EXPERIMENTAL QUANTIFICATION OF HYDROTHERMAL SOLUBILITY OF PLATINUM-GROUP ELEMENTS WITH SPECIAL REFERENCE TO PORPHYRY COPPER ENVIRONMENTS

Abstract

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° and 500°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°C is ∼ 3 ppb (∼1.9 × 10−8 moles/kg H2O), whereas the concentration of Os in a 1.5 m KCl solution also buffered by NNO at 500°C is ∼ 130 ppb (∼ 6.5 × 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 (∼ 1705 ppb; ∼ 1 × 10−5 moles/kg H2O). The palladium concentrations in a 0.1 m KCl solution buffered by RRO at 500°C and in a 0.1 m KCl solution buffered by MnO-Mn3O4 at 400°C are ∼ 40 ppb (∼ 3.8 × 10−7 moles/kg H2O) and ∼ 15 ppb (1.4 × 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 ∼ 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.