Publication Type:Book Chapter
Source:Geological Society of America, 2016 annual meeting & exposition, Geological Society of America (GSA), Boulder, CO, United States, Volume 48 (2016)
The 2740 Ma Cote 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 which suggest analogues with modern porphyry deposits versus orogenic- or syenite associated gold mineralization. The low-grade (ca. <1 g/t Au) mineralization is spatially and temporally (U-Pb titanite, Re-Os molybdenite) related to a magmatic (biotite diorite)--hydrothermal (biotite+ or -amphibole+ or -sulfide+ or -magnetite+ or -carbonate+ or -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 sequence 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 delta (super 18) O for vein quartz (+10.0 ppm) indicate that for 400 degrees C and 350 degrees C, delta (super 18) O (sub H2O) = +6.1 and =+4.7 ppm, respectively, a non-magmatic fluid invaded the dominantly magmatic hydrothermal system during mineralization, the setting of which suggests seawater. Data for delta (super 34) S (sub sulfides) are centred on 0 ppm, but excursions to lower values (-4 ppm) indicate the mineralizing fluid at times approached the pyrite-magnetite buffer. The data presented are interpreted to suggest the Cote 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, H (sub 2) O-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.
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