Mineralogical and geochemical research on Pengyang deposit: A peculiar eolian sandstone-hosted uranium deposit in the southwest of Ordos Basin

Abstract

The newly discovered Pengyang uranium prospect area in the southwest of the Ordos Basin is the world's first uranium deposit hosted in eolian sandstone, which overturned the conventional view of uranium mineralization in fluvial-delta sedimentary strata. Uranium ore body occurring as tabular or lens-shaped is hosted in the gray-green reductive eolian sandstone of Lower Cretaceous Luohe Formation at depths ranging from 700 m to 3000 m. The host eolian sandstone is characterized by the typical high angle cross-bedding, wind sand ripple and grainflow strata. In this study, we conducted borehole log, petrographic observation, whole rock geochemistry analysis, geochemical and mineralogical study of uranium occurrence (SEM), element mapping of uranium minerals and in-situ sulfur isotope determination of pyrite by LA-MC-ICP-MS to reveal the uranium enrichment mechanism. The observations by SEM show that the predominant uranium mineral is pitchblende, primarily occurring as aggregates of nano to micro crystals absorbed on amorphous TiO2 or as colloform replacing the detritus Fe-Ti oxide minerals, both demonstrating the excellent absorption property of small particles of anatase. The contents of altered minerals such as chlorite and pyrite in gray-green sandstone increased significantly under scanning electron microscope (SEM). Whole rock geochemistry analysis data of sandstone samples collected in the target layers display the strong positive correlation between U and S. Whereas, in the elements mapping images, uranium enrichments display complete dissociation with pyrite as the uranium contents adjacent to or around the pyrite grains are under detection limit, demonstrating that uranium precipitation likely occurred earlier than the sulfuration of red eolian sandstone. Sulfur isotope of the two morphological types of pyrite are both characterized by a relatively light signature, ranging from − 19.23 to − 27.22 ‰ and from − 21.60 to − 29.64‰ respectively, implying that they are generated from bacterial sulfate reduction in the open systems with an adequate supply of sulfate. Anaerobic sulfate-reducing bacteria use the hydrocarbon transferred along faults to the sandstone as energy to reduce sulfate (SO42−) to H2S. In addition to the typical vertical red - yellow - gray color zonation, we also observed distinct reverse color zonation of sandstone, which can hardly be interpreted by traditional interlayer-oxidation-zone theory. We attribute the uranium reduction to the encounter of downward penetrating oxygenated groundwater with the upward migrating oil and gas conducted by thrust faults, which are widespread distributed in the west margin of the Ordos Basin. This study gives a new insight into uranium metallogenic mechanism in eolian sandstone and underpins an effective vector for the exploration of sandstone-type uranium deposits in the western of the Ordos Basin.