Genesis of the main types of sulphide ore in the Jinchuan Ni–Cu–PGE deposit, NW China: Constraints from texture and mineral chemistry of pyrrhotite

The Jinchuan Ni-Cu-PGE deposit is one of the three largest magmatic Ni deposits in the world. Disseminated, net-textured, Cu-rich sulphide ores and massive sulphide ores are the most important ore types. The genesis of different ore types and the mineralization processes of the Jinchuan deposit remain controversial. Three hypotheses have been proposed by previous studies, including the in situ sulphide segregation of sulphide-bearing magma, deep segregation-multiple injections, and hydrothermal superimposition. To understand the genesis of different ore types, we studied the occurrence, intergrowth textures, and mineral chemistry of pyrrhotite in four types of sulphide ore using magnetic colloid experiment and electron probe microanalysis. Pure hexagonal pyrrhotites (Hpo) in disseminated and net-textured sulphide ores display higher M/S values, indicating a slow cooling process in a high-temperature and low-S magmatic system. Irregular intergrowth pyrrhotites occur in disseminated, net-textured, and Cu-rich sulphide ores. Dusty, stellate, and patchy monoclinic pyrrhotites (Mpo) occur along the fissures and grain boundaries of Hpo or the contact boundaries of silicate minerals, suggesting the interaction of Hpo and sulphur- and/or oxygen-rich hydrothermal fluids. Monoclinic pyrrhotite occurs as roughly parallel lamellae in the matrix of Hpo and lacks of pyrite porphyroblasts in massive sulphide ores, indicating a rapid reduction of temperature in a high-S magmatic system. The Cu-rich sulphide ores are characterized by a relatively high proportion of chalcopyrite and comparatively high Pt and Pd (PPGE) and low Os, Ir, Ru (IPGE), and Rh concentrations, indicating a fractional crystallization of monosulphide solid solution from sulphide melts, and then the residual sulphide melts mineralized in a local place.