The effect of platinum-group minerals on differentiation of platinum-group elements in magmatic sulfide deposits: Evidence from the Cu-Ni-PGE deposits in the Yangliuping area of the Emeishan large igneous province, SW China

Abstract

Platinum-group minerals (PGM) are important host phases for platinum-group elements (PGE) in magmatic sulfide deposits. Recent studies have demonstrated that some PGM, such as sperrylite, can crystallize early from sulfide liquids. It is important to determine whether and how these PGM particles affect the enrichment and differentiation of PGE in Cu-Ni-PGE deposits. The magmatic sulfide deposits in the Yangliuping area (mainly consisting of the Yangliuping deposit and Zhengziyanwo deposit) are the largest Cu-Ni-PGE deposits in the Emeishan large igneous province, SW China. Primitive-mantle normalized PGE patterns and fractional crystallization modelling show that massive ores in the Yangliuping deposit are enriched in Pd and Pt. In contrast, massive ores, breccia ores and Cu-rich ores in the Zhengziyanwo deposit are depleted in Pt. Scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), focused ion beam (FIB) and transmission electron microscopy (TEM) were used to constrain the occurrence of PGE in the massive ores from the two deposits. Mass balance calculation shows that <7% of Ir, Rh and Pt occur in base metal sulfides (BMS, mainly pyrrhotite, pentlandite and chalcopyrite) as solid solution in both the Yangliuping and Zhengziyanwo massive ores. 17% and 91% Pd are present in solid solution within BMS in the Yangliuping and Zhengziyanwo massive ores, respectively. In the Yangliuping massive ores, PtAs2 and Pd-PGM particles were commonly found within BMS crystals, and Ir-Rh-(Pt) sulfarsenides enclosed in cobaltite-gersdorffite solid solution (CGSS) are enriched in Pt. In contrast, in the Zhengziyanwo massive ores, no PtAs2 particles and fewer Pd-PGM particles were found, and Ir-Rh-(Pt)AsS particles enclosed in CGSS are poor in Pt. The distribution, mineralogy and microstructures of Ir-Rh-(Pt)AsS, PtAs2 and Pd-PGM particles in both the Yangliuping and Zhengziyanwo deposits suggest that they mainly crystallized from sulfide liquids or formed from Pd-Pt-semimetal-rich droplets. We suggest that the incorporation of Pd-PGM and PtAs2 particles into MSS cumulates led to the enrichment of Pd and Pt in the Yangliuping massive ores. In the Zhengziyanwo deposit, early fractional crystallization of PtAs2 reduced the Pt content in sulfide liquids and resulted in the formation of Pt-depleted sulfide ores and low-Pt Ir-Rh-(Pt)AsS particles, but no Pt-rich sulfide ores have been found to date. As a result, the role of PGM particles should be considered when using PGE to trace the formation process of Cu-Ni-PGE deposits. This study also provides additional evidence that the behavior of Pt in sulfide liquids is influenced by thermodynamic conditions and As, and suggests a potential for Pt-rich sulfide ores exploration in the Zhengziyanwo deposit.