Gold solubility, speciation, and partitioning as a function of HCl in the brine-silicate melt-metallic gold system at 800°C and 100 MPa

A vapor-undersaturated synthetic brine was equilibrated with metallic gold and a haplogranitic melt at 800degreesC and 100 MPa to examine the solubility, speciation and partitioning of gold in the silicate melt-brine-metallic gold system. The starting composition of the NaCl-KCl-HCl-H2O brine was 70 wt.% NaCl (equivalent) with starting KCl/NaCl ranging from 0.5 to 1. KCl/HCl was varied from 3.2 to 104 to evaluate the solubility and partitioning of gold as a function of the concentration of HCl in the brine. Inclusions of brine were trapped in a silicate glass during quench. Inclusion-poor and inclusion-rich portions of glass were analyzed for gold and chloride by using neutron activation analysis. The inclusion-poor glass yielded an estimate of the solubility of gold and chloride in the silicate melt. The solubility of gold in the melt, at gold metal saturation, was estimated as approximate to1 ppm. The solubility of gold in the brine was estimated by mass balance, given the concentration of gold and chloride in the inclusion-poor and inclusion-rich glasses. The solubility of gold metal at low-HCl concentrations in the brine, C-HCl(b), (3x10(3) to 1.1x10(4) ppm) is approximate to40 ppm (by weight) and is independent of the HCl concentration under those conditions. For C-HCl(b) of 1.1x10(4) ppm 4.0x10(4) ppm, the solubility of gold increased from 40 to 840 ppm, and the solubility is given by: log C-Au(b) = [2.2 . log C-HCl(b)] - 7.2 (1) These data suggest that a significant amount of gold is not chloride complexed in brines at low-HCl concentrations (<1.1x10(4) ppm), but that gold-chloride complexes, possibly AuCl2H, are important at elevated concentrations of HCl (>1.1x10(4) ppm). The calculated Nernst partition coefficient (D-Au(b/m)) for gold between a brine and melt varied from 40 to 830 over a range of brine HCl concentrations of 3x10(3) to 1.1x10(4) ppm. Our results indicate a significant amount of gold can be transported by a brine in the magmatic-hydrothermal environment independent of the fugacity of sulfur in the system. Thus brines provide an effective mechanism for the scavenging of gold from a crystallizing melt and transport into an associated magmatic-hydrothermal system, regardless of their sulfur contents. Copyright (C) 2002 Elsevier Science Ltd.