Geochemical characteristics of uranium-rich dispersed organic matter and their geological significance for uranium mineralization: A case study from the Ordos Basin

Geochemical characteristics of uranium minerals is an important content for uranium deposits, and is conducive to understand the formation mechanism, environment and genesis of sandstone-type uranium deposits. In the Diantou-Shuanglong uranium deposit from southern Ordos basin, carbonaceous debris (CD), dispersed organic matter, is widely distributed in sandstones from uranium-bearing strata, and is the dominant enrichment agent for uranium precipitation. The geochemical compositions of uranium-rich CD were investigated by using inductively coupled plasma mass spectrometry (i.e., ICP-MS) and scanning electron microscope-energy dispersive X-ray spectroscopy (i.e., SEM-EDS). The results show that element U bears a close relationship with elements W, Ta, Mo, Pb, Th, Bi, Mn, V, Ti, Co, Be, especially Mo, and Pb, indicating that these elements are rich in uranium-bearing minerals. The contents of rare-earth element (i.e., REE) and light rare-earth element (i.e., LREE) increase with the increasing uranium abundance, implying that REE primarily enrich in uranium minerals, especial for LREE. Moreover, LTEE (e.g., Nd) and Y are detected in uranium-bearing minerals. Besides, uraniumbearing CD exhibits similar REE geochemical characteristics and distribution patterns with the surrounding sandstones, indicating that they are homologous in sedimentary source, sedimentary environment, and tectonic background, and the CD is deposited during synsedimentary period. Given the distribution characteristics of trace element and REE, it is comprehensively inferred that the formation of uranium mineralization is not related to deep hydrothermal fluid below the lower crust, but is altered by the low-temperature hydrothermal fluid. Synsedimentary CD at the stage of low to medium thermal maturity is of certain adsorption and reduction, and is favorable for the precipitation and enrichment of uranium-bearing phase and the other trace element (e.g., Mo, V) similar in geophysical-chemical properties with U element, and the sedimentary environment is beneficial for preservation of uranium. It will be contributed to clarify the genesis of sandstone-type uranium deposit and to provide some guidance for the exploration of the uranium deposit in the studied area.