Enrichment mechanisms of U and Th elements in the bauxite rock series of the Benxi Formation, Lower Carboniferous, central North China Block

Objective Uranium (U) and thorium (Th) in rocks and minerals are the material basis for the generation and release of crust-derived helium.

Methods The enrichment characteristics and mechanisms of U and Th are key to understanding the accumulation of crust-derived helium reservoirs. Taking the bauxite series of the Benxi Formation in the central North China Block as the research object, this study reveals the enrichment mechanisms of U and Th in the bauxite series and calculates the helium generation rate by means of petrogeochemical and chronological research methods, combined with detrital zircon dating, sedimentary environment identification, and mass balance calculation. It provides theoretical support for exploring bauxite-type helium reservoirs.

Results and Conclusions The results show that the bauxite series of the Benxi Formation in the central North China Block is widely developed, but its planar thickness varies significantly, being relatively thicker on both the north and south sides close to the provenance area. From bottom to top, the bauxite series can be divided into an iron-rich interval, an aluminum-bearing interval, a bauxite ore interval, another aluminum-bearing interval, and a dark mudstone interval, with diaspore as the main component in the bauxite interval. The enrichment of U and Th in the bauxite series is mainly controlled by provenance supply, sedimentary environment evolution, and supergene leaching. The detrital zircons have main peak ages of 320 and 453 Ma, indicating that the provenance is magmatic rocks input from both north and south directions. After weathering and diagenesis of the source rocks, U and Th are preserved in detrital zircons in the form of isomorphism. During the formation of the bauxite series, there were dynamic changes in the oxidation-reduction environment, which provided conditions for the release, reduction, and precipitation of U. Under the influence of acidic leaching fluids, alkali metal ions (e.g., K⁺, Na⁺) and alkaline earth metal ions (e.g., Mg²⁺, Ca²⁺) exhibit high chemical reactivity and undergo significant leaching loss. In contrast, Al and Th, characterized by their chemical stability, are less mobile during the leaching process. Subject to the pH value and chemical composition of the system, U can be adsorbed and enriched by aluminum, iron, and manganese oxides/hydroxides. In the bauxite series, the U content in bauxite ranges from 13.26 to 57.1×10⁻⁶, the Th content ranges from 42.19 to 136.0×10⁻⁶, and the helium generation rate is 2.85 to 10.94×10⁻¹² cm³/(a·g), showing good helium-generating capacity. The areas with thicker bauxite rock distribution in northwestern Henan Province, as well as in southern and northern Shanxi Province, exhibit strong helium generation capacity, making them potential favorable zones for helium enrichment.