Abstract

The Clear Creek gold occurrences lie within deformed lower greenschist-facies rocks of the western Selwyn basin. They consist of auriferous, sheeted quartz veins that cut six Cretaceous stocks and their hornfels. The veins contain 1–2% combined pyrite and arsenopyrite, with lesser pyrrhotite, bismuthinite, and scheelite, as well as 2–5 g/t Au. New 40Ar/39Ar geochronology of hydrothermal micas indicates that the veins formed within 1–2 million years of granitoid emplacement. Fluid inclusion microthermometry defines a parent ore fluid of ∼81 mol.% H2O, 14 mol.% CO2, 4 mol.% NaCl ± KCl, and 1 mol.% CH4, which unmixed into a low- and high-salinity immiscible pair. This suggests a more saline parent fluid and a greater degree of fluid unmixing relative to the other occurrences in the eastern Tintina Gold Province. Inclusions trapped in As- and Bi-rich, high-gold grade veins have homogenization temperatures of 300–350°C, whereas inclusions found in more Ag- and Pb-rich veins are characterized by temperatures of 250–300°C. Fluid inclusion geobarometry suggests hydro-fracturing and gold deposition at 5–7 km depth. The δ18O values of quartz samples range from 13–16‰ (per mil) and δ34S for sulfides are also consistent between –3.0‰ and 0‰, with the exception of some outliers from the Contact Zone of the Pukelman stock that indicate a lower temperature second phase of mineralization. It remains uncertain as to whether the Clear Creek ore fluids were exsolved from magmas at depth or from devolatilization reactions within the contact metamorphic aureoles of the intrusions.

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