Geochemical characteristics, formation mechanism, and exploration potential analysis of the interlayer oxidation zone of the Upper Cretaceous Saihan Formation in the central Ulanqab Depression of the Erlian Basin

ObjectiveThe characteristics of interlayer oxidation zone restrict sandstone-type uranium mineralization. Therefore, it is of great significance to study the geochemical characteristics and genesis of interlayer oxidation zone for evaluating the potential of sandstone-type uranium mineralization. MethodsAccording to the drilling information, combined with sand dispersion system mapping, sediment debris composition, environmental sensitive parameters and elemental geochemical characteristics, the key information of interlayer oxidation zone was analyzed. ResultsThe formation mechanism of interlayer oxidation zone was discussed, and the metallogenic potential of sandstone-type uranium in the study area was evaluated. It is found that the sand bodies in the study area are mainly composed of red sandstone, yellow sandstone, gray ore-bearing sandstone and primary gray sandstone, which represent strong oxidation zone, weak oxidation zone, transition zone and reduction zone respectively. From strong oxidation zone, weak oxidation zone, transition zone, reduction zone to ore-bearing zone, the environmental sensitive parameters show a trend from small to large, and trace elements and rare earth elements are enriched in the metallogenic area of the transition zone. Conclusion When the oxygen-containing uranium-containing fluid enters the uranium reservoir, the oxygen in the fluid reacts with the reducing substances in the sand body, Fe²⁺ is oxidized to Fe³⁺, U(IV) is oxidized to U(VI), and migrates with the fluid in the form of uranyl complex ions. When the oxygen in the fluid is gradually depleted, Fe³⁺ is reduced to Fe²⁺ and combined with S^2-to form pyrite. Uranyl complex ions are reduced to form pitchblende and precipitated, and some U(IV) reacts with SiO₄^2-to form uraninite. When the mineralization occurs, the complexation degree of different complex anions in the fluid with trace elements and rare earth elements is different, which makes the four trace elements such as Y, Co, Ni and Zn and the four rare earth elements such as Sm, Gd, Dy and Er enriched at the same time. Uranium mineralization is mainly developed in the transition zone near the weak oxidation zone, indicating that the transition zone and near the oxidation zone is a favorable uranium prospecting area in the study area.