In situ major and trace elements of garnet and scheelite in the Nuri Cu–W–Mo deposit, South Gangdese, Tibet: Implications for mineral genesis and ore-forming fluid records

Abstract

Garnet and scheelite are common hydrothermal silicate mineral and metal mineral in skarn or porphyry–skarn deposits, and can aid in constraining the composition of ore-forming fluid. Based on in situ major and trace element analyses of garnet and scheelite from the Nuri Cu–W–Mo deposit, we discuss the factors and mechanisms controlling the incorporation of rare earth elements (REEs) into the garnet and scheelite, and investigate the associated hydrothermal fluid characteristics. The garnets from the Nuri deposit belong to the grossular–andradite (grandite) solid solution series and range in composition from Adr64Grs34 to Adr91Grs8. The REE patterns of garnet are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs). The REEs characteristics indicates that the incorporation of REEs into the Nuri garnet was controlled by the surface adsorption and element substitution in the mineral structure. Based on the cathodoluminescence (CL) images of scheelite, two growth stages (early and late stages) are identified. These two stages of scheelite belong to the scheelite-powellite series, and are enriched in LREEs and depleted in HREEs. However, the early scheelite exhibits negative or no Eu anomalies (0.34–0.95), whereas the late scheelite presents positive Eu anomalies (1.25–2.89). The Mo content in the early scheelite is positively correlated with the δEu value, while the Mo content in the late scheelite is negatively correlated with δEu. These characteristics indicate that the redox state of the ore-forming fluid gradually changed from the oxidized state to the reduced state. The preferred REE3+ substitution for Ca2+ in the Nuri scheelite is 3Ca2+ = 2REE3+ + [ ]Ca (where [ ]Ca is a Ca site vacancy). The REE features of the scheelite are mainly controlled by the REE nature of the ore-forming fluid. According to the characteristics of trace elements in garnet and scheelite, we suggest that the ore-forming fluid of the Nuri deposit is a magmatic fluid mixed with a slightly acidic external fluid. Chloride complexes may be the main transport ligand of the REEs, and the oxygen fugacity of the ore-forming fluid gradually decreased.