Biogenic and petroleum-related ore-forming processes in Dongsheng uranium deposit, NW China

Sandstone-hosted roll-type U deposits has recently been found in the Middle Jurassic Zhiluo Formation in the Dongsheng area, north of the Ordos Basin. In order to assess the origin of the uranium mineralization, the host sandstone has been investigated, with emphasis on δ34S values of ore-stage pyrite, δ13C of calcite, fluid inclusion oil biomarkers, X-ray diffraction, ICP-MS, as well as studies by electron microprobe and scanning electron microscope. The mineralization consists mainly of coffinite. Some of the coffinite is intimately intergrown with ore-stage pyrite, indicating co-precipitation. Ore-stage pyrite has δ34S values from − 34 to + 17‰, suggesting the pyrite sulfur originated from bacterial sulfate reduction. In the host sandstone, calcite cement exhibits δ13C values from − 27.6 to − 2.1‰ and fluid inclusion oils show evidence of biodegradation. These lines of evidence indicate petroleum was likely oxidized by sulfate-reducing bacteria (SRB) and thus supplied a source of some of the carbon in the calcite. Microbial degradation of petroleum is partially supported by the low-temperature diagenetic environment (< 70 °C). Thus, SRB were likely to reduce sulfates to sulfides simultaneously with petroleum oxidization, and caused direct or indirect reduction of hexavalent uranium [U(VI)] to tetravalent uranium [U(IV)]. U(VI) pre-concentration by adsorption and subsequent inorganic reduction is likely to be less significant than direct reduction of U(VI) by SRB in the Dongsheng deposit. This is because ore-stage solution is non-acidic (pH = 8 to 10), as indicated by abundance of ore-stage pyrite and calcite cement and lack of marcasite in the host sandstone.