MINERAL PARAGENESIS, GEOCHEMISTRY AND GEOCHRONOLOGY INVESTIGATIONS OF THE CARLIN-TYPE

GOLD DEPOSITS AT THE GOLDSTRIKE PROPERTY, NORTHERN NEVADA: IMPLICATIONS FOR ORE GENESIS, IGNEOUS PETROGENESIS AND MINERAL EXPLORATION

by

Carolina Michelin de Almeida

A thesis submitted to the Department of Geological Science and Geological Engineering

In conformity with the requirements for

the degree of Doctoral of Philosophy

Queens University

Kingston, Ontario, Canada

(September, 2009)

Copyright Carolina Michelin de Almeida, 2009

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"It is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change."

Charles Darwin

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Abstract

The Goldstrike property is located in northern Nevada and contains one of the largest and

highest-grade Carlin-type gold deposits. The majority of the Eocene Au mineralization (e.g., Ore

I) is hosted in intensely altered Paleozoic lower plate impure carbonate rocks, and is characterized

by strong to moderate silicification, higher calculated pyrite and ore-related element

concentrations (e.g., As, Cu, Hg, Ni, Tl, Sb, W, and Zn) than Ore II, which is weakly altered.

However, both ore types contain similar Au concentration in whole rock and pyrite chemistry

analyses.

Lithogeochemical and microprobe data suggest that the Paleozoic sedimentary rocks may

have been a major source of Cd, Mo, Ni, U, V, and Zn and minor As, Cu, Hg, and Se. The

Jurassic lamprophyre dikes might have been a significant source of Ba, Co, and Se, and minor

Au, and some of the Jurassic and Eocene intrusive rocks may have provided some Fe. Moreover,

the Eocene magmas are interpreted to be the main source of auriferous mineralizing fluids.

Trace element abundances and ratios of the Jurassic intrusive rocks suggest that they are

shoshonitic and formed from a metasomatized mantle-derived magma, crystal fractionation, and

crustal contamination. The Eocene dikes, also shoshonitic, are considerably more evolved and

contaminated than the studied Jurassic rocks. Furthermore, Ar-Ar results show that the Jurassic

rocks were negligibly affected by the Eocene thermal event, and that temperature of mineralizing

fluids were below the closure temperature of biotite (< 3500C).

A magmatic-related model is proposed to explain the formation of the Carlin-type gold

deposits at the studied area. In this model, Au and the ore-related elements were exsolved along

with volatiles by degassing of a deep and large plutonic complex during its early stage of

crystallization. As these magmatic-hydrothermal fluids moved upward along major conduits (e.g.,

NNW-striking faults), they may have interacted with a Fe-rich fluid, pervasively altering the

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Paleozoic impure carbonate rocks (e.g., carbonate dissolution, silicification, pyritization) and

forming Ore I. Subsequently, these fluids moved laterally further away from the major conduits,

became cooler, less acidic, and depleted in ore-related elements and interacted with the Fe-

bearing host rocks (e.g., sulfidation), favoring the precipitation of Ore II.

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Co-Authorship

This thesis including the scientific manuscripts presented herein is the result of my own

research. Dr. G. Olivo, my supervisor, is co-author of Chapters two, three and four, and during

the length of this study she has provided scientific guidance in the field and the laboratory, and

provided extensive and meticulous discussion and editorial assistance.

Dr. A. Chouinard, who is co-author of Chapter two, has assisted in sample selection, and

shared pyrite chemistry data, and also has greatly contributed to the overall discussion, and given

editorial guidance.

Mr. C. Weakly, who is co-author of Chapter two and three, has assisted in sample

selection, provided information on the local geology, and contributed to the overall discussion,

and editorial direction.

Dr. D. Archibald, who is co-author of Chapter three, has significantly contributed to the

application of 40Ar/39Ar geochronology, including selection of the samples, separation of mineral

concentrates, execution of the analyses, interpretation and discussion of the data, and editorial

assistance.

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Acknowledgements

I would like to express my sincere gratitude to Dr. Gema R. Olivo for her guidance,

inspiration, support and encouragement throughout this project. It was a great pleasure to be part

of her research team, mainly because she brings out the best in her students through her

meticulous, critical and hard work, creativity, great discussions and friendship.

Thanks are also due to Dr. Annick Chouinard for helping sampling and sharing pyrite

chemistry data. Dr. Chouinard is co-author of Chapter two and has greatly contributed to its

improvement by discussing the data, sharing her ideas, and editing.

Special thanks to Mr. Charles Weakly who is co-author of Chapters two and three and

has contributed enormously to the success of this project with his expertise during sampling, his

knowledge of the local geology, and through discussion.

I am also very thankful to Dr. Douglas A. Archibald who is co-author of Chapter three

and has significantly contributed to the selection of the samples for geochronological studies,

separation of the mineral concentrates, and interpretation of the 40Ar/39Ar dating results, and also

to Mr. Hebert Fournier and Ms. Heather Wolczanski for their kind help during the analyses.

Thanks to Dr. Albert Hofstra, anonymous reviewer and associated editors of the scientific

journal Economic Geology who, with their careful reviews, have substantially improved Chapter

two. Drs. Robert Dalrymple, Heather Jamieson, John Peacey and Richard Tosdal are also

acknowledged for their suggestions and discussions as examiners of the thesis.

I would like to also thank Barrick Goldstrike Mine and Barrick Exploration Elko for their

logistical and partial financial support, and for access to the property, drill-hole sampling and

data. A National Sciences and Engineering Research Council Discovery Grant and the Premier's

Research Excellence Award to Dr. Olivo also supported this research and are thankfully

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acknowledged. Thanks to the School of Graduate Studies, Queens University for a graduate

scholarship and an international bursary.

I also appreciate the feedback given by the Goldstrike Exploration staff during the course

of this study, with special thanks to Mr. Bob Leonardson for assisting in the selection of the study

sections, Dr. Missy Mateer for transferring data from the mine model to a compatible GIS format,

and Mr. Robert Malloy for surveying the sampled drill-holes samples.

I am grateful to Mr. Peter C. Jones and Dr. Shi Lang for their assistance with the electron

microprobe studies at Carleton University, Ottawa, and McGill University, Montreal,

respectively, and Dr. Edward Chown for editing Chapter 3.

Thank you to the many people from the Department of Geological Science and

Geological Engineering at Queens University who have contributed to this project: Mr. Mark

Badham for improving the writing style; Mr. Rogers Innes for his kind help cutting and storing

the samples, providing sample bags, and always keeping the petrographic microscope working

properly; Mr. Alan Grant for his assistance using the scanning electron microscope and X-ray

diffraction facilities; Mr. Rob Harrap for his help setting up the ArcView GIS software; Mr. Rob

Renaud for setting up my computer from Microsoft Vista to Microsoft XP and quickly solving

issues related to the network; and finally Ms. Joan Charbonneau, Ms. Linda Brown, and Ms.

Dianne Hyde for their kind assistance and logistical support during this study.

Big thanks to all my friends, close and distant, for the good times, sharing all the ups and

downs of this journey, keeping me going, and putting a smile on my face when everything

seemed so lost and far away. You know who you are. Friends are the family we choose to have.

And last, but not least, my deepest appreciation to my beautiful family, papis Toninho,

mamis Cida, Juju and Xico for their unconditional love, emotional and financial support,

encouragement, and understanding. We know this thesis is your achievement as well. Thank to all

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of you who came all the way to Canada to share some of this experience with me. Longe dos

olhos, mas sempre muito perto do corao

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Statement of Originality

I hereby certify that all of the work described within this thesis is the original work of the author.

Any published (or unpublished) ideas and/or techniques from the work of others are fully

acknowledged in accordance with the standard referencing practices.

(Carolina Michelin de Almeida)

(September, 2009)

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Table of Contents

Abstract...........................................................................................................................................iii Co-Authorship ................................................................................................................................iii Acknowledgements.........................................................................................................................vi Statement of Originality.................................................................................................................. ix Table of Contents............................................................................................................................. x List of Figures...............................................................................................................................xiv

List of Tables xvi

Chapter 1 INTRODUCTION........................................................................................................... 1 1.1 Carlin-type gold deposits: Overview ..................................................................................... 1 1.2 This study: Objectives and approach ..................................................................................... 7

1.2.1 Scope of this thesis.......................................................................................................... 9 1.3 References............................................................................................................................ 11

Chapter 2 MINERAL PARAGENESIS, ALTERATION AND GEOCHEMISTRY OF CARLIN-TYPE GOLD DEPOSITS IN THE SOUTHERN PART OF THE GOLDSTRIKE PROPERTY, NORTHERN NEVADA: IMPLICATIONS FOR SOURCES OF ORE-FORMING ELEMENTS, ORE GENESIS AND MINERAL EXPLORATION .................................................................... 17

2.1 Abstract ................................................................................................................................ 17 2.2 Introduction.......................................................................................................................... 19 2.3 Geologic Setting................................................................................................................... 21

2.3.1 Tectonic Evolution........................................................................................................ 21 2.3.2 Lithological units and the major hosts of gold mineralization...................................... 24

2.4 Methodology........................................................................................................................ 30 2.4.1 Sampling and Analytical Methods................................................................................ 30 2.4.2 Data Analysis ................................................................................................................ 31

2.5 Mineral Paragenesis ............................................................................................................. 32 2.5.1 Paleozoic diagenesis and Jurrassic metasomatism........................................................ 32 2.5.2 Eocene Carlin hydrothermal to post-Carlin events ....................................................... 34

2.6 Lithogeochemistry Investigation ......................................................................................... 41 2.6.1 Mass Balance: Isocon Diagrams ................................................................................... 44

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2.6.2 Element Association: R-Mode Factor Analysis............................................................ 51 2.6.3 Relationship between gold grade and degree of alteration ........................................... 54

2.7 Spatial Distribution of Gold, Associated Elements, and Alteration Indexes ....................... 60 2.8 Discussion............................................................................................................................ 65

2.8.1 Metal source and zonation throughout the southern part of the Goldstrike property.... 66 2.8.2 Mechanisms for metal transport and precipitation, and formation of ore I and ore II .. 70

2.9 Conclusion ........................................................................................................................... 73 2.10 References.......................................................................................................................... 75

Chapter 3 PETROGENESIS OF THE JURASSIC AND EOCENE SHOSHONITIC INTRUSIVE ROCKS FROM THE SOUTHERN PART OF THE GOLDSTRIKE PROPERTY, NEVADA AND THEIR METALLOGENETIC IMPLICATIONS TO CARLIN-TYPE GOLD DEPOSITS....................................................................................................................................................... 84

3.1 Abstract ................................................................................................................................ 84 3.2 Introduction.......................................................................................................................... 85 3.3 Regional Geological Setting ................................................................................................ 87 3.4 Sampling and Analytical Methodology ............................................................................... 90 3.5 Petrography and Mineral Chemistry.................................................................................... 94

3.5.1 Jurassic phlogopite lamprophyre dikes ......................................................................... 94 3.5.2 Jurassic hornblende-biotite gabbro-diorite-granodiorite Goldstrike intrusion and related dikes ..................................................................................................................................... 103 3.5.3 Eocene biotite-plagioclase porphyry dikes ................................................................. 105

3.6 Whole-Rock Geochemistry................................................................................................ 107 3.6.1 Jurassic phlogopite lamprophyre dikes ....................................................................... 119 3.6.2 Jurassic Goldstrike intrusion and related dikes........................................................... 119 3.6.3 Eocene biotite-plagioclase porphyry dikes ................................................................. 122

3.7 Mass Change: Isocon Diagrams......................................................................................... 123 3.7.1 Jurassic Phlogopite lamprophyre dikes....................................................................... 123 3.7.2 Jurassic Goldstrike intrusion and related dike-1: ........................................................ 126 3.7.3 Eocene Biotite-Plagioclase Porphyry dikes: ............................................................... 126

3.8 40Ar-39Ar Geochronology ................................................................................................... 127 3.9 Discussion: Petrogenesis and Metallogenesis.................................................................... 131

3.9.1 Implications for source of metals and ore-forming processes..................................... 134

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3.10 Conclusion ....................................................................................................................... 136 3.11 References........................................................................................................................ 138

Chapter 4 CHEMICAL COMPOSITION OF DIAGENETIC TO LATE HYDROTHERMAL SPHALERITE IN THE HOST ROCKS OF THE CARLIN-TYPE GOLD MINERALIZATION

FROM THE SOUTHERN PART OF THE GOLDSTRIKE PROPERTY, NEVADA .............. 146 4.1 Abstract .............................................................................................................................. 146 4.2 Introduction........................................................................................................................ 147 4.3 Geological setting .............................................................................................................. 148 4.4 Methodology: Sampling and analytical methods............................................................... 152 4.5 Mineral paragenesis of the Au-hosted sedimentary rocks and mode of occurrence of sphalerite.................................................................................................................................. 153 4.6 Mineral chemistry of sphalerite ......................................................................................... 157 4.7 Discussion.......................................................................................................................... 165 4.8 References.......................................................................................................................... 169

Chapter 5 DISCUSSION ............................................................................................................. 176 5.1 Tectonic evolution ............................................................................................................. 176

5.1.1 Paleozoic: sediment deposition and the Antler orogeny ............................................. 178 5.1.2 Jurassic: emplacement of the shoshonitic Goldstrike intrusion and related-dikes and Au-bearing lamprophyre dikes............................................................................................. 179 5.1.3 Eocene: shoshonitic magmatism and Au-bearing Carlin hydrothermal event ............ 180

5.2 Genetic model for the Carlin-type gold deposits ............................................................... 181 5.3 Implications for mineral exploration.................................................................................. 183 5.4 References.......................................................................................................................... 183

Chapter 6 FINAL CONSIDERATION........................................................................................ 188 6.1 Major conclusions.............................................................................................................. 188 6.2 Future studies ..................................................................................................................... 191

APPENDIX A: ELECTRON MICROPROBE METHODOLOGY FOR PYRITE AND SPHALERITE ANALYSES.................................................................................................... 192

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APPENDIX B: LITHOGEOCHEMISTRY ANALYTICAL METHODS, DETECTION LIMITS, QUALITY ASSURANCE AND QUALITY CONTROL........................................ 194 APPENDIX C: LITHOGEOCHEMICAL ANALYSES OF THE PALEOZOIC LOWER-PLATE SEDIMENTARY ROCKS ......................................................................................... 197 APPENDIX D: CORRELATION MATRIX OF PALEOZOIC LOWER-PLATE

SEDIMENTARY ROCKS....................................................................................................... 233 APPENDIX E: ADDITIONAL INFORMATION ON THE THEMODYNAMICS OF MERCURY, ANTIMONY, AND THALLIUM .238 APPENDIX F: 40AR-39AR GEOCHRNOLOGY METHODOLOGY .239 APPENDIX G: ELECTRON MICROPROBE ANALYSES OF THE PHLOGOPITE, HORNBLENDE AND BIOTITE IN THE JURASSIC INTRUSIVE ROCKS....................... 240 APPENDIX H: ELECTRON MICROPROBE ANALYSIS OF IGNEOUS AND HYDROTHERMAL PYRITE HOSTED BY THE JURASSIC AND EOCENE INTRUSIVE ROCKS .................................................................................................................................... 245

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List of Figures

Figure 1.1 Digital elevation model of northern Nevada.................................................................. 2 Figure 1.2 Schematic east-west cross section of northern Nevada ................................................ 3 Figure 1.3 Ore grade (g/t Au) versus metric tons of ore for Carlin-type deposits........................... 5 Figure 2.1 Simplified geological map of the southern part of the Goldstrike property, Nevada. .20 Figure 2.2 Schematic geological SW-NE cross sections in the southern part of the Goldstrike

Property, Nevada...................................................................................................................... 25 Figure 2.3 Simplified tectono-stratigraphic column for the Goldstrike property, Nevada............ 26 Figure 2.4 Paragenetic sequence for the sedimentary rocks from the Goldstrike property........... 33 Figure 2.5 Photomicrographs and back-scattered electron (BSE) images from the Paleozoic

sedimentary rocks and Jurassic intrusive rocks... .................................................................... 35 Figure 2.6 Box and whisker plots of whole-rock data Goldstrike property. ................................. 43 Figure 2.7 Al2O3 (wt %) versus TiO2 (wt %) binary diagram for the Goldstrike sedimentary

rocks......................................................................................................................................... 46 Figure 2.8 Logarithm isocon diagrams.......................................................................................... 49 Figure 2.9 Gold grade (g/t) versus change in mass (wt %) of all sedimentary rock samples. ...... 50 Figure 2.10 Gold grade (g/t) versus degree of silicification.......................................................... 55 Figure 2.11 Excess SiO2 (wt %) versus CaO+MgO+ LOI (wt %) for mineralized sedimentary

rock samples. ........................................................................................................................... 56 Figure 2.12 Binary diagram showing the relation between Fe (wt %) vs S (wt %)...................... 57 Figure 2.13 Logarithm correlation diagrams showing relations between calculated pyrite and

trace elements.. ........................................................................................................................ 58 Figure 2.14 Gold grade (g/t) versus organic C concentration (wt %) for all sedimentary rock

samples..................................................................................................................................... 59 Figure 2.15 Spatial distribution of degree of silicification (excess SiO2(wt %)/ Al2O3(wt %)),

calculated amount of pyrite and concentration of Au and select trace elements for samples from various depth project to the surface................................................................................. 64

Figure 3.1 Simplified geological map of the southern part of the Goldstrike property, Nevada. .88 Figure 3.2 Simplified tectono-stratigraphic column from the Goldstrike Property, Nevada ........ 89 Figure 3.3 Simplified NW-SE geological cross section of the Betze-Post deposit, Goldstrike

property, Nevada...................................................................................................................... 92

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Figure 3.4 Photographs of representative drill-hole samples from the intrusive rocks, Golstrike property, Nevada...................................................................................................................... 95

Figure 3.5 Photomicrographs and back-scattered electron (BSE) images of the intrusive rocks, 97 Figure 3.6 Mineral chemistry data of the phlogopite phenocrysts from the phlogopite

lamprophyre dikes plotted in a compositional mica ternary system Al-Mg-Fet+2 diagram. .. 101 Figure 3.7 Paragenetic sequence of the intrusive rocks from the southern part of the Goldstrike

property.. ................................................................................................................................ 104 Figure 3.8 TiO2 (wt %) versus Al2O3 (wt %) binary diagram for the intrusive rocks in the

Goldstrike property. ............................................................................................................... 116 Figure 3.9 Chondrite-normalized REE diagrams for the intrusive rocks in the southern part of the

Goldstrike property.. .............................................................................................................. 117 Figure 3.10 Primitive mantle-normalized trace element spider diagrams of the intrusive rocks

from the southern part of the Goldstrike property.. ............................................................... 118 Figure 3.11 Ce/Yb ratio versus Ta/Yb ratio plot......................................................................... 120 Figure 3.12 Logarithm isocon diagrams showing the relation between least altered and altered

intrusive rocks composition. .................................................................................................. 125 Figure 3.13 40Ar-39Ar age spectra of the intrusive rocks in the Goldstrike property, Nevada. ... 130 Figure 3.14 Th/Yb ratio versus Ta/Yb ratio diagram.................................................................. 132 Figure 4.1 Simplified geological map of the southern part of the Goldstrike property, Nevada.150 Figure 4.2 Simplified tectono-stratigraphic column for the Goldtrike property. ........................ 151 Figure 4.3 Paragenetic sequence for the Goldstrike property including diagenesis, and Carlin

hydrothermal (early-, syn- and late-ore) events.. ................................................................... 154 Figure 4.4 Photomicrographs and back-scattered electron (BSE) images of the various

generations of sphalerite and pyrite.. ..................................................................................... 155 Figure 4.5 Box and whisker plot of major, minor and trace elements in diagenetic, early-ore, syn-

ore, and late-ore sphalerite.. ................................................................................................... 163 Figure 4.6 Binary diagrams of major, minor and trace elements for the four generations of

sphalerite.. .............................................................................................................................. 165 Figure 4.7 Chemical variation of major, minor and trace elements of the various generations of

pyrite in the southern part of the Goldstrike property, Nevada............................................. 166 Figure 4.8 Whisker plot of (a) Fe, (b) Cd, (c) Hg, (d) Cu, and (e) Se concentrations from Carlin-

type, MVT, SEDEX, epithermal and VMS deposits ............................................................. 169 Figure 5.1 Schematic genetic model for Carlin-type Au deposits............................................... 177

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List of Tables

Table 2.1 Sedimentary Units and Their Depositional Environments, Goldstrike Property, Nevada.................................................................................................................................................. 27

Table 2.2 Mineralogy and Textures of Least Altered Samples of the Popovich and Roberts Mountains Formations in the Southern Part of the Goldstrike Property, Northern Nevada .... 28

Table 2.3 Major Characteristics of Carlin-type Au Mineralization: Ore I and Ore II................... 39 Table 2.4 Rotated Factor Loading Determined by Five-Factor Model for Lower Plate Carbonate

Rocks data from the Southern Part of the Goldstrike Property, Nevada ................................. 52 Table 2.5 Elements in each Factor of the Five-Factor Model for Barren to Mineralized Lower

Plate Sedimentary Rocks in the Southern Part of the Goldstrike Property, Nevada................ 53 Table 3.1 Major Characteristics of the Jurassic and Eocene Intrusive Rocks at the Goldstrike

Property, Nevada...................................................................................................................... 91 Table 3.2 Range Composition of Phlogopite, Biotite and Hornblende Electron Microprobe

Analyses................................................................................................................................. 100 Table 3.3. Range Composition of Igneous to Hydrothermal Pyrite from the Intrusive Rocks in the

Southern Part of the Goldstrike Property, Nevada................................................................. 102 Table 3.4 Whole-Rock Data of the Jurassic and Eocene Intrusive Rocks of the Southern Part of

the Goldstrike Property, Nevada............................................................................................ 108 Table 3.5 40Ar/39Ar Geochronology Results of the Igneous Rocks from the Southern Part of the

Goldstrike Property, Nevada.................................................................................................. 128 Table 3.6 Summary of 40Ar/39Ar Laser Step-Heating Data of the Igneous Rocks from the

Southern Part of the Goldstrike Property, Nevada................................................................. 130 Table 4.1 Composition of Diagenetic Sphalerite from the the Goldstrike Property, Nevada. .... 158 Table 4.2 Composition of Early-Ore Sphalerite from the Goldstrike Property, Nevada. ........... 159 Table 4.3 Composition of Syn-Ore Sphalerite from the Goldstrike Property, Nevada............... 160 Table 4.4 Composition of Late-Ore Sphalerite from the Goldstrike Property, Nevada.............. 161

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Chapter 1 INTRODUCTION

1.1 Carlin-type gold deposits: Overview

Carlin-type gold deposits, also known as sediment-hosted disseminated gold and invisible

gold deposits, are mainly hosted in Paleozoic impure carbonate rocks in northern Nevada, United

States (Figure 1.1). They occur in clusters, usually along trends (e.g., Carlin, Battle Mountain-

Eureka, Getchell, Jerritt Canyon and Alligator Ridge, Figure 1.1), underneath the Devonian-

Mississipian Roberts Mountain thrust, and are commonly structurally- and/or stratigraphically-

controlled (Figure 1.2) (Hofstra and Cline, 2000; Muntean et al., 2004; Cline et al., 2005; and

references therein). Moreover, the Carlin-type gold deposits are commonly larger and more

abundant in areas underlain by variably attenuated Archean crust or oceanic crust and small or

nonexistent in areas underlain by thick Proterozoic crust (Figure 1.2) (De Paolo and Farmer,

1984; Wooden et al., 1998; Tosdal et al., 2000; Hofstra and Cline, 2000).

They are associated with subtle to pervasive hydrothermal alteration, which consists of

carbonate dissolution, precipitation of hydrothermal quartz, and fine-grained porous sulfides

(mainly pyrite), and argillization of silicate minerals. Gold occurs as ionic substitution or

submicron-sized inclusions mainly in disseminated fine-grained trace element-rich pyrite (e.g.,

As, Cu, Hg, Ni, Sb, Tl, and W) and sometimes in marcasite and arsenopyrite (Fleet and Mumin,

1997; Bakken et al., 1989; Simon et al., 1999; Palenik et al., 2004; Reich et al., 2005). Auriferous

pyrite occurs both as thin rims on earlier pyrite and as tiny grains (i.e., commonly less than a few

microns in diameter) (Wells and Mullens, 1973; Arehart et al., 1993; Simon et al., 1999; Hofstra

and Cline, 2000; Chouinard et al., 2006).

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Figure 1.1 Digital elevation model of northern Nevada, showing location of major mineral belts and districts, including Carlin-type deposits (circles: Peters et al., 2003), other significant Au, Ag, Pb, Zn, or Cu deposits (crosses: Long et al., 2000), eastern limit of the Roberts Moutain allochthon (bold white line; Crafford and Grauch, 2002), cities (small circles), and highways (black lines). Inset (white box) shows the distribution of Carlin-type deposits in the northern Carlin trend (Thompson et al., 2002), and the location of the Goldstrike property. Figure provided by D. Sweetkind, U.S. Geological Survey, and modified from Hofstra et al. (2003).

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Figure 1.2 Schematic east-west cross section of northern Nevada, showing attenuated Archean crust, oceanic crust, overlying stratigraphic sequences and allochthons, fault zones, and locations of Carlin-type gold deposits (black) (Hofstra and Cline, 2000).

Mass balance studies on several deposits (Hofstra and Cline, 2000; Cail and Cline, 2001;

Yigit and Hofstra, 2003; Emsbo et al., 2003; Kesler et al., 2003; Cline et al., 2005; and references

therein) reveal that Au, As, S, Sb, Hg, and Tl are commonly introduced in the Carlin

hydrothermal system; however, Ag, Ba, Cu, Fe, Mo, Pb, Se, Si, Te, W and Zn are also enriched in

some of the studied deposits. On the other hand, C, CaO, MgO, and Sr are commonly the most

depleted elements, and Al2O3 and TiO2 are generally the most immobile pair.

The sulfidation of Fe-bearing host rocks is the proposed mechanism to explain the

precipitation of Au-bearing sulfides (Stenger, 1998; Hofstra and Cline, 2000; Emsbo et al., 2003;

Cline et al., 2005). However, recent studies in several districts have also shown that both Fe and S

4

were added to the system, suggesting that the precipitation of Au-bearing hydrothermal pyrite

also occured by pyritization (Cail and Cline, 2001; Kesler et al., 2003; Ye et al., 2003).

Limited fluid inclusion data suggest that the hydrothermal fluids were of moderate

temperature (~ 180-2400C), moderate acidity (pH ~ 5), low salinity (~ 2-3 wt % NaCl equiv),

CO2-bearing (< 4 mol %), and CH4-poor (< 0.4 mol %), with sufficient H2S (10-1-10-2 mol %) to

transport Au as bisulfide complexes (Cline et al., 2005; and references therein).

Recently, relevant field relationships and geochronological data constrainted the age of

the gold mineralization to a narrow time interval during the mid to late Eocene (42-36 Ma:

Hofstra et al., 1999; Tretbar et al., 2000; Arehart et al., 2003; Cline et al., 2005). At this time, the

tectonic setting of northern Nevada was in transition from contractional to extensional

deformation of the upper crust with associated magmatism (e.g., onset of the Basin and Range

tectonics).

Current gold production from Carlin-type gold deposits comprises around eight percent

of the worlds total production, making the United States one of the leading Au-producers

worldwide (Nevada Bureau of Mines and Geology, 2007). Although several Carlin-type gold

deposits in Nevada are world-class (> 100 t Au) and giant (> 250 t Au), both the total contained

Au amounts and ore grades vary widely across districts and within deposits (Figure 1.3). The

Carlin NNW-trend, which extends more than 60 km (Figure 1.1) and includes more than fifty

known deposits, is one of the most productive gold-mining districts in the world with total

production now exceeding 68.5 Moz (Nevada Bureau of Mines and Geology, 2007).

The Goldstrike property, located in the Carlin Trend, contains one of the largest (e.g.,

Betze-Post: 1,250 t Au, annual production plus reserve) and highest-grade (e.g., Meikle: 24.7 g/t

Au) gold deposits ever mined (Figure 1.1), which comprises around thirty percent of the total

5

annual gold production in the Carlin Trend (i.e., greater than 50 t Au: Nevada Bureau of Mines

and Geology, 2007).

Figure 1.3 Ore grade (g/t Au) versus metric tons of ore for Carlin-type deposits (small diamonds), deposits containing greater than 5 Moz (155.5 t) gold (squares), and major districts (triangles). AR = Alligator Ridge district, CCT = central Carlin trend, CTZ = Cortez district in Battle Mountain-Eureka trend (BMET), Eu = Eureka district in BMET, GB = Gold Bar district in BMET, GT = Getchell trend, JC = Jerritt Canyon district, NCT = north Carlin trend, SCT = south Carlin trend. (Cline et al., 2005). Screamer, Meikle and Betze deposits are comprised in the Goldstrike property.

Although these deposits have been extensively studied since their discovery in the early

1960s, a comprehensive and broadly accepted genetic model is still the subject of debate (Ilchik

and Barton, 1997; Hofstra and Cline, 2000; Emsbo et al., 2003; Cline et al., 2005; Ressel and

Henry, 2006; and references therein). The conflicting interpretations are related to the nature and

composition of the hydrothermal fluids, source of Au, S, Fe, and other ore-related elements (e.g.,

As, Cu, Hg, Ni, Sb, Tl, Zn, and W), mechanisms of pyrite formation (e.g., sulfidation versus

6

pyritization) and Au incorporation in Fe-bearing sulfides, and the role of magmatism in the

genesis of these deposits.

Several genetic models have been proposed for Carlin-type gold deposits, including: (1)

meteoric water circulation (Ilchik and Barton, 1997; Emsbo et al., 2003), (2) epizonal intrusion-

related (Henry and Ressel, 2000; Ressel et al., 2000; Ressel and Henry, 2006; Johnston and

Ressel, 2004), and (3) deep metamorphic magmatic fluids (Hofstra and Cline, 2000; Cline et

al., 2005).

The meteoric water model proposes that metals (S, Au, and trace elements) were

scavenged by lateral flow of meteoric water from pre-existing Au-hosting sedimentary rocks

(e.g., sediment-exhalative: Emsbo et al., 2003) or deep meteoric water convection through the

Neoproterozoic basement crustal rocks (e.g., metapelites: Seerdorff, 1991; Ilchik and Barton,

1997). Although most of the available isotopic data support this model, it does not explain the

evidence for the involvement of deep metamorphic and/or magmatic water or the presence of

certain metals reported in some districts (Hofstra and Cline, 2000; Cline, 2001; Kesler et al.,

2003; Cline et al., 2005; and references therein).

The epizonal pluton-related model proposes that the Eocene magmatism may represent

the major heat source, which has driven the circulation of the hydrothermal fluids and may have

potentially supplied metals and fluids (Ressel et al., 2000; Henry and Ressel, 2000; Ressel and

Henry, 2006; Johnston and Ressel, 2004). Moreover, Cunningham et al. (2004) proposed that the

Carlin-type gold deposits may correspond to deeper sedimentary analogs of volcanic-hosted

epithermal deposits, or distal mineralization related to porphyry-style intrusions. This model

would explain the spatial and temporal relationship between Carlin-type gold mineralization and

intrusive rocks in some districts (Ressel et al., 2000; Ressel and Henry, 2006). However, some

typical features of magmatic hydrothermal systems have not yet been identified in the Carlin-type

7

deposits, including the lack of Eocene dikes in some of the districts (e.g., Getchell: Cline, 2001),

no evidence of mineral and alteration zoning, lack of higher temperature (> 2500C) mineral

assemblages (including skarn-related alteration), and no evidence of boiling fluid inclusions with

typical magmatic-derived fluids (Hofstra and Cline, 2000; Cline et al., 2005).

The deep metamorphic and/or magmatic fluid model suggests that water and other

components of the ore fluids were provided mainly by devolatization of sedimentary rocks

through prograde metamorphism with heat supplied by magmatic centers and discharge along

crustal faults triggered during crustal extension. Although this model explains the spatial and

temporal association of the deposits and their crustal-scale structures and magmatism, as well as

radiogenetic and stable isotope data that indicate deep sources for some ore-fluid components, it

cannot account for the sedimentary and meteoric-related isotopic signatures of some elements in

some deposits, the absence of associated magmatism, and the lack of demonstrated

metamorphism during the Eocene (Hofstra and Cline, 2000; Cline et al., 2005).

Therefore, some of the most intriguing questions about the genesis of the Carlin-type

gold deposits are the source of fluids, Au and ore-related elements, the mechanisms of pyrite

precipitation and gold incorporation in the ore-stage pyrite, and the role of magmatic events

during the formation of Carlin-type gold deposits.

1.2 This study: Objectives and approach The present thesis was conducted to advance our knowledge of the factors that were

crucial during the formation of the Carlin-type gold deposits and to determine vectors towards

zones with high potential to host mineralization by addressing the following questions: (1) What

is/are the potential source(s) of fluids, Au and other ore-related elements?; (2) What are the

relationships between Au mineralization, host rocks, major structures, hydrothermal alteration,

8

metal zoning, spatial distribution at both deposit and district scale?; (3) What is/are the

mechanism(s) that caused the precipitation of auriferous hydrothermal pyrite?; and (4) What

is/are the role(s) of the Jurassic and Eocene magmatic events in the Carlin-type gold

mineralization?

The Goldstrike property is an ideal area to investigate these questions because it contains

one of the largest and highest-grade deposits, it comprises a variety of Au-host rocks, including

Paleozoic sedimentary rocks, and Jurassic and Eocene intrusive rocks, and the magmatic rocks

are spatially and temporally associated with the Au mineralization. Thus, the Goldstrike property

is the object of study of the present doctoral thesis.

In order to achieve these objectives, I conducted:

(1) Detailed petrography, mineralogical and lithogeochemical characterization of major

sedimentary and intrusive rocks throughout the property to identify mineral paragenesis, styles of

Au mineralization and hydrothermal alteration, as well as their spatial distribution;

(2) Evaluation of the potential source of Au and other ore-related elements (e.g., As, Cu,

Hg, Ni, Sb, Se, Tl, W, and Zn) based on the whole-rock composition of the least altered

sedimentary and intrusive rocks, as well as the mineral chemistry of diagenetic and igneous

sulfides (mainly pyrite and sphalerite), and magmatic silicates (e.g., phlogopite, biotite, and

hornblende);

(3) Petrogenetic investigation of the least altered intrusive rocks based on the least mobile

elements to determine the origin of the magmatic rocks temporally and/or spatially associated

with gold mineralization and their role in the mineralizing process, as possible source of metals,

fluids and/or heat;

(4) Construction of isocon diagrams to determine the elements that were enriched and

depleted in the various hydrothermally altered host rocks;

9

(5) Integration of mass balance results, factor analysis, lithogeochemistry and mineral

chemistry to evaluate the elements that were introduced during the various stages in the evolution

of these rocks (i.e., Paleozoic diagenesis, Jurassic magmatism and metasomatism, and Eocene

magmatism and gold mineralization);

(6) Quantitative assessment of the relationship between Au grade and the various types of

hydrothermal alteration (i.e., carbonate dissolution, silicification and precipitation of

hydrothermal pyrite) to identify the factor(s) that may have been the most relevant in the

formation of the gold mineralization;

(7) Integration of the Fe and S mass change results in the various rocks with mineral

compositions and textural relationships for the various generations of sulfides to discriminate

between the most probable mechanisms responsible for the formation of the Au-bearing pyrite

(e.g., sulfidation versus pyritization processes);

(8) 40Ar/39Ar geochronology to determine the age of the selected intrusive rocks and the

effects of the Eocene event on the Jurassic intrusive rocks; and

(9) Integration of all data to construct a comprehensive genetic model and to establish

exploration guidelines for Carlin-type gold deposits.

1.2.1 Scope of this thesis

This thesis is presented in a manuscript format. Three manuscripts (Chapters two to four)

comprise the main body of the thesis and are summarized below.

The thesis also includes Chapter 1 (Introduction), which gives an overview of the

geology and genetic models proposed for Carlin-type gold deposits, identifies the major issues

that are not yet well understood about the formation of this deposit type, and includes the

objectives and scope of the present thesis. Chapter 5 (Discussion) integrates all of the major

10

scientific contributions of this study with the tectonic evolution of the area where these deposits

are located and presents a comprehensive genetic model for Carlin-type gold deposits. Chapter 6

(Conclusion and Future Studies) summarizes the major findings and proposes future studies to

address some of the questions that still remain open and are fundamental to advancing our

knowledge related to the formation of Carlin-type gold deposits.

Chapter 2, titled Mineral Paragenesis, Alteration and Geochemistry of Carlin-Type Gold

Deposits in the Southern Part of the Goldstrike Property, Northern Nevada: Implications for

Sources of Ore-Forming Elements, Ore Genesis and Mineral Exploration, was prepared by C.M.

Almeida and collaborators (i.e., G.R. Olivo, A. Chouinard, and C. Weakly) and re-submitted to

Economic Geology on July 31 of 2009. To our knowledge, this study is the first attempt to

characterize thoroughly two types of hypogene Au mineralization hosted in the Paleozoic

sedimentary rocks based on mineralogical and whole-rock composition, degree of hydrothermal

alteration, chemical composition of pyrite and spatial distribution at the property. Moreover, we

have evaluated the possible sources of ore-related metals based on the compositions of the least

altered sedimentary rocks, chemical composition of diagenetic pyrite and sphalerite, and mass

balance studies showing the elements that were enriched or depleted in the alteration zones. Our

findings have allowed us to propose an integrated genetic model, including an alternative

mechanism for precipitation of the auriferous pyrite, as well as to suggest exploration guidelines

for Carlin-type gold deposits.

Chapter 3, titled Petrogenesis of the Jurassic and Eocene Shoshonitic Intrusive Rocks

from the Southern Part of the Goldstrike Property, Nevada and their Metallogenetic Implications

to Carlin-Type Gold Deposits, was prepared by C.M. Almeida and collaborators (i.e., G.R.

Olivo, D.A. Archibald, and C. Weakly) and submitted to Economic Geology on July 31 of 2009.

To our knowledge, this is the first study that integrates mineralogical (texture and composition),

11

alteration, lithogeochemistry, and 40Ar/39Ar geochronology data of various types of intrusive

rocks that are spatially and/or temporally associated with Carlin-type gold deposits. Moreover, it

is the first to thoroughly characterize the Jurassic phlogopite lamprophyre dikes, and identify

these Jurassic and Eocene intrusive rocks as shoshonitic. The integration of these data brings new

insights regarding their petrogenesis and the tectonic evolution of the studied area. In addition,

this study evaluates the possibility that the magmatic rocks might be the source of Au and ore-

related elements based on the composition of the least altered rocks and both igneous and

hydrothermal pyrite, and documents the overprinting of the Carlin hydrothermal event in both

Jurassic and Eocene magmatic rocks.

Chapter 4, titled Chemical Composition of Diagenetic to Late Hydrothermal Sphalerite

in the Host Rocks of the Carlin-Type Mineralization from the Southern Part of the Goldstrike

property, Nevada, was prepared by C.M. Almeida and G.R. Olivo for submission to

Mineralogical Magazine. This study is the first to investigate the mode of occurrence and

variation in chemical composition of diagenetic, early-ore, syn-ore, and late-ore sphalerite

associated with Carlin-type gold deposits, in order to evaluate the availability of various trace

elements during the evolution of these rocks.

1.3 References

Arehart, G.B., Foland, K.A., Naeser, C.W., and Kesler, S.,E., 1993, 40Ar/39Ar, K/Ar, and fission-

track geochronology of sediment-hosted disseminated gold deposits at Post-Betze, Carlin

Trend, northeastern Nevada.: Economic Geology, v. 88, p.662-646.

Arehart, G.B., Chakurian, A.M., Tretbar, D.R., Christensen, J.N., McInnes, B.A., and Donelick,

R.A., 2003, Evaluation of radioisotope dating of Carlin-type gold deposits in the Great Basin,

12

western North America, and implications for deposit genesis: Economic Geology, v. 98, p.

235-248.

Bakken, B.M.m Hochella, M.F., Jr., Marshall, A.F., and Turner, A.M., 1989, High-resolution

microscopy of gold in unoxidized ore from the Carlin mine, Nevada: Economic Geology, v.

84, p. 171-179.

Cail, T.L. and Cline, J.S., 2001, Alteration associated with gold deposition at the Getchell Carlin-

type gold deposit, north-central Nevada: Economic Geology, v. 96, p. 1343-1359.

Chouinard, A., Olivo, G., and Poirier, G., 2006, Chemistry and spatial distribution of

trace elements in the auriferous pyrite of the Goldstrike property, Barrick Goldstrike

mines Inc., unpublished internal report, 77p.

Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M., Hickey, K.A., 2005, Carlin-type gold

deposits in Nevada: Critical geologic characteristics and viable models: Economic Geology,

100th Anniversary Volume, p. 451-484.

Crafford, A.E.J., and Grauch, V.J.S., 2002, Geologic and geophysical evidence for the influence

of deep crustal structures on Paleozoic tectonics and the alignment of world class gold

deposits, north central Nevada, USA: Ore Geology Reviews, v. 21, p.157-184.

Cunningham, C.G., Austin G.W., Naeser, C.W., and Rye, R.O., 2004, Formation of a

paleothermal anomaly and disseminated gold deposits associated with Bingham Canyon

porphyry Cu-Au-Mo system, Utah: Economic Geology, v. 99, p. 789-806.

De Paolo, D.J., and Farmer, G.L., 1984, Isotopic data bearing on the origin of Mesozoic and

Tertiary granitic rocks in the Western United States: Philosophical Transactions of the Royal

Society of London, Series A, v. 310, p.743-753.

Emsbo, P., Hofstra, A.H., Lauha, E.A., Griffin, G.L., and Hutchinson, R.W., 2003, Origin of

high-grade gold ore, source of ore fluid components, and genesis of the Meikle and

13

neighboring Carlin-type deposits, northern Carlin trend, Nevada: Economic Geology, v. 98,

p. 1069-1105.

Fleet, M.E., and Mumin, A.H., 1997, Gold-bearing arsenian pyrite and marcasite and arsenopyrite

from Carlin trend gold deposits and laboratory synthesis: American Mineralogist, v. 82, p.

182-193.

Henry, C.D., and Ressel, M.W., 2000, Eocene magmatism of northeastern Nevada: the smocking

gun for Carlin-type gold deposits, in Cluer, J.K., Price, J.G., Struhsacker, E.M., Hardyman,

R.F., and Morris, C.L., eds., Geology and Ore Deposits 2000: The Great Basin and Beyond:

Reno, Geological Society of Nevada, Special Publication, p. 365-388.

Hofstra, A.H. and Cline, J.S., 2000, Characteristics and models for Carlin-type gold deposits:

Reviews in Economic Geology, v. 13, p. 163-220.

Hofstra, A.H., Snee, L.W., Rye, R.O., Folger, H.W., Phinisey, J.D., Loranger, R.J., Dahl, A.R.,

Naeser, C.W., Stein, H.J., and Lewchuk, M.T., 1999, Age constraints on Jerritt Canyon and

other Carlin-type gold deposits in the western United States; relationship to mid-Tertiary

extension and magmatism: Economic Geology, v. 94, p. 769-802.

Hofstra, A.H., John, D.A., and Theodore, T.G., 2003, A special issue devoted to gold deposits in

northern Nevada: Pt 2. Carlin-type deposits: Economic Geology, v. 98, p. 1063-1067.

Ilchik, R.P. and Barton, M.D., 1997, An amagmatic origin of Carlin-type gold deposits:

Economic Geology, v. 92, p. 50-75.

Johnston, M.K., and Ressel, M.W., 2004, Controversies on the origin of world-class gold

deposits, Pt. I: Carlin-type gold deposits in Nevada, II. Carlin-type and distal disseminated

Au-Ag deposits: Related distal expressions of Eocene intrusive centers in north-central

Nevada: Society of Economic Geologists Newsletter 59, p. 12-14.

14

Kesler, S.E., Fortuna, J., Ye, Z., Alt, J.C., Core, D.P., Zohar, P., Borhauer, and J., Chryssoulis, S.,

2003, Evaluation of the role of sulfidation in deposition of gold, Screamer section of the

Betze-Post Carlin-type deposit, Nevada: Economic Geology, v. 98, p. 1137-1157.

Long, K.R., DeYoung, J.H. Jr., and Ludington, S., 2000, Significant deposits of gold, silver,

copper, lead, and zinc in the United States: Economic Geology, v. 95, p. 629-644.

Muntean, J.L., Cline, J. Johnston, M.K., Ressel, M.W., Seedorff, E., and Barton, M.D., 2004,

Controversies on the origin of world class gold deposits, Part I: Carlin type gold deposits in

Nevada: Society of Economist Geologists Newsletter, n. 59, p. 1, 11-17.

Nevada Bureau of Mines and Geology, 2007, The Nevada mineral industry 2007: Nevada Bureau

of Mines and Geology Special Publication MI-2007, 182 p.

Palenik, C.S.; Satoshi, U.; Reich, M,; Kesler, S.E.; Wang, L.; Ewing, R.C., 2004,

Invisible gold revealed: Direct imaging of gold nanoparticles in a Carlin-type

deposit: American Mineralogist, v. 89, p. 1359-1366.

Peters, S.G., Armstrong, A.K., Harris, A.G., Oscarson, R.L., and Noble, P.J., 2003,

Biostratigraphy and structure of Paleozoic host rocks and their relationship to Carlin-type

gold deposits in the Jerritt Canyon mining district, Nevada: Economic Geology, v. 98, p. 317-

337.

Reich, M., Kesler, S.E., Utsunoyiya, S., Palenik, C.S., Chryssoulis, S., and Ewing, R.C., 2005,

Solubility of gold in arsenian pyrite: Geochimica and Cosmochimica Acta, v. 69, p.2781-

2796.

Ressel, M.W. and Henry, C.D., 2006, Igneous geology of the Carlin trend, Nevada: Development

of the Eocene plutonic complex and significance for Carlin-type gold deposits: Economic

Geology, v. 101, p. 347-383.

15

Ressel, M.W., Noble, D.C., Henry, C.D., and Trudel, W.S., 2000, Dike-nested ores of the Beast

deposit and the importance of Eocene magmatism in gold mineralization of the Carlin Trend,

Nevada: Economic Geology, v. 95, p. 1417-1444.

Seedorff, E., 1991, Magmatism, extension, and ore deposits of Eocene to Holoceno age in the

Great Basin-mutual effects and preliminary proposed genetic relationships, in Raines, G.L.,

Lisle, R.E., Schafer, R.W., and Wilkison, W.H., eds., Geology and ore deposits of the Great

Basin, Symposium Proceedings: Geological Society of Nevada, Reno, Nevada, p.133-178.

Simon, G., Kesler, S.E., and Chryssoulis, S., 1999, Geochemistry and textures of gold-bearing

arsenian pyrite, Twin Creek, Nevada: Implications for deposition of gold in Carlin-type

deposits: Economic Geology, v.94, p. 405-422.

Stenger, D.P., Kesler, S.E., Peltonen, D.R., and Tapper, C.J., 1998, Deposition of gold in Carlin-

type gold deposits: The role of sulfidation and decarbontion at Twin Creeks, Nevada:

Economic Geology, v.93, p. 201-215.

Thompson, T.B., Teal, L., and Meeuwig, R.O., 2002, Gold deposits of the Carlin trend: Nevada

Bureau of Mines and Geology Bulletin 111, 204 p.

Tosdal, R.M., Wooden, J.L., and Kistler, R.W., 2000, Geometry of the Neoproterozoic

continental break-up and implications for location of Nevada mineral belts: Geology and Ore

Deposits 2000: The Great Basin and Beyond Symposium, Geological Society of Nevada,

Reno-Sparks, Nevada, May 15-18, 2000, Proceedings, p. 451-466.

Tretbar, D., Arehart, G.B., and Christensen, J.N., 2000, Dating gold deposition in a Carlin-type

gold deposit, using Rb/Sr methods on the mineral galkahaite: Geology, v. 28, p. 947-950.

Wells, J.D. and Mullens, T.E., 1973, Gold-bearing arsenian pyrite determined by microprobe

analysis, Cortez and Carlin gold mines, Nevada: Economic Geology, v. 68, p. 187-201.

16

Wooden, J.L, Kistler, R.W., and Tosdal, R.M., 1998, Pb isotopic mapping of crustal structure in

the northern Great Basin and relationships to Au deposit trends: U.S. Geological Survey

Open-File Report 98-338, p. 20-33.

Ye, Z., Kesler, S.E., Essene, E.J., Zohar, P.B., and Borhauer, J.L., 2003, Relation of Carlin-type

gold mineralization to lithology, structure and alteration: Screamer zone, Betze-Post deposit,

Nevada: Mineralium Deposita, v. 38, p. 22-38.

Yigit, O. and Hofstra, A.H., 2003, Lithogeochemistry of Carlin-type gold mineralization in the

Gold Bar district, Battle Mountain-Eureka Trend, Nevada: Ore Geology Reviews v. 22, p.

201-224.

17

Chapter 2 MINERAL PARAGENESIS, ALTERATION AND GEOCHEMISTRY OF CARLIN-TYPE GOLD DEPOSITS IN THE SOUTHERN PART

OF THE GOLDSTRIKE PROPERTY, NORTHERN NEVADA: IMPLICATIONS FOR SOURCES OF ORE-FORMING ELEMENTS,

ORE GENESIS AND MINERAL EXPLORATION

2.1 Abstract

This study was undertaken to characterize the mineral paragenesis and metal zoning at

the property scale, evaluate the potential sources of ore-related metals, quantify the relationship

between intensity of alteration and gold grade, and propose a comprehensive genetic model for

the Carlin-type gold deposits at the southern part of the Goldstrike property, Nevada.

Mineralogy, textural relationships, whole-rock composition and spatial distribution of the

studied samples revealed two types of Au ore: Ore I and Ore II. The former, which is hosted by

the Roberts Mountains and Rodeo Creek formations, and the Wispy, Planar and Upper Mud units,

is the most abundant and widespread in the property and is characterized by intense hydrothermal

alteration (e.g., carbonate dissolution, silicification, and precipitation of pyrite) and high amounts

of trace elements (e.g., Ag, As, Ba, Cd, Cu, Hg, Mo, Ni, S, Sb, Se, Te, Tl and Zn). On the other

hand, Ore II, which is hosted in the Wispy, Planar and Soft Sediment Deformation units, is

mainly confined to the central-NNW portion of the Screamer deposit and is weakly altered with

low concentration of trace elements. Both Ores I and II contain similar average concentrations of

Au in whole-rock (14 and 19 g/t Au, respectively) and pyrite (460 and 430 ppm, respectively);

18

however, auriferous pyrite from Ore I is richer in trace elements (e.g., As, Hg, Sb, Se and Tl) than

Ore II, which contains higher amounts of Cu and W.

The sedimentary units are interpreted to be the major local source of Cd, Mo, Ni, U, V,

and Zn and minor As, Cu, Hg, and Se as denoted by the composition of the least altered samples

and diagenetic pyrite and sphalerite. In this study, Al2O3 and TiO2 are identified as one of the

most immobile compounds, and their distribution indicates a homogeneous source for the detrital

components in the sedimentary rocks. Isocon diagrams indicate Au, As, Cu, Hg, S, Sb, SiO2, and

Tl are added in most of the ore samples; however, C, CaO, LOI, MgO and Sr were lost in most

samples of Ore I and constant or slightly depleted in Ore II. Iron is added in mineralized samples

from the Roberts Mountains Formation and Wispy Unit, but immobile in ore samples of the

Planar unit and Rodeo Creek Formation. Potassium is either lost or immobile, U immobile, and

Ba, Cd, Mo, Se, V, Zn, and W either added or lost in most of the studied mineralized samples.

Among the ore-related trace elements, Tl best correlates with Au-grade (R2=0.69) and

shows some relationship with the calculated amount of pyrite (R2=0.49), indicating that Tl would

be the best element to vector towards high grade Carlin-type gold mineralization.

A fluid mixing model is proposed for the studied Carlin-type gold deposits, with one

fluid enriched in Au, S and trace elements and another enriched in Fe, as pyrite solubility is very

low in the low salinity Carlin-mineralizing fluids, and mass balance data indicate that both Fe and

S were introduced in most of the mineralized samples. The lack of a strong correlation of Au

grade, degree of alteration, mass change and trace element distribution suggests that the processes

related to the precipitation of Au mineralization and associated metals were complex and might

be controlled by sulfidation of Fe either by reaction with the host rock or fluid mixing (e.g.,

pyritization). Our results, integrated with available thermodynamic data for Au and ore-related

elements, lead us to suggest that the formation of Ore I occurred more proximal to the major

IV

19

mineralizing conduits as the hotter trace element-rich auriferous fluids interacted with an Fe-rich

fluid and the impure carbonate host rocks, intensely dissolving the carbonate rocks and

precipitating quartz and auriferous pyrite. As the fluids moved laterally throughout the favorable

host rocks, temperatures may have dropped and pH increased, leading to a decrease in the rate of

carbonate dissolution and in the solubility of most of the trace elements, favoring the formation of

Ore II. Significantly, the gold concentrations in whole rock and in pyrite are very similar in both

ore types, suggesting that cooler and less acidic conditions were still favorable for the

incorporation of gold in the structure of pyrite, even at lower concentrations of other trace

elements.

2.2 Introduction

The Goldstrike property is located in the northern part of the Carlin Trend, northeastern

Nevada, in the United States of America (Figure 2.1). It extends for 60 km along a NNW-trend,

and contains one of the largest (e.g., Betze-Post: ~ 1250 ton Au) and highest-grade (e.g., Meikle:

24.7 g/t Au) Carlin-type gold deposits ever discovered and mined. It comprises approximately

thirty percent of the total annual gold production in the Carlin trend (i.e., greater than 50 t Au:

Nevada Bureau of Mines and Geology, 2007).

Several studies have been carried out at the Goldstrike property, including ore-stage

mineral paragenesis at the Betze-Post deposit (Ferdock et al., 1997), characterization of the major

tectonic-deformational events in the Goldstrike property (Volk et al., 2001), mineralogical and

geochemical investigations of the Screamer deposit (Kesler et al., 2003; Ye et al., 2003), genesis

of the high-grade orebodies at the Meikle deposit (Emsbo et al., 2003), and others. However the

processes related to the formation of this giant Carlin-type deposit (Hofstra and Cline, 2000;

Muntean et al., 2004; Cline et al., 2005; and references therein), the role of magmatism (Ressel

20

Figure 2.1 Simplified geological map of the southern part of the Goldstrike property showing the location of the deposits, cross section lines, and analyzed samples (projected vertically to the surface), including least altered, barren and altered, Ore I and Ore II. UTM coordinated are given in feet.

21

and Henry, 2006; Ressel et al., 2000; and references therein), the possibility of pre-Eocene Au-

rich mineralizing events (Emsbo, 2000; Emsbo et al., 1999, 2000, 2003), and the overall sources

of metals and fluids are still the subjects of debate (Cline et al., 2005; and references therein).

This is in part due to the fact that little information is available about metal and alteration zoning

throughout the property and their relationship with rock units, structures, sulfide compositions

and paragenesis. Furthermore very few studies have investigated the sources of the metals

associated with the Carlin-type ore.

To further our understanding of the processes that formed these giant Carlin-type gold

deposits, we characterized the mineral assemblages that precipitated prior to, during and after the

gold deposition, investigated the metal zonation at the property scale, determined the elements

introduced and hosted in the various sedimentary rocks, and evaluated quantitatively the

relationship between intensity of alteration and gold grades. By integrating the various aspects of

this investigation, we assessed the factors that may have been crucial in concentrating an

enormous amount of gold in a restricted part of the Carlin trend and in a relatively short period of

time (42-36 Ma: Hofstra et al., 1999; Tretbar et al., 2000; Arehart et al., 2003).

2.3 Geologic Setting

2.3.1 Tectonic Evolution

The Goldstrike property is located in the Great Basin, at the northern end of the Carlin

Trend, near the inferred western margin of the Precambrian North American craton, as defined by

both stratigraphic and isotopic data (Cunningham, 1988; Tosdal et al., 2000; Grauch et al., 2003;

Cline et al., 2005; Emsbo et al., 2006; Lund, 2008). The long-lived and complex geological

history of the studied area is characterized by the establishment of a passive continental margin

22

during late Proterozoic to early Cambrian, followed by the deposition of Ordovician to Devonian

shallow carbonates and shales to the east, (e.g., Roberts Mountains, Popovich, and Rodeo Creek

formations, which host the Carlin-type ore), and Ordovician deep siliciclastics with minor

carbonate input to the west (e.g., Vinini Formation). The geometry of the basin and sediment

deposition may have been controlled by high-angle NNW- and NE-striking faults (Volk et al.,

2001). Emsbo (2000) and Emsbo et al. (1999, 2003) proposed that sedimentary exhalative Au-

bearing stratiform barite and base metal mineralization formed during sedimentation and

lithification of the Upper Mud Unit of the Popovich Formation during the late Devonian.

Subsequently, the region was affected by several magmatic-hydrothermal tectonic events which

are summarized below.

During the late Devonian-early Mississipian, the area was affected by the Antler orogeny,

which placed Ordovician-Devonian deep siliciclastic rocks over Ordovician-Devonian shallow

basin and platformal carbonate rocks along the Roberts Mountains Thrust (Roberts et al., 1967).

Contractional structures, which post-date the Antler orogeny and pre-date the emplacement of the

late Jurassic Goldstrike intrusion, may have formed during several events and therefore their

specific timing is somewhat uncertain. The earliest of these events (e.g., late Paleozoic Humboldt

orogeny: Bettles, 2002) may have generated the WNW-striking low-angle reverse faults (e.g.,

Dillon series) and folds of similar orientation (e.g., Betze anticline) (Volk et al., 2001; Bettles,

2002). These structures were later overprinted by NNW-trending anticlines (e.g., Post anticline),

moderately E- and W-dipping NNW-striking normal faults (e.g., Post and JB systems,

respectively) and moderate to steeply W-dipping NNE-striking reverse faults (e.g., Weird

systems) (Volk et al., 2001). At the Goldstrike property, the NNW-striking faults and folds and

the W-dipping NNE-striking faults are important local ore controls (Bettles, 2002).

23

Magmatic events at the Goldstrike property are documented to have occurred during the

late Jurassic and late Eocene (Emsbo et al., 1996; Mortensen et al., 2000; Ressel et al., 2000;

Ressel and Henry, 2006). The former comprises the intrusion of the dioritic Goldstrike intrusion

and diorite-granodiorite, rhyodacite and lamprophyre dikes and sills along high-angle NNW- and

NNE-striking faults and low-angle WNW- and NNW-striking faults. Late Eocene porphyritic

dacite, basaltic-andesite and rhyolite dikes intruded mainly along NNW-striking high-angle faults

and to a lesser extent along low-angle NNW- and WNW-striking structures (Volk et al., 2001;

Ressel and Henry, 2006). This later magmatic event is related to an E-W extension period and

reactivation of old structures that have affected the area and is interpreted to be coeval with the

formation of the Carlin mineralization (~ 42 to 36 Ma: Hofstra et al.; 1999; Tretbar et al., 2000;

Arehart et al., 2003; Ressel and Henry, 2006). Ressel and Henry (2006) suggest that the Eocene

dikes emanate from large concealed magma chambers that might represent the heat source which

has driven the Au-bearing hydrothermal fluids upwards.

Reactivation of the deep long-lived crustal structures associated with the Carlin trend,

which is supported by magnetotelluric data (Rodriguez, 1998), regional gravity surveys

(Hildenbrand et al., 2000), and Pb isotopes (Tosdal et al., 2000), might have played a significant

role in the formation of these deposits as these structures are believed to have controlled the

Paleozoic sedimentation, tectonism and several episodes of magmatism and hydrothermal activity

(Hofstra and Cline, 2000; Emsbo et al., 2006).

24

2.3.2 Lithological units and the major hosts of gold mineralization The distribution of the major rock types in the Goldstrike property is shown in the surface

geological map and selected cross sections (Figures 2.1 and 2.2A, B, respectively), and the

tectonic-stratigraphy is presented in Figure 2.3, and their characteristics are summarized below.

Sedimentary units. The autochthonous rocks (Figures 2.1-2.3) comprise Ordovician to

Devonian carbonate units and Devonian siliciclastic units (Bettles, 2002). The summary of their

depositional environment and major characteristics is presented in Table 2.1 and Figure 2.3, and

their mineralogical and textural relationships in Table 2.2. At the base of this sequence is the

Ordovician-Silurian Hanson Creek Formation (HCD), which is overlain unconformably by the

Silurian-Devonian Roberts Mountains Formation (LL). The Devonian Popovich Formation (Dp)

lies conformably above the Roberts Mountains Formation and comprises four units, from the

bottom to the top: Wispy (WS), Planar (PL), Soft Sediment Deformation (SD), and Upper Mud

(UM). It is conformably overlain by the Late Middle to late Devonian Rodeo Creek Formation

(RC), which represents the uppermost stratigraphic unit in the footwall of the Roberts Mountains

Thrust Fault in the area (Figures 2.1, 2.2). The major host rocks for Au mineralization at the

Goldstrike property are the Upper Roberts Mountains and the lower Popovich (e.g., Wispy and

Planar units) in Betze-Post, the lower Popovich Formation (e.g., Wispy and Planar units) in

Screamer, and the Wispy and Upper Mud units and Rodeo Creek Formation in Rodeo.

Allochthonous units are characterized by the Ordovician Vinini Formation (OV), and locally by

the Silurian Elder Sandstone and the Devonian Slaven formations. These sedimentary rocks occur

above the Roberts Mountains Thrust and commonly exhibit shearing and brecciated tectonic-

deformational features. These units are noncomformably overlain by Miocene rhyolite flows and

volcanoclastic rocks of the Carlin Formation (Bettles, 2002).

25

Figure 2.2 Schematic geological SW-NE cross sections in the southern part of the Goldstrike Property. (A) Annick 2: Screamer and Betze-Post deposits). (B) Annick 4: North Screamer and Rodeo deposits, showing the lithological contacts, major faults, and location of the samples analyzed. (Geological interpretations were provided by Barrick Goldstrike mine staff).

26

Figure 2.3 Simplified tectono-stratigraphic column for the Goldstrike property showing the position of the gold deposits (e.g., Betze-Post, Screamer and Rodeo) relative to stratigraphic units. Modified from Volk et al. (2001).

27

Table 2.1. Sedimentary Units and Their Depositional Environments, Goldstrike Property, Nevada.

Geological Units Depositional Environment1 Lithology1,2,3

Ordovician Vinini Fm.

Marine basinal Mudstone, siltstone, chert, sandstone with minor limestone and marine basinal flows.

Devonian Rodeo Creek Fm (RC)

Anoxic basin

Interbeds of thinly-bedded siliceous mudstone to local argillite, sandy siltstone to fine sandstone, with minor silty to muddy limestone.

Upper Mud Unit (UM)

Anoxic Fine-grained, finely plane-bedded carbonaceous calcareous mudstone to muddy limestone, with minor thin fossil hash beds and debris flows.

Soft Sediment Deformation Unit (SD)

Upper slope/shelf

Thin to thick bedded, finely laminated micritic to lime mudstone and muddy limestone, locally bioclastic beds. It is characterized by slump and slide syn-sedimentary deformational structures.

Planar Unit (PL)

Transition oxygenated to anoxic deep-water (e.g,. graptolites: Monograptus sp.)

Planar bedded carbonaceous muddy limestone and calcareous mudstone, with minor interbedded thin fossil-rich layers. The upper zone contains well-preserved graptolites, an indication of slow to non-deposition due to the rapid sea level rise and basin starvation. D

evo

nia

n Po

pov

ich

Fm

Wispy Unit. (WS)

Progressively deeping-basin, from foreslope to basin euxinic conditions.

Oxygenated conditions (e.g. Wispy laminations)

Wispy (burrowed-bioturbated laminations) laminated muddy to silty limestone, locally mudstone with debris flows and fossiliferous limestone beds

Silurian-Devonian Roberts Mountains Fm. (LL)

Anoxic, deep water shelf or slope to basin, in a tectonically stable environment

Finely laminated sandy limestone to calcareous siltstone, grading to carbonate packstone to mudstone, locally dolomitic, with minor interbedded chert. The upper zone is fossiliferous-rich limestone to dolomitic limestone (e.g. echinoderms and brachiopods)

Ordovician-Silurian Hanson Creek (HCD)

Shallow water, as the final stage of an upward shoaling sequence

Sandy to massive dolostone, locally interbedded with minor limestone.

1 Armstrong et. al. (1998), 2 Zohar (pers. comm.), 3 This study.

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Table 2.2 Mineralogy and Textures of Least Altered Samples of the Popovich and Roberts Mountains Formations in the Southern Part of the Goldstrike Property, Northern Nevada

Geological Units Mineralogy and Textural Relationship Upper Mud Unit (UM)

Finely laminated and extremely fine-grained micritic groundmass associated with carbonaceous-rich material and intraclasts of anhedral and irregular silty-size quartz grains. Pyrite occurs commonly as subhedral to euhedral (e.g. cubic) isolated grains along bedding and minor fine-grained anhedral aggregates concentrated in discontinuous layers. Sphalerite occurs as fine-grained (up to 20 m) anhedral grains parallel to bedding planes.

Soft Sediment Deformation Unit (SD)

Micritic groundmass is characterized by microcrystalline carbonate aggregates associated with minor carbonaceous material and minor angular to elongated detrital quartz and traces of fine-grained white mica. Fine-grained subhedral to euhedral diagenetic pyrite occurs disseminated in micritic groundmass along bedding planes. Traces of sphalerite are commonly associated with carbonaceous-rich discontinuous lenses.

Planar Unit(PL)

Thick carbonaceous-rich layers alternate with thin layers of coarser-grained subhedral carbonate crystals, minor sub-angular quartz and traces of fine-grained white mica. Micritic groundmass commonly exhibits diagenetic dissolution features (e.g. micro-vugs), which are usually lined with extremely fine-grained carbonate. Carbonaceous material is concentrated along bedding in discontinuous lenses and commonly fills stylolite structures. Diagenetic euhedral to subhedral pyrite occurs as isolated grains disseminated along bedding planes and in the micritic groundmass.

Dev

on

ian

Po

pov

ich

Form

atio

n

Wispy Unit. (WS)

Alternating layers of coarse-grained carbonates and carbonaceous-rich laminations. Trace amounts of angular to rounded quartz grains and fine-grained white mica along bedding and disseminated in carbonaceous laminations. Diagenetic pyrite occurs as subhedral to euhedral grains along bedding planes, commonly associated with the carbonaceous laminations. Traces of anhedral chalcopyrite and pyrrhotite are included in subhedral pyrite grains.

Silurian-Devonian Roberts Mountains Fm. (LL)

Anhedral silt-sized quartz grains, minor euhedral carbonate crystals (up to 50 m) and traces of fine-grained white mica along bedding planes in a fine-grained micritic groundmass. Trace of fine-grained diagenetic euhedral to subhedral pyrite and traces of sphalerite along bedding planes.

29

Intrusive rocks. The area has been affected by two major episodes of magmatism during

the Jurassic and Eocene. The late Jurassic (158-157 Ma: Arehart et al. 1993; Mortensen et al,

2000; Ressel et al., 2000; Ressel and Henry, 2006) magmatic event includes the massive sill-like

Goldstrike intrusion and diorite to granodiorite, rhyodacite, and lamprophyre dikes. The

Goldstrike intrusion is located in the southern part of the property and ranges in composition from

gabbro-diorite to granodiorite. The diorite, granodiorite and rhyodacite dikes are controlled by

several fault systems throughout the property and are commonly altered to quartz-muscovite-

pyrite arsenopyrite and locally host auriferous polymetallic veins, interpreted to be formed

during the Jurassic by Emsbo et al., (2000). The lamprophyre dikes occur mainly along NNW-

striking high-angle faults or as sills along bedding and formation contacts (Bettles, 2002). The

carbonate and siliciclastic rocks in contact mainly with the Goldstrike intrusion and, to a smaller

degree along the dikes, are metamorphosed to marble and calc-silicate hornfels, respectively.

Locally, high-grade Au mineralization (up to 27.02 g/t Au: this study) is hosted in some of the

Jurassic sills and dikes, representing a minor volume of the Goldstrike ore. However, gold

mineralization is widespread in the sedimentary units at the margins of the altered and brecciated

Goldstrike intrusion. The rhyodacite dikes host low to moderate Au mineralization adjacent to the

Post fault, between the Griffin and Banshee deposits. Jurassic lamprophyre dikes are locally

mineralized throughout the property and are an important Au-host in the Meikle (Emsbo et al.,

2003), South Meikle and Banshee deposits.

The late Eocene (40.1-37.3 Ma: Ressel et al., 2000; Ressel and Henry, 2006) porphyritic

dacite, basaltic andesite, and rhyolite dikes occur along the Post fault zone and are coeval with the

mineralization. Locally, in the Deep-Post deposit these dikes are strongly fractured and host some

Au mineralization.

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2.4 Methodology

2.4.1 Sampling and Analytical Methods

About 450 drill core and pit samples were collected for this study during three field

seasons in the southern part of the Goldstrike property, and include the Screamer, Betze-Post and

Rodeo deposits. Drill hole sampling was concentrated along six major section lines (e.g.,

Leonardson, Annick 1 to Annick 5) (Figures 2.1, and 2.2A, B), and samples were collected at a

maximum of 100 m spacing from these sections. Samples of the four units of the Popovich

Formation (e.g., Upper Mud, Soft Sediment Deformation, Planar and Wispy) were also collected

in the Betze-Post open pit. The selection of the sections was done in collaboration with Barrick

Goldstrike geologists and was based on lithological boundaries, major structures, ore contours,

and their spatial distribution. Samples comprise rocks from each of the autochthonous

sedimentary units with various degrees of alteration and gold grades (waste to high-grade ore).

Petrographic descriptions of representative samples from the lower plate sedimentary units, with

several degrees of alteration and ore-grades, were carried out. Key samples were characterized

further using scanning electron microscopy (SEM) and electron microprobe (EMP). The EMP

methodology and detection limits are summarized in Appendices A, A.1 and A.2. The

classification of Au-barren pyrite is based on the electron microprobe detection limit for analyses

performed at McGill University (70 ppm Au). The characterization of early-, syn-ore and late- to

post-ore mineral assemblages (i.e., paragenetic sequence) was conducted based on the

identification of Au-bearing iron sulfide using electron micropobe analyses and textural

relationships.

Whole rock geochemical analyses were carried out on 176 samples by Acme Analytical

Laboratories Ltd. in Canada. Major, minor, and trace elements, and precious metal contents were

analyzed in all sets. Platinum and Pd were analyzed only in a few samples and F was analyzed in

31

selected samples. Descriptions of the analytical methods and detection limits are in Appendix B

and selected whole-rock data in Appendix C.

2.4.2 Data Analysis

Lithogeochemical data were evaluated using a combination of statistical, spatial, and

factor analyses. Intervals for statistical and spatial purposes were determined using the Natural

Breaks classification method (9.1 ArcView GIS: Environmental Systems Research Institute:

ESRI) which has most successfully characterized the Goldstrike dataset. Intervals for ore-grade

were determined using the cut-off (1 g/t Au) and high-grade Au mineralization (10 g/t Au) values

established by the Barrick Goldstrike mine staff.

Isocon diagrams were constructed using the method described by Grant (1986, 2005) to

quantify the changes in mass and concentration of selected elements in altered rocks relative to

their fresh protoliths (or the least altered sample, when fresh samples are not available).

Factor analysis was conducted using 1.8 Statistical Power (2007) for MS EXCEL to

identify element associations in the whole-rock data, based on their mutual linear correlation

coefficients. These correlation coefficients may be explained by a specific geological process

(e.g., influence of host rock signature and/or the various hydrothermal events). The elements that

were below or at the detection limit (e.g., Ag, Be, Bi, Cr2O3, MnO, Na2O, P2O5 and Te), as well

as REE and F were not used in the factor analysis.

The isocon diagrams and factor analyses in this study were conducted using the same

approach as Hofstra and Cline (2000), Cail and Cline (2001), Emsbo et al. (2003), and Yigit and

Hofstra (2003) to allow for comparison with their results.

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2.5 Mineral Paragenesis

The mineralogical composition and textural relationships in the sedimentary rocks of the

Goldstrike property reflect the long and complex tectonic evolution of the area, including their

diagenesis, metasomatic metamorphism during the emplacement of the Jurassic Goldstrike

intrusion and associated dikes, and hydrothermal activity prior to, during and after deposition of

auriferous pyrite in the Eocene. To establish the paragenetic sequence (Figure 2.4) and

understand the multiple alteration processes that took place in the southern part of the Goldstrike

property, selected samples from the lower plate sedimentary rocks with various degrees of

alteration and ore-grades were investigated. The relevant textural relationships are shown in

Figures 2.5A-P and summarized below.

2.5.1 Paleozoic diagenesis and Jurrassic metasomatism

In the least altered rocks, thickly laminated to massive carbonate rocks comprise

carbonate rhombs intergrown with minor sub-angular to rounded silt-sized detrital quartz grains

and detrital mica flakes and a diagenetic K-bearing clay mineral along bedding planes, in an

extremely fine grained micritic groundmass (Figure 2.5A). Carbonaceous material is associated

with the micritic groundmass and is concentrated along bedding planes in the Popovich

Formation (Figure 2.5A). Dissolution features, probably related to diagenesis of the sedimentary

rocks, are also observed either as micro vugs in the micritic groundmass (Figure 2.5A) that are

commonly rimmed by fine-grained euhedral carbonate crystals, or as stylolite structures parallel

to laminations (Figure 2.5B). The latter contain high proportions of organic and detrital material.

In general, diagenetic pyrite occurs as fine-grained framboidal grains and fine to coarse-grained

subhedral to euhedral (e.g., cubic) crystals (Figure 2.5B). It is found mostly as disseminated

grains along bedding planes and less commonly as aggregates in discontinuous layers in the

33

Figure 2.4 Paragenetic sequence for the sedimentary rocks from the Goldstrike property, including diagenesis, metassomatism, and Carlin to post-Carlin hydrothermal events. The bold and thin lines represent the major and minor phases, respectively. The discontinuous lines indicates uncertainty in the determination of the paragenetic sequence due to lack of clear textural relationship.