Platinum-group minerals and tellurides from the PGE-bearing Xinjie layered intrusion in the Emeishan Large Igneous Province, SW China

Abstract

The Xinjie layered intrusion is one of a number of major ultramafic-mafic bodies hosting Fe-Ti-V deposits and Cu-Ni-PGE sulfide deposits in the Pan-Xi (Panzhihua-Xichang) area of the Sichuan Province, SW China. The Xinjie ultramafic-mafic layered intrusion, genetically related to the Permian plume-related Emeishan flood basalts, consists of three lithological cycles, each representing a sequence from ultramafic to mafic-felsic composition. The basal part of the intrusion is composed of three lithological units, namely, the Marginal Unit (MU), Peridotite Unit (PeU) and Pyroxenite Unit (PyU). In the present study, three major PGE-mineralised Cu-Ni sulfide layers were discovered within the Marginal and Pyroxenite Units. The major base-metal sulfides (BMS) comprise chalcopyrite, pyrrhotite, and pentlandite. Detailed microscopic and microprobe analyses revealed the presence of the sperrylite and Pd-Pt-Bi-Te minerals (merenskyite, moncheite, and michenerite). These PGMs are commonly associated with the BMS, or magnetite coexisting with BMS in the PGE-enriched layers. The 1:1 substitution between Pt and Pd, as well as between Te and Bi, confirms the complete solid-solution series between moncheite and merenskyite. The textural association of the PGMs with BMS and Fe-Ti oxides (magnetite) suggests that the PGMs may have crystallised slightly later than the hosting magnetite and BMS. The formation of magnetite may have played an important role in producing the sulfur-saturated melt and the PGEs thus concentrated in the sulfide liquid during the crystallisation history. It is therefore suggested that the Cu-Ni-PGE-bearing layers in the basal part of the Xinjie intrusion were generated by magma evolution processes.