Origin of Paleozoic granitoids in the Yuhai Cu–Mo deposit, Eastern Tianshan, NW China and implications for regional metallogeny

Abstract

The Yuhai porphyry Cu–Mo deposit is located at the eastern section of the Dananhu–Tousuquan Arc in Eastern Tianshan, NW China. Cu–Mo mineralization generally occurs as disseminations and veins in potassic and phyllic alteration zones, and is predominantly hosted in the Yuhai quartz diorite, granodiorite and minor syenogranite intrusions. New LA–ICP–MS zircon U–Pb dating indicates that the acidic syenogranite was emplaced at 318–320 Ma, younger than the intermediate rocks (i.e., quartz diorite and granodiorite) with Silurian ages. The acidic to intermediate intrusions exhibit geochemical affinities with normal arc rocks (e.g. high Y and Yb contents, and low Sr/Y ratios), in contrast with the subducted slab-derived adakites hosting large porphyry Cu deposits (e.g. Tuwu–Yandong) in the middle section of the Dananhu–Tousuquan Arc. The Yuhai acidic syenogranite rocks have high SiO2 (≥74.90 wt%), and K2O (≥3.48 wt%), relatively low Mg# and compatible element contents (e.g. Cr = 0.65–3.37 ppm; Ni = 0.63–1.02 ppm), and strong depletions in HFSE (e.g. Nb, Ta, and Ti). These geochemical features, coupled with isotopic data such as low initial 87Sr/86Sr (≤0.7033), positive εNd(t) (4.0 to 5.9) and εHf(t) (≥11) values, young Hf model ages, and variable 206Pb/204Pb ratios (18.549 to 19.465) reported in this study, suggest that their parental magmas originated from the partial melting of a juvenile lower crust and mixed with some old crustal components beneath the Dananhu–Tousuquan Arc. In contrast, the Yuhai intermediate intrusions exhibit relatively low SiO2 and K2O contents, high Mg# values (40–56), and a wide range of εHf(t) values, suggesting a source of sub-continental lithospheric mantle metasomatized by slab-derived fluids and melts. Zircon trace element and plagioclase compositions of the Yuhai granitoids further imply that the Silurian intermediate intrusion-related magma was moderately oxidized (mostly EuN/EuN* > 0.6) and water-rich (~9 wt%), favorable for the copper mineralization, whereas the Carboniferous syenogranite-related magma was weakly oxidized (mostly EuN/EuN* = 0.3–0.6) but also hydrous (~6 wt%). Combined with the regional tectono-magmatic activities and metallogenic processes, we suggest that the Yuhai Cu–Mo deposit and associated granitoids were formed in an arc setting, and that the magma nature and its source components are the key factors constraining the distribution, scale and style of the porphyry copper mineralization in Eastern Tianshan.