THE STABILITY OF GOLD POLYSULFIDE COMPLEXES IN AQUEOUS SULFIDE SOLUTIONS - 100-DEGREES-C TO 150-DEGREES-C AND 100 BARS

Gold solubility was measured in sulfur-saturated aqueous solutions con taining small amounts of dissolved NaHS and NaOH or HCl at 100-150 deg rees C and 100 bars. Addition of NaOH and HCl to sulfur-saturated solu tions creates systematic variation in pH and f(O2) and changes the rel ative abundances of dissolved sulfur species. Strong correlations betw een gold solubility and the abundances of aqueous polysulfide species suggest that Au-polysulfides (AuSnS- where n = 2-7) are the dominant A u-bearing complexes in sulfur-saturated solutions. A preliminary asses sment of our data suggests that Au-polysulfide species likely dominate the transport and deposition of Au at temperatures from 100-150 degre es C in a broad region surrounding the sulfur saturation field in pH-f (O2) - k(S2) space. Although Au(HS)(2)(-) complexes clearly dominate g old solubility in sulfide solutions significantly undersaturated with respect to sulfur, our results suggest that the highest gold solubilit ies should occur in solutions coexisting with elemental sulfur.