Geometry and Genesis of the Giant Obuasi Gold Deposit, Ghana
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Geometry and genesis of the giant Obuasi gold deposit, Ghana Denis Fougerouse, BSc, MSc This thesis is presented for the degree of Doctor of Philosophy. Centre for Exploration Targeting School of Earth and Environment The University of Western Australia July 2015 Supervisors: Dr Steven Micklethwaite Dr Stanislav Ulrich Dr John M Miller Professor T Campbell McCuaig ii "It never gets easier, you just go faster" Gregory James LeMond iv Abstract Abstract The supergiant Obuasi gold deposit is the largest deposit hosted in the Paleoproterozoic Birimian terranes of West Africa (62 Moz, cumulative past production and resources). The deposit is hosted in Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dykes and granites. In this study, the deformation history of the Obuasi district was re-evaluated and a three stage sequence defined based on observations from the regional to microscopic scale. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric. The D2Ob event is the main deformation stage and corresponds to a NW-SE shortening, involving tight to isoclinal folding, a pervasive subvertical S2Ob cleavage striking NE, as well as intense sub-horizontal stretching. Finally, a N-S shortening event (D3Ob) formed an ENE-striking, variably dipping S3Ob crenulation cleavage. Three ore bodies characteristic of the three main parallel mineralised trends were studied in details: the Anyankyerem in the Binsere trend; the Sibi deposit in the Gyabunsu trend, and the Obuasi deposit in the main trend. In the Obuasi deposit, two distinct styles of gold mineralisation occur; (1) gold-bearing sulphides, dominantly arsenopyrite, disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins up to 25 m wide. Both mineralisation styles are contained in high grade ore shoots classified in three groups on the basis of 3D modelling of deposit geometry from drill hole and underground development data; (1) volcanic rock controlled shoots, (2) fault intersection and bifurcation, and (3) F3Ob fold hinge controlled shoots. Strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob but folded by S3Ob, indicate that the sulphides were formed during D2Ob. In contrast, although the mineralised quartz veins formed during D2Ob, field, SEM and microtomographic observations demonstrate that the gold is hosted in microcrack networks in the veins, located in hinges of F3Ob folds. These observations provide the first evidence for multiple stages of gold deposition at the Obuasi deposit. The crystal plasticity and element mobility behaviour of the gold-bearing arsenopyrite during metamorphism (340° - 460° and 2 kbars) was investigated using quantitative electron backscatter diffraction analysis, ion microprobe imaging and synchrotron XFM mapping. The results show that the arsenopyrites remained remarkably robust during the high strain deformation (D2Ob) and preserved their gold content (300 to 3000 ppm Au). However, small amounts of crystal plasticity or intragranular microcracking, during D3Ob, enabled low volume fluid infiltration into the arsenopyrites and activation of a dissolution-replacement reaction, involving replacement by Au-poor (bellow detection limit of all techniques), Ni- i Abstract rich (up to 3000 ppm Ni) arsenopyrite coeval with Ni-rich pyrite and native gold precipitation in the microcracks. The newly formed arsenopyrite is also S- and Fe-depleted and As-enriched in comparison to the primary arsenopyrite. Thin-section scale mass balance calculations indicate gold was mobilised over distances greater than the centimetre scale; likely contributing to native gold along wall rock cleavage and at high concentrations in the fracture networks of the quartz veins. The gold remobilisation mechanism was likely controlled by strong chemical gradients at crystal-fluid interfaces induced by the salinity of the infiltrating fluid and S-release during the replacement reaction. It is suggested that the high grade native gold hosted in quartz veins was sourced from the arsenopyrite mineralisation. ii Table of content Table of content Chapter I: Introduction 1 1. Context and aim of this thesis 1 1.1. Context 1 1.2. Aim and objectives of this thesis 2 2. Methodology 3 2.1. Field observations, mapping and sample collection 3 2.2. Analytical techniques 3 2.2.1. Optical microscopy 3 2.2.2. Scanning Electron Microscopy 3 2.2.3. Electron Probe Microanalyzer 3 2.2.4. High resolution X-ray computed tomography 4 2.2.5. Electron Backscattered Diffraction 4 2.2.6. Ion probe imaging (SIMS and NanoSIMS) 4 2.2.7. Maia detector array and X-ray fluorescence microscopy imaging 4 3. Organisation of thesis 4 3.1. Thesis by papers and authorship 4 3.1.1. Chapter II 5 3.1.2. Chapter III 5 3.1.3. Chapter IV 6 3.1.4. Chapter V 6 3.1.5. Chapter VI 6 3.2. Appendixes 7 3.3. Supporting references 7 3.3.1. Appendix II 7 3.3.2. Appendix III 7 3.3.3. Appendix IV 7 iii Table of content 3.3.4. Appendix V 7 3.3.5. Appendix VI 7 3.3.6. Appendix VII 8 3.4. Summary declaration 8 4. References 8 Chapter II: Evidence for Two Stages of Mineralization in West Africa’s Largest Gold Deposit: Obuasi, Ghana 11 1. Abstract 13 2. Introduction 14 3. Geological Background 15 4. Methods 21 4.1. Field observations, mapping and sample collection 21 4.2. Petrography and microchemical analysis 21 4.3. High resolution X-ray computed tomography 22 5. Structure, Fabrics, and Deformation History of the Obuasi District 22 5.1. Early fabrics S0-S1Ob 22 5.2. Dominant fabric S2Ob and sub-horizontal stretching 23 5.3. Late crenulation fabric S3Ob 24 5.4. Quartz vein populations 25 5.5. Summary 28 6. Mineralization styles and relative timing of gold deposition at the Obuasi deposit 29 6.1. Deposit-scale geometry 29 6.2. Ore zone-scale geometry 30 6.3. Mineralization style I; arsenopyrite-hosted gold mineralization and its relationship to deformation fabrics 33 6.4. Mineralization style II; visible gold and its relationship to late-stage fracturing and folding 36 7. Discussion 42 iv Table of content 7.1. Model for structural evolution and multiple episodes of mineralization at Obuasi 42 7.2. Evidence for multiple stages of mineralization in the Birimian terrains of West Africa and worldwide 49 8. Conclusion 51 9. Acknowledgments 51 10. References 52 Chapter III: The golden ark: Arsenopyrite crystal plasticity and the retention of gold through high strain and metamorphism. 57 1. Abstract 59 2. Introduction 60 3. Samples and methods 60 4. Results 63 5. Crystal-plastic evolution of arsenopyrites and consequences for gold Retention 68 6. Acknowledgements 71 7. References 71 Chapter IV: Quantified, multi-scale X-ray fluorescence element mapping using the Maia detector array: application to mineral deposit studies 75 1. Abstract 77 2. Introduction 78 3. X-ray fluorescence microscopy (XFM) mapping methods 80 4. Results 81 4.1. Case study A 81 4.2. Case study B 84 v Table of content 4.3. Case study C 87 5. Discussion 89 6. Summary and conclusions 90 7. Acknowledgments 91 8. References 91 Chapter V: Gold remobilisation and formation of high grade ore shoots driven by dissolution-reprecipitation replacement and Ni substitution into auriferous arsenopyrite 95 1. Abstract 97 2. Introduction 98 3. Previous studies on trace element remobilisation 99 4. Sample suite and microanalytical methods 100 4.1. Sample suite 100 4.2. Analytical methods 101 5. Descriptive properties of arsenopyrites, mineral assemblages and native gold distribution 104 5.1. Arsenopyrite overgrowths and rims 104 5.2. Crystallographically controlled microtextures and chemistry 104 5.3. Mineral-filled microfractures 106 6. Discussion 113 6.1. Interpretation of mineral textures timing relationships and mass balance estimates 113 6.2. Gold remobilisation from arsenopyrite grains 114 6.3. Global Implications 121 7. Conclusions 122 8. Acknowledgments 122 9. References 123 vi Table of content Chapter VI: Conclusion 131 1. Key research outcomes 131 1.1. A new deformation scheme 131 1.2. Multistage mineralisation and alteration footprints 132 1.3. Microstructural and chemical evolution of the gold-bearing arsenopyrites 133 2. Implications for exploration 134 3. Further work 135 4. References 137 vii viii Acknowledgements Acknowledgements Here we are, 3.5 years from the start. The thesis is printed, finished, submitted. More than an ending point, I see this moment as a beginning, ready to further investigate the world of geology. I gained more than a degree here, but a wide range of skills and expertise as well as personal enrichment. This is what I am most grateful of and this has been accomplished with the help of many people. On top of the list, my supervisors, Steve Micklethwaite, Stano Ulrich, John Miller and Cam McCuaig. So much has been done with their help, particularly Steve who gave some top quality guidance and was always available for scientific and personal discussions through corridor meetings, Skype calls and countless emails. Thank you for that. The departure of Stano after the first year was heartbreaking, but Stano was always present and willing to help out. Thank you. John and Cam made this project possible and they too, were excellent advisors. Thank you both. Secondly, AngloGold Ashanti Ltd is gratefully acknowledged for its financial and logistic support. Jane Allen and Tom Gell managed this project with care and have been very comprehensive in releasing scientific communications. The Obuasi operations team logistic help, in particular Clement Asamoah-Owusu and Wisdom Kportufe, during the numerous field missions, was priceless. Thank you for everything. Many people in the dark helped with data collection, reduction and interpretation. Louise Fisher, Angela Halfpenny, Andrew Tomkins, Yuan Mei, Matt Kilburn, John Cliff, Laure Martin, Paul Guagliardo, Belinda Godel, David Adams, Janet Muhling, Richard Mazzucchelli, Mary Gee, Daryl Howard, and David Paterson, amongst others, are specially thanked.