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The mineralogical, geochemical and isotope characteristics of alteration, mineralization and metamorphism of the Red Lake Gold Mines, Ontario Stock, Elizabeth Denise


Archean lode-gold deposits are a significant source of gold. However, exploration of this deposit type is hindered by their poorly understood genetic models and geochemical features. This project investigated the geochemical expression surrounding the Archean lode-gold Red Lake Gold Mines (RLGM) in the Superior Province, Canada. Mineral chemistry, whole rock and isotope geochemistry were used to establish how hydrothermal and metamorphic events influenced ore genesis. The RLGM is a basalt-hosted lode-gold deposit that formed from multiple superposed hydrothermal and metamorphic events. This study defined three significant superposed events which were important for gold mineralization. The first event was a widespread hybridized seafloor-magmatic event which caused reduction with FeO, MnO, K2O, SO3, SiO2, Rb, As and Cu enrichment. Seawater interaction created abundant micas-clays-chlorite-carbonate-FeMn oxides. Localized acidic magmatic fluids, in syn-volcanic faults, caused advanced argillic alteration. Subsequent peak-regional metamorphism created a widespread (>7km) occurrence of metamorphosed altered basalts. The micas-clays-chlorite-carbonate-FeMn oxides were metamorphosed to form Fe-biotite-Ti-magnetite±carbonate and Fe-chlorite-Fe-amphibole-FeMn-garnet-epidote/clinozoisite-magnetite-calcite-biotite assemblages. The metamorphosed argillic alteration created a quartz-muscovite-andalusite assemblage. Overprinting the widespread metamorphosed altered basalt was the significantly narrower (<1km), and structurally-controlled main vein and mineralization event. Abundant ankerite-calcite±quartz veins with replacement sulfide assemblages of pyrrhotite-pyrite-arsenopyrite-magnetite-gold-quartz-Mg-brown biotite were formed. The replacement ore-forming fluid enriched the basalt in CaO, MgO, SiO2, K2O, Ag, Au, Ni, Cu, Sb and Zn. Sulfides were created by sulfidation reactions between the host rock and ore-forming fluid. Redox differences during sulfidation reactions between the reduced wall rock and relatively oxidized fluid were identified as the main influence on gold precipitation. Carbon isotopes from the ankerite-calcite±quartz veins ranged from -4 to +4‰ δ13C which is consistent with a metamorphic origin by devolatilization reactions of pre-existing marine carbonates. Sulfur isotope from mineralized samples ranged from δ34SH2S 0–9‰. These are consistent with a magmatic source as well as metamorphic desulfidation of pre-existing sulfides (-7 to +6‰ 34S). The stable isotopes are most consisted with an evolved ore-forming fluid derived from a magmatic intrusion emplaced during regional-metamorphism. Therefore, gold mineralization at the RLGM was formed during ongoing, but post-peak regional metamorphism from an evolved magmatic source emplaced during a regional-contact metamorphic event.

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