Petrogenesis and tectonic setting of the Bolong ore-bearing granodiorite porphyry in the Bangongco–Nujiang metallogenic belt, northwestern Tibet: Evidence from geochemistry, zircon U[sbnd]Pb ages, and Sr–Nd–Pb–Hf isotopes

Abstract

The giant Bolong porphyry Cu deposit (with proven Cu resources of 2.08 Mt. and 0.9 t Au) is located in the Bangongco–Nujiang metallogenic belt, northwestern Tibet. Copper mineralization has a genetic relationship with the granodiorite porphyry. Zircon laser ablation inductively coupled plasma mass spectrometry UPb dating gives crystallization ages of 124.5 ± 1.1 and 121.4 ± 0.6 Ma (Early Cretaceous) for the first and second stages, respectively, of the Bolong ore-bearing granodiorite porphyry. Geochemical analysis indicates that the Bolong ore-bearing granodiorite porphyry samples exhibit obvious light rare earth element enrichment, high initial ratios of 87Sr/86Sr (0.7076–0.7098) and low values of εNd(t) (−7.7 to −3.5), with TDM2(Nd) ages of 1.21 to 1.55 Ga. Ratios of (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t are 16.96–17.79, 15.48–15.52, and 36.72–37.74, respectively. Zircons yield εHf(t) values of −1.3 to 9.1 and 2.7 to 10.5 and TDM2(Hf) ages of 0.59–1.26 Ga. These isotopic signatures suggest that the primary magma of the Bolong ore-bearing granodiorite porphyry originated mainly from mixing of crust-derived felsic and metasomatized mantle-wedge-derived mafic melts. The Bolong ore-bearing granodiorite porphyry rocks have high logfO2 values ranging from −14.56 to −11.3 and −14.19 to −11.4 respectively, corresponding to △FMQ +1.7 to △FMQ +2.6 and △FMQ +2.2 to △FMQ +2.8, respectively. The Bolong porphyry CuAu deposit formed in a continental arc tectonic setting, during the northward subduction of the Bangongco–Nujiang Ocean in the Early Cretaceous.