Mineralogical and geochemical evidence for coupled bacterial uranium mineralization and hydrocarbon oxidation in the Shashagetai deposit, NW China

• Published in: Chemical geology Vol. 236; no. 1; pp. 167 - 179
• Main Authors: Cai, Chunfang, Dong, Hailiang, Li, Hongtao, Xiao, Xinjian, Ou, Guanxi, Zhang, Chunming
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2007
• Subjects: Anaerobic petroleum oxidation; Bacteria; Microfossil; Nanoparticle; Sulfate-reducing bacteria; Sulfur isotopes; Uranium deposit; Sulfate-reducing bacteria; Microfossil; Sulfur isotopes; Uranium deposit; Nanoparticle; Anaerobic petroleum oxidation
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2006.09.007

Unusual mineral structures have recently been found in a sandstone-hosted roll-type uranium deposit in the Middle Jurassic Zhiluo Formation in the Shashagetai deposit, the northern Ordos basin, NW China. The structures possess a chemical composition and crystal structure characteristic of mineral coffinite [(USiO 4) 1− x (OH) 4 x ], which occurs as nanoparticles with size ranging from 5 to 25 nm. These structures are interpreted to be fossilized microorganisms, based on mineralogical biosignatures including morphology, size, occurrence of biogenic coffinite as nano-crystals, and biological elements such as P. The intimate intergrowth of coffinite with secondary pyrite of bacterial origin, as defined by low δ 34S values, and calcite cements with petroleum-derived carbon supports its biogenic origin. Oil inclusions in the host sandstone are characterized by abundant n-alkanes, slightly increased Pr/nC 17 and Ph/nC 18 ratios, significant amounts of demethylated hopanes and tricyclic terpanes, and the existence of unresolved complex mixtures. These characteristics are interpreted to be a result of mixing of an earlier, heavily degraded oil with a later charged fresh oil; subsequently the oils were slightly degraded. These lines of evidence lead to the proposal that the reduction of sulfate and oxidization of petroleum are likely synchronous with reduction of hexavalent [U(VI)] to tetravalent [U(IV)] uranium by sulfate-reducing bacteria (SRB). The discovery of a natural association of microorganism-like structures, a uranium mineral, and biodegraded petroleum has implications for uranium biomineralization and fossil fuel exploration.