Mixing thermodynamic model of Au-Ag Alloys in gold deposits established by a modifier double defect method

The Au-Ag alloys show distinct statistical patterns across different types of gold deposits, yet the underlying thermodynamic control remain unknown. The difficulty arises from low accuracy in the mixing thermodynamic models for alloys and limited understanding of newly identified silver and gold species in hydrothermal fluids. To address this, this study introduces a updated thermodynamic mixing model using a modified Double Defect Method (DDM) and Monte Carlo simulations, and identifies key factors influencing Au-Ag alloy composition in the mineralization conditions of sulfur content (0–2 wt%), fO2 (−42 to −32), salinity (0–15 wt% NaCl), pressure (500–5000 bar), and fluid temperatures (200–500°C). Thermodynamic calculations suggest that elevated fS2, fO2, salinity, and temperature in hydrothermal fluids significantly reduce the Ag content in Au-Ag alloys. Conversely, a rise in pressure and pH result in an opposite effect. The observed double-hump pattern across a wide compositional range (0%–80%) of Au-Ag alloys in epithermal deposits is primarily attributed to much varied sulfidation states of the hydrothermal fluids, in combination with their progressive cooling and decompression under high-salinity and intermediate temperatures of 300 to 250°C. In contrast, the continuous compositional distribution within a relatively narrow range (0%–40%) in orogenic deposits is mainly driven by progressive cooling and decompression of hydrothermal fluids with a low sulfidation state, low salinity, and higher temperatures ranging from 450 to 350°C. Combined the thermodynamic traits with the mineralization process in varying deposit types, the composition of Au-Ag alloys could serve as a pragmatic indicator for gold exploration.