Abstract: Baituyingzi molybdenum deposit is located in the south of the Xilamulun molybdenum metallogenic belt, which is a new exploration discovered medium porphyry type molybdenum deposit in recent years. This paper reports the geological characteristics, Re-Os age of molybdenite, fluid inclusions and H-O-S-Pb isotope data of  Baituyingzi molybdenum deposit to determine its metallogenic age and fluid characteristics. Molybdenum mineralization mainly exists in veinlet-disseminated, breccia-type and vein-type forms in Indosinian granite porphyry. The Re-Os isochron age of (245.4 ± 4.1) Ma for the molybdenite indicates that molybdenum mineralization occurred in the Middle Triassic. Four types of inclusions were identified: gas-liquid two-phase inclusions (W), CO2-bearing three-phase inclusions (C),  daughter crystals-bearing three-phase inclusions (S) and pure gas-phase inclusions. The Ore-forming fluid in the early stage of mineralization belongs to the CO2-H2O-NaCl system with medium-high temperature and medium-low salinity, while the main mineralization stage is characterized by medium temperature, high-salt and low-salinity coeval, and the gas-phase composition is gradually transformed into H2O-based. The fluid undergoes immiscibility during the ore-forming process, accompanied by the separation and precipitation of a large number of metal minerals. With the continuous reduction of temperature and pressure, and the addition of atmospheric water, the fluid evolved into H2O-NaCl system with medium-low temperature and low salinity and low CO2 in the late stage of mineralization. Combined with the H-O-S-Pb isotopic data, it is believed that the ore-forming materials are mainly derived from the mantle, the crust-derived materials are added in the late stage of mineralization, and the source of the crust-mantle mixture as a whole. The ore-forming fluid is derived from magmatic hydrothermal solution in the early stage, atmospheric water is added in the late stage. Therefore, we suggest that  Baituyingzi porphyry-type molybdenum deposit was formed in the post-orogenic extension environment in the Early-Middle Triassic. The source of the ore-forming materials in porphyry molybdenum deposits is mainly crust-mantle mixed or crust, while the source of ore-forming materials of copper is mantle. The oxygen fugacity and pH value of the fluid are two of the main factors for the precipitation of molybdenite.

Key words:  , Baituyingzi molybdenum deposit, Triassic, metallogenic chronology, ore-forming fluid, Inner Mongolia ,