GENETIC RELATIONSHIPS BETWEEN BASE-METAL SULFIDES AND PLATINUM-GROUP MINERALS IN THE YANGLIUPING Ni-Cu-(PGE) SULFIDE DEPOSIT, SOUTHWESTERN CHINA

摘要

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 purhotite 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 percent Pt. Euhedral grains of cobaltite-gersdorffite solid solution (<30 mu m in diameter) are zoned, with a Rh-Pt-Ni-rich core containing up to 6.4 wt percent Rh and 0.5 percent Pt, and a Co-rich margin. Textural relations suggest that the Pt-bearing cobaltite-gersdorffite solid solution crystallized before exsolution of pentlandite from monosulfide solid-solution i.e., probably above >600 deg C based on the phase relations of the system FeAsS-NiAsS-CoAsS. Sperrylite and testibiopalladite crystals are up to 50-80 mu 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 deg C.