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.