Unraveling the enrichment mechanisms of U, Re, Se and V in the Bayinqinggeli Deposit, Northern Ordos Basin through in situ mineralogical analysis

The Bayinqinggeli uranium (U) deposit, located in the northern Ordos Basin, is rich in U and co-enriched with valuable elements such as rhenium (Re), selenium (Se), and vanadium (V). This study presents new data of whole-rock geochemistry, as well as in situ analysis of authigenic minerals in the Bayinqinggeli deposit using Electron Probe Microanalyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), to elucidate the enrichment mechanisms of U and associated elements in the sandstone-hosted U deposit. The clay minerals and pyrites (abbreviated as Py for classification purposes) are each categorized into two types: those associated with U mineralization (Clay1 and Py1) and those not associated (Clay2 and Py2). Rhenium is primarily hosted within clay minerals and some in U minerals. Selenium is present either as discrete minerals such as clausthalite and ferroselite, or as a lattice substitute within Py1. Vanadium is enriched in unaltered U minerals and Clay1, likely existing as lattice substitutions. Uranium predominantly occurs as coffinite that is largely cemented by ore-stage calcite, which is indicative of an alkaline, carbonate-rich ore-forming fluid. The intimate association of coffinite with iron-rich montmorillonite (FeO up to 14.14 wt%), a typically neutral-environment mineral, and their extensive replacement for the dissolved Py1 demonstrate that localized abrupt changes in redox and pH facilitate U mineralization. The dissolution of pyrite resulted in a localized acidic and reducing environment and triggered U precipitation from the ore-bearing fluids. The Bayinqinggeli deposit was controlled by multiple sources of fluids. During the ore-forming stage, the early low-temperature alkaline oxidizing fluid introduced redox-sensitive elements, including U, Re, Se, and V, into a redox transition zone, leading to their enrichment. Calcite cementation partially preserved primary U minerals from oxidation. The persistent interaction of oxidizing fluids could remobilize U minerals and extract V in permeable sandstones. During the post-ore-forming stage, changes in the sources and nature of fluids within the deposit led to increased levels of Sc, Y, and P in altered coffinite, as well as the formation of low-Se Py2 and organically derived calcite. This profound understanding on mineralization mechanism of sandstone-hosted U deposits, and occurrence of U, Re, Se, and V, is crucial for mineral exploration and resource utilization in the Ordos Basin.