Abstract

The Geant Dormant (Sleeping Giant) mine is a vein-type gold deposit that contrasts with Archean shear-related (mesothermal) deposits in the preductile deformation timing of the quartz gold-bearing veins and in their high sulfide content. Gold mineralization is centered about a felsic complex that intrudes a volcano-sedimentary succession composed of mafic sills, interbedded pelagic sediments, and mafic pillowed and felsic lavas. The felsic complex includes a dacitic subvolcanic dome and a swarm of polyphase plagioclase-porphyritic and quartz and plagioclase-porphyritic dikes. All felsic rocks are cogenetic and have calc-alkalic affinity (Zr/Y nearly equal 12), whereas all mafic rocks, including postmineralization dikes, are cogenetic and tholeiitic (Zr/Y nearly equal 2.5). Strata were built from two different volcanic sources: a proximal felsic and a distal mafic one in a deep submarine setting (>3.150 m). Three successive mineralizing events are recognized: (1) a sea floor-related gold-poor event, characterized by sericite, chlorite, and quartz alteration with disseminations and stringers of gold-bearing pyrrhotite and chalcopyrite affecting rocks of paleo-sea-floor origin; (2) an autometasomatic event affecting only the dacitic dome, expressed by chloritization and dissemination of auriferous pyrite; and (3) quartz sulfide vein-style gold mineralization, with only weak chlorite-sericite or no altered selvages. The gold-bearing vein network was formed during the last stage of a four-stage volcanogenic hydrothermal system as indicated by the parallelism between volcanic and hydrothermal evolutionary patterns. At stage 1, diffuse sea floor-related hydrothermal fluids altered and mineralized lavas and Fe-rich sediments. Stage 2 involves the sealing of the hydrothermal system by injection of a 1-km-thick stack of impermeable mafic sills. During stage 3, the mineralizing system was reactivated by the emplacement of the dacitic dome. Stage 4 involves injection of quartz and plagioclase-porphyritic dikes along active synvolcanic faults. These dikes served as conduits for the ascension of mineralizing fluids through the volcanic pile. Gold, and related quartz and sulfides, precipitated in open fractures developed mainly in mafic rocks near these dikes. Vein formation is a consequence of the specific two-source volcanic pile. The exogenic voluminous mafic sills disturbed the evolution of a volcanogenic hydrothermal system related to the felsic center. Renewed felsic activity generated fractures for vein formation, enhanced by pressurization of mineralizing fluids at near-lithostatic pressure within the impermeable host sequence. Gold enrichment from stage 1 to stage 4 is consistent with the proposed evolution and most likely related to boiling of the mineralizing fluids during vein formation.

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