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The geology and genesis of the Central Zone alkalic copper-gold porphyry deposit, Galore Creek district, northwestern British Columbia, Canada Micko, Janina

Abstract

Located in the Late Triassic Galore Creek alkalic Cu-Au porphyry district in northwestern British Columbia, the Central Zone deposit represents the end-member of the silica-undersaturated class of alkalic porphyry systems. The deposit is hosted by volcano-sedimentary rocks of the Middle to Upper Triassic Stuhini Group that were intruded by a syenite-monzonite complex and hydrothermal breccias. Post-mineral tilt (45 to 60° W-SW) provides an opportunity to examine a vertically extensive depth range of the system, and the impact of host rocks and a redox control on the precipitation of sulfide and silicate alteration minerals. Early mineralization associated with potassic alteration is dominated by gold-bearing chalcopyrite + bornite (Cu:Au ~ 2:1). A second gold-poor mineralization event is associated with calc-potassic alteration and dramatically changes the Cu:Au ratio (5:1) in the core of the Central Zone. In general, greatest Cu-Au concentrations overlap lithological contacts characterized by contrasting ferromagnesian mineral content, thus forming redox gradients. Sulfur isotopic compositions emphasize the importance of fO₂ conditions in ore deposition. Sulfides in highly mineralized centers are characterized by moderately negative δ₃₄Ssulfide values (-10.66‰ to -7.84‰), whereas sulfides deposited distally show highly negative δ₃₄Ssulfide values (-17.13‰ to -4.03‰). These data suggest that the interaction of sulfate-rich (SO₄²-(aq)) fluids with varying amounts of Fe²⁺-bearing minerals in host rocks increased H₂S/SO₄²- leading to formation of reduced S, and precipitation of sulfide minerals. Trace elements such as V and As in host rocks and Eu²⁺ in hydrothermal garnet reflect the same redox influence. Vanadium and As are soluble under highly oxidizing conditions. The shift in oxidation state facilitates their incorporation in alteration minerals. Thus, highest V (>700ppm) and As (>40ppm) concentrations form halos distally to the redox gradients and ore bodies. Hydrothermal garnets near lithologic contacts contain excess Eu²⁺. In contrast to V and As, Eu²⁺ is soluble in reduced fO₂ conditions and precipitates close to the redox gradient. This study demonstrates that redox is the dominant control on ore deposition in the Central Zone. Recognizing redox changes may provide a valuable guide for future exploration in the Galore Creek district and perhaps other alkalic Cu-Au porphyry systems worldwide.

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