Geochemical Characteristics and Uranium Mineralization Processes in the Shawan Formation of the Chepaizi Uplift, Junggar Basin, Northwestern China

Abstract

The Chepaizi Uplift, situated on the western edge of the Junggar Basin in northwestern China, has recently become a significant target area for in situ leach sandstone-type uranium exploration. The Neogene Shawan Formation, a newly identified uranium-bearing layer, has gained considerable attention for its potential. This study utilizes scanning electron microscopy (SEM), X-ray powder diffraction (XRD), whole-rock geochemistry, and electron probe microanalysis (EPMA) of uranium minerals. Combined with sedimentological and tectonic background analysis, these methods were applied to investigate geochemical characteristics and uranium mineralization processes. The sandstones in the Shawan Formation are primarily lithic sandstone and subarkose, with the provenance dominated by felsic rocks from the upper crust. Coffinite is the predominant uranium mineral, accompanied by titanium−uranium oxides and minor amounts of pitchblende. Coffinite appears as colloidal coatings around framboidal pyrite, in short-prismatic aggregates corroding albite, and as banded structures within calcite cement. Elemental ratios indicate that the Shawan Formation's paleo-hydrological environment was arid, continental, and brackish, with paleo-redox conditions reflecting a hot, dry climate. Uranium mineralization occurred in two stages: initially, uranium-containing oxygenated waters migrated laterally across slope zones, forming a redox transition zone and resulting in the pre-enrichment of uranium. Subsequently, hydrocarbons migrated along faults and unconformities, leading to secondary reduction of the interlayer oxidation zone and resulting in uranium enrichment and mineralization at the interface of grayish-green and gray sandstone layers.