Formation of the Cretaceous skarn Cu–Au deposits of the southern Gangdese belt, Tibet: Case studies of the Kelu and Sangbujiala deposits

Abstract

Mesozoic subduction of Neo-Tethyan oceanic crust caused magmatism that formed numerous Jurassic and Cretaceous intrusions within the southern Gangdese belt of Tibet. These intrusions are associated with the formation of giant Jurassic Xiongcun porphyry Cu deposits, but the prospectivity of other Cretaceous intrusions in this area remains unclear. This study presents new molybdenite Re–Os ages for the Kelu and Sangbujiala Cu–Au skarn deposits within the southern Gangdese belt as well as new geochemical, zircon U–Pb geochronological, and Sr–Nd–Hf isotopic data for the intrusions associated with these deposits. These new data indicate that (1) the Kelu deposit formed at ca. 92 Ma, coeval with emplacement of the diorite and biotite granodiorite intrusions (ca. 92–90 Ma) in this area, and (2) the Sangbujiala deposit formed at ca. 95 Ma, contemporaneous with emplacement of the mineralized biotite granodiorite (ca. 95–92 Ma) in this area. These data suggest that the skarn Cu–Au mineralization in this area formed as a result of Cretaceous magmatism in the southern Gangdese belt. The intrusions associated with the Kelu and Sangbujiala deposits have adakitic affinities with low contents of Y and heavy rare-earth elements and high Sr/Y ratios (22.4–123). These intrusions also have more primitive Sr–Nd isotopic compositions (εNd(t) = 3.99–4.81, (87Sr/86Sr)i = 0.7040–0.7044), lower εHf(t) values (8.0–13.18), and higher zircon EuN/Eu* values (mean of 0.50) than those of contemporaneous but barren adakitic intrusions in this region. This suggests that the mineralization-related intrusions were oxidized and formed from magmas that were generated predominantly by partial melting of subducted Neo-Tethyan slab material without any significant crustal contamination or assimilation of sedimentary material. However, these intrusions contain elevated Th, Ni, and Cr contents, and have high Mg# ratios (43.4–56.9), suggesting that they formed from magmas that interacted with metasomatized mantle wedge peridotite material. Combining our new data with the results of previous research suggests that the southern Gangdese belt records two stages of Cu–Au mineralization related to Neo-Tethyan subduction. The first stage generated the large Xiongcun porphyry Cu–Au deposit and was associated with normal subduction during the Jurassic, whereas the second stage was associated with the formation of the Kelu and Sangbujiala skarn deposits during Cretaceous slab rollback.