Abstract

Hypogene uytenbogaardtite, acanthite, and native gold parageneses have been revealed at the epithermal Yunoe gold-silver deposit, Magadan Region, Russia. Thermodynamic calculations in the system Si–Al–Mg–Ca–Na–K–Fe–Pb–Zn–Cu–Ag–Au–S–C–Cl–H2O were carried out at 25–400 °C and 1–1000 bars to elucidate the role of hydrothermal solutions in the formation of gold and silver sulfides. Several most probable scenarios for ore-forming processes in the deposit are considered: (1) interaction between cold and heated meteoric waters percolating along cracks from surface to depth and reacting with the host rock—rhyolite; (2) evolution of ascending postmagmatic fluid resulting in chloride–carbonic acid solution, which interacts with rhyolite at 100–400 °C; (3) stepwise cooling of hydrothermal ore-bearing solutions; (4) rapid cooling of ore-bearing hydrotherms on their mixing with cold surface waters. Rhyolite with Pb, Zn, Cu, Cl, S, Ag, and Au clarke contents was taken as an initial host rock. Calculations by model 3 showed the possible formation of uytenbogaardtite and petrovskaite at low-temperature stages. Gold and silver sulfides can be deposited during the mixing of ore-bearing acid chloride–carbonic acid hydrothermal solutions with surface alkaline waters.

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