Genetic relationships between base-metal sulfides and platinum-group minerals in the Yangliuping Ni-Cu-(PGE) sulfide deposit, southwestern China

Abstract

The Yangliuping Ni-Cu-(PGF) sulfide deposit in southwestern China is hosted by a series of mafic-ultramafic sills that intrude Devonian strata. Massive sulfides and disseminated sulfides occur at the base of the sills. Minor massive sulfide veins occur in footwall fractures. The major base-metal sulfides (BMS) are pyrrhostite, pentlandite, and chalcopyrite. The most common PGE-bearing sulfarsenide is a Pd-bearing cobaltite-gersdorffite solid solution. Other PGM identified include sperrylite, testibiopalladite, and Pd-bearing melonite. All grains of the cobaltite-gersdouffite solid solution and PGM are enclosed in pyrrhotite and pentlandite. Both the PGM and their host BMS have been partially altered in place by hydrothermal fluids. Electron-microprobe analyses show that pyrrhotite and pentlandite contain up to 0.1-0.5 wt% Pt. Euhedral grains of cobaltite-gersdorffite solid solution (<30 μm in diameter) are zoned, with a Rh-Pt-Ni-rich core containing up to 6.4 wt% Rh and 0.5% Pt, and a Co-rich margin. Textural relations suggest that the Pt-bearing coballite-gersdorffite solid solution crystallized before exsolution of pentlandite from monosulfide solid-solution i.e., probably above >600°C based on the phase relations of the system FeAsS-NiAsS-CoAsS. Sperrylite and testibiopalladite crystals are up to 50-80 μm in diameter and are compositionally homogeneous. Some testibiopalladite crystals occur in the contact of two pyrrhotite grains. Monosulfide solid-solution was the first phase to crystallize from a sulfide melt, and it then exsolved to pyrrhotite and pentlandite. Pt and Pd became enriched in the residual sulfide liquid, and crystallized as PGM and Pd-bearing cobaltite-gersdorffite solid solution at a lower temperature. Minor Pt and Pd in Mss were finally expelled from the structure below 800°C.