Metamorphism, geochronology and stratigraphy of an amphibolite-facies greenstone-hosted gold deposit: Plutonic Gold Mine, Marymia Inlier, Western Australia by Michael Fuller Gazley A thesis submitted to Victoria University of Wellington in fulfilment of the requirements for the degree of Doctor of Philosophy in Geology Geology Department School of Geography, Environment and Earth Sciences Victoria University of Wellington, New Zealand 2011 In memory of F L Fuller & W V Gazley Two men who inspired me to ask questions ii A jumbo working in the BD3959WDD. iii ABSTRACT METAMORPHISM, GEOCHRONOLOGY AND STRATIGRAPHY OF AN AMPHIBOLITE-FACIES GREENSTONE-HOSTED GOLD DEPOSIT: PLUTONIC GOLD MINE, MARYMIA INLIER, WESTERN AUSTRALIA Michael F. Gazley Under the supervision of Dr. Julie Vry and Prof. Joel Baker with Dr. Monica Handler and Dr. Marc-Alban Millet A significant proportion of the world‟s Au occurs in the metamorphosed mafic rocks of Archaean greenstone belts. In such deposits, the original stratigraphy and its possible role in localising Au mineralisation can be difficult to discern due to a lack of distinctive marker units and the mineralogically and texturally monotonous nature of the metabasaltic host rocks. Understanding the effects of metamorphism, deformation, and alteration on these largely uniform host rocks, which may have experienced multiple generations of Au mineralisation, is essential for finding and extracting Au from within those deposits, and for discovering new greenstone-hosted Au deposits. This study examines the effects of primary stratigraphy on Au mineralisation, the conditions and possible controls on metamorphism, and the timing of Au mineralising events at Plutonic Gold Mine (Plutonic), Plutonic Well Greenstone Belt (PWGB), Marymia Inlier, Western Australia. Questions that remain unresolved in over 20 years of mining can now be addressed utilising advances in portable X-Ray fluorescence (pXRF), thermodynamic modelling of mineral activities and geochronological techniques. iv The stratigraphy of the Au-mineralised amphibolite-facies metabasalts that comprise the mine sequence at Plutonic has been examined using pXRF techniques. The results illustrate a geochemical stratigraphy in which individual lava flows can be identified on the basis of element concentrations. The most evolved basalts are at the structural base of the succession, and the least evolved at the top of the sequence. This confirms previous geochemical interpretations and textural evidence that the sequence is overturned, and demonstrates for the first time that the presented section does not involve significant structural repetition. In conjunction with Au assay data, the pXRF data reveal that Au typically occurs along basalt flow boundaries. The elemental concentration data clearly demonstrate stratigraphic control on Au mineralisation that is not readily apparent at the macroscopic level. Results of P–T pseudosection calculations in the NCFMASHTOS (Na2O-CaO-FeO- MgO-Al2O3-SiO2-H2O-TiO2-O-SO2) system are presented for two typical metabasaltic rocks from the Plutonic. Those results, together with changes in mineral compositions and mineral assemblages observed in the rocks, are used to argue that a previously-unrecognised steep pressure increase (from ~ 3–4 kbar at ~ 500 °C to ≥ 8 kbar at ~ 600 °C) accompanied metamorphism to peak temperatures. Existing models for the early evolution of the PWGB involve nappe stacking supported by relatively cold strong crust, with little overall change in thickness and with peak metamorphism at temperatures similar to those reported here, but with pressures of ~ 4 kbar. Prior to this study the main episode of Au mineralisation in the PWGB was interpreted to either have accompanied or shortly followed the attainment of peak metamorphic conditions in the late Archaean at ~ 2650 Ma. New Pb-isotope results reveal that the majority of Au-associated sulphides at Plutonic are Proterozoic in age, v at ~ 2200 Ma, suggesting that Au-mineralisation may have been widespread in the inlier and associated cratonic areas at that time. Later Au-mineralising events have also been constrained at ~ 1830 Ma, and at 1730–1660 Ma. Rb-Sr data from a biotite from Plutonic possibly indicates that the metamorphism was followed by a protracted period of slow cooling. Alternatively, the biotite data may reflect some combination of resetting, probably related to metasomatic events associated with Au mineralisation at ~ 2200 Ma, or with the Capricorn Orogeny at ~ 1830 Ma, and cooling. A further metasomatic event at ~ 1720 Ma is dated by both U-Pb dating of zircon overgrowths, and a new 207Pb-206Pb age from a hydrothermal sphene in chlorite-carbonate vein of 1725 ± 26 Ma. This metasomatic event was probably associated with Au mineralisation, as the Pb-isotope ages for the final Au-mineralising event range from 1730–1660 Ma. vi ACKNOWLEDGEMENTS First and foremost, thanks must go to my primary supervisor, Julie Vry; without her guidance, effort and encouragement I could never have made it through this project. Her multi-coloured edits have gained notoriety, but they are the single most effective tool I know for bashing a student‟s writing into shape, and as painful as they were to write, they were equally painful to receive and do damage control on afterwards. I will forever be in her debt for her enthusiasm and commitment. Joel Baker, Monica Handler and Marc-Alban Millet deserve thanks for coaching and coaxing a metamorphic and economic geologist through the perils of ultra-clean lab work and geochronology. Stewart Bush, Richard Wysoczanski, John Patterson and Chelsea Tutt are acknowledged for their assistance with making, mounting and analysing thinsections in the microprobe. I have to acknowledge Matt Stevens, Katie Collins and Sarah Grain for sharing their honours and masters years with me, and providing comments and feedback on my work throughout. The three of them and coffee have kept me sane. Jamie Boorman is thanked for the hours that he spent keeping me company on the microprobe on those late-night runs and for his assistance in making THERMOCALC draw lines when I had all but given up. Thanks too, to all the other students at Victoria University who directly or indirectly have had a bearing on this project – the list of names would fill a page. vii Ettienne du Plessis is entirely responsible for bringing this study to be. Without his faith that a young Kiwi geologist who appeared on site as a vacation student had the ability to do this project (as an MSc, as it was then planned), I wonder where I would be today? His enthusiasm for rocks is infectious, and the financial and logistical backing that he, and Barrick Gold Australia Pacific have provided has been amazing. The assistance and ongoing support of Ray Swanson, Sam de Beer, Sue Murray, Kathi Dilworth and the rest of the geologists at Plutonic is also gratefully acknowledged. Roger Powell, Katy Evans and Johann Diener are gratefully acknowledged for their input with THERMOCALC and the modelling of phase equilibria in mafic rocks. Discussions with Rob Hough, John Walshe, Steve Barnes and Guillaume Duclaux have greatly improved my understanding of structure, tectonics, host rock and gold mineralising systems. Input from Kevin Cassidy on Pb-isotopes and his tour of the Alchemy Resources‟ site at Three Rivers, on the edge of the Marymia Inlier, is also gratefully acknowledged. Reviews by the following people have greatly improved the chapters: K. Howard Poulsen, Rob Kerrich, Katy Evans, Johann Diener, Steve Barnes, Richard White, and two anonymous reviewers. To my long-suffering partner throughout these last six years, Annette Pocock; you got me to Plutonic in the first place, and you have stuck with me through thick and thin. I know you most definitely wanted to be doing something more exciting with your life (like go to the beach...) than wait for me to emerge from working on my laptop to viii decide I had done enough to be allowed out for the day. Your encouragement and motivation to keep me focussed to get this thesis finished has been amazing, and your editing is always greatly appreciated – especially turning my sentences around the right way and reminding me “subject, object, verb” everytime I wrote a long convoluted sentence. I cannot thank you enough. Finally, and perhaps most importantly I have to thank my parents, Dennis and Frances Gazley, for their ongoing support and encouragement and for providing me with somewhere to live for my university career. I would never have got this far without you. Also, my brother, Scott Gazley; your humour, entertainment and camaraderie is always enjoyable. If I have omitted anyone, I apologise, and I thank you all the same. M. F. Gazley 08 November 2011 ix CONTENTS ABSTRACT ............................................................................................................................. iv ACKNOWLEDGEMENTS .................................................................................................... vii CONTENTS .............................................................................................................................. x LIST OF FIGURES .................................................................................................................. xi LIST OF TABLES .................................................................................................................. xii CHAPTER 1 - General Introduction ........................................................................................