Geological characteristics, mechanism, and metallogenic model of the Benbatu uranium deposit, Bayingobi basin, North China

Abstract

The Benbatu sandstone-type uranium deposit is in the southeast part of the Bayingobi basin. This study examines the geological characteristics, geochemistry, C-O isotopes, micro area scanning, and fluid inclusions in the ore-bearing sandstones of the Benbatu uranium deposit. It classifies the alteration mineral combination, defines the stage of uranium mineralization and establishes both the mineralization laws and the uranium mineralization model. The favorable stratigraphic structure of the Lower Cretaceous lower member of the Bayingobi Formation (falling stage system tract), 1st and 2nd members of the upper Bayingobi Formation (lowstand + transgressive system tract) laid a foundation for migrating the ore-forming fluids along faults and the inflation of supergene fluids into the basin. The uranium mineralization period can be divided into two stages. The early alteration mineral combinations primarily include hematite, calcite, automorphic dolomite, automorphic fluorapatite, and pyrite. The late stage of mineralization includes limonite, fine-grained dolomite, fine-grained fluorapatite, pyrite, calcite, and illite. The early ore-forming fluids with the faults mainly originated from the deep part of the basin, with temperatures of 105–165 ℃ and salinity levels of 5 %–9%. In the late stage of mineralization, the ore-forming fluid is primarily epigenetic fluid, with a temperature of 60–105 °C and a salinity of 1.5 %–5%. The temperature and salinity of the ore-forming fluids have the characteristic of gradually evolving from high to low from the early to late stages of mineralization. The ore has undergone significant water and rock interaction, where uranium, lithophile, chalcophile, and rare earth elements were substitute the elements in the fine crystalline fluorapatite or adsorbed on its surface by fine-crystalline fluorapatite during the mineralization process. The U content of ore is negatively correlated with SiO₂, Al₂O₃, K₂O, and NaO content and positively correlated with other elements (P, Sr, Ca, and Fe²⁺).