THE GEOLOGY AND MINERALISATION OF THE WAISOI PORPHYRY COPPER DEPOSITS, NAMOSI PROVINCE, REPUBLIC OF FIJI. PETER D. ELLIS (B.Sc. Hons.).

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1 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, This thesis is submitted in partial fulfillment for the Master of Economic Geology Degree..1. i..

2 Frontpiece. The Waisoi valley showing the position of the three mineralised zones in relation to the Waisoi camp and the Korobasabasga Range. I -"' ~ ,{._;'", I I",

3 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. i i i 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.... ii

4 ... 11,. DISCLAIMER., 'j.'. i 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 j!.... I

5 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. >. 1:.. I. 1 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 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. f I 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 iv

6 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. i! I i 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.... : I.l.. I. I v

7 '. '.... :. 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,. I i 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. j.i 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... ' vi

8 ,' ':::.... ' CONTENTS Thesis Topic Disclaimer Abstract Acknowledgements Page ii iii iv vi 1. BACKGROUND ' INTRODUCTION AIMS AND TECHNIQUES Aims Techniques REGIONAL SETI"ING 15. ' 2.1. TECTONIC SETTING 16 I 2.2. REGIONAL GEOLOGY 25 "i Fiji 25, :-: Viti Levu 26...:; General Geology 27.,...' Regional Structure Geological Setting REGIONAL MINERALISATION WAISOI GEOLOGY PROSPECT GEOLOGY General Waisoi Geology 46 i. I Wainimala Agglomerate 57.. ' asal Namosi Conglomerate 62 I Namosi Andesite 68 j Korobasabasaga Pyroclastics Porphyries 75 I Surficial Deposits 82! ' Weathering Profile Interpretation PROSPECT STRUCTURE General Waisoi Structure Structural Setting Structural Detail Structural Interpretation PROSPECT GEOLOGICAL SUMMARY WAISOI MINERALISATION AND ALTERATION PROSPECT MINERALISATION General Waisoi Mineralisation General Distribution 105 vii

9 , '.. :::,:.:::] ' Disseminated Mineralisation 108 : Vein Mineralisation 111,j Massive Sulphide Mineralisation 116 ' Secondary Enrichment PROSPECT ALTERATION General Waisoi Alteration Regional Burial Metamorphism Porphyry-related Hydrothermal 124 Alteration General Distribution Disseminated Alteration Vein Alteration Pervasive Alteration 137! 4.3. MINERALISATION AND ALTERATION SUMMARY 145 '! 4.4. PARAGENESIS General Pre-Porphyry Alteration Porphyry Alteration Sericite Alteration Silica Alteration Potassic Alteration Chlorite Alteration Carbonate Alteration Paragenesis Summary WAISOI GEOCHEMISTRY 163 I 5.1. FLUID INCLUSIONS General Inclusion Petrography Homogenisation and Freezing Temperatures Homogenisation Temperatures Freezing Temperatures and 175 Salinities Fluid Inclusion Summary GEOCHEMISTRY Soil/Drainage/Rock-chip Geochemistry Drainage Geochemistry Soil Geochemistry Wholerock Geochemistry Mineralisation Geochemistry Selected Element Geochemistry Geochemical Summary 201 l 6. GENETIC MODEL AND CONCLUSIONS GENETIC MODEL CONCLUSIONS 209 REFERENCES 212 viii

10 LIST OF FIGURES Figure Title Page 1.1 Location and tectonic setting of Fiji in the SW Pacific 3.. I 1.2 Islands of Fiji with location of Waisoi Viti Levu with the Namosi and Waisoi exploration areas Simplified gross tectonic setting of Fiji Main Namosi mineralised areas 12! 2.1 Tectonic setting of Fiji Stages in the development of Fiji Simplified geological and magmatic event geology of Fiji Tectono-structural units in Fiji Geology of Fiji Simplified geology of Namosi Province Alternate geological interpretation of Namosi Thrust interpretation of Namosi AMAX interpretation of Namosi Regional SLAR image of Viti Levu Major structures of Viti Levu Geodynamic evolution of the Vanuatu-Fiji region 39 <I I 2.13 Mineralisation of Fiji 41 I. : I 3.1 Regional geology of the Waisoi-Waivaka area Local Waisoi stratigraphy Waisoi area geological interpretation East-west cross-section of Waisoi 88800N North-south cross-section of Waisoi 37645E North-south cross-section of Waisoi 37435E North-south cross-section of Waisoi36875E North-south cross-section of Waisoi E North-south cross-section of Waisoi 35825E Distribution of AMAX porphyries Namosi area SLAR image with significant structures Major structural elements of Waisoi Cartoon of the formation of Waisoi mineralisation MainWaisoi porphyry mineralised areas 101 i 4.2 Mineralised prospects in the Namosi area 102 I 4.3 Waisoi drilling Waisoi IP survey image Classical alteration model for porphyry systems Diorite model of porphyry alteration Western Pacific porphyry mineralisation Waisoi paragenetic sequence Frequency distribution for homogenisation temperatures Frequency distribution for freezing temperatures Homogenisation/Freezing temperature relationships CMF Waisoi drainage geochemistry 184 ix

11 . i, 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 , ' ' ' l. i I X

12 ' :... LIST OF PLATES,'i I i Plate Title Page Front Waisoi Valley 3.1 Fresh Wainimala Agglomerate Alteration affected Wainimala Agglomerate Alteration affected Wainimala Agglomerate to give an 61 agglomeratic appearance 3.4 Fresh Basal Namosi Conglomerate Coarse-grained units of the Basal Namosi Conglomerate Fine-grained units of the Basal Namosi Conglomerate Fresh Namosi Andesite Variations in Namosi Andesite due to alteration Fresh Korobasabasga Pyroclastics Quartz-feldspar Porphyry Typical mineralising porphyry at Waisoi Typical mineralising porphyry at Waisoi showing alteration Examples of disseminated sulphide mineralisation Disseminated chalcopyrite, pyrite and magnetite 110 I 4.3 Disseminated chalcopyrite, bornite and pyrite 112 I I.. I 4.4 Quartz stockwork vein mineralisation Quartz micro-veinlets hosting copper mineralisation Sulphide grains in polycrystalline quartz Occurrences of pyrite Electrum grains in the structurally remobilised massive 118 sulphide zone 4.9 Regional burial metamorphic assemblages 122.j 4.10 Albite altered Wainimala Agglomerate 123. : 4.11 Alteration of the Basal Namosi Agglomerate 127 l 4.12 Disseminated Alteration Chlorite clots Disseminated pyrite Disseminated magnetite Pervasive alteration adjacent to veins Carbonate alteration Pervasive alteration Pervasive sericite alteration Pervasive biotite alteration Pervasive silica alteration Pervasive carbonate alteration Complex cross-cutting vein relationships Vein relationships Primary fluid inclusions Large secondary fluid inclusions Secondary fluid inclusions Small secondary fluid inclusions Waisoi fluid inclusion types 170 xi

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