SOLUBILITY OF GOLD IN NACL-BEARING AND H2S-BEARING AQUEOUS-SOLUTIONS AT 250-350-DEGREES-C

A total of 108 silica capsule experiments were performed in order to determine the solubility of gold in aqueous solutions containing both NaCl and H2S at temperatures of 250, 300, and 350-degrees-C, at pressures dictated by vapor/liquid coexistence. The starting materials in the capsules were H2O + S0 + gold wire + NaCl (0 to 4m) +/- Na2SO4. The pH, a(H2S)(aq), a(H2)(aq), f(H2)(g), f(O2)(g), and f(S2)(g) values of the solutions were controlled by the sulfur hydrolysis reaction (4S-degrees + 4H2O(1) = 3H2S(aq) + HSO4- + H+) and the sulfide/sulfate reaction. The quenched run products were analyzed for Au (0.1 to 66 ppm), SIGMA-H2S, SIGMA-SO4(2-), and pH. The calculated solution compositions at 250-350-degrees-C fall in the following ranges: pH = 1.9 to 5.0, log a(H2S)(aq) = -2.0 to -0.7, log a(HSO4-) = -3.8 to -1.4, and log a(H2)(aq) = -7.0 to -4.7. The results of our experiments indicate that gold solubility is independent of the activity of Cl- and H+ in the solutions, indicating that chloride complexes are not important. The gold solubility, however, increases with increasing H2S(aq) activity, indicating that gold dissolved largely as a bisulfide complex according to the reaction: Au(s) + 2H2S(aq) = HAu(HS)2(0) + 1/2H2(aq). The equilibrium constant determined from our experimental data for the above reaction is constant over a temperature range of 250 to 350-degrees-C at log K = -5.1 +/- 0.3. The above equilibrium constant, together with that for a reaction involving Au(HS)2- obtained by SHENBERGER and BARNES (1989), determines the dissociation constant of HAu(HS)2(0):HAu(HS)2(0) = H+ + Au(HS)2-. The log K value becomes -5.3 +/- 0.5 at 250-degrees-C, -5.6 +/- 0.6 at 300-degrees-C, and -6.2 +/- 0.6 at 350-degrees-C. Therefore, HAu(HS)2(0) is the dominant gold-sulfide species at pH below about 5.5, while Au(HS)2- becomes dominant at higher pH conditions. The results from our study suggest that the solubility of gold in ore-forming fluids in equilibrium with pyrite and/or pyrrhotite at 250-350-degrees-C is typically between 0.1 ppb to 1 ppm Au, transported mostly as bisulfide complexes; gold-chloride complexes do not become important unless the fluid is H2S-poor (e.g., < 10(-4)m H2S(aq) at 250-degrees-C), chloride-rich (> 0.5m SIGMA-Cl), and of low pH (< 4.5). Precipitation of gold from ore-forming solutions may occur by increasing a(H-2)(aq), such as by reactions with organic matter or ferrous-bearing minerals, or by decreasing a(H2S)(aq), such as by precipitation of sulfide minerals or by mixing of H2S-poor fluids. Simple cooling or heating of fluids without changing the H-2(aq) and H2S(aq) contents is not an effective mechanism of gold precipitation when gold is transported largely as a bisulfide complex. Effects of oxidation or of boiling on gold precipitation, however, cannot be easily evaluated from the available thermodynamic data alone.