Geology, geochemistry, and geochronology of the Zegulangbei deposit in the Jiama ore district: Implications for a polycentric, complex porphyry mineralization system model

Abstract

Jiama is a typical giant-scale collisional-type porphyry mineralization system of in the Gangdese metallogenic belt, Tibet. The Zegulangbei deposit is located in the northern part of the ore district, which was a recent discovery. In the Zegulangbei deposit, intrusions have developed in multiple stages and led to the formation of complex porphyry intrusions (2# porphyries), of which granodiorite porphyry and Ⅱ-quartz diorite porphyry occurred related to mineralization. Hydrothermal alteration surrounding the 2# porphyries is characterized by potassic, phyllic, and chlorite–sericite alteration, with minor propylitization, argillization and tourmalinization. Furthermore, veinlet-disseminated copper and molybdenum mineralization mainly occurred within the potassic and phyllic alteration zones. Partial copper mineralization developed within the chlorite–sericite alteration zone. The mineralization is comprised of porphyry Cu–Mo, hornfels Cu–Mo and skarn Cu–polymetallic ore bodies. Hydrothermal vein system developed well, including 5-staged veins (M-, A-, B-, C-, and D-type vein). The similar ages of intrusive rocks (15.8–14.5 Ma, U–Pb), biotite and sericite alteration minerals (15.04–15.02 Ma, 40Ar–39Ar), and molybdenite (14.9 Ma, Re–Os) indicate that the intermediate–felsic magmatic rock emplacement triggered the hydrothermal alteration and mineralization in the Zegulangbei deposit. Geochemistry and Sr–Nd–Pb isotopes of the intrusions in Zegulangbei are consistent with those of the intrusions in the Main deposit (1# porphyries) and the Miocene adakites that developed in the eastern Gangdese belt, implying they were generated from the partial melting of the thickened phlogopite-bearing juvenile crust that had been reworked by diapirs of the arc magma during subduction. The evidence presented in this study suggests that Zegulangbei developed a complete porphyry Cu–polymetallic mineralization system, which was related to the same magma chamber in the upper crust that caused the mineralization in the Main deposit.