probably within the
sternbergite. A discrete native Au grain was observed within a chalcedony-filled
cavity in calcite, associated with a fine-grained, Cu Fe-sulphide aggregate (Figure
6.25b).
Page 153
Chapter 6 Borehole CR149 - Sample Descriptions
6.5 Borehole CR149 - Massive Sulphide
6.5.1 Introduction
Borehole CR149 intersects the massive sulphide at a depth of 190.00 metres.
The massive sulphide extends for several metres below the gossan. Due to the
relatively low precious metals content of the core, only the first two sample
intervals were selected for examination.
The massive sulphide directly below the gossan/massive sulphide contact is
characterised by a marked decrease in the Au (1.04ppm) and Ag (83.0ppm)
content relative to the overlying gossan. Due to the highly friable nature of the
core, the contact zone between the massive and overlying gossan was not well
preserved. The massive sulphide consists predominantly of pyrite together with
subordinate amounts of quartz and galena.
6.5.2 General Mineralogy
The massive sulphide directly below the gossan consists predominantly of pyrite
(Figure 6.26a and 6.26b). The pyrite occurs as euhedral crystals, granular
aggregates and as fine-grained aggregates that often exhibit primary textural
features (Figure 6.26b). The pyrite appears relatively fresh and does not exhibit
any evidence of dissolution and/or oxidation. The pyrite is, however, extensively
fractured (Figures 6.26a and 6.26b). Fine rims of galena commonly occur along
the margins of the pyrite grains and in fractures (Figure 6.26a). Quartz is the
dominant gangue mineral and may fill or partially fill the fractures in the pyrite
grains and aggregates (Figures 6.26a and 6.26b). Examination of the thin section
prepared from the massive sulphide confirms that the quartz exhibits a medium to
coarse-grained crystallite size.
Page 154
Chapter 6 Borehole CR149 - Sample Descriptions
6.6 Borehole CR149 – Summary Diagram
Section 6.3: The Tertiary Sand is characterised by a low Cu, Pb and S content and moderate amounts of Fe. The trace and precious metal content is moderately low. This sample interval is dominated by the presence of glauconite-like clay together with subordinate amounts of siderite, chlorite, quartz and feldspar. Accessory minerals include galena, pyrite, sphalerite, ilmenite and chromite.
Section 6.4: The Gossan contains highly variable amounts of Fe, largely reflecting variations in the relative proportions of the dominant minerals, siderite and quartz. Galena and Pb(SbAs)-sulphides are present in minor amounts throughout the gossan. The variable S content largely reflects the presence of Fe- and Pb-bearing sulphides. Fine-grained native Au grains are disseminated throughout the gossan and are particularly abundant in the upper and middle portions of the core. Accessory minerals include cassiterite, native bismuth, bismuthinite and mimetite.
Section 6.5: Calcite and quartz are the dominant minerals in the Gossan/Massive Sulphide Contact, which is also characterised by an elevated Au, Ag and Hg content. Fine-grained and complex native Au and native Hg are present in sternbergite, pyrite and galena aggregates.
Section 6.6: The uppermost portion of the Massive Sulphide is high in Fe and S and consists predominantly of pyrite together with subordinate amounts of quartz and galena.
Figure 6.27 - Diagram illustrating the key mineralogical features for the 'Tertiary Sand', 'Gossan', 'Gossan /Massive Sulphide Contact' and 'Massive Sulphide'.
Page 155
Chapter 7 Borehole CR038 - Sample Descriptions
7 BOREHOLE CR038 – SAMPLE DESCRIPTIONS
7.1 Introduction
Chapter 7 describes the chemistry and mineralogy of borehole CR038. The
samples selected for examination are provided in Appendix 2. Section 7.2
describes the major and minor element chemistry of the borehole including their
relative abundance and degree of correlation.
The mineralogical descriptions are grouped into three separate sections,
summarising the mineralogy of the ‘Quartz Replaced Tuffs' (Section 7.3), ‘Quartz
Replaced Tuff/Partial Massive Sulphide Contact’ (Section 7.4) and ‘Partial
Massive Sulphide’ (Section 7.5) respectively. A summary diagram of the
geochemistry and mineralogy is provided in Section 7.6.
This borehole was selected for examination because of the extensive precious
metal mineralisation and the marginal location relative to the massive sulphide
mineralisation. The location of borehole CR038 is illustrated and described in
further detail in Chapter 3. This borehole is a vertical hole.
Samples selected for examination consist predominantly of the quartz-replaced
tuffs (as described by the field geologists) that lie directly above more sulphide-
rich materials. Borehole CR038 does not exhibit the characteristic reddish-brown
gossan-style characteristics that were logged by the field geologists in the
previous two boreholes. The Au-envelope of quartz-replaced tuffs extends
between 150.80 and 157.25 metres, where they intersect partial massive
sulphide with clay.
The core is often friable in nature and consists of fine-grained sand-like materials
together with angular rock fragments that rarely exceed a few centimetres in
maximum dimensions. The core varies in colour from light grey to light yellow-
brown in the quartz replaced tuffs to a darker grey colour in the semi-massive
sulphide (Figure 7.2). The sulphide-rich material is also typically friable in nature.
Page 156
Chapter 7 Borehole CR038 - Sample Descriptions
Core recoveries were relatively poor and contact zones are not well preserved.
Marked changes in the mineralogy of the core are, however, clearly recognised in
the polished section prepared from these materials. The friable samples were
wet screened into a number of size fractions and mounted in preparation for
examination using reflected light microscopy and SEM based techniques. A
small number of whole-rock polished sections were also prepared.
The characterisation of borehole CR038 was based on the examination of 84
polished sections and 5 thin sections. Samples of Au-bearing material from
borehole CR038 have been examined previously and the results presented in
R2644 (1996).
Page 157
Chapter 7 Borehole CR038 - Sample Descriptions
7.1 Borehole CR038 - Chemistry
7.1.1 Introduction
The variable chemistry exhibited in borehole CR038 reflects changes in the
mineralogy of the core. Only limited assay data is available for this borehole.
The assay data are provided in Appendix 3. A diagram combining the list of
selected samples together with the major and minor element chemistry is
illustrated in Figure 7.1.
Figure 7.1 - Diagram illustrating chemistry variations in borehole CR038. The sample intervals examined from borehole CR038 consist of quartz replaced massive tuffs and partial massive sulphide. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The variation in chemistry is displayed to the right of the borehole schematic. Distinct compositional zones are clearly evident, particularly at the tuff/sulphide contact, largely reflecting variations in the mineralogy of each sample interval. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. CR038 is a vertical hole. QTM - Quartz Replacement of Massive Tuff, MSPCL - Partial Massive Sulphide with Clay, MPS - Partial Massive Sulphide.
Page 158
Chapter 7 Borehole CR038 - Sample Descriptions
7.1.2 Geochemical Profile
The Cu content of the quartz tuffs is low, with marginally elevated Cu values
occurring in the underlying partial massive sulphide. The Pb content of this
borehole is also consistently low. Assay data for the Fe and S content of this
borehole were not available.
The Ag content of the tuff and sulphide intersection is low, with the exception of
the tuff/sulphide contact, where the Ag content exhibits a marked increase. This
increase is associated with a similar increase in the As content. The Hg and Au
content also increases significantly towards the base of the quartz-replaced tuff,
although slightly higher in the profile relative to the position of the elevated Ag
and As values.
Borehole CR038 exhibits highly elevated Au values close to the tuff/sulphide
contact zone. Although the Au content of the tuff intersections examined during
this investigation all exhibit significant Au values, the Au content peaks a few
metres above the contact zone. The elevated Au values are associated with a
similarly marked increase in the Sn content, with Ag, As and Hg also increasing
slightly lower in the geochemical profile.
The As content of the upper portion of the quartz replaced tuff is relatively low,
but increases significantly towards the base of the tuff, close to the contact with
the underlying sulphides. This increase in As content is closely associated with a
similar increase in the Ag content. The As content of the partial massive sulphide
is consistently high.
The Hg content of this borehole is similar to that described for Ag, occurring in
relatively minor amounts in the tuff and sulphide, but increasing significantly
above the tuff/sulphide interface.
The Bi content remains fairly constant with increasing depth, but increases
marginally with depth in the underlying sulphide intersections. The Sb content is
highly variable throughout the borehole, with no distinct zones or association with
other elements being apparent.
Page 159
Chapter 7 Borehole CR038 - Sample Descriptions
The Sn content increases markedly, several metres above the tuff/sulphide
interface. This increase in Sn content is associated with a similar increase in the
Au content and occurs slightly higher in the profile relative to marked increases in
Ag, As and Hg. The Sn content also exhibits another marked increase several
metres below the tuff/sulphide interface.
Page 160
Chapter 7 Borehole CR038 - Sample Descriptions
7.2 Borehole CR038 - Quartz Replaced Tuffs
7.2.1 Introduction
The quartz replaced tuffs occur at a depth of between 150.80 and 157.25 metres.
The 156.30 to 157.25 metre sample interval marks the contact between the tuffs
and underlying sulphide and is described separately in Section 7.4. The quartz
replaced tuffs contain only very minor to trace amounts of Cu (0.00–0.02%) and
Pb (0.06–0.58%). The Ag (3.8–22.0ppm) and Au (1.71–11.31ppm) contents of
these sample intervals are moderate but variable. Minor elements include As (0–
54ppm), Bi (19–40ppm), Hg (0.2–90.1ppm), Sb (36–252ppm) and Sn (144–
756ppm). The macroscopic examination of the core confirms that the quartz
replaced tuffs are typically friable in nature and consist predominantly of light
cream, grey and brown coloured quartz-rich rock fragments that may exhibit
some degree of replacement by siderite (Figure 7.2). XRD analysis confirms the
presence of quartz, pyrite, anatase, galena, goethite and cassiterite.
7.2.2 Transparent Gangue Mineralogy
Quartz is the dominant mineral, comprising in excess of 90 per cent of the core
locally. The bulk of the quartz is present as loosely aggregated, irregular shaped
fragments that range from a few micrometres to several millimetres in size
(Figures 7.4a and 7.6a). The highly irregular morphology of the fragments
suggests that a significant proportion of the quartz has been subjected to some
degree of dissolution, particularly the fine-grained quartz-rich matrix (Figures
7.8a, 7.8b and 7.16a).
Locally, the quartz may occur as larger, more coherent massive crystalline
aggregates (Figures 7.3a, 7.3b and 7.6b). These larger quartz fragments
commonly contain euhedral pore spaces that appear to represent the presence of
former pyrite crystals that have been subsequently oxidised and leached from the
quartz (Figures 7.3a and 7.3b). One or more of fine-grained quartz fragments
and siderite may partially or extensively fill these euhedral pore spaces (Figures
7.3a, 7.3b, 7.8a and 7.8b). Detailed examination of the quartz in thin section
confirms that it exhibits a highly variable grain size (Figures 7.4a, 7.4b, 7.5a,
7.5b, 7.6a to 7.6d and 7.7a to 7.7d).
Page 161
Chapter 7 Borehole CR038 - Sample Descriptions
The size of the quartz crystallites is also highly variable and ranges from a few
micrometres to in excess of 100µm. The quartz also exhibits fibrous textures that
have developed along the margins of the euhedral pore spaces (Figures 7.4a and
7.4b).
The highly fragmented and irregular morphology of the quartz suggests a history
of brecciation and dissolution (Figures 7.6a and 7.6b). Many larger, more
coherent quartz-rich rock fragments consist of very fine-grained, fragmented
quartz fragments that have been cemented by later stages of chalcedony (Figure
7.4a, 7.4b, 7.5a, 7.5b, 7.7a and 7.7b). The chalcedony exhibits varying degrees
of recrystallisation.
With the exception of ring-like structures of anatase that occur in the quartz, the
bulk of the quartz is free from inclusions and intergrowths. The presence of
siderite, sulphide minerals and the precious metal mineralisation is strongly
influenced by the porosity of the quartz, with the euhedral cavities and fine-
grained, porous regions of quartz acting as conduits for the mineralisation.
Siderite is common and occurs along fractures and infilling cavities in the quartz-
rich rock fragments (Figures 7.3a, 7.3b, 7.8a, 7.8b, 7.9a and 7.9b). The bulk of
the siderite is relatively unweathered and exhibits little or no evidence of oxidation
and replacement by limonite. Several stages of siderite mineralisation are
evident, with euhedral crystals often being overgrown by later stages of siderite
(Figure 7.8a). The earlier formed siderite may also contain needle-like crystals of
a PbSb-sulphide that are either absent or less abundant in the later overgrowths
(Figure 7.8a). The later siderite overgrowths often exhibit anhedral
morphologies, despite being developed in cavities, possibly indicating that they
have been subjected to dissolution. Cassiterite (Figure 7.8b), anatase (Figure
7.9a), Fe-sulphides (Figure 7.9b) and native Au (Figure 7.16a) are closely
associated with the siderite mineralisation. Quantitative SEM analyses confirm
that the siderite contains variable amounts of CaO (5.8-9.2%) and MgO (5.5-
6.2%). Compositional zoning is also evident within some of the siderite.
Page 162
Chapter 7 Borehole CR038 - Sample Descriptions
Minor amounts of Fe-rich clay (nontronite) were also observed along fractures in
the quartz (Figure 7.11a). A number of Au grains were locate within the
nontronite.
7.2.3 Ore Mineralogy
Fe-sulphides occur in minor amounts throughout the quartz replaced tuffs. The
Fe-sulphides occur as euhedral crystals and as needle-like grains that typically
form radiating aggregates within fractures and cavities in the fine-grained quartz-
rich matrix (Figures 7.9a, 7.9b and 7.11b). The Fe-sulphides may also be
intimately associated with the precious metal-bearing grains (Figures 7.12b,
7.12d, 7.13b, 7.14b, 7.15c and 7.16b).
Galena is the dominant Pb-bearing sulphide mineral and typically occurs as finely
disseminated aggregates within pore spaces and fractures. Discrete galena
grains rarely exceed a few micrometres in size. The galena is often intimately
associated with native Au (Figures 7.11b, 7.12c, 7.13a and 7.13c) and PbSb-
bearing sulphides. The PbSb-sulphides may also occur as rhomb-shaped
crystals and aggregates of radiating acicular crystals (Figures 7.8a, 7.8b and
7.11b) that rarely exceed 10µm in size. The PbSb-sulphides were not positively
identified due to their very fine grain size.
Galena may partially replace quartz along fractures and grain boundaries
(Figures 7.10a, 7.10b and 7.10c). Minor amounts of a Pb-selenide mineral
(probably clausthalite, ideally PbSe), was also identified.
Cassiterite hosts the bulk of the Sn content and occurs as granular aggregates
and euhedral crystals that typically occur within cavities and fractures in the fine-
grained quartz-rich matrix (Figures 7.8b, 7.13d and 7.14c). The cassiterite is
rarely associated with native Au (Figures 7.13d, 7.14a and 7.14c). Cassiterite
aggregates and crystals commonly exceed 20µm in size.
Anatase is a common accessory mineral and occurs as angular grains and
euhedral crystals that typically occur within the fine-grained quartz-rich matrix
(Figure 7.9a). The anatase often forms ring-like structures within the quartz that
Page 163
Chapter 7 Borehole CR038 - Sample Descriptions
clearly represent neoformation of TiO2 species in cavities and are evidence of Ti
dissolution and reprecipitation (Figures 7.15c and 7.15d). The cavities within
which the anatase formed are largely filled by later stages of chalcedony
mineralisation. Numerous native Au grains were also observed as inclusions and
in close association with the anatase (Figures 7.15a, 7.15b and 7.15d).
Anatase and cassiterite are common resistate phases and may reflect
components of the primary mineralisation that have survived the extensive
reworking and dissolution that have affected these ores. However, the euhedral
nature of some of the cassiterite and anatase and the intimate nature of some of
the intergrowths with native Au clearly indicate that these phases formed in situ.
7.2.4 Precious Metal Mineralisation
The Ag content of the quartz replaced tuffs is moderately low (3.8–22.0ppm).
Minor amounts of native Ag, sternbergite, Ag(SbAs)-sulphide (Figure 7.12a) and
iodargyrite (ideally AgI) were recognised and typically occur in association with
one or more of native Au, native Bi, galena, PbSb-sulphides and Fe-sulphides.
A large number of native Au grains were observed during this investigation. The
bulk of the native Au grains rarely exceed a few micrometres in size (Figures
7.10, 7.11 and 7.12), but may exceed 30µm locally (Figure 7.14d). The
morphology of the native Au grains is highly variable and ranges from subhedral
to irregular (Figures 7.10 through to 7.16).
The native Au typically occurs within cavities and along fractures in the fine-
grained quartz-rich matrix (Figure 7.16a), often occurring within euhedral cavities
(Figures 7.14a, b, c and d). This emphasises their secondary origin, as the bulk
of the Au present in the primary ores is present in a sub-microscopic form. There
is an intimate association between native Au and galena/PbSb-sulphosalts
(Figures 7.10, 7.11b, 7.12c, 7.13a and 7.13c), Fe-sulphides (Figures 7.12b,
7.12d, 7.13b, 7.14b, 7.15c, 7.15d and 7.16b), anatase and cassiterite (Figures
7.13d, 7.14a and 7.15a, b, c and d). Qualitative SEM analysis of the native Au
grains confirms that they are typically Au-rich, with the Ag content commonly
being below the detection limits for Energy Dispersive X-ray analysis (~0.5%).
Page 164
Chapter 7 Borehole CR038 - Sample Descriptions
7.3 Borehole CR038 - Quartz Replaced Tuff/Partial MassiveSulphide Contact
7.3.1 Introduction
The contact between the quartz replaced massive tuffs and massive sulphide
occurs at a depth of between 156.30 and 157.25 metres and is characterised by
a marked change in the mineralogy of the core. The Cu (0.04%) and Pb (0.29%)
contents remain relatively low. The Ag (1240ppm) content exhibits a marked
increase. The Au content (1.33ppm) is moderate. Minor elements include As
(918ppm), Bi (26ppm), Hg (16.8ppm), Sb (126ppm) and Sn (216ppm). The
contact zone appears essentially similar to the silicified massive tuffs exhibiting a
light grey colour in hand specimen. The core is highly friable and rubble-like in
nature. The contact was not well preserved. XRD analysis confirms the
presence of pyrite and quartz.
7.3.2 Transparent Gangue Mineralogy
The transparent gangue mineralogy is essentially similar to that described for the
quartz replaced tuffs. Quartz is the dominant gangue mineral and exhibits similar
textures to those described previously.
The quartz may contain euhedral cavities that represent the presence of former
pyrite crystals (Figure 7.17a). The quartz is often extensively fractured and the
highly irregular morphology of the quartz fragments may be indicative of some
degree of dissolution.
7.3.3 Ore Mineralogy
Pyrite typically occurs along the margins of the quartz grains and in fractures
(Figures 7.17a, 7.17b and 7.18b). With increasing depth, the core becomes
progressively more pyrite-rich and quartz-poor (Figures 7.19a and 7.19b).
Siderite is largely absent from the contact zone. The pyrite aggregates may be
fractured with quartz and/or goethite partially filling the fractures (Figure 7.19b).
The pyrite may also exhibit compositional zoning that largely reflects the
presence of minor amounts of arsenic.
Page 165
Chapter 7 Borehole CR038 - Sample Descriptions
Goethite is common in the pyrite-rich portions of the contact zone (Figures 7.19a
and 7.19b). Euhedral pyrite does not oxidise as readily as other types of pyrite
(collomorphic overgrowths for example). The possibility exists that later stages
pyrite overgrowths once existed on the euhedral crystals, and these were
oxidised to produce the interstitial goethite. The fine galena rims that form
around the margins of the pyrite crystals provide further evidence for this theory
(Figure 7.19a). These rims appear suspended within the goethite and do not
touch the euhedral pyrite crystals. The fine galena rims may have originally
formed on a later stage pyrite overgrowth that has subsequently been oxidised.
During oxidation, galena is one of the last sulphide minerals to be oxidised, and
would therefore survive the partial oxidation of the pyrite.
Qualitative SEM analysis of the goethite also revealed the presence of minor
amounts of Si. This may represent minor amounts of quartz or FeSi-rich clay that
is present within the goethite.
7.3.4 Precious Metal Mineralisation
Only a single, micrometre-sized native Au inclusion in pyrite was identified. A
significant proportion of the Au may therefore be present in a sub-microscopic
form, possibly in association with the Ag-bearing sulphide minerals. A HgAg-
sulphide (possibly imiterite) is present in the pyrite in very minor amounts (Figure
7.17b). A Ag-sulphide (possibly acanthite) was also recognised during this
investigation.
Sternbergite and proustite/pyrargyrite are the dominant Ag-bearing minerals
(Figures 7.17a, 7.17b, 7.18a and 7.18b). The sternbergite and
proustite/pyrargyrite are intimately associated with pyrite (Figures 7.17a, 7.18a
and 7.18b). Quantitative SEM analysis of a number of proustite/pyrargyrite
grains confirms that they are largely As-rich (14.6 - 16.7%) with the Sb content
being below detection limits and are therefore close to the proustite end member
composition (Appendix 5, analyses #1 to #6). A number of the
proustite/pyrargyrite grains are, however, compositionally zoned (Figure 7.18a)
with pyrargyrite (analysis #7, 19.1% Sb and 1.9% As) being intimately associated
with the proustite.
Page 166
Chapter 7 Borehole CR038 - Sample Descriptions
7.4 Borehole CR038 - Partial Massive Sulphide
7.4.1 Introduction
The partial massive sulphide occurs at a depth of 157.25 metres. A single sample
of the partial massive sulphide ore from the 157.25 to 158.25 metre sample
interval was selected for characterisation. The Cu (0.21%) and Pb (0.05%)
contents are low. The Ag (9.9ppm) and Au (0.23ppm) contents are low. The As
(360ppm) content is moderate. The Bi (21ppm), Hg (7.9ppm) and Sb (54ppm)
contents are relatively low. The Sn (216ppm) content is moderate. This sample
exhibits a medium-dark grey colour and is highly friable. XRD analysis confirms
the presence of pyrite and quartz.
7.4.2 Transparent Gangue Mineralogy
Quartz is the dominant gangue mineral and occurs along fractures and grain
boundaries associated with extensively fractured pyrite (Figures 7.20a and
7.20b). The quartz exhibits a medium to coarse-grained crystallite size. Fibrous
quartz is also present. The quartz is largely free from inclusions and
intergrowths. Minor amounts of TiO2 are also present.
7.4.3 Ore Mineralogy
Pyrite is the dominant sulphide mineral, is granular in nature and has been
extensively fractured. The pyrite may exhibit compositional zoning reflecting the
presence of minor amounts of As. With increasing depth, supergene Cu-
sulphides may also occur along the fractures in the pyrite (Figure 7.20b). The Cu-
sulphides are often highly porous in nature.
Page 167
Chapter 7 Borehole CR038 - Sample Descriptions
7.5 Borehole CR038 – Summary Diagram
Section 7.3: The Quartz Replaced Tuffs consist predominantly of quartz fragments and subordinate amounts of siderite. Accessory minerals include galena, PbSb-sulphide, Fe-sulphides, cassiterite and anatase. Ag-bearing minerals include native Ag, sternbergite, Ag(SbAs)-sulphides and iodargyrite. Abundant, fine-grained native Au grains occur in cavities and along the margins of the quartz fragments.
Section 7.4: The Quartz Replaced Tuff/Partial Massive Sulphide Contact consists predominantly of quartz fragments and subordinate amounts of pyrite and goethite. Accessory minerals include proustite/pyrargyrite and sternbergite, which host the bulk of the Ag content. Trace amounts of galena and AgHg-sulphide are also present. Native Au is rare.
Section 7.5: The Partial Massive Sulphide consists predominantly of pyrite and quartz together with subordinate amounts of secondary Cu-sulphides. Minor amounts of galena and TiO2 are also present.
Figure 7.21 - Diagram illustrating the key mineralogical features for the 'Quartz Replaced Tuffs', 'Quartz Replaced Tuff/Partial Massive Sulphide Contact' and 'Partial Massive Sulphide'.
Page 168
Chapter 8 Borehole CR191 - Sample Descriptions
8 BOREHOLE CR191 – SAMPLE DESCRIPTIONS
8.1 Introduction
Chapter 8 describes the chemistry and mineralogy of borehole CR191. The
sample list is provided in Appendix 2. Section 8.2 describes the major and minor
element chemistry of the borehole and includes details on their relative
abundance and any apparent correlation.
Section 8.3 describes the mineralogy of the ‘Tertiary Polymict
Conglomerate/Gossan Contact'. The mineralogy of the gossan is described in
Sections 8.4 (Upper Gossan), 8.5 (Middle Gossan) and 8.6 (Lower Gossan).
Section 8.7 describes the mineralogy of the ‘Partial Massive Sulphide'. A
summary diagram of the geochemistry and mineralogy is provided in Section 8.8.
Borehole CR191 is situated away from the main supergene ore body but contains
high levels of Au over a depth of more than 10 metres. The location of borehole
CR191 is illustrated and described in Chapter 3. This borehole is an inclined hole,
the angle of dip being approximately 70 degrees.
The mineralogical characterisation of this borehole is particularly focussed on the
Au and/or Ag-rich intersections. The field logs describe much of the gossan as
'extensively leached'. The core is often friable and exhibits a wide range in
textures, colour and associated mineralogy (Figure 8.2). The core ranges in
colour from pale grey and cream, through to red, dark reddish brown, dark grey
and black. The light coloured rock fragments are typically quartz-rich, with the
red and reddish brown colours typically reflecting the presence of siderite and/or
limonite. The dark grey and black core typically contains a larger proportion of
galena and Fe-sulphides. The Tertiary conglomerate situated directly above the
conglomerate/gossan contact zone was not available for characterisation.
The contact zones were often not clearly preserved. However, marked changes
in the mineralogy of the contact zones are evident. Sample selection starts at the
contact between the Tertiary conglomerate and the underlying gossan and
extends into the first three intersections of the supergene enriched massive
Page 169
Chapter 8 Borehole CR191 - Sample Descriptions
sulphide, below which the precious metal content was significantly depleted.
Borehole CR191 intersects the fossil gossan at a depth down hole of 134.25
metres. The massive sulphide ore is intersected at a depth of 153.85 metres
down hole. The characterisation of borehole CR191 was based on the
preparation and examination of 85 polished sections and 7 thin sections.
Page 170
Chapter 8 Borehole CR191 - Sample Descriptions
8.1 Borehole CR191 - Chemistry
8.1.1 Introduction
Variation in the element chemistry marks the prominent boundary between the
gossan and massive sulphide with significant variations in chemistry within the
gossan reflecting areas of extensive leaching and/or element mobility. The assay
data are provided in Appendix 3.
Figure 8.1 - Diagram illustrating chemistry variations in borehole CR191. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. The variation in chemistry is displayed to the right of the borehole schematic. Distinct compositional zones are clearly evident at the upper and lower portions of the gossan. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. Borehole CR191 is an inclined hole. TCP – Tertiary Polymict Conglomerate, GHW - Weak Hematitic Gossan, GEM - Moderately Leached Gossan, GMS - Strong Magnetic Gossan, GES - Strongly Leached Gossan, MMPXM - Massive Sulphide with Shale.
Page 171
Chapter 8 Borehole CR191 - Sample Descriptions
8.1.2 Geochemical Profile
The geochemical profile of borehole CR191 is illustrated in Figure 8.1. The Cu
content of the Tertiary conglomerate and gossan intersections is very low, with
marginally higher Cu values occurring in the underlying massive sulphide. Lead
occurs in relatively minor amounts in the Tertiary units, but exhibits a marked
increase, reaching a maximum in the upper portion of the gossan, several metres
below the Tertiary/gossan contact zone. The spike in the Pb value is strongly
correlated with a similar spike in the Ag, Bi, Sb and As values. Although other
elements may occur in significant amounts in this region, they peak at slightly
different positions in the geochemical profile. Notably, the increase in Pb content
is associated with a significant decrease in the Fe content. The Pb content is
significantly depleted in the middle portion of the gossan, but increases slightly
towards the base of the gossan. The Pb content of the underlying sulphides is
relatively low.
The geochemical profile for Fe is essentially similar to that for S, and the two
elements exhibit a strong correlation. The Fe and S content of the Tertiary
intersections is relatively low, but increases significantly in the upper portion of
the gossan. The elevated levels of Fe and S are also associated with similar
increases in the Pb, Au, Ag, Bi, Hg, Sn, Sb and As values. The central portion of
the gossan is characterised by consistently low levels of Fe and S. The Fe and S
content increases markedly towards the base of the gossan and into the
underlying massive sulphide intersections.
The geochemical profile for Ag is characterised by two prominent peaks,
occurring in the upper and lower portion of the gossan. The peak in Ag in the
upper gossan is strongly correlated with Pb, Bi, Sb and As. The Ag content of
the middle gossan region is low, but increases markedly, reaching a maximum at
the contact with the massive sulphide. At this point, the Fe and S content
increases significantly, but none of the minor elements appear to strongly
correlate with the elevated Ag values.
Page 172
Chapter 8 Borehole CR191 - Sample Descriptions
Au exhibits a very strong correlation with Sn, and this correlation is evident in the
upper portion of the gossan and closer to the base of the gossan, approximately
two metres above the contact with the massive sulphide where minor increases
in the Ag, Hg and Sb contents are also recognised. The elevated levels of Au in
the upper gossan region are also associated with elevated levels of most of the
other elements, although their positions in the geochemical profile are slightly
different.
The As content of the Tertiary units is low, but increases significantly in the upper
portion of the gossan exhibiting a strong correlation with the other minor
elements, in particular Pb, Ag, Sb and Bi. The As content of the middle and
lower gossan is relatively low but increases to more significant levels in the
underlying massive sulphides.
The geochemical profile for Bi is closely associated with that of Ag, Pb, Sb and
As, peaking in the upper portion of the gossan. The Bi content also increases
very slightly towards the base of the gossan, where it is more closely associated
with increases in Au and Sn.
The Hg content of the gossan is consistently low, but very slightly increases in
the upper portion of the gossan, possibly associated with an increase in the Fe
content. A more prominent increase occurs close to the base of the gossan
where an association between Hg, Au and Sn may be evident. The elevated Hg
levels occur throughout the gossan/sulphide contact zone and into the upper
portion of the underlying sulphides, decreasing with increasing depth.
The Sb profile in the upper portion of the gossan is strongly correlated with high
levels of other elements, in particular Ag, Pb, Bi and As. Very minor increases in
the Sb content are also evident in the lower portion of the gossan, where a
correlation between Sb, Au and Sn may occur. The Sb content of the Tertiary
units, middle gossan and massive sulphide is low. The Sn content is strongly
associated with the Au content, occurring in elevated amounts in the upper
gossan and lower gossan regions.
Page 173
Chapter 8 Borehole CR191 - Sample Descriptions
Page 174
Chapter 8 Borehole CR191 - Sample Descriptions
8.2 Borehole CR191- Tertiary Polymict Conglomerate/Gossan Contact
8.2.1 Introduction
The contact zone between the Tertiary conglomerate and gossan occurs at a
depth of between 134.25 and 135.25 metres. No sample material above this zone
was available for examination. The Fe (13.26%) content is moderate and reflects
the presence of siderite and/or limonite. The S content is very low (0.09%), with
minor amounts of galena/PbAsSb-sulphide and barite being observed. The Pb
content is moderately low (0.73%). The Cu (0.01%) content is low and no
discrete Cu-bearing minerals were recognised. The Ag (0.8ppm), Au
(<0.01ppm), As (81ppm), Bi (18ppm), Hg (0.7ppm), Sb (171ppm) and Sn
(41ppm) contents are also low.
8.2.2 General Mineralogy
This sample exhibits a pale cream colour and contains numerous reddish brown
veinlets. Transmitted and reflected light microscopy and SEM based techniques
confirm that this sample consists predominantly of extensively fractured
polycrystalline quartz aggregates together with interstitial siderite and limonite
(Figure 8.3a).
The quartz fragments range from only a few micrometres to several millimetres in
size (Figures 8.3a and 8.3b). The larger quartz aggregates typically exhibit a
medium-grained crystallite size of between 50µm and 300µm. The smaller
fragments typically exhibit a finer crystallite size with the quartz crystallites rarely
exceeding 100µm in size.
The quartz crystallites in the Tertiary conglomerate/gossan contact often exhibit
sutured grain boundaries, particularly in the smaller fragments that contain the
finer crystallites. This may partly relate to the stresses involved during the
extensive fracturing. The bulk of the quartz in the larger fragments exhibits
simple grain boundary relationships. The larger quartz crystals may exhibit
growth zoning that is marked by rows of tiny fluid inclusions.
Page 175
Chapter 8 Borehole CR191 - Sample Descriptions
The fractures in the quartz may be partially filled by one or more of siderite,
galena and limonite. Siderite is present along the margins of the quartz fragments
and appears to partially replace the fine-grained quartz-rich matrix (Figure 8.3b).
The siderite exhibits a wide variation in the degree of oxidation and replacement
by limonite. Quantitative SEM analysis of the siderite, as provided in Appendix 5,
confirms that it typically contains minor amounts of CaO (3.4-5.8%) and MgO
(1.2-2.2%).
Locally, minor amounts of nontronite clay are present. Fine-grained aggregates
of galena are relatively common, occurring along the margins of the quartz
fragments associated with the siderite mineralisation (Figure 8.3b). Minor
amounts of a fine-grained and porous PbCa-phosphate (possibly a member of
the crandallite group of minerals) and TiO2 were also identified.
Page 176
Chapter 8 Borehole CR191 - Sample Descriptions
8.3 Borehole CR191 - Upper Gossan
8.3.1 Introduction
The bulk mineralogy of the upper, middle and lower gossans for borehole CR191
is essentially similar to that described for borehole CR038. Variations in the
major elements in the gossan largely reflect the relative proportions of quartz,
siderite and limonite, together with the presence of accessory phases that host
the bulk of the trace elements and precious metals.
The upper gossan occurs at a depth of between 135.70 and 141.65 metres.
Examination of the core confirms that it is highly friable in nature and exhibits a
wide range in colour from dark reddish brown through to dark grey/black (Figure
8.4).
The Cu content of the upper gossan is consistently low (0.01–0.02%). No
discrete Cu-bearing phases were observed. The Pb (0.86–17.52%), Fe (12.00–
36.26%) and S (0.25–9.75%) contents are significantly higher than the middle
and lower gossans and largely reflect the presence of galena, Fe-sulphide and
siderite. The upper gossan is characterised by an elevated precious metal
content, exhibiting particularly high Au (0.01–12.04ppm) and moderate Ag (1.2–
58.6ppm) contents relative to the middle gossan, which is comparatively barren
(Figure 8.1). The As (99–16700ppm), Bi (65–3227ppm), Sb (342–5963ppm) and
Sn (54–9072ppm) contents are also high. The Hg content (0.2–9.8ppm) is
moderate. XRD analysis confirms the presence of quartz, goethite, rutile,
lepidocrocite, native sulphur, siderite, anglesite, galena, greigite, marcasite and
calcite.
8.3.2 Gangue Mineralogy
Quartz is common, but generally subordinate in abundance to siderite (Figure
8.6b). The quartz typically occurs as angular or irregularly shaped fragments that
may exhibit some degree of replacement by siderite. The quartz fragments
exhibit a wide range in grain size from millimetre-sized fragments to micrometre-
sized interstitial grains. The quartz fragments contain both medium- and fine-
grained polycrystalline aggregates.
Page 177
Chapter 8 Borehole CR191 - Sample Descriptions
The upper portion of the gossan is particularly siderite-rich (Figures 8.5a and
8.5b). The siderite is similar to that described for previous samples and typically
contains minor, but variable amounts of CaO (4.2-4.6%) and MgO (4.6-5.5%).
Minor amounts of Mn are also rarely present in the siderite. The siderite may
exhibit some degree of oxidation and replacement by limonite (Figures 8.5a)
typically resulting in an increase in porosity.
The siderite may exhibit botryoidal textures (Figure 8.5b), or occur as discrete
euhedral crystals (Figure 8.5a). The siderite is often intimately associated with
galena (Figures 8.5b, 8.6a and 8.6b). Late stages of siderite mineralisation
reduce the porosity of the gossan locally.
8.3.3 Ore Mineralogy
Fe-sulphides are common and accounts for the magnetic properties of the core.
Only greigite and marcasite were positively identified by XRD analysis. The Fe-
sulphides are both fine-grained and complex and are therefore described in
greater detail in Chapter 10. The Fe-sulphides typically occur as granular
aggregates that are developed within cavities and are often intimately associated
with siderite and galena (Figure 8.5a). Fe-sulphides are locally abundant (Figure
8.7a) and often exhibit partial and extensive oxidation to limonite. XRD analysis
confirms that the oxidation of the Fe-sulphides typically results in the formation of
goethite, lepidocrocite and native sulphur as oxidation products (Figures 8.7a and
8.8a).
Galena typically occurs as fine-grained, skeletal crystals that are disseminated
throughout the siderite (Figures 8.6a and 8.6b). Skeletal galena may fill euhedral
cavities formed as a result of the dissolution of siderite (Figure 8.7b). Galena is
locally abundant, occurring as more coherent aggregates that occur along the
margins of siderite (Figure 8.5b). In the lower part of the upper gossan, where the
bulk of the Fe-sulphide has been oxidised to goethite, anglesite (ideally PbSO4) is
present (Figures 8.8a and 8.8b). Anglesite, is a common oxidation product of
galena. The anglesite often retains the skeletal textures of the former galena
crystals. Subordinate amounts of a PbSb-sulphide are also present locally.
Page 178
Chapter 8 Borehole CR191 - Sample Descriptions
Fine-grained, TiO2 (largely rutile) and cassiterite are common accessory minerals
(Figures 8.8a and 8.8b) and are particularly abundant in the lower portion of the
upper gossan, where the degree of oxidation also appears to be high. The
cassiterite and TiO2 typically occurs as sub-rounded grains that probably
represent resistate phases. Discrete cassiterite and TiO2 grains rarely exceed
20µm in size (Figures 8.8a and 8.8b). Qualitative SEM analysis revealed the
presence minor amounts of Sn and V in the TiO2 grains as well as minor Ti in the
cassiterite, further highlighting the close association between these Sn- and Ti-
bearing phases.
8.3.4 Precious Metal Mineralisation
Fifteen occurrences of native Au grains were located in the upper gossan
(Figures 8.8a to 8.10d). The grains rarely exceed a few micrometres in size and
exhibit subhedral and anhedral morphologies (Figures 8.8a to 8.10d). Ten native
Au grains were located in samples of the extensively oxidised and porous lower
portion of the upper gossan relative to only 5 grains in the upper portion.
The native Au grains may be firmly encapsulated within siderite (Figures 8.9a,
8.9b, 8.9c, 8.9d and 8.10b), or occur within cavities in the partially and
extensively oxidised matrix where they were probably originally associated with
Fe-sulphides (Figures 8.8a, 8.8b, 8.10a and 8.10c). A single occurrence of Au in
quartz (Figure 8.10d) was also observed. Native Au grains may be intimately
associated with galena (Figure 8.9b). The Ag content of the Au grains was below
the detection limit for EDX analysis (~0.5%). No discrete Ag-bearing grains were
located.
Due to the relatively small number of Au grains located during this study, it is
likely that a significant proportion of the Au content of this core may be present in
a sub-microscopic form.
Page 179
Chapter 8 Borehole CR191 - Sample Descriptions
8.4 Borehole CR191 - Middle Gossan
8.4.1 Introduction
This precious and trace metal-depleted horizon occurs at a depth of between
141.65 and 149.15 metres. The core is friable in nature but typically exhibits a
pale grey colour with only very localised ferruginisation (Figure 8.11). The middle
section of the gossan is distinctively pale relative to the upper gossan (Figure
8.4). This section of core was logged by the field geologist as ‘leached gossan’.
The Cu content of the middle gossan is low (<0.01–0.03%). No discrete Cu-
bearing phases were observed. The Pb (0.17–1.87%), Fe (1.99–5.97%) and S
(0.30–2.69%) contents are consistently lower than that of the upper gossan. The
Au (0.60–4.96ppm) and Ag (5.3–12.8ppm) contents are also significantly lower
than the upper gossan (Figure 8.1). The As (117–1269ppm), Bi (38–208ppm),
Hg (0.5–7.4ppm), Sb (252–1107ppm) and Sn (1017–3209ppm) contents are
moderate but also notably lower compared to the upper gossan. XRD analysis
confirms the presence of quartz, siderite, rutile, anatase, galena, greigite, calcite,
marcasite, native sulphur and lepidocrocite.
8.4.2 Gangue Mineralogy
The gangue mineralogy is essentially similar to the quartz-replaced tuffs of
CR038 and consists of a jumble of millimetres-sized quartz fragments in a finer-
grained matrix (Figures 8.12a, 8.12b and 8.13b). The aggregates of quartz are
commonly cemented by later stages of chalcedony that give the quartz a more
massive appearance when observed using SEM based techniques. The majority
of the quartz aggregates are angular or irregularly shaped. The quartz crystallite
size ranges from a few hundred micrometres to a few micrometres.
A subordinate portion of the quartz fragments are rounded. These rounded
'clasts' of quartz consist of polycrystalline aggregates that typically exhibit a
medium crystallite size. The rounded nature of these clasts suggests that they
have travelled some distance from their original source. Siderite may also
partially cement the angular quartz fragments and may partially replace the fine-
grained quartz-rich matrix (Figures 8.13a and 8.13b). Quartz veinlets may
Page 180
Chapter 8 Borehole CR191 - Sample Descriptions
traverse the fine-grained and porous quartz-rich portions of the core (Figure
8.12b). Euhedral pore spaces are common in the quartz (Figures 8.12a and
8.12b), reflecting the presence of former pyrite crystals that have been
subsequently oxidised and leached. Siderite, Fe-sulphides and Pb-rich
aggregates may fill or partially fill the pores (Figure 8.12a).
Siderite is locally abundant and is often associated with Fe-sulphides, galena,
PbSb-sulphides, cassiterite and TiO2 (Figure 8.13a). The siderite typically
exhibits varying degrees of oxidation and replacement by limonite (Figure 8.14a).
The siderite also occurs as euhedral crystals that have developed in cavities
(Figure 8.14b). The siderite crystals commonly exhibit compositional zoning that
largely reflects variations in FeO (46.7-53.2%) and CaO (0.7-6.6%) with the MgO
content being more consistent (4.1-4.8%). The quantitative SEM analyses are
provided in Appendix 5.
8.4.3 Ore Mineralogy
Fe-sulphides are present in relatively minor amounts and occur as granular
aggregates and euhedral crystals that are present within cavities between the
quartz fragments (Figure 8.14b) often in close association with siderite (Figure
8.13a). The Fe-sulphides may also exhibit some degree of oxidation and
replacement by goethite and lepidocrocite (Figure 8.14b). Marcasite and greigite
were positively identified by XRD analysis, although it is likely that other Fe-
sulphides are present.
Galena is a common accessory mineral and occurs as fine skeletal grains,
euhedral crystals and as porous aggregates that are disseminated throughout the
core. The galena typically contains minor amounts of Sb and As and may reflect
the presence of discrete Pb(SbAs)-sulphides, although the fine-grained nature of
the intergrowths inhibited positive identification. The galena-rich aggregates
typically occur in cavities and within siderite-rich aggregates (Figures 8.12a,
8.12b and 8.13a). Fine, needle-like crystals of a PbSb-sulphide are also
occasionally present in the siderite (Figure 8.13a).
Page 181
Chapter 8 Borehole CR191 - Sample Descriptions
Granular cassiterite and TiO2 are common accessory minerals, with discrete
grains rarely exceeding 50µm in size. The cassiterite and TiO2 occur within
siderite-rich aggregates and within the fine-grained and porous quartz-rich matrix
(Figures 8.13a and 8.13b). The cassiterite and TiO2 occur as rounded, sub-
rounded and irregularly shaped grains that probably represent both resistate and
neoformed phases (Figure 8.13b). Rutile and anatase were positively identified
by XRD techniques.
8.4.4 Precious Metal Mineralisation
Only two native Au grains were located. This is not surprising considering the
low Au content of this portion of the core. The Au grains were present as
inclusions in quartz and are Au-rich, with Ag being below detection limits (~0.5%)
(Figure 8.14a).
Page 182
Chapter 8 Borehole CR191 - Sample Descriptions
8.5 Borehole CR191 - Lower Gossan
8.5.1 Introduction
The lower gossan occurs at a depth of between 149.15 and 153.85 metres. The
core is friable in nature and exhibits a pale grey colour with some localised
ferruginisation (Figure 8.15). The lower gossan is similar in overall appearance
to the middle gossan. This section of core was logged by the field geologist as
‘leached gossan’.
The Cu content of the lower gossan is low (<0.01–0.01%). No discrete Cu-
bearing phases were observed. The Pb (0.37–0.93%) is also low relative to the
middle and upper gossans. The Fe (2.51–6.26%) and S (1.91–5.66%) contents
are notably lower than the upper gossan. The Au (0.85–10.74ppm) and Ag
(10.6–19.6ppm) contents are elevated relative to the middle gossan. The As
(396–495ppm), Bi (102–364ppm) and Sb (446–1301ppm) are similar to that of
the middle gossan. The Hg (7.7–22.5ppm) and Sn (882–6809ppm) contents are
relatively high. XRD analysis confirms the presence of quartz, galena, rutile,
anatase, marcasite, greigite, pyrite, native sulphur, lepidocrocite, jarosite and
szomolnokite, which is an oxidation product of pyrite.
8.5.2 Gangue Mineralogy
The lower gossan is quartz-rich. Transmitted light microscopy confirms the
quartz consists of angular, irregularly shaped and less commonly rounded grains
that are often cemented by later stages of chalcedony (Figures 8.18b, 8.18c,
8.19a and 8.19b). The wide variety of textures, size and morphology observed in
the quartz suggests that the fragments have derived from several sources. The
quartz fragments range in size from several millimetres (Figure 8.19a) to a few
micrometres.
The quartz commonly contains abundant, euhedral cavities that appear to
represent former sulphide minerals (Figures 8.16b, 8.17a and 8.17b). The
cavities may be partially filled by Fe-sulphides, galena and siderite (Figures
8.16a, 8.16b, 8.17a and 8.17b). Quartz veinlets also traverse the core (Figure
8.17a).
Page 183
Chapter 8 Borehole CR191 - Sample Descriptions
Fibrous textures are also a common feature of the quartz (Figure 8.18a). These
textures are particularly abundant in the large, irregularly shaped quartz
fragments (Figures 8.19a and 8.19b) and are also evident in the quartz veinlets.
Siderite is present in moderate amounts and represents a late-stage phase that
partially fills pores within the quartz-rich rocks (Figure 8.16a). Quantitative SEM
analyses (Appendix 5) confirm that the siderite typically contains minor amounts
of CaO (2.1-2.5%) and MgO (3.3-3.9%).
8.5.3 Ore Mineralogy
Fe-sulphides are present in minor amounts (Figure 8.16b) and occur as euhedral
crystals and granular aggregates within cavities in the quartz-rich rock fragments.
XRD and optical analysis confirms that the Fe-sulphides consist predominantly of
greigite that is often partially replaced by marcasite, although the nature of these
phases is fine-grained and complex. The Fe-sulphides may exhibit some degree
of oxidation and replacement by limonite (largely lepidocrocite). The Fe-
sulphides are often intimately intergrown with galena (Figure 8.16b). Pyrite
becomes increasingly abundant with increasing depth, largely at the expense of
greigite and marcasite (Figures 8.17a and 8.17b). Pyrite is also typically present
within cavities in the quartz fragments, or finely disseminated throughout the less
porous quartz-rich rocks (Figures 8.17a and 8.17b).
The cassiterite and TiO2 are finely disseminated throughout the core, occurring in
the fine-grained, quartz-rich matrix and throughout the more coherent quartz-rich
areas (Figures 8.17b, 8.20a, 8.20b and 8.21a). The cassiterite and TiO2 grains
range in size from a few micrometres to tens or hundred of micrometres (Figures
8.17b, 8.20a and 8.20b). The cassiterite may also occur as radiating aggregates
of euhedral crystals (Figure 8.21a). Cassiterite and TiO2 may be intimately
intergrown, possibly indicating neoformation of these phases (Figure 8.20b). The
cassiterite may also contain minor amounts of Ti in solid solution.
There is a common association between quartz, cassiterite, TiO2 and to a lesser
extent, zircon (Figures 8.20a and 8.20b). Examination of the quartz in thin
section confirms that the cassiterite and TiO2 are a component of the fine-grained
Page 184
Chapter 8 Borehole CR191 - Sample Descriptions
matrix that has been cemented by later stages of chalcedony, giving the
appearance that the cassiterite and TiO2 are inclusions. The quartz, cassiterite
and TiO2 appear to have a resistate component and a neoformed component
precipitated from solution.
Minor amounts of fine-grained cinnabar are also present in the lower gossan, and
typically occur within the less porous quartz fragments as finely disseminated
grains. Cinnabar may exhibit replacement relationships with cassiterite (Figure
8.20a). Barite is present in minor amounts.
8.5.4 Precious Metal Mineralisation
Only 3 microscopic native Au grains were located (Figures 8.21b, 8.22a and
8.22b). The largest Au grain exceeded 15µm in size (Figure 8.21b). The Au
grains exhibit subhedral (Figure 8.21b) and anhedral morphologies (Figures
8.22a and 8.22b) and are present in cavities in the quartz-rich aggregates
(Figures 8.21b, 8.22a and 8.22b). Minor amounts of sternbergite and
proustite/pyrargyrite were also present in close association with the largest Au
grain (Figure 8.21b). The Au grains were Ag-poor with only one of the grains
containing detectable amounts of Ag (Figure 8.21b).
Page 185
Chapter 8 Borehole CR191 - Sample Descriptions
8.6 Borehole CR191- Partial Massive Sulphide
8.6.1 Introduction
The partial massive sulphide occurs at a depth of 153.85 metres and is
characterised by a marked increase in the Fe (26.55-30.29%) and S (31.11-
35.50%) content relative to the overlying leached gossan. The Cu (0.33-1.18%),
As (2408-3965ppm) and Sn (603-1130ppm) contents are moderate. The Ag
content (3.9-91.5ppm) exhibits a marked increase at the upper end of the
massive sulphide, close to the contact with the gossan. The Au (0.37-2.73ppm),
Pb (0.05-0.17%), Bi (44-63ppm), Hg (9.2-10.8ppm) and Sb (239-369ppm)
content is relatively low. XRD analysis confirms the presence of quartz, pyrite,
tennantite and chalcopyrite.
8.6.2 General Mineralogy
Quartz is the dominant gangue mineral (Figures 8.23a, 8.23b, 8.24a and 8.24b).
The quartz is present along fractures in the larger pyrite aggregates (Figure
8.23a) and also occurs as granular aggregates that form porous, rubble-like
intergrowths with pyrite (Figures 8.23b, 8.24a and 8.24b). The quartz fragments
rarely exceed 500µm in size. The quartz often exhibit euhedral morphologies
indicative of growth within open pores. Examination of the quartz in transmitted
light confirms that it exhibits a medium to coarse-grained crystallite size. Fibrous
quartz is also present along the margins of the pyrite and within fractures.
Pyrite is the dominant sulphide mineral and occurs as granular aggregates
(Figures 8.23a and 8.24a), angular fragments (Figure 8.23b) and euhedral
crystals (Figure 8.24b). The pyrite may be extensively fractured (Figures 8.23a
and 8.24a). The pyrite may also exhibit primary textural features that typically
exhibit some degree of recrystallisation. Enargite is a common accessory
mineral and typically occurs along the margins of the quartz and pyrite grains
(Figure 8.24a). Quantitative SEM analyses of the enargite are provided in
Appendix 5 (analyses #8 to #11) and confirm that the enargite consists
predominantly of Cu (40.4 - 42.2%), As (15.9 - 18.5%) and S (31.8 - 32.4%).
However, the enargite differs from those analysed in previous samples as it
contains moderate but significant amounts of Fe (7.0 - 7.8%) and Sb (0.3 - 3.7%).
Page 186
Chapter 8 Borehole CR191 - Sample Descriptions
Tennantite and chalcopyrite were also detected using XRD techniques. Granular
cassiterite aggregates are common in the partial massive sulphide and are often
intimately associated with pyrite (Figure 8.24b). TiO2 is finely disseminated
throughout the porous, rubble-like quartz and pyrite aggregates (Figure 8.24b).
Page 187
Chapter 8 Borehole CR191 - Sample Descriptions
8.7 Borehole CR191 – Summary Diagram
Section 8.3: The Tertiary Conglomerate/Gossan Contact is characterised by very low levels of Cu, Pb, S, Ag, Au, As, Bi, Hg, Sb and Sn and consists predominantly of fragmented quartz together with subordinate amounts of siderite and galena-rich aggregates.
Section 8.4: The Upper Gossan is characterised by elevated of Pb, S, Fe, Ag, Au, As, Bi, Hg, Sb and Sn consists predominantly of siderite together with subordinate amounts of limonite, Fe-sulphides and galena/PbSb-sulphide. Cassiterite is disseminated throughout the upper gossan and accounts for the high Sn content. The small number of very fine-grained native Au grains suggest that the bulk of the Au is present in a sub-microscopic forms.
Section 8.5: The Middle Gossan is depleted in Cu, Pb, Fe, S, Ag, Au, As, Bi, Hg, Sb and Sn reflecting the quartz-rich nature of the core and relative paucity of those minerals that occur in abundance in the upper gossan.
Section 8.6: The Lower Gossan is characterised by elevated levels of Au, Hg, Ag and Sn. Quartz is the dominant mineral, together with subordinate amounts of Fe-sulphide, siderite and galena. Accessory minerals include cassiterite, TiO2, cinnabar and sternbergite. The bulk of the Au is present as microscopic and possibly sub-microscopic grains.
Section 8.7: The Partial Massive Sulphide consists predominantly of pyrite and quartz together with subordinate amounts of cassiterite, TiO2, enargite and tennantite.
Figure 8.25 - Diagram illustrating the key mineralogical features for the 'Tertiary Conglomerate/Gossan Contact', ‘Upper Gossan', 'Middle Gossan', 'Lower Gossan’ and ‘Partial Massive Sulphide’.
Page 188
Chapter 9 Borehole CR123 - Sample Descriptions
9 BOREHOLE CR123 – SAMPLE DESCRIPTIONS
9.1 Introduction
Chapter 9 describes the chemistry and mineralogy of borehole CR123. The
sample suite, field geologists' core log lithocodes and lens descriptions are
provided in Appendix 2. Section 9.2 describes the major and minor element
chemistry. Section 9.3 describes the mineralogy of the ‘Tertiary Polymict
Conglomerate'. The mineralogy of the gossan is described in Sections 9.4
(Upper Siderite Gossan), 9.5 (Middle Calcite Gossan) and 9.6 (Lower Siderite
Gossan). The ‘Gossan Contact with Shale Conglomerate’ and ‘Shale
Conglomerate Contact with Gossan’ are described in Sections 9.7 and 9.8
respectively. Section 9.9 describes the mineralogy of the ‘Partial Massive
Sulphide/Shale’. A summary diagram is provided in Section 9.10.
Borehole CR123 was selected for examination because of the extensive Au
mineralisation and the marginal position relative to the main supergene massive
sulphide mineralisation. The location of borehole CR123 is illustrated in Chapter
3. This borehole is a vertical hole. The precious metal mineralisation extends for
a depth of approximately 20 metres. The core is often friable in nature and
recoveries are poor.
Sample selection begins in the Tertiary conglomerate overlying the gossan and
extends into the first three intersections of the underlying shale. Borehole CR123
intersects the fossil gossan at a depth of 153.95 meters. The pyritised shale is
intersected at a depth of 176.00 metres. The characterisation was based on the
examination of 60 polished sections and 5 thin sections.
Page 189
Chapter 9 Borehole CR123 - Sample Descriptions
9.1 Borehole CR123 - Chemistry
The chemistry of borehole CR123 is variable, marking the prominent boundary
between the Tertiary conglomerate, gossan and shale. The assay data are
provided in Appendix 3. A diagram of borehole CR123 and the major and minor
element graphs are provided in Figure 9.1.
Page 190
Chapter 9 Borehole CR123 - Sample Descriptions
Figure 9.1 - Diagram illustrating chemistry variations in borehole CR123. The sample intervals examined from borehole CR123 consist of Tertiary polymict conglomerate, gossan and quartz-rich shales that have been partially replaced by pyrite. The sample intervals are displayed on the left of the illustration, together with the lithocode as detailed in Appendix 2. A marked change in the chemistry of the borehole is evident at the contact between the shales and the gossan. The sample intervals examined and the sections of this thesis in which the mineralogy is described are provided on the right of the illustration. The borehole depths are equivalent to depth from surface. TCP - Tertiary Polymict Conglomerate, GMS - Strong Magnetic Gossan, QXM - Quartz Replacement of Massive Shale, SXM - Massive Shale, EQU – Quartz Vein.
Page 191
Chapter 9 Borehole CR123 - Sample Descriptions
9.1.1 Geochemical Profile
The Cu content of the Tertiary conglomerate and gossan is low. The Cu values
are marginally higher in the underlying shales The Pb content of the Tertiary
conglomerate is low The available information suggests that the Pb content
varies significantly over relatively short distances down hole and peaks in the
central portion of the gossan. The elevated levels of Pb are associated with a
similar peak in the Fe content, with a less prominent rise in the S and Sb contents
and a decrease in the As content. The Pb content of the core rises again at the
contact between the gossan and underlying shale. This increase is associated
with a similar increase in the Au, Ag, Bi, Sn, Sb and Hg content. The Pb content
of the shale is low.
The Fe content of the Tertiary conglomerate is relatively high. The gossan
contains variable amounts of Fe. Some relationship between Fe and S is also
evident. The Fe content of the shale is also highly variable and increases
significantly in the middle portion of the shale, exhibiting a strong correlation with
S. Sulphur follows a similar pattern to Fe throughout the borehole, occurring in
moderate but variable amounts in the Tertiary conglomerate, gossan and shale.
Silver is present in minor amounts throughout the Tertiary conglomerate, gossan
and shale. The geochemical profile of Ag, although not particularly clear from the
diagram, follows a similar pattern to that of Hg. The Ag content reaches a
maximum value at the contact between the gossan and shale. Au is also present
in moderate amounts throughout the lower portion of the Tertiary conglomerate
and into the gossan, where it reaches a maximum value at the contact between
the gossan and shale. The Au content of the shale is low.
The As content of the Tertiary marl is low, but occurs in significant amounts in the
gossan and moderate amounts in the shale. Arsenic is not strongly correlated
with the other analysed elements. Bismuth follows a similar pattern of
abundance to Au, Ag, Sn, Hg and Sb. Bismuth is present in moderate amounts
in the lower portion of the shale.
Page 192
Chapter 9 Borehole CR123 - Sample Descriptions
9.2 Borehole CR123- Tertiary Polymict Conglomerate
9.2.1 Introduction
The Tertiary conglomerate is situated directly above the gossan and occurs in the
upper portion of the 152.40 to 153.95 metres sample interval. This sample is
relatively poorly mineralised and contains minor amounts of Pb (1.12%) and Cu
(0.08%). The Fe (13.95%) and S (7.74%) contents are moderate. Minor but
significant amounts of Au (2.77ppm) and Ag (2.5ppm) are present. The As
(558ppm), Bi (148ppm), Sb (270ppm) and Sn (86ppm) contents are moderate.
The Hg content is low (0.8ppm). XRD analysis confirms the presence of quartz,
calcite, pyrite, glauconite, plagioclase and rutile.
9.2.2 Gangue Mineralogy
The Tertiary polymict conglomerate (Figure 9.2a) consists predominantly of dark
green, rounded, poorly crystalline glauconite aggregates (ideally (K,Na)
(Fe3+,Al,Mg)2(Si,Al)4(OH)2) and angular quartz and feldspar fragments in a calcite-
rich matrix (Figures 9.2b and 9.3b). The glauconite exhibits a size range of
between 200µm and 60µm (Figure 9.2b and 9.3b).
Quartz is abundant and occurs as angular and irregularly shaped grains that
range in size from a few micrometres to several millimetres (Figures 9.2a, 9.3a
and 9.3b). The bulk of the quartz fragments are monocrystalline, with a small
proportion consisting of polycrystalline aggregates with discrete crystallites of
only a few micrometres in size. The quartz is typically cemented by calcite
(Figures 9.2a, 9.3a and 9.3b). The quartz presumably forms a component of the
original sediment, along with the glauconite, feldspars and shell debris.
Plagioclase and K-feldspar are subordinate in abundance to quartz and also
typically occur as angular and irregularly shaped grains that range in size from a
few micrometres to fragments that exceed 200µm (Figures 9.4a and 9.4b).
Calcite occurs as angular fragments that exhibit similar grain size and textures as
the quartz and feldspar (Figure 9.3a). Calcite also occurs in the form of shell and
coral fragments that commonly exceed 500µm in size (Figure 9.3a). A significant
Page 193
Chapter 9 Borehole CR123 - Sample Descriptions
portion of the calcite occurs as fine-grained, crystalline cement that binds the
glauconite, quartz, calcite and feldspar fragments (Figures 9.2b, 9.3a, 9.3b, 9.4a
and 9.4b). Qualitative SEM analysis confirms that the calcite is often
compositionally zoned, representing the presence of minor amounts of Fe, Mn
and Mg.
9.2.3 Ore Mineralogy
Pyrite is the dominant sulphide mineral and occurs as granular aggregates and
as partially recrystallised framboidal aggregates (Figures 9.3a, 9.3b and 9.4a).
The pyrite also occurs as acicular crystals that probably represent pseudomorphs
after marcasite or pyrrhotite (Figure 9.4a). The pyrite typically occurs along the
margins of the glauconite, quartz, feldspar and calcite fragments and may
develop within cavities or form overgrowths on the common gangue minerals
(Figures 9.3a, 9.3b and 9.4a). The pyrite exhibits compositional zoning, reflecting
the presence of minor amounts of As. Galena is present in very minor amounts
and occurs as finely disseminated euhedral crystals (Figures 9.3a, 9.4a and 9.4b)
and as porous aggregates.
9.2.4 Accessory Mineralogy
Minerals observed in minor amounts include apatite, chlorite, a CaAl-phosphate
(probably crandallite, ideally CaAl3(PO4)2(OH)5.H2O) and TiO2. A single grain of
native Au was also observed. This grain is present along the margins of an albite
fragment, within the calcite-rich cement (Figure 9.4b). Qualitative SEM analysis
confirmed the presence of minor amounts of Ag and Cu within the native Au
grain.
Page 194
Chapter 9 Borehole CR123 - Sample Descriptions
9.3 Borehole CR123 - Upper Siderite Gossan
9.3.1 Introduction
The contact between the gossan and Tertiary conglomerate was not well
preserved. The upper siderite gossan commences in the lower portion of the
152.40 metre interval and extends into the 154.85 metre interval. The gossan is
characterised by a marked increase in the Pb (16.49%) content and a significant
increase in the Au (4.47ppm), Ag (47.9ppm), As (2075ppm), Bi (338ppm), Sb
(1319ppm), Hg (66.3ppm) and Sn (388ppm) contents relative to the poorly
mineralised Tertiary conglomerate. The Fe (10.79%) and S (2.76%) contents are
lower than the Tertiary conglomerate. The Cu content of this sample is low
(0.2%). XRD analysis confirms the presence of siderite, anglesite, cerussite,
hematite, galena, pyrite, calcite, greigite and native sulphur.
9.3.2 General Mineralogy
This sample interval is similar to the siderite-rich core described for previous
boreholes and exhibits a distinctive reddish brown colour in hand specimen. It is
porous and friable and consists predominantly of siderite together with
subordinate amounts of Fe-sulphide and galena (Figures 9.5b and 9.6a). The
siderite is often extensively oxidised and replaced by limonite (largely hematite)
(Figures 9.5b, 9.6a and 9.8a). The oxidation of the siderite has resulted in a
volume change and increase in porosity of the core. Later stages of relatively
unoxidised siderite mineralisation are present locally. Quantitative SEM analyses
confirm that the siderite typically contains moderate amounts of CaO (6.7-8.0%)
and lesser MgO (3.0-4.8%). Minor amounts of MnO are also present locally in
the siderite (maximum 0.8% MnO).
Fe-sulphides are the dominant sulphide and occur predominantly as aggregates
of plate-like crystals (basal sections) within cavities and along the margins of the
siderite grains and aggregates (Figures 9.6a and 9.8b). The Fe-sulphide crystals
rarely exceed a few micrometres in thickness but may exceed 100µm in length
(Figures 9.6b and 9.8b). Granular aggregates and euhedral crystals of Fe-
sulphide are also present locally. The Fe-sulphides, at least in part, are strongly
Page 195
Chapter 9 Borehole CR123 - Sample Descriptions
magnetic. Greigite and pyrite were identified by XRD techniques. The Fe-
sulphides are described in greater detail in Chapter 10.
Galena occurs as fine-grained and porous aggregates and tiny stringer-like
veinlets in cavities and along the margins of the porous siderite aggregates
(Figures 9.7a, 9.7b, 9.8a and 9.8b). XRD confirms the localised oxidation of
galena to anglesite and cerussite.
9.3.3 Precious Metal Mineralisation
A small number of native Au grains were located (Figures 9.6b, 9.7a, 9.7b, 9.8a
and 9.8b) and are typically present in the porous, oxidised siderite in close
association with galena. The Au grains are typically subhedral and fine-grained
in nature, with the largest grain exceeding 25µm (Figure 9.7b). The native Au
grains exhibit a deep yellow colour when observed in reflected light (Figure 9.6b),
indicative of a low Ag content. Qualitative SEM analysis confirms that the Ag
content is typically less than 0.5 weight percent.
Page 196
Chapter 9 Borehole CR123 - Sample Descriptions
9.4 Borehole CR123 - Middle Calcite Gossan
9.4.1 Introduction
This zone is situated directly below the siderite-rich gossan and occurs at a depth
of 157.05 metres, extending for approximately 3 metres where the core once
again becomes progressively more siderite-rich. The core consists of both
competent and more friable, rubble-like material. The core exhibits a distinctive
dark grey/black metallic appearance that is largely due to the presence of
abundant galena and Fe-sulphides. The presence of Fe-sulphides accounts for
the magnetic properties of the core. Core recoveries were extremely poor.
The Cu content of the core is low (0.03%-0.04%). The Pb (5.46-9.20%), Fe
(11.24-13.75%) and S (5.29-7.11%) contents are moderate and largely reflect the
presence of galena and Fe-S. The Ag (13.6-35.6ppm), Au (2.08-2.27ppm), As
(5136-5839ppm), Bi (348-403ppm), Sb (1413-1809ppm) and Sn (382-471ppm)
contents are also moderate. The Hg (8.8-20.0ppm) content is low. XRD analysis
confirms the presence of pyrite, galena, calcite, marcasite, harmotome, cerussite
and anglesite.
9.4.2 Gangue Mineralogy
Calcite is the dominant gangue mineral and occurs as angular fragments and
granular aggregates that are intimately intergrown with galena and pyrite (Figures
9.9a through to 9.14b). The calcite occurs predominantly as angular fragments
that have been cemented or partially cemented by later stages of calcite
mineralisation (Figures 9.9a, 9.9b and 9.10b). The fragmented nature of the
calcite is more evident where galena and pyrite occur along the margins of the
calcite fragments (Figures 9.9a, 9.9b, 9.10b and 9.13a). Discrete calcite
fragments exceed 500µm (Figure 9.10b).
A subordinate portion of the calcite occurs as narrow veinlets that traverse the
core (Figure 9.10a). These veinlets may be several hundred micrometres in
width. These textures are similar to those observed between galena and siderite
in previous boreholes.
Page 197
Chapter 9 Borehole CR123 - Sample Descriptions
The zeolite mineral harmotome (ideally (Ba,K)(SiAl)9O16.6H2O) is locally abundant
and occurs as radiating aggregates of acicular crystals in calcite. The
harmotome aggregates commonly exceed several hundred micrometres in size
(Figures 9.13a, 9.13b, 9.14a and 9.14b). The harmotome is often partially
replaced by pyrite (Figure 9.14a). The presence of harmotome was confirmed by
XRD. Quartz is present in very minor amounts in the calcite-rich gossan,
occurring as irregularly shaped grains that are disseminated throughout the
calcite (Figure 9.11b).
9.4.3 Ore Mineralogy
Galena is the dominant Pb-bearing sulphide mineral and occurs as fine-grained,
porous and skeletal aggregates that occur within cavities, along the margins of
the calcite grains and in calcite veinlets (Figures 9.9 and 9.10). XRD confirms the
localised oxidation of galena to anglesite and cerussite. A single occurrence of
native Bi was observed within the galena.
Pyrite is abundant and typically occurs as granular and/or porous aggregates that
are present along the margins of the calcite grains and partially fill cavities
(Figures 9.9a, 9.10a and 9.10b). The pyrite appears to replace porous Fe-
sulphide aggregates that are too fine grained and poorly crystalline for a positive
identification (Figures 9.12a and 9.12b). Fe-sulphides (excluding pyrite) are
present in subordinate amounts to the pyrite and occur as extremely porous
aggregates that are present along the margins of the calcite grains.
9.4.4 Precious Metal Mineralisation
No discrete Au-bearing grains were located during this investigation, suggesting
that the Au may be present in a sub-microscopic form.
Page 198
Chapter 9 Borehole CR123 - Sample Descriptions
9.5 Borehole CR123 - Lower Siderite Gossan
9.5.1 Introduction
This zone occurs at a depth of approximately 160.2 metres and extends for
approximately 3 metres. The core recovery below this zone was extremely poor
and no information is available on the nature of the core between 163.4 and
168.2 metres.
The core consists of centimetre-sized dark grey/black metallic fragments that are
extensively fractured and cemented by distinctive red-brown siderite (Figure
9.15). These fragments are essentially similar to the calcite/sulphide mineral
assemblage described for the calcite-rich gossan.
The Cu content is low (0.01–0.12%). The Pb (8.90–27.23%) and Fe (19.63–
32.64%) contents exhibit a marked increase relative to the overlying core. The S
(2.87–8.46%) content is also marginally higher than the calcite-rich gossan. The
Au (1.47–2.14ppm) and Ag (16.4–20.4ppm) contents are similar to the calcite
gossan, suggesting that the Au mineralisation is not directly associated with the
late-stage siderite that cements the calcite-rich fragments in this core. The Bi
(275–374ppm), Hg (5.4–9.1ppm), Sb (1481–2138ppm) and Sn (270–356ppm)
contents are also similar to the calcite gossan although the As (396–495ppm) is
significantly lower. XRD analysis confirms the presence of siderite, calcite,
galena, pyrite, greigite, marcasite, anglesite and native sulphur. The native
sulphur appears to be intimately associated with the oxidation of greigite.
9.5.2 Gangue Mineralogy
Siderite is abundant and cements the fractured calcite/sulphide fragments
(Figures 9.15 and 9.16a). The siderite exhibits a high degree of crystallinity and
may also exhibit some localised degree of oxidation and replacement by limonite
(Figure 9.19a). The siderite is often intimately associated with fine-grained,
skeletal galena (Figure 9.17a and 9.17b) as well as a number of other Pb-bearing
phases, including cerussite (ideally PbCO3) and mimetite (ideally Pb5(AsO4)3Cl)
(Figures 9.18a and 9.18b). The siderite also appears to partially replace barite
(Figures 9.18a and 9.19a).
Page 199
Chapter 9 Borehole CR123 - Sample Descriptions
Siderite may also form delicately banded botryoidal aggregates, indicative of
precipitation within open cavities. At least two stages of siderite mineralisation
are evident, with early-formed, often oxidised siderite occurring in close
association with later stages of unoxidised siderite. This is particularly evident in
Figure 9.19a, where siderite has extensively replaced barite and subsequently
been fragmented, reworked or subjected to dissolution, possibly more than once,
and recemented by later siderite mineralisation. The rounded morphology of the
siderite/barite ‘clasts’ may be indicative of dissolution rather than mechanical
transportation.
Compositional zoning is relatively common in the siderite, particularly in the later
stage mineralisation, notably the veinlets (Figure 9.18b). Quantitative SEM
analysis of the siderite confirms that it contains minor amounts of CaO (0.9-1.1%)
and MgO (1.2-1.7%) (see Appendix 5).
Calcite is common, but subordinate in abundance relative to siderite, occurring
predominantly as granular aggregates that are intimately associated with galena
and Fe-sulphides (largely pyrite) (Figures 9.16a and 9.16b). Nontronite is a
common accessory, occurring as a cavity filling, closely associated with siderite
(Figures 9.17a and 9.17b).
Barite is relatively common, forming coarse-grained aggregates that may exceed
several hundred micrometres in size (Figure 9.19a) The barite exhibits highly
irregular morphologies that appear to be the result of extensive dissolution and
replacement by siderite (Figures 9.18a and 9.19a).
9.5.3 Ore Mineralogy
Galena is the dominant sulphide mineral and occurs within the metallic grey/black
fragments associated with calcite, Fe-sulphides/pyrite (Figures 9.16a, 9.16b and
9.19b) and siderite (Figures 9.17a, 9.17b, 9.18a, 9.18b and 9.19a). The galena
typically occurs along the margins of the calcite grains as fine-grained and porous
aggregates of skeletal crystals, often infilling cavities (Figures 9.17a, 9.17b, 9.18a
and 9.18b). Discrete galena grains rarely exceed a few micrometres in size.
Galena may exhibit localised oxidation to anglesite.
Page 200
Chapter 9 Borehole CR123 - Sample Descriptions
Fe-sulphides are abundant, are typically fine-grained and porous in nature and
also occur as granular aggregates (Figures 9.16a and 9.19b). The Fe-sulphides
exhibit less replacement by pyrite than the calcite-rich gossan. The Fe-sulphides,
at least in part, account for the magnetic properties of this portion of the core.
9.5.4 Precious Metal Mineralisation
No discrete precious metal-bearing phases were located during this investigation
and it is assumed that the bulk of the Au may be present in a sub-microscopic
form.
Page 201
Chapter 9 Borehole CR123 - Sample Descriptions
9.6 Borehole CR123- Gossan/Shale Conglomerate Contact
9.6.1 Introduction
This sample interval occurs at a depth of 168.20 to 169.00 metres and consists of
varying proportions of fine-grained galena and Fe-sulphide, together with
subordinate amounts of transparent gangue (Figures 9.20a to 9.23b). The core
is porous in nature. The gossan/shale conglomerate contact is characterised by
a marked increase in the Ag (69.7ppm) and Au (31.85ppm) contents. The Bi
(1578ppm), Hg (1160ppm), Sb (4536ppm) and Sn (1100ppm) contents are
markedly higher than the previous sample intervals. The As (2002ppm) and Cu
(0.09%) contents are similar to the siderite-rich gossan. The bulk of the S
(4.12%), Pb (6.4%) and Fe (9.60%) are present in the form of galena and Fe-
sulphides. XRD analysis confirms the presence of calcite, anglesite, galena,
pyrite, gypsum, quartz, cerussite, marcasite, greigite and barite. Gypsum
appears to represent a reaction/oxidation product associated with the Fe-
sulphides and calcite.
9.6.2 General Mineralogy
The contact zone consists of fragmented calcite aggregates that exhibit partial
and extensive replacement by galena and Fe-sulphides (Figures 9.20a, 9.22a
and 9.22b). Cerussite is also common and typically occurs as extensively
corroded aggregates that are intimately intergrown with galena and Fe-sulphide
(Figure 9.20b). The irregular morphology of the cerussite is indicative of
dissolution. Qualitative SEM analysis also revealed the presence of minor
amounts of Sr in the cerussite. Subordinate amounts of dolomite are also
present in this portion of the core (Figure 9.20b).
Galena is the dominant Pb-bearing phase and may extensively replace the
transparent gangue (largely calcite) (Figure 9.20a). The galena typically occurs
as fine-grained and porous aggregates (Figures 9.20a, 9.20b, 9.22a and 9.23b)
and as micrometre-sized skeletal crystals (Figure 9.22b). The skeletal galena
also occurs in narrow veinlets that traverse the core (Figures 9.21a and 9.21b).
Siderite is notably absent in the core. The galena may exhibit localised oxidation
Page 202
Chapter 9 Borehole CR123 - Sample Descriptions
to anglesite. Qualitative SEM analysis revealed minor amounts of Sb within a
number of the galena aggregates.
Fe-sulphides are common and occur as acicular crystal and as porous and
granular aggregates that typically exhibit partial replacement by pyrite and
marcasite (Figure 9.22a). Greigite, marcasite and pyrite were identified by XRD.
A proportion of the Fe-sulphides are magnetic.
Sternbergite is the dominant Ag-bearing mineral and is intimately intergrown with
galena and Fe-sulphides (Figures 9.21a, 9.21b, 9.22a and 9.23b). Unresolved
intergrowths of Ag, Sn, Se, Pb, As, Hg and S-bearing phases are also present in
this core (Figures 9.23a and 9.23b)
Two native Au grains were located (Figure 9.23a), occurring as fine-grained
(<20µm) subhedral grains that are intimately associated with the galena
aggregates. Semi quantitative SEM analysis of these grains confirmed that they
contain approximately 20 weight percent Ag. Due to the paucity of Au grains
relative to the high Au content, it is assumed that the bulk of the Au is present in
a sub-microscopic form, probably associated with galena.
Page 203
Chapter 9 Borehole CR123 - Sample Descriptions
9.7 Borehole CR123 – Shale Conglomerate/Gossan Contact
9.7.1 Introduction
This sample interval occurs at a depth of between 169.00 and 172.85 metres and
is markedly different from the gossan/shale conglomerate contact. This zone
contains significant amounts of Pb (6.94-13.95%), Fe (9.59-16.18%) and S
(16.03-21.75%). The Ag (175.3-181.0ppm) and Hg (3061-9525ppm) contents
are high and exhibit a marked increase relative to the overlying gossan. The Au
content is high (11.68-56.55ppm), although very few Au grains were located. The
Bi (758-1920ppm), Sb (801-4536ppm) and Sn (623-1437ppm) contents are
similar to the previous sample. The As content (174-210ppm) is notably lower
than the previous sample, whereas the Cu content (0.45-1.89%) is markedly
higher. XRD analysis confirms the presence of calcite, galena, anglesite,
gypsum and pyrite.
9.7.2 Transparent Gangue
The core consists of angular, millimetre-sized fragments of quartz and to a lesser
extent calcite that are present in a matrix of fine-grained calcite, subordinate
quartz and a host of sulphide minerals (Figures 9.24a, 9.24b, 9.24c, 9.25a and
9.26a). Examination of the quartz-rich clasts in transmitted light confirms that
they typically contain fine-grained crystallites that exhibit some degree of
replacement by calcite (Figure 9.27a).
A proportion of the quartz is also associated with a pyritisation event, where
euhedral crystals of pyrite and fibrous quartz replace the fine-grained and porous
quartz-rich matrix (Figures 9.27b, 9.28a and 9.28b). The fibrous quartz has been
encountered in most of the boreholes examined during this investigation,
occurring predominately as reworked fragments associated with euhedral voids.
The quartz fragments in the upper part of the conglomerate are typically pale
grey in colour (Figures 9.24a and 9.24b). The quartz within the lower part of the
conglomerate is typically dark grey, similar in appearance to the underlying
shales (Figure 9.24c). The pale colour is probably largely due to acid leaching of
the quartz.
Page 204
Chapter 9 Borehole CR123 - Sample Descriptions
9.7.3 Pyrite
The relative proportions of pyrite and transparent gangue vary considerably. The
pyrite occurs along the margins of the calcite grains and is often associated with
one or more of cinnabar, proustite/pyrargyrite, pyrite and sternbergite (Figures
9.25b, 9.30a and 9.30b). The pyrite may be locally abundant, occurring as
massive sulphide fragments that exhibit partial replacement by galena, secondary
Cu-sulphides and Hg-tetrahedrite (Figures 9.29a and 9.29b).
9.7.4 Cinnabar and Sulphosalt Minerals
Cinnabar is a common accessory and typically occurs within the fine-grained
calcite matrix exhibiting a highly irregular morphology, probably indicating
dissolution and replacement (Figures 9.25a, 9.25b, 9.26a, 9.26b, 9.30a and
9.30b). Cinnabar aggregates may exceed 1mm (Figure 9.26). The cinnabar
aggregates are often intimately intergrown with proustite/pyrargyrite, pyrite and
sternbergite (Figures 9.25a, 9.25b, 9.26a, 9.26b, 9.30a and 9.30b). Minor
amounts of stibnite (Figure 9.26b) and native Au (Figures 9.26b and 9.31a) were
also observed in the cinnabar.
A host of other very fine-grained Ag, Pb, Cu, Hg, Se, Sn, Bi and As-bearing
phases were located but not positively identified, including a CuBi-sulphide
(possibly wittichenite, ideally Cu3BiS3) and a Hg-Se-sulphide (possibly
metacinnabar, ideally Hg(Se,S)). These phases typically occur within the calcite
matrix and within cavities associated with the massive sulphide fragments.
9.7.5 Precious Metal Mineralisation
Proustite/pyrargyrite and sternbergite host the bulk of the Ag content, occurring
within the fine-grained calcite matrix associated with cinnabar and pyrite (Figures
9.25b, 9.26b, 9.30a and 9.30b). Quantitative SEM analyses (Appendix 5,
analyses #8 to #10) of the proustite (Ag 66.8%; Sb <0.5%; As 14.0% and S
18.1%) and pyrargyrite (Ag 60.1 - 61.1%; Sb 16.1 - 18.3%; As 5.1%; S 16.4 -
16.7%) confirm that they typically exhibit near end-member compositions.
Page 205
Chapter 9 Borehole CR123 - Sample Descriptions
Only two discrete native Au grains were observed, including one occurrence
within a large cinnabar aggregate (Figures 9.26b and 9.31a) and a one in
association with calcite, galena and Hg-tetrahedrite (Figure 9.31b). The native
Au grains are less than 20µm in size and exhibit anhedral morphologies.
Semi-quantitative SEM analysis of the Au confirmed approximately 15 weight
percent Ag in the grain associate with cinnabar and approximately 30 weight
percent Ag in the grain associated with galena and tetrahedrite. The paucity of
microscopically visible Au grains suggests the bulk of the Au is probably in a
submicroscopic form.
Page 206
Chapter 9 Borehole CR123 - Sample Descriptions
9.8 Borehole CR123 – Partial Massive Sulphide/Shale
9.8.1 Introduction
This sample interval occurs at a depth of 176.00 metres and extends for
approximately 5 metres to a depth of 181.50 metres, below which the shale is
largely unmineralised. The Cu (0.58-2.56%) content is variable, but significant.
The Fe (3.67-21.51% and S (4.11-26.98%) contents are variable and largely
reflect local variations in the degree of pyrite replacement of the shale. The Pb
(0.22-0.72%) content is low. The Au (<0.01-1.49ppm) content is significantly
lower than the previous sample interval. The Ag (1.4-115.1ppm) content is
moderately high. The As (63-846ppm), Bi (24-236ppm), Hg (0.8-134.2ppm), Sb
(171-509ppm) and Sn (38-135ppm) contents are all markedly lower than the
previous sample interval. XRD analysis confirms the presence of quartz, galena,
pyrite and covellite.
9.8.2 General Mineralogy
The upper portion of this sample interval is fragmented and consists of dark
grey/black shale-like rock fragments that have been partially replaced by pyrite
and late-stage quartz. The core becomes less fragmented with increasing depth
and the degree of pyritisation generally increases. The shale typically exhibits a
high degree of porosity (Figure 9.32a). The porosity decreases in areas of pyrite
and quartz replacement (Figures 9.32a and 9.33a). Examination of the quartz in
transmitted light confirms that it is fibrous in nature (Figure 9.33b). The bulk of
the fine-grained quartz matrix exhibits primary sedimentary layering and
represents an original component of the shale.
Pyrite is the dominant sulphide mineral and occurs as euhedral crystals and
granular aggregates that are disseminated throughout the shale. The pyrite is
typically intergrown with fibrous quartz and commonly exhibits a preferred
orientation (Figure 9.32b). Partially recrystallised framboidal pyrite is also a
common feature (Figure 9.34a). The pyrite may exhibit some degree of
replacement by galena and secondary Cu-sulphides (largely covellite) (Figures
9.32a and 9.34a). Minor amounts of TiO2 and carbon are also present (Figure
9.34b).
Page 207
Chapter 9 Borehole CR123 - Sample Descriptions
9.9 Borehole CR123 – Summary Diagram
Section 9.3: The Tertiary Polymict Conglomerate is poorly mineralised and consists of glauconite, quartz and feldspar and minor amounts of pyrite and galena. Native Au grains are rare.
Section 9.4: The Upper Siderite Gossan exhibits a marked increase in the Pb, Au, Ag, As, Bi, Sb, Hg and Sn contents and consists predominantly of extensively oxidised siderite and subordinate amounts of Fe-sulphide and galena. Native Au grains are rare.
Section 9.5: The Middle Calcite Gossan is characterised by high Pb, Fe and S contents, largely reflecting the presence of galena and Fe-sulphides. Calcite is the dominant gangue mineral. Accessory minerals include harmotome, quartz and bismuth. The Ag, Au, As, Bi, Sb and Sn contents are moderate. The Cu and Hg contents are low. No discrete Au grains were located.
Section 9.6: The Lower Siderite Gossan consists of fragments of calcite-rich gossan that have been extensively fragmented and cemented by later stage siderite. Siderite, Fe-sulphides and galena are abundant and host the bulk of the Fe, S and Pb content. Calcite is common. The Bi, Hg, Sb, Sn, Ag and Au contents are similar to the calcite-rich gossan. The As content is significantly lower. Accessory minerals include nontronite, barite, mimetite and cerussite. The Cu content is low. No discrete Au grains were located
Section 9.7: The Gossan/Shale Conglomerate Contact is essentially similar to the calcite-rich gossan and is characterised by elevated Ag, Au, Bi, Hg, Sb and Sn contents. Galena and Fe-sulphides host the bulk of the S, Pb and Fe. The As content is moderate and the Cu content is low. Accessory minerals include cerussite, dolomite, sternbergite and Ag-Sn-Se-Pb-As-Ag-Hg sulphosalts. Native Au grains are rare.
Section 9.8: The Shale Conglomerate/Gossan Contact consists of angular, millimetre-sized fragments of massive sulphide, pyritised shale, quartz and calcite in a matrix of fine-grained calcite and quartz. The bulk of the Pb, Fe and S occur in pyrite and galena. The high Ag and Hg contents reflect the presence of sternbergite, proustite/pyrargyrite, Hg-tetrahedrite and cinnabar. The Bi, Sb and Sn contents are similar to the gossan component. Accessory minerals include secondary Cu-sulphides, stibnite and Cu-Bi-Hg-Se-bearing sulphosalts. Native Au grains are rare.
Section 9.9: The Partial Massive Sulphide/Shale exhibits variable Fe and S contents reflecting pyritisation and silicification of porous quartz-rich shales. The Cu and Pb contents are low, reflecting the presence of minor amounts of secondary Cu-sulphides and galena. The low Au, As, Bi, Hg, Sb and Sn contents reflect the marked decrease in accessory sulphosalt minerals that typically host these elements. Accessory minerals include graphitic carbon and TiO2. No discrete precious metal grains were located.
Figure 9.35 - Diagram illustrating the key mineralogical features for the ‘Tertiary Polymict Conglomerate’, ‘Upper Siderite Gossan', 'Middle Calcite Gossan', 'Lower Siderite Gossan’, ‘Gossan/Shale Conglomerate Contact’, ‘Shale Conglomerate/Gossan Contact’ and ‘Partial Massive Sulphide/Shale’.
Page 208
Chapter 10 Environment and Formational Mechanisms
10 ENVIRONMENT AND FORMATIONAL MECHANISMS
10.1 Introduction
The presence of abundant siderite and subordinate amounts of associated galena
and Fe-sulphide (greigite) separate the Las Cruces gossan from other VMS
gossans described in the literature. Understanding the environment and
mechanisms behind the formation of the siderite and greigite in the Las Cruces
gossan is key to understanding the genesis of the present day deposit.
Siderite and greigite are unstable in oxidising environments and are not likely to
have formed during near-surface weathering conditions. In the presence of O2,
siderite is oxidised to goethite or hematite. The formation of stable siderite
requires an anoxic environment where reduced Fe (Fe2+) can exist in solution
(Berner, 1981). Similarly, greigite and associated Fe-sulphides only form under
very restricted set of Eh/pH conditions. The nature of the environment and
mechanisms of formation of siderite and Fe-sulphides, notably greigite, are
therefore summarised in this chapter. The references have been selected as
they have bearing on the processes that are being proposed in this thesis for the
formation of this unusual gossan mineral assemblage.
The literature covering the low temperature formation of siderite and Fe-sulphides
is focussed largely on sedimentary marine deposits and burial diagenesis.
However, other environments where these minerals are formed include aquifers,
lakes, swamps, soils and waste ponds. The mechanisms of formation largely
involve the microbial metabolism of organic matter.
Page 209
Chapter 10 Environment and Formational Mechanisms
10.2 Siderite Formational Environment
10.2.1 Introduction
Organic matter is modified by several processes operating at different depths
during burial diagenesis (Irwin et al., 1977). Anaerobic (anoxic/reducing)
conditions occur in sedimentary environments when organic matter is deposited
at a rate exceeding the supply of dissolved oxygen. This depth, below which
there is no dissolved oxygen, marks the boundary between regimes of aerobic
and anaerobic metabolism (Claypool and Kaplan, 1974).
The interrelationships between sedimentological and ecological factors bring
about three distinct biogeochemical environments (Claypool and Kaplan, 1974).
These three distinct zones have been described by a number of authors, notably
Berner (1981), Curtis (1986) and Chapelle and Lovely (1992), with some
ambiguity and discrepancies among them. However, it is generally considered
that these zones consist of the following:-
1. Oxic zone
2. Sulphate reduction zone
3. Methanogenic zone
These zones are illustrated in Figure 10.1. Discrepancies in the nomenclature
used between authors are also described in this chapter. These three zones are
dominated by bacterial processes operating at low temperatures. At higher
temperatures (>50oC) and greater depths (>1000m), a fourth, abiotic zone
referred to as the 'decarboxylation zone' may dominate. Decarboxylation is also
described briefly in this chapter.
As sediments are buried, they pass successively through zones 1 to 3, within
which organic matter is being altered and carbon dioxide produced. The carbon
dioxide produced by all these reactions dissolves readily in porewater to increase
bicarbonate concentrations, often resulting in the precipitation of carbonate
minerals with distinctive carbon isotope values. The carbon isotopic composition
of the precipitated carbonate minerals, however, must be anticipated to be very
Page 210
Chapter 10 Environment and Formational Mechanisms
different from that of marine reservoir bicarbonate (δ13C 0%o PDB) since the
source in each case is organic matter (δ13C -25%o PDB) (Irwin et al., 1977;
Curtis et al., 1986).
Reduction of Fe3+ to Fe2+ is also considered to be one of the most important
geochemical reactions in anaerobic aquatic sediments because of its many
consequences for the organic and inorganic chemistry of these environments
(Coleman et al., 1993). δ13C, Mn2+ and Fe2+ (Mn is largely absent in the Las
Cruces gossan) are the parameters most likely to record interpretable changes in
original pore water chemistry independent of carbonate mineralogy (Curtis et al.,
1986). Stable carbon isotopes and the behaviour of Fe during siderite formation
are therefore examined in detail in this Chapter.
Page 211
Chapter 10 Environment and Formational Mechanisms
Figure 10.1 – A diagram illustrating the three distinct biogeochemical environments that mark the boundaries between regimes of aerobic and anaerobic metabolism. The schematic illustrates the approximate depths that the oxic, sulphate reducing and methanogenic zones occur, together with the typical δ13C values associated with the CO2 generated from the decomposition of organic matter (modified from Irwin et al., 1977 and Claypool and Kaplan, 1974).
Page 212
Chapter 10 Environment and Formational Mechanisms
10.2.2 Oxic Zone (Berner, 1981)
This zone (Figure 10.26), also referred to as the 'aerobic zone' (Claypool and
Kaplan, 1974) or the 'bacterial oxidation zone' (Irwin et al., 1977; Spiro et al.,
1993) occurs in the uppermost part of the sediment, close or at the
sediment/water interface. The oxic zone is relatively open to downward diffusion
of O2 and tends to be no more than a few millimetres to centimetres thick (Curtis
et al., 1986). However, the overall depth of this zone is determined by the extent
of downward diffusion of O2 from overlying waters (Irwin et al., 1977).
This zone is distinguished from other zones by the absence of organic matter,
which has been completely decomposed by aerobic microorganisms prior to
burial (Berner, 1981). Berner (1981) also notes that ferric (Fe3+) oxide minerals
such as hematite are not necessarily an indication of an oxic environment, as
they can persist stably at relatively low levels of O2.
Aerobic respiration using organic matter is the most efficient energy-yielding
metabolic process (Claypool and Kaplan, 1974). The process of aerobic
respiration (the process of degradation of organic matter by aerobic organisms)
produces CO2 and may be expressed by the following formula (Claypool and
Kaplan, 1974):-
CH2O + O2 CO2 + H2O
Bicarbonate activities sufficient to cause carbonate super-saturation are unlikely
to be reached in the oxic zone because of upward diffusion into depositional
waters (Irwin et al., 1977) and as a result, siderite precipitation is unlikely.
Irwin et al. (1977) suggest that bacterial oxidation of the organic matter seem to
impose little fractionation of the C during aerobic respiration, such that very light
bicarbonate is to be anticipated, indicating δ13C of approximately -25%o (i.e. the
same as that of the original organic matter). Spiro et al. (1993) indicate a
moderately heavier range, of between -8 and -13%o for δ13C.
Page 213
Chapter 10 Environment and Formational Mechanisms
10.2.3 Sulphate Reduction Zone (Curtis et al., 1986; Irwin et al., 1977)
This is referred to as the 'anoxic sulphidic zone' by Berner (1981) (Figure 10.1).
Within this zone, sulphidic conditions are brought about almost entirely by the
bacterial reduction of sulphate accompanying organic matter decomposition. The
reaction may be expressed by the following formula (Irwin et al., 1977; Claypool
and Kaplan 1974):-
2CH2O + SO42– S2- + 2CO2 + 2H2O
This process can proceed only under anoxic conditions after all dissolved oxygen
has been consumed by aerobes and therefore only takes place in anoxic basins
at or below the sediment/water interface in organic-rich sediments deposited in
oxygenated water.
Berner (1981) notes that O2 is rapidly depleted upon deposition in sediments rich
in organic carbon due to exhaustive aerobic decay. Even if the overlying water is
oxygenated, aerobic decay within the upper few millimetres of organic-rich
sediment will maintain anoxic reducing conditions. Given anoxic conditions and
reduced Fe, the primary factor that determines whether Fe-sulphides or siderite
will form is the presence of sulphide (H2S and HS-). The overall reaction for pyrite
formation is given by (Berner, 1981):-
3H2S + S + Fe2O3 → 2FeS2 +3H2O
Sulphate reduction is very common in marine waters because of the abundance
of dissolved sulphate in seawater. The first minerals formed as a result of the
reaction between hydrogen sulphide and detrital Fe minerals are a number of
monosulphide phases, chiefly mackinawite (ideally Fe1+xS) and greigite (ideally
Fe3S4). In the presence of excess H2S, these minerals are unstable and are
eventually converted to pyrite. However, if non-sulphidic conditions are attained
by the exhaustion of all sulphate and sulphide, the monosulphides can persist for
long periods of time and therefore may accompany siderite and vivianite (ideally
Fe3(PO4)2.8H2O) (Berner, 1981).
Page 214
Chapter 10 Environment and Formational Mechanisms
The sulphate reduction zone extends down to depths of 1 to 10 metres in
sediments containing significant organic matter (Curtis et al., 1986).
Irwin et al. (1977) note that sulphate reduction seem to impose little fractionation
such that very light bicarbonate is to be anticipated, indicating a δ13C of
approximately -25%o. Above and within the sulphate reduction zone, the organic
matter decomposition adds 12C, steadily decreasing δ13C. Often, δ13C reaches a
minimum of ~ -20%o near the base of sulphate reduction (Malone et al., 2002).
10.2.4 'Methanic' or methanogenic zone (e.g. Berner 1981, Curtis et al., 1986)
This zone is also referred to as the bacterial fermentation zone by Irwin et al.
(1977) and Spiro et al. (1993) (Figure 10.1).
When the sulphate concentration of the water buried with the sediment is low, as
in brackish to fresh water environments, or in marine sediments below the zone of
sulphate reduction, carbonate or CO2 reduction replaces sulphate reduction as
the preferred process of anaerobic respiration (Claypool and Kaplan, 1974).
Berner (1981) suggests that non-marine sediments may more readily produce the
environments for formation of methanic siderite because the initial sulphate
content is on average 100 times less than that of seawater. Methane production
appears to occur immediately after sulphate reduction ceases, possibly because
the bacteria cannot tolerate dissolved sulphides (Irwin et al., 1977).
If sufficient reducible Fe is present as detrital minerals, all H2S formed from
sulphate reduction is precipitated to form Fe-sulphides in the sulphate reduction
zone. Consequently, continued Fe reduction at depth results in the build up of
Fe2+ in the interstitial water because insufficient H2S is present to precipitate it due
to a lack of interstitial sulphate, the primary source of the H2S. Ultimately,
depending on conditions, saturation with siderite is attained (Berner, 1981).
Accompanying the build-up of Fe2+, continued organic matter decomposition
results in the formation of dissolved methane. In this way the formation of siderite
is accompanied by methane formation (Berner, 1981).
Page 215
Chapter 10 Environment and Formational Mechanisms
The sulphide minerals and the bacteria that produce H2S in sediments cannot
tolerate traces of oxygen without conversion to oxide minerals and death
respectively. In addition, H2S and O2 cannot coexist with one another in solution
as their coexistence is unfavourable thermodynamically and kinetically they
rapidly react (Berner, 1981).
Siderite is also inhibited from forming in marine environments because Ca2+
reacts preferentially with bicarbonate at normal marine concentrations. The
Fe2+/Ca2+ ratio in normal marine waters is two orders of magnitude too small to
permit siderite precipitation (Matsumoto, 1981). The presence of siderite is
therefore indicative of the absence of sulphate and the presence of organics and
is more commonly associated with freshwater (Berner, 1981).
Berner (1981) suggests that siderite forms through the combined effects of Fe
reduction and bacterial methanogenesis of organic carbon compounds. This
process can be expressed by the following reaction (Curtis et al., 1986):-
7CH2O + 2Fe2O3 3CH4 + 4FeCO3 + H2O
As with pyrite, the source of Fe is the reduction of detrital Fe oxides in a strongly
reducing, organic-rich sedimentary environment (Curtis et al., 1986).
Claypool and Kaplan (1974) believed that reduction of CO2 by biologically
produced hydrogen is the single most important mechanism. Borowski et al.
(1999) suggest that within marine sediments, microbially mediated methane
production generally occurs through two distinct pathways:-
1. CO2 reduction (CO2 + 4H2 CH4 + 2H2O) and
2. acetate fermentation (CH3COOH CH4 +CO2)
The methanic or methanogenic zone is not limited by external supply of oxidants
and may descend some hundreds of metres into the sediment column (Curtis et
al., 1986).
Page 216
Chapter 10 Environment and Formational Mechanisms
Malone et al. (2002) note that methanogenesis in the marine environment
proceeds by CO2 reduction, typically producing CH4 with δ13C between -60 and
-80%o. Consequently, the residual δ13C increases throughout the zone of
methanogenesis, often reaching positive values of ~ +10%o.
Irwin et al., (1977) also suggest that methanogenesis imposes a very large
fractionation with bacterial methane (CH4) values of -75%o commonplace.
Consequently, carbon dioxide produced in this reaction must be heavy and is
estimated by Irwin et al., (1977), Curtis et al., (1986) and Spiro et al., (1993) to be
in the region of +15%o.
10.2.5 Methane Oxidation
Two additional processes of potentially significant relevance to the Las Cruces
gossan are methane oxidation and Fe reduction. These processes also have a
significant impact on the distinct biogeochemical environments under which
diagenetic carbonate minerals may form. Of potentially less significance,
although they should not be discounted, are the processes of nitrate reduction
and the higher temperature process of decarboxylation.
Malone et al. (2002) suggest that the upward transport of CH4 from the zone of
methanogenesis into the sulphate reduction zone and its subsequent oxidation
produces 12C-rich Dissolved Inorganic Carbon (DIC) and as a result the δ13C can
be much less than -20%o near the sulphate/methane interface.
The anaerobic oxidation of methane at the sulphate/methane interface (SMI) is
also noted by Borowski et al. (1999). Borowski et al. (1999) propose that this
reaction, involving sulphate, occurs within a localised horizon at the interface
between the sulphate reduction zone and methanic zone. Here, sulphate and
methane are consumed at the base of the sulphate reduction zone by the net
reaction:
CH4 + SO42– HCO3
– + HS– + H2O
Page 217
Chapter 10 Environment and Formational Mechanisms
in a process called anaerobic methane oxidation.
Irwin et al. (1977) and Spiro et al. (1993) propose that methane oxidation
produces very isotopically light δ13C of between -70%o and -75%o. The anaerobic
oxidation of methane produces bicarbonate, increasing carbonate alkalinity and
saturation with respect to carbonate minerals. However, this zone is considered
by Borowski et al. (1999) to be a localised horizon and the precipitation of more
extensive volumes of very isotopically light carbonate (with respect to δ13C), such
as that encountered in the Las Cruces gossan, may be the result of other factors
such as the oxidation of methane derived from external sources.
Malone et al. (2002) describes highly negative δ13C values for calcite (as low as
-41.7%o) from the New Jersey shelf that must have formed from waters with a
large component of dissolved inorganic carbon derived from methane oxidation.
The authors suggest that the methane may have been oxidised or vented from
shelf sediments, perhaps during sea-level fluctuations during the Neogene.
Claypool and Kaplan (1974) and Malone et al. (2002) recognise that modern
continental shelves contain abundant methane produced through the bacterial
degradation of organic matter.
Lundegard (1994) suggests that highly negative δ13C compositions of calcite
cement in the Oseberg Formation, Norway, resulted from methane migration from
adjacent shale deposits, with the methane being derived from shallow biogenic
processes (methanogenesis). Lundegard (1994) concludes that considering the
probable burial depths at the time of calcite cementation (<500m), it is highly
unlikely that significant HCO3– would have been produced by thermal
decarboxylation of kerogen in adjacent mudrocks or that methane could have
been produced in adjacent mudrocks by thermogenic processes.
Lundegard (1994) notes that several additional factors indicate that methane was
oxidised during anaerobic sulphate reduction. The concentrations of Fe and Mn
in the calcite indicate that precipitation took place under reducing conditions. The
ubiquitous association of small amounts of pyrite and siderite with the calcite
Page 218
Chapter 10 Environment and Formational Mechanisms
cement not only confirm that pore water was reducing but indicate that dissolved
sulphide was present at the time of calcite precipitation. Pyrite from calcite
cemented zones has sulphur isotopic compositions suggestive of an association
with sulphate reduction.
Larrasoaña et al. (2007) describe greigite and pyrrhotite formed as a by product
of microbially-mediated reactions in the sulphate, anaerobic oxidation of methane
(AOM), and the methanic/gas hydrate zones in diagenetically modified sediments.
They suggest that greigite formed in the strongly reducing conditions below the
sulphate zone as a result of the metabolic activity of microorganisms whose
populations are enhanced in the presence of gas hydrate. The authors conclude
that geochemical conditions favourable for formation and preservation of greigite
are a limited source of sulphide, so that pyritisation reactions are not driven to
completion.
10.2.6 Fe3+ Reduction
Berner (1981) describes not three, but four distinctive biogeochemical zones in
his original classification scheme. The fourth zone described by Berner is named
the 'Post Oxic' zone. With the exception of Spiro et al. (1993), who describes this
zone as 'suboxic', this zone is generally not ascribed a separate zone
classification as it contains several other distinctive processes within it, including
nitrate, Fe and Mn reduction (e.g. Malone et al., 2002; Curtis et al., 1986;
Coleman et al., 1993; Chapelle and Lovley 1992).
Nitrate, Fe and Mn reduction may be prevalent in the sediments of freshwater and
brackish water environments or other similar situations where availability of
dissolved sulphate may be the key limiting factor to sulphate reduction. In marine
environments, where dissolved sulphate is generally abundant, sulphate reducing
bacteria (SRB) produce H2S, which can reduce Fe oxyhydroxides to form Fe
sulphides. However, in non-marine environments, or where availability of
sulphate is low, nitrate, Fe and Mn reduction may dominate.
Of potentially greatest relevance to the Las Cruces gossan is the process of
organic matter oxidation by Fe3+ reduction (due to the abundance of available Fe).
Page 219
Chapter 10 Environment and Formational Mechanisms
Mn reduction, although thermodynamically more favourable than Fe reduction
(and therefore naturally precedes Fe reduction) is likely to be a very limited
process in the Las Cruces gossan due to the low availability of reducible Mn. The
two processes are, however, essentially similar. Only Fe reduction is considered
here.
Curtis et al. (1986) note that Fe reduction is a major oxidant of organic matter
(especially in non-marine environments), where reduction of Fe3+ releases Fe2+ to
the diagenetic pore waters. The authors conclude that Fe3+ reduction may not be
mutually exclusive with other zones and may overlap with deeper diagenetic
reactions. Curtis et al. (1986) also note that aerobic oxidation, sulphate reduction
and methanogenesis reactions increase pore water bicarbonate activity and also
acidity, whereas Fe3+ reduction raises Fe activity but decreases acidity.
Combinations of these two reactions (Fe reduction with sulphate reduction or
methanogenesis) will therefore favour sulphide and/or carbonate precipitation
respectively (Curtis et al., 1986; Spiro et al., 1993). The authors express the
reduction of Fe3+ to Fe2+ with the following formula:-
2Fe2O3 + CH2O + 3H2O HCO3- + 4Fe2+ + 7OH-
The generation of Fe2+, bicarbonate and hydroxyl ions increases alkalinity and
carbonate mineral precipitation is favoured (Curtis et al.,1986; Coleman et al.,
1993):-
Fe2+ + OH- + HCO3- FeCO3 +H2O
Examples of Fe reduction in the literature are numerous. The formation of
siderite in organic-rich marine mudrocks has previously been associated with the
degradation of organic matter by anaerobic, methanogenic bacteria (Coleman et
al., 1993). Carbonate resulting from this process has a characteristic positive
isotope signature of between 10 and 15%o. Coleman et al. (1993) describe
geochemical and microbiological studies that suggest that contemporary
formation of siderite concretions in a salt marsh sediment resulting from the
activity of SRB. Instead of reducing Fe3+ indirectly through the production of Fe
Page 220
Chapter 10 Environment and Formational Mechanisms
sulphides, some of these bacteria can reduce Fe3+ directly through an enzymatic
mechanism, producing siderite rather than Fe sulphides. The siderite concretions
are found in sediments that have been deposited in the last 50 years.
The results suggest that a high proportion of the micro-organisms living in the
concretion survive by anaerobic respiration such as Fe3+ or sulphate reduction
(Coleman et al., 1993). The authors also conclude that SRB may be an important
catalyst for Fe3+ reduction in other sedimentary environments as they are
abundant in the Fe3+ reduction zones of deep aquifers in the Atlantic Coast Plain
of the USA. As there is no apparent sulphate reduction in these zones and the
SRB cannot have been recently introduced to these deeply isolated
environments, the authors speculate that the SRB must survive by reducing Fe3+.
Lovley et al. (1990) investigate the possibility that microorganisms are catalysing
the ongoing reduction of Fe3+ in the sediments of deep (20-250m) aquifers in the
Atlantic Coastal Plain. The Fe3+ reducing microorganisms were capable of
reducing ferric oxides present in deep subsurface sediments. The authors
conclude that acetate-oxidising, Fe3+ reducing microorganisms were present in
sediments from all sites where Fe3+ reduction was still active, with
methanogenesis and sulphate reduction being the predominant terminal electron-
accepting processes in sites where microbially reducible Fe3+ had been depleted.
Lovley et al. (1990) propose that the enzymatic reduction of Fe3+ by
microorganisms reported here is the first mechanism of any kind actually shown
to have the potential to couple the oxidation of organic matter to carbon dioxide
with the reduction of Fe3+ under environmental conditions typically found in deep
aquifers. The authors also comment that despite its age (Late Cretaceous in this
case), organic matter in deep sediments can be slowly metabolised by anaerobic
microorganisms.
Chapelle and Lovley (1992) describe competitive behaviour between Fe3+
reducing bacteria and SRB, where the Fe reducing bacteria in zones of high Fe in
the Middendorf aquifer (South Carolina) maintain levels of dissolved hydrogen,
formate and acetate in the groundwater at levels lower than thresholds required
Page 221
Chapter 10 Environment and Formational Mechanisms
by SRB. Where Fe is less abundant, the process is reversed and the levels of
hydrogen, formate and acetate increase to levels that allow SRB to become
active. There is a direct correlation between Dissolved Inorganic Carbon (DIC)
and dissolved Fe2+ within the high Fe zone, indicating that organic matter is being
oxidised to carbon dioxide during the reduction of Fe3+.
The authors conclude that based on their field evidence and laboratory
experiments, the results indicate that the activity and interaction of Fe3+ and
sulphate reducing micro-organisms in the Middendorf aquifer are responsible for
the development and localisation of high Fe groundwater. The results also
indicate that microbial activity is strongly influenced by hydrological factors such
as direction of groundwater flow and geological factors such as distribution of
sedimentary depositional environments in the aquifer (Chapelle and Lovley,
1992).
Murphy et al. (1992) comment that micro-organisms that utilise Fe3+ as a terminal
electron acceptor would have to come into direct contact with Fe3+ to reduce it.
This is because most Fe3+ in sediments is in solid form because of its low
solubility, noting that the less crystalline forms of Fe, such as amorphous Fe
hydroxides, are reduced most easily.
Irwin et al. (1977) found no information relating to fractionation during oxidation by
ferric compounds and have assumed that none occurs. Spiro et al. (1993)
propose a δ13C of approximately -25%o (i.e. the same as that for organic matter).
10.2.7 Nitrate Reduction
The degradation of organic matter through nitrate reduction is often considered
only briefly in the literature, presumably because, although it is a very efficient
energy yielding metabolic process (and therefore favoured by bacteria), the
availability of significant nitrate for the reduction process is generally limited and
other processes would therefore more likely dominate.
Claypool and Kaplan (1974) express the process of organic matter oxidation by
nitrate reduction with the following formula:-
Page 222
Chapter 10 Environment and Formational Mechanisms
5CH2O + 4NO3– + 4H+ 2N2 + 5CO2 + 7H2O
10.2.8 Abiotic reactions - Thermally induced decarboxylation
At high temperatures (>50oC) (Claypool and Kaplan, 1974) and greater depths
(>500m; >1000m) (Lundegard, 1994; Irwin et al., 1977 respectively) the early
stages of kerogen maturation (catagenesis) liberate CO2 (Curtis et al., 1986) and
methane and other hydrocarbons are produced by non-biological reactions
(Claypool and Kaplan, 1974). In this decarboxylation zone δ13C values are
negative, ranging between -10 and -25%o (Curtis et al., 1986).
Evidence presented within this thesis suggests that siderite and greigite formation
is active in the present day gossan as a result of interactions with the Niebla
Posadas aquifer. This is discussed in further detail in Chapter 12. Maximum
temperatures measured within the aquifer are typically less than 40oC (Knight,
2000) at burial depths of approximately 150 metres. Thermally induced
decarboxylation should therefore be considered as an unlikely source of methane
and subsequent siderite formation.
Page 223
Chapter 10 Environment and Formational Mechanisms
10.3 Formation of Fe-sulphides
10.3.1 Introduction
In ambient aqueous systems the iron sulphides constitute a diverse group of
minerals, the most abundant of which are amorphous Fe2+S (FeSam), mackinawite
(tetragonal FeS), greigite (cubic Fe2+Fe3+2S4), pyrrhotite (Fe1-xS) and
pyrite/marcasite (cubic/orthorhombic FeS2 respectively) (Rickard et al. 2001).
FeS(am), mackinawite and greigite are collectively known as Acid Volatile
Sulphides (AVS).
The interrelationships between the various forms of iron sulphide have been the
subject of extensive investigations as all but pyrite and pyrrhotite are metastable
at ambient temperatures (Rickard et al., 2001).
10.3.2 Formation Mechanisms
Schoonen and Barnes (1991) discuss pyrite and marcasite formation by
replacement of an FeS precursor as the probable mechanism in low-temperature
environments. Rickard et al. (2001) suggest that the FeS(am) precursor rapidly
transforms to metastable mackinawite. The formation of pyrite and marcasite by
precursor FeS is also discussed by Lennie et al., (1997), (citing Berner, 1964)
who consider that pyrite and marcasite are formed in sedimentary and
hydrothermal systems by the reaction sequence:
mackinawite → greigite → pyrite/marcasite
thus allowing the relatively complex crystal structures of pyrite and marcasite to
nucleate readily at low temperatures from the simpler structures of the precursors
mackinawite or amorphous FeS.
Oxidised surfaces of precursors FeS or of pyrite seeds speed up the
transformation (Benning et al., 2000). The formation of marcasite over pyrite is
thought to be related to crystal growth kinetics and pH, with marcasite often
forming at pH<5. Classical nucleation theory predicts that nucleation will be
faster for the more soluble mineral. In a solution supersaturated with respect to
Page 224
Chapter 10 Environment and Formational Mechanisms
amorphous FeS and pyrite, the highly soluble FeS(am) rapidly nucleates,
preventing the nucleation of pyrite. The presence of pyrite seeds, however, lifts
the nucleation barrier. Little effort is made in the literature to distinguish between
crystal growth mechanisms and nucleation kinetics, despite their differences, and
this area of investigation remains largely unresolved. However, investigations
suggest that pyrite can grow in the presence of pyrite seeds without the presence
of an FeS precursor (Schoonen, 2004).
Schoonen and Barnes (1991) and Lennie et al. (1997) propose that the iron
monosulphide precursor (either amorphous FeS(am) or mackinawite) is formed by
reaction of aqueous hydrogen sulphide ions with aqueous Fe2+ ions according to
the reaction:
Fe2+(aq) + H2S(aq) → FeS(am or mackinawite) + 2H+
The hydrogen sulphide needed for formation of iron sulphides in anoxic
environments is produced via bacterial reduction of sulphate (SO42-), which
enables biogenic oxidation of organic matter by anaerobic microorganisms. As
discussed previously, hydrogen sulphide can also be produced during anaerobic
oxidation of methane (AOM). Molecular, isotopic, and molecular biological
approaches have revealed that AOM is performed by methanotrophic archaea
(methane-loving organisms) and sulphate reducing bacteria. The coupled
reaction is proposed to proceed according to the following equation (Van Dongen
et al., 2007):
CH4 (methane) + SO42- (sulphate) → HCO3
– (bicarbonate) + HS- (hydrogen sulphide) + H2O
The rate of hydrogen sulphide formation via sulphate reduction is constrained by
the availability of metabolisable organic matter. In turn, the rate of hydrogen
sulphide formation in relation to the availability of reactive iron exerts an important
control on the initial AVS/pyrite ratio (Schoonen, 2004).
Page 225
Chapter 10 Environment and Formational Mechanisms
The conversion of FeS(am) to pyrite requires an electron acceptor to oxidise S2- to
S-. In addition, the Fe/S ratio must decrease via either the loss of Fe or the gain
of S. A number of potential mechanisms of pyrite transformation at low
temperatures have been proposed (Neretin et al., 2004; Schoonen, 2004):
1. Polysulfide pathway: FeS conversion via sulphur addition, with the added
sulphur acting as the electron acceptor
2. Ferrous iron loss pathway: FeS conversion via Fe loss combined with an
electron acceptor
3. H2S pathway: FeS conversion via sulphur addition, with a non-sulphur
electron acceptor
Polysulphide Pathway
Benning et al. (2000) suggest that the dominant formation path of pyrite is by
reactions between a precursor monosulphide and zero-valent sulphur species via
the ‘polysulphide pathway’. This reaction pathway is rooted in experimental
studies which demonstrate that precipitation of amorphous FeS in the presence of
native S produces pyrite, according to the reaction: (Schoonen, 2004)
FeS + S →FeS2
Schoonen (2004) notes that this mechanism has been called into question, and
that polysulphide species produced by the hydrolysis of sulphur or reactions
between sulphur and H2S are more likely the reactants in this process.
Metastable sulphur oxyanions have also been suggested as reactants.
Ferrous Fe Loss Pathway
Wilkin and Barnes (1996) indicate that under certain conditions, iron loss may
dominate over sulphur addition. Wilkin and Barnes (1996) show that iron
disulphide nucleation may proceed via loss of ferrous iron from the precursor
monosulphide rather than via addition of zero-valent sulphur according to the
following reaction:
Page 226
Chapter 10 Environment and Formational Mechanisms
8FeS + 2H2O + 3O2 → 4FeS2 + 4FeO(OH)
Experimental work by Lennie et al. (1997) shows that mackinawite can readily
transform to greigite via iron loss, but the transformation to pyrite has not been
demonstrated.
H2S Pathway
Schoonen (2004) notes that in most anoxic sediments, hydrogen sulphide is by
far the most abundant source of dissolved sulphur. Wachtershauser (1988 and
1993) propose that H2S could also oxidise monosulphides. This
thermodynamically favourable pathway involving reactions between H2S and
possibly HS- to produce pyrite is also considered by Rickard (1997), according to
the reaction:
FeS + H2S → FeS2 + H2
However, this reaction has consistently failed to produce pyrite during
experimental conditions with only mackinawite being produced. This mechanism
continues to be an area of ongoing research and controversy. Observations in
nature indicate that in the deep, anoxic, sulphidic sediments the conversion of
AVS to pyrite proceeds slowly, with estimated rates of decades to centuries. It is
possible that the conversion proceeds via a direct reaction between AVS and
H2S, producing H2, but it is also possible that the reaction takes place with H2S as
the sulphur source and a non-sulphur electron acceptor (e.g. bicarbonate).
Despite significant progress in understanding the mechanisms behind pyrite
formation, some aspects are yet to be resolved, particularly those involving the
interactions between H2S and FeS2. This may partly be due to the slow
conversion of precursor FeS to FeS2 and the difficulties in reproducing the
reaction in laboratory experiments (Schoonen, 2004).
Page 227
Chapter 10 Environment and Formational Mechanisms
10.3.3 The Role of Biological Processes
The formation of Fe-sulphides is intimately related to biological processes,
because the overwhelming source of sulphide in natural environments is bacterial
sulphate reduction. This close association with biological processes has been
recognised through their association with the early evolution of life and their role
in the intracellular processes of some micro-organisms (Rickard et al., 2001).
The two most common magnetic iron sulphide minerals are greigite and
monoclinic pyrrhotite (Fe7S8) (Larrasoaña et al., 2007). Bazylinski and Moskowitz
(1997) suggest that bacteria mediate the formation of the magnetic Fe-sulphides
in two fundamentally different ways:
1) Biologically Induced Mineralisation (BIM) - the mineralisation is not
controlled by the organism and the mineral grains form extracellularly.
2) Biologically Controlled Mineralisation (BCM) – the organisms control the
mineralisation to a high degree and the mineral grains are normally formed
intracellularly and exhibit a very restricted size range and range of
morphologies. Examples include magnetotactic bacteria.
As cellular textures are not observed in the Las Cruces magnetic Fe-sulphides,
BCM is not considered further here.
The biomineralisation associated with BIM occurs indirectly as a result of
metabolic activity and subsequent chemical reactions. The mineral formed may
be poorly crystalline and exhibit a wide range in grain size and lacking in specific
crystalline morphology. Examples include Fe- and S-reducing bacteria that use
Fe and S as terminal electron acceptors for energy generation and produce
greigite and magnetite. Sulphate reducing bacteria (SRB) respire with sulphate
anaerobically, releasing H2S which reacts with excess Fe present in the
environment producing mackinawite, greigite, pyrrhotite, pyrite and marcasite.
Two common examples of other minerals formed by BIM are siderite and vivianite
Page 228
Chapter 10 Environment and Formational Mechanisms
(ideally Fe3(PO4)2.8H2O). There is no apparent function to the minerals formed by
BIM (Bazylinski and Moskowitz, 1997).
AVS conversion studies have largely been abiotic and consideration of the role of
SRB in pyrite formation has largely been restricted to the production of H2S.
Bacteria may also play an important role in the conversion of organic sulphur
compounds to reactants that can take part in pyrite formation. However, the
complexity of the natural system inhibits meaningful experimental studies.
Nonetheless, biotic studies corroborate the abiotic studies in highlighting the
important role that FeS precursors play in pyrite formation (Schoonen, 2004).
Experimental studies by Rickard et al. (2001) have suggested that in the
presence of the organic aldehydic carbonyl group of compound, FeSam is oxidised
in a solid state reaction to form greigite which retains the original morphology of
the FeS precursor. The authors note that in the absence of the organic
compound, FeSam is transformed to pyrite via an aqueous FeS complex, with no
greigite being formed.
Work by Donald and Southam (1999) showed rapid pyrite formation and
incorporation of sulphur initially added to the system as the sulphur-bearing
amino acid cysteine. Donald and Southam (1999) proposed that the cysteine was
possibly converted to H2S before it became incorporated, indicating addition of
sulphur to FeS. Wilkin and Barnes (1996) evaluated cysteine as a sulphur source
in a strictly abiotic system and found no evidence for incorporation of cysteine-
bound sulphur in pyrite. The work by Donald and Southam (1999) therefore points
to the role bacteria can play in converting organic sulphur compounds to
reactants that can take part in pyrite formation and indicate that it is increasingly
evident that SRB play a more important role than simply providing H2S for the
reaction.
Page 229
Chapter 10 Environment and Formational Mechanisms
10.4 Mineral stability fields
10.4.1 Siderite
Chapter 4 describes how, in natural environments, under near surface weathering
conditions, the stable Fe-sulphides typically oxidise to form Fe-oxides, hydroxides
and sulphates. The presence of siderite in the Las Cruces gossan therefore
suggests that the Eh/pH conditions and chemistry of the mineralising fluids differ
markedly from those normally encountered in sub-aerially weathered gossans.
-
Figure 10.2 – Eh/pH diagram illustrating the stability of hematite, magnetite and siderite at 25oC and 1 atmosphere total pressure and pCO2 = 10-2 atmosphere with total activity of dissolved species = 10-6 (Garrels and Christ, 1965).
Page 230
Chapter 10 Environment and Formational Mechanisms
Garrels and Christ (1965) consider the influence of CO2 on the stability of the Fe-
oxides. The authors note that at pCO2 of 10-3.5 atmosphere, the partial pressure
of CO2 in the earth’s atmosphere, siderite has only a small field of stability and
only if the total dissolved carbonate increases does the siderite field expand.
Given a total activity of dissolved species of 10-6 with pCO2 of about 10-1.4, the
magnetite stability field is completely displaced by siderite. The authors therefore
consider pCO2 of 10-2 (Figure 10.26). Figure 10.26 confirms that under these
conditions, siderite is indicative of a strongly reducing environment, near neutral
to alkaline pH and the presence of CO2 in amounts greater than atmosphere.
Under decreasing pH, siderite is unstable and Fe2+ will be released into solution,
particularly under strongly reducing conditions. At moderate to high pH and
increasing Eh, siderite will oxidise to form either magnetite or hematite,
depending on the pCO2.
10.4.2 Fe-sulphides
Schoonen (2004) describes pe/pH diagrams for the Fe-sulphides and notes that
pyrite is the stable iron sulphide in anoxic, low temperature environments (Figure
10.27A). Therefore the Fe-monosulphides identified in this study in the Las
Cruces gossan are predicted to be stable under very limited Eh/pH conditions.
Marcasite is metastable with respect to pyrite under all pressures and
temperatures and therefore does not show up on stability diagrams unless pyrite
is excluded (Figure 10.27B). The stability field areas do not change when pyrite
is excluded due to the very subtle difference in free energy between pyrite and
marcasite (~2kJ/mole). Only by exclusion of pyrite, marcasite and the stable iron
monosulphides pyrrhotite and troilite are the metastability fields for amorphous
FeS, mackinawite and greigite seen (Figures 10.27C, D and E). The shape and
size of the stability field is dependent on solution composition. Figure 10.27C
gives stability fields for world average seawater composition. The equivalent
pe/pH diagram for world average river water shows a decrease in the sulphur
activity and a subsequent reduction of the metastability fields for greigite,
mackinawite and FeS(am) (Figure 10.27E). Increasing the carbon and Fe activity
produces a stability field for siderite (Figure 10.27F).
Page 231
Chapter 10 Environment and Formational Mechanisms
Figure 10.3 - Pe/pH diagrams illustrating the stability relations for iron sulphides in seawater at 25oC, 1 atmosphere total pressure. A) Iron activity 10−6, sulphur activity 10−2.551, C(IV) activity 10−3.001, troilite and pyrrhotite suppressed. B) Same as A with pyrite also suppressed. C) Same as B with marcasite suppressed. D) Same as C with greigite and mackinawite suppressed. E) Same as C but solution changed to world average river water with iron activity 10−6, sulphur activity 10−3.902, C(IV) activity 10−3.06. F) Same as C but iron activity 10−3 and C(IV) activity 10−2.5
(Schoonen, 2004).
Page 232
Chapter 10 Environment and Formational Mechanisms
On the basis of these thermodynamic considerations, one would expect only
pyrite to form in low temperature environments if equilibrium was to be
maintained. However, this is clearly not the case as marcasite and Fe-
monosulphide precursors are widely reported in a range of environments
(Schoonen, 2004).
10.4.3 Siderite/Fe-Sulphide Relationships
Figure 10.4 – Eh/pH diagram illustrating the stability relations between iron oxides, sulphides and carbonates in water at 25oC and 1 atmosphere total pressure at ΣCO2 of 100 and ΣS of 10-6
(Garrels and Christ, 1965).
Page 233
Chapter 10 Environment and Formational Mechanisms
Garrels and Christ (1965) consider the influence of a combination of CO2 and
sulphur on iron-water-oxygen relations. Figure 10.28 illustrates the relations at
ΣCO2 of 100 and ΣS of 10-6 and confirms that if siderite is to have a considerable
field of stability then ΣCO2 must be very high and ΣS very low. If the total
dissolved CO2 is reduced to 10-2, siderite does not appear on the Eh/pH diagram.
Under the conditions illustrated in Figure 10.28, siderite is indicative of both very
strongly reducing conditions and moderate reducing conditions. The field of
pyrrhotite is eliminated. The presence of siderite in ores may therefore be
indicative of the absence of appreciable divalent sulphur and the presence of
relatively large amounts of dissolved carbonate.
The mineral paragenesis, fluid geochemistry and relevance of Eh/pH conditions
with respect to the Las Cruces gossan mineral assemblage are discussed in
greater detail in Chapters 11 and 12.
Page 234
Chapter 11 Mineralogy: Key Features and Paragenesis
11 MINERALOGY: KEY FEATURES AND PARAGENESIS
11.1 Introduction
This chapter aims to highlight the key mineralogical features of the gossan,
including the dominant gangue, ore and precious metal-bearing minerals and
their paragenesis. Although differences exist between the five boreholes
examined during this investigation, a common mineral assemblage has been
ascertained for the gossan.
The present day Las Cruces gossan is defined by the presence of a reddish
brown coloured mineral assemblage dominated by siderite and associated
weathering products. The base of the gossans, at the contact with the underlying
massive sulphide is characterised by an absence of siderite and the presence of
secondary pyrite together with sternbergite and proustite/pyrargyrite. Although
there is clearly an overlap between these two mineral assemblages, the latter
suite is essentially a component of the supergene zone and is not discussed in
significant detail here.
The common gossan assemblage consists of resistate quartz that exhibits
varying degrees of replacement by a late-stage siderite-rich assemblage
containing galena, Fe-sulphides and precious metal mineralisation. This
assemblage is described in greater detail in this chapter.
Page 235
Chapter 11 Mineralogy: Key Features and Paragenesis
11.2 Quartz
11.2.1 Relative Abundance
Quartz is the dominant Si-bearing phase present in the Las Cruces gossan and,
apart from subordinate amounts of Fe-rich clay occurring locally, no other
significant Si-bearing phases were located during this investigation.
There is a strong correlation between the abundance of quartz in the gossan and
the lateral distance from the supergene massive sulphide. Boreholes CR194,
CR149 and CR123 are the least quartz-rich gossans, consisting predominantly of
siderite together with accessory amounts of quartz. These boreholes are also the
closest to the supergene massive sulphide ore, which itself contains only minor
amounts of quartz and is also the source of the remobilised Fe. Conversely,
boreholes CR191 and CR038, which appear to represent ferruginised wall rocks,
are the most quartz-rich gossans, containing only accessory amounts of siderite,
and are the most distant boreholes from the supergene massive sulphide and the
original source of the remobilised Fe. The distance from source and mobility of
Fe plays a key role in the relative abundance of quartz in the gossans.
Quartz is often found in greater abundance in the lower portions of the gossan,
close to the contact with the massive sulphide. This vertical concentration
appears to relate partly to the concentration of resistate quartz grains during
mass wasting of the massive sulphide (e.g. in the galena-rich layer of borehole
CR194) and also partly to the precipitation of secondary silica (chalcedony)
towards the base of the gossan profile (e.g. borehole CR191).
11.2.2 Grain Size, Shape and Texture
The grain size determinations include both the size of quartz fragments and also
information on grain size of the crystallites that make up fragments of rock or the
rock as a whole. The former is based largely on SEM examination and the latter
is determined by transmitted light microscopy. Grain shape is used here to
describe the shape of the quartz fragments. Grain shape may provide
information on the degree of reworking, sorting and evidence of chemical
dissolution. Where quartz is present in its primary form, such as a quartz vein or
Page 236
Chapter 11 Mineralogy: Key Features and Paragenesis
as a component of a tuff or shale it is described in terms of ‘crystallite shape’ or
‘texture’. Crystallite texture provides an indication of the source of the quartz
fragments and also helps correlate the occurrences of quartz between the
different boreholes. Texture is also important in determining evidence of
deformation. The texture of the quartz crystallites is based on transmitted light
microscope examination.
The grain size of the quartz fragments is highly variable throughout the gossans
and highlights the relatively poorly sorted nature of the quartz. The larger,
millimetre-sized quartz fragments typically exhibit angular morphologies or highly
irregular margins. The textures in the quartz fragments associated with the
gossans are varied and indicate a number of different and distinct sources. The
angular nature of the fragments is indicative of extensive fracturing and also
suggests the fragments have not been transported far from their original source.
The highly irregular morphologies exhibited by some of the fragments may be
indicative of dissolution and/or replacement, which is particularly evident in close
association with the siderite. The fine-grained quartz is more reactive than larger
grains and is significantly more prone to replacement by siderite.
The variable fragment size, angular shape and varied source has implications for
the nature of deposition of the quartz fragments and is suggestive of an erratic,
high-energy environment such as that described by Kosakevitch et al. (1993) for
the Rio Tinto deposit. The theory of Kosakevitch et al. (1993) is based on
regional climatic conditions at the time of gossan formation that would have also
impacted on the Las Cruces deposit.
Boreholes CR038 and CR191 consist predominantly of fine-grained quartz and
fibrous fragments that have been recemented by later stages of chalcedony.
These rocks appear bleached and are largely devoid of sulphide minerals that
appear to have been oxidised and leached, leaving euhedral voids. The primary
mineralogy, particularly of the associated shales, has also been extensively
leached of other silicate components that would have presumably been present
prior to oxidation and leaching. This would have resulted in a very porous and
possibly unstable matrix that may well have collapsed under the weight of the
Page 237
Chapter 11 Mineralogy: Key Features and Paragenesis
overlying rocks. The result would have been a jumbled mass of fine-grained
quartz from the shale fragments together with the fibrous quartz that formed as
part of the massive sulphide replacement of the shale, much like that observed in
the core in the present day. The jumbled mass of quartz has subsequently been
cemented by later stages of chalcedony mineralisation and lesser siderite.
The poorly sorted nature of the quartz may therefore have resulted from a
combination of exposure to a high energy environment, and collapse of the
groundmass under pressures formed during oxidation and leaching of sulphide
and gangue components by highly acidic Fe-rich solutions.
Rounded, pebble-like grains of quartz are evident locally. These are often
associated with angular and irregularly shaped fragments of quartz and provide
further evidence of poor sorting, multiple sources and greater distance of travel
than the bulk of the quartz in these ores.
Probably the most characteristic texture observed in the quartz is that of the
fibrous aggregates. These fibrous aggregates are also often associated with
euhedral voids. The source of these fibrous aggregates appears to be the
underlying shales (Figure 11.1), where the fibrous quartz aggregates form around
the margins of pyrite. Minor amounts of fibrous quartz are also associated with
the massive sulphide.
Page 238
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.1 - Borehole CR123 - A colour transmitted light photomicrograph of fibrous quartz (white and grey shades) developed around the margins of pyrite crystals (black). The surrounding matrix is fine-grained quartz and pore spaces. This image was taken in crossed polarised light from the shale. The width of view is approximately 1100µm.
These textures have been documented and described by Williams (1933-34),
occurring at the Rio Tinto deposit, Spain. Williams (1933-34) describes pyrite
crystals enwrapped in fibrous quartz, which is drawn out parallel to the schistosity
in a manner suggesting that it may have crystallised under the influence of a
direct stress.
The development of the fibrous quartz aggregates along the margins of pyrite is
well documented in the more recent literature, including Ramsay and Huber
(1983) and Passchier and Trouw (1996). These textures are described as
antitaxial pressure fringes. These studies suggest that the quartz has developed
around the margins of the rigid body of the pyrite as a result of pressure solution.
The textures developed within these fibrous growths often indicate the pressure
history of the surrounding wall rocks.
Page 239
Chapter 11 Mineralogy: Key Features and Paragenesis
It is clear that the fibrous quartz aggregates in the shales are essentially similar to
those observed in the overlying gossans. However, the quartz and pyrite have
developed in situ in the shales, whereas the fibrous quartz in the overlying
gossans is clearly reworked and often cemented and/or partially replaced by one
or more of chalcedony, siderite and, to a lesser extent, calcite. The fine-grained
quartz that is present as the matrix in gossans may well be derived from the
shale, however, the textures are less distinctive than that of the fibrous quartz and
their origins are therefore less clear.
A subordinate portion of the quartz in the gossan occurs in the form of partially
recrystallised chalcedony that often cements the surrounding matrix (Figure 11.2).
The partially recrystallised chalcedony may be locally abundant and appears
massive in nature when observed in hand specimen. A subordinate portion of the
chalcedony occurs as a cavity filling and typically exhibits delicately banded,
fibrous textures (Figure 11.3).
Figure 11.2 - Borehole CR038 - A colour transmitted light photomicrograph illustrating more coarsely crystalline quartz fragments (white and grey shades, far left and far right) that are cemented by fine-grained, partially recrystallised chalcedony (mottled grey/black shades centre of field). This image was taken in crossed polarised light from the quartz replaced tuff. The width of view is approximately 1100µm.
Page 240
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.3 - Borehole CR149 - A colour, crossed polarised transmitted light photomicrograph from the gossan/massive sulphide contact illustrating a cavity (centre field) that has been filled by fibrous chalcedony (white/grey shades). The surrounding matrix is predominantly calcite (pinkish brown shades). The width of view is approximately 2mm.
The fine, medium and coarse-grained polycrystalline quartz aggregates that occur
in the gossans are less characteristic than the fibrous aggregates and their
origins are less readily determined. Similar textures are, however, observed in
the quartz that is present in the underlying massive sulphide and it is likely that
they have derived, at least in part, from the reworking of surrounding wall rocks or
from oxidation and weathering of the massive sulphides.
11.2.3 Fluid Inclusion and Isotope Analysis
Knight (2000) suggested that the microthermometric data and δ18O signatures for
the silica cap, have indicated that a significant proportion of the quartz formed
during late-stage or retrograde activities as conditions declined in the seafloor
hydrothermal system. However, the abundance of monophase fluid inclusions
indicative of ambient temperatures and low salinities consistent with meteoric
waters suggest that a subordinate portion of the quartz in the silica cap appears
Page 241
Chapter 11 Mineralogy: Key Features and Paragenesis
to have formed conventionally in response to gossan formation during pre-
Tertiary, sub-aerial weathering. These conflicting data have been explained by
Knight (2000) as resulting from two separate processes, with the conventionally
formed, sub-aerial silica cap being reworked during the Miocene and with
seawater penetrating the more permeable parts of the deposit.
Petrographic studies during this investigation have, however, shown that the
quartz 'cap', or silica enrichment at the base of the gossan unit comprises largely
reworked shales that have subsequently been cemented by late-stage, low
temperature quartz and chalcedony. The quartz in this horizon can therefore be
characterised as two discrete types;
A mechanically reworked, resistate quartz component derived from
extensive reworking and leaching of Palaeozoic sediments, characterised
by higher temperatures of formation and higher salinities.
Chemically precipitated low salinity, low temperature quartz/chalcedony
cement associated with the downward migration of silica resulting from
dissolution of quartz and other silicate minerals during gossan formation.
This accounts for the unexpected range of temperatures (ambient to a maximum
of ~200oC), salinities (1.7 to 6.1 wt.% NaCl) and δ18O values observed by Knight
(2000).
Page 242
Chapter 11 Mineralogy: Key Features and Paragenesis
11.3 SIDERITE
11.3.1 Relative Abundance
Siderite is a common accessory mineral in the Tertiary sand of borehole CR149,
where it occurs as compositionally zoned euhedral crystals. Siderite was not
observed in the Tertiary deposits in any of the other boreholes examined during
this investigation and is essentially confined to the gossan. Siderite dominates
the gossan of boreholes CR194 and CR149. Siderite is less abundant in the
quartz dominated gossans of boreholes CR038, CR191 and CR123. The relative
proportions of siderite, limonite and quartz vary locally within the boreholes.
A significant portion of the siderite in borehole CR194 occurs as ‘fragments’.
These are less common in other boreholes, where the bulk of the siderite occurs
as a cavity filling, cement and less commonly along fractures. Significant
porosity is often developed as a result of the oxidation of the earlier stages of
siderite mineralisation. Later stages of unoxidised siderite typically replace the
oxidised matrix, resulting in a significant decrease in the porosity of the core. The
relative abundance of the siderite appears to be controlled largely by the porosity
of the gossan and distance from the original source of Fe, namely the massive
sulphide orebody.
11.3.2 Grain Size, Shape and Textures
The grain size, as described in this thesis, includes the size of siderite 'fragments'
as well as information on the size of the crystallites associated with the
fragments, veinlets and void fillings. The latter is determined by transmitted light
microscopy, whereas the more general description of the size of the siderite
fragments is based largely on SEM examination. Unfortunately, due to the
extensive oxidation of much of the siderite, crystallite size is not always readily
observed. Therefore, the bulk of the information on crystallite size is based on
the less oxidised, and often later stages of siderite mineralisation. Grain shape
describes the shape of the siderite fragments. The siderite ‘fragments’ appear to
represent reworked materials and these fragments have often been subjected to
oxidation and replacement that may mask their original morphology.
Page 243
Chapter 11 Mineralogy: Key Features and Paragenesis
Fragments of siderite are most evident in the gossan of borehole CR194. These
fragments typically exhibit and angular morphology and commonly exceed
several millimetres in size. They are set in a poorly sorted siderite- and quartz-
rich matrix (Figure 11.4). A small proportion of the siderite fragments are
relatively unoxidised and consist of polycrystalline siderite aggregates with
discrete crystallites exceeding 100µm (Figure 11.4). This crystallite size is typical
for all five boreholes.
Figure 11.4 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of angular siderite ‘fragments’ (pinkish white) in a matrix of quartz (light and dark grey shades) and oxidised siderite (black). The siderite fragments are medium-grained, with discrete crystallites exceeding 100µm in size. The width of view is approximately 2mm.
The siderite ‘fragments’ become less apparent with depth in borehole CR194.
This is largely as a result of the oxidation and replacement of the siderite by
limonite and a gradual destruction of the textures. Although the siderite often
occurs as clast-like fragments (Figure 11.4), the morphology of the siderite
aggregates is pseudomorphous and related to the morphology of former rock and
mineral fragments that the siderite has replaced or the cavities within which it has
precipitated (Figures 11.5a, b and c).
Page 244
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.5 - Borehole CR194 – False colour backscattered electron images illustrating a) a siderite ‘fragment’ that actually represents a cavity filling. The width of view is approximately 2mm. b) Compositionally zoned siderite filling a euhedral cavity in quartz. The width of view is approximately 600µm. c) Siderite that appears to have extensively replaced barite (light grey). The width of view is approximately 450µm. d) Siderite filling cavities in botryoidal limonite. The width of view is approximately 250µm. Voids are black.
Page 245
Chapter 11 Mineralogy: Key Features and Paragenesis
An excellent example of this pseudomorphous replacement can be seen in the
replacement of former quartz fragments by siderite (Figure 11.6). The presence
of siderite as mechanically 'transported' clasts would have a significant impact on
the interpretation of the timing and hence the mechanisms behind the
precipitation of siderite. However, the replacement/cavity filling textures imply that
the bulk of the siderite in the Las Cruces gossan appears to have been
precipitated in situ and is chemically rather than mechanically transported.
Figure 11.6 - Borehole CR194 – a digitised photograph showing apparent ‘fragments' of siderite (brown, outlined in red). These clasts are pseudomorphs after quartz-rich rock fragments (white/light grey). An example of a quartz-rich rock fragment partially replaced by siderite is outlined in black. The width of core is approximately 50mm.
The siderite aggregates also commonly exhibit highly irregular margins that are
interpreted here as evidence of dissolution (Figure 11.7). Galena often lines the
margins of the siderite aggregates and allows different stages of siderite
mineralisation to be recognised (Figure 11.7).
Page 246
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.7 - Borehole CR194 - False coloured backscattered electron image illustrating the presence of galena (white) replacing siderite along grain boundaries and highlighting different generations of siderite mineralisation. Limonite (light brown, red arrow) is also present. The width of view is approximately 2.3mm.
The later stages of unoxidised siderite in all five boreholes exhibit a medium
grained crystallite size that typically ranges between 50µm and 150µm. The
crystallites in each of the five boreholes are essentially similar and typically
exhibit simple grain boundary relationships that do not show any evidence of
deformation (Figures 11.4 and 11.8).
Where cavities have not been completely filled by siderite, euhedral crystals are
often present (Figures 11.5a and c). These euhedral crystals are a relatively
common feature in the gossans and the majority range between 20µm and 70µm
with the largest exceeding 100µm. They typically exhibit evidence for at least two
stages of growth with early-formed, oxidised crystals being overgrown by later
stages of unoxidised siderite (Figure 11.9).
Page 247
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.8 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of late-stage, unoxidised siderite (light and dark grey-brown shades) filling a cavity in an oxidised, opaque siderite matrix (black). Tiny skeletal galena crystals (white arrow, black) are often present in the siderite. The siderite also exhibits growth zoning (red arrows). The width of view is approximately 4mm.
Figure 11.9 - Borehole CR194 - A colour transmitted light photomicrograph with crossed polars illustrating the presence of early-formed siderite crystals (dark brown) that have formed in a cavity (dark grey). The early formed siderite crystals have been oxidised and replaced by hematite and then overgrown by later stages of unoxidised siderite (white). The width of view is approximately 1100µm.
Page 248
Chapter 11 Mineralogy: Key Features and Paragenesis
11.3.3 Associations
Several other phases are often present in close association with the siderite. This
provides an insight into the nature and composition of the transporting fluids from
which the siderite precipitated. The associations observed between siderite and
other minerals in the gossan are extensive. There are both direct associations
where siderite may well have been precipitated along with other phases, notably
the Pb- and Fe-bearing sulphides, and indirect associations, whereby siderite
appears to have partially replaced earlier formed and relict minerals, notably
quartz and barite.
Siderite is often intimately associated with Pb-sulphide mineralisation, notably
galena. The siderite veinlets and siderite-filled voids commonly contain abundant
fine-grained skeletal galena (Figure 11.13) and, to a lesser extent, micrometre-
sized, acicular crystals of Pb(AsSb)-sulphides.
The relationships between the siderite and galena are complex but the strong
association between these two minerals is clear. It appears that galena and
siderite mineralisation occurred cyclically and in multiple stages. Figure 11.10
shows the presence of galena partially filling cavities in botryoidal Fe-oxides both
with and without the presence of siderite. The absence of siderite in a number of
these cavities is probably a result of localised dissolution, a feature that is
common throughout the gossan. Numerous examples of galena replacing
siderite (e.g. Figure 11.7), highlight the multiple stages evident between these two
phases. A similar association is evident between siderite and the Fe-sulphide
mineralisation, with euhedral crystals of Fe-sulphide occurring within siderite-filled
cavities. Siderite replacement of Fe-sulphide is also evident locally (Figure
11.11).
Nontronite also exhibits a close association with the late-stage siderite
mineralisation. This commonly occurs along the interface between the oxidised
siderite/hematite matrix and the late stage siderite mineralisation. The
nontronite/siderite association is most evident in borehole CR194. Siderite may
also replace the porous, fine-grained nontronite.
Page 249
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.10 - Borehole CR194 – False colour backscattered electron image illustrating the presence of siderite (dark brown) and galena (white) filling and partially filling cavities in hematite (light brown shades). The galena exhibits characteristic skeletal textures. Siderite is only present filling some of the cavities in this sample and appears to have been leached from the galena-filled cavities in the lower left portion of this image. The width of view is approximately 310µm.
Siderite appears to partially and often extensively replace the quartz fragments
and the fine-grained quartz-rich matrix associated with the gossan and may also
cement the fine-grained quartz-rich matrix, particularly in boreholes CR149,
CR191 and CR038. Relict barite is also often extensively replaced by siderite.
The siderite often exhibits pseudomorphous textures after barite.
A close association also exists between hematite and siderite. Oxidation and
replacement of siderite by hematite is a common feature of the gossan and is
particularly evident in the finer-grained (and hence more reactive) siderite matrix.
Nonetheless, the coarse-grained siderite also exhibits some degree of oxidation,
particularly along grain boundaries and/or the margins of the siderite aggregates.
Siderite-anglesite and siderite-cerussite veinlets are also occasionally present in
the gossan.
Page 250
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.11 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of skeletal galena (pale grey) in euhedral Fe-sulphide crystals (cream-white). Siderite (dark grey/black background) is selectively replacing the Fe-sulphide. The width of view is approximately 85µm.
11.3.4 Mineral Chemistry
The results of the quantitative SEM analyses of the siderite are provided in
Appendix 5. The siderite exhibits some degree of compositional variation both
within and between the boreholes. There is no evidence to suggest any
correlation exists between the compositions of the siderite between boreholes.
Indeed, the compositional variations highlight different stages of siderite
mineralisation, of which there appear to be many.
Compositional zoning of the siderite is also evident throughout the gossan. The
changes in composition largely reflect minor variations in the relative proportions
of Fe, Mg and Ca, with most other elements being present below detection limits
(0.5%). The MgO and CaO content of the siderite ranges between an effective
lower limit of less than 0.5 per cent to the highest values that may exceed 8.00
Page 251
Chapter 11 Mineralogy: Key Features and Paragenesis
per cent. It should be noted that the majority of semi-quantitative and qualitative
EDX analyses were performed on the relatively late stage siderite mineralisation,
with the bulk of the earlier siderite being too oxidised to provide suitable material
for analysis.
The zoned siderites in the Tertiary sand of borehole CR149 exhibit compositions
that are largely chemically indistinct to those of the gossan samples. However,
the very prominent two-stages of zoning exhibited by these siderites, with a
CaMg-rich core and Fe-rich rim is a feature that distinguishes them from the
majority of the gossan siderites, which typically exhibit either relatively uniform
compositions or a more varied compositional zoning. Siderite dissolution is
common and the degree of dissolution appears to be affected by the composition
of the siderite, with selective leaching occurring within compositionally zoned
siderite crystals (Figure 11.12).
Figure 11.12 - Borehole CR191 – False colour backscattered electron image illustrating the selective leaching of compositional zones within siderite crystals (brown shades). The siderite is present along margins of quartz fragments (mauve) and within voids (black). Minor galena (white) and Fe-sulphide (light khaki) are also present. The width of view is approximately 310µm.
Page 252
Chapter 11 Mineralogy: Key Features and Paragenesis
Locally, minor amounts of Pb are present in the siderite. The orthorhombic Pb-
carbonate mineral cerussite (ideally PbCO3) does not exhibit solid solutions with
rhombohedral siderite and the presence of minor amounts of Pb therefore most
likely reflects the presence of sub-microscopic grains of cerussite and/or galena
within the siderite. The presence of sub-microscopic Pb-rich zones within the
siderite is further evidence for a strong association between the siderite and Pb
mineralisation.
11.3.5 Isotope Analysis
The presence of siderite as the dominant Fe-bearing mineral in the Las Cruces
gossan clearly indicates that this deposit has been subjected to processes that
are not typical of sub-aerially formed gossans, which are more commonly
dominated by the presence of Fe-oxides, hydroxides and sulphates. In order to
provide additional evidence to aid in the interpretation of mineral textures
observed in this deposit, three small, carefully selected samples of siderite were
submitted to Dr. Steve Crowley of the University of Liverpool for isotope analysis.
Due to the extensive oxidation of much of the siderite, sample selection was, to a
large degree, based on the ability to select relatively clean mineral separates.
Fresh, relatively unweathered siderite was carefully hand picked from wet
screened, sized mineral fractions of the gossan material from boreholes CR194
and CR123. These materials were ground in a pestle and mortar and checked for
purity using XRD analysis. XRD confirmed that the materials consisted
predominantly of siderite together with subordinate amounts of galena. No other
carbonate minerals were present confirming that the samples were suitable for
isotope analysis. Analysis of the siderite samples from boreholes CR194 and
CR123 confirm that the 13C and 18O ratios are as follows:-
Table 11.1 - Siderite 13C and 18O Ratios
Sample 13C 18O
CR123 -35.5 -3.4
CR194 #1 -33.0 -4.9
CR194 #2 -41.7 -5.1
Page 253
Chapter 11 Mineralogy: Key Features and Paragenesis
The carbon isotope ratios are particularly low, strongly indicating the involvement
of methane oxidation as an important source of HCO3. Crowley (pers. comms.)
warns of placing too much emphasis on the oxygen isotope data, noting that too
little is known about the temperature dependence of siderite-H2O oxygen isotope
fractionation. Professor Max Coleman from the University of Reading, suggests
that the oxygen isotope value may well be more negative than equilibrium if
produced by rapid microbial activity (Coleman, Pers. comms.). Coleman
suggests that estimating local groundwater at -7 SMOW would give formation
temperatures for the siderite at below 20oC. It must be emphasised, however,
that this is only indicative and information on the nature of the groundwater in this
region was unavailable.
11.3.6 Fluid Inclusion Analysis
Two samples from boreholes CR194 and CR123 respectively were analysed.
The main aim of this analysis was to provide constraints on the nature of the
fluids responsible for siderite precipitation.
Two doubly polished wafers approximately 100µm thick were prepared from the
two polished sections. Difficulties in identifying the presence of fluid inclusions in
the wafers were encountered due to the extremely fine-grained and oxidised
nature of much of the sample. Fluid inclusions were identified and described with
respect to their host mineral and any textural features present in the samples.
No fluid inclusions were observed in the oxidized material. Very small inclusions
forming cloudy, inclusion-rich growth bands occur in the first stage siderite
overgrowing opaque, oxide-rich material. This is overgrown by more coarsely
crystalline, transparent siderite that is inclusion-free and contains disseminated
opaque grains. Some vapour-rich inclusions appear to be present but these show
signs of possibly having leaked and are therefore considered unreliable.
Insufficient data were obtained from the fluid inclusion measurements to provide
any reliable information on the formation temperature. Wilkinson (pers. comms.)
notes that although the absence or paucity of fluid inclusions is not necessarily
diagnostic of a low temperature of formation, it might tend to be the case.
Page 254
Chapter 11 Mineralogy: Key Features and Paragenesis
11.4 Galena
11.4.1 Relative Abundance
Galena is the dominant Pb-bearing mineral in the Las Cruces Gossan. The Pb
content of the sample intervals provides a good measure of the abundance of
galena, with other Pb-bearing phases occurring in relatively minor amounts.
Galena is a very common accessory mineral in the gossans with the exception of
Borehole CR038, where the Pb content rarely exceeds 0.5 per cent. The relative
proportion of galena increases towards the base of the gossans of boreholes
CR194 and CR149, but is enriched at the top of the gossan of borehole CR191.
A number of galena-rich horizons exist in the gossan of borehole CR123, with the
Pb content reaching a maximum of 27 per cent in one sample interval, equivalent
to approximately 34 per cent galena.
Galena remains a common accessory mineral in the uppermost portion of the
massive sulphide of boreholes CR194, CR149, but decreases markedly with
depth relative to the overlying gossan for all of the boreholes examined during
this investigation.
11.4.2 Grain Size and Shape
Locally, the galena occurs as discrete euhedral crystals that rarely exceed a few
tens of micrometres in size. The bulk of the galena is present as fine-grained,
massive aggregates that largely consist of densely packed micrometre-sized
skeletal crystals or porous aggregates that do not exhibit any discrete crystal
form. Collomorphic textures are also locally present in the fine-grained and
porous galena aggregates.
Fine-grained, skeletal galena is one of the most striking and prominent textures
observed in the galena, occurring in all five boreholes examined during this
investigation. The most prominent and well-developed skeletal crystals occur in
the gossan/massive sulphide contact zone of borehole CR149.
Page 255
Chapter 11 Mineralogy: Key Features and Paragenesis
The fine-grained nature of the galena, the presence of skeletal crystals and
collomorphic textures and the absence of large, euhedral crystals are indicative of
rapid precipitation.
Skeletal crystals form by rapid mineral growth along corners and edges of the
crystal due to greater exposure to the mineralising solutions. Skeletal crystals
that form more extensive tree-like intergrowths are referred to as dendrites. The
rapid crystallisation may result from rapid cooling of the mineralising fluids or as a
result of supersaturation of the solution. Often, after the first rapid growth of
skeletal crystals, the skeletal texture is infilled by further crystallisation of galena
and the skeletal nature of the crystals may only be evident after chemical etching
or natural selective leaching (Ramdohr, 1980).
The skeletal galena in the Las Cruces gossan appears to have formed as a result
of rapid crystallisation followed by continued precipitation of less well developed,
porous galena aggregates. Later stages of siderite and to a lesser extent calcite
mineralisation selectively replace the fine-grained and porous galena aggregates,
leaving only the well formed skeletal galena which appears to be more resistant
to replacement by carbonate. There are several clear examples of this
replacement process (Figure 11.13). The resistance to replacement of the
skeletal galena is also seen in Figure 11.11.
The development of more coarsely crystalline galena was also probably inhibited
by a combination of fluid chemistry, notably the lack of available Pb and/or S for
further crystal growth, formation temperature and rate of precipitation.
Page 256
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.13 - Borehole CR191 – A colour, reflected light photomicrograph illustrating the presence of a fine-grained galena aggregate (pale cream) that is being progressively replaced from the upper left to lower right by siderite (dark grey background). The siderite is selectively replacing the finer-grained galena, with only the skeletal galena surviving as a relict phase. The width of view is approximately 375µm.
11.4.3 Associations
A range of typically associations between galena and other minerals are
illustrated in Figures 11.14a to k). The bulk of the galena in the gossan exhibits a
close association with siderite and Fe-sulphides (Figures 11.14a, b and c). The
nature of these associations is complex and suggests cyclical stages of galena
and siderite precipitation with various stages of dissolution and replacement also
being evident. Similar cyclical replacement and precipitation textures are
observed between galena and calcite in borehole CR123.
The fine-grained and porous galena aggregates typically contain variable (up to
several percent) amounts of Sb and As, that may reflect the presence of
unresolved PbAsSb-sulphides. Mimetite and cerussite are locally present in
close association with the galena.
Page 257
Chapter 11 Mineralogy: Key Features and Paragenesis
Galena is also closely associated with the precious metal mineralisation in the
gossan and may form micrometre-sized rims on the native Au and Au-amalgam
(Figure 11.14e). This feature is evident in all five boreholes. In borehole CR194,
there is a marked increase in the abundance of galena in the base of the gossan,
which, in turn, is associated with a marked increase in the precious metal content
with discrete Au-bearing grains being common in the galena-rich aggregates.
However, elevated levels of Pb in the core are not always associated with
elevated levels of Au and there is conflicting evidence to suggest that galena and
native Au-bearing grains formed from the same mineralising fluids. It is likely that
there have been several stages of galena mineralisation, not all of which have
been associated with precious metal mineralisation. Fe-sulphides are commonly
intergrown with galena in all five boreholes and complex replacement
associations may exist between these two minerals (Figure 11.11).
Towards the base of the gossan in borehole CR194, close to the underlying
massive sulphide, chalcopyrite and members of the tetrahedrite-tennantite solid
solution series are also intimately associated with the galena. The galena
appears to partially replace these phases along the margins of the sulphide-rich
aggregates and along grain boundaries (Figure 11.14i). At the contact between
the gossan and underlying massive sulphide in borehole CR149, euhedral galena
is present in sternbergite (Figure 11.14g). Similarly in borehole CR123, narrow
veinlets/fractures containing skeletal galena are also partially filled by
sternbergite. In the uppermost portions of the massive sulphide in boreholes
CR194, CR149, CR038 and CR191, galena forms thin rims on the primary pyrite
and also appears to replace the pyrite along grain boundaries and fractures
(Figure 11.14a).
Minor amounts of galena are also typically associated with the secondary Cu-
sulphide mineralisation in the supergene enriched massive sulphides associated
with each borehole.
Page 258
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.14 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of associations between galena and other minerals (various scales). a) Replacement of partially leached, relict primary pyrite by late-stage galena. b) Fine-grained and porous galena aggregates with intergrown Fe-sulphide. c) Partial replacement of siderite by vermicular galena. d) Au-bearing grains in galena. e) Fine galena rims on Au-bearing grains. f) Galena overgrowth on native Au. g) Euhedral galena crystals in sternbergite, Au and pyrite. h) Skeletal galena in cerussite, mimetite and siderite-bearing vein. i) Galena replacing tetrahedrite. j) Galena replacing quartz along grain boundaries. k) Galena replacing calcite along margins of fragments with later calcite and Fe-sulphide filling pore spaces (black).
Page 259
Chapter 11 Mineralogy: Key Features and Paragenesis
11.5 Fe-Sulphide Phases
11.5.1 Introduction
The Las Cruces gossan is characterised by a fine-grained Fe-sulphide
assemblage that is intimately associated with the siderite and galena
mineralisation. The Fe-sulphides, as described here, refer to Fe-sulphides other
than marcasite and pyrite, unless otherwise stated.
Optical microscopy and textural interpretation of the Las Cruces gossan Fe-
sulphide mineral assemblage, as presented in this Chapter, has recognised four
discrete Fe-sulphide phases that exhibit varying degrees of replacement by
marcasite and pyrite:
Type 1 - Colloidal, isotropic Fe-sulphide, probably amorphous FeS.
Type 2 - Colloidal/feathery, anisotropic Fe-sulphide, probably mackinawite.
Type 3 - Euhedral, isotropic Fe-sulphide, probably greigite.
Type 4 - Platy, anisotropic Fe-sulphide, probably pyrrhotite.
The Fe-sulphide assemblage is, at least in part, magnetic. Due to the paucity and
extremely fine-grained nature of the Fe-sulphides, positive identification was
considerably hampered. Nonetheless, XRD has confirmed the presence of
greigite (Figure 11.15), marcasite and pyrite. Mackinawite and pyrrhotite have
not been identified by XRD, although a detailed examination of the assemblage
using reflected light microscopy suggests that both mackinawite and pyrrhotite
may be present in these samples. The presence of a
mackinawite/greigite/marcasite/pyrite assemblage is also consistent with the
literature review on the formation of Fe-sulphides.
Page 260
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.15 - An X-ray diffractogram clearly illustrating the presence of greigite (peaks donated with blue vertical lines). The unlabelled peaks reflect the presence of quartz, lepidocrocite and sulphur. The lepidocrocite and sulphur represent the oxidation products of the Fe-sulphide assemblage. The broad peak width for greigite is indicative of a poorly crystalline nature.
11.5.2 Relative Abundance
The Fe-sulphide phases are confined to the gossan and show increased
replacement by pyrite and marcasite lower in the profile toward the contact with
the underlying massive sulphide. The Fe-sulphides rapidly oxide to form
lepidocrocite and native sulphur and are therefore largely confined to less
oxidised portions of the gossans and the later stages of siderite/galena
mineralisation. Due to their strongly magnetic nature, the presence of Fe-
sulphides is often evident in hand specimen and the magnetic nature of the core
has been logged by the field geologists (Appendix 2). Fe-sulphides are most
abundant in the middle portion of the gossan of CR194, the upper and lower
portions of the CR149 gossan and throughout the CR123 gossan. Fe-sulphides
are sparingly present throughout the quartz replaced tuffs of CR038 and are a
common accessory in the upper gossan of borehole CR191.
Page 261
Chapter 11 Mineralogy: Key Features and Paragenesis
11.5.3 Reflected Light Characterisation
Detailed examination of the Fe-sulphide phases by reflected light microscopy has
revealed additional information on the nature of this fine-grained mineral
assemblage and four discrete Fe-sulphide mineral types are evident.
Fe-sulphide ‘Type 1’ consists of radiating feathery aggregates that appear to be
colloidal, possibly amorphous in nature (Figure 11.16). This phase appears to be
anisotropic although the optical properties are somewhat masked by the
extremely fine-grained nature. This phase, tentatively described as FeS(am),
exhibits a distinctive pinkish brown colour in reflected light relative to pyrite and
marcasite. Although the polished section from which this material was located
exhibits strong magnetic properties, other forms of Fe-sulphide are intimately
present and it is not possible to identify the individual magnetic species.
Figure 11.16 – Colour, reflected light photomicrograph illustrating Type 1 Fe-sulphide, consisting of feathery, colloidal radiating aggregates of Fe-sulphide (FeSam or mackinawite/nanoparticulate mackinawite of Wolthers et al. (2003)). This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.
Page 262
Chapter 11 Mineralogy: Key Features and Paragenesis
Locally, anisotropic effects are observed in these feathery masses, particularly
along the margins of the aggregates where the feathery crystals exhibit a more
coarse-grained texture (Figure 11.17). These aggregates most likely consist of
amorphous FeS (FeS(am)) or nanoparticulate mackinawite (Wolthers et al., 2003)
that has been replaced by mackinawite or subjected to some degree of
recrystallisation and grain coarsening. This phase is Fe-sulphide ‘Type 2’.
Figure 11.17 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3 Fe-sulphide, circled) with marcasite/pyrite inclusions (paler yellow-white, white arrow) forming overgrowths on a colloidal Fe-sulphide aggregate (Type 1). The colloidal Fe-sulphide exhibits paler coloured, feathery intergrowths, towards the margins which appear strongly anisotropic (Type 2, possibly mackinawite, very weakly defined, red arrows). This polished section is magnetic. Borehole CR149, 151.75m. 100x oil, 50% zoom ppl, width of view 105um.
As well as the colloidal, radiating aggregates of Fe-sulphide (Type 1), fine-
grained, idiomorphic crystals are disseminated throughout the late-stage siderite
(Figures 11.18, and 11.19). These ‘Type 3’ euhedral Fe-sulphide crystals are
anisotropic and commonly contain inclusions of pyrite/marcasite. The idiomorphic
crystals also appear to replace the colloidal, radiating aggregates of Type 1 Fe-
sulphide (Figure 11.18).
Page 263
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.18 – Colour, reflected light photomicrograph illustrating colloidal radiating aggregates of Fe-sulphide (Type 1, white arrow) and finely disseminated euhedral Fe-sulphide crystals (Type 3, light grey, yellow arrow) in siderite (dark brown transparent gangue). The euhedral Fe-sulphides may form as a replacement or recrystallisation product of the colloidal Fe-sulphide (white circle). The black regions within the centre of these aggregates are voids. This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.
The euhedral Fe-sulphide crystals often exhibit porous textures, possibly
indicative of volume changes during replacement (Figure 11.20). The euhedral
crystals appear to either replace the colloidal aggregates, or form as a result of
recrystallisation. The presence of pyrite/marcasite inclusions probably reflects
replacement of these euhedral Fe-sulphide crystals. The euhedral Fe-sulphide
crystals appear to represent greigite, as they are isotropic and are associated
with magnetism in the core (Figure 11.21). Greigite is, however, cubic and these
crystals rarely exhibit cubic morphologies. Nonetheless, greigite is known to
pseudomorphously replace mackinawite.
Page 264
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.19 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3, cream/yellow) in siderite (black). The porous cores of the crystals may have resulted from volume changes during replacement or be relicts of recrystallisation/replacement of colloidal aggregates. This sample is magnetic. Borehole CR194, 156.70m (upper). 100x oil, ppl, width of view 150um.
Figure 11.20 - Colour, reflected light photomicrograph illustrating euhedral Fe-sulphide crystals (Type 3, pale pinkish brown) with marcasite/pyrite inclusions (paler yellow-white, white arrow) in siderite (black background). The crystals exhibit a marked porosity (red arrow), possibly indicative of volume changes during replacement. This sample is magnetic. Borehole CR194, 151.75m. 100x oil, 100% zoom, ppl, width of view 85um.
Page 265
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.21 - Colour, reflected light photomicrograph illustrating feathery, strongly anisotropic Fe-sulphide (Type 2, pinkish cream shades, mackinawite?) with cubic overgrowths of Fe-sulphide crystals (Type 3, greigite?, circled) in siderite (dark brown/black background). Cores of pyrite/marcasite are present (pale yellow). This sample is magnetic. Borehole CR149, 151.75m. 100x oil, 100% zoom, ppl, width of view 85um.
In addition to the key textural features described above, platy crystals of Fe-
sulphide also occur (Figure 11.22). These crystals, described here as ‘Type 4’
Fe-sulphide, consist of thin, strongly anisotropic platelets that appear to be
magnetic. This phase may represent monoclinic pyrrhotite, the other common
magnetic Fe-sulphide. The presence of pyrrhotite could not be confirmed by XRD
and due to the presence of other Fe-sulphides in the polished section, it could not
be confirmed that the magnetic nature of the material was directly associated with
this phase. Although this phase occurs in association with Types 1 to 3 Fe-
sulphide locally, it is largely present as a very separate phase of mineralisation,
whereas Types 1 to 3 are often intimately associated with each other. The platy
Fe-sulphide exhibits almost identical textures to the pyrrhotite recognised by
Larrasoaña et al. (2007) in marine sediments from Cascadia Margin, offshore
Oregon.
Page 266
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.22 - Colour, reflected light photomicrograph illustrating platelets of an anisotropic Fe-sulphide phase (Type 4) with minor pyrite/marcasite (poorly resolved, white arrow) in a matrix of siderite (dark brown background). This section is magnetic. Borehole CR123, 152.40m (lower). 100x oil, 100% zoom, ppl, width of view 85um.
Lower in the gossan profiles, towards the contact with the underlying massive
sulphides, the Fe-sulphides typically show a greater degree of pseudomorphous
replacement by marcasite and pyrite (Figures 11.23 and 11.24).
Page 267
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.23 - Platy textures in pyrite (pale yellow) that has pseudomorphously replaced Type 4 Fe-sulphide (pyrrhotite? darker pinkish brown, white arrow) in siderite (dark brown/black background). This section is weakly magnetic, probably due to the presence of disseminated greigite crystals in the siderite (not present in this image). Borehole CR149, 187.40m (middle). 100x oil, 100% zoom, ppl, width of view 85um.
Figure 11.24 - Marcasite (pale yellow, white arrow) extensively replaces the strongly anisotropic Fe-sulphide phase (probably mackinawite +/- greigite, darker pinkish yellow, red arrow) that is partially filling a euhedral cavity in quartz (undifferentiated black background). This section is magnetic. Borehole CR191, 150.10m. 40x air, ppl, width of view 375um.
Page 268
Chapter 11 Mineralogy: Key Features and Paragenesis
11.5.4 Optical Properties and Occurrences of the Fe-sulphides
Greigite is not a mineral commonly described in gossans, however, greigite
appears to be one of the dominant Fe-sulphide phases, contributing significantly,
if not entirely to the magnetic properties of the Las Cruces gossan. Therefore an
understanding of how it forms will help to identify the processes of formation of
this rather unusual gossan.
Greigite is a relatively recently discovered mineral (Skinner et al., 1964), and its
poor stability and often fine-grained nature make it difficult to identify. Failure to
identify greigite or incorrect identification is undoubtedly widespread with XRD
probably being one of the more reliable methods for positive identification of this
phase.
Greigite is a thiospinel, which shares the same crystal structure as magnetite and
is therefore strongly ferromagnetic (Roberts and Weaver, 2005). It often occurs
as tiny grains and crystals (Lennie et al., 1997), rarely as cubes, and as balls of
intergrown octahedral (known as framboids) with curved faces up to 0.5 mm in
size (Anthony et al., 1990). Greigite is pale creamy white in reflected, plane
polarised light, is isotropic and exhibits no internal reflections (Uytenbogaardt and
Burke, 1971). It is often formed in lacustrine beds and hydrothermal vein deposits
(Anthony et al., 1990) and may, partly, be biogenic in origin, being found as
inclusions in magnetotactic bacteria. Greigite forms authigenically in anoxic
sedimentary environments as a precursor to pyrite (FeS2) in association with
chemical reactions driven by bacterial degradation of organic matter (Roberts and
Weaver, 2005).
In nature, mackinawite typically occurs as a poorly crystalline precipitate (Lennie
et al., 1997), as well-formed thin tabular crystals, to 1 mm and as fine-feathery
massive aggregates (Anthony et al., 1990). It sometimes occurs as idiomorphic
crystals and due to its perfect basal cleavage, sometimes flakes like graphite.
Mackinawite exhibits a pinkish grey colour in reflected, plane polarised light,
similar to pyrrhotite, and a moderate to strong bireflectance, very strong
anisotropy and no internal reflections (Uytenbogaardt and Burke, 1971). It is
Page 269
Chapter 11 Mineralogy: Key Features and Paragenesis
formed by hydrothermal activity in mineral deposits, during serpentinisation of
peridotites, and in the reducing environment of river bottom muds. Mackinawite
may be produced by magnetotactic and sulphate-reducing bacteria (SRB).
Mackinawite occurs rarely in iron and carbonaceous chondrite meteorites
(Anthony et al., 1990).
Pyrite is the most widespread of the sulphide minerals. It is cubic, isotropic and
exhibits a pale brass-yellow in reflected light (Anthony et al., 1990). The crystal
form of pyrite is usually idiomorphic but may also be granular, colloidal,
concretionary and cryptocrystalline (Uytenbogaardt and Burke, 1971). Pyrite
forms in a wide variety of environments, including hydrothermal deposits and as
diagenetic deposits in sediments (Anthony et al., 1990).
Marcasite is orthorhombic and is distinguished from isotropic pyrite by its strong
anisotropy and strong bireflectance (Uytenbogaardt and Burke, 1971). Marcasite
typically forms under low temperature, highly acidic conditions including
sedimentary environments and hydrothermal veins. Marcasite typically occurs as
idiomorphic crystals, often as laths and aggregates of radiating crystals, or
colloform aggregates (Anthony et al., 1990).
Pyrrhotite is monoclinic or hexagonal. Pyrrhotite occurs as granular aggregates,
or commonly as tabular or platy crystals that may form rosettes. In reflected light
pyrrhotite is pinkish brown, exhibiting strong bireflectance and strong anisotropy
(Uytenbogaardt and Burke, 1971). Although pyrrhotite is largely found in mafic
igneous rocks, it is also associated with hydrothermal veins, sedimentary and
metamorphic rocks and meteorites (Anthony et al., 1990). Monoclinic pyrrhotite is
magnetic.
Optical investigations suggest that Type 1 likely represents FeS(am) or
nanoparticulate mackinawite and Type 2 is likely to be mackinawite. The
relationship between Type 1 and 2 is likely to be one of recrystallisation and/or
replacement. Type 3 is tentatively identified as greigite. Greigite is known to be
present due to confirmation by XRD techniques. The morphology of the greigite
is not necessarily consistent with a cubic mineral, but the greigite may well be
Page 270
Chapter 11 Mineralogy: Key Features and Paragenesis
pseudomorphous after mackinawite. The optical properties of Type 4 are
consistent with pyrrhotite, and the textures observed are almost identical to those
recognised by Larrasoaña et al. (2007).
Page 271
Chapter 11 Mineralogy: Key Features and Paragenesis
11.6 Au-Bearing Phases
11.6.1 Relative Abundance
The Au content of the gossan is variable, but is high relative to the overlying
Tertiary deposits and the underlying massive sulphides. In borehole CR194, the
Au content increases towards the middle portion of the gossan with discrete
native Au grains being observed. The Au content increases markedly towards
the base of the gossan at the contact with the underlying massive sulphide where
the dominant Au-bearing phase is Au-amalgam.
In boreholes CR149 and CR191, the Au content is relatively high in the upper
gossan, decreasing towards the middle gossan and then increasing again
towards the base of the gossan at the contact with the underlying massive
sulphide. Elevated levels of Au are also present throughout the quartz-replaced
tuffs of borehole CR038 and at the base of the gossan in borehole CR123.
Native Au is the dominant Au-bearing phase in all but borehole CR194, where
both Au and Au-amalgam is present.
The Au content of the massive sulphide is low in all five boreholes, but increases
markedly in the massive sulphide/shale horizon in borehole CR194. The increase
in Au content in this highly porous zone has resulted from the penetration of
supergene solutions. The Au content of the shale in borehole CR194 is very low.
There is enrichment of Au close to the base of the gossan at the contact with the
massive sulphide in all of the boreholes. These two features are consistent with a
model of supergene Au enrichment during oxidation and mass wasting of the
massive sulphide deposit. Other Au-rich horizons are, however, also developed
in the middle and upper portions of the gossan in some boreholes. This suggests
that other processes may be causing Au mobilisation and precipitation.
Alternatively, these horizons may represent relict supergene zones that once
represented contact zones between the gossan and massive sulphide, with
subsequent oxidation events resulting in a lowering of the water table and deeper
oxidation of the sulphides.
Page 272
Chapter 11 Mineralogy: Key Features and Paragenesis
11.6.2 Grain Size and Shape
The grain size of the native Au grains in the gossan is generally very fine, with
discrete grains rarely exceeding a few micrometres in maximum dimensions.
This and the general paucity of microscopically visible grains suggest that a
significant portion of the Au may be present in a sub-microscopic form. In
particularly Au-rich sample intervals, native Au grains may occasionally exceed
30µm (e.g. CR038 and CR123).
Within the galena-rich layer of borehole CR194, the Au-bearing amalgam may
exceed 100µm in size, although the majority of grains are typically <20µm. In
borehole CR149, rhythmically precipitated Au grains in sternbergite range from a
few micrometres in size to below the limits of optical resolution (see Figure
11.14g), indicating that at least some of the Au may be present in a sub-
microscopic form in the Ag-rich sulphides. Despite the relatively high Au content,
no discrete Au-bearing grains were recognised in the massive sulphide/shale
zone of borehole CR194, and the bulk of the Au is therefore likely to be present in
a sub-microscopic form.
The native Au grains in the gossans range from euhedral to highly irregular
(Figures 11.25a to 11.25i). Grains that are typically less than a few micrometres
in size more commonly exhibit rounded morphologies. The irregularly shaped,
cuspate margins of some of the Au/Au-amalgam grains associated with galena
suggests they may have been subjected to some degree of dissolution and/or
replacement (Figures 11.25a, 11.25b and 11.25c). A single, rounded and
compositionally zoned native Au grain was also located in the gossan of borehole
CR194. The bulk of the microscopic Au grains located in the gossan/massive
sulphide contact zone of borehole CR149 occurs as very fine-grained and
irregularly shaped grains in sternbergite, although the grains are often on the
limits of resolution (~1µm) (Figure 11.25i). The very minor amounts of fine-
grained native Au located in the lower gossan of borehole CR191 exhibits highly
irregular morphologies. This, at least in part, reflects the irregular shape of the
cavities in which the Au has precipitated, occurring largely along the margins of
angular and irregularly shaped quartz fragments.
Page 273
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.25 - A montage of false colour, backscattered electron images selected from Chapters 5 to 9, illustrating a wide range of morphologies and associations of the Au and Au-bearing grains (various scales). a) An aggregate of irregularly shaped Au-amalgam grains in galena. b) Two native Au grains rimmed and possibly replaced by galena. c) Au-amalgam grains with cuspate margins that appear to have been extensively replaced by galena. d) Irregularly shaped native Au grains in cinnabar. e) A euhedral native Au grain in lepidocrocite. f) A euhedral native Au grain in siderite. g) Anhedral native Au in siderite and Fe-sulphide. h) Native Au in a euhedral cavity (black) in quartz. i) Euhedral native Au in galena replacing quartz. j) Au in Fe-sulphide. k) Euhedral Au with cassiterite. l) Au in Fe-sulphide and anatase.
Page 274
Chapter 11 Mineralogy: Key Features and Paragenesis
11.6.3 Associations
A striking feature of the native Au and Au-amalgam observed throughout the
gossan is the intimate association with galena (Figures 11.25a, b, c and i) which
commonly rims Au-bearing grains (Figure 11.25b). Less common are Pb(SbAs)-
sulphide rims on native Au. Similarly, native Au inclusions and intergrowths with
Fe-sulphides were also observed in lesser amounts but throughout the gossan
(Figures 11.25g, j and l), especially in borehole CR038.
Native Au grains are commonly present in euhedral cavities and along the
margins of the quartz-rich matrix, particularly in borehole CR038, highlighting the
importance of porosity to fluid migration (Figure 11.25h). Later stages of
mineralisation commonly fill or partially fill the cavities within which the Au is
present. Siderite and chalcedony are examples of these later stages of cavity-
filling mineralisation. Less common associations include nukundamite and
bismuthinite, both of which have been observed in close association with native
Au (Appendices 6 to 10).
The gossan contact with the massive sulphide in borehole CR149 contains
extremely fine-grained and complex aggregates of native Au that occur within
sternbergite (Figure 11.14g). Au-sternbergite associations were also recognised
in other boreholes in this suite. Borehole CR038 exhibited some associations
that were not observed in any of the other boreholes in this suite, including two
discrete occurrences of Au with cassiterite (Figure 11.25k) and numerous
examples of fine-grained native Au in anatase (Figure 11.25l).
Native Au grains were also located in a cinnabar aggregate in borehole CR123
(Figure 11.25d). These grains exhibit an irregular morphology and cuspate grains
boundaries that may indicate partial replacement of the Au. Although no discrete
microscopic grains of native Au were located in the porous massive
sulphide/shale of borehole CR194, the high levels of Au associated with this zone
coincide with the elevated levels of Ag, Bi, Hg and Sb associated with the
presence of supergene tetrahedrite. It is therefore likely that the Au is present in
a submicroscopic form associated with the supergene mineralisation.
Page 275
Chapter 11 Mineralogy: Key Features and Paragenesis
11.6.4 Chemistry
The majority of the native Au grains located in the gossans contain less than five
weight per cent Ag, with the majority of grains containing less than 0.5 weight
percent Ag. A single, spheroidal grain with a Ag-rich rim was located in borehole
CR194 and several Ag-rich Au grains (electrum), were identified within the
galena-rich aggregates. The low Ag content of the bulk of the native Au grains is
consistent with supergene Au precipitated from solution in an acidic and oxidising
environment. The presence of at least some electrum grains suggests that the
conditions under which the Au was mobilised may have been quite variable.
The gossan contact with the massive sulphide in borehole CR194 is
characterised by the presence of Au-amalgam grains. No native Au grains were
located during the examination of the polished sections from this sample interval
and it is presumed that the bulk of the Au is present as Au-amalgam. The Au, Ag
and Hg contents between grains are quite variable (Appendix 5). The Au-
amalgam grains in the middle portion of this contact zone typically contain
subordinate amounts of Au (range 5.4–11.6%). The Au content of the Au-
amalgam grains located in the lower portion of this contact zone typically contain
more significant amounts of Au (range 27.7–56.5%).
In the galena-rich layer of borehole CR194 the Au content of Au-amalgam grains
exhibits a more restricted compositional range (16.4–21.0%). A single,
compositionally zoned grain, depleted of Ag towards the margins, is possibly
indicative of leaching. Au-amalgam was not observed in any other of the
boreholes selected for examination during this investigation and may reflect
restricted Hg migration away from the central supergene sulphide orebody.
Elevated levels of Hg and Se were associated with native Au in borehole CR149.
A small number of Ag-bearing native Au grains were located in the gossan/shale
conglomerate contact of borehole CR123. The Ag content of the native Au grains
ranges from between approximately 15 and 30 per cent.
Page 276
Chapter 11 Mineralogy: Key Features and Paragenesis
11.7 Gossan Paragenesis
11.7.1 Introduction
The dominant gossan mineral paragenesis is described here. The paragenesis
has been determined by a detailed examination of mineral associations and
textural observations documented in this thesis. These observations also aid in
the understanding of the geochemistry of the mineralising fluids. The paragenetic
sequence for the Las Cruces gossan is as follows:
Quartz (resistate) → Native Au (± Au amalgam) → Galena (± Pb(SbAs)-
sulphosalts) → Fe-sulphides → Siderite
This mineral paragenesis is rarely observed in the gossan as a complete
sequence. This appears to be due to a number of factors including:
Multiple stages of mineralisation/changes in fluid chemistry
Oxidation, particularly of Fe-monosulphides and siderite
Replacement (e.g. siderite replaces Fe-monosulphides and galena)
Dissolution (e.g. of siderite)
Partial sequences are observed consistently in all five boreholes examined during
this investigation, examples of which are illustrated in Figure 11.26. Mineralogical
examination confirms that the bulk of the quartz is resistate in nature and is
therefore not strictly part of the siderite, sulphide and precious metal
mineralisation that forms the dominant gossan assemblage.
Native Au is commonly found in relative isolation within cavities and along grain
boundaries associated with relict quartz (Figure 11.26a). The native Au may well
have originally precipitated with galena (Figure 11.26b), Fe-monosulphides
(Figure 11.26e) and siderite, but extensive dissolution, replacement and oxidation
has, in many cases, removed these phases. Native Au is extremely inert and the
least mobile of the phases in this paragenetic sequence. It is resistant to
oxidation and dissolution and its survival in relative isolation is therefore not
unexpected.
Page 277
Chapter 11 Mineralogy: Key Features and Paragenesis
Figure 11.26 – Montage of false colour backscattered electron images illustrating partial and complete paragenetic sequences observed during this investigation. a) Au (yellow) is frequently located in isolation along the margins of relict quartz grains (mauve). b) Au with siderite (brown) cementing relict quartz. c) Au with overgrowths of galena (pale blue/white) cementing relict quartz. d) Au with overgrowths of galena (pale blue/white) and siderite cementing relict quartz. e) Au inclusion in Fe-monosulphide (light grey/brown) in quartz. f) Au with euhedral Fe-monosulphide crystals and siderite cement. g) A rare example of a complete paragenetic sequence consisting of Au → galena → Fe-monosulphide → siderite.
In the absence of complexing ions, native Au is insoluble across the entire Eh/pH
range. In acid, oxidising environments, Au is generally considered to be soluble
as a Au-chloride complex, but only under relatively high chloride concentrations.
In the presence of free sulphur, under near neutral to alkaline reducing
conditions, Au is most likely mobile as a thiosulphate complex. Subtle changes in
Eh, pH and solution chemistry would therefore likely result in the rapid
precipitation of native Au from solution. Au-chloride complexes may also be
Page 278
Chapter 11 Mineralogy: Key Features and Paragenesis
reduced in the presence of Fe2+. Au-amalgam was only observed in borehole
CR194 and is therefore not considered as part of the dominant mineral
assemblage.
Native Au is observed in close and direct association with siderite (Figure
11.26b). Examples illustrated in Figure 11.26d, 11.26f and 11.26g demonstrate
that siderite represents the final stage of mineralisation in this sequence.
However, the example illustrated in Figure 11.26b may be the result of two
discrete mineralisation events involving the initial precipitation of native Au (±
other phases removed via oxidation, replacement e.t.c.) followed by a later,
discrete stage of siderite mineralisation.
The most common and widespread association with native Au (or Au-amalgam) is
that of galena. Galena clearly forms later overgrowths on native Au (Figures
11.26c and d). Pb is extremely immobile under most Eh/pH conditions, only
becoming soluble as Pb2+aq and HPbO2
-aq under extreme acid and alkaline
oxidising conditions respectively. Galena is the stable phase under strongly
reducing conditions and rapidly oxidises to form anglesite and cerussite under
acid and alkaline oxidising conditions. The early precipitation of galena from
solution is therefore not unexpected due to the highly immobile nature of Pb. The
galena would have precipitated under strongly reducing conditions over a wide
pH range.
Locally within the gossan, anglesite and cerussite are present with the siderite
mineralisation. A small proportion of the anglesite, and to a lesser extent the
cerussite, has clearly formed as an oxidation product of the galena. However,
some of the cerussite and anglesite has formed from the same mineralising
solutions as the siderite, indicating elevated Eh conditions relative to those under
which the galena has precipitated. The precipitation of cerussite may also occur
if the sulphur activity of the solution is insufficient (relative to the CO2 activity) to
precipitate galena. Several, discrete Pb(Sb,As)-bearing sulphosalts are present
in the gossan. These, however, appear to be localised occurrences, probably
derived from more As and Sb-rich fluids than those directly responsible for the
Page 279
Chapter 11 Mineralogy: Key Features and Paragenesis
galena mineralisation and are rarely observed directly associated with the
precious metal mineralisation.
The precipitation of galena would affect the mineralising fluid chemistry by
reducing the Pb and S activity to an extent that galena mineralisation would
eventually cease. If the Pb activity became the first limiting factor for galena
precipitation, Fe-sulphides may then become the stable phase. If the S activity
was the limiting factor, then cerussite might form. Siderite is commonly observed
in association with the Au/galena intergrowths (Figure 11.26d), possibly as a
result of the lowering of Pb and S activity (due to the precipitation of galena), or a
later and discrete stage of siderite mineralisation.
The second most common native Au association is that of Au and Fe-
monosulphides (Figures 11.26e and f). A complete paragenetic sequence
consisting of native Au → galena → Fe-monosulphides → siderite is extremely
rare (Figure 11.26g). However, Au → Fe-monosulphide associations are
common and may occur either with or without later stages of siderite (Figures
11.26e and f respectively). Due to the greater mobility of Fe2+ relative to Pb2+,
particularly under medium to low pH conditions, any Pb in the mineralising
solutions would be expected to precipitate as galena prior to the formation of Fe-
sulphides. A decrease in Pb activity resulting from the precipitation of galena
may produce solutions that still contain a sufficiently high S activity for the
formation of Fe-monosulphides. Galena → Fe-monosulphide associations are
relatively common and are illustrated throughout Chapters 5 to 9. The formation
mechanisms for Fe-sulphide formation are discussed in detail in Chapter 10. A
significant portion of the Fe-monosulphides have been replaced by marcasite and
pyrite.
Siderite is the final stage of mineralisation observed in the Las Cruces gossan.
The siderite may occur with or without galena and Fe-monosulphides. Multiple
stages of siderite mineralisation are evident by the presence and absence of
associated phases and also by the variation in mineral chemistry. The siderite
may be compositionally zoned. The chemistry of the different siderite generations
is not characteristic enough to provide any genetic links between the different
Page 280
Chapter 11 Mineralogy: Key Features and Paragenesis
stages of mineralisation either within or between boreholes. Siderite will only
precipitate once the S activity of the mineralising solutions has been significantly
reduced by the precipitation of galena or Fe-monosulphides as siderite will only
have a significant field of stability if the CO2 activity is very high and the S activity
very low.
It is clear that some stages of mineralisation were relatively Pb-poor and
occurrences of Au → Fe-monosulphide and Au → Fe-monosulphide → siderite
mineralisation without galena are relatively common. Similarly, some stages of
mineralisation consist almost entirely of galena that often replaces earlier stages
of siderite and/or Fe-sulphide mineralisation. These differences in fluid chemistry
further highlight localised variations in solution chemistry and the multiple stages
of mineralisation that are evident in the Las Cruces gossan. These variations
may, in part, reflect the differences in mobility of the elements/stability of the
discrete mineral species. This difference in stability of the mineral species is
most clearly observed between the relatively unstable siderite and acid volatile
sulphides (greigite, mackinawite e.t.c.) and the relatively stable galena and Au.
Siderite will only form under a very restricted set of Eh/pH conditions and its
stability is very dependant on Eh and pH conditions. Under reducing conditions,
a very slight decrease in pH may result in the dissolution of siderite. The AVS are
also very prone to dissolution under these conditions. However, galena is stable
at moderate to low pH under reducing conditions and is often retained in the
gossan where selective leaching of siderite and Fe-sulphides is clearly evident.
Page 281
Chapter 12 Discussion and Conclusions
12 DISCUSSION AND CONCLUSIONS
11.8 Introduction
This Chapter compares and contrasts the information gathered during the
literature review on gossans with that of the mineralogy of the Las Cruces
gossan. Information gathered on the Eh/pH of the dominant gossan mineral
assemblage, paragenesis and formational mechanisms and environment are also
discussed in terms of the genetic history of the gossan and a model of formation
is described.
Knight (2000) concluded that, based on mineralogical and textural evidence,
stable isotopes, noble gas geochemistry and fluid inclusion studies, the formation
of Las Cruces included seven distinct events:
1. A primary hydrothermal event
2. Oxidation during the waning stages of the hydrothermal system
3. Burial by a thick sequence of culm sediments
4. Sub-aerial supergene enrichment following uplift and erosion
5. Reworking of the sub-aerially gossan by seawater during the Miocene.
6. Burial by a thick sequence of Miocene sediments
7. Possible interactions by the present day water table
The current investigation is focussed predominantly on the formation of the
gossan. Knight's model for the formation of the massive sulphide, together with
the limited geological information provided in the internal Rio Tinto reports is
therefore used as a basis for events that predated the formation of the gossan.
The evidence collected during this investigation suggests that the Las Cruces
gossan has formed as a result of the processes described in Sections 12.2 to
12.7.
Page 282
Chapter 12 Discussion and Conclusions
11.9 Seafloor Gossan Formation
Knight (2000) suggests that oxidation, similar to that described for modern
seafloor sulphide deposits, occurred during the waning stages of the
hydrothermal system resulting in the formation of secondary
Fe-oxides/hydroxides and secondary Cu sulphides on the ancient seafloor.
Supporting evidence for the theory that the present day Las Cruces gossan
formed as a result of seafloor oxidation, includes:
greater mobility of Pb and Au in Cl-rich environments
presence of Cl-bearing minerals mimetite, pyromorphite and rare
nadorite
Fe-oxide dustings on silica (Knight, 2000)
This, however, is by far outweighed by the evidence against seafloor oxidation as
a dominant formation mechanism.
Knight (2000) suggests that burial of the Las Cruces deposit during the late
Carboniferous was followed by tilting during the Hercynian, with uplift and erosion
being followed by sub-aerial weathering and the development of the gossan,
silica cap and supergene Cu-sulphides. This resulted in a hinge zone that
effectively separates the steeply dipping primary mineralisation from the largely
horizontal secondary mineralisation (Figure 12.1).
The essentially horizontal gossan and secondary mineralisation is oriented to the
post Hercynian palaeo-surface and present day surface (and water table). This
effectively discounts ancient seafloor weathering as having a significant influence
on the present day gossan, as the ancient seafloor gossan would be present as a
steeply dipping ore zone along the upper edge of the massive sulphide zone,
quite separate from the present day gossan and associated supergene zone
(Figure 12.1). To date, no such zone has been recognised.
Page 283
Chapter 12 Discussion and Conclusions
Figure 12.1 - Diagram illustrating a) Primary massive sulphide and seafloor gossan preserved under culm sediments produced by continued volcanic activity. b) Tilting of the deposit during the Hercynian would have resulted in a steeply dipping primary massive sulphide and preserved seafloor gossan quite distinct from the sub-aerially derived, horizontal gossan, silica cap and supergene mineralisation (modified from Knight, 2000).
The mature geochemical profile of the Las Cruces gossan is more akin to the
extensive weathering of a massive sulphide deposit under sub aerial, near
surface weathering conditions. The absence of low temperature Mn-rich
hydrothermal deposits, marine fossils, pillow lavas, the lack of graded bedding of
the gossan fragments and the absence of clastic sulphides and Fe-oxide debris
are also an indication that the Las Cruces gossan did not form on the seafloor.
Carvalho (1999) notes that the striking difference between most Iberian Pyrite
Belt deposits and present day seafloor sulphide deposits is the lack of significant
oxidation and sedimentary dilution. The IPB VMS deposits typically consist of
truly massive sulphides and therefore differ markedly from the oxidised sulphide
rubble mixed with sediment and rock fragments that are commonly observed on
the seafloor (citing Rona and Scott, 1993). Carvalho (1999) suggests that this
may be due to the formation of IPB massive sulphides below the palaeo-seafloor,
protected by a thin cap of impermeable sediments.
Siderite, galena and greigite, the dominant mineral assemblage in the Las Cruces
gossan, can only form under strongly reducing conditions, and the high sulphate
content of seawater would favour the precipitation of Fe-sulphides, not siderite,
which is more likely to form in freshwater environments.
Page 284
Chapter 12 Discussion and Conclusions
11.10 Sub-Aerial Gossan Formation
CR194 is the only borehole examined during this investigation that exhibits
evidence of relict Fe-oxyhydroxides that are typical of a mature, sub-aerially
derived gossan. The bulk of the present day gossan has been extensively
replaced by a siderite and sulphide dominated assemblage. Despite the lack of
'typical' relict sub-aerial gossan, significant evidence remains to suggest that the
Las Cruces gossan was initially formed as a result of sub-aerial weathering,
including the geological and climatic history of area, the local geology of gossan
and supergene zone, the presence of a relict/resistate mineralogy, the primary
geology of massive sulphide, the presence of a large Fe dispersion halo around
the massive sulphide orebody and the nature of precious metal mineralisation.
The geological history of the area indicates that the massive sulphide was
partially exposed before and during the Tertiary. The climate at the time was
warm with high rainfall, creating ideal conditions for oxidation and the
development of a deep weathering profile.
The local geology also indicates that the orientation of the present day gossan
and supergene zone indicates that they formed after the tilting, uplift and
subsequent erosion that occurred during the Hercynian.
The vertical extent of the present day gossan and supergene zone, the degree of
metal leaching and concentration lower in the profile is indicative of a mature
gossan profile that is only known to occur under strongly acidic conditions
resulting from near-surface weathering. The concentration of precious metals
and relict resistate phases such as quartz, TiO2 and cassiterite and the
reprecipitation of secondary chalcedony (from leached quartz grains and other
silicates), cassiterite (derived from Sn2+ from oxidised sulphides) and anatase (Ti
from leached oxides, micas, amphiboles and TiO2 e.t.c.) at the base of the
gossan profile are further evidence of extensive leaching of primary gangue and
ore minerals and a high degree of mass wasting as a result of the oxidation of the
original massive sulphides.
Page 285
Chapter 12 Discussion and Conclusions
The relict pyrite at the contact between the gossan and massive sulphide zone of
borehole CR194 exhibits highly irregular morphologies that are typical of partial
oxidation and dissolution under highly acidic conditions. The relict quartz and
ferruginised host rocks in all five boreholes also exhibit evidence of extensive acid
leaching. The absence of boxwork textures in the gossan is also characteristic of
a low pH environment.
The primary geology of the Las Cruces massive sulphide deposit is pyrite-rich
and acid-buffering gangue-poor. Extensive oxidation of the massive pyrite body
would have resulted in low pH conditions due to the high Fe-sulphide content and
the absence of significant acid buffering minerals. This would have resulted in a
significant degree of dissolution and remobilisation of Fe, together with base and
precious metals from the massive sulphide deposit and silica from surrounding
wall rocks. As a result, a geochemical profile, typical of a mature gossan profile,
would have developed.
Blain and Andrew (1977) note that the mobility of Fe is pH dependent, with
greater Fe mobility occurring under very low pH conditions. The gossan
developed at Las Cruces appears to have undergone not only mechanical
dispersion, but also significant degrees of chemical dispersion, with the gossan
halo developed some distance from the central massive sulphide orebody. This
may, however, at least in part, reflect the oxidation of more disseminated sulphide
mineralisation that is present in the associated shales and other wallrocks.
The extremely fine-grained nature of the precious metal mineralogy and the
apparent in situ growth of precious metal-bearing grains within cavities and
fractures are common features of secondary Au. Recent experimental works by
Vlassopoulos and Wood (1990) show that in groundwaters circulating through
oxidising orebodies, Au(S2O3)23-, AuHS0 and Au(HS)2
- are the stable solution
species. Bacteria in the natural environment may play an important role in both
the mobilisation and reprecipitation of Au and other metals (Lengke and Southam,
2005; Reith and McPhail, 2006). However, it is generally considered (Webster
and Mann, 1984; Koshman and Yugay, 1973; Williams, 1933-34; Mann, 1984;
Ross, 1997) that under strongly acidic, oxidising conditions the most likely
Page 286
Chapter 12 Discussion and Conclusions
mechanism for Au and Ag mobilisation during the initial, near-surface weathering
of sulphide deposits would be that of chloride complexes, with high fineness
native Au precipitating lower in the gossan profile following reduction by Fe2+.
The Ag chloride complex is typically precipitated further down in the gossan
profile due to the comparatively high solubility of the Ag chloride relative to Au
chloride. Numerous high fineness Au grains were observed in the Las Cruces
gossan, with Ag typically present as discrete phases, notably members of the
proustite-pyrargyrite solid solutions series and sternbergite lower in the profile. It
should be noted, however, that not all of the Au in the Las Cruces gossan exhibits
a high fineness, with Ag-bearing Au and Au-amalgam being present locally.
Several factors would have had the effect of slowing the rate of oxidation of the
Las Cruces orebody including:-
Recrystallisation of the pyrite in the primary orebody.
Buffering of acidic solutions by surrounding wallrocks.
The low porosity of the primary massive sulphide, limiting the diffusion rate
of oxygenating groundwaters.
The absence of highly reactive pyrrhotite.
Despite these limiting factors, the evidence suggests that the original Las Cruces
gossan formed under near surface weathering conditions, developing a mature
gossan profile under low pH conditions.
The key differences between the Las Cruces gossan and mature gossans
described in the literature relate to the mineralogy, which largely reflect the
reducing conditions that are prevalent at Las Cruces. These differences are
summarised in Table 12.1.
Page 287
Chapter 12 Discussion and Conclusions
Table 12.1 - Comparison of Mature Gossans and Las Cruces Gossan Mineralogy
Mature Gossans Las Cruces Gossan
Fe Mineralogy Goethite, hematite, jarosite Siderite, hematite, greigite
Au Mineralogy High fineness Au High fineness Au, Ag-bearing Au, Au-amalgam
Ag Mineralogy Ag-halides, acanthite, Ag-jarosite
Proustite, pyrargyrite, amalgam, sternbergite (largely in supergene zone)
Pb Mineralogy Anglesite, cerussite, Pb-sulphates
Galena
As-Sb Mineralogy PbSbAs-sulphates PbSbAs-sulphides
The siderite dominated mineral assemblage seen in the Las Cruces gossan
clearly represents a late stage of mineralisation that extensively replaces the
original gossan mineral assemblage. The bulk of the limonite in the Las Cruces
gossan represents the oxidation product of siderite. It is evident that the relict
limonite dominated, sub-aerial gossan that remains partially evident in borehole
CR194, formed under a very different environment relative to the later, siderite-
dominated environment, which could not have formed under oxidic, near surface
weathering conditions. A significant portion of the Au is also present associated
with the siderite/galena/Fe-sulphide assemblage and therefore did not precipitate
under oxidising conditions.
Page 288
Chapter 12 Discussion and Conclusions
11.11 Gossan reworking
Mechanically reworked gossans are a common feature of the Iberian Pyrite Belt
and are known locally as gossan transportado. If it is assumed that the dominant
stages of sulphide weathering at Las Cruces occurred prior to burial by the
Tertiary marl, then the climate immediately preceding the Tertiary was relatively
warm and wet (Knight, 2000; citing Sanz de Galdeano and Vera, 1992 and
Moreno, 1993). Kosakevich et al. (1993) describe the Rio Tinto gossan
transportado as comprising Fe oxide precipitates that have been brecciated,
reworked and redeposited in more recent sediments by solution transport.
Kosakevich et al. (1993) suggest that the bedding of these deposits indicates a
sudden discharge, possibly indicative of erosion under a warm wet
Mediterranean-type climate, with brief, violent seasonal rainfall.
The initial reworking of the Las Cruces gossan appears to have occurred prior to
burial by the Tertiary conglomerate, with the contact between the gossan and
marl being sharply defined in all of the boreholes examined during this
investigation. Some fragments of gossan are found in the Tertiary conglomerate
in places (Knight, 2000), but these are generally rare. Minor amounts of siderite
are present in the Tertiary conglomerate, but the siderite is relatively rare and
differs somewhat from the gossan mineralisation due to the nature of the very
distinctive two-stage zoning with particularly Ca- and Mg-rich cores and Fe-rich
rims. The Tertiary conglomerate siderite may therefore be unrelated to the
gossan mineralisation.
The Rio Tinto gossan transportado exhibits remarkably similar larger scale
textural features to the Las Cruces gossan, with assorted shale-like rock
fragments occurring within a distinctive red coloured matrix. The main difference
is that the Rio Tinto gossan is dominated by jarosite and Fe-oxyhydroxides,
whereas the Las Cruces gossan is dominated by the presence of siderite.
However, the relict Fe-oxyhydroxide gossan of borehole CR194 also exhibits the
characteristic reworked/fragmented texture similar to that described for the Rio
Tinto gossan, suggesting that the Las Cruces gossan may well have been
Page 289
Chapter 12 Discussion and Conclusions
essentially similar in mineral composition and texture to the Rio Tinto gossan,
prior to extensive replacement by siderite.
The relationship and timing between the gossan reworking and siderite
mineralisation has important implications for the paragenesis of the gossan.
Initial examination of the gossan suggested that the siderite had been subjected
to reworking as it typically exhibits a 'clast-like' appearance. This would suggest
that the siderite precipitated prior to the pre-Tertiary reworking event. Further
examination confirms that the siderite 'clasts' actually consist of cavity infills and
the pseudomorphous replacement of former rock fragments. This suggests that
the siderite mineralisation could post-date the reworking of the gossan.
Textural evidence suggests that continued cycles of dissolution and
reprecipitation of the siderite, together with the oxidation of disseminated
sulphides may have resulted in fragmentation of the gossan as these events
would have left large voids and an unstable structure that may have collapsed
under the weight of the overlying rocks. Therefore, some degree of reworking of
the gossan may have occurred post burial, suggesting two very discrete stages of
gossan reworking, one pre Tertiary and one post Tertiary.
The gossan reworking has not significantly affected the geochemical profile
developed during near-surface weathering conditions and the concentration of
resistate phases and precious metals near the base of the gossan appears
largely preserved. This possibly indicates that oxidation and/or geochemical
mobilisation continued during and after the period of reworking, with some
reworking of sulphide-rich fragments also possibly occurring during this time.
These sulphide-rich fragments would have subsequently been oxidised during
later stages of oxidation.
Page 290
Chapter 12 Discussion and Conclusions
11.12 Marine incursion and seawater alteration
Major climate change resulted in a significant rise in sea level during the Tertiary
and subsequent burial of the Las Cruces deposit by glauconite sand and marl,
eventually sealing the gossan from further near-surface weathering. The
incursion of significant volumes of seawater into the region may have played a
major role in the alteration of the Fe-oxyhydroxide dominated, sub-aerially formed
gossan. The possible oxidising effects of sea water, the introduction of other
elements into the gossan environment, including chlorine and CO2 may, at least
in part, provide some explanation for the complex mineralogy found in the modern
day gossan. However, the mineralogy of the Lagoa Salgada gossan, an Iberian
Pyrite Belt deposit also buried under the Tertiary marl, contains predominantly
goethite and hematite (Oliveira et al., 1998), and no siderite.
As discussed previously, seafloor gossans are dominated by the presence of Fe-
oxides and hydroxides, with siderite being rare or absent. The evidence of minor
amounts of Cl-bearing minerals, including pyromorphite and mimetite is by far
outweighed by the evidence against seawater alteration as an important
formation mechanism, with the dominant Las Cruces gossan mineral assemblage
of siderite, galena and greigite only forming under strongly reducing conditions.
Siderite is also inhibited from forming in marine environments because the
Fe2+/Ca2+ ratio in normal marine waters is two orders of magnitude too small to
permit siderite precipitation.
During the seawater incursion, the environment would have remained an
oxidising one, with goethite and hematite remaining the stable and dominant Fe-
bearing species. Continued oxidation of the sulphides and subsequent element
mobilisation may have occurred and would have been enhanced in the strongly
oxidising, Cl-rich environment. This may have resulted in the precipitation of
discrete Cl-rich species, including atacamite, pyromorphite and mimetite. Some
Cl-rich species are, however, often extremely soluble (e.g. atacamite) and
evidence of their formation may well have been destroyed by later stages of
mineralisation or circulating groundwater.
Page 291
Chapter 12 Discussion and Conclusions
11.13 Deep burial by Tertiary sediments
The presence of approximately 150 metres of Tertiary marine sediments
overlying the Las Cruces gossan is a clear indication that the deposit was
covered by a marine transgression that lasted for a significant period of time.
Knight (2000), estimates that during the Miocene, up to 1000 metres of marl may
have covered the Las Cruces deposit, prior to uplift and erosion to the present
day position and suggests a temperature in and around the deposit during burial
of ~100oC, although this would assume a relatively steep geothermal gradient.
Knight (2000) proposes that the increase in geothermal gradient that occurred
during burial has brought about a marked change in the sulphide mineral
assemblage of the secondary Cu zone, with retrograde replacement of digenite
by bornite and chalcopyrite (Knight, 2000). In addition, the burial of the gossan
by the Tertiary sediments resulted in both a high degree of preservation of the
oxide zone and a marked change in Eh.
The burial of the Las Cruces gossan, elevated temperatures and a shift from a
high to low Eh environment may have resulted in the marked changes in the
gossan mineralogy. However, although the reduction of anglesite might form
galena under such conditions, siderite and greigite form under very specific
conditions that burial alone cannot explain. Under decreasing Eh conditions,
goethite may dehydrate to form hematite and hematite may be reduced to form
magnetite. Magnetite is essentially absent from the Las Cruces gossan,
suggesting conditions were unsuitable for magnetite formation. Therefore, the
Fe-oxyhydroxides are unlikely to be significantly affected by burial except perhaps
under more extreme reducing conditions or significantly elevated temperatures.
This lack of alteration of the gossan minerals is evident at Lagoa Salgada,
another buried gossan in the IPB, where the gossan is dominated by the
presence of hematite.
The burial of the gossan does not explain the cyclical oxidation, reduction events
associated with the multiple stages of metal mobilisation and siderite dissolution
and reprecipitation that is evident in the core. Although the burial of the gossan
may have created suitable reducing conditions for siderite formation, other factors
Page 292
Chapter 12 Discussion and Conclusions
must have been active for the formation of the siderite, galena and greigite
dominated assemblage observed in the present day gossan. Burial would also
not explain the strong indication that bacterial Fe- and/or sulphate-reduction has
played an important role in both the siderite and greigite formation.
There is, however, significant evidence to suggest that the present day aquifer
provides the ideal environment for the formation of the mineral assemblage that is
currently observed in the Las Cruces gossan. The most significant of these
changes is the potential for the replacement of the primary, oxidic gossan
minerals by the carbonate- and sulphide-dominated assemblage, consisting of
siderite, galena and greigite.
Page 293
Chapter 12 Discussion and Conclusions
11.14 Modern day gossan and aquifer
11.14.1 Introduction
There is compelling evidence to suggest that the Niebla Posadas aquifer plays a
key role in the formation of the siderite/greigite/galena/precious metal mineral
assemblage currently observed in the Las Cruces gossan.
The aquifer lies directly above the Las Cruces gossan, within the Tertiary
conglomerate/glauconite sand unit. The water in this regionally important aquifer
is mainly a calcium bicarbonate type, probably as a result of the large volumes of
marl within the area from which, at least in part, the aquifer drains. In the deeper
parts of the basin, water quality decreases and is more typically a sodium chloride
type, with trace amounts of metals and sulphate also occurring in places,
suggesting metal mobilisation (R2795, 1998). Elevated water temperatures of
~40oC have been recorded in the aquifer (Knight, 2000).
Mineralogical evidence suggests that there have been several stages of
siderite/greigite/galena/precious metal mineralisation resulting from fluctuating
Eh/pH conditions, where the dominant environment is that of a reducing one,
induced by consumption of O2 and production of CO2 by biogenic processes.
Fluctuations in the level of the aquifer during periods of drought and high rainfall
and/or bacterially driven processes of oxidation and reduction might explain the
changes in Eh and pH in the region of the gossan, resulting in changes in O2,
chlorine, CO2 and metal content of the water.
11.14.2 Siderite and Greigite
The processes of siderite and greigite formation described in Chapter 10 has
significant implications for the formation of the Las Cruces gossan as they are
intimately linked to biological, anaerobic mechanisms that occur at or below the
water table. A detailed review of the literature, together with limited stable isotope
analyses, have shown that biogenic processes within the aquifer are the likely
mechanisms behind the extensive siderite mineralisation, with a significant
influence from the biogenic anaerobic oxidation of methane.
Page 294
Chapter 12 Discussion and Conclusions
These biological processes, including bacterial sulphate reduction,
methanogenesis, methane oxidation and Fe-reduction produce the key
components for carbonate and sulphide formation, including CO2/HCO3, CH4 and
H2S/HS-. These processes are summarised in Figure 12.2.
In summary, the oxidation of organic matter to CO2 occurs whenever oxygen is
present, however, once it is consumed, other, less energy producing substances
are utilised by anaerobic bacteria. This gives rise to the following succession in
the processes of organic matter decomposition:
Oxygen consumption (respiration)
Sulphate reduction
Methanogenesis
In each of these zones, the dominant microbial population exploits the
environment, creating a new environment that favours other species. Thus, the
transition from aerobic sediment, to anaerobic sulphate-reducing sediment, to
anaerobic methane-producing sediment is (at least in part) geochemical
consequences of species induced environmental changes (Claypool and Kaplan,
1974).
The aerobic oxidation of organic matter is a CO2 producing reaction. However,
siderite precipitation is unlikely in this zone because carbonate activities sufficient
to cause carbonate super-saturation are unlikely to be reached due to upward
diffusion into depositional waters. Siderite and greigite are also only stable under
reducing conditions.
Siderite and greigite formation are therefore only likely in the anaerobic zone of
sulphate reduction and methanogenesis, with a significant influence from
methane oxidation and Fe-reduction.
Page 295
Chapter 12 Discussion and Conclusions
Figure 12.2 – A schematic illustrating the distinct biogeochemical and abiotic environments that mark the boundaries between regimes of aerobic and anaerobic metabolism and subsequent carbonate and/or sulphide mineral precipitation. The schematic illustrates the approximate depths, changes in temperature and typical δ13C values associated with the CO2 generated from the decomposition of organic matter. In addition, the competitive and/or complementary processes of nitrate reduction and Fe-/Mn-reduction are also included (modified from Irwin et al., 1977 and Claypool and Kaplan, 1974).
Page 296
Chapter 12 Discussion and Conclusions
The δ13C values of -33, -36 and -42%o for the Las Cruces siderite indicate that
methane oxidation was a source of at least some of the carbon, diluted by
isotopically heavier carbon, probably produced by one or more of sulphate
reduction, Fe-reduction and methanogenesis. Methane oxidation is considered to
play a key role in the formation of the Las Cruces siderite as the upward transport
of CH4 from the zone of methanogenesis into the sulphate reduction zone
typically produces CH4 with δ13C between -60 and -80%o. No other processes
produce such isotopically light δ13C values.
The anaerobic oxidation of methane is performed by methanotrophic archaea
and sulphate reducing bacteria where sulphate and methane are consumed at
the base of the sulphate reduction zone. The anaerobic oxidation of methane
produces bicarbonate, increasing carbonate alkalinity and saturation with respect
to carbonate minerals and siderite may form. The reduction of sulphate will also
favour the precipitation of sulphides.
Although the source of the methane in the Las Cruces gossan has not been
determined during this investigation, the aquifer provides the ideal environment
for shallow biogenic processes to predominate.
The bacterial reduction of sulphate accompanied by organic matter
decomposition produces S2- and CO2. The carbon dioxide produced by this
reaction dissolves readily in pore water to increase bicarbonate concentrations,
often resulting in the precipitation of carbonate minerals with distinctive carbon
isotope values (δ13C typically -25%o).
The presence of significant levels of sulphur will inhibit the precipitation of siderite
and sulphides will preferentially form. Hence, sulphide precipitation, namely that
of galena and greigite, always precedes siderite mineralisation in the Las Cruces
gossan, assuming sufficient sulphur activity of the mineralising fluids
The presence of galena and greigite +/- other Fe-monosulphides is therefore
indicative of sulphate reduction. Crystal growth kinetics is considered to play an
important role in the initial formation of Fe monosulphides over pyrite and
Page 297
Chapter 12 Discussion and Conclusions
marcasite. The fact that greigite persists in the gossan and exhibits only partial
replacement by marcasite and pyrite is likely an indication that non-sulphidic
conditions are attained by the exhaustion of all sulphate and sulphide, so that
pyritisation reactions are not driven to completion. A limited source of sulphate is
typical of brackish to fresh water environments.
Methanogenesis is essentially a microbial process involving the production of
methane. Microbially mediated methane production generally occurs via CO2
reduction and/or acetate fermentation. Berner (1981) suggests that siderite
forms through the combined effects of Fe reduction and bacterial
methanogenesis of organic carbon compounds. δ13C values of +10 to +15%o are
typical of siderite formed as a result of methanogenesis.
Where the availability of Fe3+ outweighs that of sulphate, Fe-reduction will
predominate, releasing Fe2+ to the diagenetic pore waters. Fe reduction may
occur in conjunction with sulphate reduction and methanogenesis and the
subsequent generation of Fe2+, bicarbonate and hydroxyl ions increases alkalinity
and siderite precipitation is favoured.
Mineralogical evidence suggests that there have been several stages of late-
stage siderite/sulphide/precious metal mineralisation within the gossan, with
cyclical oxidising and reducing events and subsequent changes in pH, consistent
with what might be expected in a fluctuating water table associated with a semi-
arid, Mediterranean climate. The aquifer would not create a suitable environment
for the development of the mature gossan profile seen in the Las Cruces gossan
due to strong buffering of acid solutions by siderite and the relatively low metal
sulphide content of the gossan. This suggests that the original mature gossan
profile developed under near-surface weathering conditions eventually being
replaced by the siderite/secondary sulphide mineralisation during interaction with
the aquifer.
Siderite has formed as a result of chemical transportation of Fe. Porosity of the
host rocks has significantly influenced Fe migration within the gossan. Siderite
occurs as a late-stage phase that commonly forms along the margins of the
Page 298
Chapter 12 Discussion and Conclusions
friable, residual silica-rich rock fragments and within open pore spaces, further
indication of chemical transportation. The aquifer provides an ideal mechanism
for the chemical dispersion of Fe that is observed within the gossan.
Siderite is the final stage of mineralisation observed in the Las Cruces gossan.
Multiple stages of siderite mineralisation are evident by the presence and
absence of associated phases, notably galena and Fe-monosulphides, the
variation in siderite chemistry, evidence of overgrowths and degrees of oxidation.
The siderite may be compositionally zoned. The chemistry of the different
siderite generations is not characteristic enough to provide any genetic links
between the different stages of mineralisation either within or between boreholes.
Given the mechanisms behind siderite and greigite formation and the strong
influence that biological processes have on creating the ideal Eh/pH conditions
and products for siderite and greigite formation, the aquifer remains the only likely
environment throughout the history of the Las Cruces deposit for the formation of
this late-stage mineral assemblage.
11.14.3 Pb-bearing sulphides
Pb is mobile as Pb2+aq and HPbO2
-aq under extreme acid and alkaline oxidising
conditions respectively. At low Eh, galena is the stable phase, with anglesite and
cerussite occurring under oxidising acid and alkaline conditions respectively. The
presence of galena in the Las Cruces gossan mineral assemblage is therefore
further indication of strongly reducing conditions.
The close association between galena and the siderite/greigite mineralisation
suggest that Pb mobilisation and precipitation may be related to bacterial
processes within the gossan. Recent studies (Wu et al., 2006; Jensen-Spaulding
et al., 2004; Lui et al., 2008) show that bacteria may pose both positive and
negative impacts on the mobility of heavy metals. The abundance of galena in
the Las Cruces gossan and the close association with the siderite/greigite
mineralisation suggest that galena precipitation may have occurred as a result of
bacterial sulphate reduction within the aquifer.
Page 299
Chapter 12 Discussion and Conclusions
The mobility of Pb, one of the least mobile elements in the gossan profile is
enhanced by the presence of chlorine in the transport medium. The presence of
accessory mimetite and pyromorphite also provide evidence for the presence of
chlorine, although these phases are largely confined to the relict gossan of
borehole CR194 and may relate to the near-surface weathering of the primary
sulphide. Anglesite pseudomorphs after galena are evident locally within the
core, indicative of localised oxidising conditions. Increased chlorine levels and
localised oxidation/remobilisation of Pb are consistent with what might be
expected in a cyclical oxidising/reducing environment brought about by a
fluctuating water table.
It is likely that increased mobility of Pb within the gossan occurred as a result of
localised oxidising conditions resulting from either a decrease in the level of the
aquifer and/or by the actions of sulphur oxidising bacteria. The subsequent
decrease in pH and probable increase in the concentration of dissolved salts,
including chlorine, would have resulted in the dissolution of galena from the
supergene zone and/or primary ore. Contact with diluting groundwater and
sulphate reducing bacteria resulted in the rapid precipitation of Pb2+aq as galena.
The rapid precipitation of galena may also account to some degree for the
extremely fine-grained nature of this phase within the gossan.
The presence of anglesite and cerussite within some siderite veinlets is further
evidence of local variations in Eh during precipitation of the late-stage mineral
assemblage. The precipitation of cerussite may also occur if the sulphur activity
of the mineralising solution is insufficient, relative to the CO2 activity, to precipitate
galena. Several, but minor amounts of discrete Pb(Sb,As)-bearing sulphosalts
are present in the gossan and are probably derived from more As and Sb-rich
fluids than those directly responsible for the galena mineralisation.
11.14.4 Precious metals
A large number of Au and Ag-bearing grains were identified in the Las Cruces
gossan, providing a significant amount of information on the nature and mode of
occurrence of the precious metal mineralisation. Several potential mechanisms
of Au and Ag mobilisation have been identified and although Mann (1984)
Page 300
Chapter 12 Discussion and Conclusions
suggests that it is generally accepted that only one of these mechanisms may be
operating in a single deposit, there is evidence at Las Cruces that several
mechanisms of Au dissolution, remobilisation and reprecipitation may have
occurred during different stages of weathering.
Au is one of the least mobile of elements in the gossan and as such, the aquifer
may have had little effect on the high fineness Au that precipitated under near
surface weathering conditions. However, a portion of the Au is alloyed with Ag
and is also present as micrometre- and sub-micrometre grains that are intimately
associated with the siderite/galena/Fe-sulphide mineralisation. The low fineness
of the Au and the close association with the siderite, Fe-sulphide and galena
mineralisation indicates that remobilisation and reprecipitation of precious metals
has occurred since the initial near surface weathering event that formed the
Fe-oxyhydroxide dominated gossan.
Although cyclical fluctuations in the aquifer level may explain the various stages
of oxidation and reduction evident in the Las Cruces gossan, the close
association between Au and the siderite/greigite mineralisation suggests that
bacteria may play an important role in the mobilisation and precipitation of Au
within this assemblage. Although bacterially mediated Au mobilisation and
precipitation remains and area of ongoing research, Reith and McPhail (2006),
Lengke and Southam (2005) and Southam and Beveridge (1996) have shown
that bacteria in the natural environment play an important role in the mobilisation
and reprecipitation of Au.
In carbon poor environments, such as the Las Cruces gossan, Au release
appears to be linked to Fe or sulphide oxidation. Bacterially mediated Au
mobilisation could proceed by via number of mechanisms, including initial release
of Au from the supergene zone via sulphide oxidation and subsequent
mobilisation as Au-thiosulphate or Au-organic complex. Subsequent precipitation
of the Au may occur via bacterially mediated sulphate and/or Fe reduction. This
theory allies closely with the bacterially mediated precipitation of siderite and
greigite discussed previously in this study. In addition, Au has been shown to play
Page 301
Chapter 12 Discussion and Conclusions
a functional role in the oxidation of methane (Levchenko et al., 2002), a process
strongly associated with the siderite mineralisation at Las Cruces.
Similarly, localised oxidising conditions within the gossan resulting from cyclical
fluctuations in the water table may have resulted in localised oxidation of the
sulphides associated with the supergene mineralisation. The nature and
mechanisms behind the mobilisation of Au and Ag under such conditions would
be in marked contrast to those conditions under which the initial, sub-aerial
weathering took place. Instead of strongly acidic conditions, resulting from the
oxidation of a pyrite-dominated orebody, the remobilisation of Au in the siderite-
dominated gossan would be under near-neutral to alkaline conditions and may
therefore proceed via thiosulphate or AuOH(H2O)0 complexes rather than AuCl4- .
The co-precipitation of Au and Ag is greatest under near neutral to alkaline
conditions, particularly in the presence of free sulphur. This may account for the
presence of abundant Au-bearing amalgam and the presence of Ag-rich rims and
Ag-rich Au grains that are also observed locally in the present day gossan.
Vlassopoulos and Wood (1990), Webser and Mann (1984) and Thornber (1992)
have observed that in the presence of incompletely oxidised sulphides,
thiosulphate is the stable species under alkaline oxidising conditions and Au of
low fineness is re-precipitated by reduction at the water table.
Garrels and Christ (1965) suggest that, given sufficient sulphur activity, Au is
mobile as a Au-sulphur complex under reducing conditions and near neutral-
alkaline pH. It is pertinent to consider that although localised oxidation within the
gossan (whether biotic or abiotic) may have resulted in the release of metals into
the surrounding groundwaters, the fluids from which this assemblage is
associated are predominantly associated with reducing conditions. As such, it is
likely that the Au, mobile as a thiosulphate complex, precipitated along with
galena and Fe-sulphides as a result of a decrease in sulphur activity of the
mobilising fluid as well as possibly reduction to native Au by interactions with
Fe2+.
Page 302
Chapter 12 Discussion and Conclusions
The Ag content of the siderite/galena/Fe-sulphide gossan assemblage is
relatively low compared to the sternbergite and proustite/pyrargyrite dominated
supergene zone. Although the sternbergite and proustite/pyrargyrite are
considered as part of the supergene zone, there is a distinct possibility that this
Ag-rich assemblage has also formed as a result of interactions with the aquifer,
an area for possible future investigations.
Page 303
Chapter 12 Discussion and Conclusions
11.15 Conclusions
Figure 12.3 illustrates an idealised cross section through the Las Cruces gossan,
supergene zone and underlying primary massive sulphide. The gossan is
overlain by approximately 100-150m of Tertiary sediments, the lower portion of
which contains the Niebla Posadas aquifer within a Tertiary sand unit. The
aquifer lies in direct contact with the porous gossan zone.
Tertiary marine deposits: consisting of rounded glauconite aggregates, quartz and feldspar fragments, calcite cement and shell debris and accessory pyrite. This unit hosts the aquifer, which extends into the porous gossan zone. Vertical extent: 100-150m.
Gossan: Concentration of chemically and physically resistate Au, quartz, cassiterite and TiO2 has occurred due to mass wasting during near surface weathering. Biogenic activity within the Niebla Posadas aquifer resulted in extensive replacement of quartz-rich wall rocks and relict sub-aerial Fe-oxyhydroxide gossan by siderite, greigite, galena and Au mineralisation. Vertical extent: 0-20m.
Gossan/Sulphide contact: Some boreholes exhibit a pyrite, sternbergite, proustite/pyrargyrite assemblage with some native Au that may represent a biogenic supergene zone, distinct from the underlying supergene Cu-sulphide. Vertical extent: 0-10cm.
Supergene Cu-sulphide: Developed during sub-aerial weathering in the pre-Tertiary and consists predominantly of secondary Cu-sulphides and pyrite. Accessory minerals include tetrahedrite/tennantite and enargite. Vertical extent: 40-60m.
Primary Massive Sulphide: Pyrite dominated massive sulphide with minor but significant amounts of chalcopyrite, galena and sphalerite. Accessory minerals include tetrahedrite/tennantite quartz and calcite. Vertical extent: undetermined.
Figure 12.3 – Diagram illustrating an idealised cross section through the Las Cruces deposit.
Page 304
Chapter 12 Discussion and Conclusions
The gossan overlying the supergene massive sulphide typically consists of
reworked relict Fe-oxyhydroxide and quartz-rich rock fragments that exhibit
extensive replacement by a late-stage siderite, greigite, galena and Au mineral
assemblage. The gossan mineralisation also extends away from the massive
sulphide where the chemically precipitated siderite-rich assemblage typically
replaces quartz-rich wall rocks. The base of the gossan is often marked by a
narrow Ag-rich supergene zone consisting of secondary pyrite, sternbergite,
proustite/pyrargyrite and native Au.
The secondary Cu zone consists of supergene-enriched massive sulphide or
supergene-enriched wall rocks. The primary massive sulphide zone is a tabular
structure dipping to the north at an angle of approximately 35o and consists
predominantly of pyrite together with accessory galena, chalcopyrite and
sphalerite.
There is no evidence to suggest that the present day Las Cruces gossan formed
as a result of seafloor oxidation during the waning stages of massive sulphide
mineralisation.
Local geology, notably the alignment of the gossan to the present day water table
indicates that the gossan developed after the tilting that occurred during the
Hercynian orogeny. Following uplift during the Hercynian, a mature gossan profile
developed under low pH and high Eh conditions as a result of extensive near-
surface weathering of the primary massive sulphide orebody. This resulted in
significant acid leaching and mobilisation of the more mobile elements and a
fixing of Fe as Fe-oxyhydroxides above the water table. Mass wasting resulted in
a concentration of chemically immobile elements and physically resistate
minerals, notably Au, Si (quartz), Sn (cassiterite) and Ti (TiO2). Au mobilisation
likely occurred as a Au-chloride complex.
Some gossan reworking probably occurred before and during the Tertiary prior to
burial by Tertiary marine sediments resulting in a jumbled mass of Fe-
oxyhydroxides, shale debris and quartz-rich rock fragments.
Page 305
Chapter 12 Discussion and Conclusions
The original sub-aerially weathered, Fe-oxyhydroxide dominated gossan has
been extensively replaced by siderite, greigite, galena and Au mineralisation.
Gossan mineral paragenesis occurred in the order Au→galena→Fe-
sulphides→siderite. Early precipitation of Au and galena was controlled largely
by their highly immobile nature. Siderite precipitation only proceeded once the
CO2 activity of the mineralising fluids was sufficiently high and the S activity was
reduced via the precipitation of galena and greigite.
Siderite/greigite mineralisation is intimately linked to anaerobic bacterial
processes. Microbial Fe and sulphur oxidation, Fe- and sulphate reduction and
methane oxidation coupled with a fluctuating water table within the Niebla
Posadas aquifer provide the mechanisms for cyclical metal release and
subsequent siderite and greigite mineralisation. The aquifer provides the
mechanism for the hydromorphic dispersion of Fe and other metals within the Las
Cruces region.
The Eh/pH conditions and processes involved in metal mobilisation/precipitation
within the carbonate-rich and sulphide poor gossan would be in marked contrast
to those involved in the initial, sub-aerial gossan forming process.
Although little is known about the bacterial mobilisation of Au and Pb in the
complex natural environment, experimental studies and the close association
between Au/galena and the siderite/greigite mineralisation suggests that the
mobilisation and precipitation of these metals is also either directly or indirectly
associated with bacterial processes within the gossan.
Greigite exhibits partial and extensive replacement by marcasite and pyrite,
particularly with increasing depth in the gossan. This replacement process
extends down to the contact with the supergene Cu-sulphide zone and is often
marked by the presence of supergene pyrite, sternbergite, proustite/pyrargyrite,
Au mineralisation. This Ag-rich zone is marked by the absence of siderite, but
may also be intimately linked to bacterial activity within the aquifer. Possible
interactions between microbiota within the aquifer and the underlying supergene
Cu mineralisation should be examined in any future investigations.
Page 306
Chapter 12 Discussion and Conclusions
11.16 Future Investigations
The Las Cruces deposit, and in particular the gossan mineralisation, is clearly
complex in nature and significant further studies would be required to gain a more
comprehensive understanding of the processes that have resulted in the
formation of the modern day deposit. The interpretation of larger scale textural
features, in particular, those of the gossan, would become more apparent once
mining of the deposit has commenced. This investigation has focussed on the
detailed documentation of the siderite, greigite, galena and precious metal
mineralisation of the gossan. This information provides a sound base for any
future investigations.
There is a distinct possibility that the aquifer and associated bacterial activity has
also had a significant impact on the supergene mineralisation, in particular the
sternbergite, pyrite and proustite/pyrargyrite assemblage. This area provides and
exciting opportunity for future investigation.
The more traditional mineralogical techniques, including optical microscopy, x-ray
powder diffraction and electron microscopy have been extensively used during
the current study. A significant degree of information may be gained by focussing
future investigations on the analysis of specific phases and/or stages of
mineralisation, possibly incorporating additional fluid inclusion studies and stable
isotope analyses to gain a greater understanding of the nature of the fluids that
resulted in the formation of siderite and the late-stage sulphide mineralisation.
In addition, analysis of the aquifer in the Las Cruces area may also provide useful
information on the nature of dissolved species, in particular, dissolved cations,
chloride content, CO2, pH and Sulphate Reducing Bacteria (SRB) activity,
together with the possibility that the aquifer is still active in the dissolution,
oxidation, replacement and alteration of the Las Cruces gossan.
Page 307
References
References
ADAMS, L. K., LLOYD, J. R., MACQUAKER, J. H. S. 2003. Microbial iron
reduction: From bacteria to cement. Goldschmidt conference abstracts. A7.
AL, T. A., MARTIN, C. J., BLOWES, D. W. 2000. Carbonate/water interactions in
sulphide-rich mine tailings. Geochimica et Cosmochimica Acta, 64, pp. 3933-
3948.
ALMODOVAR, G. R., SAEZ, R., PONS, J. M., MAESTRE, A. TOSCANA, M.,
PASCUAL, E. 1998. Geology and genesis of the Aznalcollar massive sulphide
deposits. Mineralium Deposita, 33, pp. 111-136.
AMOROS, J. L., LUNAR, R., TAVIRA, P. 1981. Jarosite: a silver-bearing mineral
of the gossan of Rio Tinto (Huelva) and La Union (Cartagena, Spain). Mineralium
Deposita, 2, pp. 205-213.
ANDERKO, A. and SHULER, P.J. 1997. A computational approach to predicting
the formation of iron sulphide species using stability diagrams. Computers and
Geosciences. 23, pp. 647-658.
ANDERSON, J.A. 1990. Characteristics of leached capping and techniques of
appraisal. In: WILLIAMS, P. A. ed. Oxide Zone Geochemistry. London, Ellis
Horwood Ltd, pp. 275-295.
ANDREW, R.L. 1984. The Geochemistry of Selected Base Metal Gossans,
Southern Africa. Journal of Geochemical Exploration, 22, pp. 161-192.
ANGELICA, R.S., DA COSTA, M. L., POLLMANN, H. 1996. Gold, wolframite,
tourmaline-bearing lateritized gossans in the Amazon region, Brazil. Journal of
Geochemical Exploration, 57, pp. 201-215.
Page 308
References
ANTHONY, JW, BIDEAUX, RA, BLADH, KW AND NICHOLS, MC. 1990.
Handbook of Mineralogy, Volume I. Elements, Sulfides, Sulfosalts. Tucson:
Mineral Data Publishing.
BATEMAN, A. M. 1927. Ore Deposits of the Rio Tinto (Huelva) district, Spain.
Economic Geology, 22, pp. 569-614.
BAZYLINSKI, D.A and MOSKOWITZ, B.M. 1997. Microbial biomineralization of
magnetic iron minerals: microbiology, magnetism, and environmental
significance. In: BANFIELD, J.F., NEALSON, K.H. eds. Geomicrobiology:
interactions between microbes and minerals (Rev Mineral Vol 35), Washington,
DC: Mineralogical Society of America, pp. 181-223
BELOGUB, E. V., NOVOSELOV, C. A., SPIRO, B., YAKOVLEVA, B. A. 2003.
Mineralogical and S isotope features of the supergene profile of the Zapadno-
Ozernoe massive sulphide and Au-bearing gossan deposit, South Urals.
Mineralogical Magazine. 67, pp. 339-354.
BENNING, L. G., WILKIN, R. T., BARNES, H. L. 2000. Reaction pathways in the
Fe-S system below 100 degrees C. Chemical Geology, 167, pp. 25–51.
BERNER, R.A. 1964. Iron sulphides formed from aqueous solution at low
temperatures and atmospheric pressure. Journal of Geology, 72, pp. 293-306.
BERNER, R. A. 1981. A new geochemical classification of sedimentary
environments. Journal of Sedimentary Petrology. 51 (2), pp. 359-365.
BINNS, R. A., SCOTT, S. D., BOGDANOV, Y. A., LISITZIN, A. P., GORDEEV, V.
V., GURVICH, E. G., FINLAYSON, E. J., BOYD, T., DOTTER, L. E., WHELLER,
G. E., MURAVYEV, K. G. 1993. Hydrothermal oxide and gold-rich sulphate
deposits of Franklin Seamount, Western Woodlark Basin, Papua New Guinea.
Economic Geology, 88, pp. 2122-2153.
Page 309
References
BISCHOFF, J.L., ROSENBAUER, R.J., ARUSCAVAGE, P.J., BAEDECKER, P.A.
AND CROCK, J.G. 1983. Seafloor massive sulphide deposits from 21oN, East
Pacific Rise; Juan de Fuca Ridge and Galapagos rift: Bulk chemical composition
and economic implications. Economic Geology, 78, pp. 1711-1720.
BLAIN, C.F. and ANDREW, R.L. 1977. Geochemical Weathering and the
Evaluation of Gossans in Mineral Exploration. Minerals Science Engineering, 9,
(3), pp. 119-150.
BLAIN, C.F. 1978. Mineralisation and gossans in the Wadi Wassat-Wadi Qatan
region, Kingdom of Saudi Arabia. Institution of Mining and Metallurgy,
Transactions, Section B: Applied Earth Science. 87, pp. 14-20.
BLOWES, D. W., PTACEK, C. J. 1994. Acid neutralisation mechanisms in
inactive mine tailings. In: JAMBOR, J. L., BLOWES, D. W. eds. The
environmental geochemistry of sulphide mineral wastes. Ottawa, Ontario:
Mineralogical Association of Canada. pp. 271-292.
BOROWSKI, W.S., PAULL, C.K., USSLER, W. 1999. Global and local variations
of interstitial sulphate gradients in deep water, continental margin sediments:
Sensitivity to underlying methane and gas hydrates. Marine Geology, 159, pp.
131-154.
BOYLE, D. R. 1995. Geochemistry and Genesis of the Murray Brook precious
metal gossan deposit, Bathurst Mining Camp, New Brunswick. Exploration and
Mining Geology, 4, (4), pp. 341-363.
BOYLE, R.W. JONASSON, I.R. 1973. The geochemistry of arsenic and its use as
an indicator element in geochemical prospecting. Journal of Geochemical
Exploration, 2, pp. 251-296.
BRESHENKOV, B.K. 1946. On the problem of the genesis of jarosites: Compt.
Rend (Doklady) de l'Acad. des Scieces de l' URSS. 52, (4), pp. 329-332.
Page 310
References
BROOKINS, D.G. 1988. Eh-pH diagrams for geochemistry. New York: Springer-Verlag,
BROWNELL, G.M. and KLINKELL, A.R. JR. 1935. The Flin Flon mine, geology
and paragenesis of the ore deposits. Canadian Institute Mining and Metallurgy
Transactions, 38, pp. 261-286.
BRUCE, J. L. 1948. Cyprus Mines copper again. Mining Technology A.IME. 12,
pp. 1-28.
BUTT, C.R.M. and ZEEGERS, H. 1992. Climate, geomorphological environment
and geochemical dispersion models. In: BUTT, C.R.M. and ZEEGERS, H. eds.
Regolith Exploration Geochemistry in Tropical and Sub-Tropical Terrains.
Amsterdam: Elsevier, pp 3-23.
CARVALHO, D. BARRIGA, F. J. A. S. MUNHA J. 1999. Bimodal siliclastic
systems - The case of the Iberian Pyrite Belt. In: BARRIE, T. and HANNINGTON,
M., eds. Volcanic-associated massive sulphide deposits: Processes and
examples in modern and ancient setting., United States: Society of Economic
Geologists, 8, pp 375-408.
CHAPELLE, F. H., LOVLEY, D. R. 1992. Competitive exclusion of sulphate
reduction by Fe (III)-reducing bacteria: A mechanism for producing discrete
zones of high-iron ground water. Groundwater 30, pp 29-36.
CLAYPOOL, G.E., KAPLAN, I.R. 1974. The origin and distribution of methane in
marine sediments. In: Kaplan, I.R. ed. Natural gases in marine sediments. New
York: Plenum Press, pp. 99-140.
COLEMAN, M. L., HEDRICK, D. B., LOVLEY, D. R., WHITE, D. C., PYE, K.
1993. Reduction of Fe(III) sediments by sulphate-reducing bacteria. Nature. 361,
pp. 436-438.
Page 311
References
CONSTANTINOU, G. GOVETT, G. J. S. 1972. Genesis of sulphide deposits,
ochre and umber of Cyprus. Institute of Mining and Metallurgy Transactions
Section B: Applied Earth Science, 81, pp. B34-B46.
COSTA, M.L. ANGELICA, R.S. COSTA N. C. 1999. The geochemical
association Au-As-B-(Cu)-Sn-W in latosol, colluvium, lateritic iron crust and
gossan in Carajas, Brazil: importance for primary ore identification. Journal of
Geochemical Exploration, 67, pp. 33-49.
CRAIG, H. 1957. Isotope standards for carbon and oxygen correction factors for
mass-spectrometric analysis of carbon dioxide. Geochimica et Cosmochimica
Acta, 12, pp. 133-149.
CURTIS, C. D., COLEMAN M. L., LOVE L. G. 1986. Pore water evolution during
sediment burial from isotopic and mineral chemistry of calcite, dolomite and
siderite concretions. Geochimica et Cosmochimica Acta, 50, (10), pp. 2321-2334.
DEWEY, J. F., HELMAN, M. L., TURCO, E., HUTTON, D. H. W., KNOTT, S.D.
1989. Kinematics of the western Mediterranean. In: COWARD, M. P., DEITRICH,
D., PARK, D. G. Eds. Alpine Tectonics. London: Geological Society special
publication. 45, pp. 265-283.
DIMANCHE, F. BARTHOLOME, P. 1976. The alteration of ilmenite in sediments.
Mineral Science Engineering, 8, pp. 187-201.
DONALD, R., and SOUTHAM, G. 1999. Low temperature anaerobic bacterial
diagenesis of ferrous monosulphide to pyrite. Geochimica et Cosmochimica Acta,
63, pp. 2019–2023.
DOYLE, M., MORRISSEY, C., SHARP, G. 2003. The Las Cruces orebody,
Seville Province, Andalucia, Spain. In: KELLY, J. C., ANDREW, C. J., ASHTON,
J. H., BOLAND, M. B., EARLS, G., FUSCIARDI, L. STANLEY, G. eds. Europe's
major base metal deposits. Ireland: Irish association for economic geology, pp.
381-389.
Page 312
References
DRUSCHEL, G.K., LABRENZ, M., THOMSEN-EBERT, T., FOWLE, D.A., AND
BANFIELD, J.F. 2002. Geochemical modelling of ZnS in Biofilms: An example of
ore depositional processes. Economic Geology. 97, pp. 1319-1329.
DURNEY, D.W. and RAMSAY, J.G. 1973. Incremental strains measured by
syntectonic crystal growths. In: DE JONG K.A. and SCHOLTEN R. eds. Gravity
and tectonics, New York: Wiley, pp. 67-96.
ENZWEILLER, J. and JOEKES, I. 1991. Adsorption of colloidal gold on colloidal
iron oxides. Journal of Geochemical Exploration. 40, pp. 133-142.
FERNANDEZ, M., BERASTEGUI, X., PUIG, C., GARCIA-CASTELLANOS, D.,
JURADO, M. J., TORNE, M. BANKS, C. J. 1998. Geophysical and geological
constraints on the evolution of the Guadalquivir Basin, Spain. In: MASCLE, A.,
PUIGDEFABREGAS, C., LUTERBACHER, H. P., FERNANDEZ, M., eds.
Cenozoic Foreland Basins of Western Europe. London: Geological Society
special publication, 134, pp. 29-48.
FINLAYSON, A. M. 1910a. The pyritic deposits of Huelva, Spain. Economic
Geology 5, pp. 403-437.
FINLAYSON, A. M. 1910b. Secondary enrichment in the copper deposits of
Huelva, Spain. Transactions of the Institute of Mining and Metallurgy, 20, pp. 61-
72.
FUJII, T. and HARAMURA, H. 1976. Gold chloride complex in silica solution at
low temperature and pressure. 25th International Geological Congress, Abstract,
2, pp. 563-564.
FUJII, T., HONMA, H. and NISHIDA, N. 1977. Geochemistry of colloidal gold-
silica deposition at low temperature and pressure. Proceedings, Japan
Academy, Science B, Physical and Biological Sciences, LIII, 7, pp. 267-271.
Page 313
References
GARCIA PALOMERO, F, BEDIA FERNANDEZ, J. L. GARCIA MAGARINO, M.
SIDES E. J. 1986. Nuevas investigaciones y trabajos de evaluacion de reservas
de gossan en minas de Rio Tinto. Boletin Geologico y Minero. Sp., 97 (5), pp.
622-642.
GARCIA PALOMERA, F. 1990. Rio Tinto deposits - geology and geological
models for their exploration and ore-reserve evaluation. In: GRAY P. M. J.,
BOWYER, G.J., CASTLE, J.F., eds. Sulphide deposits - their origin and
processing. London: Institute Mining and Metallurgy.
GARRELS, R.M., AND CHRIST, C.L. 1965. Solutions, minerals and equilibria,
New York: Harper and Row.
GASPAR, O. C. 1992. Estudo de microscopia de minerios de sondagem LS4 de
Lagoa Salgada. General Directorate for Geology and Mines, Porto. 27pp.
GASPAR, O. C., OLIVEIRA, V. M. J., MATOS, J. M. X. 1993. Nota preliminar
sobre os sulfuretos macicos da jazida da Lagoa Salgada, F. P. I. Portugal, 11
Congresso de geochima dos paises de lingua Portuguesa. Museu e laboratorio
Mineralogico e Geoogicol. 3, pp. 239-242.
GASPAR, O.C., FERREIRA, J.A. and BOWLES, J.F.W. 1998. Contribution of the
mineralogical studies of Ag-Hg alloys from the gossan of Lagoa Salgada to the
history of ancient mining in the Iberian Pyrite Belt. IV Simpósio Internacional de
Sulfuretos Polimetálicos da Faixa Piritosa Ibérica. Lisboa, A1, pp. 1-6.
GOMEZ, M. S., MEDINA, F. T. 2003. Recent tectonic activity on the south margin
of the Guadalquivir basin, between Cabra and Quesada towns (provinces of Jaen
and Cordoba, Spain). In:. ESPINAR, M., ESQUIVEL, J. A., PEÑA, J. A., eds.
Historia del Observatorio de Cartuja 1902 - 2002 - Nuevas investigaciones.
Instituto Andaluz de Geofísica.
GUILBERT, J.M., PARK, C.F. 1986. The geology of ore deposits. New
York. .W.H. Freeman and Co.
Page 314
References
GRAY D.J., C.R.M. BUTT, LAWRENCE L.M. 1992. The Geochemistry of Gold in
Lateritic Terrains. In: Butt, C.R.M. and Zeegers, H., eds. Regolith Exploration
Geochemistry in Tropical and Sub-Tropical Terrains. Amsterdam: Elsevier, pp.
461-481.
HALBACH, P. BLUM, N., MÜNCH, U., PLÜGER, W., GARBE-SCHÖNBERG, D.
and ZIMMER, M. 1998. Formation and decay of a modern massive sulphide
deposit in the Indian Ocean. Mineralium Deposita, 33 (3), pp. 302-309.
HANNINGTON, M.D. PETER, J.M. SCOTT, S.D. 1986. Gold in sea-floor
polymetallic sulphide deposits. Economic Geology, 81, pp. 1867-1883.
HANNINGTON, M.D. THOMPSON, G. RONA, P.A. SCOTT, S.D. 1988. Gold
and native copper in supergene sulphides from the Mid-Atlantic Ridge. Nature
333, pp. 64-66.
HANNINGTON, M.D., HERZIG, P.M., GREGOIRE, D.C., THOMPSON, G.,
RONA, P.A. 1991a. The Mineralogy and Geochemistry of Submarine Gossans:
Part I. Fe-Oxide Assemblages from the TAG Hydrothermal Field, Mid-Atlantic
Ridge In: Geological Association of Canada, Miner. Assoc. of Canada joint
annual meeting with the Soc. of Econ Geologists; program with abstracts.
HANNINGTON, M.D., HERZIG P.M., THOMPSON G., RONA P.A. 1991b.
Metalliferous Sulphide-Oxide Sediments from the TAG Hydrothermal Field
(26oN), Mid-Atlantic Ridge. In: Geological Association of Canada, Miner. Assoc.
of Canada joint annual meeting with the Soc. of Econ Geologists; program with
abstracts.
HANNINGTON, M.D., HERZIG P.M., SCOTT, P., THOMPSON G., RONA P.A.
1991c. Comparative mineralogy and geochemistry of gold-bearing sulphide
deposits on the Mid Ocean Ridges, Marine Geology, 101, pp. 217-248.
Page 315
References
HANNINGTON, M.D., HERZIG P.M., SCOTT, P. 1991d. Auriferous hydrothermal
precipitates on the modern seafloor. In: Foster R. P., ed. Gold metallogeny and
exploration. Glasgow and London: Blackie, pp.249-282.
HEALY R., PETRUK W. 1990. Petrology of Au-Ag-Hg alloy and "invisible" gold in
the Trout Lake massive sulfide deposit Flin Flon, Manitoba. Canadian
Mineralogist. 28, pp. 189-206.
HEKINIAN, R., HOFFERT, M., LARQUE, P., CHEMINEE, J. L., STOFFERS, P.,
BIDEAU, D. 1993. Hydrothermal Fe and Si oxyhydroxide deposits from South
Pacific intraplate volcanoes and East Pacific Rise axial and off-axial regions.
Economic Geology 88. 2099-2121.
HERZIG, P.M., HANNINGTON, M.D., SCOTT, S.D. MALIOTIS, G., RONA, P.A.
THOMPSON, G. 1991. Gold-rich sea-floor gossans in the Troodos ophiolite and
on the Mid-Atlantic Ridge. Economic Geology, 86, pp. 1747-1755.
HILL, P.A. 1954. Mina Margot, Matanzas, Cuba. Unpublished work. A
preliminary report to Diaz de Villegas Contrastistas, S.A., Havana, Cuba
HILL, P. A. 1962. The gossans of Minas Carlota, Cuba. Economic Geology 57,
pp. 168-194.
HUNGER, S., NEWTON, R.J., BOTTRELL, S., BENNING, L.G. 2006. The
formation and preservation of greigite. Geochimica et Cosmochimica Acta, 70,
(18), p. A273.
HUTTON, J.T., TWIDALE, C.R, MILNES, A.R. AND ROSSER, H. 1972.
Composition and genesis of silcretes and silcrete skins from the Beda Valley,
southern Arcoona. Plateau, South Australia, Journal of the Geological Society of
Australia, 19, pp. 31-39.
Page 316
References
IRWIN, H., CURTIS, C. AND COLEMAN, M., 1977. Isotopic evidence for source
of diagenetic carbonates formed during burial of organic-rich sediments. Nature,
269, pp. 209-213.
JENSEN-SPAULDING, A., SHULER, M.L., LION, L.W. 2004. Mobilisation of
adsorbed copper and lead from naturally aged soil by bacterial extracellular
polymers. Water Research, 38, pp. 1121–1128.
KARNACHUK, O. V., KUROCHKINA, S. Y., TUOVINEN, O. H. 2002. Growth of
sulphate-reducing bacteria with solid-phase electron acceptors. Applied
Microbiology and Biotechnology. 58, pp. 482-486.
KAO, S.J., HORNG, C.S., ROBERTS, A.P., LIU, K.K. 2004. Carbon–sulfur–iron
relationships in sedimentary rocks from southwestern Taiwan: influence of
geochemical environment on greigite and pyrrhotite formation. Chemical
Geology, 203, pp. 153– 168.
KNIGHT, F. C., RICKARD, D. BOYCE, A. J. 1999. Multigenic origin for secondary
enrichment in Las Cruces VMS deposit, Iberian Pyrite Belt. In Stanley et al. (eds),
Mineral deposits: Processes to processing, Rotterdam: Balkema, pp.543-546.
KNIGHT, F. 2000. The mineralogy, geochemistry and genesis of the secondary
sulphide mineralisation of the Las Cruces Deposit, Spain. Unpublished PhD
Thesis. Cardiff: University of Wales.
KNOTT, R. 1994. Hydrothermal Diagenesis. Unpublished PhD Thesis. Cardiff:
University of Wales.
KNOTT, R., FALLICK, A. E., RICKARD, D., BACKER, H. 1995. Mineralogy and
sulphur isotope characteristics of a massive sulphide boulder, Galapagos Rift,
85o55'W. In: PARSON, L. M., WALKER, C. L., DIXON, D. R., eds. Hydrothermal
vents and processes. London: Geological Society special publication. 87, pp.
207-222.
Page 317
References
KOSAKEVITCH, A., GARCIA PALOMERO, F., LECA, X., LEISTEL J.M. 1993.
Climatic and geomorphological controls on the gold concentrations of the Rio
Tinto gossans (Huelva Province, Spain). C.R. Academie Sciences Paris. 316, (2),
pp. 85-90.
KOSHMAN P.N., YUGAY T.A. 1972. The causes of variation in fineness levels
of gold placers. Geochemistry International. 9 (3), pp. 481-484.
LARRASOAÑA, J.C., ROBERTS, A.P., MUSGRAVE, R.J., GRÀCIA, E.,
PIÑERO, E., VEGA, M., MARTÍNEZ-RUIZ, F. 2007. Diagenetic formation of
greigite and pyrrhotite in gas hydrate marine sedimentary systems. Earth and
Planetary Science Letters, 261, pp. 350–366.
LEISTEL, J. M., BONIJOLY, D., BRAUX, C., FREYSSINET, P., KOSAKEVITCH,
A., LECA, X., LESCUYER, J. L., MARCOUX, E., MILESI, J. P., PIANTONE, P.,
SOBOL, F., TEGYEY, M., THIEBLEMONT, D., VIALLEFOND, L. 1994. The
massive sulphide deposits of the South Iberian Pyrite Province: Geological
setting and exploration criteria. Bureau de Recherces Geologiques et Minieres.
Doc 234, 236.
LEISTEL, J. M., MARCOUX, E., THIEBLEMONT, D., QUESADA, C., SANCHEZ,
A., ALMODOVAR, G. R., PASCUAL, E., SAEZ, R. 1998. The volcanic hosted
massive sulphide deposits of the Iberian Pyrite Belt. Review and preface to the
thematic issue. Mineralium Deposita, 33, pp. 2-30.
LEISTEL, J. M., MARCOUX, E., DESCHAMPS, Y. 1998. Chert in the Iberian
Pyrite Belt. Mineralium Deposita, 33, pp. 59-81.
LENGKE, M.F. and SOUTHAM, G. 2005. The effect of thiosulphate-oxidizing
bacteria on the stability of the gold-thiosulphate complex. Geochimica et
Cosmochimica Acta, 69 (15), pp. 3759–3772.
Page 318
References
LENNIE, A.R., REDFERN, S.A.T., CHAMPNESS, P.E, STODDART, C.P.,
SCHOFIELD, P.F., VAUGHAN, D.J.. 1997. Transformation of mackinawite to
greigite: An in situ X-ray powder diffraction and transmission electron microscope
study. American Mineralogist, 82, pp. 302-309.
LEVCHENKO, L.A., SADKOV, A.P., LARIONTSEVA, N.V., KOLDASHEVA, E.M.,
SILOVA, A.K., SHILOV, A.E. 2002. Gold helps bacteria to oxidize methane.
Journal Inorganic Biochemistry, 88. pp. 251–253.
LIU, Y.G., ZHOU, M., ZENG, G.M., WANG, X., LI, X., FAN, T., XU, W.H. 2008.
Bioleaching of heavy metals from mine tailings by indigenous sulphur-oxidising
bacteria: Effects of substrate concentration. Bioresource Technology, 99, pp.
4124–4129.
LOPEZ GARCIA, J.A., LUNAR R., OYARZUN, R. 1988. Silver and lead
mineralogy in gossan-type deposits of Sierra de Cartagena, southeast Spain.
Transactions of the Institute of Mining and Metallurgy. Section B Applied Earth
Science. 97, pp. B82-B88.
LOUGHLIN, G.F. 1914. The oxidised zinc ores of the Tintic District, Utah.
Economic Geology, 9, pp. 1-19.
LOVLEY, D.R., CHAPELLE, F.H., PHILLIPS, E.J.P. 1990. Fe(III)-reducing
bacteria in deeply buried sediments of the Atlantic Coastal Plain. Geology. 18,
pp. 954-957
LUNDEGARD, P.D. 1994. Mixing zone origin of 13C-depleted calcite cement:
Oseberg Formation sandstones (Middle Jurassic), Veslefrikk Field, Norway.
Geochimica et Cosmochimica Acta, 58 (12), pp. 2661-2675.
MALONE E.J. 1979). Nature, distribution and relationships of the mineralisation
at Woodlawn, NSW. J. Geol. Soc. Aust. 26:, pp. 141-153.
Page 319
References
MALONE, M.J., CLAYPOOL, G., MARTIN, J.B., DICKENS, G.R. 2002. Variable
methane fluxes in shallow marine systems over geologic time. The composition
and origin of pore waters and authigenic carbonates on the New Jersey shelf.
Marine Geology, 189, pp 175-196.
MANN, A.W. 1984. Mobility of Gold and Silver in Lateritic Weathering Profiles:
Some Observations from Western Australia. Economic Geology, 79, pp 38-49.
MANN, A. W. 1988. Oxidised gold deposits; relationships between oxidation and
relative position of the water table. Australian Journal of Earth Sciences, 45 (1),
pp. 97-108.
MARCOUX, E. 1998. Lead isotope systematics of the giant massive sulphide
deposits in the Iberian Pyrite Belt. Mineralium Deposita, 33, pp. 45-58.
MAY, E.R. 1977. Flambeau - A Precambrian supergene enriched massive
sulphide deposit. Geoscience Wisconsin, 1, pp. 1-26.
MCCREA, J.M. 1950. On the isotopic chemistry of carbonates and a
paleotemperature scale. Journal of Chemical Physics, 18, pp. 849-857.
MIDDLETON, H.A. and NELSON, C.S. 1996. Origin and timing of siderite and
calcite concretions in late Palaeogene non- to marginal-marine facies of the Te
Kuiti Group, New Zealand. Sedimentary Geology, 103, pp. 93-115.
MILNES A.R. and R.W. FITZPATRICK 1989. Titanium and Zirconium minerals. In
Dixon, J.B. and Weed, S.B., eds. Minerals in Soil Environments. Madison,
Wisconsin: 2nd ed. Soil Sci. Soc. America Book Series 1. SSSA, pp.1131-1205.
MONHEMIUS, A.J. 1977. Precipitation diagrams for metal hydroxides, sulphides,
arsenates and phosphates. Transactions of the Institute of Mining and
Metallurgy. Section C, 86, pp. 202-206.
Page 320
References
MORIN, K. A., CHERRY, J. A. 1986. Trace amounts of siderite near a Uranium
tailings impoundment, Elliot Lake, Ontario, Canada, and its implication in
controlling contaminant migration in a sand aquifer. Chemical Geology, 56, pp.
117-134.
MORRIS, R.C. FLETCHER, A.B. 1987. Increased solubility of quartz following
ferrous-ferric iron reactions. Nature, 330, pp. 558-561.
MORENO C. 1993. Post volcanic Palaeozoic of the Iberian Pyrite Belt: An
example of basin morphological control on sediment distribution in a turbidite
basin. Journal of Sedimentary Petrology, 63 (6), pp. 1118-1128.
MORENO C., SIERRA S., SAEZ R., 1996. Evidence for catastrophism at the
Famennian-Dinantian boundary in the Iberian Pyrite Belt. In: Strogen P.,
Sommerville I. D., Jones, G. L., eds. Recent advances in Lower Carboniferous
geology. London: Geological Society Special Publication, 107, pp. 153-162.
MOSSMAN D.J., REIMER T. and DURSTLING H. 1999. Microbial processes in
gold migration and deposition: Modern analogues to ancient deposits.
Geoscience Canada. 26. pp. 131-140.
MOZELY, P.S. 1989. Relation between depositional environment and the
elemental composition of early diagenetic siderite. Geology, 17, pp. 704-706.
MOZLEY, P.S. WERSIN, P. 1992. Isotopic composition of siderite as an Indicator
of depositional environment. Geology, 20, pp. 817-820.
MOZELY, P. S., CAROTHERS, W. 1992. Geochemistry of siderite in the Kuparuk
Formation, Alaska: Influence of water/sediment interaction and microbial activity
on early pore-water chemistry. Journal of sedimentary petrology, 62, pp. 681-692.
MUNHA, J. 1983. Low grade regional metamorphism in the Iberian Pyrite Belt.
Comunicacoes dos Servicos Geologicos de Portugal, 69, pp. 3-35.
Page 321
References
MURPHY E. M., SCHRAMKE, J. A., FREDRICKSON, J. K., BLEDSOE, H. W.,
FRANCIS, A. J., SKLAREW, D. S., LINEHAN, J. C. 1992. The influence of
microbial activity and sedimentary organic carbon on the isotope geochemistry of
the Middendorf Aquifer. Water Resources Res., 28, pp 723-740.
NERETIN, L.N., BOTTCHER, M.E., JØRGENSEN, B.B., VOLKOV, I.I.,
LUSCHEN, H. and HILGENFELDT, K. 2004. Pyritization processes and greigite
formation in the advancing sulfidization front in the Upper Pleistocene sediments
of the Black Sea. Geochimica et Cosmochimica Acta, 68, (9), pp. 2081–2093.
NICKEL, E.H. 1984. The Mineralogy and Geochemistry of the Weathering Profile
of the Teutonic Bore Cu-Pb-Zn-Ag Sulphide Deposit. Journal Geochemical
Exploration, 22, pp. 239-264.
NICKEL, E. H., THORNBER, M. R. 1977. Chemical constraints on the
weathering of serpentinised containing nickel-iron sulphides. Journal
Geochemical Exploration, 8, pp. 235-245.
NUNEZ, C., ROCA, A., ESPIELL, F. 1986. Improved gold and silver recovery
from Spanish gossan ores by sulphidisation prior to cyanidation. Transactions of
the Institute of Mining and Metallurgy. 95, pp. C195-C198.
OHMOTO H. 1986. Stable isotope geochemistry of ore deposits. Reviews in
Mineralogy. 16, pp. 491-559.
OKAMOTO, G., OKURA, T., GOTO, K. 1957. Properties of silica in water.
Geochimica et Cosmochimica Acta, 12, pp. 123-132.
OLIVEIRA, J. T. 1983. The Marine Carboniferous of South Portugal: A
stratigraphic and sedimentological approach. In: SOUSA, M. J. L., OLIVEIRA, J.
T. eds, The Carboniferous of Portugal. Memorias dos Servicos Geologicos de
Portugal, 29, pp. 3-37.
Page 322
References
OLIVEIRA, V., MATOS, J., BENGALA, M., SILVA, N., SOUSA, P., TORRES, L.
1998. Geology and geophysics as successful tools in the discovery of the Lagoa
Salgada Orebody (Sado Tertiary Basin - Iberian Pyrite Belt), Grandola, Portugal.
Mineralium Deposita, 33, pp.170-187.
PAKTUNC, A. D., DAVE, N. K. 2002. Formation of secondary pyrite and
carbonate minerals in the Lower Williams Lake tailings basin, Elliot Lake, Ontario,
Canada. American Mineralogist,. 87, pp. 593-602.
PASSCHIER, C.W. AND TROUW, R.A.J. 1996. Microtectonics. Heidelberg:
Springer.
PRICHARD, H.M. and MALIOTIS, G. 1998. Gold mineralisation associated with
low temperature, off-axis, fluid activity in the Troodos ophiolite, Cyprus. Journal of
the Geological Society London, 155, pp 223-231.
QUESADA, C. 1991. Geological constraints on Palaeozoic tectonic evolution of
stratigraphic terrains in the Iberian Massif. Tectonophysics. 185, pp. 225-245.
QUESADA, C. BELLIDO, F., DALLMEYER, R. D., GIL IBARGUCHI, J. I.,
OLIVEIRA, J. T., PEREZ ESTAUN, A., RIBEIRO, A., ROBARDET, M., SILVA, J.
B. 1991. Terranes within the Iberian Massif: Correlations with West African
sequences. In: DALLMEYER, R.D., LECORCHE, J. P. eds. The West African
Orogens and Circum-Atlantic Correlations. Springer-Verlag. pp 267-293.
RAMSAY, J. HUBER, M.I. 1983. The techniques of modern structural geology, 1:
Strain analysis. London: Academic press.
RAN, Y., FU, J., RATE, A.W., GILKES, R.J., 2002. Adsorption of Au(I, III)
complexes on Fe, Mn oxides, and humic acid. Chemical Geology, 185. pp 33–49.
RAVIZZA, G., BLUSZTAJN, J., PRICHARD, H. M. 2001. Re-Os systematics and
PGE distribution in metalliferous sediments from the Troodos ophiolite. Earth and
Planetary Science Letters, 188, pp. 369-381.
Page 323
References
REITH, F., and MCPHAIL, D.C. 2006. Effect of resident microbiota on the
solubilisation of gold in soil from the Tomakin Park Gold Mine, New South Wales,
Australia Geochimica et Cosmochimica Acta, 70. pp 1421–1438.
RIBEIRO, A., QUESADA, C. DALLMEYER, R. D. 1990. Geodynamic evolution of
the Iberian Massif. In: DALLMEYER R. D., MARTINEZ G. eds. Pre-Mesozoic
geology of Iberia. Springer-Verlag. pp 399-409.
RICKARD, D. 1997. Kinetics of pyrite formation by the H2S oxidation of iron (II)
monosulphide in aqueous solutions between 25 and 125°C: The rate equation:
Geochimica et Cosmochimica Acta, 61, pp. 115–134.
RICKARD, D. 1999. European Phanerozoic metallogenesis. Mineralium
Deposita. 34, pp. 417-421.
RICKARD, D., BUTLER, I.B., OLDROYD, A. 2001. A novel iron sulphide mineral
switch and its implications for Earth and planetary science. Earth and Planetary
Science Letters, 189, pp. 85-91.
RICKARD, D. 2006. The solubility of FeS. Geochimica et Cosmochimica Acta,
70, pp. 5779–5789.
RICKARD, D. 2006. Characteristics of mackinawite, tetragonal FeS. Goldschmidt
Conference Abstracts, A533.
ROBERTS, A.P. and WEAVER, R. 2005. Multiple mechanisms of
remagnetization involving sedimentary greigite (Fe3S4), Earth and Planetary
Science Letters, 231, pp. 263–277.
ROBERTSON, A. H. F. and BOYLE, J. F. 1983. Tectonic setting and origin of
metalliferous sediments in the Mesozoic Tethys Ocean. In: RONA, P.,
BOSTROEM, K., LAUBIER, L. SMITH, K. Eds. Hydrothermal processes at
seafloor spreading centres. Marine Sciences. 12, pp. 595-663.
Page 324
References
RODEN E. E. 2004. Analysis of long-term bacterial vs. chemical Fe(III) oxide
reduction kinetics. Geochimica et Cosmochimica Acta, 68, (15), pp. 3205–3216.
RONA, P. A. and SCOTT, S. D. 1993. Preface to special issue on seafloor
hydrothermal mineralisation: New perspectives. Economic Geology. 88, pp.
1935-1976.
RONA, P. A., HANNINGTON, M. D., RAMAN, C. V., THOMPSON, G., TIVEY, M.
K., HUMPHRIS, S. E., LALOU, C., PETERSEN, S. 1993. Active and relict sea-
floor hydrothermal mineralisation at the TAG hydrothermal field, Mid Atlantic
Ridge. Economic Geology. 88, pp. 1989-2017.
ROSENBAUM, J. and SHEPPARD, S.M.F. 1986. An isotopic study of siderites,
dolomites and ankerites at high temperatures. Geochimica et Cosmochimica
Acta, 50, pp. 1147-1150.
ROSS, A.M. 1997. Supergene gold enrichment of the Precambrian aged
Flambeau gossan, Flambeau Mine, Rusk County, Wisconsin. Unpublished work.
University of Utah: MSc Thesis.
RYALL, W.R., NICHOLAS, T. 1979. Surface geochemical and biogeochemical
expression of base-metal mineralisation at Woodlawn, NSW, Australia. Journal of
the Geological Society of Australia. 26, pp. 187-195.
R2643. 1996. Las Cruces Deposit: Gold And Silver Mineralogy of Selected
Samples from Boreholes CR059 and CR060. Unpublished work. Rio Tinto
Internal Report.
R2644. 1996. Las Cruces Deposit: Gold and Silver Mineralogy of Miscellaneous
Samples. Unpublished work. Rio Tinto Internal Report.
R2696. 1997. Las Cruces Deposit: Gold and Silver Leaching Testwork and
Mineralogy. Unpublished work. Rio Tinto Internal Report.
Page 325
References
R2795. 1998. Las Cruces Feasibility Study Phase 1. Unpublished work. Rio Tinto
Internal Report.
R2796. 1998. Las Cruces Feasibility Study, Geology and Mineral Resources.
Unpublished work. Rio Tinto Internal Report.
R2703. 1998. Las Cruces Conceptual study. Unpublished work. Rio Tinto
Internal Report
SAEZ, R., PASCUAL, E., ALMODOVAR, G. R. 1996. Geological constraints on
massive sulphide genesis in the Iberian Pyrite Belt. Ore Geology Reviews. 11,
pp. 429-451.
SAEZ, R, PASCUAL, E., TOSCANA, M., ALMODOVAR, G. R. 1999. The Iberian
type of volcan-sedimentary massive sulphide deposits. Mineralium Deposita 34,
pp. 549-570.
SANZ DE GALDEANO, C. VERA J.A. 1992. Stratigraphic record and
palaeogeographical context of the Neogene basins in the Betic Cordillera, Spain.
Basin Research, 4, pp. 21-36.
SATO, M. 1960. Oxidation of sulphide ore bodies: Pt 1. Geochemical
environments in terms of Eh and pH. Economic Geology, 55 (5), pp. 928-961.
SAUNDERS, J.A. 1993. Supergene oxidation of bonanza Au-Ag veins at the
sleeper Deposit, Nevada, USA: implications for hydrochemical exploration in the
Great Basin. Journal Geochemical Exploration, 47, pp. 359-375.
SCHOONEN, M.A.A. and BARNES H.L. 1991. Reactions forming pyrite and
marcasite from solution: II. Via FeS precursors below 100°C. Geochimica et
Cosmochimica Acta, 55, pp. 1505-1514.
Page 326
References
SCHOONEN, M.A.A. 2004. Mechanisms of sedimentary pyrite formation. In:
AMEND, J.P., EDWARDS, K.J., LYONS, T.W. Eds. Sulphur biogeochemistry:
Past and present. Boulder, Colorado. Geological Society America. pp. 117-134.
SCOTT, K.M., ASHLEY, P.M., LAWIE, D.C. 2001. The geochemistry, mineralogy
and maturity of gossans derived from volcanogenic Zn-Pb-Cu deposits of the
eastern Lachlan Fold Belt, NSW, Australia. Journal Geochem. Explor., 72, pp.
169-191.
SCHERMERHORN, J. L. G. 1971. An outline stratigraphy of the Iberian Pyrite
Belt. Boletín Geológico y Minero, 82, pp. 239-268.
SKINNER, B.J., ERD, R.C., GRIMALDI, F.S.. 1964. Greigite, the thio-spinel of
iron; a new mineral. American Mineralogist, 49, pp. 543–555.
SKRABAL, S.A. 1995. Distributions of dissolved titanium in Chesapeake Bay
and the Amazon River Estuary. Geochimica et Cosmochimica Acta 59, pp. 2449-
2458.
SOLOMON, M. 1967. Fossil gossans at Mt. Lyell, Tasmania. Economic Geology,
62, pp. 757-772.
SOUTHAM, G., and BEVERIDGE, T. J. 1996. The occurrence of sulphur and
phosphorus within bacterially derived crystalline and pseudocrystalline octahedral
gold formed in vitro. Geochimica et Cosmochimica Acta, 60, pp. 4369–4376.
SPIRO, B., GIBSON, P.J., SHAW, H.F. 1993. Eogenetic siderites in lacustrine oil
shales from Queensland, Australia, a stable isotope study. Chemical Geology,
106, pp. 415-427.
STICKNEY, A.W. 1915. Pyritic copper of Kyshtim, Russia: Economic Geology,
10, pp. 593-633.
Page 327
References
STRAUSS G. K. 1970. Sobre la geologia de la provincia piritifera del suroeste de
la Peninsula Iberica y de sus yacimentos, en especial sobre la mina de piritia de
lousal (Portugal). Memoires de Institute Geologico y Minero Espana, 77, 266p.
STRAUSS, G. K. MADEL, J. 1974. Geology of massive sulphide deposits in the
Spanish-Portuguese Pyrite Belt. Geologische Rundschau, 63, pp. 191-211.
STRAUSS, G. K. BECK, J.K. 1990. Gold Mineralisation in the SW Iberian Pyrite
Belt. Mineralium Deposita, 25 (4), pp. 237-245.
TAYLOR, J.H. 1958. Formation of supergene galena at Broken Hill, N. Rhodesia.
Mineralogy Magazine, 31, pp. 908-913.
TAYLOR, P., RUMMERY, T.E., OWEN D.G. 1979. On the conversion of
mackinawite to greigite. Journal of Inorganic and Nuclear Chemistry, 41, (4), pp.
595-596.
TAYLOR, G.F., SYLVESTER, G.C. 1982. Analysis of a weathered profile on
sulphide mineralisation at Mugga Mugga, Western Australia. Journal
Geochemical Exploration, 16, pp. 105-134.
TAYLOR, G.F., APPLEYARD, E.C. 1983. Weathering of the Zinc-Lead Lode,
Dugald River, North-West Queensland: 1. The Gossan Profile. Journal
Geochemical Exploration, 18, pp. 87-110.
TAYLOR, G.F., THORNBER M.R. 1992. Gossan and Ironstone Surveys. In:
C.R.M. BUTT AND H. ZEEGERS, eds. Regolith Exploration Geochemistry in
Tropical and Sub-Tropical Terrains. Amsterdam: Elsevier, pp. 139-201.
THORNBER, M.R. 1975. Supergene alteration of sulphides II. A chemical study
of the Kambalda nickel deposit. Chemical Geology, 15, pp. 117-144.
THORNBER, M.R. 1976. Supergene alteration of sulphides. III The composition
of associated carbonates. Chemical Geology, 17, pp. 45-72.
Page 328
References
THORNBER, M.R. 1992. The Chemical Mobility and Transport of Elements in the
Weathering Environment. In: C.R.M. BUTT AND H. ZEEGERS, eds. Regolith
Exploration Geochemistry in Tropical and Sub-Tropical Terrains. Amsterdam:
Elsevier, pp. 79-95.
THORNBER M.R., TAYLOR, G.F. 1992. The Mechanism of Sulphide Oxidation
and Gossan Formation. In: C.R.M. BUTT AND H. ZEEGERS, eds. Regolith
Exploration Geochemistry in Tropical and Sub-Tropical Terrains. Amsterdam:
Elsevier, pp. 110-138.
THORNBER, M.R. WILDMAN, J.E. 1984. Supergene alteration of sulphides, VI.
The binding of Cu, Ni, Zn, Co and Pb with gossan (iron-bearing) minerals.
Chemical Geology, 44, pp. 399-434.
THORNBER, M.R., WILDMAN, J.E. 1984. The mechanisms of forming iron-base
metal gossan minerals. Journal Geochemical Exploration, 22, pp. 349-350.
TRESCASES J.J. 1992. Chemical Weathering. In: C.R.M. BUTT AND H.
ZEEGERS, eds. Regolith Exploration Geochemistry in Tropical and Sub-Tropical
Terrains. Amsterdam: Elsevier, pp. 25-38.
UYTENBOGAARDT, W. and BURKE, E.A.J. 1971. Tables for identification of
ore minerals. 2nd edn. New York: Dover Publications, Inc.
VAN DEN BOOGARD, M., SCHERMERHORN, L. J. G. 1980. Conodont faunas
from Portugal and southwestern Spain. Part 4: a Famennian conodont fauna near
Nerva (Rio Tinto). Scripta Geol. 56, pp. 1-14.
VAN DONGEN, B.E., ROBERTS, A.P., SCHOUTEN, S., JIANG, W-T.,
FLORINDO, F., PANCOST, R.D. 2007. Formation of iron sulphide nodules during
anaerobic oxidation of methane. Geochimica et Cosmochimica Acta, 71, pp.
5155–5167.
Page 329
References
VINALS, J., ROCA, A., CRUELLS, M., NUNEZ, C. 1995. Characterisation and
cyanidation of Rio Tinto Gossan Ores. Canadian Metallurgical Quarterly, 34, (2)
pp. 115-122.
VINK, B.W. 1996. Stability relations of antimony and arsenic in the light of revised
and extended Eh-pH diagrams. Chemical Geology, 130, pp. 21-30.
VLASSOPOULOS, D., WOOD, S.A. 1990. Gold speciation in natural waters:I.
solubility and hydrolysis reactions of gold in aqueous solution. Geochimica et
Cosmochimica Acta, 54, pp. 3-12.
VLASSOPOULOS, D., WOOD, S.A., MUCCI, A. 1990. Gold speciation in natural
waters: II. The importance of organic complexing- Experiments with some simple
model ligands. Geochimica et Cosmochimica Acta, 54, pp. 1575- I586.
WEBSTER, J.G., MANN, A.W. 1984. The influence of climate, geomorphology
and primary geology on the supergene migration of gold and silver. Journal
Geochemical Exploration 22, pp. 21-42.
WACHTERSHAUSER, G. 1988. Pyrite formation, the first energy source of life: a
hypothesis. Systematics Applied Microbiology, 10, pp. 207–210.
WACHTERSHAUSER, G. 1993. The cradle chemistry of life: on the origin of
natural products in a pyrite-pulled chemoautotrophic origin of life. Pure and
Applied Chemistry, 65, pp. 1343–1348.
WEIJMA, J., DE HOOP, K., BOSMA, W., DIJKMAN, H. 2002. Biological
conversion of anglesite (PbSO4) and lead waste from spent car batteries to
galena (PbS). Biotechnology Progress. 18, pp. 770-775.
WILKIN, R.T. and BARNES, H. L. 1996. Pyrite formation by reactions of iron
monosulfides with dissolved inorganic and organic sulphur species. Geochimica
et Cosmochimica Acta, 60, pp. 4167–4179.
Page 330
References
WILLIAMS, D. 1933-34. The geology of Rio Tinto Mines, Spain. Bulletin of the
Institute of Mining and Metallurgy. 43, pp. 594-640.
WILLIAMS, D. 1950. Gossanized breccia ores, jarosites and Jaspers at Rio Tinto
mines, Spain. Bulletin of the Institute of Mining and Metallurgy. 59, pp. 1-12.
WILLIAMS, P. A. 1990. Oxide Zone Geochemistry. London, Ellis Horwood Ltd.
WILMSHURST, J.R. 1975. Surface rocks in the vicinity of the Woodlawn deposit.
Unpublished work. CSIRO Minerals research Laboratories Restricted
Investigation Report 679R.
WILMSHURST, J.R. 1977. Notes on some representative gossans from the
Woodlawn deposit. Unpublished work. CSIRO Minerals Research Laboratories
Restricted Investigation Report 914R.
WILMSHURST, J.R. 1979. Woodlawn Cu-Pb-Zn deposit. In: RAMSDEN, A.R.,
RYALL, W.R., eds. The Lachlan Fold Belt: Contributions to Mineral Exploration.
CSIRO Institute Earth Resources Investigation Report 128, pp.11-21.
WOLTHERS, M., VAN DER GAAST, S.J., RICKARD, D., 2003. The structure of
disordered mackinawite. American Mineralogist, 88, pp. 2007–2015.
WU, S.C., LUO, Y.M., CHEUNG, K.C., WONG, M.H. 2006. Influence of bacteria
on Pb and Zn speciation, mobility and bioavailability in soil: A laboratory study
Environmental Pollution, 144, pp 765-773.
96/934. 1996. Mineralogical Characterisation of a Sample of Tertiary Marl from
the Guadalquivir Basin, Las Cruces, Spain. Unpublished work. Rio Tinto Internal
Report.
Page 331
Appendix 1 List of Mineral Formulae
Appendix 1: List of mineral formulae
AcanthiteAguilariteAluniteAmalgamAnataseAnglesiteApatiteArgentojarosite ArsenopyriteAtacamiteBariteBeudantiteBindheimiteBismuthiniteBorniteCalciteCassiteriteCerussiteChalcociteChalcopyriteChlorargyriteCinnabarClausthaliteCoronaditeCovelliteCrandalliteCristobaliteDigeniteDjurleiteEnargiteGalenaGeocroniteGlauconiteGoethiteGreigiteHarmotomeHematiteIdaiteImiteriteIodargyriteJarositeJordaniteKaoliniteKesteriteKoutekiteLaffittiteLaurioniteLepidocrociteLudlockiteLuzoniteMackinawiteMarcasiteMetacinnabarMimetiteNadoriteNaumanite
Ag2SAg4SSeKAl3(SO4)2(OH)6 AgHg-alloyTiO2
PbSO4
Ca5(PO4)3(F,Cl,OH) AgFe3+
3(SO4)2(OH)6
FeAsSCu2Cl(OH)2
BaSO4
PbFe3(AsO4)(SO4)(OH)6
Pb2Sb2O6(O,OH)Bi2S3
Cu5FeS4
CaCO3
SnO2
PbCO3
Cu2SCuFeS2
AgClHgSPbSePb(Mn4+,Mn2+)8O16
CuSCaAl3(PO4)2(OH)5.H2O SiO2
Cu1.805S Cu1.9SCu3AsS4 PbSPb14(Sb,As)6S23 (K,Na)(Fe3+,Al,Mg)2(Si,Al)4(OH)2
α-Fe3+O(OH)Fe2+Fe2
3+S4
(Ba,K)(SiAl)9O16.6H2OFe2O3
Cu4FeS6
Ag2HgS2
AgIKFe3(SO4)2(OH)6
Pb14(As,Sb)6S23
Al2Si2O5(OH)4
Cu2(Zn,Fe)SnS4
Cu5As2
AgHgAsS3
PbClOHγ-Fe3+O(OH)(Fe,Pb)As2O6
Cu3AsS4
Fe9S8
FeS2
Hg (Se,S)Pb5(AsO4)3Cl PbSbO2ClAg2Se
A1
Appendix 1 List of Mineral Formulae
NontroniteNovakiteNukundamitePhilipsbornitePlumbogummitePlumbojarositeProustitePyrargyritePyritePyromorphitePyrrhotiteQuartzRhodochrositeRobinsoniteRutileScoroditeSideriteSphaleriteStanniteSternbergiteStibiconiteTennantiteTetrahedriteWitticheniteZinkeniteZircon
Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O
(Cu,Ag)21As10
(Cu,Fe)4S4
PbAl3(AsO4)2(OH)5.H2OPbAl3(PO4)2(OH)5.H2OPbFe6(SO4)4(OH)12
Ag3AsS3
Ag3SbS3
FeS2
Pb5(PO4)3ClFe1-xSSiO2
MnCO3
Pb4Sb6S13
TiO2
FeAsO4.2H2OFeCO3
ZnSCu2FeSnS4 AgFe2S3
Sb3+Sb5+2O6(OH)
(Cu,Fe)12As4S13
Cu12Sb4S13 Cu3BiS3
Pb9Sb22S42
ZrSiO4
A2
Appendix 2 Sample List
Appendix 2: Sample List (Data courtesy of Rio Tinto Limited)
Borehole CR194
Depth (m) LITHOCODE/DESCRIPTION LENS
From To149.80 150.80 GHS - Strong Hematitic Gossan150.80 151.75 GHS - Strong Hematitic Gossan HWL (Hangingwall)151.75 152.70 GHS - Strong Hematitic Gossan HWL (Hangingwall)152.70 153.70 GHS - Strong Hematitic Gossan HWL (Hangingwall)153.70 154.75 GHS - Strong Hematitic Gossan HWL (Hangingwall)154.75 155.75 GBM - Moderate Hematite Magnetic HWL (Hangingwall)155.75 156.70 GBM - Moderate Hematite Magnetic AU1 (Au)156.70 157.30 GBM - Moderate Hematite Magnetic HWP (Hangingwall Pb)157.30 158.70 GBM/GHS - Moderate/Strong Hematitic Magnetic HWP (Hangingwall Pb)158.70 159.75 GHS - Strong Hematitic Gossan HWP (Hangingwall Pb)159.75 160.75 GHS - Strong Hematitic Gossan AU (Au)160.75 161.75 GHS - Strong Hematitic Gossan AU (Au)161.75 162.75 GHS - Strong Hematitic Gossan AU (Au)162.75 163.75 GHS - Strong Hematitic Gossan AU (Au)163.75 164.60 GHS - Strong Hematitic Gossan AU (Au)164.60 165.80 MMP - Massive Sulphide HCH (Secondary Cu)165.80 166.80 MMP - Massive Sulphide HCH (Secondary Cu)166.80 167.75 MMP - Massive Sulphide HCH (Secondary Cu)167.75 168.70 MMP - Massive Sulphide HCH (Secondary Cu)170.70 171.60 MMP - Massive Sulphide HCH (Secondary Cu)171.60 172.50 MMP - Massive Sulphide HCH (Secondary Cu)172.50 173.50 MMP/QXM - Massive Sulphide/Quartz/Shale HCH (Secondary Cu)173.50 174.50 QXM/MMP - Quartz/Shale/Massive Sulphide HCH (Secondary Cu)174.50 175.45 MMP - Massive Sulphide HCH (Secondary Cu)175.45 176.35 MMP - Massive Sulphide HCH (Secondary Cu)176.35 177.20 MMP - Massive Sulphide HCH (Secondary Cu)177.20 178.50 MMP - Massive Sulphide HCH (Secondary Cu)178.50 180.00 SXM - Massive Shale MCL (Secondary Cu)
A3
Appendix 2 Sample List
Borehole CR149
Depth LITHOCODE/DESCRIPTION LENS
From To
170.20 170.90 TSA - Tertiary Sand AU (Au)170.90 172.40 GHS - Strong Hematitic Gossan AU (Au)172.40 174.10 GHS/GMS - Strong Hematitic/Strong Magnetic AU (Au)174.10 175.10 GEM - Moderately Leached Gossan AU (Au)175.10 175.90 GEM - Moderately Leached Gossan AU (Au)175.90 176.90 GLM/GEW - Moderate Limonitic /Weakly Leached AU (Au)176.90 178.05 GHM/GLS - Moderate Hematitic/Strong Limonitic AU (Au)178.05 179.00 GEM - Moderately Leached Gossan AU (Au)179.00 180.35 GEM/GLS - Moderately Leached/Strong Limonitic AU (Au)180.35 182.00 GLS/GHS - Strong Limonitic/Strong Hematitic AU (Au)182.00 182.85 GHS - Strong Hematitic Gossan AU (Au)182.85 183.90 GHS - Strong Hematitic Gossan AU (Au)183.90 185.40 GMS - Strong Magnetic Gossan AU (Au)185.40 186.80 GLW - Weak Limonitic Gossan AU (Au)186.80 187.40 GHS - Strong Hematitic Gossan AU (Au)187.40 188.90 GMS - Strong Magnetic Gossan AU (Au)188.90 190.00 GMS - Strong Magnetic Gossan AU (Au)190.00 190.90 MMP - Massive Sulphide HCH (Secondary Cu)190.90 191.90 MMP - Massive Sulphide HCH (Secondary Cu)
Borehole CR038
Depth LITHOCODE/DESCRIPTION LENS
From To
150.80 151.45 QTM - Quartz Replacement of Massive Tuff AU (Au)151.45 152.40 QTM - Quartz Replacement of Massive Tuff AU (Au)152.40 153.20 QTM - Quartz Replacement of Massive Tuff AU (Au)153.20 154.20 QTM - Quartz Replacement of Massive Tuff AU (Au)154.20 155.20 QTM - Quartz Replacement of Massive Tuff AU (Au)155.20 156.30 QTM - Quartz Replacement of Massive Tuff AU (Au)156.30 157.25 QTM/MSPCL - Quartz/Tuff/Sulphide/Clay AU (Au)157.25 158.25 MSPCL - Partial Massive Sulphide with Clay E (Envelope)
A4
Appendix 2 Sample List
Borehole CR191
Depth LITHOCODE/DESCRIPTION LENS
From To
134.25 135.25 TCP/GHW - Conglomerate/Weak Hematitic Gossan HWL (Hangingwall)135.25 135.70 GHW/GEM - Weak Hematitic/Moderately Leached HWL (Hangingwall)135.70 136.85 GEM - Moderately Leached Gossan HWL (Hangingwall)136.85 137.95 GMS - Strong Magnetic Gossan HWP (Hangingwall Pb)137.95 138.90 GMS - Strong Magnetic Gossan AU1 (Au)138.90 139.85 GMS - Strong Magnetic Gossan AU1 (Au)139.85 141.00 GMS - Strong Magnetic Gossan AU1 (Au)141.00 141.65 GES - Strongly Leached Gossan AU1 (Au)141.65 142.65 GES - Strongly Leached Gossan AU1 (Au)142.65 143.60 GES - Strongly Leached Gossan AU1 (Au)143.60 144.70 GES - Strongly Leached Gossan144.70 145.75 GES - Strongly Leached Gossan145.75 146.90 GES - Strongly Leached Gossan146.90 148.15 GES - Strongly Leached Gossan148.15 149.15 GES - Strongly Leached Gossan149.15 150.10 GES - Strongly Leached Gossan AU (Au)150.10 150.90 GES - Strongly Leached Gossan AU (Au)150.90 151.75 GES - Strongly Leached Gossan AU (Au)151.75 153.85 GES - Strongly Leached Gossan AU (Au)153.85 155.35 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)155.35 156.25 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)156.25 157.20 MMPXM - Massive Sulphide with Shale HCL (Secondary Cu)
Borehole CR123
Depth LITHOCODE/DESCRIPTION LENS
From To
152.40 153.95 TCP - Tertiary Polymict Conglomerate AU (Au)153.95 154.85 GMS - Strong Magnetic Gossan AU (Au)154.85 157.05 No Core Recovery157.05 158.65 GMS - Strong Magnetic Gossan AU (Au)158.65 160.20 GMS - Strong Magnetic Gossan AU (Au)160.20 161.40 GMS - Strong Magnetic Gossan AU (Au)161.40 161.80 GMS - Strong Magnetic Gossan AU (Au)161.80 163.40 GMS - Strong Magnetic Gossan AU (Au)163.40 168.20 No Core Recovery168.20 169.00 GMS - Strong Magnetic Gossan AU (Au)169.00 169.65 QXM - Quartz Replacement of Massive Shale AU (Au)169.65 172.85 QXM - Quartz Replacement of Massive Shale AU (Au)172.85 176.00 No Core Recovery176.00 178.35 SXM - Massive Shale MCL (Secondary Cu)178.35 180.00 QXM - Quartz Replacement of Massive Shale MCL (Secondary Cu)180.00 181.50 SXM - Massive Shale MCL (Secondary Cu)
A5
Appendix 3 Assay Data
Appendix 3: Assay Data
Au Analysis – Fire assay with AAS/ICP finish
The sample is roasted and mixed with a suitable flux, transferred to a fireclay crucible and fused. Lead oxide in the flux is reduced, and the lead globules formed collect the precious metals. The lead button is cleaned and cupelled, the precious metal prill dissolved in aqua regia and the analytes of interest are analysed by AAS (Thermo Jarrell Ash Atomic Absorption Spectrophotometer model Video 12 E) or ICP (Philips PV8060 simultaneous ICP-emission spectrometer) against reagent matched standards.
Examination of the analysis results of MA2 and SARM-7 indicates that the precision and accuracy appears to be concentration and matrix dependent. Accuracy of the method was determined by examining analysis results of certified reference materials MA2 (1.86g/t Au) and SARM-7 (0.31g/t). A random series of analytical results from 24 month analyses of MA2 and SARM-7 were used as a basis for the determinations. MA2 is accurate to 1.1% and 6.4% for SARM-7. The uncertainty for MA2 at the 95% confidence level is 1.6%, while for a lower grade material SARM-7 the uncertainty at the 95% confidence level increases to 4.7%.
Cu, Pb, Zn, Fe, Ag - Atomic Absorption Spectrometry
The sample is digested by acid treatment and the solution evaporated to incipient dryness. The residue is then re dissolved in hydrochloric acid, and ammonium acetate solution and diluted to volume. The solution is then analysed by atomic absorption spectrometry (Thermo Jarrell Ash Atomic Absorption Spectrophotometer model Video 12 E) using flame atomisation.
The accuracy and precision (at 95% confidence levels) of the method for the elements Cu, Pb, Zn, Fe, and Ag was determined from the results of the analysis of the Certified Reference Standard MP-1a and CCU-1b.
Precision and accuracy of AAS analytical method
Element AccuracyCCU1b
AccuracyMP-1a
PrecisionCCU1b
PrecisionMP-1a
Cu +0.25% ±1.4% +0.71% ±0.86%
Ag +1.52% ±0.3% +0.72% ±1.3%
Fe +0.35% ±3.9% +0.98% ±1.6%
Pb +5.94% ±0.4% +1.09% ±0.44%
Zn +0.16% ±0.8% +0.85% ±1.66%
A6
Appendix 3 Assay Data
Sulphur Analysis – Leco
An appropriate weight of sample is ignited in a stream of oxygen. The sulphur present in the sample is converted to sulphur dioxide and absorbed into dilute hydrochloric acid and starch solution. This solution is then titrated against potassium iodate solution.The precision and accuracy of this method is dependent on the photoelectric cell which controls the titration rate. Without regular cleaning and with time the photoelectric cell will deteriorate, thus reducing recovery and therefore reducing accuracy. Results should agree to within 0.1% for low levels of sulphur and within 0.2% for high levels of sulphur.
As, Sb, Sn - XRF
The powdered sample is mixed with a diluent and a binder in fixed proportions, and the mixture is pressed at 25 KN. The pellets are analysed by X-ray fluorescence spectroscopy using a Philips PW1400 and Philips PW2400. Precision and accuracy of this method is equipment, concentration and matrix dependant. Additional details of typical analyses of standards are provided below.
Certified reference standards MRG-1, SO-1, SO-2, SY-2, SY-3 and internal standard STD5 have been used to validate the Work Instruction for the PW1400 and PW2400.
Standard MRG-1
MRG-1
MRG-1
SO-1 SO-1 SO-1 SO-2 SO-2 SO-2
Element As (ppm)
Sb (ppm)
Sn (ppm)
As (ppm)
Sb (ppm)
Sn (ppm)
As (ppm)
Sb (ppm)
Sn (ppm)
Theoretical
values
1 1 4 2 1 3 1 1 3
No of values 52 52 52 53 53 53 106 106 106
Max 3 3 7 8 5 7 4 4 7
Min -3 -3 1 1 -2 -2 -3 -3 0
Mean 0 1 4 3 1 3 1 0 3
S.D. 1.540 1.572 1.607 1.674 1.420 2.057 1.531 1.656 1.733
%R.S.D. 153.98 157.18 40.18 83.69 141.98 68.57 153.11 165.60 57.77
Uncertainty 0.43 0.44 0.45 0.46 0.39 0.57 0.30 0.32 0.34
%Uncertainty 92.53 43.59 10.26 15.72 41.35 18.04 34.27 110.00 9.91
A7
Appendix 3 Assay Data
Standard SY-2 SY-2 SY-2 SY-3 SY-3 SY-3 STD5 STD5 STD5
Element As (ppm)
Sb (ppm)
Sn (ppm)
As (ppm)
Sb (ppm)
Sn (ppm)
As (ppm)
Sb (ppm)
Sn (ppm)
Theoretical
values
17 1 6 19 1 7 1000 1000 1000
No of values 108 108 108 52 52 52 50 50 50
Max 22 7 12 30 2 12 1093 1064 1090
Min 13 -3 2 15 -3 6 948 945 983
Mean 18 1 7 20 0 8 1016 1015 1033
S.D. 2.764 1.838 2.549 4.299 1.488 1.935 39.958 35.626 30.122
%R.S.D. 16.26 183.76 42.48 22.63 148.84 27.64 3.70 3.56 3.01
Uncertainty 0.53 0.35 0.49 1.19 0.41 0.54 10.45 10.08 8.52
%Uncertainty 2.92 26.90 7.55 5.92 2146.59 6.69 1.03 0.99 0.82
Hg – AFS Cold vapour method
The sample is digested in an acid mixture and then placed in a water-bath at approximately 60oC for 1.5 to 2 hours. On cooling the sample is then oxidised with KMnO4 and then further oxidised with (NH
4)2S2O8. At this stage the process can be left
overnight.
Boric acid is then added to neutralise the HF in the solution. Finally, just prior to analysis, hydroxylammonium sulphate/sodium chloride solution is added to decolorise the solution and to dissolve any precipitated MnO2. A suitable aliquot of sample solution is diluted if
required and reduced with stannous chloride using a PSA vapour generator. The mercury vapour produced is then determined by Atomic Fluorescence Spectrophotometry using a PSA Merlin mercury fluorescence detector.
When analysing for mercury using this Work Instruction against the designated certified reference standards the following results were obtained:
Certified reference standard CCU-1b CPB-1
Certified value (ppm) 72 +6 5.5
Range found 71.17 - 79.92 5.63 - 6.75
Mean 76.7 5.9
Standard deviation 2.797 0.419
Number of measurements 12 12
Uncertainty at 95% confidence level +1.78 +0.27
% Uncertainty at 95% confidence level
+2.3% +4.5%
A8
Appendix 3 Assay Data
Major Element Assay Data for Borehole CR194
Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)149.80 150.80 0.06 0.95 33.62 5.01 GHS150.80 151.75 0.03 1.33 27.06 2.51 GHS HWL151.75 152.70 0.03 1.41 31.49 1.54 GHS HWL152.70 153.70 0.10 1.56 28.02 1.56 GHS HWL153.70 154.75 0.02 1.39 31.47 1.34 GHS HWL154.75 155.75 0.01 1.64 40.22 6.14 GBM HWL155.75 156.70 0.03 5.31 41.20 7.40 GBM AU1156.70 157.30 0.34 3.79 40.80 7.90 GBM HWP157.30 158.70 0.02 3.40 44.83 3.43 GBM/GHS HWP158.70 159.75 0.09 2.78 52.87 2.28 GHS HWP159.75 160.75 0.01 3.01 62.74 0.34 GHS AU160.75 161.75 0.01 3.95 61.13 0.70 GHS AU161.75 162.75 0.04 6.36 51.31 0.97 GHS AU162.75 163.75 0.03 7.92 53.49 1.11 GHS AU163.75 164.60 0.04 7.31 58.87 0.88 GHS AU164.60 165.80 7.42 5.75 37.74 45.89 MMP HCH165.80 166.80 16.91 4.27 35.67 44.51 MMP HCH166.80 167.75 13.65 1.04 36.64 47.74 MMP HCH167.75 168.70 12.25 1.63 43.92 47.02 MMP HCH170.70 171.60 19.40 0.21 37.93 39.51 MMP HCH171.60 172.50 17.04 0.45 31.24 45.55 MMP HCH172.50 173.50 18.35 2.00 28.01 38.79 MMP/QXM HCH173.50 174.50 15.58 0.44 28.66 39.92 QXM/MMP HCH174.50 175.45 20.09 0.17 39.20 46.79 MMP HCH175.45 176.35 19.03 0.36 36.82 42.61 MMP HCH176.35 177.20 15.27 0.14 40.18 43.52 MMP HCH177.20 178.50 20.03 0.10 31.91 45.33 MMP HCH178.50 180.00 12.85 0.16 7.37 12.47 SXM MCL
A9
Appendix 3 Assay Data
Minor/Trace Element Assay Data for Borehole CR194
Depth Ag Au As Bi Hg Sb Sn Lithocode LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)
149.80
150.80 5.6 0.32 774 68 0.9 486 59 GHS
150.80
151.75 4.2 0.26 329 84 0.2 495 50 GHS HWL
151.75
152.70 3.5 0.21 504 73 0.2 437 50 GHS HWL
152.70
153.70 3.2 0.31 626 49 0.9 293 41 GHS HWL
153.70
154.75 3.3 0.18 887 86 0.2 252 32 GHS HWL
154.75
155.75 7.9 0.14 1476 25 0.9 306 50 GBM HWL
155.75
156.70 6.4 2.23 1391 55 0.9 558 68 GBM AU1
156.70
157.30 9.4 0.12 1625 133 4.2 1125 95 GBM HWP
157.30
158.70 6.2 0.08 2066 252 0.7 1670 185 GBM/GHS HWP
158.70
159.75 4.3 0.07 2633 336 0.9 1710 189 GHS HWP
159.75
160.75 1.9 5.91 5126 400 0.2 2025 221 GHS AU
160.75
161.75 5.0 5.29 10224 592 2.8 1625 239 GHS AU
161.75
162.75 22.5 5.03 17897 800 21.3 2295 225 GHS AU
162.75
163.75 178.5 7.39 10305 1388 69.0 3375 450 GHS AU
163.75
164.60 1114.4 14.42 4550 1629 645.9 5135 626 GHS AU
164.60
165.80 546.4 5.43 4892 517 69.4 927 203 MMP HCH
165.80
166.80 236.9 0.67 3213 65 206.5 450 <3 MMP HCH
166.80
167.75 149.9 0.52 2844 60 72.2 302 <3 MMP HCH
167.75
168.70 150.2 0.82 3609 61 86.2 360 <3 MMP HCH
170.70
171.60 205.0 0.84 2867 116 162.1 437 <3 MMP HCH
171.60
172.50 273.3 2.23 3416 140 834.2 428 <3 MMP HCH
172.50
173.50 495.3 13.03 2826 795 1817.9 1364 99 MMP/QXM HCH
173.50
174.50 108.7 2.19 3488 88 597.8 338 <3 QXM/MMP HCH
174.50
175.45 91.4 1.25 4419 106 133.7 311 <3 MMP HCH
175.45
176.35 71.8 1.78 4095 256 153.8 500 <3 MMP HCH
176.35
177.20 45.9 1.80 3695 110 112.4 333 <3 MMP HCH
177.20
178.50 51.4 1.14 3933 83 136.0 369 <3 MMP HCH
178.50
180.00 27.4 0.72 2894 64 238.1 513 <3 SXM MCL
A10
Appendix 3 Assay Data
A11
Appendix 3 Assay Data
Major Element Assay Data for Borehole CR149
Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)170.20 170.90 0.03 0.16 10.95 0.75 TSA AU170.90 172.40 0.11 2.85 22.75 2.99 GHS AU172.40 174.10 0.11 2.65 20.03 2.27 GHS/GMS AU174.10 175.10 0.64 0.48 9.05 3.34 GEM AU175.10 175.90 0.01 0.31 6.09 1.11 GEM AU175.90 176.90 0.02 0.45 4.34 0.57 GLM/GEW AU176.90 178.05 0.05 1.04 8.10 1.41 GHM/GLS AU178.05 179.00 0.10 1.28 11.28 2.50 GEM AU179.00 180.35 0.85 0.91 19.24 4.58 GEM/GLS AU180.35 182.00 0.02 2.79 36.06 3.02 GLS/GHS AU182.00 182.85 0.03 1.25 44.15 1.60 GHS AU182.85 183.90 0.05 2.14 41.39 5.86 GHS AU183.90 185.40 0.04 2.37 20.01 8.87 GMS AU185.40 186.80 0.05 3.70 26.45 4.73 GLW AU186.80 187.40 0.02 1.64 36.76 2.81 GHS AU187.40 188.90 0.24 3.70 17.07 7.54 GMS AU188.90 190.00 0.07 5.02 17.41 12.86 GMS AU190.00 190.90 0.12 3.35 42.48 52.19 MMP HCH190.90 191.90 0.32 0.41 42.64 50.18 MMP HCH
Minor/Trace Element Assay Data for Borehole CR149
Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)170.20
170.90 3.6 1.57 113 55 0.3 261 77 TSA AU
170.90
172.40 15.7 21.79 711 586 0.6 2088 248 GHS AU
172.40
174.10 35.2 48.54 864 756 10.2 2925 396 GHS/GMS AU
174.10
175.10 7.0 47.67 239 238 14.5 2399 122 GEM AU
175.10
175.90 3.3 24.82 63 178 5.5 1922 68 GEM AU
175.90
176.90 6.9 47.99 86 191 8.9 1269 270 GLM/GEW AU
176.90
178.05 29.5 9.17 356 700 3.4 2993 981 GHM/GLS AU
178.05
179.00 20.0 7.11 855 537 3.7 1922 540 GEM AU
179.00
180.35 28.0 2.83 765 581 4.6 1755 734 GEM/GLS AU
180.35
182.00 71.7 20.51 3420 3497 7.4 7556 1071 GLS/GHS AU
182.00
182.85 35.1 2.23 536 921 1.8 2250 716 GHS AU
182.85
183.90 64.0 1.75 468 1052 2.5 2853 1409 GHS AU
183.90
185.40 34.9 8.51 1035 1052 4.0 2268 4262 GMS AU
185.40
186.80 24.3 9.93 2007 244 8.0 1112 7326 GLW AU
186.80
187.40 32.7 0.67 572 236 1.5 1413 428 GHS AU
187.4 188.90 69.2 2.66 4892 1066 14.5 2129 1206 GMS AU
A12
Appendix 3 Assay Data
0188.90
190.00 735.8 42.75 882 1414 83.5 1967 1440 GMS AU
190.00
190.90 83.0 1.04 981 331 5.9 585 243 MMP HCH
190.90
191.90 48.4 1.00 1580 329 1.5 405 261 MMP HCH
A13
Appendix 3 Assay Data
Assay Data for Borehole CR038
Depth Cu Pb Ag Au As Bi Hg Sb SnFrom To (%) (%) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)150.8
0151.4
5<0.01 0.58 6.3 3.37 36 23 0.7 252 216
151.45
152.40
<0.01 0.09 3.8 2.94 36 40 0.5 90 252
152.40
153.20
<0.01 0.09 7.5 4.19 54 29 0.2 54 144
153.20
154.20
0.02 0.09 13.5 11.08 54 19 1.3 108 666
154.20
155.20
<0.01 0.07 17.7 11.31 <5 22 1.7 36 756
155.20
156.30
0.01 0.06 22.0 1.71 36 22 90.1 108 396
156.30
157.25
0.04 0.29 1240 1.33 918 26 16.8 126 216
157.25
158.25
0.21 0.05 9.9 0.23 360 21 7.9 54 216
158.25
159.25
0.94 0.07 28.0 0.22 1098 38 8.5 180 198
159.25
160.25
0.86 0.08 7.8 0.16 936 32 2.4 72 216
160.25
161.25
1.18 0.05 12.5 0.27 1008 37 0.8 54 198
161.25
162.25
1.57 0.06 9.6 0.15 864 31 3.0 72 216
162.25
163.25
1.38 0.08 8.2 0.18 1170 34 0.6 198 378
163.25
164.25
1.64 0.23 8.5 0.22 1404 48 3.3 126 666
164.25
165.35
0.91 0.2 7.0 0.17 972 42 0.2 198 936
165.35
166.30
1.86 0.22 9.0 0.16 1242 72 1.3 180 468
166.30
167.30
0.89 0.19 7.1 0.16 1188 74 0.4 144 216
Major Element Assay Data for Borehole CR191
Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)133.25 134.25 0.01 0.73 24.61 0.45 TCP HWL134.25 135.25 <0.01 0.67 13.26 0.09 TCP/GHW HWL135.25 135.70 0.02 0.78 14.06 0.25 GHW/GEM HWL135.70 136.85 0.01 0.86 12.00 0.31 GEM HWL136.85 137.95 0.01 2.82 36.26 6.34 GMS HWP137.95 138.90 0.02 13.37 34.00 7.54 GMS AU1138.90 139.85 0.01 17.52 19.37 7.20 GMS AU1139.85 141.00 0.01 10.51 32.27 9.75 GMS AU1141.00 141.65 0.01 1.41 32.68 7.37 GES AU1141.65 142.65 <0.01 1.87 2.11 0.60 GES AU1142.65 143.60 0.02 1.07 2.62 0.37 GES AU1143.60 144.70 0.01 0.58 1.99 0.30 GES144.70 145.75 0.03 0.60 3.08 0.37 GES145.75 146.90 0.01 0.17 4.79 1.56 GES146.90 148.15 <0.01 0.38 4.63 0.43 GES
A14
Appendix 3 Assay Data
148.15 149.15 <0.01 0.32 4.57 0.70 GES149.15 150.10 0.01 1.39 5.97 2.69 GES AU150.10 150.90 0.01 0.56 2.51 1.91 GES AU150.90 151.75 <0.01 0.93 6.26 5.66 GES AU151.75 153.85 0.01 0.37 3.05 2.75 GES AU153.85 155.35 0.33 0.13 30.29 35.50 MMPXM HCL155.35 156.25 0.69 0.05 29.51 32.15 MMPXM HCL156.25 157.20 1.18 0.17 26.55 31.11 MMPXM HCL157.20 158.15 3.44 1.02 29.19 33.76 MMPXM HCL158.15 159.05 3.66 0.45 21.98 27.21 MMPXM HCL159.05 159.60 3.52 0.16 23.53 27.37 MMPXM HCL
A15
Appendix 3 Assay Data
Minor Element Assay Data for Borehole CR191
Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)133.25
134.25 6.4 0.07 554 119 0.2 365 293 TCP HWL
134.25
135.25 0.8 <0.01 81 18 0.7 171 41 TCP/GHW HWL
135.25
135.70 1.2 0.01 99 65 0.2 342 54 GHW/GEM HWL
135.70
136.85 2.1 0.01 221 145 0.7 666 149 GEM HWL
136.85
137.95 4.5 0.22 1733 785 8.4 2588 1098 GMS HWP
137.95
138.90 9.1 2.39 10305 1501 9.8 4838 1809 GMS AU1
138.90
139.85 58.6 2.89 16700 3227 7.9 5963 2993 GMS AU1
139.85
141.00 25.3 12.04 1836 1092 4.2 4050 9072 GMS AU1
141.00
141.65 15.0 0.82 666 386 3.0 1656 4874 GES AU1
141.65
142.65 9.9 4.96 1269 39 6.0 792 1706 GES AU1
142.65
143.60 12.8 2.94 446 38 3.3 441 1215 GES AU1
143.60
144.70 9.2 0.61 230 39 1.4 356 1017 GES
144.70
145.75 5.5 0.99 248 46 1.9 324 1494 GES
145.75
146.90 5.7 0.60 144 58 0.5 252 1364 GES
146.90
148.15 5.3 0.66 221 123 0.9 275 1251 GES
148.15
149.15 6.3 0.90 117 156 0.7 419 1476 GES
149.15
150.10 11.3 3.61 873 208 7.4 1107 3209 GES AU
150.10
150.90 10.6 0.85 396 120 7.7 819 882 GES AU
150.90
151.75 19.6 10.74 419 364 20.4 1301 6809 GES AU
151.75
153.85 11.8 5.01 495 102 22.5 446 3569 GES AU
153.85
155.35 91.5 2.73 2408 63 9.3 270 918 MMPXM HCL
155.35
156.25 10.9 0.91 3695 56 10.8 369 1130 MMPXM HCL
156.25
157.20 3.9 0.37 3965 44 9.2 239 603 MMPXM HCL
157.20
158.15 7.1 0.81 4320 126 4.0 432 117 MMPXM HCL
158.15
159.05 6.2 0.37 3236 43 4.0 284 225 MMPXM HCL
159.05
159.60 4.5 0.30 2363 40 2.0 293 513 MMPXM HCL
Major Element Assay Data for Borehole CR123
A16
Appendix 3 Assay Data
Depth Cu Pb Fe S Lithocode LensFrom To (%) (%) (%) (%)152.40 153.95 0.08 1.12 13.95 7.74 TCP AU153.95 154.85 0.02 16.49 10.79 2.76 GMS AU154.85 157.05 No Core Recovery157.05 158.65 0.04 9.20 11.24 5.29 GMS AU158.65 160.20 0.03 5.46 13.75 7.11 GMS AU160.20 161.40 0.01 27.23 28.69 8.46 GMS AU161.40 161.80 0.02 8.90 19.63 2.87 GMS AU161.80 163.40 0.12 25.63 32.64 6.66 GMS AU163.40 168.20 No Core Recovery168.20 169.00 0.09 6.40 9.60 4.12 GMS AU169.00 169.65 0.45 13.95 9.59 16.03 QXM AU169.65 172.85 1.89 6.94 16.18 21.75 QXM AU172.85 176.00 No Core Recovery176.00 178.35 1.33 0.53 5.11 5.95 SXM MCL178.35 180.00 2.38 0.22 19.75 26.93 QXM MCL180.00 181.50 2.56 0.61 21.51 26.98 SXM MCL181.50 183.00 1.1 0.70 6.46 8.20 SXM MCL183.00 184.40 0.58 0.72 3.67 4.11 QXM MCL184.40 185.70 0.76 0.12 1.04 1.46 EQU E185.70 187.60 0.57 0.43 1.17 5.23 SXM E
A17
Appendix 3 Assay Data
Minor Element Assay Data for Borehole CR123
Depth Ag Au As Bi Hg Sb Sn Code LensFrom To (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm)152.40 153.95 2.5 2.77 558 148 0.8 270 86 TCP AU153.95 154.85 47.9 4.47 2075 338 66.3 1319 388 GMS AU154.85 157.05 No Core Recovery157.05 158.65 13.6 2.27 5839 348 20.0 1809 471 GMS AU158.65 160.20 35.6 2.08 5136 403 8.8 1413 382 GMS AU160.20 161.40 16.4 1.81 1454 374 5.4 2138 270 GMS AU161.40 161.80 20.4 1.47 2488 275 9.1 1481 356 GMS AU161.80 163.40 18.1 2.14 2728 362 5.7 1701 331 GMS AU163.40 168.20 No Core Recovery168.20 169.00 69.7 31.85 2002 1578 1160 4536 1100 GMS AU169.00 169.65 181.0 56.55 174 1920 9525 4536 1437 QXM AU169.65 172.85 175.3 11.68 210 758 3061 801 623 QXM AU172.85 176.00 No Core Recovery176.00 178.35 115.1 1.49 624 83 134.2 171 38 SXM MCL178.35 180.00 1.4 <0.01 63 24 2.3 207 77 QXM MCL180.00 181.50 82.1 0.75 99 201 112.8 509 135 SXM MCL181.50 183.00 1.8 0.12 122 236 0.8 203 63 SXM MCL183.00 184.40 1.5 0.05 846 129 1.3 216 41 QXM MCL184.40 185.70 1.6 <0.01 693 357 2.4 95 45 EQU E185.70 187.60 0.8 <0.01 482 147 0.5 23 5 SXM E
A18
Appendix 4 XRD Data
Appendix 4: XRD Data
A19
Appendix 4 XRD Data
A20
Appendix 4 XRD Data
A21
Appendix 4 XRD Data
A22
Appendix 4 XRD Data
A23
Appendix 4 XRD Data
A24
Appendix 4 XRD Data
BoreholeFrom (m)
To(m) Description ID Major ID Minor ID Trace
CR194 149.80 150.80 Limonite Yellow Clast Qz, Goeth Gal, Sid Angle, AnatCR194 149.80 150.80 Matrix Qz, Sid, Goeth Gal AnatCR194 149.80 150.80 Sid Clast Qz, Sid Hem, Gal Goeth, possible PyrrhotiteCR194 150.80 151.75 Sid-rich Area Magnetic Qz, Sid Hem, GalCR194 150.80 151.75 Sid-rich Area Qz, Sid Hem, Gal Barite, GoethCR194 151.75 152.70 Green-Grey Matrix Qz, Sid Gal Angle, GoethCR194 154.75 155.75 Whole Rock Strong Magnetic Sid Qz Gal, GoethCR194 154.75 155.75 Whole Rock Weak Magnetic Sid, Qz Gal Hem, GoethCR194 155.75 156.70 Whole Rock Strong Magnetic Gal, Sid Qz AngleCR194 155.75 156.70 Whole Rock Weak Magnetic Gal, Sid, Qz Hem CR194 157.30 158.70 Whole Rock 'Red' Sid Sid Hem, GalCR194 158.70 159.75 Whole Rock Sid, Gal Hem GoethCR194 159.75 160.75 Whole Rock Hem Gal, Sid, AngleCR194 161.75 162.75 Whole Rock Sid, Hem Gal, AngleCR194 163.75 164.60 Black Clay Band Sid, Gal Hem, Qz, AngleCR194 163.75 164.60 Upper Whole Rock Hem, Gal Sid, AngleCR194 163.75 164.60 Middle Whole Rock Yellow Goeth, Sid, Hem Gal, AngleCR194 164.60 165.80 Upper Contact Black Layer Gal Sid, QzCR194 164.60 165.80 Upper Contact Black Layer Lwr Gal Py, Angle, SidCR194 164.60 165.80 Upper Contact Yellow Layer Goeth, Gal Angle, SidCR194 164.60 165.80 Whole Rock Py Tennantite, Chalcopyrite, Melanterite,CR194 165.80 166.80 Whole Rock Py DjurleiteCR194 168.70 169.70 Whole Rock Py DjurleiteCR194 172.50 173.50 Lower Whole Rock Py, Gal, Qz, DjurleiteCR194 173.50 174.50 Whole Rock Py, Gal, Qz Djurleite
Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,
S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.
A25
Appendix 4 XRD Data
BoreholeFrom (m)
To(m) Description ID Major ID Minor ID Trace
CR149 170.20 170.90 Whole Rock Qz, Albite, Glauc Sid Anat, Gal, ChloriteCR149 170.90 172.40 Lower Green/Grey Matrix Qz, Gal, Angle SidCR149 170.90 172.40 Upper Whole Rock Qz, Hem, Sid GalCR149 172.40 174.10 Middle Whole Rock Qz Sid, Gal, poorly crystalline clayCR149 175.10 175.90 Lower Whole Rock Chalky White Qz Rut, Gal, SidCR149 175.10 175.90 Upper Whole Rock Chalky White Qz Rut, SidCR149 176.90 178.00 Lower Whole Rock Qz, Goeth Sid Gal CR149 179.00 180.35 Lower Whole Rock Qz, Goeth Sid, GalCR149 180.35 182.00 Upper Whole Rock Sid, Qz Gal, Hem, Goeth RutCR149 182.00 182.85 Lower Whole Rock Sid Qz, Hem, GalCR149 182.00 182.85 Upper Whole Rock Sid Qz, Hem GalCR149 183.90 185.40 Lower Whole Rock Qz Py, S, Lepid, GreigCR149 183.90 185.40 Upper Magnetic Qz Py, S, Lepid, Greig, Goeth SidCR149 185.40 186.80 Upper Whole Rock Qz, Sid Goeth Rut, CassiteriteCR149 187.40 188.90 Lower Whole Rock Qz Angle, Gal Lepid, S, Anat, RutCR149 187.40 188.90 Middle Fe-Sulphide Qz Lepid, S, Py, Marc, Gal, Goeth, Greig CalcCR149 188.90 190.00 Lower Whole Rock Py, Qz, Gal AngleCR149 188.90 190.00 Fe-Sulphide HCl Leached Py, Qz Greig, SCR149 188.90 190.00 Fe-Sulphide plus AgFe-Sulphide Py, Qz, Calc GreigCR149 188.90 190.00 Upper Whole Rock Calc, Qz Py possible GreigCR149 190.00 190.90 Upper Whole Rock Py, Qz Gal AngleCR149 190.90 191.90 Upper Whole Rock Py, Qz, Gal Angle
Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,
S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.
A26
Appendix 4 XRD Data
BoreholeFrom (m)
To(m) Description ID Major ID Minor ID Trace
CR038 150.80 151.45 Upper Whole Rock Qz Py, Sid Gal, Goeth, AnatCR038 151.45 152.40 Upper Whole Rock Qz Sid AnatCR038 152.40 153.20 Upper Whole Rock Qz Sid AnatCR038 153.20 154.20 Cyclosizer Heavy Mineral Conc. Qz, Anat, Sid Py CassiteriteCR038 153.20 154.20 Upper Whole Rock Qz Sid AnatCR038 154.20 155.20 Upper Whole Rock Qz Sid, AnatCR038 155.20 156.30 Upper Whole Rock Qz Sid, AnatCR038 156.30 157.25 Upper Whole Rock Qz Py AnatCR038 157.25 158.25 Upper Whole Rock Qz Py Anat
Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,
S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.
A27
Appendix 4 XRD Data
BoreholeFrom (m)
To(m) Description ID Major ID Minor ID Trace
CR191 135.20 135.70 Whole Rock Qz, Goeth RutCR191 136.85 137.95 Whole Rock Sid Gal, Goeth, AngleCR191 137.95 138.90 Lower Whole Rock Gal, Sid QzCR191 137.95 138.90 Magnetic Sid S, Gal, Goeth, Lepid Greig CR191 137.95 138.90 Upper Whole Rock Sid, Qz S, Angle, Goeth, LepidCR191 137.95 138.90 Whole Rock Sid, Qz S, Angle, LepidCR191 138.90 139.85 Whole Rock Gal, Angle, Sid QzCR191 139.85 141.00 Lower Whole Rock Sid, Qz Gal, Lepid Angle, S, RutCR191 139.85 141.00 Upper Whole Rock Gal, Sid Qz Angle, RutCR191 141.00 141.65 Whole Rock Qz, Sid Gal Marc, Rut, Calc, possible CassiteriteCR191 141.65 142.65 Whole Rock Qz Rut, Anat, Gal, SidCR191 143.60 144.70 Whole Rock Qz Rut, Anat, Gal, SidCR191 145.75 146.90 Whole Rock Qz Sid, S, Marc, Lepid Greig, CalcCR191 148.15 149.15 Whole Rock Qz, Sid Gal, MarcCR191 150.10 150.90 Whole Rock Qz Gal Rut, Marc, Greig, S, LepidCR191 150.90 151.75 Lower Whole Rock Qz Cassiterite, Gal Rut, Anat CR191 150.90 151.75 Upper Whole Rock Qz Py, S, Anat, Lepid JarositeCR191 151.75 153.85 Lower Whole Rock Qz Cassiterite, Py, AnatCR191 151.75 153.85 Upper Whole Rock Qz Szomolnokite (oxidation product of Py), Py Anat, Rut, CassiteriteCR191 153.85 155.35 Whole Rock Qz, Py CR191 155.35 156.25 Whole Rock Qz, Py Tennantite Chalcopyrite
Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,
S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.
A28
Appendix 4 XRD Data
BoreholeFrom (m)
To(m) Description ID Major ID Minor ID Trace
CR123 152.40 153.95 Lower Whole Rock Sid Angle, Gal Greig, SCR123 152.40 153.95 Upper Whole Rock Qz, Calc, Py,Glauc, Albite RutCR123 153.95 154.85 Lower Whole Rock Gal, Py, Calc Angle, CerussCR123 153.95 154.85 Upper Whole Rock Sid, HemCR123 157.05 158.65 Upper Whole Rock Gal, Py, Calc QzCR123 158.65 160.20 Lower Low Mag Frantz Calc, Gal Py HarmotomeCR123 158.65 160.20 Lower Magnetic Py, Gal, Calc Angle possible MarcCR123 158.65 160.20 Upper Whole Rock Calc, Gal, Angle Py, Gyp CerussCR123 160.20 161.40 Upper Whole Rock Gal, Py, Calc MarcCR123 161.80 163.40 Upper Metallic Clasts Gal, Sid AngleCR123 161.80 163.40 Lower Magnetic Sid, Gal Angle Greig, SCR123 161.80 163.40 Upper Whole Rock Sid, GalCR123 168.20 169.00 Lower Gangue Calc, Angle, Gal Gyp, PyCR123 168.20 169.00 Lower Magnetic Calc, Angle, Gal Py Sid, GypCR123 168.20 169.00 Lower Massive Pb-Sulphide Ceruss, Gal Calc QzCR123 168.20 169.00 Upper Massive Fe-Sulphide Gal, Py, Calc Greig, Marc, Gyp Barite, possible pyrrhotiteCR123 168.20 169.00 Upper Massive Pb-Sulphide Ceruss, Gal, Calc QzCR123 169.00 169.65 Upper Green/Grey Matrix Calc, Angle, Gal Gyp CR123 169.00 169.65 Upper Massive Sulphide Gal, Py AngleCR123 180.00 181.50 Whole Rock Qz, Py, Gal Covellite
Abbreviations: Qz = Quartz, Goeth = Goethite, Sid = Siderite, Gal = Galena, Angle = Anglesite, Anat = Anatase, Hem = Hematite, Py = Pyrite, Glauc = Glauconite, Rut = Rutile,
S = Sulphur (native), Lepid = Lepidocrocite, Greig = Greigite, Marc = Marcasite, Calc = Calcite, Ceruss = Cerussite, Gyp = Gypsum.
A29
Appendix 4 XRD Data
5 10 15 20 25 30 352Theta (°)
0
1000
2000
3000
4000
5000
6000Inte
nsity
(co
unts
)
air dry 001; 15.0
glycol-solvated 001; 16.9
heated 001; 9.99
CR194 - Black Fe-rich Clay: Oriented Si crystal mount, whole powder scans (black -air dry, red - ethylene glycol-solvated, green - heated 550degC/2 hours) which clearly indicate a smectite-group mineral due to c.15ang 001 peak (air dry) swelling to 16.9ang on glycolation and collapsing to c.10ang on heating.
A30
Appendix 4 XRD Data
71 72 73 74 75 76 77 78 792Theta (°)
0
1000
2000
3000
4000
5000
Inte
nsity
(co
unts
)
?nontronite 060; 1.52
CR194 - Black Fe-rich Clay: Random powder Si crystal mount, scan over the diagnostic 060 diffraction band possibly suggests a spacing of 1.52ang - indicative of nontronite.
A31
Appendix 5 SEM Analyses
Appendix 5: SEM Analyses: Borehole CR194: Siderite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14Compound Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)
FeO 52.9 52.8 55.2 56.8 53.2 52.6 53.6 55.3 59.4 54.1 54.0 53.3 59.1 58.1MgO 1.6 1.6 1.2 1.0 1.7 1.8 1.9 1.2 0.5 1.3 1.1 1.1 0.3 0.2CaO 3.5 3.5 2.4 1.4 3.8 4.4 3.4 2.8 0.4 4.8 4.3 5.6 0.2 0.1CO2 42.0 42.0 41.1 40.9 41.3 41.2 41.1 40.7 39.7 39.8 40.6 40.1 40.4 41.7
TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %
C 21.0 21.0 20.8 20.8 20.8 20.7 20.7 20.7 20.5 20.3 20.6 20.4 20.8 21.2Mg 0.9 0.9 0.7 0.6 1.0 1.0 1.0 0.7 0.3 0.7 0.6 0.6 0.2 0.1Ca 1.4 1.4 1.0 0.6 1.5 1.7 1.4 1.1 0.1 1.9 1.7 2.2 0.1 0.0Fe 16.2 16.2 17.1 17.7 16.4 16.2 16.5 17.2 18.8 16.9 16.8 16.6 18.6 18.1O 60.5 60.5 60.4 60.4 60.4 60.4 60.4 60.3 60.3 60.1 60.3 60.2 60.4 60.6
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
C 1.1 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1Mg 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0Ca 0.1 0.1 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.0 0.0Fe 0.8 0.8 0.9 0.9 0.8 0.8 0.8 0.9 0.9 0.8 0.8 0.8 0.9 0.9O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 0.9 0.9 0.9 0.9 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.9 0.9
C+O 4.1 4.1 4.1 4.1 4.1 4.0 4.0 4.0 4.0 4.0 4.0 4.0 4.1 4.1TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
A32
Appendix 5 SEM Analyses
Borehole CR194: Siderite - Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 149.80 150.80 Gossan FeS-lined cavity filling,
#2 149.80 150.80 Gossan Galena-lined cavity filling
#3 149.80 150.80 Gossan Galena-lined cavity filling
#4 149.80 150.80 Gossan Veinlet
#5 149.80 150.80 Gossan ‘Fragment’
#6 149.80 150.80 Gossan Euhedral cavity filling
#7 149.80 150.80 Gossan Cavity filling
#8 149.80 150.80 Gossan ‘Fragment’
#9 155.75 156.70 Gossan ‘Fragment’, zoned
#10 155.75 156.70 Gossan Cavity filling
#11 155.75 156.70 Gossan ‘Fragment’
#12 155.75 156.70 Gossan ‘Fragment’
#13 163.75 164.60Gossan contact with massive sulphide Late veinlet with anglesite
#14 164.60 165.80 Massive sulphide contact with gossan Cavity filling
A33
Appendix 5 SEM Analyses
Borehole CR149: Siderite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12
CompoundWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
FeO 43.0 51.1 45.5 50.1 44.7 49.7 50.3 51.3 49.0 51.2 51.0 49.9MgO 6.0 2.7 4.9 3.4 6.2 3.1 3.3 3.3 4.2 3.4 3.8 3.5CaO 7.6 3.3 5.4 3.6 5.3 3.6 3.8 3.5 4.3 4.2 4.8 4.9CO2 43.4 42.9 44.2 42.9 43.9 43.5 42.6 41.9 42.5 41.2 40.5 41.7
TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %
C 20.9 21.2 21.3 21.1 21.1 21.3 21.0 20.8 20.9 20.6 20.3 20.7Mg 3.1 1.4 2.6 1.8 3.3 1.7 1.8 1.8 2.3 1.9 2.1 1.9Ca 2.9 1.3 2.0 1.4 2.0 1.4 1.5 1.4 1.7 1.6 1.9 1.9Fe 12.7 15.5 13.4 15.1 13.1 14.9 15.2 15.6 14.7 15.6 15.6 15.2O 60.4 60.6 60.7 60.6 60.5 60.7 60.5 60.4 60.4 60.3 60.1 60.4
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
C 1.0 1.1 1.1 1.1 1.1 1.1 1.1 1.0 1.0 1.0 1.0 1.0Mg 0.2 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1Ca 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Fe 0.6 0.8 0.7 0.8 0.7 0.7 0.8 0.8 0.7 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 1.0 1.0 1.0
C+O 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.1 4.0 4.0 4.0TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
A34
Appendix 5 SEM Analyses
Borehole CR149: Siderite – Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 170.20 170.90Tertiary conglomerate Zoned crystal - core
#2 170.20 170.90Tertiary conglomerate Zoned crystal - rim
#3 170.20 170.90Tertiary conglomerate Zoned crystal - core
#4 170.20 170.90Tertiary conglomerate Zoned crystal - rim
#5 170.20 170.90Tertiary conglomerate Zoned crystal - core
#6 170.20 170.90Tertiary conglomerate Zoned crystal - rim
#7 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
#8 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
#9 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
#10 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
#11 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
#12 175.10 175.90 GossanAssociated with Fe-S and PbSb-sulphide, replacing quartz matrix
A35
Appendix 5 SEM Analyses
Borehole CR038: Siderite
Analysis #1 #2 #3 #4 #5 #6 #7
CompoundWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
FeO 44.7 42.8 43.0 46.5 44.0 44.8 46.4MgO 5.6 6.2 5.5 5.6 5.7 5.9 5.8CaO 7.9 8.7 9.5 5.8 9.2 7.5 6.4CO2 41.8 42.3 42.1 42.0 41.0 41.8 41.4
TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0At %
C 20.4 20.5 20.5 20.5 20.1 20.4 20.3Mg 3.0 3.3 2.9 3.0 3.1 3.1 3.1Ca 3.0 3.3 3.6 2.2 3.5 2.9 2.5Fe 13.4 12.7 12.8 13.9 13.2 13.4 14.0O 60.2 60.2 60.2 60.3 60.1 60.2 60.2
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
C 1.0 1.0 1.0 1.0 1.0 1.0 1.0Mg 0.2 0.2 0.1 0.2 0.2 0.2 0.2Ca 0.2 0.2 0.2 0.1 0.2 0.1 0.1Fe 0.7 0.6 0.6 0.7 0.7 0.7 0.7O 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 1.0 1.0 1.0 1.0 1.0 1.0 1.0
C+O 4.0 4.0 4.0 4.0 4.0 4.0 4.0TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0
A36
Appendix 5 SEM Analyses
Borehole CR038: Siderite – Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 150.80 151.45Quartz replaced tuff Euhedral cavity filling
#2 150.80 151.45Quartz replaced tuff Euhedral crystal with Pb(SbAs)-sulphide needles
#3 150.80 151.45Quartz replaced tuff
Later anhedral overgrowth on #2 without Pb(SbAs)-sulphide needles
#4 150.80 151.45Quartz replaced tuff Euhedral crystal, zoned
#5 150.80 151.45Quartz replaced tuff Euhedral crystal with Pb(SbAs)-sulphide needles
#6 150.80 151.45Quartz replaced tuff Later anhedral overgrowth without Pb(SbAs)-sulphide needles
#7 150.80 151.45Quartz replaced tuff Later anhedral overgrowth without Pb(SbAs)-sulphide needles
A37
Appendix 5 SEM Analyses
Borehole CR191: Siderite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14
CompoundWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
FeO 50.8 53.6 53.1 51.7 48.8 48.6 47.7 53.2 46.7 53.2 50.9 52.3 51.5 51.9MgO 1.2 2.0 2.2 2.1 5.5 4.6 4.8 4.8 4.8 4.1 3.9 3.8 3.6 3.3CaO 5.8 3.4 4.1 4.1 4.2 4.6 5.5 0.7 6.6 0.7 2.5 2.2 2.1 2.5CO2 42.3 41.0 40.7 42.1 41.6 42.3 42.0 41.3 42.0 42.0 42.7 41.7 42.8 42.3
TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0At %
C 21.1 20.7 20.5 21.0 20.5 20.8 20.6 20.6 20.6 20.9 21.1 20.8 21.1 21.0Mg 0.6 1.1 1.2 1.2 2.9 2.5 2.6 2.6 2.6 2.2 2.1 2.0 2.0 1.8Ca 2.2 1.3 1.6 1.6 1.6 1.8 2.1 0.3 2.5 0.3 1.0 0.9 0.8 1.0Fe 15.5 16.5 16.4 15.8 14.7 14.6 14.4 16.3 14.0 16.2 15.4 16.0 15.6 15.8O 60.5 60.3 60.3 60.5 60.2 60.4 60.3 60.3 60.3 60.4 60.5 60.4 60.6 60.5
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
C 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.1 1.0 1.1 1.0Mg 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Ca 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0Fe 0.8 0.8 0.8 0.8 0.7 0.7 0.7 0.8 0.7 0.8 0.8 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms Mg+Ca+Fe 0.9 1.0 1.0 0.9 1.0 0.9 1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.9
C+O 4.1 4.0 4.0 4.1 4.0 4.1 4.0 4.0 4.0 4.1 4.1 4.1 4.1 4.1TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
A38
Appendix 5 SEM Analyses
Borehole CR191: Siderite – Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 134.25 135.25Conglomerate contact with gossan Massive siderite
#2 134.25 135.25Conglomerate contact with gossan Massive siderite
#3 134.25 135.25Conglomerate contact with gossan Massive siderite
#4 134.25 135.25Conglomerate contact with gossan Massive siderite
#5 139.85 141.00 Upper gossan Euhedral crystals
#6 139.85 141.00 Upper gossan Euhedral crystals
#7 141.00 141.65 Middle gossanCompositional zoning
#8 141.00 141.65 Middle gossanCompositional zoning
#9 141.00 141.65 Middle gossanCompositional zoning
#10 141.00 141.65 Middle gossanCompositional zoning
#11 150.10 150.90 Lower gossan Cavity infilling
#12 150.10 150.90 Lower gossan Cavity infilling
#13 150.10 150.90 Lower gossan Cavity infilling
#14 150.10 150.90 Lower gossan Cavity infilling
A39
Appendix 5 SEM Analyses
Borehole CR123: Siderite
Analysis #1 #2 #3 #4 #5 #6 #7 #8
CompoundWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
FeO 45.8 46.0 48.1 47.8 56.7 55.0 54.6 55.2MgO 4.8 3.9 3.0 3.5 1.5 1.7 1.7 1.2CaO 6.7 8.0 7.7 7.7 1.1 1.1 1.2 0.9MnO 0.8 0.6 0.1 0.3 bdl bdl bdl bdlCO2 41.9 41.5 41.1 40.8 40.7 42.2 42.5 42.7
TOTAL 100.0 100.0 100.0 100.1 100.0 100.0 100.0 100.0At %
C 20.6 20.5 20.5 20.3 20.7 21.2 21.3 21.4Mg 2.6 2.1 1.6 1.9 0.8 0.9 0.9 0.7Ca 2.6 3.1 3.0 3.0 0.4 0.4 0.5 0.3Mn 0.3 0.2 0.0 0.1 bdl bdl bdl bdlFe 13.8 13.9 14.7 14.6 17.7 16.9 16.7 16.9O 60.2 60.2 60.2 60.1 60.4 60.6 60.6 60.7
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
C 1.0 1.0 1.0 1.0 1.0 1.1 1.1 1.1Mg 0.1 0.1 0.1 0.1 0.0 0.0 0.0 0.0Ca 0.1 0.2 0.2 0.2 0.0 0.0 0.0 0.0Mn 0.0 0.0 0.0 0.0 bdl bdl bdl bdlFe 0.7 0.7 0.7 0.7 0.9 0.8 0.8 0.8O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0No. Atoms
Mg+Ca+Fe+Mn 1.0 1.0 1.0 1.0 1.0 0.9 0.9 0.9C+O 4.0 4.0 4.0 4.0 4.0 4.1 4.1 4.1
TOTAL 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0
bdl = below detection limits (~0.5%)
Borehole CR123: Siderite – Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 153.95 154.85Upper siderite gossan
Massive siderite, Mn-bearing
#2 153.95 154.85Upper siderite gossan
Massive siderite, Mn-bearing
#3 153.95 154.85Upper siderite gossan Massive siderite
#4 153.95 154.85Upper siderite gossan Massive siderite
#5 161.80 163.40Lower siderite gossan Massive siderite, trace Pb
#6 161.80 163.40Lower siderite gossan Massive siderite, trace Pb
#7 161.80 163.40Lower siderite gossan Massive siderite, trace Pb
#8 161.80 163.40Lower siderite gossan Massive siderite, trace Pb
A40
Appendix 5 SEM Analyses
Boreholes CR194 and CR191: Enargite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11
ElementWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Cu 46.1 46.1 46.5 49.4 47.5 50.0 48.1 40.6 40.7 40.4 42.2Fe 0.9 1.3 0.9 0.0 0.0 0.0 0.0 7.3 7.8 7.5 7.0Sb 0.4 0.4 0.2 0.0 0.0 0.0 0.0 3.7 1.9 2.2 0.3As 18.5 18.5 19.9 19.6 20.9 17.7 19.9 15.9 18.5 18.2 18.4S 33.7 32.8 33.0 31.5 32.3 31.6 32.2 32.3 31.8 32.4 32.4
TOTAL 99.6 99.0 100.6 100.5 100.7 99.3 100.2 99.8 100.7 100.7 100.2At %Cu 35.5 35.9 35.8 38.5 36.8 40.3 37.4 31.6 31.5 31.1 32.4Fe 0.8 1.1 0.8 0.0 0.0 0.0 0.0 6.5 6.8 6.6 6.1Sb 0.2 0.2 0.1 0.0 0.0 0.0 0.0 1.5 0.8 0.9 0.1As 12.1 12.2 13.0 13.0 13.7 11.1 13.1 10.5 12.2 11.9 12.0S 51.4 50.6 50.4 48.6 49.5 48.6 49.5 49.9 48.8 49.5 49.3
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
Cu 2.8 2.9 2.9 3.1 2.9 3.2 3.0 2.5 2.5 2.5 2.6Fe 0.1 0.1 0.1 0.0 0.0 0.0 0.0 0.5 0.5 0.5 0.5Sb 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.0As 1.0 1.0 1.0 1.0 1.1 0.9 1.0 0.8 1.0 1.0 1.0S 4.1 4.0 4.0 3.9 4.0 3.9 4.0 4.0 3.9 4.0 3.9
TOTAL 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0No. Atoms
Cu+Fe 2.9 3.0 3.0 3.1 2.9 3.2 3.0 3.0 3.1 3.0 3.1As+Sb 1.0 1.0 1.0 1.0 1.1 0.9 1.0 1.0 1.0 1.0 1.0
S 4.1 4.0 4.0 3.9 4.0 3.9 4.0 4.0 3.9 4.0 3.9TOTAL 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0
A41
Appendix 5 SEM Analyses
Boreholes CR194 and CR191: Enargite – Analysis Locations
Analysis BoreholeFrom(m)
To(m) Location
#1 CR194 164.60 165.80Massive sulphide contact with gossan
#2 CR194 164.60 165.80Massive sulphide contact with gossan
#3 CR194 164.60 165.80Massive sulphide contact with gossan
#4 CR194 173.50 174.50 Massive sulphide/shale
#5 CR194 173.50 174.50 Massive sulphide/shale
#6 CR194 173.50 174.50 Massive sulphide/shale
#7 CR194 173.50 174.50 Massive sulphide/shale
#8 CR191 155.35 156.25 Partial massive sulphide
#9 CR191 155.35 156.25 Partial massive sulphide
#10 CR191 155.35 156.25 Partial massive sulphide
#11 CR191 155.35 156.25 Partial massive sulphide
A42
Appendix 5 SEM Analyses
Borehole CR194: Hg-tetrahedrite/tennantite
Analysis #1 #2 #3 #4 #5 #6
ElementWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Cu 41.2 36.7 41.4 35.4 34.5 32.6Fe 0.8 0.8 1.0 1.1 1.5 0.4Hg 7.7 7.9 5.5 11.8 12.8 21.7Zn 2.0 4.0 3.6 2.7 1.8 0.1Sb 9.2 21.5 10.6 22.2 22.1 23.4As 12.8 4.1 11.6 3.2 3.4 0.5S 25.8 24.6 25.5 23.8 23.8 22.3
TOTAL 99.4 99.6 99.2 100.2 99.7 101.0At %Cu 36.4 34.2 36.4 33.9 33.3 33.7Fe 0.8 0.8 1.0 1.2 1.6 0.4Hg 2.2 2.3 1.5 3.6 3.9 7.1Zn 1.7 3.6 3.0 2.5 1.7 0.1Sb 4.2 10.5 4.9 11.1 11.1 12.6As 9.6 3.2 8.7 2.6 2.8 0.4S 45.1 45.4 44.5 45.1 45.6 45.6
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0No Atm
Cu 10.6 9.9 10.6 9.8 9.7 9.8Fe 0.2 0.2 0.3 0.3 0.5 0.1Hg 0.6 0.7 0.4 1.0 1.1 2.1Zn 0.5 1.0 0.9 0.7 0.5 0.0Sb 1.2 3.0 1.4 3.2 3.2 3.7As 2.8 0.9 2.5 0.8 0.8 0.1S 13.1 13.2 12.9 13.1 13.2 13.2
TOTAL 29.0 29.0 29.0 29.0 29.0 29.0No. Atoms
Cu+Fe+Zn+Hg 11.9 11.9 12.2 12.0 11.8 12.0As+Sb 4.0 4.0 3.9 4.0 4.0 3.8
S 13.1 13.2 12.9 13.1 13.2 13.2TOTAL 29.0 29.0 29.0 29.0 29.0 29.0
Borehole CR194: Hg-tetrahedrite/tennantite – Analysis Locations
AnalysisFrom(m)
To(m) Location
#1 170.70 171.60 Massive sulphide
#2 170.70 171.60 Massive sulphide
#3 170.70 171.60 Massive sulphide
#4 170.70 171.60 Massive sulphide
#5 170.70 171.60 Massive sulphide
#6 173.50 174.50Massive sulphide/shale
A43
Appendix 5 SEM Analyses
Borehole CR194: Tetrahedrite/tennantite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11
ElementWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Cu 40.6 40.5 40.4 40.2 40.4 44.4 45.8 36.8 36.3 35.7 36.0Fe 7.9 7.5 7.6 7.6 7.6 6.2 5.8 5.6 4.9 5.8 4.8Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.5 2.1 2.5 1.7Sb 1.0 0.6 0.9 1.4 1.1 0.1 0.0 29.2 29.0 30.5 30.0As 22.2 21.8 21.2 21.9 21.3 21.6 20.6 1.6 1.1 0.4 0.9S 28.4 29.8 28.4 28.5 28.7 28.3 27.8 25.3 25.6 25.2 26.0
TOTAL 100.0 100.0 98.4 99.5 99.0 100.6 100.0 99.9 99.0 100.0 99.4At %Cu 32.4 31.9 32.6 32.3 32.4 35.3 36.6 33.1 32.8 32.2 32.4Fe 7.2 6.7 7.0 6.9 6.9 5.6 5.3 5.7 5.1 5.9 5.0Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.3 1.9 2.2 1.5Sb 0.4 0.2 0.4 0.6 0.5 0.1 0.0 13.7 13.7 14.4 14.1As 15.0 14.6 14.5 14.9 14.5 14.6 14.0 1.2 0.9 0.3 0.6S 44.9 46.5 45.5 45.3 45.7 44.5 44.1 45.0 45.7 45.0 46.4
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0
A44
Appendix 5 SEM Analyses
Borehole CR194: Tetrahedrite/tennantite (continued)
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11No Atm
Cu 9.4 9.3 9.5 9.4 9.4 10.2 10.6 9.6 9.5 9.3 9.4Fe 2.1 1.9 2.0 2.0 2.0 1.6 1.5 1.7 1.5 1.7 1.4Zn 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.4 0.5 0.6 0.4Sb 0.1 0.1 0.1 0.2 0.1 0.0 0.0 4.0 4.0 4.2 4.1As 4.4 4.2 4.2 4.3 4.2 4.2 4.1 0.3 0.2 0.1 0.2S 13.0 13.5 13.2 13.1 13.3 12.9 12.8 13.1 13.3 13.1 13.5
TOTAL 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0No. Atoms Cu+Fe+Zn 11.5 11.2 11.5 11.4 11.4 11.9 12.2 11.6 11.5 11.7 11.3
As+Sb 4.5 4.3 4.3 4.5 4.3 4.2 4.1 4.3 4.2 4.3 4.3S 13.0 13.5 13.2 13.1 13.3 12.9 12.8 13.1 13.3 13.1 13.5
TOTAL 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0 29.0
A45
Appendix 5 SEM Analyses
Borehole CR194: Tetrahedrite/tennantite – Analysis Locations
AnalysisFrom(m)
To(m) Location
#1 164.60 165.80Massive sulphide contact with gossan
#2 164.60 165.80Massive sulphide contact with gossan
#3 164.60 165.80Massive sulphide contact with gossan
#4 164.60 165.80Massive sulphide contact with gossan
#5 164.60 165.80Massive sulphide contact with gossan
#6 164.60 165.80Massive sulphide contact with gossan
#7 164.60 165.80Massive sulphide contact with gossan
#8 164.60 165.80Massive sulphide contact with gossan
#9 164.60 165.80Massive sulphide contact with gossan
#10 164.60 165.80Massive sulphide contact with gossan
#11 164.60 165.80Massive sulphide contact with gossan
A46
Appendix 5 SEM Analyses
Boreholes CR038 and CR123: Proustite/pyrargyrite
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10
ElementWt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Wt (%)
Ag 67.4 66.2 65.9 67.5 66.9 65.4 62.3 60.1 66.8 61.1Sb 0.0 0.0 0.3 0.0 0.0 0.3 19.1 18.3 0.4 16.1As 14.8 15.8 15.6 14.6 15.1 16.7 1.9 5.1 14.0 5.1S 18.8 18.2 18.2 18.4 18.6 18.2 17.5 16.4 18.1 16.7
TOTAL 100.9 100.2 100.0 100.5 100.6 100.5 100.8 100.0 99.2 99.0At %Ag 44.4 44.1 44.0 44.9 44.2 43.3 44.3 43.2 45.1 44.0Sb 0.0 0.0 0.2 0.0 0.0 0.2 12.0 11.7 0.2 10.3As 14.0 15.1 15.0 14.0 14.4 16.0 2.0 5.3 13.6 5.3S 41.6 40.8 40.8 41.1 41.4 40.5 41.8 39.8 41.1 40.5
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
Ag 3.1 3.1 3.1 3.1 3.1 3.0 3.1 3.0 3.2 3.1Sb 0.0 0.0 0.0 0.0 0.0 0.0 0.8 0.8 0.0 0.7As 1.0 1.1 1.0 1.0 1.0 1.1 0.1 0.4 0.9 0.4S 2.9 2.9 2.9 2.9 2.9 2.8 2.9 2.8 2.9 2.8
TOTAL 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0No. Atm
Ag 3.1 3.1 3.1 3.1 3.1 3.0 3.1 3.0 3.2 3.1As+Sb 1.0 1.1 1.1 1.0 1.0 1.1 1.0 1.2 1.0 1.1
S 2.9 2.9 2.9 2.9 2.9 2.8 2.9 2.8 2.9 2.8TOTAL 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0
Boreholes CR038 and CR123: Proustite/pyrargyrite – Analysis Locations
Analysis BoreholeFrom(m)
To(m) Location
#1 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#2 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#3 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#4 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#5 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#6 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#7 CR038 156.30 157.25Quartz replaced tuff/partial massive sulphide
#8 CR123 169.00 169.65 Shale/gossan contact
#9 CR123 169.00 169.65 Shale/gossan contact
#10 CR123 169.00 169.65 Shale/gossan contact
A47
Appendix 5 SEM Analyses
Borehole CR194: Cu-arsenides
Analysis #1 #2 #3 #4 #5 #6 #7 #8Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)
Cu 59.1 59.1 59.7 59.6 60.1 59.4 64.9 64.9As 36.7 34.9 35.6 35.0 35.1 37.8 33.3 33.4Ag 4.4 4.3 4.5 0.0 0.2 0.0 0.0 0.0
TOTAL 100.2 98.3 99.8 94.6 95.4 97.2 98.2 98.3At %Cu 63.7 64.8 64.5 66.7 66.8 64.9 69.7 69.6As 33.6 32.5 32.7 33.3 33.1 35.1 30.3 30.4Ag 2.8 2.8 2.9 0.0 0.1 0.0 0.0 0.0
TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0No Atm
Cu 19.7 20.1 20.0 20.7 20.7 20.1 4.9 4.9As 10.4 10.1 10.1 10.3 10.2 10.9 2.1 2.1Ag 0.9 0.9 0.9 0.0 0.0 0.0 0.0 0.0
TOTAL 31.0 31.0 31.0 31.0 31.0 31.0 7.0 7.0No. Atoms
Cu+Ag 20.6 20.9 20.9 20.7 20.8 20.1 4.9 4.9As 10.4 10.1 10.1 10.3 10.2 10.9 2.1 2.1
TOTAL 31.0 31.0 31.0 31.0 31.0 31.0 7.0 7.0
Borehole CR194: Cu-arsenides – Analysis Locations
AnalysisFrom(m)
To(m) Location Comments
#1 164.60 165.80Massive sulphide contact with gossan
Steel grey colour, identified as novakite
#2 164.60 165.80Massive sulphide contact with gossan
Steel grey colour, identified as novakite
#3 164.60 165.80Massive sulphide contact with gossan
Steel grey colour, identified as novakite
#4 164.60 165.80Massive sulphide contact with gossan
Crimson tarnish, identified as novakite
#5 164.60 165.80Massive sulphide contact with gossan
Crimson tarnish, identified as novakite
#6 164.60 165.80Massive sulphide contact with gossan
Crimson tarnish, identified as novakite
#7 164.60 165.80Massive sulphide contact with gossan
Blue-grey colour, identified as koutekite
#8 164.60 165.80Massive sulphide contact with gossan
Blue-grey colour, identified as koutekite
No. AtomsTheoreticalNovakite
TheoreticalKoutekite
Cu+Ag 21.0 5.0
As 10.0 2.0
TOTAL 31.0 7.0
A48
Appendix 5 SEM Analyses
Borehole CR194: As-bearing pyrite
Analysis #1 #2 #3 #4
Element Wt (%) Wt (%) Wt (%) Wt (%)
Fe 46.2 44.2 44.3 45.6
As 2.4 2.7 1.8 1.1
S 52.3 53.5 53.6 54.0
TOTAL 101.0 100.4 99.6 100.7At %Fe 33.2 31.7 31.9 32.5As 1.3 1.5 1.0 0.6S 65.5 66.8 67.2 67.0
TOTAL 100.0 100.0 100.0 100.0No Atm
Fe 1.0 1.0 1.0 1.0As 0.0 0.0 0.0 0.0S 2.0 2.0 2.0 2.0
TOTAL 3.0 3.0 3.0 3.0No.
AtomsFe 1.0 1.0 1.0 1.0
As+S 2.0 2.0 2.0 2.0TOTAL 3.0 3.0 3.0 3.0
Borehole CR194: As-bearing pyrite – Analysis Locations
AnalysisFrom(m)
To(m) Location
#1 173.50 174.50Massive sulphide/shale
#2 173.50 174.50Massive sulphide/shale
#3 173.50 174.50Massive sulphide/shale
#4 173.50 174.50Massive sulphide/shale
A49
Appendix 5 SEM Analyses
Borehole CR194: Amalgam
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13
Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)
Ag 54.7 54.2 56.4 54.6 53.9 52.9 59.8 59.0 59.9 58.1 56.4 55.2 55.8
Hg 45.5 46.7 44.2 45.1 46.7 46.6 40.8 41.4 39.7 42.9 44.3 44.9 44.2
TOTAL 100.2 100.9 100.5 99.7 100.6 99.5 100.7 100.4 99.6 101.0 100.7 100.1 100.0
AnalysisFrom(m)
To(m) Location
#1 161.75 162.75 Gossan
#2 161.75 162.75 Gossan
#3 161.75 162.75 Gossan
#4 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#5 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#6 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#7 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#8 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#9 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#10 163.75 164.60Gossan contact with massive sulphide - Lower Portion
#11 164.60 165.80 Massive sulphide contact with gossan
#12 164.60 165.80 Massive sulphide contact with gossan
#13 164.60 165.80 Massive sulphide contact with gossan
A50
Appendix 5 SEM Analyses
Borehole CR194: Au-Amalgam
Analysis #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 #14 #15 #16
Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)
Fe 2.8 2.5 2.0 2.1 nd nd nd nd nd nd nd nd nd nd nd nd
Ag 48.5 50.5 50.7 50.6 42.9 35.3 27.1 29.8 33.0 22.8 24.8 47.5 46.4 46.1 46.2 48.1
Au 11.6 8.5 6.5 5.4 27.7 37.2 48.6 47.0 44.0 56.5 46.0 18.5 16.4 19.4 18.8 17.0
Hg 38.0 39.3 40.5 42.6 28.9 27.1 23.8 21.6 23.6 20.5 30.0 34.9 36.8 35.0 33.8 34.3
TOTAL 100.9 100.8 99.7 100.8 99.5 99.7 99.5 98.4 100.6 99.8 100.8 100.9 99.7 100.5 98.8 99.4
Analysis #17 #18 #19 #20 #21 #22 #23 #24
Element Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%) Wt (%)
Ag 45.7 46.0 50.7 49.9 34.4 41.9 31.9 31.6
Au 21.0 19.0 11.3 8.3 17.0 15.4 31.9 22.9
Hg 34.1 34.7 38.4 41.3 47.6 43.0 35.7 45.2
TOTAL 100.8 99.8 100.3 99.5 99.1 100.3 99.5 99.7
A51
Appendix 5 SEM Analyses
Borehole CR194: Au-Amalgam Locations
AnalysisFrom(m)
To(m) Location Analysis
From(m)
To(m) Location
#1 163.75 164.60Gossan contact with massive sulphide - Middle #14 164.60 165.80 Massive sulphide contact with gossan
#2 163.75 164.60Gossan contact with massive sulphide - Middle #15 164.60 165.80 Massive sulphide contact with gossan
#3 163.75 164.60Gossan contact with massive sulphide - Middle #16 164.60 165.80 Massive sulphide contact with gossan
#4 163.75 164.60Gossan contact with massive sulphide - Middle #17 164.60 165.80 Massive sulphide contact with gossan
#5 163.75 164.60 Gossan contact with massive sulphide - Lower #18 164.60 165.80 Massive sulphide contact with gossan
#6 163.75 164.60 Gossan contact with massive sulphide - Lower #19 164.60 165.80 Massive sulphide/gossan contact, zoned grain (core)
#7 163.75 164.60 Gossan contact with massive sulphide - Lower #20 164.60 165.80 Massive sulphide/gossan contact , zoned grain (core)
#8 163.75 164.60 Gossan contact with massive sulphide - Lower #21 164.60 165.80Massive sulphide/gossan contact, zoned grain (margin)
#9 163.75 164.60 Gossan contact with massive sulphide - Lower #22 164.60 165.80Massive sulphide/gossan contact, zoned grain (margin)
#10 163.75 164.60 Gossan contact with massive sulphide - Lower #23 164.60 165.80Massive sulphide/gossan contact, zoned grain (margin)
#11 163.75 164.60 Gossan contact with massive sulphide - Lower #24 164.60 165.80Massive sulphide/gossan contact, zoned grain (margin)
#12 164.60 165.80 Massive sulphide contact with gossan
#13 164.60 165.80 Massive sulphide contact with gossan
A52