Orogenic gold systems in 3‐D space and time

International Symposium on Deep Earth Exploration and Practices Beijing, China - October 24-26, 2018 Orogenic gold systems in 3-D space and time John A. Percival1, Wouter Bleeker1 1Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8 Canada Lode gold deposits (structurally hosted gold-bearing quartz vein systems) occur in deformed, low- to medium-grade metamorphosed rocks of ages ranging from Archean to Cenozoic. They commonly form clusters along regional-scale faults or fault systems, deposited by low salinity, mixed H2O-CO2 fluids, representing a distinct mineral system1. Hronsky et al.2 proposed a unifying model for orogenic systems in which three key variables must coincide in order to concentrate significant gold resources to economic grades: 1) long-term fertilization of the upper mantle through subduction-related fluids and magmas; 2) lithosphere-scale structures providing mantle-to-crust pathways; and 3) a transient remobilization event in which gold is transported upward by fluids or magmas. Additionally, changes in ambient stress such as extensional collapse may be necessary to both trigger variables 2 and 3, and to prevent erosion of the high structural levels at which gold is trapped3. Decades of research on Canadian gold deposits have resulted in characterization and understanding at the deposit, camp and district scales4,5,6. The complex structural, lithological and geochemical nature of lode gold deposits is reflected in the variety of models proposed for the source of gold, the transport media and structural controls. A recent model, developed for greenstone-hosted deposits of the Timmins and Kirkland Lake camps (~100 million ounces Au) in the southern Abitibi greenstone belt, recognizes a complex series of steps necessary to form and preserve significant gold deposits3. Key among these is transient synorogenic extension, which created deep-penetrating faults, triggered alkaline magmatism, opened synorogenic basins that accumulated conglomerates, and, through crustal thinning, limited post-orogenic erosion of the goldbearing structural levels. In the Abitibi, the extensional phase (2686-2672 Ma) followed ca. 2687 Ma early thrust imbrication by a few million years and was succeeded by renewed thrusting and compression (26702660 Ma) and younger strike-slip faulting along structural ‘breaks’3. The main features observed on detailed seismic reflection profiles across Abitibi gold camps are antiformal culminations of thrust stacks, truncated by steeply-dipping fault zones7. These sub-vertical truncation zones extend to depths of a few kilometres, where they are underlain by sub-horizontal reflectors, inferred to represent ductile extensional fabrics in gneisses, developed after 2660 Ma8. A significant gold deposit is present at these deep structural levels: the Borden Lake deposit occurs in stretched-pebble conglomerate equivalent in age to the upper crustal extensional basins. Can such a region-specific model be applied elsewhere? Bleeker3 drew comparisons to the Archean Slave and Yilgarn cratons and their gold-producing regions. In west Africa, the Birimian (2.2-2.06 Ga) craton exhibits many similar features in its gold-bearing greenstone belts9. Recently, Honsberger and Bleeker10 documented a striking resemblance between Paleozoic gold systems of the Newfoundland Appalachians (ca. 0.45 Ga) and those of the Abitibi belt, suggesting that a recurring sequence of tectonic processes may be responsible for localizing and preserving structurally-hosted gold deposits within accretionary terranes. References Wyman, D., Cassidy, K.F. and Hollings, P., 2016. Orogenic gold and the mineral systems approach: Resolving fact, fiction and fantasy. Ore Geology Reviews 78: 322–335. 2 Hronsky, J.M.A., Groves, D.I., Loucks, R.R. and Begg, G.C., 2012. A unified model for gold mineralisation in accretionary orogens and implications for regional-scale exploration targeting methods. Mineralium Deposita 47: 339-358. 3 Bleeker, W., 2015. Synorogenic gold mineralization in granite-greenstone terranes: the deep connection between extension, major faults, synorogenic clastic basins, magmatism, thrust inversion, and long-term preservation; in Targeted Geoscience Initiative 4: Contributions to the Understanding of Precambrian Lode Gold Deposits and 1 International Symposium on Deep Earth Exploration and Practices Beijing, China - October 24-26, 2018 Implications for Exploration, (ed.) B. Dubé and P. Mercier-Langevin; Geological Survey of Canada, Open File 7852, pp. 25–47. 4 Poulsen, K.H., Robert, F., and Dubé, B., 2000. Geological classification of Canadian gold deposits; Geological Survey of Canada, Bulletin 540, 106 p. 5 Dubé, B., and Gosselin, 2007. Greenstone-hosted quartz-carbonate vein deposits; in Mineral Resources of Canada: A Synthesis of Major Deposit-types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods, (ed.) W.D. Goodfellow; Geological Association of Canada, Mineral Deposits Division, Special Publication 5, pp. 49–73. 6 Dubé, B., and . Mercier-Langevin, P., 2015. Targeted Geoscience Initiative 4: Contributions to the Understanding of Precambrian Lode Gold Deposits and Implications for Exploration. Geological Survey of Canada, Open File 7852 7 Snyder, D.B., Bleeker, W., Reed, L.E., Ayer, J.A., Houlé, M.G. and Bateman, R., 2008. Tectonic and metallogenic implications of regional seismic profiles in the Timmins mining camp. Economic Geology 103: 1135-1150. 8 Moser, D.E., Heaman, L.M., Krogh, T.E., and Hanes, J.A., 1996, Intracrustal extension of an Archean orogen revealed using single-grain U-Pb zircon geochronology: Tectonics 15: 1093–1109. 9 Béziat, D., Dubois, M., Debat, P., Nikiema, S., Salvi, S. and Tollon, F., 2008. Gold metallogeny in the Birimian craton of Burkina Faso (West Africa). Journal of African Earth Sciences 50: 215-233. 10 Honsberger, I., and Bleeker, W., 2018. Orogenic comparison of structurally controlled gold systems of the Abitibi greenstone belt and central Newfoundland Appalachians: Implications for Newfoundland gold potential and recurring tectonic drivers of gold mineralization; in Targeted Geoscience Initiative: 2017 report of activities, volume 2, (ed.) N. Rogers; Geological Survey of Canada, Open File 8373, pp. 65–70. http://doi.org/10.4095/306602