In situ trace-element and Sr isotopic characteristics of scheelite and their implications for the genesis in the Nuri Cu-W-Mo deposit, Xizang

The Nuri deposit in Xizang is currently the only large deposit with Cu-W-Mo mineralization discovered in the Gangdese metallogenic belt. However, since its discovery in the last century, its genetic type has been highly controversial. Based on geological features, this study precisely classifies the metallogenic stages of the deposit. As study objects, two types of scheelite (hereinafter referred to as Sch–A and Sch–B) in the oxide stage and quartz-sulfide stage are analyzed to investigate the origins of the metallogenic fluids, the deposit's evolutionary process, and its genesis utilizing Sr isotope and LA-ICP-MS trace element analyses. Scanning electron microscopy-cathodoluminescence (SEM-CL) reveals that Sch–A has two generations, with dark homogeneous Sch–A1 being irregularly replaced by light homogeneous Sch–A2; while Sch–B has a "core-edge" structure, with Sch–B1 possessing dark gray uniform growth rings in the core and light gray homogeneous Sch–B2 in the edge. The distribution pattern of rare earth elements standardized by chondrite in Sch–A and Sr isotope data indicate that the ore-forming fluid originated from granodiorite porphyry in the early stage, and in the late stage, mixed with surrounding rock materials due to strong water-rock interactions, which is a key mechanism for the massive precipitation of scheelite. The scheelite in this study has high Mo and low Sr contents, which is consistent with the characteristics of magmatic-hydrothermal scheelite. Therefore, combining the trace-element and Sr isotope geochemical characteristics exhibited by Nuri scheelite in this study and the indications of the source of ore-forming fluid, the evolutionary process, and the genesis of the deposit, all of these indicate that the Nuri deposit belongs to the porphyry-skarn type deposit.