EXPERIMENTAL INVESTIGATION OF THE HYDROTHERMAL GEOCHEMISTRY OF PLATINUM AND PALLADIUM .2. THE SOLUBILITY OF PTS AND PDS IN AQUEOUS SULFIDE SOLUTIONS TO 300-DEGREES-C

A large number of experiments (n = 185) were used to measure the solub ilities of synthetic PtS (cooperite) and PdS (vysotskite) in aqueous H 2S-rich solutions at 30 to 300-degrees-C. The experiments were perform ed in sealed silica tubes (T > 100-degrees-C) or in Pyrex vessels equi pped with gas-tight PTFE valves (T < 100-degrees-C). The solutions con tained SIGMAS2- (aq) = 0.01 to 1.0 m, with pH controlled by HCl/Cl-(pH 200-degrees-c = 0.5 to 1.0); H3PO4/H2PO4- (3.0 to 4.3); H2S/HS- (6.4 t o 7.9); or H2PO4-/HPO42- (7.7 to 8.2). The solubilities of both sulfid es increased systematically with increase in SIGMAS2-. Maximum solubil ities were 11.1 ppm Pd and 2.4 ppm Pt, for 200-degrees-C solutions con taining 1.0 m SIGMAS2- and pH values near the H2S/HS- boundary. Much l ower solubilities were measured in acid solutions (<10 ppb). In genera l, solubilities decreased with increase in temperature, especially bet ween 200 and 300-degrees-C. The results were used to calculate the fol lowing equilibrium constants (I = 0.0, P = P(sat)): [GRAPHICS] A polyn omial was obtained for reaction (c): log K = -11.10 + 0.0183T - 9.523 x 10(-5)T2 (T = degrees-C. valid between 30 and 300-degrees-C). These data indicate that Pd(HS)2 and Pt(HS)2 will dominate over most of the range of pH, SIGMAS2, and redox state found in nature. Although solubi lities as these species are quite low (<0.1 ppb), they are many orders of magnitude greater than the contribution from chloride complexes fo r solutions in which H2S(aq) is dominant (e.g., Salton Sea brines). Pr ecipitation of PGE minerals will occur in response to a decrease in a( H2S) or a shift in f(O2) away from the stability fields of PtS and PdS . In contrast, cooling, pH change, and dilution should have little eff ect. Comparison with previous work indicates that the PGE are orders o f magnitude less soluble in H2S-rich solutions than Au and Ag. Therefo re, formation of deposits with a high (Pt + Pd)/(Au + Ag) ratio requir es either a PGE-preconcentration step, undersaturation with respect to Au and Ag during metal precipitation, or remobilization of Au and Ag during a later hydrothermal event.