PETROGENESIS OF THE PT–PD MINERALIZED JINBAOSHAN ULTRAMAFIC INTRUSION IN THE PERMIAN EMEISHAN LARGE IGNEOUS PROVINCE, SW CHINA

Abstract

The Jinbaoshan ultramafic intrusion is a sheet-like body with a thick wehrlite unit in the center and thin pyroxenite units at the margins. PGE are enriched in several disseminated sulfide zones in the intrusion. Olivine from the intrusion has low Fo and depleted Ni contents compared to olivine from coeval Emeishan picrites. Whole rock major and trace element concentrations suggest that the Jinbaoshan wehrlites originally contained <30% trapped liquid. The total amount of sulfide in the rocks exceeds that which could have been dissolved in the trapped liquid. The Jinbaoshan wehrlites are interpreted to represent residual assemblages formed by dissolution of plagioclase by passing magma. No clear evidence of crustal contamination is indicated by S, Nd and Os isotopes. We envision that sulfide saturation occurred at depth due to olivine and chromite crystallization. Immiscible sulfide droplets were transported to the Jinbaoshan conduit where they accumulated and reacted with magma successively passing through the conduit to achieve high PGE concentrations.