Trapping of uranium by organic matter within sandstones during mineralization process: A case study from the Shuanglong uranium deposit, China

Abstract

Carbonaceous debris (i.e., CD), dispersed organic matter (i.e., DOM) within sandstones, has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Relations of uranium enrichment and organic matter (i.e., OM) were investigated through maceral analyses, proximate analysis, vitrinite reflectance measurements, and morphology analyses of uranium minerals. The results show that CD widely distributed in sandstones, is associated with pyrite and uranium minerals. Macerals are mostly composed of telinite, collotelinite, collodetrinite, semifusinite, and inertodetrinite, including that collotelinite is dominant (contents more than 60%). The CD is at the stage of medium coalification, and is of low moisture, high ash, high volatile matter, medium fixed carbon, and high sulfur. Uranium occurs in uranium minerals (i.e., pitchblende and coffinite) and amorphous uranium. Moreover, uranium minerals partly or completely replace macerals. Statistical analyses show that porous inertinite and higher organic matter maturity is more advantageous to uranium enrichment. Additionally, uranium is unevenly distributed in macerals along certain directions, which is related to the physicochemical properties of macerals. Porous inertinite could provide channel for uranium-bearing fluid migration and space for uranium preconcentration. The CD at the stage of gas coal is of reducibility to be favorable for the forming of uranium minerals. Moreover, the relative atomic content of uranium (IV) increases with the increasing organic matter maturity. Uranium-bearing fluid could permeate into macerals under certain physical and chemical conditions, and react, i.e., adsorption followed by reduction. It is beneficial to comprehend the mechanism of uranium mineralization, providing certain guides for prospection.