Effect of selenium on silver transport and precipitation by hydrothermal solutions: Thermodynamic description of the Ag-Se-S-Cl-O-H system

An internally consistent thermodynamic database has been compiled for the Ag-Se (?II, 0, +IV)-S (?II, +VI)-Cl-O-H system on the basis of published experimental data. This database is fit for predicting the chemical properties of species over a broad interval of temperatures (0–600°C) and pressures (1–3000 bar). The stability constant of the hydroselenide silver complexes [AgHSe_aq] and [Ag(HSe) ₂ ^? ] has been estimated. A thermodynamic model of the effect of Se on the transport properties of Ag-bearing hydrothermal fluid has been developed. Selenium has been found to be a good precipitant even if its concentration in the fluid is five orders lower than the sulfur concentration. When the temperature falls, the role of selenium as a precipitant becomes increasingly important. It has also been shown that the effect of hydroselenide complexes on Ag behavior in natural hydrothermal fluids may be ignored.