Metallogeny of the Poyi magmatic Cu-Ni deposit: revelation from the contrast of PGE and olivine composition with other Cu-Ni sulfide deposits in the Early Permian, Xinjiang, China

Abstract

Permian mafic-ultramafic intrusions in or besides the Central Asian Orogenic Belt (CAOB) have been the focus of several recent studies, mainly located in Altay, East Tianshan, and Beishan Rift from north to south. Some of them host economic magmatic sulfide deposits. Among those, Poyi is a large magmatic Cu-Ni sulfide deposit related to ultramafic rock in Beishan Rift. Poyi does not have massive ore, but only disseminated ore in hornblende-peridotite, so Poyi is not as rich as the Karatongk Cu-Ni deposit in the Altay area or the Huangshandong and Tulargen Cu-Ni sulfide deposits in Eastern Tianshan. To address these questions, this paper contrasts the feature of PGE (Platinum Group Element) and olivine composition in magmatic Cu-Ni deposits between three regions (Altay, East Tianshan, and Beishan Rift) to provide a view of the metallogenic process of the Poyi magmatic Cu-Ni deposit. The degree of sulfide segregation (DSS) of Poyi is around 0.01%; the value of R (the ratio of the mass of silicate magma to the mass of sulfide that reached equilibrium together) is 500–5000, and is 2333 on average; the R value of Cu-Ni sulfide deposits in East Tianshan is in the range of 100–500, and the degree of sulfide segregation is about 0.013%; Poyi Cu-Ni deposit is not as rich as the magmatic Cu-Ni deposits in East Tianshan, probably because the degree of sulfide segregation of the former (0.01%) is lower than that of the latter (0.013%). From Beishan area to East Tianshan and then to Altay Region, the R value changed from 2333 to 100–500 and then to 100 according to the theory of Naldrett (2011). Poyi does not have the potential to be a PGE deposit. The olivine of Poyi has higher contents of MgO and SiO2 and lower contents of FeO and CaO, which reach sulfur saturation more easily, but due to the lack of sulfur, it is still very difficult to reach sulfur saturation, and therefore the olivine of Poyi has the highest Ni content among the Permian magmatic Cu-Ni deposits in Xinjiang.