Abstract

Combining thermodynamic data at 25 °C and 1 bar with correlation algorithms enables prediction of equilibrium dissociation constants for aqueous platinum-group-element (PGE) complexes at high pressures and temperatures. We used these data (1) to calculate the speciation of Pd(II) in NaCl solutions at temperatures to 750 °C, pressures to 2 kbar, and values from 3 to 8, and (2) to predict the solubility of palladium at 2 kbar as a function of temperature, oxygen fugacity (fO2), pH, and total chloride concentration (mTCl-). Calculations indicate that as pH increases there is a sharp transition from chloride- to hydroxide-complex dominance at all chloride concentrations. This shift occurs at or below neutral pH in dilute solution, but at mTCl- ≥1.0, Pd(II) chloride complexes account for the majority of dissolved Pd(II) well into alkaline conditions. Solubility of palladium increases with increasing temperature, mTCl-, and fO2. At 700 °C, 2 kbar, and the fO2 set by the FMQ buffer, palladium solubility in a 3.0 m NaCl solution is ∼0.3 ppm at a pH of 4, and a one-unit pH variation can change palladium solubility by two orders of magnitude. These results suggest that PGE can be transported as chloride complexes in Cl-rich magmatic-hydrothermal solutions like those inferred from petrologic studies of the Stillwater and Bushveld complexes.

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