Fluid inclusion and organic geochemistry characteristics of the Bashibulake uranium deposit in Kashi Sag, Xinjiang

Abstract：Located in Kashi Sag on the northwestern margin of Tarim basin, Xinjiang, the Bashibulake uranium deposit is hosted in clastic sedimentary rocks of continental facies, where lots of organic matters are widely spread, such as residues of oil, gas and asphalt. This paper mainly presents analytical data of fluid inclusions and organic geochemistry of asphalt sandstone ore bodies, with the purpose of constraining the ore-forming fluid temperature, salinity and ore-forming physical-chemical background, tracing the organic matter source and discussing uranium mineralization. Fluid inclusions in the ore-hosting sandstone under microscope are predominantly petroleum inclusions, composed mostly of liquid hydrocarbon inclusions with minor hydrocarbon-bearing brine inclusions. The homogenization temperature of the ore-forming fluid changes from 71 to 193℃, and the salinity varies from 0.71 to 23.05 (wt% NaCl), indicating a kind of low temperature and low salinity ore-forming fluid. The mineralization pressure varies from 77.90 to 211.75 ×105Pa, and the mineralization depth varies from 0.26 to 0.71 km. The content of chloroform bitumen “A” in organic matter changes insignificantly from 0.0019 to 0.0026 wt%, suggesting that the organic matter should be derived from the marine algae. The ratio of pristine to phytane (Pr/Ph) from samples changes from 0.77 to 1.01, 0.89 on average, indicating a deoxidization environment for the organic matter. Odd-even predominance index (OEP) varies from 0.72 to 0.84, 0.78 on average, implying that the maturity of the organic matter is relatively high. Carbon preference index (CPI) is from 1.16 to 1.35, 1.25 on average, indicating a higher degree in thermal evolution. Combined with field geological characteristics, the authors hold that the uranium mineralization of this ore deposit was intimately related to the reduction of oil and gas which migrated upward along fractures and rock pores.