Evolution of magmatic sulfide of the giant Jinchuan Ni-Cu deposit, NW China: Insights from chalcophile elements in base metal sulfide minerals

Abstract

Chalcophile elements differentially distributed among base metal sulfides (BMS) and other discrete minerals are useful for assessing the evolution of magmatic sulfides. The world-class Jinchuan Ni-Cu sulfide deposit is composed of the western and eastern intrusions, but the differences in the distribution of chalcophile elements among BMS between two intrusions have not been well investigated. This study provides new data of in-situ chalcophile elements and platinum-group minerals (PGM) hosted in BMS of two intrusions, and discusses the discrepancy of chalcophile elements distribution and sulfide melt evolution. The variations of all compatible elements (Re, Co, Os, Ir, Ru, and Rh) and most incompatible elements (Pt, Ag, Zn, Cd, As, Sn, Sb, and Bi) in BMS versus whole-rock (Pt + Pd)/(Ir + Rh + Rh) ratios suggest these chalcophile elements were fractionated by progressive crystallization of the sulfide melt. Micro-textures of PGM suggested that (Ir-Rh-Pt)AsS crystallized almost simultaneously with the monosulfide solid solution, and Pd-semimetal minerals crystallized after the loop-textured pentlandite. As for the same type of sulfide mineralization, the contents of chalcophile elements in BMS minerals of the western intrusion are generally higher than those in BMS minerals of the eastern intrusion, which is consistent with the whole-rock variations. However, chalcopyrite of the eastern intrusion hosts significantly higher proportions of Pd and Te than chalcopyrite of the western intrusion with comparable amounts of Pd-semimetal micro- and nano-particles, indicating the diffusion of Pd and Te during the peritectic reactions in the eastern intrusion was inhibited. In terms of the negative correlation between Metal/S ratios of pyrrhotite and elemental ratios of Pd and Te between pentlandite and chalcopyrite, it is likely attributed to the lower sulfur fugacity (fS2) in the eastern intrusion which has been quantitatively calculated by pyrrhotite composition. We propose schematic models of the crystallization history of sulfide melts at Jinchuan, in which fractional crystallization of sulfide melts, crystallization of PGM and other discrete minerals, and formation of loop-textured pentlandite jointly control the distribution of chalcophile elements. The difference of fS2 between western and eastern intrusions resulted in the differential diffusion of Pd and Te from fractionated sulfide melt into loop-textured pentlandite.