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

Gold solubility and speciation in low density H2O-HCl vapour were investigated experimentally at temperatures between 300 and 450 degrees C, and pressures up to 366 bar using batch-type titanium autoclaves. Concentrations of total dissolved gold in the experimental condensates ranged from 0.9 ppb at 300 degrees C and 48 bar to 4.6 ppm at 400 degrees C and 297 bar. 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, Au-s + xHCl(g) + yH(2)O(g) = AuClx(H2O)(y) + x/2H(2,g) K-s,K-y 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 K-s,y with reciprocal temperature compare well with published experimental data for the solubility of gold at 1000 degrees 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 degrees C. In fluids with densities of 0.20 and 0.10 g cm(-3) the corresponding solubility maxima are reached at similar to 400 degrees C, and are of 14.8 and 0.49 ppm, respectively. (C) 2013 Elsevier Ltd. All rights reserved.