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 solubilities of synthetic PtS (cooperite) and PdS (vysotskite) in aqueous H2S-rich solutions at 30 to 300-degrees-C. The experiments were performed in sealed silica tubes (T > 100-degrees-C) or in Pyrex vessels equipped with gas-tight PTFE valves (T < 100-degrees-C). The solutions contained SIGMAS2- (aq) = 0.01 to 1.0 m, with pH controlled by HCl/Cl-(pH200-degrees-c = 0.5 to 1.0); H3PO4/H2PO4- (3.0 to 4.3); H2S/HS- (6.4 to 7.9); or H2PO4-/HPO42- (7.7 to 8.2). The solubilities of both sulfides increased systematically with increase in SIGMAS2-. Maximum solubilities were 11.1 ppm Pd and 2.4 ppm Pt, for 200-degrees-C solutions containing 1.0 m SIGMAS2- and pH values near the H2S/HS- boundary. Much lower solubilities were measured in acid solutions (<10 ppb). In general, solubilities decreased with increase in temperature, especially between 200 and 300-degrees-C. The results were used to calculate the following equilibrium constants (I = 0.0, P = P(sat)): [GRAPHICS] A polynomial 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 solubilities as these species are quite low (<0.1 ppb), they are many orders of magnitude greater than the contribution from chloride complexes for solutions in which H2S(aq) is dominant (e.g., Salton Sea brines). Precipitation 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 effect. Comparison with previous work indicates that the PGE are orders of magnitude less soluble in H2S-rich solutions than Au and Ag. Therefore, formation of deposits with a high (Pt + Pd)/(Au + Ag) ratio requires 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.