Relative Cu/Ni Enrichment of the Baixintan Magmatic Ni–Cu Deposit in the Eastern Tianshan Orogen (NW China): Insights from S, Pb, Cu, and Lu–Hf Isotopes

Baixintan is a magmatic Ni–Cu sulfide deposit discovered in the central Tuwu–Yandong porphyry Cu belt of the Eastern Tianshan Orogen (ETO) of NW China in 2016. It is in close proximity (~5 km) to the Tuwu Cu deposit, the largest Carboniferous porphyry Cu deposit (~336 Ma) in Xinjiang. The Baixintan Ni–Cu ore is characterized by a high Cu/Ni ratio, but the reason for it remains unclear. To resolve this question, we present petrographic, geochronological, whole-rock geochemical, and S, Pb, Cu, and Lu–Hf isotopic data. Ore-related hornblende olivine websterite (HOW) and hornblende olivine gabbro (HOG) were emplaced at 296.6 ± 1.1 Ma and 289.7 ± 1.2 Ma, respectively, which were formed in an Early Permian post-collisional extensional setting. Whole-rock Pb and zircon Lu–Hf isotopes suggest that the parental magmas were predominantly mantle-derived. The Baixintan HOW and HOG contain abundant hornblende and are enriched in LILEs and LREEs but depleted in HFSEs, suggesting subduction-related metasomatism in their magma source. The sulfide (chalcopyrite, pyrrhotite, and pentlandite) δ34S values cluster around 0‰ (–0.13 to 0.11, n = 11), and the chalcopyrite has uniformly negative but variable δ65Cu values (–0.96 to –0.13, n = 6), which suggest that the ore-forming materials were mainly derived from the subduction-metasomatized mantle. Olivine Ni contents are significantly lower than those crystallized under sulfide-unsaturated conditions, implying that olivine fractional crystallization was an important trigger for sulfide melt segregation at Baixintan. Baixintan is the only known magmatic Ni–Cu sulfide deposit in the ETO that shares a Cu-preconcentrated, metasomatized mantle source with a giant porphyry Cu system. Copper pre-enrichment in the magma source may be the main factor for the relatively high Cu/Ni ratio observed in the Baixintan deposit.