Organic molecules based palaeoenvironmental inferences and U mineralization in Palaeoproterozoic Bijawar basins of Central India

Abstract

Palaeoproterozoic, siliclastic, intracratonic rocks of the Bijawar and Sonrai basins contain anomalous U concentration and abundant Organic Matter (OM), although, earlier studies documented rare data on commonality between high U content and closely associated OM. Moreover, OM contains several biomolecules of complex carbonaceous compounds with macro-molecular structures, but insignificant information is available on the origin and associated palaeoenvironmental conditions. Thus, present study on organo-uranyl-complexes is relevant and carried out to understand palaeoenvironmental reasons accountable for U mineralization. For this purpose, OM (isolated from each bulk sample) was analyzed for Total Organic Carbon (TOC), n-alkanes, n-fatty acids and Polycyclic Aromatic Hydrocarbon compounds (PAHs). Obtained TOC values range between 0.02 and 0.23 wt%. Mostly, calculated [(C/S)/12] and [(C/N)/12] values are low; indicate algal source and anoxic depositional conditions, but, Rohini carbonate, Bandai sandstone and Chloritic shale high values signify sub-oxic conditions. Also, low (<1.25) U/Th and high authigenic U data of mineralized Chloritic shale imply anoxic conditions and post-depositional enrichment of U ions (mostly as organo-uranyl-complexes), respectively.GC-MS data show high concentration of n-alkane, fatty acid methyl ester (FAME) and PAH molecules in the OM associated with the rocks signifying high U values. The predominance of Short Chain (SC) n-alkanes together with the SC n-fatty acids revealed derivation of OM from marine algae and bacteria. The Rohini carbonate and Chloritic shale show low isoprenoids, Pristane (Pr)/Phytane (Ph), CPI, OEP, Pr/n-C-17, Ph/n-C18, but high TC-/(Pr/Ph), TC/(Pr + Ph), MP, MPI, MPR and VR values, suggest biodegraded and thermally matured OM which has undergone hydrothermal alteration under anoxic to sub-anoxic conditions. Considerably, high degree of OM oxidization occurred synchronously with the reduction of U6+ to U4+ by sulfate-reducing bacteria. The consistent loss of diaromatic PAHs is also noticed from older to younger rock-units. Further, scanty appearance of PAHs in the lower Gorakalan shale and their disappearance in the upper Chloritic and black shale units, indicate progressive maturation of sediments. The Low Molecular Weight (LMW)-PAHs predominate over the High Molecular Weight (HMW)-PAHs. Comparatively, high phenanthrene abundance is also noticed in the Rohini carbonate, Chloritic shale and -black shale (with high U content). The high proportion of LMW-PAHs and absence of HMW-PAHs (>6 rings) is pointing towards the presence of hydrothermally derived bitumen. Moreover, partial combustion of OM (pyrogenic) contributed to the formation of PAHs. The Chloritic shale and Rohini carbonate are coeval with the Bijawars of the Sonrai basin, representing almost similar organo-molecular records. Organo-molecular abundance and their structural attributes when plotted across the stratigraphic successions, revealed alternate humid conditions facilitated U4+oxidation and formation of U6+ rich solutions, although arid/semiarid climatic conditions assisted precipitation, sorption and absorption of U4+. Almost, similar clay mineral attributes for both the basins lend support to cyclic humid to arid/semiarid palaeoenvironmental conditions. Moreover, majority of the OM derived from the algal mats has played critical role in U mineralization by trapping and adsorption of tiny uraninite grains as well as U ions onto the OM.