Gold solubility and speciation in low density H2O-HCl vapour were investigated experimentally at temperatures between 300 and 450°C, and pressures up to 366bar using batch-type titanium autoclaves. Concentrations of total dissolved gold in the experimental condensates ranged from 0.9ppb at 300°C and 48bar to 4.6ppm at 400°C and 297bar. The hydrated gold monochloride species (AuCl(H2O)y) is the dominant gold species under the experimental conditions. Gold solubility increases with increasing water vapour pressure and can be expressed by the reaction,Aus+xHClg+yH2Og=AuClxH2Oy+x/2H2,gKs,yThe 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 logKs,ywith 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 0.1bar is similar to that in a vapour with approximately 50bar 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 (0.35gcm-3) aqueous fluids shows that gold solubility reaches a maximum of 253ppm at 500°C. In fluids with densities of 0.20 and 0.10gcm-3the corresponding solubility maxima are reached at ~400°C, and are of 14.8 and 0.49ppm, respectively. © 2013 Elsevier Ltd.
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