Genesis of the Nuri Cu‐W‐Mo Deposit, Tibet, China: Constraints from in situ Trace Elements and Sr Isotopic Analysis of Scheelite

Abstract

AbstractThe Nuri deposit is the only Cu‐W‐Mo polymetallic deposit with large‐scale WO3 resources in the eastern section of the Gangdese metallogenic belt, Tibet, China. However, the genetic type of this deposit has been controversial since its discovery. Based on a study of the geological characteristics of the deposit, this study presents mineralization stages, focusing on the oxide stage and the quartz‐sulfide stage where scheelite is mainly formed, referred to as Sch‐A and Sch‐B, respectively. Through LA‐ICP‐MS trace element and Sr isotope analyses, the origin, evolutionary process of the ore‐forming fluid and genesis of the ore deposit are investigated. Scanning Electron Microscope‐Cathodoluminescence (SEM‐CL) observations reveal that Sch‐A consists of three generations, with dark gray homogenous Sch‐A1 being replaced by relatively lighter and homogeneous Sch‐A2 and Sch‐A3, with Sch‐A2 displaying a gray CL image color with vague and uneven growth bands and Sch‐A3 has a light gray CL image color with hardly any growth band. In contrast, Sch‐B exhibits a ‘core‐rim’ structure, with the core part (Sch‐B1) being dark gray and displaying a uniform growth band, while the rim part (Sch‐B2) is light gray and homogeneous. The normalized distribution pattern of rare earth elements in scheelite and Sr isotope data suggest that the early ore‐forming fluid in the Nuri deposit originated from granodiorite porphyry and, later on, some country rock material was mixed in, due to strong water‐rock interaction. Combining the O‐H isotope data further indicates that the ore‐forming fluid in the Nuri deposit originated from magmatic‐hydrothermal sources, with contributions from metamorphic water caused by water‐rock interaction during the mineralization process, as well as later meteoric water. The intense water‐rock interaction likely played a crucial role in the precipitation of scheelite, leading to varying Eu anomalies in different generations of scheelite from the oxide stage to the quartz‐sulfide stage, while also causing a gradual decrease in oxygen fugacity (fO2) and a slow rise in pH value. Additionally, the high Mo and low Sr contents in the scheelite are consistent with typical characteristics of magmatic‐hydrothermal scheelite. Therefore, considering the geological features of the deposit, the geochemical characteristics of scheelite and the O‐H isotope data published previously, it can be concluded that the genesis of the Nuri deposit belongs to porphyry‐skarn deposit.