Pt and Pd mobility in hydrothermal fluids: Evidence from komatiites and from thermodynamic modelling

Komatiite host rocks and ores provide a natural laboratory for the study of Pt and Pd during hydration and carbonation of ultramafic rocks. The degree of correlation between Pt and Pd, and of those elements with lithophile elements such as Ti, in komatiitic rocks and ores is strongly dependent on the magmatic sulfide content of the rock. Pt and Pd are typically very strongly correlated within S-poor komatiites, and in disseminated ores containing 5 volume % or less magmatic sulfides. In contrast, they are almost completely decoupled in sulfide-rich ore samples.Thermodynamic modelling based on the existing body of published experimental data on Pt and Pd solubilities suggests that Pd and Pt can be readily transported as bisulfide complexes in acidic-neutral solutions under reduced and moderate oxidation condition at 300 °C, and chloride complexes are only important under extremely acidic/oxidised conditions. Pd is much more soluble than Pt in the case of fluids in sulfide rich environments, in equilibrium with the characteristic assemblages found within altered and metamorphosed komatiites. Solubilities of both elements are much lower in sulfide-poor environments, especially where pH values are constrained to neutral values by the near-ubiquitous presence of carbonate minerals in the alteration assemblage. This accounts for the evident near-immobility of Pt and Pd in even the most altered silicate host rocks, as compared with their wide dispersion in sulfide-rich ores.