Metasedimentary influence on metavolcanic-rock-hosted greenstone gold deposits: Geochemistry of the Giant mine, Yellowknife, Northwest Territories, Canada

The Giant mine is a mesothermal, greenstone-hosted gold deposit that has pr oduced similar to 250 metric tons of gold, principally from sulfide ores in altered metavolcanic rocks. Previous studies concluded that mineralizing f luids acquired metals and other ore-forming components from within the ore- hosting metavolcanic rocks and ascended a steep-dipping shear zone to the s ite of ore deposition. Our studies indicate that although the metavolcanic host rocks were important geochemically in the precipitation of gold, exten sive metasedimentary rocks to the east were a more important conduit and/or source of fluids, metals, and ore-forming constituents. Geochemical analys es 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 delta(18)O(quartz) values of 11.7 parts per thousand to 14.1 parts per thousand. These data indicate that wall-rock-hosted gold mineralization was deposited where fluids emerging from metasedimentary ro cks encountered highly reactive Ti-rich tholeiitic basalts, From a geochemi cal standpoint, this ore system represents a metasedimentary-type gold depo sit hosted in metavolcanic rocks. Documentation of a metasedimentary influe nce on formation of the minerals of the Giant mine helps explain why smalle r greenstone belts can host substantial economic gold mineralization and ha s important implications for exploration for giant (>150 t) gold deposits.