Distinct Enrichment Mechanisms of PGE in Different Orebodies of the Jinchuan Ni-Cu-PGE Sulfide Deposit, Western China

Abstract

The Jinchuan magmatic Ni-Cu-PGE deposit is the largest single Ni sulfide deposit in the world. It consists primarily of orebody-24 in segment I, and orebody-1 and orebody-2 in segment II. The contents of platinum-group elements (PGE) in these orebodies decrease significantly from west to east across the deposit. However, the PGE characteristics of platinum-group minerals (PGM) and alloys, as well as their roles during mineralization in different orebodies, remain unclear. In this study, PGM and alloy occurrences in orebody-24 and orebody-2 had been observed by scanning electron microscopy (SEM) and spherical-aberration corrected scanning transmission electron microscope (Cs-STEM). The PGE contents were analyzed by SEM–EDS for submicron-scale PGM and alloys. The results show that olivine in lherzolite mainly hosts PGM of PtTe and PdBi₂, whereas pyroxene contains small amounts of PdBiTe. Pyrrhotite predominantly hosts (Ir-Rh-Pt)AsS, Pd(BiTe), and other PGM, as well as PtSn and PtOs alloys. Pentlandite mainly encloses Pd(BiTe), PdBi/PdBi₂ and other PGM, as well as PtSn and PtFe alloys. Chalcopyrite primarily encloses PdBi/PdBi₂, and other PGM, along with PtSn and IrOs. Distinct distribution patterns of PGE in PGM and alloys had been observed between different orebodies. Orebody-24 contains more (Ir-Rh-Pt)AsS minerals and PtFe/PtSn alloy grains, whereas orebody-2 has a higher proportion of Pt- and Pd-bearing PGM. The presence of euhedral alloys in silicate minerals from orebody-24 suggests that its parent magma had a higher PGE content before sulfide saturation than that of orebody-2. More than 90% of PGM and alloys in both orebody-24 and orebody-2 contain Pt and Pd, emphasizing their contributions to the elevated Pt and Pd concentrations. The different PGE distributions of PGM and alloys in the two orebodies suggest that thermodynamic conditions (fO₂ and fS₂) and semimetals, especially As, play critical roles in controlling PGE behavior and occurrence.