Origin of Dispersed Organic Matter within Sandstones and Its Implication for Uranium Mineralization: A Case Study from Dongsheng Uranium Ore Filed in China

Carbonaceous debris (CD), common dispersed organic matter (i.e., DOM), is widely disseminated in sandstones from uranium-bearing strata from the Dongsheng uranium ore field of the northern Ordos Basin. Compositions of maceral, element and biomarkers of CD were investigated through a series of methods with optical microscope, elemental analyzer and gas chromatography-mass spectrometry analyses (GC-MS) to study origin of CD. The results show that CD, centrally distributed nearby channel erosion surface, decreases with the increased distances to channel erosion surface, which indicates the CD might be related to the coal seam from the upper unit of the J 2 y Formation or synsedimentary plant from the J 2 z Formation. Macerals of CD are composed of vitrinite (i.e., V), inertinite (i.e., I), and minerals, including that V is primary. Compared with the coals from the J 2 y Formation classed into vitrinertite-V (V+I>95%, V>I), CD is grouped into vitrite (V>95%). Although, CD and coal are similar in element composition, the former is of lower organic carbon, H, N, and higher S. The (C 27 +C 29 )/(C 31 +C 33 ) ratios of n -alkanes biomarkers indicate that the percentage of woody plants accounting for vegetation composition of CD predominate over that of coal, which is also evidenced by the higher C/N ratios and oleanane contents of CD. The evidence is also supported by plant branch buried in sandstones. The distribution characteristics of CD and differences in vegetation types between CD and coal suggest that CD might be not from the coal seam from J 2 y . The tissue preservation index, gelification index, ground water level index, and vegetation index reflect that the paleoenvironment of CD is controlled by fluctuating water, which is also supported by the existences of round CD. Compared with peat, sedimentary paleoenvironment where CD deposits is of weaker reducibility, higher salinity by analyzing Pr/Ph ratios and gammacerane index. Distributions of n -alkanes carbon number of CD with the presence of unknown complex mixtures show that microbial activities exist in sand bodies. Differences in hydrodynamic intensity, redox condition, and microbial activity intensity between sedimentary paleoenvironment of CD and peat, show that CD is born in synsedimentary sandstone environment not in peat. Hence, it comprehensively draws conclusions that immature ‘non-peatborn’ CD is formed from the trunk, stem, branch, root fragments buried in sandstones, depositing in (micro) allochthonous positions by the influences of fluctuating water. The DOM from synsedimentary plant debris might play more roles in adsorption and complexation, and microorganisms may participate in uranium mineralization, which could provide certain guidance for uranium exploration and mining.