Petroleum-related origin for sandstone-hosted uranium deposits in the Dongsheng area, Ordos Basin (China)

Abstract

Sandstone-hosted roll-type uranium deposits were found to occur in the Middle Jurassic Zhiluo Formation sandstone in Dongsheng area, north of the Ordos basin. The uranium occurs mainly as coffinite. Some of the coffinite is intergrown intimately with sec- ondary pyrite, indicating simultaneous precipitation. The pyrite has δ34S values from -34‰ to +18‰, suggesting an origin of bacterial sulfate redu ction. Calcite cement of the host sandstones has δ13C values from +0.3‰ to -27.6‰. The most negative δ13C value is more negative than those of terrestrial higher plants-derived organic mat- ter in the Jurassic in the basin, thus the carbon of the calcite cement is likely derived from oil and natural gas, most likely from mixing of preexisting inorganically derived CO2 and methane-derived CO2. The hypothesis is supported by the great number of oil/gas inclusions found in the calcite cement and healed fractures through quartz grains. Interestingly, in the sandstone with high content of uranium, secondary pyrite has δ34S as light as -34‰, calcite cement has δ13C lower than -10‰. Thus, it can be concluded that sulfate reducing bacteria likely oxidized hydrocarbons, and directly or indirectly re- duced uranium (VI) to uranium (IV).