Origin and evolution of ore-forming fluids of the Larong W-(Mo) deposit, eastern Tibet: Constraints from fluid inclusions, H-O isotopes, and scheelite geochemistry

Abstract

The Larong porphyry W-(Mo) deposit, formed in the main collisional orogenic stage of Lhasa and Qiangtang terranes, is the first-discovered giant W-(Mo) deposit in the Leiwuqi-Zogang metallogenic belt, Sanjiang region. The W-Mo mineralization occurs as veins and stock-works mainly hosted in the granodiorite porphyry, monzogranite porphyry, biotite granite porphyry, and surrounding schist. Four types of fluid inclusions (FIs) including PC-type (pure vapor CO2), C-type (CO2-H2O), W-type (aqueous two-phase) and L-type (pure liquid H2O) FIs have been identified from Stage I quartz-scheelite ± alkaline feldspar veins, Stage II (including three sub-stages) quartz-molybdenite ± pyrite ± chalcopyrite veins, and Stage III quartz ± calcite ± pyrite veins. The total homogenization temperatures of FIs vary widely (187–437 °C) with multiple peaks, indicating multistage fluid activities. The ore-forming fluids of the main mineralization stages are characterized by CO2 enrichment, high-medium temperature (~270–440 °C), and medium–low salinity (2.4–14.4 wt% NaCl equiv.). The homogenization temperature of W-type inclusions is obviously higher than that of C-type inclusions, and the salinity is relatively lower, implying multiple immiscibility of fluids with continuous escape of CO2. H-O isotopic compositions suggest that the primary ore-forming fluids dominantly originated from magma water. Mo concentrations and δEu values in scheelite indicate a decreasing trend of redox state of ore fluids. From Stage I to Stage II to Stage III, the relatively higher-temperature, CO2-rich, and oxidized fluids gradually evolved into relatively lower-temperature, CO2-poor, and reduced fluids. Here we propose that the decrease of temperature and oxygen fugacity and increase of S2- activity and PH values of the ore fluids caused by continuous fluid-rock interaction and loss of CO2 could account for the successive mineralization of W and Mo at the Larong deposit.