An experimental study of the transport of gold through hydration of AuCl in aqueous vapour and vapour-like fluids

The hydration number (y) increases with increasing pressure, thereby indicating that solvation by H2O molecules in the gas-phase is analogous to that in liquid-like fluids. Results of extrapolation of the data using a linear relationship of log Ks,y with reciprocal temperature compare well with published experimental data for the solubility of gold at 1000 °C in dilute HCl-bearing water vapour. At high water vapour pressure, the solubility of gold in an aqueous vapour with an HCl fugacity of 0.1 bar is similar to that in a vapour with approximately 50 bar H2S, in which AuS is the dominant gaseous gold species. This indicates that hydrated gold monochloride species may play an important role in magmatic-hydrothermal systems dominated by low density aqueous fluids with high HCl concentrations. Modelling of the cooling and decompression of HCl-bearing intermediate-density (0.35 g cm−3) aqueous fluids shows that gold solubility reaches a maximum of 253 ppm at 500 °C. In fluids with densities of 0.20 and 0.10 g cm−3 the corresponding solubility maxima are reached at ∼400 °C, and are of 14.8 and 0.49 ppm, respectively.