Solubility of gold in NaCl-and H 2S-bearing aqueous solutions at 250–350°C

Abstract

A total of 108 silica capsule experiments were performed in order to determine the solubility of gold in aqueous solutions containing both NaCl and H 2S at temperatures of 250, 300, and 350°C, at pressures dictated by vapor/liquid coexistence. The starting materials in the capsules were H 2 O + S° + gold wire + NaCl (0 to 4 m) ± Na 2 SO 4. The pH, a H 2 S ( aq), aH 2( aq), ƒ H 2(g) ,ƒ O 2(g) , and ƒ S 2(g) values of the solutions were controlled by the sulfur hydrolysis reaction (4 S ° + 4 H 2 O(1) = 3 H 2 S( aq) + HSO − 4 + H +) and the sulfide/sulfate reaction. The quenched run products were analyzed for Au (0.1 to 66 ppm), ΣH 2S, ΣSO 2− 4, and pH. The calculated solution compositions at 250–350°C fall in the following ranges: pH = 1.9 to 5.0, log aH 2 S( aq) = −2.0 to −0.7, log a HSO 4 − = −3.8 to −1.4 , and log a H 2( 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 H 2S(aq) activity, indicating that gold dissolved largely as a bisulfide complex according to the reaction: Au(s) + 2H 2S(aq) = HAu(HS) 0 2 + 1 2 H 2(aq) . The equilibrium constant determined from our experimental data for the above reaction is constant over a temperature range of 250 to 350°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) 0 2: HAu( HS) 0 2 = H + + Au( HS) − 2. The log K value becomes −5.3 ± 0.5 at 250°C, −5.6 ± 0.6 at 300°C, and −6.2 ± 0.6 at 350°C. Therefore, HAu(HS) 0 2 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°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 H 2S-poor (e.g., <10 −4 m H 2S(aq) at 250°C), chloride-rich (>0.5 m 2Cl), 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 H 2 S( aq), such as by precipitation of sulfide minerals or by mixing of H 2S-poor fluids. Simple cooling or heating of fluids without changing the H 2(aq) and H 2S(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.