Magmatic Origin And Tectonic Setting Of Giant Duolong Cu–Au Deposit, Tibet

We constrain the origin and tectonic setting of the giant Duolong porphyry–epithermal Cu–Au deposit in the South Qiangtang Terrane of northern Tibet, based on new zircon U–Pb ages and Hf isotopic data as well as whole-rock major and trace element data from ore-associated intrusions in the Duolong area. The ore-associated rocks collected from the Bolong, Tiegelong, West Tiegelong and Saijiao plutons were selected for zircon LA–ICPMS U–Pb dating in this study, and the results indicate that these ore-associated rocks formed between 121 and 126 Ma. The ore-associated rocks consist of diorites, quartz-diorites, and granodiorites, and they are dominated by normal arc igneous rocks. These ore-associated rocks can be further classified into low-K tholeiitic M-type granitoids and mid- to high-K, calc-alkaline I-type granitoids. The mid- to high-K, calc-alkaline I-type granitoids, which have variable zircon εHf(t) values(-1.4 to +12.1), originated from magma mixing of mantle-derived mafic and hybrid lower crust-derived felsic melts. The low-K tholeiitic M-type granitoids, which have positive zircon εHf(t) values(+8.9 to +15.6), originated from fractional crystallization of mantle-derived mafic melts. Detailed regional geological survey reveals that the country rocks of the ore-associated rocks are flysch formations that are deposited on the continental slope(rise) of the South Qiangtang Terrane. Moreover, the ore-associated rocks have high and variable(87Sr/86Sr)i values, old two-stage Nd model ages(T2DM(Nd)) and crustal model ages(TC DM(Hf)), and considerable inherited zircons with older ages, indicates that the involvement of ancient crustal materials in the genesis of the ore-associated rocks. Thus, we argue that the Duolong porphyry–epithermal Cu–Au deposit formed within an ?ensialic forearc‘ of an active continental margin as a result of the northwards subduction of the Bangong–Nujiang Ocean crust beneath the South Qiangtang Terrane.