We have conducted a series of experiments on the solubility of metallic osmium and palladium in supercritical aqueous KCl fluids in which pH was buffered by the assemblage K-feldspar-quartz-muscovite and oxygen fugacity was controlled by various oxygen buffers at 400 degrees and 500 degrees C. An important objective of these well-constrained experiments is to evaluate current theoretical models of PGE solubility. Our experimental results indicate that the concentration of Os in a 1.0 m KCl solution buffered by the Ni-NiO (NNO) assemblage at 500 degrees C is similar to 3 ppb (similar to 1.9 x 10(-8) moles/kg H2O), whereas the concentration of Os in a 1.5 m KCl solution also buffered by NNO at 500 degrees C is similar to 130 ppb (similar to 6.5 x 10(-7) moles/kg H2O). As the pH in these two experiments is nearly the same, this finding implies that Os is present in solution as a chloride complex. The concentration of Os in a 1.5 m KCl solution but buffered by a higher oxygen fugacity buffer (Re-ReO2 assemblage) (RRO) is about one order of magnitude higher (similar to 1705 ppb; similar to 1 x 10(-5) moles/kg H2O). The palladium concentrations in a 0.1 m KCl solution buffered by RRO at 500 degrees C and in a 0.1 m KCl solution buffered by MnO-Mn3O4 at 400 degrees C are similar to 40 ppb (similar to 3.8 x 10(-7) moles/kg H2O) and N 15 ppb (1.4 x 10(-7) moles/kg H2O), respectively. Our experimental results on the solubility of Pd metal in combination with field observations show that the theoretical calculations of Sassani and Shock (1990, 1998) on palladium underestimate the solubility of palladium by many orders of magnitude under supercritical conditions. Our experimental results are directly applicable to porphyry copper systems because in such systems temperature, pH, salinity and oxygen fugacity are similar to the experimental parameters. Application of our experimental results indicate that a typical porphyry systems can transport at least 40 tons of Pd if sources of platinum-group elements (PGE) are available and the solubility-controlling phase is metallic Pd. Similarly, the amount of osmium that can be transported in a typical porphyry system is calculated to be at least similar to 3 tons. Consequently, porphyry systems have the capacity to transport sufficient PGE to form at least mid-sized PGE deposits. The absence of important Os mineralization in known porphyry deposits may be due to the lack of availability of source materials, or to the presence of Os in less soluble forms, such as sulfides.
