Gold and Silver Minerals and Conditions of Their Formation at the Dorozhnoye Deposit (Magadan Region, Russia)

Abstract

This paper presents the brief geological and mineralogical characteristics of the Dorozhnoye mesothermal gold ore deposit (Magadan Region, Russia). The gold and silver minerals are re-presented by electrum (530‰ - 700‰), freibergite, stephanite, pyrargyrite and Bi-diaphorite. The homogeneous fluid inclusions enable one to determine that the quartz-molybdenite veins were formed at a temperature of 340˚C to 140˚C, and that the quartz-sulfide gold ore veins were formed at a temperature of 165˚C to 125˚C. It is assumed that the fluid had a mixed composition and evolved from K-chloride and K-carbonate to Na-chloride. The hypergenesis zone contains: high fineness gold (800‰ - 950‰), petrovskaite (AgAuS) and uytenbogaardtite (Ag3AuS2) associated with goethite and melanterite. The surface waters contain: Ca++ up to 293 mg/L, HCO3 up to 125 mg/L, SO4 up to 777 mg/L; рН is 5.8 to 7.2. Thermodynamic calculations have been conducted and Eh-pH diagrams have been built for the Au-Ag-S-H2O system at 25˚C and 100˚C (p = 1 bar) to reveal gold and silver stability fields. Au-Ag sulfides at the Dorozhnoye deposit are formed under oxidizing conditions and an acidic environment with dominating sulfates.