Uranium Occurrence State and Its Implication for Sandstone-Type Uranium Mineralization within the Hanbazhai Area of the Longchuanjiang Basin, China

The Mangbang Formation in the Hanbazhai area is part of the uranium ore field in the Longchuanjiang Basin, China. Uraniferous sandstones from this formation are examined in this study. The type and mode of occurrence of uranium are investigated in detail using an experiment for the sequential extraction of uranium, as well as an electron probe, scanning electron microscopy, and energy spectrum analyses. The sequential extraction experiment indicates that the proportion of uranium minerals is significantly greater than that of the adsorbed uranium in the samples, with the latter being largely present in framboidal pyrites and clay minerals. The results show that these uranium minerals are mainly composed of coffinite and uranium phosphosilicates, which closely coexist with framboidal pyrites, carbon debris, feldspar minerals, and clay minerals. The discovery of coffinite and uranium phosphosilicates is discussed in context with their symbiotic relationship and geochemical environment. Uraniferous sandstones are considered to have undergone at least two stages of mineralization: the sedimentary-diagenetic stage and the later uranium enrichment by fluid. The geochemical environment of the sedimentary-diagenetic stage is generally a sulfide-reducing environment, and the later fluids are rich in U, Si, P, and Y.