| Summary: | Epithermal veins of the southern Dawson Range, Yukon, are hosted by a wide range of lithologies, ranging from Proterozoic metasedimentary rocks to Late Cretaceous volcanic rocks. Both the 105 Ma calc-alkaline Mount Nansen volcanic group and the 70 Ma shoshonitic Carmacks volcanic group also host small Au-Cu(+/-Mo) porphyry deposits. Although the structurally-controlled mineralized veins are spatially associated with Mount Nansen felsic dykes, K/Ar dates for sericitically and argillically altered dykes proximal to mineralization are --70 Ma, indicating thermal resetting by a Carmacks-age hydrothermal event. These altered dykes are depleted in Na, Pb, Zn, and Cu, but enriched in As and Sb, relative to unaltered dykes. The base metal-rich epithermal veins are transitional between classic low and high sulphidation deposit types. They were deposited by a Na +-dominated fluid with an average salinity of ~6 wt.% NaCl equiv., a temperature of --300°C, a high S2-- (0.20 m) concentration, and significant CO2 (~1 mol.%). The isotopic composition of inclusion fluids (delta18O = ~--10‰; deltaD = ~--95‰) indicate that the hydrothermal fluid was dominantly meteoric, but a magmatic fluid contribution is suggested by the heavy delta18O, delta34S values close to zero, and the high temperatures, salinities, SigmaS, and CO2. The isotopic compositions of Pb in galena correlate well with initial whole rock values for Carmacks volcanic rocks, suggesting that much of the Pb, and other base metals, were leached from the Carmacks volcanics. As and Sb, and by inference Au, are not locally derived. The log fO2 and pH conditions of mineralization were between --34 and --36, and 3--5, respectively. The deposition of base metal sulphides appears to have been caused by an increase in pH due to the consumption of H+ during the alteration of the host rocks. The decrease in SigmaS caused by sulphide precipitation destabilized Au(HS)2-- complexes, precipitating native gold. The mineralization was caused by the hydrothermal circulation of acidic, meteoric water, probably driven by Carmacks magmatism.
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