GEOLOGIC FEATURES AND FLUID INCLUSIONS OF NO. 431 URANIUM DEPOSIT IN ERGUNA AREA, INNER MONGOLIA

To recognize the metallogenic features and genetic type of the uranium deposits in Erguna area, No. 431 uranium deposit is adopted for case study. Based on geological features of the deposit, microthermometry of fluid inclusions is researched for silicified quartz veins closely related to the genesis of uranium mineralization. The results show that the deposit occurs in the contact zone between the Indosinian granite and the Neoproterozoic Ergun metamorphic rocks, obviously controlled by faults. The wallrocks of ore bodies are generally developed with such alterations as silicification, chloritization, kaolinization and carbonation. The uranium mineral is dominantly autunite in the surface. Comparison studies of this deposit with some typical granitic uranium deposits indicate that No. 431 deposit is associated with granitic magmatism. The silicified quartz veins contain two types of inclusions, i.e. gas-liquid two-phase and CO₂-bearing three-phase inclusions, corresponding to homogenization temperatures of 132.2-301.5℃ and 322.5-408.9℃, salinities(NaCl) of 12.65%-2.73% and 4.04%-16.94%, and fluid densities of 0.76-0.94 g/cm³ and 0.60-0.78g/cm³. It is concluded that the uranium ore-forming fluid belongs to a NaCl-H₂O-CO₂ fluid system with medium temperature, mid-low salinity and low density. The mineralization depth of the uranium deposits in this area estimated by related formulas ranges from 0.4 to 1.7 km, which suggests a mesothermal deposit.