Geochronology, petrogenesis and tectonic implications of the newly discovered Cu–Ni sulfide-mineralized Yueyawan gabbroic complex, Kalatag district, northwestern Eastern Tianshan, NW China

Abstract

The early Permian Cu–Ni sulfide-bearing mafic-ultramafic intrusions from North Xinjiang in NW China help to provide constraints on the late Paleozoic crustal growth in the Central Asian Orogenic Belt (CAOB) and improve the understanding of the genesis of magmatic sulfide mineralization in orogenic settings and the identification of future prospecting targets. The Cu–Ni sulfide-mineralized Yueyawan gabbroic complex in Kalatag represents a new breakthrough for Cu–Ni prospecting in covered areas in the northwestern part of the Eastern Tianshan orogenic belt, North Xinjiang. Rocks from this intrusion have similar LA-ICP-MS zircon U–Pb ages (282.4 ± 1.1–281.3 ± 1.2 Ma) to other mafic-ultramafic intrusions containing magmatic sulfide mineralization in North Xinjiang. These early Permian gabbroic intrusions are thus important targets for Cu–Ni mineralization exploration in this region. The rocks of the Yueyawan gabbroic complex are a ca. 1 km2 elongated NW-trending crescent-shaped at the junction of two E-W- and NW-SE-trending faults. The intrusion has the morphology of a conduit tube. The rocks include troctolite, olivine gabbro, hornblende gabbro, and gabbro, and the Cu–Ni sulfides occur as steeply dipping (65–70°) lenses with disseminated textures in the contact zone between the olivine gabbro and hornblende gabbro, indicating that sulfide precipitation was coeval with the crystallization of the magmas giving rise to these rocks. The rocks investigated in this study are gabbroic in composition; they are characterized by moderate concentrations of SiO2 (44.8–48.8 wt%) and low to moderate concentrations of MgO (1.69–13.4 wt%, Mg# values of 39.6–73.3), Ni, and Cr. Furthermore, the rocks have high positive εHf(t) values (+13.1 to +15.9) and flat rare earth element patterns ((La/Yb)N = 2.01–2.89), with marked positive Eu anomalies, high Ti/Zr ratios (70–164), and low Ce/Pb (3.65–14.83) and Nb/Ta values (9–16), indicating very restricted amounts (5–10%) of crustal assimilation. This complex was likely derived from 10 to 20% partial melting (as evidenced by the low Sm/Yb values of 1.48–2.02 and La/Sm values of 1.61–2.23) of weakly slab melt-metasomatized (as evidenced by the low Ba/Nb and Ba/Zr ratios) subcontinental lithospheric mantle (with 2% garnet and 2% spinel lherzolite) at depths greater than 85 km in a post-collisional extensional source that contained sufficient Ni and Cu for the saturation and segregation of immiscible sulfides to concentrate the metals. The magma underwent sulfide saturation and segregation together with olivine fractionation (as evidenced by Ni and TFe2O3 increase and SiO2, CaO, and Zr decrease with increasing MgO content) in the conduit-type intrusion. Therefore, regional fault structures, ore-bearing rock types, and geochemical ratios that reflect high degrees of partial melting are features of the Cu–Ni-mineralized gabbroic rocks in adjacent areas that help support exploration.