Abstract

The Giant mine is a mesothermal, greenstone-hosted gold deposit that has produced ∼250 metric tons of gold, principally from sulfide ores in altered metavolcanic rocks. Previous studies concluded that mineralizing fluids acquired metals and other ore-forming components from within the ore-hosting metavolcanic rocks and ascended a steep-dipping shear zone to the site of ore deposition. Our studies indicate that although the metavolcanic host rocks were important geochemically in the precipitation of gold, extensive metasedimentary rocks to the east were a more important conduit and/or source of fluids, metals, and ore-forming constituents. Geochemical analyses reveal an east-dipping Na depletion zone extending from the ore zone to within the metasedimentary sequence that coincides with enrichments in Ag, As, S, and Sb and with δ180quartz values of 11.7‰ to 14.1‰. These data indicate that wall-rock–hosted gold mineralization was deposited where fluids emerging from metasedimentary rocks encountered highly reactive Ti-rich tholeiitic basalts. From a geochemical standpoint, this ore system represents a metasedimentary-type gold deposit hosted in metavolcanic rocks. Documentation of a metasedimentary influence on formation of the minerals of the Giant mine helps explain why smaller greenstone belts can host substantial economic gold mineralization and has important implications for exploration for giant (>150 t) gold deposits.

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