Pyroxenite source for the ultramafic−mafic intrusions in the giant Xiarihamu Cu-Ni sulfide deposit, China

Magmatic Cu-Ni sulfide deposits are usually associated with mantle-derived ultramafic−mafic intrusive rocks, yet the role of the magma source in their formation is often overshadowed by the focus on shallow magmatic processes. A combined geochemical and isotopic study was performed on the ore-barren ultramafic−mafic igneous rocks from the Xiarihamu deposit, East Kunlun orogenic belt, China. This study included analyses of zircon Hf-O isotopes, olivine major and trace elements, and whole-rock major and trace elements, along with Sr-Nd-Hf isotopes. Zircon U-Pb dating yielded ages of 420−418 Ma, synchronous with exhumation of the deeply subducted continental crust. The samples display arc-like trace-element characteristics, enriched Sr-Nd-Hf isotopic signatures, and high zircon δ18O values. These geochemical features resemble those of Neoproterozoic orthogneiss, which suggests that the Xiarihamu ultramafic−mafic rocks could have originated from the mantle source modified by melts derived from subducting continental crust. In addition, olivine phenocrysts in peridotite display high Ni content, high Fe/Mn ratios, and low Ca and Mn contents, indicative of a pyroxenite source transformed from peridotite via crustal metasomatism induced by subducting continental materials. Furthermore, the volatiles were likely introduced into the mantle source as a result of continental subduction. Subsequent partial melting of the pyroxenite source, induced by the upwelling asthenospheric mantle, would effectively release large amounts of Ni into the mafic magma, while the subducting volatiles (e.g., S) would function as the main complexing agent for Cu. These conditions are essential for the final formation of Cu-Ni sulfide deposits. Therefore, this study provides valuable insight into the crust−mantle interactions in continental subduction zones and establishes a geochemical connection between metasomatized lithospheric mantle, derivative mafic magma, and giant Cu-Ni sulfide deposits.