Evolution of oxidized Ore-Forming fluids and uranium mineralization mechanisms in Sandstone-Type uranium Deposits: Insights from the Lenghu Area, Qaidam Basin

Understanding the multiphase mineralization mechanisms of sandstone-type uranium deposits remains a significant challenge in economic geology. To address this issue, we performed a comprehensive mineralogical (SEM-EDS and XRD) and geochemical (carbon–sulfur content, C–O–S isotopes, uranium valence states, and uranium speciation) analysis of 101 drill core samples from various redox zones in the Lenghu deposit, Qaidam Basin. The results reveal the evolution of a single-phase paleo-oxidizing fluid mineralization system. The findings indicate that organic matter and minerals responded to fluid evolution through three distinct stages: (1) Pre-mineralization stage: In the oxidation zone, organic matter was oxidized and dissolved (TOC loss of 17.78 %), whereas in the transition zone, strong acid precipitation led to TOC enrichment (388.63 %), accompanied by kaolinitization, iron oxide alteration, and calcite dissolution. (2) Mineralization stage: Uranium was initially adsorbed (83.68 % bound to organic matter or pyrite), then reduced (80.29 % as U⁴⁺), forming primarily coffinite within organic matter and secondarily pitchblende around framboidal pyrite. (3) Post-mineralization stage: Enrichment of calcite (258.69 % TIC) and pyrite (938.03 % TS) in the transition zone suggests the cessation of fluid activity. Significant negative δ³⁴S shifts in pyrite (–47.8 ‰ to –12.2 ‰) and δ¹³C shifts in calcite (–19.70 ‰ to –10.70 ‰) indicate that uranium enrichment occurred in a microbially mediated reducing environment, rather than by direct biological reduction. The evolution of ore-forming fluids is categorized into four stages, with uranium precipitation taking place during the third stage under weakly acidic and weakly reducing conditions. The interaction between low-temperature (<50 °C) groundwater and coal-bearing clastic rocks promoted long-distance transport and high-concentration enrichment of organically bound uranium. This study clarifies complex geochemical interactions and highlights the synergistic roles of organic, inorganic, and microbial processes, offering a reference framework for understanding multiphase mineralization in similar deposits.