The Multiple Roles of Sulfate-Reducing Bacteria and Fe-Ti Oxides in the Genesis of the Bayinwula Roll Front-Type Uranium Deposit, Erlian Basin, NE China
Résumé
The Bayinwula roll front-type uranium deposit is hosted in sandstones of the Early Cretaceous Saihan Formation deposited during the postrift stage of the.Erlian basin, in northeast China. The present study aims to characterize: (1) the roles of sulfate-reducing bacteria and (2) the roles of Fe-Ti oxides in the genesis of the Bayinwula deposit. Drill cores of the host sandstone have been investigated with emphasis on petrographic observations, whole-rock geochemistry, and geochemical and/or mineralogical study of Fe-Ti oxides, iron disulfide, uranium minerals (EMP, LA-ICPMS), and organic matter (rock-eval pyrolysis). The delta S-34 value has been measured in situ by SIMS on the different generations of iron disulfide. Within regional sandstones, preore uranium enrichment (U-mean = 30.4 ppm in whole rock) was identified on altered Fe-Ti oxides (36.5% of the whole-rock U content) and constitutes a major source of uranium for the mineralization. Petrographic observations and rock-eval data indicate that organic matter occurring in the host sandstone is mainly inherited from land plants and corresponds to type III or type IV kerogens. Organic matter fragments disseminated in sandstones may also contain significant preore uranium concentration. Framboidal and collomorph ore-stage iron disulfides have moderate to high concentrations of As, Ni, and Co and have a light sulfur isotope signature characterized by delta S-34 values from -30.5 to -7.5 parts per thousand, suggesting that sulfur originated from bacterial sulfate reduction, which was mainly responsible for (1) the liberation of U from Fe-Ti oxides and organic matter, (2) the generation of ore-stage iron disulfides, and (3) the production of a secondary H2S-rich reducing barrier involved in the reduction of U(VI) and the precipitation of U(IV). Uranyl and sulfate ions were transported through the host sandstone by low-temperature oxygenated groundwater and U(IV) was precipitated at the redox front as P-rich coffinite and ningyoite, dominantly as replacement of ore-stage iron disulfides which have partly to totally replaced organic matter and Fe-Ti oxides. Therefore, the combined mineralogical, geochemical, and isotopic characteristics of the Bayinwula roll-front uranium deposit support the theory that biogenic processes have widely contributed to the genesis of the uranium mineralization.