GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 212-11
Presentation Time: 4:20 PM


KONTAK, Daniel J.1, KATZ, Laura1 and DUBE, Benoit2, (1)Department of Earth Sciences, Laurentian University, Sudbury, ON P3E 2C6, Canada, (2)Geological Survey of Canada, 490 rue de la Couronne, Quebec, QC G1K 9A9, Canada,

The 2740 Ma Côté Gold Au(-Cu) deposit (8.65 Moz Au) lies in the Au-rich Archean Abitibi greenstone belt, Canada, but differs from the other gold deposits in its geological setting and attributes whichr suggest analogues with modern porphyry deposits versus orogenic- or syenite associated gold mineralization. The low-grade (ca. <1g/t Au) mineralization is spatially and temporally (U-Pb titanite, Re-Os molybdenite) related to a magmatic (biotite diorite) – hydrothermal (biotite ± amphibole ± sulfide ± magnetite ± carbonate ± apatite) breccia complex, part of the 2740 Ma diorite-tonalite suite of the Chester Intrusive Complex (CIC). The CIC is part of a subvolcanic feeder system which fed a sequnce basic to felsic, subaqueous volcanic rocks interbedded with marine sediments, including iron formation; trace-element chemistry of the basalts suggests a back arc extensional setting. The Au-Cu mineralization occurs as disseminated, stockwork and vein types which are co-spatial with well-developed biotite- and sericite alteration types which are overprinted by a pervasive, fabric destructive sodic alteration and localized propylitic alteration. The chemistry of biotite from magmatic through hydrothermal samples record a reversal in its mg# which combined with abundant magnetite suggests oxidation may have been an important control on Au mineralization. Results of δ18O for vein quartz (+10.0‰) indicate that for 400°C and 350°C, δ18OH2O = +6.1 and = +4.7‰, respectively, a non-magmatic fluid invaded the dominantly magmatic hydrothermal system during mineralization, the setting of which suggests seawater. Data for δ34Ssulfides are centred on 0‰, but excursions to lower values (-4‰) indicate the mineralizing fluid at times approached the pyrite-magnetie buffer. The data presented are interpreted to suggest the Côté Gold deposit reflects a high-level, subvolcanic setting wherein the release of metal-rich, in this case Au(-Cu), fluids were synchronous with over-pressuring of an evolved, H2O-rich dioritic magma. Release of the fluid resulted in formation of a mineralized, biotite-rich breccia body and related stockwork and vein type mineralization. The features described best equate therefore with younger analogues referred to as porphyry-type deposits.