Western Australian School of Mines Department of Applied Geology Geological Controls on the Distribution of Arsenic in the Perseverance Nickel Deposit, Leinster, WA Alexandre Albiero Diragitch This thesis is presented for the Degree of Master of Philosophy (Geology) of Curtin University, Perth, Western Australia November, 2014 To the best of my knowledge and belief this thesis contains no material previously published by any other person except where due acknowledgment has been made. This thesis contains no material which has been accepted for the award of any other degree or diploma in any university. Alexandre Albiero Diragitch Date 29/11/2014 1 ABSTRACT The Perseverance deposit is a highly deformed ultramafic disseminated and semi- massive sulphide orebody, resulting in significant mobilization of primary magmatic sulphide ores into axial planar shear zones that have been subsequently refolded (Barnes et al 2011 and references therein). The bulk of the Perseverance orebody comprises a basal accumulation of matrix ores (heavily disseminated sulphide mineralisation), occupying an arcuate channel feature, with an extensive asymmetric halo of disseminated sulphides. The Perseverance Ultramafic Complex (PUC) is interpreted as a high-flux, flow-through conduit formed by evolving magmas and there is a pervasive signature of country-rock contamination throughout the complex (Barnes et al, 2011). The most common arsenic-bearing minerals in the Perseverance deposit are Co- Ni-Fe sulpharsenide phases, Gersdorffite (NiAsS) and Cobaltite (CoAsS). High arsenic concentrations within main Perseverance orebody were previously known in the massive sulphide ore shoots and along the footwall massive sulphide rocks in areas with intense shear zones. The main matrix sulphide mineralisation was considered previously as an essentially arsenic “free” mineralisation, however, based on this study, it has been confirmed that the arsenic zone is not only confined to the remobilized massive sulphides, it is also found within the main matrix and heavily disseminated sulphide ore of PUC, with samples reaching excess of over 50,000ppm As (total whole rock analyses). Arsenic-bearing minerals within the PUC are found within: olivine-tremolite-chlorite rocks containing carbonate, which correspond to metamorphosed komatiite flows, with the variation in present olivine content reflecting an original alternation between spinifex textured A zones and olivine-enriched cumulate B-zones; massive sulphides (which includes massive sulphides with brecciated rocks); matrix and disseminated sulphide olivine-cumulate rocks containing carbonates; the PUC basal contact unit, which formed within a mylonitic zone; and a serpentinised olivine-cumulate rock, located at the basal portion of the PUC. Cobaltite is interpreted to have minimum crystallisation temperatures between 500ºC to 550ºC. Gersdorffite has a wider crystallisation temperature range, with an interpreted minimum crystallisation temperature estimated from approximately 250ºC to 550ºC. Arsenic-bearing minerals whose minimum crystallisation temperature lies at approximately 550ºC are inferred to be associated with the i peak metamorphic event, with lower minimum temperatures recording remobilisation by later retrograde events. The arsenic source and its influx into the Perseverance Ultramafic Complex remains unclear, and it may realistically be linked to any of the events from magmatism to peak metamorphism, including early serpentinisation and talc- carbonate alteration prior to the de-hydration reactions that occurred during peak metamorphism. Evidence of the earlier (magmatic and/or early alteration) processes is now overprinted by the ubiquitous metamorphic and metasomatic events throughout the study area. ii ACKNOWLEDGMENTS First of all, I would like to thank my supervisors Dr Katy Evans and Dr Belinda Godel for their guidance, supervision and discussions during this research, especially for the last year. I also would like to thank especially Dr Peter Collins for his help, guidance and support during the early stages of this project and he may have his deserved rest in peace now. The author thanks BHP Billiton (Nickel West), Leinster Nickel Operations for the opportunity to undertake this project and for financial and logistic support. Special thanks to geologist Dr Carlos Spier for encouraging the start of this project and extra support; Keith Rogers and Sean Grimshaw for sample preparation and selection; Martin Lewis for the support with drawings and maps; geologists Michael Bryant, Cameron Chambers, Steven Guy and Marcel Menicheli for their support and discussion regarding the geology of the deposit and arsenic controls. The author thanks Elaine Miller and Dr William Rickard for their support and attention during the long sessions using the SEM at the Centre for Materials Research, Department of Physics, Curtin University. Some of the ideas and geological knowledge of this project rely on works developed by previous geologists from BHP Billiton, Western Mining Corporation and academic researchers. Their work during the long history of the Perseverance Nickel Deposit is acknowledged here. Special thanks for Fernanda Veraldo for her support; help with the laboratory activities and patience during the entire project. iii TABLE OF CONTENTS 1. INTRODUCTION ........................................................................................... 1 1.1. Location and Access ............................................................................... 2 1.2. Objective ................................................................................................. 2 1.3. Background............................................................................................. 3 1.4. Research Methodology ........................................................................... 4 2. GEOLOGY OF THE PERSEVERANCE DEPOSIT ............................................ 7 2.1 Regional Geology - Stratigraphy and Metamorphism ............................... 7 2.2 Local Geology .........................................................................................11 2.2.1 Lithostratigraphy ..............................................................................11 2.2.1.1 Footwall Rocks ..............................................................................11 2.2.1.2 Perseverance Ultramafic Complex ................................................12 2.2.2 Nickel Sulphide Mineralisation..........................................................13 2.2.3 Structural Geology ...........................................................................15 2.2.4 Metamorphism and alteration of ultramafic rocks .............................16 3. MACROSCOPIC DISTRIBUTION OF ARSENIC AT THE PERSEVERANCE DEPOSIT .............................................................................................................19 3.1. High arsenic domain definition and macro distribution ...........................19 3.2. Correlation of arsenic and other commom chalcophile elements in the high arsenic domain ......................................................................................28 3.3. Summary of macrodistibution of the high arsenic domain ......................31 4. SAMPLE SELECTION AND WHOLE ROCK DATA ......................................32 4.1. Sample criteria and spatial location ........................................................32 4.2. Whole Rock Geochemistry of the Selected Drillholes from Footwall Rocks .....................................................................................................................39 4.3. Whole Rock Geochemistry of the Selected Samples from PUC .............41 4.4. Summary of whole rock geochemistry ...................................................47 5. PETROLOGY AND MINERALOGY OF THE PERSEVERANCE DEPOSIT ......49 5.1. Footwall Rocks ......................................................................................51 5.1.1. Amphibolite schists (C-54, C-55, C-59) ...........................................51 5.1.2. Meta-sedimentary formation (C-04, C-05, C-13, C-53, C-57) ..........52 iv 5.1.3. Rhyodacite porphyry (C56, C-58) ....................................................53 5.2 Perseverance Ultramafic Complex ..........................................................54 5.2.1 Tremolite-chlorite-olivine rocks .........................................................55 5.2.1.1 Tremolite-chlorite-olivine rocks with carbonate rich zones ............57 5.2.2 Massive sulphide .............................................................................59 5.2.3 Matrix and disseminated sulphide olivine-cumulate ..........................62 5.2.3.1 Altered matrix and disseminated sulphide olivine-cumulate rock with carbonate rich zones .................................................................................64 5.2.4 Olivine-adcumulate with sulphides ...................................................66 5.2.5 PUC basal contact – Mylonitic zone .................................................67 5.2.6 Serpentinised olivine-cumulate sulphide rock ...................................69 5.3. Summary of petrology and mineralogy at the Perseverance deposit inside the study area ...............................................................................................72 6. SULPHIDES, SULPHARSENIDES AND OXIDES COMPOSITIONS ................75 6.1. Sulphides ...............................................................................................75 6.1.1