Mineralogy, geochemistry, and fluid action process of uranium deposits in the Zhiluo Formation, Ordos Basin, China

Abstract

Some controversies still exist regarding the genesis of sandstone-type uranium deposits in the northeastern Ordos Basin. Herein, based on boreholes and well logging data, we selected sandstone samples representing different epigenetic alteration environments as the research object and comprehensively utilized polarizing microscopy, scanning electron microscopy, electron probe analysis, elemental analysis, and other test methods to research sandstone genesis. The main altered minerals in the sandstone of this area include limonite, pyrite, achavalite, carbonate, sulfate, clay, and uranium minerals. The complex characteristics of alteration minerals indicate multi-stage fluid mixed mineralization in this area. According to the relationship between the major elements in different types of sand bodies, the components were classified into three groups. The first is the reduction media group, including CaO, MnO, total oxygen carbon (TOC), S, U, and burning loss. The second is the detrital particle group, including SiO2, K2O, and Na2O, and the third contains the clay and volcano component group, including Fe2O3, P2O5, TiO2, FeO, MgO, and Al2O3. Among these, the content of reduction media is positively correlated with uranium enrichment, whereas detrital particles and clay components reflect the influence of alteration on the sand body composition, which indirectly affects the uranium adsorption and precipitation. Through the normalization of rare earth element (REE) chondrites, we find that the distribution patterns of REEs in rocks with different geochemical environments show a fractionation of light and heavy rare earth elements (HREEs), a right-leaning enrichment of light rare earth elements (LREEs), and relatively flat HREE. Further, via the Post Archean Australian Shale (PAAS) normalization, we observe that the mudstone of the Zhiluo Formation exhibits a relatively flat curve, whereas the sandstones have strongly positive Eu anomalies. The difference in REE values for different types of sand bodies indicates that, although all types of rocks have a similar sedimentary environment, their original sediments underwent fluid alterations such as oxidation and reduction. Based on the uranium mineralization evolution history in the study area, the water-rock reaction process and mineral-alteration sequence were determined on the basis of alteration types and elemental concentrations. Our findings provide new insight into the petrological significance, mineralogy, and geochemical composition of uranium deposits in the Ordos Basin.