Abstract:

Uraniumbearing sandstone diagenesis is a complicated process in the Middle Jurassic Zhiluo Formation, north of Ordos Basin, and mineralization is inconsistent with typical interlayer redox model. On the basis of the study of diagenetic minerals, stable isotope composition (δ13C and δ18O) of calcite cements and fluid inclusions in Hangjinqi uranium deposit, north of Ordos Basin, we discussed the features of fluid activities and the process of uranium mineralization. The result shows that three fluid types were identified in the uraniumbearing sandstone: oxidizing meteoric water, acidic fluid generated by biodegradation of hydrocarbon, and alkaline hydrothermal fluid. The peak homogenization temperatures of primary aqueous inclusions trapped in sparry calcite are 140160 ℃, with the salinities ranging from 8.00% to 16.34%. The sparry calcite in sandstone has negative values of δ13C between -7.1‰ to -18.3‰(PDB). These data indicate that hightemperature, highsalinity and hydrocarbon related fluid occurred at the late stage of fluid evolution history. Previous studies have proved bacterial activity caused the uranium reduction in Zhiluo Formation sandstonehosted deposit, north of Ordos basin. However, such bacterial activity cannot occur in the hightemperature environment. Considering the uranium mobility characteristics in groundwater, we concluded that hydrothermal fluid activity was not contradictory with the microbial reduction of uranium. Hightemperature alkaline fluid changed the physicochemical condition of fluid environment, which facilitated the formation of stable CaU(VI)CO3 complex, enriching the concentration of soluble uranium in solution. Bacteria exerted the reduction of the uranium after the hydrothermal fluidrock interaction.

Key words: fluid rock interaction, alkaline hydrothermal fluid, sandstonetype uranium deposit, Ordos Basin