THE 2740 MA CôTé GOLD AU(-CU) DEPOSIT, CANADA: EXAMPLE OF PORPHYRY-TYPE MAGMATIC-HYDROTHERMAL ORE-FORMING PROCESSES IN THE ARCHEAN

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, δ¹⁸O_{H2O =} +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 δ³⁴S_sulfides 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.