Petrogenesis and mineralization of the Hulu Ni-Cu sulphide deposit in Xinjiang, NW China: constraints from Sr- Nd isotopic and PGE compositions

The Permian Hulu intrusion is one of several sulphide-bearing Permian mafic-ultramafic intrusions in the eastern part of the eastern Tianshan located at the southern margin of the Central Asian Orogenic Belt (CAOB) in Xinjiang, NW China. The intrusion is composed of lherzolite, olivine websterite, gabbro, and gabbro-diorite. Disseminated and net-textured Ni-Cu sulphide ores are located at the bottom of the lopolith complex. Negative Zr, Hf, Nb, and Ta anomalies, whole-rock epsilon Nd(t) values of +5.7to+8.8, and variable (Th/Nb)(PM) values (from 1.06 to 8.13) suggest that the source of the Hulu complexes is depleted mantle metasomatized by subducted slab-derived fluid and/or melt (similar to 5% global subducted sediment and 15% slab fluid) that has experienced approximately 3% lower crustal and 10% upper crustal contamination. The Hulu intrusion is characterized by low PGE abundances i.e. 0.03-1.08ppb Ir, 0.04-0.69ppb Ru, 0.02-2.15ppb Rh, 0.30-48.71ppb Pt, and 0.21-344ppb Pd. Our calculations indicate that if the Pd, Os, Ir, and Cu contents of the primary magma were 2.1ppb, 0.03ppb, 0.05ppb, and 200ppm, respectively, a variable R-factor between 200 and 1600 with residual magma that had experienced 0.01% early-sulphide segregation can explain the variation in Pd, Os, and Ir contents of sulphide-poor and disseminated sulphide samples of the Hulu deposit. Basaltic magma fractionation and assimilation and/or contamination of sulphur-bearing crustal materials might have triggered sulphur saturation to form Cu-Ni sulphide ores. Tarim basaltic PGE contents cannot be used as the mineralized parent magma for the Hulu intrusion because of the differing evolutionary trends of the Ni/Pd and Cu/Ir values. However, similar Cu/Ni and Pd/Ir values in Tarim basalts and Hulu Cu-Ni sulphide ores, as well as the same early sulphide segregation process, show that certain genetic relationships between them and magma sources are probably similar to each other.