Uranium occurrence state in the Tarangaole area of the Ordos Basin, China: Implications for enrichment and mineralization

The Zhiluo Formation in the Tarangaole area is a part of the Dongsheng uranium ore-field in the northeast Ordos Basin, China. Uraniferous sandstones from this formation are examined in this study. The type and mode of occurrence of uranium minerals is investigated in detail using an electron probe, α-track etching, energy spectrum, backscattering, a scanning electron microscope, and an experiment for sequential extraction of uranium. The results show that these uranium minerals are mainly composed of coffinite, a small amount of pitchblende, and some minerals containing titanium and uranium. Coffinite closely coexists with pyrite, altered ilmenite or leucoxene, charcoal, biotite and clay minerals. Some coffinite occurs along mineral edges as burrs or fine columns; it also occurs in biotite cleavage cracks with granulars. In addition, some uranium minerals are also observed within grains. α-Track etching shows that adsorbed uranium is either distributed at the edge of the debris or scattered among the grains. The sequential extraction experiment indicates that the proportion of uranium minerals was significantly greater than that of the adsorbed uranium in the samples, with the latter being largely present in pyrite and organic matter. Residual, oxidizable and weak acid-extractable uranium occur in the deposit. Scan-mapping of uranium and titanium reveales that brannerite is almost nonexistent. Strongly altered ilmenite is considered a highly effective aggregating agent for uranium. A large number of altered minerals, including clays, carbonaceous clastics, biotite, pyrite and ilmenite, are widely developed in the uraniferous sandstone, and play a significant role in the complex process of adsorption–reduction–precipitation of uranium. The discovery of pitchblende and two types of coffinite (high-Y and low-Y) are discussed in context with their symbiotic relationship with the altered minerals. It is suggested that the uraniferous sandstone underwent at least two stages of fluid activity.