Petrogenesis and Economic Potential of the Sangong Mafic-Ultramafic Intrusion in the Eastern Tianshan, Central Asian Orogenic Belt: Constraints from Mineral, Whole-Rock, and PGE Geochemistry

Abstract

The Sangong Cu-Ni sulfide mineralized mafic-ultramafic intrusion is located on the southern margin of the Bogeda-Harlik belt, eastern Tianshan, China. The intrusion is a well-differentiated complex and is comprised of leucogabbro, gabbro, olivine gabbro, Pl-bearing peridotite, and Pl-bearing pyroxenite. The Pl-bearing pyroxenite hosts both irregularly disseminated sulfide and round droplet sulfide. The intrusive rocks have a wide range of SiO₂ (42.1 wt.%–50.48 wt.%) and MgO (6.21 wt.%–22.11 wt.%), and are enriched in light rare earth elements (LREE), large-ion lithophile elements (LILE; e.g., Rb, Ba, Sr, and Pb), and palladium platinum group elements (PPGE) but depleted in high-field-strength elements (HFSE; e.g., Nb, Ta, and Ti) and iridium PGEs (IPGE). These geochemical characteristics indicate that the Sangong mafic-ultramafic intrusion was derived from high degree of partial melting of depleted mantle and interacted with subduction-related material. The low Pd/Ir (3.21–27.44) but high Ni/Cu (1.64–24.16) ratios, combined with the olivine crystals with low Fo (60.88–78.65) and Ni (54.99 ppm–1 688.87 ppm) concentrations suggest that the parental magma of the Sangong intrusion were likely high MgO basaltic in composition that experienced extensive evolution prior emplacement. The Ce/Pb ratios (5.8–13.6) and Nb/U ratios (11.6–30.3) of the intrusive rocks all range between MORB and crustal values, the Nb/Yb and Th/Yb values are close to the lower crust values, together with the low Se/S ratios [(17–100) × 10⁻⁶)] suggest that the magma experienced assimilation not only in mantle source but also in conduit, but the degree of crustal contamination is limited. The Cu/Pd ratios of the rocks range from 3.9 × 10⁴ to 10.8 × 10⁴, and the Cu/Zr ratios of Pl-bearing pyroxenite in the Sangong intrusion are >1, combined with the presence of sulfide droplets in the Pl-bearing pyroxenite, indicating the parental magma experienced sulfide saturation and the economical ore bodies may present in the depth of the intrusion. Furthermore, given the discovery of the Baixintan and Yueyawan deposits, we propose the Dananhu-Harlik belt as an essential prospecting target for Cu-Ni mineralization in North Xinjiang.