THE GEOLOGY AND MINERALISATION

OF THE

WAISOI PORPHYRY COPPER DEPOSITS,

NAMOSI PROVINCE,

REPUBLIC OF FIJI.

by

PETER D. ELLIS (B.Sc. Hons.).

CODES KEY CENTRE,

UNIVERSITY OF TASMANIA,

1996.

This thesis is submitted in partial fulfillment for the Master of Economic Geology Degree.

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Frontpiece. The Waisoi valley showing the position of the three mineralised zones in relation to the Waisoi camp and the Korobasabasga Range.

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THESIS TOPIC

This thesis covers recent petrological, mineralogical, alteration and fluid

inclusion investigations combined with field observations of geology, structure

. and veining which led to the defining of a paragenetic sequence for the Waisoi

porphyry copper deposit mineralisation at Namosi.

The above is translated into Fijian by Fereti lnoke.

NA VAKASAMA OQO E OKATI TU KINA NA VULICI Nl VEIKA ME BALETA NA

VATU KEI NA YAU TALE!. E OKATI TALEGA EKE NA VEIMATAQALI VEISAU

ERA SA SOTAVA. NA VEIKA OQORI KEI NAVEl TUKUTUKU E SA KUMUNI

TU MAl NA CAKACAKA KA SA VAKAYACORI MAl WAISOI, E

VAKAMACALATAKA VEl KEDA NA VEIKAA YACO KEI NA GAUNA ERA DUI

YACO KINA ME YACOVA Nl SA KUNEI E NA VATU NA YAUTALEI MAl

NAMOSI.

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DISCLAIMER

This thesis contains no material that has been accepted for the award of any

higher degree or graduate diploma in any Tertiary Institution and contains no

material previously published or written by another person, except when due

reference is made in the text of the thesis.

Signed

Peter D. Ellis

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ABSTRACT

The Waisoi porphyry copper/gold deposits at Namosi, in the Republic of Fiji, are

hosted by several units of strongly faulted, fractured and altered volcanic rocks.

The lower host volcanic unit consists of coherent and autobrecciated basaltic

andesite lavas of the Late Oligocene to Late Miocene (32 to 10 Ma), Wainimala

basement (early island arc magmatic affinities), whereas the upper host volcanic

unit is dominated by Late Miocene (8 to 5.5 Ma), calc-alkaline andesitic dacite

lavas and volcaniclastics of the Namosi Andesite (mature island arc magmatic

affinities). These two units are separated by a +5my time break, with a mass

flow conglomerate unit (Basal Namosi Conglomerate) marking the beginning of

the calc-alkaline volcanism. These rocks are intruded (at 5.5 - 6.0 Ma) by dacitic

quartz diorites of similar chemistry to the Namosi Andesite volcanics. Fluids

associated with these intrusives caused extensive alteration in the Namosi area

and led to formation of the 930 million tonne Waisoi porphyry mineralisation,

averaging 0.43% copper and 0.14 ppm gold.

Petrological, mineralogical, alteration and fluid inclusion investigations,

combined with field observations of temporal relationships between geological

features, demonstrated that the paragenetic sequence consists of early,

regional, greenschist facies, burial metamorphism and albitisation followed by

porphyry-related alteration. The latter (porphyry-related alteration) consists of

early sericitisation, followed by silicification, before the main mineralising fluids

were introduced. These resulted in a potassic alteration event, during which

most of the copper and gold was deposited, followed by a chloritisation event,

also with significant copper mineralisation. The variable phase mineralising

event was followed by several carbonate-rich alteration events.

Minor copper mineralisation was deposited as chalcopyrite during the

silicification event, but most of the copper and gold was introduced and

deposited as chalcopyrite and bornite during the subsequent main

mineralisation episode. This involved a series of events, from those with highly

saline fluids at temperatures in excess of 500°C, to high salinity (+45 wt.% NaCI

equivalent) fluids at temperatures above 380°C, to fluids with lower salinities (10

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wt.% NaCI equivalent) and temperatures around 300°C. Minor copper was also

deposited as chalcopyrite during the early stages of the carbonate alteration

event.

Mineralisation distribution is controlled by lithology and structure. Regional

structure controlled the location of the mineralising porphyries, the fluid

pathways and the porosity/permeability of the host rocks, whereas the

lithologies controlled the degree of chemical reaction and the style of

mineralisation. The dominant structures controlling the emplacement of the

porphyries are northeast-southwest and northwest-southeast trending corridors,

with lesser influence of east-west zones. East-west and north-south structures

tended to occur later, and disrupt the mineralisation on a series of block faults.

The high erosion rate of the Waisoi area has resulted in the exposure of the

Waisoi mineralisation, but lower erosion at nearby Waivaka has only exposed

the top of a similarly mineralised system.

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ACKNOWLEDGEMENTS

I would like to thank the following individuals and organisation for their

assistance during the production of this thesis:

Placer Exploration Limited for allowing me to use data, samples and

results I obtained while I was working in Fiji and on which this study is

based,

Allan Jay (Senior Project Geologist) for his encouragement, discussions,

intellectual stimulation and critical evaluation during my work in Fiji,

Dr Khin Zaw for help in the completion of fluid inclusion studies,

Fereti lnoke, Siamoni Tuibau, Anitra Ross and Lenore Jepson for

stimulating discussions and keen observations,

My supervisors, Drs Bruce Gemmell, Anthony Crawford and David

Cooke, for their assistance, encouragement and discussions throughout

the project,

Alicia Verbeeten for discussions, especially in simplifying the tectonic

setting and regional geology of Fiji,

CODES staff for their assistance, ideas and discussions during the

project and the associated Master of Economic Geology Short courses,

Liza Gazis and Bob White for drafting figures and diagrams,

My wife, Margaret, to whom I am especially thankful, for support,

encouragement and editing drafts of this thesis, and the rest of my family,

Georgina and Antonia, for putting up with me being away from home

even more than usual.

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CONTENTS

Page

Thesis Topic ii Disclaimer iii Abstract iv Acknowledgements vi

1. BACKGROUND 1 ..

· .. '1 1.1. INTRODUCTION 2 1.2. AIMS AND TECHNIQUES 9

1.2.1. Aims 9 1.2.2. Techniques 11

2. REGIONAL SETI"ING 15 . ' 2.1 . TECTONIC SETTING 16

I 2.2. REGIONAL GEOLOGY 25 I ·"i

2.2.1. Fiji 25 ·, :-:. 2.2.2. Viti Levu 26

·.· .. :; 2.2.2.1.General 27 2.2.2.2.Geology 27

·.·,·· .. ·.' 2.2.3. Regional Structure 35 2.2.4. Geological Setting 35

2.3. REGIONAL MINERALISATION 40

3. WAISOI GEOLOGY 44 3.1. PROSPECT GEOLOGY 45

3.1.1. General 45 3.1.2. Waisoi Geology 46

i 3.1.2.1.Wainimala Agglomerate 57 ·. I .. ··'

3.1.2.2.8asal Namosi Conglomerate 62 I 3.1.2.3.Namosi Andesite 68 j

3.1.2.4.Korobasabasaga Pyroclastics 71

I 3.1.2.5.Porphyries 75 3.1.2.6.Surficial Deposits 82

! ·' 3.1.3. Weathering Profile 82

3.1.4. Interpretation 83 3.2. PROSPECT STRUCTURE 85

3.2.1. General 85 3.2.2. Waisoi Structure 88

3.2.2.1.Structural Setting 88 3.2.2.2.Structural Detail 89 3.2.2.3.Structural Interpretation 93

3.3. PROSPECT GEOLOGICAL SUMMARY 96

4. WAISOI MINERALISATION AND ALTERATION 99 4.1. PROSPECT MINERALISATION 100

4.1.1. General 100 4.1.2. Waisoi Mineralisation 103

4.1.2.1.General Distribution 105

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4.1.2.2.Disseminated Mineralisation 108 : 4.1.2.3.Vein Mineralisation 111 ' '

,j 4.1.2.4.Massive Sulphide Mineralisation 116 4.1.2.5.Secondary Enrichment 119

4.2. PROSPECT ALTERATION 120 4.2.1. General 120 4.2.2. Waisoi Alteration 121

4.2.2.1.Regional Burial Metamorphism 121 4.2.2.2.Porphyry-related Hydrothermal 124

Alteration 4.2.2.2.1.General Distribution 124 4.2.2.2.2. Disseminated Alteration 128 4.2.2.2.3.Vein Alteration 133 4.2.2.2.4.Pervasive Alteration 137

! 4.3. MINERALISATION AND ALTERATION SUMMARY 145

'! 4.4. PARAGENESIS 151 4.4.1. General 151 4.4.2. Pre-Porphyry Alteration 151 4.4.3. Porphyry Alteration 153

4.4.3.1. Sericite Alteration 153 4.4.3.2. Silica Alteration 157 4.4.3.3. Potassic Alteration 158 4.4.3.4.Chlorite Alteration 159 4.4.3.5. Carbonate Alteration 159

4.4.4. Paragenesis Summary 160

5. WAISOI GEOCHEMISTRY 163 I 5.1. FLUID INCLUSIONS 164

5.1 .1. General 164 5.1.2. Inclusion Petrography 164 5.1.3. Homogenisation and Freezing Temperatures 172

5.1.3.1. Homogenisation Temperatures 173 5.1.3.2. Freezing Temperatures and 175

Salinities 5.1.4. Fluid Inclusion Summary 180

5.2. GEOCHEMISTRY 183 5.2.1. Soil/Drainage/Rock-chip Geochemistry 183

5.2.1.1.Drainage Geochemistry 183 5.2.1.2.Soil Geochemistry 187

5.2.2. Wholerock Geochemistry 192 5.2.3. Mineralisation Geochemistry 198

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5.2.4. Selected Element Geochemistry 201 5.2.5. Geochemical Summary 201

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6. GENETIC MODEL AND CONCLUSIONS 203 6.1. GENETIC MODEL 204 6.2. CONCLUSIONS 209

REFERENCES 212

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LIST OF FIGURES

Figure Title Page

1.1 Location and tectonic setting of Fiji in the SW Pacific 3 1.2 Islands of Fiji with location of Waisoi 4

.. • I 1.3 Viti Levu with the Namosi and Waisoi exploration areas 5 1.4 Simplified gross tectonic setting of Fiji 10 1.5 Main Namosi mineralised areas 12

2.1 Tectonic setting of Fiji 17 2.2 Stages in the development of Fiji 18 2.3 Simplified geological and magmatic event geology of Fiji 19 2.4 Tectono-structural units in Fiji 24 2.5 Geology of Fiji 28 2.6 Simplified geology of Namosi Province 30 2.7 Alternate geological interpretation of Namosi 31

! 2.8 Thrust interpretation of Namosi 33 2.9 AMAX interpretation of Namosi 34

2.10 Regional SLAR image of Viti Levu 36 2.11 Major structures of Viti Levu 37 2.12 Geodynamic evolution of the Vanuatu-Fiji region 39

<I 2.13 Mineralisation of Fiji 41 I

3.1 Regional geology of the Waisoi-Waivaka area 47 I

3.2 Local Waisoi stratigraphy 49 3.3 Waisoi area geological interpretation 50 3.4 East-west cross-section of Waisoi 88800N 51 3.5 North-south cross-section of Waisoi 37645E 52 3.6 North-south cross-section of Waisoi 37 435E 53

. : I 3.7 North-south cross-section of Waisoi36875E 54 3.8 North-south cross-section of Waisoi 361 05E 55 3.9 North-south cross-section of Waisoi 35825E 56

3.10 Distribution of AMAX porphyries 77 3.11 Namosi area SLAR image with significant structures 86 3.12 Major structural elements of Waisoi 90 3.13 Cartoon of the formation of Waisoi mineralisation 94

4.1 MainWaisoi porphyry mineralised areas 101

i 4.2 Mineralised prospects in the Namosi area 102 I 4.3 Waisoi drilling 104

4.4 Waisoi IP survey image 106 4.5 Classical alteration model for porphyry systems 147 4.6 Diorite model of porphyry alteration 148 4.7 Western Pacific porphyry mineralisation 149 4.8 Waisoi paragenetic sequence 152

5.1 Frequency distribution for homogenisation temperatures 176 5.2 Frequency distribution for freezing temperatures 177 5.3 Homogenisation/Freezing temperature relationships 179 5.4 CMF Waisoi drainage geochemistry 184

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5.5 5.6 5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15

AMAX Waisoi drainage geochemistry AMAX regional drainage geochemistry AAA Waisoi drainage geochemistry AMAX Waisoi ridge and spur soil Cu geochemistry AMAX Waisoi grid soil Cu geochemistry AMAX Waisoi grid soil Mo geochemistry AMAX Waisoi grid soil Au geochemistry AMAX Waisoi grid soil Zn geochemistry Scatter plots of elements from the wholerock geochemistry Ternary diagram for Cu, Mo and Au at Waisoi Scatter plot of copper and gold for Waisoi drilling

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185 186 188 190 191 193 194 195 197 199 202

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LIST OF PLATES

Plate Title Page

Front Waisoi Valley

I 3.1 Fresh Wainimala Agglomerate 59 3.2 Alteration affected Wainimala Agglomerate 60 3.3 Alteration affected Wainimala Agglomerate to give an 61

agglomeratic appearance 3.4 Fresh Basal Namosi Conglomerate 63 3.5 Coarse-grained units of the Basal Namosi Conglomerate 64 3.6 Fine-grained units of the Basal Namosi Conglomerate 66 3.7 Fresh Namosi Andesite 70 3.8 Variations in Namosi Andesite due to alteration 72 3.9 Fresh Korobasabasga Pyroclastics 73 3.10 Quartz-feldspar Porphyry 76

i 3.11 Typical mineralising porphyry at Waisoi 79 3.12 Typical mineralising porphyry at Waisoi showing alteration 80

,'i 4.1 Examples of disseminated sulphide mineralisation 109 4.2 Disseminated chalcopyrite, pyrite and magnetite 110

I 4.3 Disseminated chalcopyrite, bornite and pyrite 112 I I 4.4 Quartz stockwork vein mineralisation 113

4.5 Quartz micro-veinlets hosting copper mineralisation 114 4.6 Sulphide grains in polycrystalline quartz 115 4.7 Occurrences of pyrite 117 4.8 Electrum grains in the structurally remobilised massive 118

sulphide zone 4.9 Regional burial metamorphic assemblages 122

.. I 4.10 Albite altered Wainimala Agglomerate 123 .j

.··: 4.11 Alteration of the Basal Namosi Agglomerate 127 l 4.12 Disseminated Alteration 129

4.13 Chlorite clots 130 4.14 Disseminated pyrite 131 4.15 Disseminated magnetite . 132 4.16 Pervasive alteration adjacent to veins 134 4.17 Carbonate alteration 136 4.18 Pervasive alteration 138 4.19 Pervasive sericite alteration 139 4.20 Pervasive biotite alteration 141 4.21 Pervasive silica alteration 142 4.22 Pervasive carbonate alteration 143 4.23 Complex cross-cutting vein relationships 154 4.24 Vein relationships 155

5.1 Primary fluid inclusions 165 5.2 Large secondary fluid inclusions 167 5.3 Secondary fluid inclusions 168 5.4 Small secondary fluid inclusions 169 5.5 Waisoi fluid inclusion types 170

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