Thermodynamic calculations of AuxAg1−x – Fluid equilibria and their applications for ore-forming conditions

• Solubilities of Au–Ag solid solutions are calculated at wide conditions.
• Ratios of dissolved Au and Ag depend only on pH at intermediate pH.
• Fluid conditions for high gold finenesses have been examined.

Concentrations of dissolved gold and silver species in hydrothermal fluids equilibrated with Au–Ag solid solutions have been calculated at wide conditions on the well known fO2–pH spaces. Ratios of the total concentrations of dissolved gold and silver species (∑Au/∑Ag) are higher as pH higher and fO2 lower. The ratios are constant at very low and high pH conditions where major dissolved species of both gold and silver are chloride complexes and thio complexes, respectively, while the ratios practically depend only on pH at intermediate pH conditions where Au(HS)2− and AgCl2− are major.The calculated results indicate that the solid solutions of high gold finenesses may precipitate from the fluids of low ratios of the total concentrations of dissolved gold and silver species when the conditions are (1) low pH’s and/or (2) high concentration ratios of dissolved chlorine and sulfur and/or (3) high temperatures.