New insights into uranium source and mineralization process of the world-class jingchuan sandstone-hosted uranium deposit, ordos basin, China: Evidence from geology, carbonate textures and geochemistry

Abstract

The world-class Jingchuan sandstone-type uranium deposit is situated in the Lower Cretaceous gray sandstone strata of the Luohe Formation, Ordos Basin, China, distinguished by its exceptionally thick orebodies (3–280 m) and extensive spatial distribution (about 2000 square kilometers), with relatively high ore grades of 4.3–17.4 kg/m², suggesting exceptionally high uranium reserves. Uranium mineralization is preferentially hosted within reduced gray sandstone zones that exhibit calcareous, ferruginous, and siliceous cementation. In contrast, the extensive red beds, present in the middle and lower parts of the gray sandstones, are generally barren of uranium. The uranium mineralization in basin sandstones is widely attributed to redox processes; however, the precise mechanisms and detailed processes involved remain undetermined. The presence of carbonates persists throughout the uranium mineralization process, effectively recording the properties of ore-forming fluids, mineralization processes, and precipitation conditions. This study integrates detailed petrographic observations with in situ geochemical analyses (LA-ICP-MS) to constrain the genesis of this exceptional uranium deposit. Three distinct types of carbonates have been identified: (1) highly rounded dolomite and calcite clasts (pre-ore stage; detrital grains; Type 1); (2) shell-like, euhedral microcrystalline, or cement-bound Fe-bearing dolomite (ore stage; spatially and genetically associated with uranium mineralization; Type 2); and (3) pore-filling sparry calcite (post-ore stage; formed during supergene fluid infiltration; Type 3). LA-ICP-MS analyses of the ore-stage carbonates (Type 2) reveal elevated U contents and prevalent existence of U-rich mineral inclusions, corroborating their direct association with U mineralization. Additionally, elevated total rare earth element (ƩREE) and phosphorus (P) concentrations in the Type 2 carbonates indicate mixture of oxidizing basinal fluid and reducing hydrocarbon-bearing fluid. Type 1 and Type 2 carbonates share similar chondrite-normalized REE patterns and Th/U ratios, indicating that the latter inherited its geochemical signature from the former. The similarity in geochemical signatures suggests that Type 1 carbonate clasts may have served as a source of uranium and other metals for the ore-forming fluids. In contrast, Type 3 shows a distinct geochemical signature, indicative of a different fluid source and a shift in the geochemical conditions during its formation. The combined geochemical (Y/Ho ratios, δCe values, and other trace element data) and mineralogical evidence suggest that the uranium mineralization may have originated from intra-basinal stratigraphic uranium-bearing carbonate clasts. We propose that prolonged interaction between oxidizing basinal fluids and these carbonate clasts resulted in the progressive enrichment of U, Fe, Mg, Ca, and other elements within the ore-forming fluids. Subsequent mixing of these oxidizing, uranium-rich fluids with reducing, hydrocarbon-bearing fluids, migrating upward along deep-seated faults, triggered redox reactions, leading to the widespread precipitation of Fe-bearing dolomite and the formation of the Jingchuan uranium deposit. The post-mineralization stage, marked by the deposition of uranium-poor calcite, reflects a waning of mineralizing hydrothermal activity and the influx of meteoric water into the system.