Geological setting, geochemistry and genesis of the Sepon gold … · 2016-08-03 · deposits...

Geological setting, geochemistry and genesis of

the Sepon gold and copper deposits, Laos

by

Paul W. Cromie

B.Sc. (Hons.), M.Sc. (Econ. Geol.)

A thesis submitted in fulfilment of the requirements for the

degree of Doctor of Philosophy

CODES ARC Centre of Excellence in Ore Deposits

University of Tasmania (UTAS), Australia

June 2010

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ABSTRACT

This study documents the geology, mineralogy, geochronology and geochemistry of the

Sepon Mineral District (SMD) gold and copper deposits. The SMD is located in the Sepon Basin

along the Truong Son Fold Belt on the NE margins of the Indochina Terrane in south-eastern Laos.

The geology of the SMD is dominated by Ordovician, Silurian and Devonian-aged continental

fluvial and shallow to deep marine sedimentary rocks that were deposited in a half graben basin.

Intrusion of rhyodacite porphyry (RDP) mainly occurred along pre-existing faults during the Early

Permian, constrained by U-Pb dating of zircons to between 2806 and 2977 Ma.

Three main hypogene mineralisation styles are recognised in the SMD, comprising distal

sedimentary-rock hosted gold (SHGD), proximal skarn (Cu+Au) and central porphyry (Cu-Mo).

Exploration programs in the SMD conducted by CRA/RioTinto (1993-1999), Oxiana Limited/OZ

Minerals Limited (2000-2008) resulted in the discovery of a mineral district containing resources of

83 Mt @ 1.8 g/t Au for 4.75 million ounces of gold in seven separate but adjacent SHGD, and

supergene copper at three deposits, namely the Khanong (27 Mt @ 4.3 % Cu), Thengkham North

(11.4 Mt @ 2.7 % Cu) and Thengkham South (9.8 Mt @ 2.3 % Cu) deposits.

Gold in the SMD is predominantly hosted by Devonian Discovery Formation calcareous

shale with lesser amounts reported in turn for the underlying Devonian Nalou Formation bioclastic

dolomite, the Silurian-Devonian Kengkeuk Formation calcareous shale and the Ordovician-Silurian

Nampa Formation claystone and siltstone sequence. The Nalou Formation bioclastic dolomite and

the Kengkeuk Formation calcareous shale mainly host the known SMD copper deposits.

Hypogene gold and copper ore-types occurring in the SMD deposits are epigenetic and

often occur along steep faults and/or veins cutting all of the Ordovician to Middle Devonian aged

carbonate and siliciclastic rocks and Early Permian RDP dykes and sills. The principal structural

trends controlling and hosting gold mineralisation in the SMD SHGD comprise WNW-striking

normal faults with steep dips and high-angle ENE-striking normal and reverse faults, cutting all of

the Ordovician to Devonian-aged carbonate and siliciclastic rocks and Early Permian RDP dykes

and sills. Gold ore zone geometries in the SMD include (1) ribbon-like bodies that are strike

continuous, moderate to shallow dipping sheets that are not always connected to faults, and fault

controlled steep sheet-like bodies.

Common major sulphide minerals in the SMD gold and copper deposits include, pyrite,

arsenic-rich pyrite, chalcopyrite, and minor sphalerite, galena, bornite and stibnite, but no realgar,

orpiment or cinnabar have been observed. Alteration types occurring in the SMD gold deposits

include (a) variable decarbonatisation of carbonate units, (b) silicification (jasperoid formation)

along permeable horizons and faults, (c) locally developed argillisation along RDP contacts, and (d)

variable dolomitsation of carbonate units. Primary sulphide zones hosting copper mineralisation in

skarns proximal to RDP intrusions are associated with sericite alteration along intrusion margins, in

addition to (a) early prograde garnet and pyroxene skarn alteration cut by later retrograde chlorite-

epidote alteration of carbonate host units, and (b) hornfels in non-calcareous sedimentary rocks.

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At least 5 main paragenetic mineral assemblage stages were observed across the SMD; they

are collectively grouped together as SMD Stages 1 to 5 based on the gold stage paragenetic

observations from this study: (a) the adjacent to distal type SHGD, (b) the proximal Cu (-Au) skarn

deposits underlying supergene copper, and (c) the porphyry Cu (-Mo) deposits. The pre-main gold

ore stages include SMD Stages 1, 2 and 3A. Main gold ore comprises SMD Stages 3B, 3C, 4A and

4B, and SMD Stage 5 is post-main gold deposition.

The SMD Stage 1 assemblage is interpreted to form early in the diagenesis of the Sepon

Basin (i.e. syn- to post-sedimentation); it does not contain any known gold and mainly comprises

rare disseminations of framboidal pyrite (Pyrite 1) hosted by calcareous shale (CSH) rocks, with

minor ferroan dolomite occurring along cleavage and stylolites in the CSH rocks, which are in turn

cut by small late stage milky white calcite veins.

The SMD Stage 2 consists of three types of gold-poor diagenetic pyrite, comprising:

(a) semi-massive nodular-shaped pyrite (Pyrite 2A); (b) euhedral spongy-textured pyrite (Pyrite

2B), and (c) euhedral angular pyrite (Pyrite 2C). These pyrite types contain very low levels of gold,

generally <0.3 ppm Au in the cores and <0.1 ppm in the rims. Characteristic pink calcite (Calcite 2)

filled fractures cutting SMD Stage 2 pyrite and also cuts both cleavages and stylolites containing

Stage 1 ferroan dolomite. The SMD Stage 2 is interpreted to form post-sedimentation and late in the

diagenesis of the Sepon Basin.

The SMD Stage 3 represents the combined main base metal dominant group of

assemblages in the SMD and consists of three sub-stages, namely: (a) the SMD Stage 3A: early

carbonate-hosted pyrite-galena-sphalerite-dolomite veins (i.e. in the SMD SHGD), followed by (b)

the SMD Stage 3B: low grade gold-bearing RDP intrusion-hosted early retrograde veins with

pyrite-sphalerite-galena-quartz-dolomite (SMD SHGD and Cu skarn deposits), and (c) the SMD

Stage 3C: late low grade gold-bearing RDP intrusion-hosted retrograde veins with quartz-

chalcopyrite+bornite+molybdenite (SMD Cu skarn and porphyry Cu-Mo deposits). The SMD Stage

3B mineral assemblage contains low grades of gold ranging from 0.13 to <3 ppm Au that mainly

occur in veins containing pyrite and sphalerite, which were observed in all three deposit types,

namely: (a) central porphyry (in Pyrite 3B), (b) proximal skarn (in Pyrite SKN1 and sphalerite), and

(c) distal SHGD (in Pyrite 3B1 and sphalerite). SMD Stage 3C typically comprises chalcopyrite

with inclusions containing low grade gold ranging from 0.06 to 0.9 ppm Au in the central porphyry

and proximal skarn settings.

The SMD Stage 4 is the main high grade gold phase in the SMD. At least two sub-stages

have been observed, namely: (a) the SMD Stage 4A pyrite comprising high grades of gold

concentrated along (i) the growth rims of pyrite cores that also occur along fractures cutting SMD

Stage 3 sulphides in the SMD SHGD, or (ii) associated with rough-textured pyrite cutting SKN

Stage 3C assemblages in the SMD copper deposits, and (b) the SMD Stage 4B Hg-Au telluride

filling fractures in the SMD Stage 3 and SMD Stage 4A sulphides. Both the SMD Stage 4A pyrite

types observed in the SMD SHGD and the SMD proximal copper skarn deposits contained gold

values ranging from >1 and up to 293 ppm Au.

The post-main high grade gold stage assemblages are grouped into SMD Stage 5 and

comprises at least three vein assemblages, namely: (a) the SMD Stage 5A: quartz-stibnite-dolomite,

(b) the SMD Stage 5B: quartz-dolomite, and (c) the SMD Stage 5C: calcite-quartz-fluorite calcite-

quartz. No gold grades were observed in the SMD Stage 5 sulphides.

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The timing of the Cu-Mo mineralisation at SMD is constrained by Re-Os dating of Stage

3C molybdenite which is coeval with Stage 3C chalcopyrite to be between 2871 and 2801 Ma at

the Thengkham South deposit and at Padan Prospect. The absolute timing of the SMD gold

mineralization is not well constrained but the distal nature of the SMD Stage 4A main gold ore to

Pyrite SKN1 suggest that the age of the SMD gold mineralization is broadly similar or slightly

younger than the SMD skarn (Cu+Au) formation.

Detailed textural, paragenetic and LA-ICPMS trace element investigations of sulphides

indicate that within the SMD SHGD, primary gold is not visible, but is associated with pyrite and

occurs in: (1) SMD Stage 2B Pyrite 2B (diagenetic), which contains <0.5 ppm Au in pyrite cores,

(2) SMD Stage 3B Pyrite 3B (in base metal veins), which contains <0.3 to 3 ppm Au as inclusions,

(3) SMD Stage 4A Pyrite 4A, which contains >3 to 200 ppm Au as overgrowths on As-rich rims,

and (4) SMD Stage 4B sulphosalts (in veins), which contain Hg-Au-telluride. The later high grades

of gold in Pyrite 4A could have been derived from the early SMD Stage 2 diagenetic pyrites in the

SMD and/or from the later SMD Stages 3B and 3C pyrite types during syn- to post-emplacement of

the SMD RDP intrusions.

Gold in the SMD copper deposits is not visible, it is within pyrite and occurs in: (1) SMD

Stage 3B Pyrite SKN1 (in retrograde veins), which contains <0.7 ppm Au as inclusions, (2) SMD

Stage 3C Chalcopyrite 3C (in retrograde veins), which contains <0.9 ppm Au as inclusions, and

(3) SMD Stage 4A Pyrite SKN2 (fractures), which contains >1 to 293 ppm Au in As-rich pyrite.

Gold deposited in the SMD Stage 3B and 3C pyrites syn- to post-emplacement of the SMD RDP

intrusions provides potential gold sources for the high grade gold formed in Pyrite SKN2.

Both the LA-ICPMS and PIXE NMP analytical methods established that the pyrite types in

the distal SMD SHGD comprise the following trace element signatures: (a) SMD Stage 2B Pyrite

2B (pre-main gold ore): Pb-Ni-Co-As-Ti, (b) SMD Stage 3A Pyrite 3A (pre-main gold ore): As-Cu-

Ni-Pb, and (c) SMD Stage 4A Pyrite 4A1 to 4A4 (main gold ore): Au-As-Sb-Pb-Cu-Ni-Ti-Zn-Mn-

Ag-Tl. In contrast, pyrite types from the proximal skarn mineral assemblages in the SMD copper

deposits contain the following trace element signatures: (a) SMD Stage 3B Pyrite SKN1 (pre-main

gold ore): Cu-Zn-Co-Se-Ni-Bi-Mn-TeAu, (b) SMD Stage 3C Chalcopyrite 3C (pre-main gold

ore): Cu-Zn-Se-Bi+Au, and (c) SMD Stage 4A Pyrite SKN2 (main gold ore): Au-Cu-As-Bi-Co-Se-

Pb-Zn-Ag. The presence of Cu-Zn-Se-Bi in primary copper ore stage Chalcopyrite 3C (SMD Stage

3C) probably implies derivation from magmatic sources during RDP intrusion emplacement.

The SMD Stage 2B diagenetic pyrite yielded a wide range of 34S values from -11.6 to

+33‰ and these results are comparable to the known ranges of 34S values for sedimentary pyrite

types in other Carlin-type SHGD in Nevada, USA and China. In contrast, the SMD Stage 3 group

base metal sulphides have 34S compositions that are more similar to those derived from deep

magmatic or metamorphic sources centred at 05‰. Furthermore, the light 34S values for SMD

Stage 4A pyrite (Pyrite 4A) ranging from -28.9 to 2.5‰ are comparable to 34S values reported for

(a) late gold ore stage pyrite in the Nevada Carlin-type gold depositsand (b) main gold ore stage

pyrite in the Chinese Carlin-type SHGD. However, the post-main gold ore SMD Stage 5 stibnite has

34S values more similar to magmatic systems.

Fluid inclusion studies combined with textural cathodoluminescence investigation

indicate that main primary copper ore stage quartz (SMD Stage 3C) yielded minimum

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homogenisation temperatures ranging from 175 to 260oC. The presence of primary hematite

with this mineral assemblage suggests oxidising fluid conditions during mineralisation, thereby

possibly enabling copper to be transported at these lower temperatures. The quartz from the

main primary copper ore (SMD Stage 3C) yielded moderate salinity values ranging from 7.7 to

11.2 wt % NaCl equiv. Bi-variate plots of the SMD 18O and D compositions from the SMD

Stages 3B and 3C pre-main gold ore assemblages have ore fluid characteristics similar to

metamorphic and magmatic waters. Overall, the SMD Stage 3C ore fluids in quartz associated

with Chalcopyrite 3C are probably derived from magmatic sources considering (a) their close

relationship with the SMD RDP intrusions, (b) their isothermal mixing characteristics, and

(c) the presence of CO2.

The main gold ore SMD Stage 4A quartz yielded homogenisation temperature (Th)

values ranging from 180 to 290oC; these results are similar to Th values in the distal

disseminated gold deposit (DDGD) types, but are hotter than those in typical Nevada Carlin-

type deposits. Salinity values from the main gold ore SMD Stage 4A quartz yielded two groups

at 4.3 to 8.6 wt % NaCl equiv. and 12.1 to 13.7 wt % NaCl equiv., indicating the possible

presence of two fluids. Additionally, the SMD Stage 4A main-gold ore fluids in quartz yielded

calculated 18O values ranging from 9.3 to 13.8 ‰, which are in the range for metamorphic and

magmatic waters. Furthermore, surface fluid dilution trends confirmed the presence of at least

two saline fluids present in fluid inclusions from the SMD Stage 4A quartz associated with main

gold stage Pyrite 4A, indicating the involvement of ore fluids that are similar to (1) evolved

meteoric fluids for those with low-moderate salinities <9 wt % NaCl equiv., and (2) magmatic

fluids comprising moderate-high salinities >9 wt % NaCl equiv., although involvement of

amagmatic (i.e. metamorphic) fluids and/or connate brines cannot be ruled out.

All of the RDP, galena and pyrite Pb isotope results from the SMD imply a crustal Pb

source. The pre-main gold ore-stage mineralisation shows evidence for at least two distinct Pb

isotope sources, comprising (a) less radiogenic Pb in SMD Stage 3A galena associated with a

paragenetically early carbonate-hosted style of base metal mineralisation mainly hosted by

Devonian Nalou Formation bioclastic dolomite, and (b) paragenetically later but also more

radiogenic Pb occurring in SMD Stage 3B galena and SMD RDP samples that are hosted by

younger Early Permian RDP intrusions. However, The main gold ore-stage Pyrites 4A (SMD Stage

4A) contain a wide range of Pb isotopic sources that are interpreted to be derived from both (a)

early less radiogenic sedimentary Pb sources and (b) later more radiogenic Pb from RDP intrusions.

The overall geological setting, mineral assemblages and geochemistry of the SMD SHGD

share several broad characteristics with those in the Nevada (USA) and Dian-Qian-Gui (Southern

China) Carlin-type gold deposits and also some with the Nevada DDGD-types. In conclusion,

geochemical data from the SMD gold and copper deposits suggests a genetic model involving fluid

mixing processes, comprising (1) early magmatic and minor associated metamorphic fluids, mainly

contributing Cu, Au, Pb and S during the formation of the SMD copper skarn and porphyry deposits

(SMD Stage 3C), and (2) later interaction of the early deep magmatic and minor associated

metamorphic fluids with circulating evolved meteoric fluids, possibly also sourcing S, Pb, Au and

associated trace elements from sedimentary rocks for the gold deposits (SMD Stage 4A).

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ACKNOWLEDGEMENTS

First and foremost, I wish to thank my wife, Hongbing (Ellen) and son Aaron for all of

their support, encouragement and love throughout the journey during my Ph.D. I’m eternally

grateful for their patience and commitment for providing me with the opportunity to pursue my

goal whilst putting things on hold during my studies, especially the career opportunities,

holidays and our family time together.

Throughout my Ph.D. I was also given the great opportunity to travel to Southeast Asia

and meet numerous people, who have contributed in more ways than one to the successful

completion of the project. In particular, I would like to thank my principal supervisor Khin Zaw

for recommending an interesting project and also for the excellent research discussions,

technical guidance, support and encouragement throughout my studies. Many thanks also to his

family for their support and kind hospitality whilst we were based in Hobart. I’m also grateful

to my co-supervisors Noel White and David Cooke for their mentorship, interesting ideas,

guidance, and critical reading of thesis chapters.

This project would not have been possible if it were not for the principal financial

support for research and logistics initially provided by Oxiana Limited and then continued by

the subsequent owners of the Sepon Project by OZ Minerals Limited and MMG. Many thanks

to Tony Manini who approved and implemented Oxiana’s commitment to the project and also

for his time to discuss and share the knowledge about Sepon. Thanks also to Stuart Smith for

his technical support and guidance on behalf of the companies managing Sepon. Furthermore,

thanks to the numerous people from Oxiana, OZ Minerals and MMG who assisted my project,

including: Dan Olberg, Mark Allen, James Patterson, Steve Ryan, Doug Morris, James Cannell,

Mark Lindsay, Michael Feldman, Terry McMahon, Rob Curtis, Mike Carter, Chris Gertisen,

Craig Michael, Henry Agupitan, Chantone, Khampone, Bounoume, Thongmeuane, Kingkham,

Eda, Aris Lupis and Viengxay.

Many thanks also to the CSIRO who provided funding to me through a top-up Post-

graduate Scholarship and access to the Proton Induced X-ray Emission (PIXE) and Nuclear

Microprobe (NMP). A special thanks to Chris Ryan for his supervision and assistance during

the PIXE trace element analyses at the University of Melbourne. I’m also grateful to the

Society of Economic Geologists who awarded me a student research grant during 2006 towards

the Re-Os dating of a single molybdenite sample. Hence, thanks to Holly Stein and the team at

the AIRIE, Department of Earth Resources at Colorado State University (CSU) for the

subsequent age dating of a Sepon molybdenite sample.

Additionally, a large number of people and groups are also thanked for assisting with

the research for this project, including: Jon Woodhead (University of Melbourne; LA-MC-

ICPMS Pb isotopes); Sebastian Meffre (CODES; pyrite LA-ICPMS Pb isotopes and LA-

ICPMS U-Pb age dating of zircons), Terry Mernagh (Geoscience Australia, Canberra; Laser

Raman Spectrometric analyses of fluid inclusions), Sue Golding (University of Queensland;

oxygen and hydrogen isotopes), Sarah Gilbert (CODES; pyrite LA-ICPMS trace element

investigations and solution Pb isotopes), Keith Harris (CSL; LA-sulphur isotopes),

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Christine Cook (CSL; conventional sulphur isotopes), Philip Robinson, Nilar Hlaing, and Kate

McGoldrick (CODES; whole rock XRF analyses), Karsten Gomann (CSL; electron microprobe

analyses), Simon Stephens and his team (UTAS; lapidary) and Milan Rosker (Exploration

Graphics, Melbourne; map and figure drafting).

I am also grateful for the opportunity to conduct my research at CODES, especially the

ability to discuss research in a team oriented environment. Hence, I would like to thank:

Ross Large, Bruce Gemmell, Jocelyn McPhie, Steve Walters, Tony Crawford, Leonid

Danyushevshy, Dima Kamenetsky, Peter McGoldrick, Ron Berry, Clive Burrett, Garry

Davidson, Sebastian Meffre, Anthony Harris, Zhaoshan Chang, Karin Orth, Rob Scott and

Wally Herman. Thanks also to the assistance from the CODES support staff, in particular Nilar

Hlaing, June Pongratz, Christine Higgins, Diane Stephens, Keith Dobson and Peter Cornish.

Many thanks also for the friendship, support and encouragement from my past and current

postgraduate colleagues, especially: Ben Jones, Singoyi Blackwell, Weerapan Srichan, Rod

Maier, Mawson Croaker, Bryan Bowden, Dave Braxton, Wallace Mackay, Steve Lewis,

Takayuki Manaka, Abhisit Salam, Claire McMahon, Terra Kamvong and Bronto Sutopo.

I would also like to take this opportunity to thank both Tony Manini and Steve Ryan

(Oxiana/OZ Minerals) and also Ian Sandl (Teck) who assisted me during the final stages of my

studies by providing the flexibility between work commitments to enable me the time to

complete the writing of this thesis.

A special thanks to my parents who have encouraged and helped my geological journey

during the last couple of decades and also my extended family and friends whom have all

supported me in many ways during my studies.

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CONTENTS ABSTRACT…………………………………………………………………………………..ii ACKNOWLEDGEMENTS…………………………………………………………………vi LIST OF FIGURES………………………………………………………………..……..xviii LIST OF TABLES…………………………………………………………………………xxvii

CHAPTER 1: INTRODUCTION…………………………………………………………….1 1.1 INTRODUCTION………………………………………………………………………1 1.2 GEOGRAPHY AND ACCESS…………………………………………………………2 1.3 SMD EXPLORATION HISTORY AND MINING DEVELOPMENT………………..4 1.4 GOLD AND COPPER RESOURCES IN THE SMD………………………………….7 1.5 PREVIOUS STUDIES………………………………………………………………….8 1.6 AIMS……………………………………………………………………………………9 1.7 RESEARCH METHODS……………………………………………………………….9

1.7.1 Field investigation methods…………………………………………………...10 1.7.2 Laboratory research methods…………………………………………………10

1.8 THESIS STRUCTURE AND CONVENTIONS……………………………………...11 CHAPTER 2: REGIONAL GEOLOGICAL SETTING…………………………………..12

2.1 INTRODUCTION……………………………………………………………………..12 2.2 TECTONIC SETTING………………………………………………………………...12

2.2.1 Principal tectonic components of Mainland Southeast Asia………………….12 2.2.2 Indochina Terrane……………………………………………………………..14

2.2.2.1 Kontum Massif………………………………………………………………………14 2.2.2.2 Truong Son Fold Belt…………………………………………………………..…..15 2.2.2.3 The Loei and Sukhothai Fold Belts…………………………………………...15

2.2.3 Tectonic evolution of the Indochina Terrane…………………………………16 2.2.4 Mineralisation epochs along the margins of the Indochina Terrane………….19

2.2.4.1 Early Permian mineralisation (300 - 250 Ma)…………………………………...19 2.2.4.2 Late Permian to Late Triassic (250 - 220 Ma)…………………………………...19 2.2.4.3 Late Triassic to Jurassic (220 - 200 Ma)………………………………...............20 2.2.4.4 Post Jurassic (<200 Ma)…………………………………………………...............20

2.3 REGIONAL GEOLOGY OF LAOS…………………………………………………..24 2.3.1 Precambrian and Phanerozoic metamorphic rocks…………………………...24 2.3.2 Palaeozoic sedimentary rocks………………………………………………...27

2.3.2.1 Cambrian………………………………………………...…………………....27 2.3.2.2 Ordovician……………………………………………...……………….…….27 2.3.2.3 Silurian …………………………………………………………...…………...27 2.3.2.4 Devonian………………………………………………………...…................27 2.3.2.5 Carboniferous…………………………………………………...…………….28

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2.3.2.6 Permian……………………...………………...……………………………...28 2.3.3 Mesozoic sedimentary rocks………………………………………………….29

2.3.3.1 Triassic………………………………….………………………………….....29 2.3.3.2 Jurassic to Cretaceous…………………………….…………………………..29 2.3.3.3 Cretaceous………………………………………………….…………………29

2.3.4 Cenozoic………………………………………………………………………30 2.3.5 Volcanic Activity……………………………………………………………..30 2.3.6 Igneous Intrusions…………………………………………………………….31 2.3.7 Regional structure of Laos……………………………………………………32

CHAPTER 3: DISTRICT-SCALE GEOLOGICAL SETTING OF THE SEPON MINERAL DISTRICT…………..………………………………………34

3.1 INTRODUCTION……………………………………………………………………..34 3.2 SEPON BASIN STRATIGRAPHY……...……………………………………………34

3.2.1 Palat Formation ……………………………………………………………….39 3.2.2 Payee Formation………………………………………………………………39 3.2.3 Houay Bang Formation.………………………………………………………39 3.2.4 Nampa Formation……………………………………………………………41 3.2.5 Vang Ngang Formation………………………………………………………41 3.2.6 Namphuc Volcanics…………………………………………………………41 3.2.7 Kengkeuk Formation………………………………………………………….42 3.2.8 Nalou Formation………………………………………………………………42 3.2.9 Discovery Formation…………………………………………………………44 3.2.11 Nan Kian Formation…………………………………………………………..45 3.2.12 Mesozoic Khorat Group ………………………………………………………45

3.3 SMD IGNEOUS ROCKS……………………………………………………………..46 3.3.1 Rhyodacite-porphyry…………………………………………………………46

3.3.1.1 Occurrence…………………………………………………………………………….46 3.3.1.2 RDP petrology………………………………………………………………………48 3.3.1.3 RDP whole rock geochemistry………………………………………………………50

3.3.2 Granite………………………………………………………………………...53

3.3.2.1 Occurrence…………………………………………………………………………….53 3.3.2.2 Granite petrology……………………………………………………………………54 3.3.2.3 Granite whole rock geochemistry…………………………………………………54

3.3.3 Mafic intrusions……………………………………………………………….57 3.4 GEOCHRONOLOGY OF SMD IGNEOUS ROCKS………………………………59

3.4.1 Introduction…………………………………………………………………...59 3.4.2 U-Pb analytical methodology used for SMD zircon geochronology………....59 3.4.3 Zircon petrology………………………………………………………………60 3.4.4 Geochronology results………………………………………………………62 3.4.5 SMD RDP geochronology results……………………………………………64 3.4.6 BSK granite geochronology results……………………………………….......65 3.4.7 SMD mafic dyke geochronology……………………………………………..66 3.4.8 Comparison of SMD zircon data to previous regional geochronology

Studies………………………………………………………………………...66

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3.5 DISTRICT-SCALE STRUCTURAL SETTING OF THE SMD…………………….67

3.5.1 Architecture of the Sepon Basin………………………………………………67 3.5.2 Major faults in the SMD………………………………………………………68 3.5.3 District-scale folding………………………………………………………….72 3.5.4 District-scale fault history summary…………………………………….........73

CHAPTER 4: DEPOSIT GEOLOGY………………………………………………..……..74

4.1 INTRODUCTION……………………………………………………………………..74 4.2 SMD SEDIMENTARY ROCK-HOSTED GOLD DEPOSITS……………………….76

4.2.1 Introduction…………………………………………………………………...76 4.2.2 Nalou gold deposit…………………………………………………………….76

4.2.2.1 Location and background……………………………………………………………76 4.2.2.2 Geology and structure………………………………………………………………77 4.2.2.3 Surface mineralisation……………………………………………………………….77

4.2.3 Discovery West gold deposit………………………………………………….83

4.2.3.1 Location and background……………………………………………………………83 4.2.3.2 Geology and structure………………………………………………………………83 4.2.3.3 Surface mineralisation………………………………………………………………87

4.2.4 Discovery Colluvial gold deposit……………………………………………..89

4.2.4.1 Location and background……………………………………………………………89 4.2.4.2 Geology and structure………………………………………………………………..89 4.2.4.3 Structure……………………………………………………………………………….91 4.2.4.4 Surface mineralisation……………………………………………………………….93

4.2.5 Discovery Main and Discovery East gold deposits…………………………...95

4.2.5.1 Location and background…………………………………………………………...95 4.2.5.2 Geology and structure………………………………………………………………..95 4.2.5.3 Surface mineralisation summary …………………………………………………...96

4.2.6 Namkok West and Namkok East gold deposits………………………………99

4.2.6.1 Location and background……………………………………………………………99 4.2.6.2 Geology and structure………………………………………………………………..99 4.2.6.3 Surface mineralisation……………………………………………………………...100

4.2.7 Vang Ngang gold deposit……………………………………………………102

4.2.7.1 Location and background…………………………………………………………102 4.2.7.2 Geology and structure………………………………………………………………103 4.2.7.3 Surface mineralisation ……………………………………………………………104

4.2.8 Phavat Gold-Copper Prospects………………………………………………105

4.2.8.1 Location and background………………………………………………………….105 4.2.8.2 Geological setting…………………………………………………………………...105 4.2.8.3 Mineralisation……………………………………………………………………….106

4.2.9 Nakachan Gold Prospect…………………………………………………….108

4.2.9.1 Location and background………………………………………………………….108 4.2.9.2 Geological setting…………………………………………………………………...108 4.2.9.3 Surface mineralisation……………………………………………………………...108

4.2.10 Discussion of SMD SHGD mineralisation…………………………………..110

4.2.10.1 Introduction………………………………………………………………………..110

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4.2.10.2 Host rocks………………………………………………………………………….110 4.2.10.3 Structural controls on gold deposit geometries………………………………...110 4.2.10.4 Surface trace-element associations…………………………………………….112 4.2.10.5 Mineralisation characteristics………………………………………………….113 4.2.10.6 Summary……………………………………………………………………………115

4.3 SMD COPPER DEPOSITS…………………………………………………………..116 4.3.1 Introduction………………………………………………………………….116 4.3.2 Khanong copper deposit ……………………………………………………117

4.3.2.1 Location and background……………………………………………………........117 4.3.2.2 Geology and structure………………………………………………………………117 4.3.2.3 Mineralisation……………………………………………………………………….119

4.3.3 Thengkham South copper deposit……………………...……………………124

4.3.3.1 Location and background………………………………………………………….124 4.3.3.2 Geology and structure………………………………………………………………125 4.3.3.3 Mineralisation……………………………………………………………………….127

4.3.4 Padan Prospect (Cu-Mo)…………………………………………………….130

4.3.4.1 Location and Background………………………………………………………….130 4.3.4.2 Geology and structure………………………………………………………………131 4.3.4.3 Mineralisation……………………………………………………………………….132

4.3.5 Discussion of the SMD copper deposits geology and mineralisation……….134

4.3.5.1 Introduction………………………………………………………………………….134 4.3.5.2 Host rocks…………………………………………………………………………….134 4.3.5.3 Structural controls…………………………………………………………………..134 4.3.5.4 Surface trace-element associations……………………………………………….136 4.3.5.5 Alteration……………………………………………………………………………..136 4.3.5.6 Mineralisation characteristics………………………………………………….....136 4.3.5.7 Summary……………………………………………………………………………...137

CHAPTER 5: MINERALOGY, TEXTURES AND PARAGENESIS ………………….138

5.1 INTRODUCTION……………………………………………………………………138 5.1.1 Previous petrology investigation …………………………………………….138 5.1.2 Petrology research methods………………………………………………….139

5.1.2.1 Methods used to investigate the nature of gold occurrence…………………...139 5.1.2.2 Carbonate mineral identification…………………………………………………139 5.1.2.3 Feldspar mineral identification using chemical stains…………………………140 5.1.2.4 Compositional variation investigations of sphalerite and garnet…………….140

5.2 SMD SHGD MINERALISATION AND PARAGENESIS…………………………141 5.2.1 Introduction………………………………………………………………….141 5.2.2 Stage 1: Framboidal pyrite + calcite + dolomite…………………………….143

5.2.2.1 Framboidal pyrite (Pyrite 1)………………………………………………………143 5.2.2.2 Calcite (Calcite 1)…………………………………………………………………..144 5.2.2.3 Dolomite (Dolomite 1)……………………………………………………………...144

5.2.3 Stage 2: Diagenetic pyrite-calcite………………………………………….146

5.2.3.1 Semi-massive nodular pyrite (Pyrite 2A) ……………………………………146 5.2.3.2 Euhedral spongy pyrite (Pyrite 2B)……………………………………………….146 5.2.3.3 Euhedral angular pyrite (Pyrite 2C)……………………………………………...146

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5.2.3.4 Calcite (Calcite 2)………………………………………………………………......148 5.2.4 Stage 3: Pyrite-sphalerite-galena + sulphosalts + dolomite + quartz +

sericite + Au…………………………………………………………………149

5.2.4.1 Stage 3A……………………………………………………………………..149 5.2.4.2 Stage 3B………………………………………………………………………………152 5.2.4.3 Stage 3C………………………………………………………………………………155

5.2.5 Stage 4: Disseminated pyrite-Au-quartz + telluride (Hg-Au)……………….157 5.2.5.1 Stage 4A: Quartz-disseminated pyrite-Au (high-grade)……………………….157 5.2.5.2 Stage 4B: Hg-Au telluride-quartz…………………………………………………165

5.2.6 Stage 5: Quartz-stibnite-calcite……………………………………………...166

5.2.6.1 Stage 5A: Stibnite-quartz-dolomite……………………………………………….166 5.2.6.2 Stage 5B: Quartz-dolomite…………………………………………………………167 5.2.6.3 Stage 5C: Calcite-quartz…………………………………………………………...167

5.2.7 SMD SHGD mineral paragenesis summary………………………………....168 5.3 SMD COPPER DEPOSIT MINERALISATION AND PARAGENESIS…………...169

5.3.1 Introduction………………………………………………………………….169 5.3.2 SKN Stage 1: Prograde skarn………………………………………………..172 5.3.3 SKN Stage 2: Retrograde chlorite-epidote-pyrite-sphalerite-

carbonate-quartz skarn………………………………………………………174

5.3.3.1 SKN Stage 2A………………………………………………………………………174 5.3.3.2 SKN Stage 2B………………………………………………………………………..175 5.3.3.3 SKN Stage 2C………………………………………………………………………..176

5.3.4 SKN Stage 3: Retrograde quartz-chalcopyrite-bornite- molybdenite -Au skarn………………………………………………………177

5.3.5 SKN Stage 4: Late retrograde pyrite-quartz-calcite-Au skarn…………........178 5.3.6 SKN Stage 5: Calcite-quartz-fluorite………………………………………..179 5.3.7 SMD copper deposits mineral paragenesis summary………………………..180

5.4 SMD PORPHYRY MINERALISATION AND PARAGENESIS………………......181 5.4.1 Introduction………………………………………………………………….181 5.4.2 Porphyry Stage 1: Potassium feldspar……………………………….............183 5.4.3 Porphyry Stage 2: Sericite-chlorite………………………………………….184 5.4.4 Porphyry Stage 3: Quartz infiltration veins………………………………….185 5.4.5 Porphyry Stage 4: Massive quartz veins…………………………………….188 5.4.6 SMD porphyry style mineral paragenesis summary………………………...189

5.5 RHENIUM - OSMIUM DATING OF SMD STAGE 3 MOLYBDENITE…….……190 5.5.1 Introduction………………………………………………………………….190 5.5.2 Re-Os analytical method……………………………………………………190 5.5.3 SMD Re-Os results…………………………………………………………..192 5.5.4 Discussion and summary…………………………………………………….193

5.6 COMPOSITIONAL VARIATION IN SPHALERITE………………………………194 5.6.1 Introduction………………………………………………………………….194 5.6.2 Textural features of SMD sphalerite………………………………………...194 5.6.3 Analytical method…………………………………………………………...194 5.6.4 FeS content of SMD sphalerite …………………………………………….195 5.6.5 Comparison of the SMD sphalerite zinc contents …………………………..197 5.6.6 Discussion and summary…………………………………………………….198

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5.7 SMD COMBINED MINERAL PARAGENESIS SUMMARY……………………..199 5.7.1 Introduction………………………………………………………………….199 5.7.2 Discussion of the interpreted SMD combined mineral paragenesis…………199

5.7.2.1 SMD Stage 1 - Early basin diagenesis……………………………………………199 5.7.2.2 SMD Stage 2 - Late basin diagenesis………………………………………….....201 5.7.2.3 SMD Stage 3 - Main base metal phase…………………………………………...201 5.7.2.4 SMD Stage 4 - Main high-grade gold phase…………………………………….202 5.7.2.5 SMD Stage 5 - Post-main high-grade gold phase ……………………………...202

5.7.3 SMD combined mineral paragenesis summary……………………………...202

CHAPTER 6: TRACE ELEMENT CHEMISTRY………………………………….…204 6.1 INTRODUCTION……………………………………………………………………204 6.2 TRACE ELEMENT GEOCHEMISTRY OF SMD PYRITE………………………..205

6.2.1 Introduction………………………………………………………………….205 6.2.2 Analytical methods…………………………………………………………..206

6.2.2.1 Sample selection and preparation………………………………………………...206 6.2.2.2 LA-ICPMS analysis technique…………………………………………………….206

6.2.3 Morphology of SMD pyrite types…………………………………………...207 6.2.4 LA-ICPMS trace element results from the SMD pyrite types……………....208

6.2.4.1 Background…………………………………………………………………………..208 6.2.4.2 Pyrite 2B composition………………………………………………………………209 6.2.4.3 Pyrite 3B trace element composition……………………………………………..212 6.2.4.4 Pyrite 4A1-4A4 compositions……………………………………………………...215 6.2.4.5 Comparison of trace elements from Pyrite 2B, Pyrite 3B and Pyrite 4A…….219 6.2.4.6 Pyrite SKN1………………………………………………………………………….223 6.2.4.7 Chalcopyrite 3C……………………………………………………………………..226 6.2.4.8 Pyrite SKN2………………………………………………………………………….229 6.2.4.9 Comparison of trace elements from the SMD SHGD and SMD copper

Deposits………………………………………………………………………………232 6.2.5 Discussion…………………………………………………………………...236

6.2.5.1 SMD gold occurrences……………………………………………………………..236 6.2.5.2 Gold-arsenic relationships in SMD pyrites………………………….................237 6.2.5.3 Gold and silver concentration in SMD pyrites………………………………….239 6.2.5.4 Gold-silver ratios in SMD pyrite ………………………………………………….240 6.2.5.5 Characteristic trace element associations in SMD pyrite types………………241

6.3 PIXE TRACE ELEMENT STUDY OF SMD SULPHIDE-ORES…………………243 6.3.1 Introduction……………………………………….…………………………243 6.3.2 PIXE analytical method……………………………………………………...243 6.3.3 SMD PIXE NMP Results……………………………………………………245

6.3.3.1 SMD Stage 2 (Pyrite 2B) overprinted by late SMD Stage 4A and SMD Stage 4B……………………………………………………………………....246

6.3.3.2 SMD Stage 3 (Pyrite 3B) cut by late SMD Stage 4A……………………………251 6.3.3.3 SMD Stage 4 (Pyrite 4A1)………………………………………………………….253 6.3.3.4 SMD Stage 4 (Pyrite 4A2)………………………………………………………….256 6.3.3.5 SMD Stage 4 (Pyrite 4A3)…………………………………………………….……259 6.3.3.6 SMD Stage 4 (Pyrite 4A4)………………………………………………………….262

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6.3.3.7 SMD Skarn Stage 2B and Stage 4 pyrite…………………………………………265 6.3.4 Discussion and summary of observations from the PIXE NMP pyrite

Study…………………………………………………………………………268 6.4 SUMMARY………………………………………………………………………….271

6.4.1 LA-ICPMS trace element investigations…………………………………….271 6.4.2 PIXE NMP investigations…………………………………………………...272

CHAPTER 7: ISOTOPE AND FLUID CHEMISTRY…………………………………..273

7.1 INTRODUCTION……………………………………………………………………273 7.2 STABLE ISOTOPE STUDY………………………………………………………...273

7.2.1 Introduction………………………………………………………………….273 7.2.2 Stable isotope investigation aims……………………………………………274 7.2.3 Sulphur isotope study………………………………………………………..274

7.2.3.1 Introduction………………………………………………………………….274 7.2.3.2 Sulphur isotope analytical methods………………………………………….275 7.2.3.3 SMD sulphur isotope results………………………………………………...276 7.2.3.4 Discussion and comparison of sulphur isotope results………………………281

7.2.4 Oxygen and Hydrogen isotopes……………………………………………..285

7.2.4.1 Introduction…………………………………………………………………285 7.2.4.2 Oxygen and hydrogen isotope analytical methods……………………………..286 7.2.4.3 SMD oxygen and hydrogen isotope results………………………………………287 7.2.4.4 Discussion and comparison of the SMD oxygen and hydrogen isotope

Results………………………………………………………………………………...289 7.2.5 Summary…………………………………………………………………….290

7.3 LEAD ISOTOPE STUDY……………………………………………………………291 7.3.1 Introduction………………………………………………………………….291 7.3.2 Aims of study………………………………………………………………..292 7.3.3 Methods of study…………………………………………………………….292

7.3.3.1 Reagents used………………………………………………………………..292 7.3.3.2 Aqua regia acid digestion (Galena solution method)………………………292 7.3.3.3 HF/H2SO4 PicoTrace high pressure digestion (Whole rock

solution method)……………………………………………………………...……..293 7.3.3.4 LA-ICPMS technique (Pyrite)……………………………………………….…….293 7.3.3.5 LA-Multi collector ICPMS technique (Pyrite)……………………………..……294

7.3.4 Types of SMD samples analysed for lead isotope compositions……………294 7.3.5 SMD lead isotope data………………………………………………………296 7.3.6 SMD lead isotope results…………………………………………………….297 7.3.7 Discussion…………………………………………………………………...299

7.4 FLUID INCLUSION INVESTIGATIONS…………………………………………..302 7.4.1 Introduction………………………………………………………………….302 7.4.2 Aims…………………………………………………………………………304 7.4.3 Methods of study…………………………………………………………….304

7.4.3.1 Microthermometric method…………………………………………………….….304 7.4.3.2 Laser Raman Spectrometry method………………………………………………305

7.4.4 Fluid inclusion petrography………………………………………………….306 7.4.5 Microthermometric results…………………………………………………..309

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7.4.5.1 SMD SHGD homogenisation temperature results…………………………..310 7.4.5.2 SMD Copper deposits - copper skarn zone homogenisation temperature

Results………………………………………………………………………………...310 7.4.5.3 SMD Padan porphyry (Cu-Mo) Prospect homogenisation temperature

Results………………………………………………………………………………...312 7.4.5.4 Observations from the SMD fluid inclusion freezing temperature

Analyses………………………………………………………………………………312 7.4.5.5 SMD SHGD salinity results………………………………………………………..312 7.4.5.6 SMD Copper deposits - copper skarn zone salinity results……………………314 7.4.5.7 SMD Padan porphyry (Cu-Mo) Prospect salinity results……………………...314

7.4.6 Laser Raman Spectrometry results…………………………………………..314 7.4.7 Discussion…………………………………………………………………...316

7.4.7.1 Temperature of homogenisation…………………………………………………..316 7.4.7.2 Salinity ……………………………………………………………………………….319 7.4.7.3 Fluid processes………………………………………………………………………321 7.4.7.4 Gas composition…………………………………………………………………….321

7.4.8 Summary…………………………………………………………………….323 7.5 SUMMARY………………………………………………………………………….324

7.5.1 SMD stable isotope investigations…………………………………………..324 7.5.2 SMD lead isotope study……………………………………………………..324 7.5.3 SMD Fluid inclusion investigations…………………………………………325

CHAPTER 8: DISCUSSIONS, GENETIC MODEL AND CONCLUSIONS…………..326 8.1 INTRODUCTION……………………………………………………………………326 8.2 MODE OF OCCURRENCE…………………………………………………………327

8.2.1 Regional geological setting………………………………………………….327 8.2.2 Age of host stratigraphy.…………………………………………………..329 8.2.3 Host rock types………………………………………………………………330 8.2.4 The association of igneous rocks…………………………………………….330 8.2.5 Structural controls…………………………………………………………...332

8.3 GOLD ORE MINERALOGY………………………………………………………..333 8.3.1 Prelude……………………………………………………………………….333 8.3.2 Alteration…………………………………………………………………….333 8.3.3 Paragenetic comparison of gold and associated minerals in the

SMD SHGD…………………………………………………………………334

8.3.3.1 Pre-main gold ore stage mineral assemblages in Carlin-type deposits…...…334 8.3.3.2 Main gold ore mineral stages in Carlin-type deposits…….…………………...337 8.3.3.3 Post-main gold ore stage mineral assemblages in the Carlin-type

Deposits………………………………………………………………………………339 8.3.4 Gold occurrence in the SMD copper skarn zones…...………………………339 8.3.5 SMD gold ore summary ……………………………………………………..340 8.3.6 Timing of SMD Au and Cu mineralisation…………………………….……342

8.4 TRACE ELEMENT, FLUID AND ISOTOPE GEOCHEMISTRY…………………344 8.4.1 Trace elements……………………………………………………………….344

8.4.1.1 Trace elements: Source of metals…………………………………………………344 8.4.1.2 Trace element spatial distribution: SMD gold and copper deposits………….347

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8.4.2 Fluid chemistry………………………………………………………………348

8.4.2.1 Ore-fluid homogenisation temperatures…………………………..……………..348 8.4.2.2 Ore-fluid salinities……………………………………………………..…………...349 8.4.2.3 Depths of ore deposition and fluid inclusion gas compositions……………….351 8.4.2.4 Source of ore fluid components: Presence of CO2 in fluid inclusions……….351 8.4.2.5 Source of ore fluid components: Oxygen and Hydrogen isotope studies…….352

8.4.3 Sulphur isotopes: Source of sulphur…………………………………………353 8.4.4 Lead isotopes………………………………………………………………...354 8.4.5 Principal characteristics and source of the possible SMD ore fluids………..355

8.5 SHGD GENETIC MODELS…………………………………………………………356 8.5.1 Introduction………………………………………………………………….356 8.5.2 Meteoric water model………………………………………………………..357 8.5.3 Metamorphic (orogenic) ore-fluid model ……………………………………359 8.5.4 Magmatic (intrusion related) ore fluid model……………………………….360 8.5.5 Evolved fluid mixing model (multiple ore fluids)…………………………...361

8.6 GENETIC MODEL FOR GOLD IN THE SMD DEPOSITS……………………….363 8.6.1 Overview…………………………………………………………………….363 8.6.2 Proposed genetic model for gold in the SMD gold and copper deposits……363

8.6.2.1 Pre-main gold ore stages…………………………………………………………..363 8.6.2.2 Main gold ore stages………………………………………………………………..365 8.6.2.3 Post-main gold ore stage…………………………………………………………..368

8.7 SUMMARY AND CONCLUSIONS………………………………………………...369 8.7.1 Geological setting……………………………………………………………369 8.7.2 Mineralogy…………………………………………………………………..369 8.7.3 Gold occurrence……………………………………………………………..371 8.7.4 Timing of SMD mineralisation……………………………………………...371 8.7.5 Geochemistry………………………………………………………………...372

8.7.5.1 SMD SHGD………………………………………………………………………….372 8.7.5.2 SMD Copper Deposits……………………………………………………………...372

8.7.6 Lead isotope characteristics………………………………………………….373 8.7.7 Genetic model………………………………………………...……………...373

8.8 IMPLICATIONS FOR EXPLORATION……………………………………………374 8.9 FUTURE RESEARCH RECOMMENDATIONS…………………………………...376

REFERENCES……………………………………………………………………………..377

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APPENDICES……………………………………………………………………………….396

APPENDIX 1.1.0: List of SMD drill holes logged and sampled (2004 and 2005)……...………AI

APPENDIX 1.2.0: List of SMD samples collected (Rock Catalogue).…………………...……..AII

APPENDIX 3.3.1: XRF major and trace element geochemical data for SMD

geochronology samples investigated………………………..………..……AIII

APPENDIX 3.4.1: Zircon and heavy mineral separation method used at CODES, UTAS…….AIV

APPENDIX 3.4.2: SMD LAICPMS zircon U-Pb geochronology data…………………...…….AV

APPENDIX 4.1.0: SMD deposit-scale structural data measurements………………………….AIV

APPENDIX 4.2.0: LXML Padan Cu-Mo Prospect logs for DDH PDN001 and PDN002……AVII

APPENDIX 5.1.0: Electron microprobe analyses of carbonate bearing mineral

compositions (Oxide %) from the SMD SHGD, Laos…………………...AVIII

APPENDIX 5.2.0: Electron microprobe analyses of sphalerite grain compositions from

the SMD SHGD and Khanong copper deposit, Laos……………………..AIX

APPENDIX 5.3.0: Electron microprobe analyses (Oxide %) from SMD Stage 3 garnets,

SMD copper deposits, Laos…………………………………..……………..AX

APPENDIX 5.4.0: PIMA spectral plots for samples analysed from the SMD…………………AXI

APPENDIX 6.2.1: SMD LA-ICPMS data obtained from pyrite and chalcopyrite…………..AXI1

APPENDIX 6.2.2: SMD LA-ICPMS study: Trace element data correlation matrices obtained

from SMD pyrite and chalcopyrite………………………………..……AXIII

APPENDIX 7.2.1: Summary of sulphur isotope results from the SMD, Laos………….……AXIV

APPENDIX 7.2.2: Geobarometry using SMD sulphur isotope results………………………..AXV

APPENDIX 7.2.3: Measured oxygen and hydrogen isotope data from the SMD SHGD,

Cu skarn zones in the SMD Cu deposits and Padan Cu-Mo porphyry

Prospect……………………………………………………..…………..AXVI

APPENDIX 7.3.1: SMD lead isotope results……………………………………………..…AXVII

APPENDIX 7.4.1: Fluid inclusion petrographic and microthermometric data from

the SMD SHGD, Laos………………………………...………………AXVIII

APPENDIX 7.4.2: Laser Raman Spectral (LRS) plots from Geoscience Australia

(Canberra) for fluid inclusion gases detected within Stage 3 quartz

from the SMD Cu deposits………………………………………….….AXVIV

(0.5_Figures_PWC_Final_Rev_100618pr)

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FIGURES

CHAPTER 1

Fig.1.1. Location of the Sepon project area in south-central Laos……………………………...........1

Fig.1.2. Photos showing the geographic setting of the Sepon Mineral District, Lao PDR…………..2

Fig. 1.3. Map showing the location of gold and copper areas in the SMD…………………………..3

Fig. 1.4. Geochemical images of the SMD (Cu and Au)…...………………………………………...4

Fig. 1.5. Comparison of the average ore grade (g/t Au) versus metric tonnes ore for

individual Carlin-type gold deposits in Nevada, USA and the combined known

gold resources of the Sepon Mineral District (SMD) SHGD…………………….......……...5

CHAPTER 2

Fig. 2.2.1. Map showing the tectonic setting of mainland SE Asia and the present

location of Continental terranes…………………………....…...…..……..........………..13

Fig. 2.2.2. Sections through time showing tectonic development of the Indochina

Terrane during the Silurian to Permian period…………….……………………..……..17

Fig. 2.2.3. Reconstruction maps showing the Phanerozoic positions of the Indochina

Terrane, commencing in the Early Carboniferous to Late Triassic……...………...…....18

Fig. 2.2.4. Location map of known mineral deposits along the margins of the

Indochina Terrane……....……………………………………………………………….20

Fig. 2.2.5. Summary time-space plot showing stratigraphic columns with the currently

known representative sequences of volcano-sedimentary and igneous rocks that

occur in the Central Laos, Loei, Petchabun-Pitchit, Lopburi and Srae Keo regions……23

Fig. 2.3.1. Regional geology map of Laos showing the location of the SMD………………………25

Fig. 2.3.2. Regional structural geology map of Laos………………………………………………..32

CHAPTER 3

Fig. 3.1. Regional-scale geology map of the southern Truongson Fold Belt in Laos………...…….35

Fig. 3.2. District-scale geology map of the Sepon Mineral District (SMD) showing the

location of the main gold and copper deposits…………..………………………………36

Fig. 3.3. Stratigraphic column from the basal Payee Formation to the upper Nam Kian

Formation in the SMD…………………………...………………………………………38

Fig. 3.4. Photographs showing lithological features of the Palat, Payee and Houay Bang

Formations occurring in the SMD, Laos………..……………………………………….40

Fig. 3.5. Photographs showing in turn the lithological features of the Nampa Formation,

Vang Ngang Formation, Namphuc Volcanics, Kengkeuk Formation and Nalou

Formation occurring in the SMD……………………...…………………………………43

Fig. 3.6.1. Photographs showing lithological features of the Discovery Formation occurring

in the SMD……………………………………………………………………………….44

Fig. 3.6.2. Photographs showing the Nan Kian Formation and the underlying transitional

zone between the Discovery Formation and the Nan Kian Formation…………………..45


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Fig. 3.7. Satellite image map with K-Th radiometric data showing the outline of RDP

intrusions in the SMD occurring along an E-W trending corridor in the Padan-

Thengkham (P-T) sector………………………………………………………………..47

Fig. 3.8. SMD rhyodacite porphyry represented in outcrop, drill core and thin section……...…..49

Fig. 3.9. Plot of immobile elements Zr, Ti, Nb, Sc, V and Y versus SiO2 using SMD

whole-rock analysis data……………………………….………………..……………….51

Fig. 3.10. Geochemical data plotted from the SMD RDP samples listed in Table 3.3.2…………52

Fig. 3.11. Comparison of geochemical data from the SMD RDP samples listed in Table 3.3.2

using the classification of Pearce et al. (1984)………………….……………………….52

Fig. 3.12. Regional geology map of showing the location of the SMD and granite samples

collected along the margins of the Truongson Fold Belt at Ban Kengkok and

Ban Sopmi……………………………………….…………………………………........53

Fig. 3.13. Photographs showing lithological features of granite in the Ban Sopmi-Kengkok

(BSK) area…………………………………..…………………………………………...55

Fig. 3.14. Diagrams showing rock type classification, using geochemical data for the two

granite samples listed in Table 3.3.3…………..………………………………………..56

Fig. 3.15. Diagrams showing comparison of geochemical data from the SMD RDP and BSK

granite samples listed in Tables 3.3.2 and 3.3.3 respectively…………………………..56

Fig. 3.16. Textural features of a mafic dike cutting rhyodacite porphyry (RDP) at the

Discovery Colluvial deposit…………………………………………..…………………58

Fig. 3.17. Photomicrographs showing textural features of SMD zircons from RDP

and granite intrusions for age dating using U-Pb LA-ICPMS at CODES, UTAS……....61

Fig. 3.18. A relative probability histogram of the SMD RDP zircon ages obtained from the

198 zircons listed in Table 3.4.3 and Appendix 3.4.3…………..……………….………63

Fig. 3.19. Plot of the range of LA-ICPMS U-Pb zircon isotopic ages determined for the

SMD samples listed in Table 3.4.3 against their easting location in the SMD….......63

Fig. 3.20. Thengkham West RDP examples analysed by LA-ICP-MS for their U-Pb

zircon ages……………………………………………...……………………………65

Fig. 3.21. Broadly defined basin architecture of the SMD……………………………………..68

Fig. 3.22. Simplified model for the development of the SMD in an E-W oriented pull-apart

basin in a NW-directed sinistral transpresional zone………………………………..69

Fig. 3.23. District-scale geology map of the Sepon Mineral District (SMD) showing the major

Faults and locations of the main gold and copper deposits…………..……….…………71

Fig. 3.24. Photographs of small-scale folds in the SMD……………………………………………72

Fig. 3.25. Interpreted models for the structural evolution of the Sepon Basin, including the SM….73

CHAPTER 4

Fig. 4.1.1. District-scale geology map of the Sepon Mineral District (SMD) showing the

location of the main gold and copper deposits…………..……….………………...……75

Fig. 4.2.1. Geological map of the Nalou gold deposit, SMD……………………….………………78

Fig. 4.2.2. Geological cross-section of the Nalou gold deposit along line 603550E………..............78

Fig. 4.2.3. Geological map of the eastern section of the open-pit at the Nalou gold deposit…….…79

Fig. 4.2.4. Photographs showing lithological and structural features of the Nalou gold deposit…...80


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Fig. 4.2.5. Photographs and drill hole logging section along DDH NLU072 showing the

Stratigraphic position of gold mineralisation in relation to structurally prepared

rheological zones at the Nalou gold deposit……………………………………..………81

Fig. 4.2.6. Photographs showing examples of rock types containing mineralisation at the

Nalou gold deposit…………………………………………………………………….…82

Fig. 4.3.1. Geological map of the Discovery West gold deposit (DSW)……………………………84

Fig. 4.3.2. Geological cross-section of the DSW gold deposit looking east along line 604350mE...84

Fig. 4.3.3. Discovery west gold deposit geological mapping of the open-pit……………………....85

Fig. 4.3.4. Photographs showing lithological features of Discovery West gold deposit

(DSW) rock type examples………………………………………………………………86

Fig. 4.3.5. Photographs and drill hole logging section along DDH DIS056 showing the

stratigraphic position of gold mineralisation in relation to structurally prepared

rheological zones at the Discovery West deposit (DSW)………………………………..88

Fig. 4.4.1. Geological map showing the location of the Discovery Colluvial gold deposit (DSC)…90

Fig. 4.4.2. Geological cross-section through the Discovery Colluvial deposit looking east

along line 605500mE…………………………………………………………………….90

Fig. 4.4.3. Photographs showing examples of Discovery Formation calcareous shale at the

Discovery Colluvial deposit (DSC)………………….......................................................91

Fig. 4.4.4 Geological map of the Discovery Colluvial gold deposit (DSC)………………………...92

Fig. 4.4.5. Photographs showing examples of weathered sulphide veins occurring in rhyodacite

porphyry (RDP) at the Discovery Colluvial deposit (DSC)……………………………..93

Fig. 4.4.6. Photographs and drill hole logging section along DDH DIS001 at the Discovery

Colluvial deposit…………………………………………………………………………94

Fig. 4.5.1. Geological map showing the location of the Discovery Main (DSM) and

East (DSE) gold deposits………………………………………………………………...96

Fig. 4.5.2. Geological cross-section through the Discovery Main (DSM) deposit looking east

along line 606800mE…………………………………………………………………….97

Fig. 4.5.3. Geological cross-section through the Discovery East DSE deposit looking east

along line 607500m………………………..…………………………………………….97

Fig. 4.5.4. Photographs showing hostrock lithology and mineralisation at the Discovery Main

and East deposits…………………………………………………………………………98

Fig. 4.6.1. Geological map of the Namkok West (NKW) and East (NKE) gold deposits

in the SMD.......................................................................................................................100

Fig. 4.6.2. Cross-sections looking east, showing the geology of the Namkok West (A) and

Namkok East (B) gold deposits………………………………………………………...101

Fig. 4.7.1. Geological map of the Vang Ngang gold deposit and surrounding areas……………...102

Fig. 4.7.2. Geological cross-section of the Vang Ngang East gold deposit looking west along

section 608650E………………………………………………………………………...103

Fig. 4.7.3. Photographs showing lithology and mineralisation at the Vang Ngang (VNG)

gold deposit……………………………………………………………………………..104

Fig. 4.8.1. Geological map of the Phavat and Phavat North gold prospect areas in the SMD…….106

Fig. 4.8.2. Photographs showing lithology and mineralisation at Phavat Prospect in drill holes

VAT001 and VAT006………………………………………………………………….107

Fig 4.9.1 Geological map of the Nakachan Prospect area in the SMD…………………..………..109


xxi

Fig. 4.9.2. Photographs showing lithology and mineralisation at Nakachan Prospect in

drill holes NAK001 and NAK006…………………………………….………………..109

Fig. 4.10.1. Diagrammatic examples of gold ore zone geometries that have been observed

in the SMD SHGD……………………………………………………………………...112

Fig. 4.3.1.1. Map showing the location of known copper resources and prospects in the SMD,

including the location of the SMD SHGD……………………………………………...116

Fig. 4.3.2.1. Geological map of the Khanong copper deposit area in the SMD…………………...118

Fig. 4.3.2.2. Geological section through the Khanong copper deposit along line 608250E………120

Fig. 4.3.2.3. Geological section through the Khanong copper deposit along line 608500E………120

Fig. 4.3.2.4. Photographs showing characteristics of supergene mineralisation at the

Khanong copper deposit………………………………………………………………..122

Fig. 4.3.2.5. Photographs showing nature of supergene and skarn associated mineralisation

along DDH KHN013 at the Khanong copper deposit………………………………….123

Fig. 4.3.3.1. Geological map of the greater Thengkham copper resource area, showing the

locations of the Thengkham South, Thengkham North and Phabing copper deposits…124

Fig. 4.3.3.2. Geological map of the Thengkham South copper deposit area, showing the

location defined copper resources…………………………………………………..….126

Fig. 4.3.3.3. Cross-section line 597100mE showing copper mineralised intervals and skarn

zones at the Thengkham South copper deposit…………………………………………128

Fig. 4.3.3.4. Photographs showing nature of supergene, skarn and sulphide mineralisation at

the Thengkham South copper deposit…………………………………………………..128

Fig. 4.3.3.5. Photographs showing characteristics of skarn mineralisation and paragenetic

Stages 1 to 4 observed in Thengkham South drill hole TKM035……………………...129

Fig. 4.3.4.1. Geological map of the Padan Cu-Mo Prospect area in the SMD………………….…130

Fig. 4.3.4.2. Easterly view towards Mt Padan (horizon) and the Padan Cu-Mo Prospect area

in the SMD……………………………………………………………………………...131

Fig. 4.3.4.3. Textural features, including characteristics of alteration and mineralisation

along drill hole PDN002 at the Padan Prospect…………………………………….…..133

CHAPTER 5

Fig. 5.2.1. Paragenetic diagram showing the hypogene mineral assemblages from Stages 1 to

Stage 5 occurring in the SMD SHGD…………………………………………………..139

Fig. 5.2.2. Photomicrographs showing textural features of SMD Stage 1 framboidal pyrite

(Pyrite 1)……………………………………………………………………………..…143

Fig. 5.2.3. Examples of Dolomite 1 (ferroan dolomite) observed at the Nalou gold deposit……...145

Fig. 5.2.4 Photomicrographs showing textural features of Stage 2 diagenetic types Pyrite 2A,

2B and 2C from the SMD SHGD....................................................................................147

Fig. 5.2.5. Textural features and chemistry of pink calcite (Calcite 2) identified by carbonate

staining and microprobe analyses at the SMD SHGD………………………………….148

Fig. 5.2.6. Photographs of drill core showing textural features of Stage 3A veins hosted by

Nalou Formation dolomite, at the SMD SHGD………………………………………..149

Fig. 5.2.7. Photomicrographs showing textural features of Stage 3A at the SMD SHGD………...151

Fig. 5.2.8 Photographs showing textural features examples of Stage 3B mineralisation hosted

in rhyodacite porphyry (RDP) and calcareous shale (CSH) at the SMD SHGD……….152


xxii

Fig. 5.2.9. Photomicrographs showing textural features of Stage 3B mineralisation at the

SMD SHGD…………………………………………………………………………….154

Fig. 5.2.10. Photomicrographs showing textural features of Stage 3B base metal veins cut by

Stage 3C sulphosalts at the SMD SHGD……………………………………………….156

Fig. 5.2.11. Textural characteristics of Stage 4A gold bearing pyrite types Pyrite 4A1 and

Pyrite 4A2 hosted in Discovery Formation calcareous shale (CSH) at the Nalou

SHGD in Sample NLU0060300………………………………………………………..158

Fig. 5.2.12. Textural features of Stage 3B base metal veins with low gold grades (<1 ppm Au)…158

Fig. 5.2.13. Photomicrographs showing textural features of sulphide and gold-ores from the

Nakachan gold Prospect hosted by brecciated bioclastic dolomite, with early Stages

3b base-metal veins, cut by fractures filled with high-Au bearing Stage 4A pyrite…....159

Fig. 5.2.14. Photomicrographs showing textural features of Stage 4A Pyrite 4A1………..............160

Fig. 5.2.15. Photomicrographs showing textural features of Stage 4A disseminated euhedral

pyrite (Pyrite 4A2)……………………………………………………………………...161

Fig. 5.2.16. Photomicrographs showing textural features of Stage 4A euhedral Pyrite 4A3

(Py 4A3)………………………………………………………………………………...162

Fig. 5.2.17. Photomicrographs showing textural features of Stage 4A colloform banded

Pyrite 4A4 (Py 4A4)…………………………………………………………………...163

Fig. 5.2.18. Photomicrographs showing examples of Stage 4A quartz associated with Stage 4A

gold-bearing pyrite……………………………………………………………………...164

Fig. 5.2.19. Photomicrographs showing textural features of Stage 4B (Hg-Au telluride-quartz)

at the Nalou SHGD…………………………………………………………………..…165

Fig. 5.2.20. Textural features associated with the Stage 5A stibnite-quartz-dolomite assemblage..166

Fig. 5.2.21. Textural features associated with both the Stage 5B stibnite-quartz-dolomite and

Stage 5C calcite-quartz assemblages…………………………………………………...167

Fig. 5.3.1. A combined SMD hypogene mineral paragenesis chart for SKN Stages 1 to 5……….170

Fig. 5.3.2. Summary of the hypogene mineral assemblages and paragenesis for SKN

Stages 1 to 5 present in the SMD copper deposits…………………………...…………171

Fig. 5.3.3. Textural features of SKN Stage 1 prograde garnet skarn in DDH TKM035,

Thengkham South deposit……………………………………………………………...172

Fig. 5.3.4. Textual features of SKN Stage 1 garnet skarn from the Khanong and Discovery

East deposits……………………………………………………………………………173

Fig. 5.3.5. Photographs showing textural features of the SKN Stage 2A mineral assemblage in

drill core from the Khanong, Thengkham South and Discovery East deposits………...174

Fig. 5.3.6. Photographs showing textural features of the SKN Stage 2B mineral assemblage

overprinting the SKN Stage 1 and SKN Stage 2A assemblages from the Khanong,

Thengkham South deposits……………………………………………………………..175

Fig. 5.3.7. Textural features of SKN Stage 2B sulphides from the Khanong copper deposit…..…176

Fig. 5.3.8. Photomicrographs showing the textural features of SKN Stage 3 sulphides,

SMD Cu deposits……………………………………………………………………….177

Fig. 5.3.9. Photomicrographs showing the textural characteristics of SKN Stage 4 gold-bearing

pyrite (Py-SKN2)……………………………………………….………………………178

Fig. 5.3.10. Photographs showing textural features of SKN Stage 5 veins in the SMD copper

deposits……………………………………………………………….………………..179


xxiii

Fig. 5.4.1. Paragenetic diagram showing the Porphyry Stages 1to 4 established for the SMD

Porphyry style Cu-Mo mineralisation………………………………………………….181

Fig. 5.4.2. Photographs showing textural features associated with the Porphyry Stage 1

mineral assemblage, SMD………………………………………..…………………….183

Fig. 5.4.3. Photographs showing textural features associated with Porphyry Stage 2 in the SMD..184

Fig. 5.4.4. Photographs showing textural features associated with SMD Porphyry Stage 3A

to 3D veins……………………………………………………………….……………..185

Fig. 5.4.5. Textural features showing Porphyry Stage 3C sulphides containing chalcopyrite…….186

Fig. 5.4.6. Textural features showing Porphyry Stage 3C sulphides containing molybdenite…….187

Fig. 5.4.7. Photographs showing textural features of SMD Porphyry Stage 4 quartz veins………188

Fig. 5.5.1. Photographs showing textural features of the Padan and Thenkham South RDP

samples containing Stage 3C quartz veins hosting molybdenite that were submitted

to AIRIE (CSU) for Re-Os geochronology determination……………………………..191

Fig. 5.5.2. Comparison of the SMD Re-Os and U-Pb results………………………………….…..193

Fig. 5.6.1. Histogram of FeS mole % in sphalerite from the SMD gold and copper deposits…….195

Fig. 5.6.2. Plot of FeS mole % in sphalerite versus the average grade of gold (Au ppm) in

drill core containing sphalerite from the SMD gold and copper deposits…………...…197

Fig. 5.6.3. Plot of Zn wt % versus Fe wt % in sphalerite types from the SMD SHGD and

Khanong copper deposit……………………………………………………..…………197

Fig. 5.6.4. Plot of Zn wt % versus Cu wt % in sphalerite types from the SMD SHGD and

Khanong copper deposit………………………………………………………………..198

Fig. 5.7.1. A combined hypogene mineral paragenesis diagram of SMD Stages 1 to 5…….…….200

Fig. 5.7.2. A summary diagram for SMD Stages 1 to 5 showing the main mineral assemblages

hosting gold-bearing sulphides and their associated gold grades as determined by

LA-ICPMS analyses during this study……………………………………..…………..203

CHAPTER 6

Fig. 6.2.1. Photomicrographs showing the textural variations of the seven main types of SMD

pyrite analysed by LA-ICPMS, namely Pyrite 2B, Pyrite 3B, Pyrite 4A1 to -4A4

and Pyrite SKN1 and -SKN2…………………………………………………………...207

Fig. 6.2.2. Box plots showing the ranges and averages of trace element compositions in

Pyrite 2B (SMD Stage 2) from the SMD……………………………………………….210

Fig. 6.2.3. Details of spot LA-ICPMS analyses on pre-main gold ore stage Pyrite 2B

(SMD Stage 2) collected from the Nalou SHGD in drill core, Sample NLU0060300...211

Fig. 6.2.4. Box plot showing the ranges and averages of trace element compositions in

Pyrite 3B (SMD Stage 3) from the SMD……………………………………………….213

Fig. 6.2.5. Details of spot LA-ICPMS analyses on pre-main gold ore stage Pyrite 3B

(SMD Stage 3) from the Discovery West SHGD in drill core Sample DIS0561142…..214

Fig. 6.2.6. Details of a LA-ICPMS profile from a laser burn traverse from point A to D across a

main gold ore stage Pyrite 4A3 (SMD Stage 4A) grain, Discovery West SHGD..….…215

Fig. 6.2.7. Details of a LA-ICPMS profile from a laser burn traverse from point A to D across a

main gold ore stage Pyrite 4A3 (SMD Stage 4A) grain, Discovery Colluvial SHGD…216

Fig. 6.2.8. Box plots showing the ranges and averages of trace element compositions in

Pyrite 4A (SMD Stage 4) from the SMD………………………………………………218


xxiv

Fig. 6.2.9. Bi-variate plots for (A) Au vs Ag and (B) Au vs As from the LA-ICPMS spot

analysis data for Pyrite 2B, Pyrite 3B and Pyrite 4A………………………………......219

Fig. 6.2.10. Bi-variate plots for (A) Au vs Cu and (B) Au vs Sb from the LA-ICPMS spot

analysis data for Pyrite 2B, Pyrite 3B and Pyrite 4A…..................................................220

Fig. 6.2.11. Bi-variate plots for (A) Au vs Tl, (B) Au vs Mo and (C) Au vs W from the

LA-ICPMS spot analysis data for Pyrite 2B, Pyrite 3B and Pyrite 4A…………..…….221

Fig. 6.2.12. Bi-variate plots for (A) Au vs Bi, (B) Au vs Co, (C) Au vs Ni, (D) Au vs Pb,

(E) Au vs Se, (F) Au vs Ti, (G) Au vs V and (H) Au vs Zn from the LA-ICPMS spot

analysis data from the SMD SHGD pyrite types Pyrite 2B, Pyrite 3B and Pyrite 4A…222

Fig. 6.2.13. Box plot showing the ranges and averages of trace element compositions in Pyrite

SKN1 (SMD Stage 3B) from the SMD………………………………………………...224

Fig. 6.2.14. Details of a spot LA-ICPMS analysis of Pyrite SKN1 from the SMD Stage 3B

skarn assemblage in Thengkham South copper deposit from Sample TKM0050311.…225

Fig. 6.2.15. Box plot showing the ranges and averages of trace element compositions in

Chalcopyrite 3C (SMD Stage 3C) from the SMD……………………………………...227

Fig. 6.2.16. Details of a spot LA-ICPMS analysis on Chalcopyrite 3C from the SMD Stage 3C

skarn assemblage in Khanong copper deposit drill core Sample KHN1790830……….228

Fig. 6.2.17. Box plot showing the ranges and averages of trace element compositions in Pyrite

SKN2 (SMD Stage 4) from the SMD…………………………………………………..230

Fig. 6.2.18. Details of a spot LA-ICPMS analysis on Pyrite SKN2 from the skarn assemblage

(SMD Stage 4A), Sample DIS0231396 from the Discovery East SHGD……………...231

Fig. 6.2.19. Bi-variate plots for Au vs As from the LA-ICPMS spot analysis data from the SMD

SHGD and the SMD copper deposits……………………………………..……………232

Fig. 6.2.20. Bi-variate plots for Au vs Sb, -Tl and -Ag. from the LA-ICPMS spot analysis data

from the SMD SHGD and the SMD copper deposits………………………………..…233

Fig. 6.2.21. Bi-variate plots for Pyrite 4A with respect to Au vs Cu, -Mo and -W from the

LA-ICPMS spot analysis data from the SMD SHGD and the SMD copper deposits….234

Fig. 6.2.22. Bi-variate plots of Au vs Bi, Se, Ni, Pb, V and Zn from the LA-ICPMS spot

analysis data from the SMD SHGD and the SMD copper deposits…………………....235

Fig. 6.2.23. Range and mean Au contents for the different SMD pyrite generations……….…….237

Fig. 6.2.24. Bi-variate plot of As vs Au using the LA-ICPMS data from the SMD pyrite types

tabulated in Appendix 6.2.1……………………………………...……………………..238

Fig. 6.2.25. Histogram of Au concentration in all SMD pyrite types analysed by LA-ICPMS…...239

Fig. 6.2.26. Histogram of Ag concentration in all SMD pyrite types analysed by LA-ICPMS…...239

Fig. 6.3.1. Photograph of the CSIRO-GEMOC Nuclear Microprobe (NMP) facility (UMelb)…...243

Fig. 6.3.2. Photograph of the X and Y co-ordinate measuring equipment (UMelb)………………243

Fig. 6.3.3. Images showing the trace element distributions within Pyrite 2B (SMD Stage 2)…….246

Fig. 6.3.4. Images showing the SMD Stage 4A trace element distributions of As, Cu, Sb and

Se surrounding Pyrite 2B (SMD Stage 2)…………………………………………..…..247

Fig. 6.3.5. PIXE NMP trace element profiles across Pyrite 2B, SMD Stage 2…………...........….248

Fig. 6.3.6. Images showing the interpreted SMD Stage 4B trace element distributions of Au,

Hg, and Te filling a fracture in Pyrite 2B (SMD Stage 2)………………………..…….249

Fig. 6.3.7. PIXE NMP trace element profiles across a fracture containing Au-Hg-Telluride

(SMD Stage 4B) between two grains of Pyrite 2B, SMD Stage 2……...…………..….250


xxv

Fig. 6.3.8. Trace element distribution images of As and Au for SMD Stage 4A pyrite (Py 4A)

filling a fracture cutting SMD Stage 3B pyrite (Py 3B) and galena (Gn 3B)……..……251

Fig. 6.3.9. PIXE NMP Trace element profiles and images for SMD Stage 3B pyrite (Py 3B)

and SMD Stage 4A gold and arsenic bearing pyrite (Py 4A)………………..…………252

Fig. 6.3.10. Images showing the PIXE NMP trace element distributions for Fe, As and Au for

SMD Stage 4A pyrite (Py 4A1)……………………………………………...…………253

Fig. 6.3.11. PIXE NMP images showing the distribution of As in SMD Stage 4A Pyrite 4A1…...254

Fig. 6.3.12. PIXE NMP Trace element profiles and images for Pyrite 4A1 (SMD Stage 4A)……255

Fig. 6.3.13. Images showing the PIXE NMP trace element distributions of As, Au and Sb for

SMD Stage 4A pyrite (Py 4A2)…………………………………………………..…….256


Fig. 6.3.15. PIXE NMP Trace element profiles and images for Pyrite 4A2 (SMD Stage 4A)…....258

Fig. 6.3.16. Images showing the PIXE NMP trace element distributions of As, Au and Cu for

Pyrite 4A3 (SMD Stage 4A)……………………………………………………………259


Fig. 6.3.18. PIXE NMP Trace element profiles and images for Pyrite 4A3 (SMD Stage 4A)……261

Fig. 6.3.19. Images showing the PIXE NMP trace element distributions of Ag, As, Au, Cu,

Pb, Sb and Tl for Pyrite 4A4 (SMD Stage 4A)…………………………...……………262

Fig. 6.3.20. PIXE NMP trace element data and images showing the distribution of As in

Pyrite 4A4 (SMD Stage 4A)……………………………………………………………263

Fig. 6.3.21. PIXE NMP trace element profiles and images for Pyrite 4A4 (SMD Stage 4A)……..264

Fig. 6.3.22. Images showing the PIXE NMP trace element distributions of Fe, Ag, As, Au, Bi,

Cu, Mo, Sb and Pb for SMD pyrite types Py-SKN1 and Py-SKN2…………………....265

Fig. 6.3.23. A PIXE NMP image showing the distribution of iron in Sample DIIS2991102-3

from the copper skarn zone at the Discovery East SHGD……………………..…….266

Fig. 6.3.24. PIXE NMP trace element profiles and images across Py-SKN1 (SKN Stage 2B)

and Py-SKN2 (SKN Stage 4)………………………………………………………..267

CHAPTER 7

Fig. 7.2.1. Histogram of δ34S values for sulphides from SMD Stages 2B to 5………………...…277

Fig. 7.2.2. Plot showing the ranges of δ34S values for sulphides from SMD Stages 2B to 5……..278

Fig. 7.2.3. Plot of Au (ppm) versus the corresponding δ34S values for sulphides from SMD

Stages 2, 3A, 3B, 3C and 4A…………………………………………………………...280

Fig. 7.2.4. Comparison of sulphur isotope data (δ34S) from SMD Stages 2 to 5 with the

ranges of known δ34S values from ore stage sulphides in other deposit types………...282

Fig. 7.2.5. Histogram of the measured and calculated δ18O isotopic compositions in quartz

from SMD Stages 3B, 3C and 4A……………………………………………………...288

Fig. 7.2.6. Histogram of the measured δD isotopic compositions of Stage 3B and Stage 3C

quartz from the SMD…………………………………………………….…………….288

Fig. 7.2.7. Plot of δD values versus the calculated δ18O values for the ore-bearing fluids in

quartz from the SMD Stages 3B and 3C data…………………………………..………289

Fig. 7.3.1. Photographs of the six types of SMD samples submitted for Pb isotope analysis……..295

Fig. 7.3.2. Plot of lead isotopic compositions for 207Pb/204Pb versus 206Pb/204Pb from

SMD Stages 2B, 3A, 3B sulphides and SMD RDP…………………………………….298


xxvi


SMD Stages 2B, ,3A, 3B sulphides and SMD RDP……………………………………298


SMD Stages 2B, 3A,3B, 4A sulphides and SMD RDP……………………………...…300


SMD Stages 2B, 3A,3B, 4A sulphides and SMD RDP determined in this study…...…300


SMD Stages 2B, 3A, 3B, 4A sulphides and SMD RDP determined in this study

compared to the reported fields from other deposits in areas surrounding the

Indochina Terrane………………………………………………………………………301

Fig. 7.4.1. Photomicrograph and image examples of SMD Stage 3B and 3C quartz……………...307

Fig. 7.4.2. Photomicrographs showing fluid inclusion characteristics from the SMD deposits…...308

Fig. 7.4.3. Histograms of the SMD temperature of homogenisation data for SMD Stage 3B,

SMD Stage 3C, SMD Stage 4A and SMD Stage 5………………………………...…..311

Fig. 7.4.4. Histograms of the SMD salinity data for SMD Stage 3B, SMD Stage 3C,

SMD Stage 4A, and SMD Stage 5………………………………………….…………..313

Fig. 7.4.5. An example of a Laser Raman Spectral profile of CO2 compositions identified in

a Type III fluid inclusion in SMD Stage 3C quartz from sample PDN0022342……….315

Fig. 7.4.6. Comparative ranges of SMD temperature of homogenisation (ThoC) data with

other gold systems……………………………………………………………...………318

Fig. 7.4.7. Comparative ranges of SMD salinity data with other gold systems……………..…….320

Fig. 7.4.8. Plots of salinity (wt % NaCl equiv.) versus homogenisation temperatures (Th oC)

from the SMD fluid inclusion Types I to III……………………………………...…….322

CHAPTER 8

Fig. 8.1. Schematic diagram showing a simplified long section interpretation of the SMD

geology and the spatial distribution of the characteristic trace element patterns and

the associated sulphur isotope values for each mineral stage for the gold and copper

deposit types in the SMD………………………………………………………….……347

Fig. 8.2. Carlin-type gold deposit ore-fluid models (modified after Seedorff and Barton, 2004)...358

Fig. 8.3. The evolved fluid mixing model developed by Cline et al. (2005) illustrating the

interpreted spatial relationships between, magmatic, metamorphic and meteoric

fluids during the formation of Carlin-type deposits….……………………………...…362

Fig. 8.4. Simplified geology and genetic model showing the formation of the SMD SHGD

and copper skarn deposits……………………………………………….…...…………364

xxvii

TABLES

CHAPTER 1

Table 1.1. Pre-mining and current gold resources in the SMD at a 0.5 g/t Au cut-off grade.……......7

Table 1.2. Pre-mining and current copper resources in the SMD at a 0.5 % Cu cut-off grade…...….7

CHAPTER 2

Table 2.1A. Geological characteristics of known mineral deposits occurring along the margins

of the Indochina Terrane……….………………………………………………...…..21

Table 2.1B. Geological characteristics of known mineral deposits occurring along the margins

of the Indochina Terrane…………………………...……………………………...…..21

CHAPTER 3

Table 3.2.1. Stratigraphy comparison of former and current Formation names in the SMD,

Lao PDR………………………………………………………….……………………37

Table 3.3.2. Whole rock data for a total of fourteen RDP intrusions investigated from the SMD....50

Table 3.3.3. Whole rock XRF data for two granite intrusions from the margins of the

Sepon Basin………………………………………………………………...……….…55

Table 3.3.4. Whole rock geochemical data for a mafic dyke with doleritic composition, collected

from drill hole DD94DIS001 at the Discovery Colluvial gold deposit………………58

Table 3.4.1. Summary of LA-ICPMS U-Pb isotopic ages for zircons occurring in SMD RDP

intrusions and granite intrusions from along the margins of the Sepon Basin………...62

Table 3.5.1. Summary of the major faults in the SMD, Lao PRD…………………………………..70

CHAPTER 4

Table 4.1. Geological characteristics summary of the SMD SHGD…………………………..111

Table 4.3.1. Sepon copper resources summary…………………………………………………..116

Table 4.3.2. Khanong copper deposit mineralisation types……………………..………………...119

Table 4.3.3. Comparison of geological and mineralisation characteristics of the SMD

copper deposits…………………………………………………..…………………...135

CHAPTER 5

Table 5.1. Summary list of possible staining colours for carbonate minerals……..……...……..140

Table 5.5.1. AIRIE (CSU) Re-Os geochronology results for samples from Padan and

Thengkham South…………………………………..………………………………..192

Table 5.5.2. Comparison of the SMD Re-Os (AIRIE, CSU) and U-Pb (CODES, UTAS)

geochronology results from the Thengkham South Cu deposit and the Padan

Cu-Mo prospect…………………………………………………………………….192

xxviii

Table 5.6.1. Summary of microprobe analysis of the average FeS mole % composition in

sphalerite with the average gold grade of diamond drill core samples from the

SMD gold and copper deposits………………………………………………………196

CHAPTER 6

Table. 6.2.1. Statistical summary of the trace element concentrations of Pyrite 2B determined

from 37 LA-ICPMS points………………………………………………….……….209

Table. 6.2.2. Statistical summary of the trace element concentrations of Pyrite 3B determined

from 15 LA-ICPMS points………………………………………………….……….222

Table 6.2.3. Statistical summary of the trace element concentrations of Pyrite 4A determined

from a total of 110 LA-ICPMS spot analyses………………………………..…..…..217

Table 6.2.4. Statistical summary of the trace element concentrations of Pyrite SKN1 determined

from a total of 44 LA-ICPMS spot analyses……………………………………........223

Table 6.2.5. Statistical summary of the trace element concentrations of Chalcopyrite 3C

determined from a total of 35 LA-ICPMS spot analyses……………………..…...…226

Table 6.2.7. Statistical summary of the trace element concentrations of Pyrite SKN2

determined from a total of 17 LA-ICPMS spot analyses……………………..…...…229

Table 6.2.6. Statistics for LA-ICPMS spot analyses of gold in pyrite types from the SMD

SHGD and SMD Cu deposits………………………………………………...……....236

Table 6.2.8. Statistics for Au/Ag ratios determined from LA-ICPMS analyses of SMD

pyrite types………………………………………………………...………..………..240

Table 6.2.9. Characteristic trace element signatures of the SMD pyrite types…………...………..242

Table. 6.3.1. List of SMD pyrite types tested by PIXE at the CSIRO-GEMOC Nuclear

Microprobe facility at the University of Melbourne………………………..…….….245

Table 6.3.2. Summary table of trace element associations detected by PIXE NMP for pyrite

types in SMD mineral assemblage Stages 2B, 3B, 4A and 4B……………….……...270

CHAPTER 7

Table 7.2.1. Summary chart showing the minimum and maximum ranges of δ34S values

for sulphides from SMD Stages 2B to 5……………………………………..……..278

Table 7.2.3. Measured oxygen (δ18O) and hydrogen (δD) isotope data from the SMD SHGD,

SMD Cu deposits and Padan Cu-Mo porphyry Prospect…………………………..287

Table 7.3.1. Lead isotope data from SMD Stages 2B, 3A, 3B, 4A sulphides and SMD RDP

wall rock……………………………………………………………………………296

Table 7.3.2. Lead isotope ranges from SMD Stages 2B, 3A, 3B, 4A sulphides and SMD RDP….297

Table 7.4.1. Summary of previous SMD fluid inclusion results from the studies by Comsti

(1995) and APS (2004)……………………………………………………………….303

Table 7.4.2. Summary of the microthermometric results yielded from primary, pseudo-

secondary and secondary fluid inclusions in SMD Stages 3B, 3C, 4A and 5………..309

Table 7.4.3. Laser Raman Spectral analyses showing the fluid inclusion gas compositions in

SMD Stage 3C quartz from the Thengkham South copper deposit (TKM) and

Padan (PDN) porphyry Cu-Mo Prospect……………………………………………..315

CHAPTER 8

xxix

Table 8.1. Comparison of the major geological and geochemical characteristics of sedimentary

rock-hosted gold deposits in Laos, Carlin-type gold deposits in Southern China and

Nevada, and also distal disseminated gold deposits in Nevada, USA……………….…328

Table 8.2. Comparison of the mineral assemblages and stages for the Carlin-type gold deposits

in Nevada (USA) and the Dian-Qian-Gui (China) with the SMD SHGD (Laos)…...…335

Table 8.3. Comparison of the SMD main mineral groups and deposit settings with other known

mineral districts containing distal-disseminated gold deposits (DDGD), skarn (Zn-Pb,

Cu+Au) and porphyry (Cu-Mo+Au) deposits………………………………………….341

Table 8.4. Comparison of fluid chemistry and sulphur isotope data from the SMD gold and

copper deposits with other genetic models……………………………………………350

Geological setting, geochemistry and genesis of the Sepon gold … · 2016-08-03 · deposits...

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Transcript of Geological setting, geochemistry and genesis of the Sepon gold … · 2016-08-03 · deposits...