Sulfur isotope, fluid inclusions, and quartz solubility model decipher multistage hydrothermal fluids evolution and ore precipitation mechanism at the Nantai porphyry Mo deposit, Qinling, Central China

Abstract

Qinling orogenic belt hosts many porphyry Mo deposits, which were formed by ore-forming fluids exsolved from oxidized magmas. Detailed processes of ore fluids evolution and metal deposition mechanisms of porphyry Mo deposits remain unclear. The quartz solubility model and cathodoluminescence (CL) images combined with fluid inclusions (FIs) and isotope analyses at the Nantai Mo deposit provide new insights into processes and physicochemical conditions for Mo mineralization. Based on field and microscopy of ore and alteration assemblages together with CL images, four types of hydrothermal veins (i.e., A, B, C, and D veins) have been identified. The quartz in A veins intergrown with K-feldspar precipitated from high-temperature and low- to intermediate-salinity fluids under near-lithostatically conditions. B vein comprises quartz and K-feldspar and forms at 336–466 °C, pH from 6.30 to 11.02, and oxygen fugacity (fO2) of −30.81 to −24.26. Hydrothermal fluids during this stage underwent phase separation under ductile to brittle transition. C veins include muscovite and most molybdenite without quartz, indicating that they formed in the retrograde quartz solubility field at 353 to 398 °C. Thermodynamic calculation suggests molybdenite mainly precipitated under fO2 from −32.37 to −29.05 and pH from 2.68 to 6.54. Quartz in D veins formed from low-temperature and low-salinity fluids. Sulfur isotopic analyses suggest that the ore components are derived from the old crust. The main factors controlling Mo mineralization at Nantai are the decreases in fO2 and pH values of hydrothermal fluids.