Geochronological and geochemical study of the Baixintan magmatic Ni-Cu sulphide deposit: New implications for the exploration potential in the western part of the East Tianshan nickel belt (NW China)

Abstract

Several newly discovered magmatic Ni-Cu sulphide deposits are located in the East Tianshan nickel belt in the southern part of the Central Asian Orogenic Belt in NW China. The Baixintan magmatic sulphide deposit is one of these new discoveries. The host mafic-ultramafic intrusion of the Baixintan deposit is composed of plagioclase-bearing lherzolite, Pl-bearing olivine websterite, olivine norite and gabbro. Zircon crystals from olivine norite yielded a U-Pb age of 286.0 ± 1.6 Ma, similar to the ages of other magmatic Ni-Cu sulphide ore deposits in this belt. Disseminated sulphide mineralization is present in olivine-bearing rocks. The Fo and Ni contents of olivine crystals from the sulphide-barren samples vary from 81.7 to 85.6 mol% and from 956 to 2837 ppm, respectively, exhibiting a positive Fo-Ni correlation that is consistent with the fractional crystallization of olivine plus removal of small amounts of immiscible sulphide liquid. The Baixintan mafic-ultramafic rocks are characterized by light REE enrichment, pronounced negative Nb–Ta anomalies, low initial 87Sr/86Sr ratios from 0.7035 to 0.7039, positive εNd(CHUR, 286 Ma) values from 6.0 to 7.4 and zircon εHf(CHUR, 286 Ma) values from +11.6 to +15.8. These data indicate that the parental magmas were derived from a depleted mantle source with moderate degrees of contamination by the upper crust. The variations in PGE tenor in the Baixintan deposit, which are characterized by positive correlations between IPGE and PPGE, can be explained by variable R factors (R = 400–3000). Mass balance calculations indicate that the parental magma was severely depleted in PGE, which could be likely due to prior sulphide segregation at depth or sulphide retention in the mantle during partial melting. Both fractional crystallization and crustal contamination may have played a role in triggering second stage sulphide saturation in the Baixintan magma. The similarities in ages, lithological controls and bulk sulphide composition between the Baixintan deposit and other known magmatic Ni-Cu sulphide ore deposits in the East Tianshan nickel belt encourage further exploration in the Baixintan mafic-ultramafic intrusion, as well as expansion of the on-going regional Ni exploration program.