EXPERIMENTAL INVESTIGATION OF THE HYDROTHERMAL GEOCHEMISTRY OF PLATINUM AND PALLADIUM .3. THE SOLUBILITY OF AG-PD ALLOY PLUS AGCL IN NACL/HCL SOLUTIONS AT 300-DEGREES-C

The solubility of the assemblage AgCl(s) + Ag-Pd alloy was measured in NaCl/HCl solutions at 300-degrees-C using the silica tube method. Oxidation state was controlled by the solid phases, according to the reaction Ag(alloy) + 1/4O2(g) + Cl- + H+ = AgCl(s) + 1/2H2O. (A1) The SIGMACl concentrations of the solutions ranged from 0.2 to 4.0 m, with calculated pH300-degrees-C = 0.47 to 1.67. The surfaces of the alloys after the experiments were corroded and showed a substantial depletion in Ag relative to the initial compositions (either Ag.5Pd.5 or Ag.7Pd.3). SIGMAAg solubilities ranged from 0.026 to 0.39 m, whereas SIGMAPd solubilities were 3 to 4 orders of magnitude lower (max. of 10(-3.41) m). The results are in excellent agreement with previous studies of the solubility of AgCl (SEWARD, 1976; ZOTOV et al., 1986) and Pd-sulfide phases (GAMMONS et al., 1992) in hydrothermal brines. For the following reaction: Pd(alloy) + 2H+ + 4Cl- + 1/2O2(g) PdCl42- + H2O, (A2) a value of log K300-degrees-c = 12.81 +/- .18 was obtained, which compares with a value of 12.65 +/- 0.5 obtained by GAMMONS et al. (1992). The agreement is strong evidence in favor of the validity of both experimental investigations. The results of the present study indicate that Ag will be far more soluble than Pd over most of the range of pH, f(O2), and SIGMACl concentration found in nature. Nonetheless, Pd-rich alloys may form, owing to the strong thermodynamic partitioning of Pd into the solid phase. Scavenging of dissolved palladium by pre-existing Au-Ag alloy is a viable precipitation mechanism and may limit PGE mobility to extremely low levels in many natural hydrothermal systems. Because this is the first detailed study of its type, the methods and results presented in this study have important applications to future experimental work in the hydrothermal geochemistry of precious metal alloys.