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‐(PGE) 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 pyrrhotite, 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‐gersdorffite 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 ( 600°C based on the phase relations of the system FeAsS‐NiAsS‐CoAsS. Sperrylite and testibiopalladite crystals are up to 50‐80 � m 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.