Traces of hydrocarbon-bearing fluid and microbial activities and their implications for uranium mineralization in southern Ordos basin, China

The sandstone-type uranium mineralization in the Shuanglong area of the Ordos basin, has been studied from the point view of organic geochemistry and mineralogy. Acidolysis hydrocarbon compositions within sandstones, and occurrences of uranium minerals were analyzed by using optical microscope, gas chromatograph, scanning electron microscopy coupled with energy dispersive spectroscopy, and electron probe. The results show the methane is dominant among acidolysis hydrocarbon components, and the natural gas is oil-type gas. According to the escape time of oil -gas, the main mineralization period, and spatial relations of oil-gas and ore bodies, it is inferred that the hydrocarbon-bearing fluid is mainly from the lacustrine source rocks of the Triassic Yanchang Formation (T3y), and the uranium-bearing stratum might undergo hydrocarbon reduction. Uranium minerals occur in forms of microspheres outlining at the cell lumens of organic matter, and uranyl phosphate minerals are also observed not alongside Fe-oxyhydroxides, indicating that bio-mineralization exits. The fact that carbonaceous debris (i.e., CD) partly underwent biodegradation is evidenced by the front-peak type n-alkanes compounds with the presence of unknown complex mixtures. Besides, numbers of framboidal pyrites with the diameter of 1 mu m occur in CD, implying that microbial activities exist in uranium-bearing stratum. The maximum paleogeothermal temperature of the Jurassic Zhiluo Formation (J2z) shows that the geological environment is favorable for the activity of microorganisms. Under the combined actions of deep hydrocarbon-bearing fluid, microbial activities, CD, and pyrites, aqueous uranium is precipitated and enriched in forms of adsorption and reduction, which is helpful for further understanding the mechanism of sandstone-type uranium mineralization, thus providing certain guide for uranium exploration.