Solubility of Pt in sulphide mattes: Implications for the genesis of PGE-rich horizons in layered intrusions

Abstract

The partitioning of Pt in sulphide melt (matte) has been studied as a function of fS 2 and fO 2 at 1200 and 1300 °C. The results show that the solubility of Pt in mattes increases strongly with increasing fS 2 and decreases weakly with increasing fO 2. The increase in Pt solubility with increasing fS 2 is attributed to Pt dissolving in the melt as a sulphide species and the weak inverse dependence of Pt solubility on fO 2 to the diluting effect of increasing O in the melt at high fO 2. These results, coupled with measurements of Pt solubility in silicate melts taken from the literature, allow the calculation of Pt matte/silicate-melt partition coefficients ( D Pt matte – sil ) for a range of conditions pertinent to the formation of Pt-rich horizons in layered intrusions. The calculated values range between 10 7 and 10 11, depending on fO 2 and fS 2, several orders of magnitude higher than previously published values. Our preferred value for D Pt matte – sil for conditions appropriate to the Merensky Reef is 10 7 and for the Stillwater Pt-rich horizon 10 8. The new results are consistent with the magmatic hypothesis for Pt-rich horizons in layered intrusions.