Geochemistry and geochronology of the Late Cretaceous Tongchanggou Mo–Cu deposit, Yidun Terrane, SE Tibet; implications for post-collisional metallogenesis

Abstract

The Tongchanggou porphyry Mo–Cu deposit is located within the Zhongdian Arc of the southern Yidun Terrane of SE Tibet and is a typical porphyry deposit formed in a post-collisional setting. However, the geodynamic and ore-forming processes that controlled the formation of the deposit remain poorly understood, primarily as a result of a lack of systematic research on the petrogenetic and mineralizing processes. This study presents new whole-rock geochemical, geochronological, zircon trace element and Hf isotopic, and molybdenite Re–Os isotopic data for the Tongchanggou porphyry Mo–Cu deposit, and uses these data to discuss the petrogenesis of the granites, ore-forming processes that generated the deposit, geodynamic setting of the deposit, and broaderimplications for the genesis of post-collisional mineralization. The zircon U–Pb and molybdenite Re–Os data indicate that the Tongchanggou porphyry Mo–Cu mineralization formed during the Late Cretaceous (82.9–84.3 Ma), contemporaneous with the formation of the Xiuwacu Mo–W, Relin Mo–W–Cu, and Hongshan Cu deposits elsewhere in the Zhongdian Arc. The Tongchanggou porphyry intrusions are weakly peraluminous to metaluminous I-type granites, that are high-K and calc-alkaline seriers, show subduction signatures and adakitic affinity. They have low εHf(t) (−8.1 to −1.7) and ancient two-stage Hf model ages (TDM2: 1.11–1.47 Ga). These features indicate that the magmas that formed the Tongchanggou porphyry intrusions were derived from a subduction-modified mantle wedge and overthickened ancient lower crustal material. The mixing of these mantle-derived Cu-rich and crust-derived Mo-rich melts would have generated high magmatic oxygen fugacity conditions and hydrous magmas, both of which are critical for the formation of this type of porphyry mineralization. These parental magmas attained highMo/Cu values as after the significant crystallization process during magma ascent, a process that led to the formation of the Tongchanggou Mo–Cu deposit. This study demonstrates that post-collisional porphyry deposits generally formed during a rapid tectonic change from compressional to extensional setting and changing tectonic regimes may be key for the formation of this type of deposits.