Abstract

The Poyi ultramafic intrusion is part of the 278 Ma Pobei mafic-ultramafic complex located in the northeastern margin of the Tarim craton. Significant Ni-Cu sulfide mineralization associated with ultramafic rocks has been discovered through an ongoing state-funded drilling campaign. The Poyi intrusion is a pipe-like body with a surface exposure measuring 1 × 3 km and a downward extension >1,600 m. It is composed of dunite, lherzolite, and olivine websterite. Minor plagioclase is ubiquitous in the ultramafic rocks. Sulfide mineralization occurs as concordant layers or zones of disseminated sulfides with thicknesses varying from 10 to 80 m within the steeply dipping layers of lherzolite. Olivine in dunite and lherzolite contains forsterite and Ni up to 89 mol % and 3,000 ppm, respectively. Olivine in olivine websterite has slightly lower forsterite content (86 mol %) and significantly lower Ni content (1,100 ppm). The rapid decrease in olivine Ni contents from lherzolite to olivine websterite indicates cotectic sulfide segregation during olivine crystallization. The Poyi ultramafic rocks are characterized by nearly flat REE patterns, negative Nb anomalies, arc-like Th/Yb and Nb/Yb ratios, positive ɛNd values (4.7–6.6) and low initial 87Sr/86Sr ratios (0.7037–0.7066). The arc-like trace element ratios coupled with positive ɛNd values for the Poyi ultramafic rocks indicate the involvement of a subduction-modified lithospheric mantle or a granitic melt derived from a juvenile arc crust. The contents of PGE in the Poyi sulfide separates are significantly higher than in sulfide-poor rocks, indicating that immiscible sulfide liquids were the primary collectors of PGE in the intrusion. Sulfide-poor samples from the Poyi intrusion have low γOs values (10–18). In contrast, sulfide separates from mineralized lherzolites have distinctly higher γOs values (54–154). It is perplexing that the sulfide separates have mantle-like δ34S values (−0.3 to −2‰). Based on the contrasting γOs values between sulfide-poor rocks and sulfide separates, we suggest that external sulfur played a role in triggering sulfide saturation in the Poyi magma.

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