Gold solubility in alkaline and ammonia-rich hydrothermal fluids: Insights from ab initio molecular dynamics simulations

Abstract

Knowledge of speciation and thermodynamic properties for aqueous gold complexes are important for both the understanding and numerical modelling of the formation of hydrothermal gold deposits. While gold speciation in chlorine- and sulfur-rich hydrothermal fluids has been investigated by considerable theoretical and experimental studies, the complexation of gold with other ligands that may be important for transporting gold in hydrothermal fluids, such as hydroxyl (OH−) and ammonia (NH3) complexes, have received limited attention so far.In this study, we conduct ab initio molecular dynamics (MD) simulations to calculate the coordination structures of Au(I)–OH and Au(I)–NH3 complexes. Our simulations show linear structures of Au(I) complexes with two bonded ligands (OH−/NH3/H2O), consistent with previous experimental and theoretical studies of Au(I) complexation (e.g., Liu et al., 2014). The new speciation models show that Au(I)–OH−/NH3/H2O complexes are stable in hydrothermal fluids and vapors. We also use thermodynamic integration to determine the formation constants of these species at temperatures up to 350 °C and at water-saturated pressures; furthermore, we extrapolate these properties to wider temperatures and pressures range based on the Modified Ryzhenko–Bryzgalin equation of state parameters. In particular, this study, for the first time, reports the speciation and formation constants of mix-ligand [Au(NH3)(OH)]0(aq) complexes. The derived formation constants show that the stability of Au(I)–NH3 complexes decreases progressively with increasing temperature. The quantitative modelling of gold solubility based on the new thermodynamic data shows that gold can be transported as Au(I)–OH−/NH3 complexes in sulfur-poor and ammonia-rich hydrothermal fluids. In sulfur-bearing ore fluids, ammonia is only likely to transport small amounts of gold as [Au(NH3)(OH)]0(aq) in hydrothermal fluids in the near-neutral to alkaline fluids under extremely reduced conditions.