THE GEOLOGY AND GEOCHEMISTRY OF THE OMAI GOLDFIELD, GUYANA. ROY GRAHAM ELLIOTT A thesis submitted in partial fulfilment of the requirements of Oxford Brookes University* for the degree of Doctor of Philosophy. This research programme was carried out in collaboration with Guyana Geology and Mines Commission (GGMC) and Golden Star Resources (GSR) Ltd., Alberta, Canada. December 1992 * formerly Oxford Polytechnic. PAGES NOT SCANNED AT THE REQUEST OF THE UNIVERSITY SEE ORIGINAL COpy OF THE THESIS FOR THIS MATERIAL Now we see but a poor reflection as in a mirror; then we shall see face to face. Now I know in part; then I shall know fully, even as I am fully known. I CorinthiilRs 13:12. The geology and geochemistry of the Omai goldfield, Guyana. by R. G. Elliott ABSTRACT The Omai goldfield consists of mesothermallode gold mineralisation and associated saprolite- alluvial placer deposits, hosted within Palaeoproterozoic granite-greenstone terrain of the Guiana Shield. Total mining reserves are estimated at 44.8 million tonnes, grading at 1.43 glt Au. The goldfield lies along an ESE-trending, regional-scale structure referred to as the Issano-Appaparu shear zone. At Omai, the gold deposits are located in two discrete ore zones - the Omai Stock Zone and the Wenot Lake Zone. The bulk of primary mineralisation is centred on a high-AI, quartz diorite-tonalite boss (the Omai Stock), where wall-rock alteration is dominated by a hydrothermal sericite-carbonate assemblage. The primary ore package of Au-W-Te-S mineralisation is contained in a series of narrow (1-5cm), quartz- carbonate (ankerite) veins. Visible gold is commonly associated with galena and microscopic tellurides. Provisional fluid inclusion studies have indicated that the parent hydrothermal fluid was H20-C02 -bearing (- 5.0 mol% CO2), of low salinity (0-1.8 wt. % NaCI equiv.) and moderate density (0.96 g/cm3). The depositional temperature of the fluid was probably in the region of 200-400oC. Preliminary 6'80 values are consistent with a magmatic andlor metamorphic source. The Majuba Suite greenstones adjacent to the Omai Stock are also partially mineralised. These rocks are dominated by primitive, low-K, high-Fe tholeiitic (HFT) basalts and minor basic intrusives which are characterised by flattish REE patterns of around 10 times chondrite. A subordinate, calc-alkaline series (CAS), of mostly andesitic composition, is intercalated within the volcanic pile. In terms of trace element geochemistry, the Majuba Suite is chemically comparable to volcanic rocks from modern island arc settings. The REE geochemistry, which proved to be extremely diagnostic throughout, indicates that the Omai Stock is genetically related to the regional Tigri pluton. This and other Trans- Amazonian soda granitoid plutons in the area have chemical affinities akin to calc-alkaline volcanic arc granitoids of Phanerozoic orogenic belts. Several lamprophyric (appinite) pipes are spatially and temporally associated with the Omai Stock. Although the appinites are only weakly mineralised, they may have a genetic significance regarding the source of the gold. Both the Omai Stock and Majuba Suite are cut by a series of partially mineralised, ultrabasic to intermediate dykes (the Gilt Creek Suite) which intruded along shallow-dipping fracture zones. These rocks are typically Mg-rich, LREE enriched (LanlYbn = 7.84-32.6) and appear to be the product of alkaline arc magmatism. A few post-orogenic mafic dykes (POMDs), of probable Mesozoic age, are also identified in the area. These rocks are strictly non- mineralised and are chemically correlatable with tholeiitic basalts from continental rift-related settings. In the Wenot Lake Zone, lode mineralisation is hosted within thin bands of high-silica rhyolites (HSRs), also of the Majuba Suite, close to a sheared contact with phyllitic tuffs. The HSRs have a distinct chemistry characterised by elevated Si02 (> 75.0%), Na, Nb, Zr and REE contents with anomalously low concentrations of Mg, Ti, Ca, K, Rb and Sr. These rocks are further characterised by wing-shaped REE patterns containing deep negative Eu anomalies. ACKNOWLEDGEMENTS Anyone who has undertaken a research degree will know that John Donne was right in saying "no man is an island". For sure, this thesis would never have been completed without the help and co-operation of numerous people. Firstly, I would like to thank Chris Barron for introducing me to life and geology in Guyana, way back in 1984, and for his friendship since then. I would also like to thank my former colleagues at the Guyana Geology and Mines Commission (GGMC) where I spent several happy years working as a geologist and where this research was initiated. Dave Fennell, Hilbert Shields and Carlos Bertoni, all of Golden Star Resources (GSR) Ltd, helped to get the project off the ground and provided invaluable logistic support along the way. Funds for the research and production of this thesis were made available by Oxford Brookes University (OBU)·. My thanks are also extended to my main supervisors, Prof. Howard Colley (OBU) and Or. Bill Perkins (University College of Wales, Aberystwyth) for their guidance and counsel throughout the course. Anton Kearsley provided the expertise with the SEM-EDS facilities at OBU and freely gave countless hours of his time. Jon Wells, Dave Hicks, Simon Deadman and Chris Gilbert, all of OBU, supplied invaluable technical support in the respective fields of thin-sectioning, information technology, photography and geochemistry. Thanks are also due to Or. Dave Alderton (Royal Holloway) who supervised the fluid inclusion and stable isotope studies and to Ron Hardy (University of Durham) who ran the XRF analyses. I am also very grateful to Drs. Robert De Vletter (Netherlands), Fred Barnard (USA), Asit Choudhuri (Brazil) and Xafi (Brazil) who kindly supplied inaccessible reference material, as did Catherine Kehoe and Liz Longshaw of the Inter-Library Loan (ILL) office at OBU. Finally, I would like to express my appreciation to my parents, Francis and Margaret, for all their love and support over the years. • formerly Oxford Polytechnic. II TABLE OF CONTENTS • page Abstract I Acknowledgements II list of Figures VIII List of Tables XIV List of Plates XV CHAPTER 1: INTRODUCTION 1 1 .1 Opening Remarks 1 1.2 Organisation of this research 1.3 Aims 1 1.4 Guyana - "the land of many waters" 2 1.5 The Omai goldfield 6 1 .5. 1 Location and access 6 1.5.2 Mining and exploration history (1886-1992) 6 1.5.3 Previous research (1899-1992) 9 CHAPTER 2: TECTONIC SETTING AND GEOLOGICAL FRAMEWORK 13 2.1 Aims of chapter 13 2.2 Tectonic setting 13 2.3 Geological outline of Guyana 15 2.4 Regional geology and structure of the Omai area 17 2.4.1 Geology 17 2.4.2 Structure 22 2.5 The geology of the Omai goldfield 23 2.5. 1 The Omai Stock Zone 23 III page 2.5.2 The Wenot Lake Zone 45 2.5.3 The oxidised zone 48 CHAPTER 3: GEOCHEMISTRY OF THE MAJUBA SUITE AND THE CAPTAIN 53 MANN SILL 3.1 Aims of chapter 53 3.2 Description of the geochemical subset 53 3.3 Elemental analyses 53 3.4 Constraints 54 3.5 Alteration 54 3.6 Element mobility in the Majuba Suite (Omai Stock Zone) 58 3.6.1 Major elements 58 3.6.2 Trace elements 59 3.6.3 Element mobility: summary 62 3.7 Geochemistry of the Majuba Suite 71 3.7.1 Compositional classification 71 3.7.2 Magmatic affinity 72 3.7.3 Geochemical characteristics 75 3.7.4 Petrogenesis 81 3.7.5 Tectonic association 85 3.7.6 Regional correlation 94 3.8 Geochemistry of the Captain Mann Sill 98 CHAPTER 4: GEOCHEMISRTY OF THE OMAI GRANITOID ROCKS 106 4.1 Aims of chapter 106 4.2 Description of the geochemical subset 106 4.3 Elemental analyses 107 4.4 Constraints 107 4.5 Trans-Amazonian granitoid complexes - a regional overview 108 IV page 4.6 Alteration and element mobility 109 4.7 Magmatic affinity and peraluminosity 119 4.8 Geochemical characteristics 119 4.8.1 The Omai Stock 119 4.8.2 The Tigri pluton 120 4.8.3 The Mariaba pluton 128 4.8.4 The Mowasi pluton 129 4.9 Petrogenesis 129 4.10 Tectonic association 138 4.11 Regional correlation 143 CHAPTER 5: GEOCHEMISTRY OF THE OMAI MAFIC DYKE SUITES 148 5.1 Aims of chapter 148 5.2 Description of the mafic dyke subset 148 5.3 Constraints 148 5.4 General overview 149 5.5 Minor basic intrusives of the Majuba Suite 149 5.6 Appinite pipes (Badidku Suite 1) 149 5.6.1 Geochemical characteristics 149 5.6.2 Petrogenesis 152 5.6.3 Tectonic association 155 5.6.4 Regional correlation 155 5.7 The Gilt Creek Suite 155 5.7.1 Geochemical characteristics 156 5.7.2 Petrogenesis 156 5.7.3 Tectonic association 157 5.7.4 Regional correlation 157 5.8 Post-orogenic mafic dykes (POMDs) 165 5.8.1 Geochemical characteristics 167 v page 5.8.2 Petrogenesis and tectonic association 167 5.8.3 Regional correlation 169 CHAPTER 6: MINERALISATION 180 6.1 Aims of chapter 180 6.2 Ore-sample subset 180 6.3 Aspects of the primary ore zones 180 6.3.1 Style of mineralisation 180 6.3.2 Structural controls on mineralisation 181 6.3.3 Timing of mineralisation 181 6.3.4 Alteration paragenesis 182 6.3.5 Mineral paragenesis (Omai Stock Zone) 182 6.3.6 Mineral paragenesis (Wenot Lake Zone) 190 6.4 Fluid inclusion studies 191 6.4.1 Analytical technique 191 6.4.2 Sample description 191 6.4.3 Results and interpretation of data 191 6.4.4 Comparison with global gold deposits 198 6.5 Light stable isotope characteristics (0 and C) 199 6.5.1 6'3C and 6'80 systematics of hydrothermal carbonate 199 6.5.2 613C and 6'80 systematics of the hydrothermal fluid 200 6.5.3 Origin of the Omai hydrothermal fluid 203 6.5.4 Summary of 0 and C