Petrological and geochemical constraints on genesis of the Tel’aobao sandstone-hosted uranium deposit, NW Ordos Basin

Abstract

Abstract The Tel’aobao uranium deposit, which was recently discovered within the oxidizing Huanhe Formation, northwestern Ordos Basin, represents a new stratigraphic target for uranium exploration. This paper investigates the ore genesis through petrological, geochemical and chronological analyses of various sandstone and mudstone samples. Results show that the ore-hosting strata, sourced from intermediate-acidic volcanic rocks, were deposited under the semi-arid and hot paleoclimatic conditions. The host sandstones exhibit several distinct characteristics: a limited initial reducing capacity, with total organic carbon (TOC) below 0.1% and total sulphur content (TSC) ranging from 0.002% to 0.074%; low kaolinite (averaging c. 8%) and chlorite content (averaging c. 13%); a high proportion of chlorite–smectite (43.7% to 70.3%) and illite–smectite mixed layers (40.5% to 53.6%); and elevated uranium content in both red and green non-mineralized sandstones, with averages of 6.8 and 7.46 ppm, respectively. In addition, these sandstones appear to have been altered by the ternary fluids system comprising oxidizing groundwater, reducing hydrocarbon and alkaline hydrothermal fluid, leading to the formation of some authigenic minerals and epigenetic indicators: uneven colour gradation, oxidized carbonaceous remnants, hematitization, pyritization, calcitization, chloritization, elevated acid-hydrolysed hydrocarbon content (24.6 to 6452 μL/kg) and pale-yellow fluorescence. The negative δ 1 3CV-PDB (−7.6‰ to −1.1‰) and δ 34S (−38.7‰ to −7‰) values support the occurrence of bacterial sulphate reduction (BSR), and abundant nano- to micro-sized coffinite (P-, Zr- or Y-bearing) and U-Ti aggregates are intimate related to pyrite, chlorite, sphalerite and iron oxides, reflecting two distinct mineralization mechanisms: low-temperature redox and thermochemical precipitation. Whole-ore U-Pb dating yields a reliable isochron age of 53.8 ± 6.3 Ma, whereas electron microprobe analysis traces a continuous dynamic mineralization process from 134.43 to 4.45 Ma. Based on these findings, a metallogenic model is proposed for this deposit: (1) uranium pre-enrichment during sedimentation stage; (2) secondary hydrocarbon reduction at the end of the Early Cretaceous; (3) regional interlayer oxidation-reduction mineralization from the Late Cretaceous to Eocene; and (4) hydrothermal reworking and deficient oxidation since the Oligocene. This research offers an effective vector for future uranium exploration within red-variegated strata.