Fluid inclusions and H–O–S–Pb isotope systematics of the Baishan porphyry Mo deposit in Eastern Tianshan, China

Abstract

The Baishan porphyry Mo deposit formed in the Middle Triassic in Eastern Tianshan, Xinjiang, northwestern China. Mo mineralization is associated with the Baishan monzogranite and granite porphyry stocks, mainly presenting as various types of hydrothermal veinlets in alerted wall rocks, with potassic, phyllic, propylitic, and fluorite alteration. The ore-forming process can be divided into four stages: stage I K-feldspar–quartz–pyrite veinlets, stage II quartz–molybdenite±pyrite veinlets, stage III quartz–polymetallic sulfide veinlets and stage IV barren quartz–calcite veins. Four types of fluid inclusions (FIs) can be distinguished in the Baishan deposit, namely, liquid-rich two-phase (L-type), vapor-rich two-phase (V-type), solid-bearing multi-phase (S-type) and mono-phase vapor (M-type) inclusions, but only the stage I quartz contains all types of FIs. The stages II and III quartz have three types of FIs, with exception of M-type. In stage IV quartz minerals, only the L-type inclusions can be observed. The FIs in quartz of stages I, II, III and IV are mainly homogenized at temperatures of 271–468°C, 239–349°C, 201–331°C and 134–201°C, with salinities of 2.2–11.6wt.% NaCl equiv., 1.1–10.2wt.% NaCl equiv., 0.5–8.9wt.% NaCl equiv. and 0.2–5.7wt.% NaCl equiv., respectively. The ore-forming fluids of the Baishan deposit are characterized by high temperature, moderate salinity and relatively reduced condition, belonging to a H2O–NaCl±CH4±CO2 system. Hydrogen and oxygen isotopic compositions of quartz indicate that the ore-forming fluids were gradually evolved from magmatic to meteoric in origin. Sulfur and lead isotopes suggest that the ore-forming materials came predominantly from a deep-seated magma source from the lower continental crust. The Mo mineralization in the Baishan deposit is estimated to have occurred at a depth of no less than 4.7km, and the decrease in temperature and remarkable transition of the redox condition (from alkalinity to acidity) of ore-forming fluids were critical for the formation of the Baishan Mo deposit.