Gold solubility was measured in sulfur-saturated aqueous solutions containing small amounts of dissolved NaHS and NaOH or HCl at 100-150°C and 100 bars. Addition of NaOH and HCl to sulfur-saturated solutions creates systematic variation in pH and fo2 and changes the relative abundances of dissolved sulfur species. Strong correlations between gold solubility and the abundances of aqueous polysulfide species suggest that Au-polysulfides (AuSnS- where n = 2-7) are the dominant Au-bearing complexes in sulfur-saturated solutions. A preliminary assessment of our data suggests that Au-polysulfide species likely dominate the transport and deposition of Au at temperatures from 100-150°C in a broad region surrounding the sulfur saturation field in pH-fo2-fs2 space. Although Au(HS)-2 complexes clearly dominate gold solubility in sulfide solutions significantly undersaturated with respect to sulfur, our results suggest that the highest gold solubilities should occur in solutions coexisting with elemental sulfur.
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