Age and genesis of the Pusangguo skarn Cu-dominated polymetallic deposit, Gangdese metallogenic belt, Tibet

The Pusangguo skarn Cu-polymetallic deposit is a newly discovered deposit in the western Gangdese porphyry copper belt (GPCB). The orebodies are hosted along the contact between limestone of the Early Cretaceous Taken Formation and a biotite granodiorite intrusion. The mineralization is associated with the biotite granodiorite intrusion and is divided into four-stages: (1) a prograde skam stage; (2) a pre-ore retrograde skarn stage; (3) a quartz-sulfides stage; and (4) a post-ore carbonate stage. These skarns have the characteristics of both calcareous and manganese skams. The mineral compositions of the garnets and pyroxenes indicate that the oreforming environment gradually evolved from a high-temperature, oxidative and alkaline environment into a low-temperature and reductive environment. The sulfur and lead isotopic compositions of the ore sulfides suggest a magmatic source for the sulfur and a crust-dominated source for the ore-forming metals. The hydrogen and oxygen isotopic compositions of the quartz and garnets in the ores indicate that the ore-forming fluids may have originated from a mixture of magmatic and meteoric fluids. The geochronological data show that both the magmatism (14.8 +/- 0.4 Ma) and mineralization (13.2 +/- 0.7 Ma) occurred during the Miocene in a post-collisional tectonic setting. The ore-forming biotite granodiorite is a high-K calc-alkaline I-type granite and has an adakitic geochemical affinity similar to the Gangdese Miocene adakites, which were generated from thickened lower crust in a post-collisional tectonic setting. The low positive epsilon(Hf) (t) values (0 to +2.6), young two-stage Hf model zircon ages, and whole-rock geochemical and lead isotopic compositions indicate that the magma of the ore-related biotite granodiorite may have originated from the thickened lower crust in the Lhasa terrane.