Evolution from sea floor-related to sulfide-rich quartz vein-type gold mineralization during deep submarine volcanic construction: The Geant Dormant gold mine, Archean Abitibi Belt, Canada

The Geant Dormant (Sleeping Giant) mine is a vein-type gold deposit that co ntrasts with Archean shear-related (mesothermal) deposits in the preductile deformation timing of the quartz gold-bearing veins and in their high sulf ide content. Gold mineralization is centered about a felsic complex that in trudes a volcano-sedimentary succession composed of mafic sills, interbedde d pelagic sediments, and mafic pillowed and felsic lavas. The felsic comple x 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 approximate to 12), wher eas all mafic rocks, including postmineralization dikes, are cogenetic and tholeiitic (Zr/Y approximate to 2.5). Strata were built from two different volcanic sources: a proximal felsic and a distal mafic one in a deep submar ine setting (> 3,150 m). Three successive mineralizing events are recognize d: (1) a sea floor-related gold-poor event, characterized by sericite, chlo rite, and quartz alteration with disseminations and stringers of gold-beari ng pyrrhotite and chalcopyrite affecting rocks of paleo-sea-floor origin: ( 2) an autometasomatic event affecting only the dacitic dome, expressed by c hloritization and dissemination of auriferous pyrite; and (3) quartz sulfid e vein-style gold mineralization, with only weak chlorite-sericite or no al tered selvages. The gold-bearing vein network was formed during the last stage of a four-st age volcanogenic hydrothermal system as indicated by the parallelism betwee n 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 inje ction 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 al ong active synvolcanic faults. These dikes served as conduits for the ascen sion of mineralizing fluids through the volcanic pile. Gold, and related qu artz and sulfides, precipitated in open fractures developed mainly in mafic rocks near these dikes. Vein formation is a consequence of the specific tw o-source volcanic pile. The exogenic voluminous mafic sills disturbed the e volution for vein formation, enhanced by pressurization of mineralizing flu ids at near-lithostatic pressure within the impermeable host sequence. Gold enrichment from stage 1 to stage 4 is consistent with the proposed evoluti on and most likely related to boiling of the mineralizing fluids during vei n formation.