VARIATIONS IN SILICATE AND SULPHIDE MINERAL CHEMISTRY BETWEEN FREE-MILLING "METALLIC"AND REFRACTORY "INVISIBLE" GOLD ORES, CON MINE, YELLOWKNIFE, N.W.T. by John P. Armstrong Depanment of Earth Sciences Submitted in partial fulfdment of the requirements for the degree of Doctor of Philosophy Faculty of Graduate Studies The University of Western Ontario London. Ontario Apnl 1997 O John P. .Armstrong. 1997 National Library Bibliothèque nationale du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellingtm ChtawaON K1AON4 Ottawa ON K1A ON4 Canada Canada The author has granted a non- L'auteur a accorde une licence non exclusive licence allowing the exclusive permettant à la National Library of Canada to Bibliothèque nationale du Canada de reproduce, loan, distribute or sell reproduire, prêter, distribuer ou copies of this thesis in microform, vendre des copies de cette thèse sous paper or electronic formats. la forme de microfiche/film, de reproduction sur papier ou sur format électronique. The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fiom it Ni la thèse ni des extraits substantiels may be printed or otherwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son permission. autorisation. -4BSTRACT The Con Mine is an epigenetic mesothermal Archean lode gold deposit sited in deep seated crustal shear zones that transect 2.70-2.65 Ma volcanic rocks of the Yellowknife Greenstone Belt. The economic gold is concentrated within the greenschist facies rocks of the Con and Campbell Shear Systems that retrograde amphibolite facies metabaalts within the contact metamorphic aureole of the rnultiphase (2634-2604 Ma) Western Plutonic Cornplex. Shear deformation and fluid mubility comrnenced at peak metamorphic conditions. Protracted retrograde fluid evolution within the shear zones occurred in an environment of steep and inverted geothemal gradients. The Con Mine hosts both free-milling "meiallic" and refractory "invisible1*gold mineralisation. There is no continuum between ore types: free-milling ores display heterogeneity in texture. rnineralogy, and alteration both dong strike and down dip reflecting prolonged. episodic vein evolution within a brittle ductile deformational regime. Free-milling ores are characterised by a muscovite-phengitic muscovite-chlonte-calcite- albite-pyrite-pyrrhotite-native gold assemblage associated with folded and boudinaged quartz carbonate veins. Temperatures caiculated by the chlorite geothermometry, a subtle decrease in goid fineness with depth, and previously cdculated fluid compositions indicate that free-rnillinp ores formed at temperatures of less than 375°C with NalK c 1 . 8'0 of 6.8%~. Refractory ores occur structurally higher and footwall to hee-rnilling ores in productive shear strands. Re fractory ores are characterised by a paragonitic muscovite-AI chlorite-FeNg carbonate-arsenopynte-pyrite assemblage associated with ductile deformation of pre-existing quartz-carbonate vein systems. The "invisible" refrzctory gold iii is hostrd in As-rnriched nms of As/S zoned arsenopyrire (up to 1900 ppm Au). and locally within As-enriched domains of pyrite grains (up to 450 ppm Au). Calculated temperatures of mineralisation are 300-420°C. Na/K >I. and &"O and GL3Cfluid values of 9.1%. and -2.27% respectively. The arnphiboiite facies environment. greenschist retrogression and presence of refractory gold ores makes the Con Mine a unique member among Archean lode gold deposits spanning the panulite to subgreenschist facies ternes. The refractory mineraiisation at the Con Mine specificaily relates to an environment that straddles structurally deeper arnphiboiite facies and structurally shailower greenschist facies goid depositional environments within the Archean gold continuum. ACKNOWLEDGMENT This thesis work would not have been possible without the financial assistance of NERCO Minerals which provided employment ( 1990.9 1.92) and analytical support ( 1990,199 1 ). a University of Western Ontario entrance scholarship ( 1WO), several teaching assistantshi ps, and National Science and Engineering Research Council gants. The author wishes to thank NERCO Minerals and MIRAMAR Mining for their financial and logistical support of this study. Robert L. Hauser and Dean McDonald are gratefully acknowledged for their patience, and the opportunity to cary out this research. The geology department at the mine showed exceptional hospitaiity, and provided a very capable introduction, education, and enlightenment on the dark world of underground geology. To the miners I've met and who take such pride in their workmanship, thanks for teaching the skill of decision making on the muck pile. standing your ground and the lesson to never go back to the face. A special thanks to the Heimbach farnily for their generous hospitaiity over the last seven years. G. Mount and C. Weisner from Surface Science Western conducted the SIMS analyses and provided many useful discussions. Dr. F.J. Longstaffe and P. Middlestead are thanked for their assistance. encouragement and funding of the carbonate stable isotope data. C. Wu carried out the whole rock and trace element analyses. J. Forth and Ci. Wood provided timely polished thin section preparation. Many thanks to my former colleagues in 15G and B&G for the opportunities to discuss and exchange ideas in a relaxed and at times boisterous atmosphere. Pamela Strand is thanked for her assistance in London and Yellowknife and her kindness toward my farnily. R.L. Barnett and Dave Kingston gave invaluable assistance with the collection and v interpretation of microprobe data. They provided hours of lively debate and outstanding advice. Bob has shown continued interest in the study and freely shared his vast kno wlédge of rninenl chernistry and expertise with a petrographic microscope. My supervisor. Dr. Norman Duke, is generously acknowledged for introducing me to Yellowknife, the Con Mine. the Gold Range. and for spearheading the start of this study. He has provided many useful discussions on pology and other more esoteric subjects. His guidance, patience, perseverance, and ability to provide editing advice across the miles has ailowed the thesis to be maintained across country over the past three years. 1 am indebted to my parents and brothers for their continued support and belief that it could be done. To my children Emily, Amanda, and Clara who wonder why Daddy is still in school, 1 sincerely hope they will never lose their curiosity about al1 things. Finally, 1 would like to acknowledge the tremendous unwavenng and undying support of my wife. Allison. For her patience. loving support. and sacrifices 1 am forever in her debt. Thank you, Allison. TABLE OF CONTENTS .. CERTiFICATE OF EXAMINATION . ri ... ABSTRACT . iii ACKNOWLEDGMENT . v TABLE OF CONTENTS . vii LISTOFAPPENDICES . x LIST OF PHOTOGRAPHIC PLATES . xii .. LIST OF TABLES . xi11 LIST OF FIGURES . xiv CHAPTER 1 INTRODUCTION 1.1 General Statement and Purpose of Study . 1 1.2 Location. Access. and Physiography . 3 1-3 Previous Geological Investigations . 5 1.4 Field work and Methodology . 6 1.5 Metallurgicd Classification of Gold Ore Types . 7 CHAPTER 2 GEmRAL GEOLOGY 2.1 Introduction . 10 2.2 Geology of the Yellowknife Greenstone BeIt . 15 2.3 Prograde Metarnorphism . 26 2.4 Retrograde Shear Systerns . 28 2.5 BrittIe FauIt Structures . 32 CHAPTER 3 CON MINE GEOLOGY 3.1 Introduction . 35 3.2 Host Lithologies . 35 3.2.1 Volcanic flows . 35 3.32 Gabbroic Intrusives . 37 3.2.2.1 Arsenic Pond Gabbro . 38 3.2.3.2 Late Gabbro dyke suite . 38 3.2.3 Granite Intrusives . 43 3.3 Prograde Contact Metamorphisrn ............................. 44 3.4 Peak Metamorphic Vein and Schist Development .................. 49 3.1.1 Amphibolitic Schists ............................... 49 3.4.2 Quartz-Plagioclase-Amphibole Veins .................... 51 3.5 Post-peak Potassium Metasomatism ........................... 54 3.6 Post-peak Retrograde BrittIe-DuctiIe Shear Systems ................ 60 3.6.1 Metallurgy of Con Mine Ore ......................... 61 3.6.2 Negus-Rycon Shears ............................... 64 3.6.3 Con Shear System ................................. 65 3.6.3.1 Shaft System .............................. 68 3.6.3.2 C32 Shear ................................ 72 3.6.3.3 C4 Shear ................................. 72 3.6.3.4 C34 Shear ................................ 87 3.6.3.5 C36 Shear ................................ 89 3.6.3.6 C39 Shear ................................ 92 3.6.4 Campbell Shear ................................... 93 3.6.4.1 Negus Zone ............................... 93 3.6.4.2 102 Zone ................................. 98 3.6.4.3 103 Zone ................................. 107 3.6.4.4 101 Zone ................................. 111 CHAPTER 4 GEOCHEMISTRY 4.1 Bulk Rock Geochemistry ................................... 132 4.1.1 Introduction ..................................... 132 3.1.2 Major Element Geochemistry ......................... 132 4.1.3 Major Element Geochemicai F'luxes ..................... 140 4.1.3.1 Pud Stock ................................ 140 4.1.3.2 102 Zone ................................. 141 4.1.3.3 101 Zone ................................. 141 4.2 Mineral Chemistry ....................................... 149 4.3.1 Amphibole .....................................