Sedimentary Environment and Uranium Mineralization of the Neogene Shawan Formation in the Beisantai Uplift, Eastern Junggar Basin

The Beisantai Uplift, situated on the eastern margin of the Junggar Basin, has recently attracted attention due to the discovery of uranium-bearing strata within the Neogene Shawan Formation. This study investigates the depositional and diagenetic controls on uranium mineralization using back-scattered electron (BSE) microscopy, whole-rock geochemistry, and in situ trace element and sulfur isotope analyses of pyrite. Coffinite is the dominant uranium mineral, typically occurring along pyrite margins, within detrital fractures, and associated with calcite cement. Titanium–uranium oxides are distributed on rutile and pyrite surfaces or within calcite dissolution pores, indicating that uranium precipitation was governed by localized reducing microenvironments. Geochemical proxies such as Cr, V, Ni, Sr, and Ba suggest that the Shawan Formation was deposited under semisaline, arid conditions with high evaporation and weak oxidation─favoring early uranium mobility. Subsequently, δ34S values of pyrite (−48.08‰ to −40.26‰) indicate formation via bacterial sulfate reduction (BSR), with hydrocarbons providing both reducing agents (e.g., CH4, H2S) and sulfur sources. This hydrocarbon-driven BSR process led to early pyrite formation under strongly reducing the conditions. Uranium minerals are frequently observed replacing or surrounding pyrite, suggesting that pyrite acted as a redox buffer and facilitated U6+ reduction. The enrichment of redox-sensitive elements (Mo, U, Ni, Co) in ore-related pyrite further supports the development of sulfidic anoxic microenvironments. Collectively, mineralogical, isotopic, and geochemical evidence reveals a two-stage mineralization model: uranium was first introduced under oxidizing conditions and then locally precipitated in response to hydrocarbon-induced reducing environments. The close spatial and temporal association among hydrocarbons, pyrite, and uranium highlights their synergistic role in controlling mineralization within the Shawan Formation.