Tectonic setting and metallogenic chronology of the Ashele Cu–Zn deposit in Xinjiang, NW China: Constraints from Re-Os dating of pyrite, U-Pb dating of zircon and Hf isotopes

Abstract

The Ashele Cu-Zn deposit, an important volcanogenic massive sulfide (VMS) deposit, is located in the centre of the Ashele volcano-sedimentary basin of the Altay metallogenic belt, North Xinjiang, NW China. The No. 1 orebody is located between the roof basalt and the base tuff. The roof basalt yielded a zircon U-Pb age of 342 ± 4.6 Ma (MSWD = 1.5). Four pyrite samples from massive ores sulfides yielded a Re-Os isochron age of 342 ± 11 Ma (MSWD = 9.2), while the vein sulfide has a model age of 315 Ma. This shows that massive sulfide orebodies developed contemporaneously with the emplacement of the roof basalt. The basalts have geochemical characteristics of tholeiitic basalts with relatively high Mg# values (62.7–63.9), Cr and Ni, a slightly negative Eu anomaly (δEu = 0.75–0.87), and relatively low concentrations of Sr, Nb/Ta and (La/Yb)N. The dacite has characteristics similar to those of sanukite, in terms of high concentrations of Mg# values (52.7–53.0), a weakly negative Eu anomaly (δEu = 0.90–1.04), and low concentrations of Cr, Ni, Sr and (La/Yb)N. Hf isotope analysis in the basalt exhibits the εHf(t) value of (+9.68 to + 17.03). The two-stage Hf model age (TDM2) shows 259–727 Ma with an average of 500 Ma. These geochemical characteristics indicate that the deposit formed in the magmatic arc setting, whereas the basalt originated from young (regenerated) mantle material. Re/Os and common Os imply that crust-derived and S-rich sediments also contributed to the ore formation. Accordingly, based on the results of this study and review of the regional magmatic framework, we can conclude that the Ashele deposit is diplogenetic, with VMS epigenetic and hydrothermal mineralization expressed.