Geology and factors controlling the formation of the newly discovered Beimulang porphyry Cu deposit in the western Gangdese, southern Tibet

Abstract

The porphyry Cu deposits in the Gangdese metallogenic belt were mainly formed in the eastern portion. The Beimulang is a newly discovered deposit formed in post-collisional setting in the western part of the belt and adjacent to the super-large Zhunuo porphyry Cu deposit. There are three epochs of magmatism at Beimulang, including pre-mineralization quartz porphyry (PQP; 49.7 ± 0.4 Ma), inter-mineralization monzogranite porphyry (IMGP; 14.8 ± 0.2 Ma), monzogranite (IMG; 14.1 ± 0.2 Ma), and late-mineralization granite porphyry (LGP; 11.7 ± 0.1 Ma). The various degrees of alteration and sulfide occurred at the IMG(P) and LGP indicate that there are at least two episodes of fluid exsolution events associated with different intrusions spanning ∼ 3 m.y. at Beimulang, which are unlike other Gangdese porphyry Cu deposits showing single fluid exsolution around only one porphyry at Miocene (e.g., Qulong, Zhunuo, Dabu). The PQP is characterized by high SiO2 (75.6–81.4 wt%) and K2O (2.1–4.3 wt%) concentrations, high Nb/Ta (8.4–11.3) ratios, and low Mg# (22–43), Ni (1.5–2.7 ppm) contents. They have high (87Sr/86Sr)i (0.705098–0.707767) ratios and low εNd(t) (-4.9 to −1.9) values. These geochemical characteristics indicate that they were probably derived from a juvenile basaltic lower crust during the ongoing Indian-Asian collision. The IMGP and IMG show similar petrography and geochemical characteristics, such as high SiO2 (65.0–69.8 wt%), K2O (3.6–4.3 wt%), Sr (550–809 ppm) contents, high Sr/Y (79–89) ratios, and low Mg# (43–48), Th/Yb, and Nb/Yb values, combined with high (87Sr/86Sr)i (0.707254–0.708017) and low εNd(t) (-7.9 to −6.0), implying a similar source probably originated in the subduction-modified lower crust metasomatized by Neo-Tethyan oceanic slab-derived fluids. The LGP displays trends produced by fractionation of magma with a composition similar to the IMG(P), but wall rock assimilation plays a significant role during magmatic evolution. Zircons from the IMG(P) show higher Eu/Eu* (>0.3), 10000*(Eu/Eu*)/Y (>4), and (Ce/Nd)/Y (>0.04) ratios than those of the PQP, indicating the fertile magmas were more oxidized and hydrous. The high magmatic oxidation state for the IMG(P) is also supported by using the zircon (ΔFMQ = +4.1 to + 6.1), and amphibole (ΔFMQ =+2.6 to + 3.4) oxybarometers. Their high magmatic water contents are supported by evaluations using amphibole (∼3.5 wt%) and plagioclase (∼6.7 wt%) compositions, respectively. However, the LGP has higher magmatic oxygen fugacity (Eu/Eu* >0.3, ΔFMQ = +4.2 to + 7.3) caused probably by auto-oxidation of the magmas, but lower magmatic water contents (Sr/Y = 31 ± 15; V/Sc = 6 ± 4) caused presumably by previous fluid exsolution and/or fractionation of hydrous silicate minerals during magma ascent than those of the IMG(P). Therefore, the highly hydrous and oxidized magmas with juvenile material are favorable to form porphyry Cu deposits in Tibetan orogen. On the contrary, magmatic evolution can probably reduce the water content of magma chamber, which is not beneficial to porphyry Cu mineralization.