The electrum of the Agatovskoe deposit (Northeast Russia) and its formation conditions

This paper discusses the largest electrum units precipitated in calcite of the final (“nonproductive”) stage of ore formation. Notably, during examination of the chemical composition of these gold grains, sphalerite and other minerals belonging to the early (“productive”) association have been found. The results of thermo- and cryometric investigations of fluid inclusions in calcite of the Agatovskoe deposit have indicated that in the composition of two-phase fluid solutions of admixtures, NaCl, KCl, and MgCl dominated. This is evidenced by chloride eutectics of the admixture solutions in the temperature interval from −35 to −39°C. The complete homogenization of the fluid admixtures takes place in a narrow temperature interval (from 206 to 213°C) and concentration of salts is 3.7–6.0 mass % in NaCl equivalent. The density of the fluid changes starting from 0.88–0.89 g/cm³. The derived homogenization temperatures correspond to the temperatures of gold deposition, since the pressure during formation of gold veins in the deposit was small.