Carbonaceous substances associated with the Paleoproterozoic natural nuclear fission reactors of Oklo, Gabon: paragenesis, thermal maturation and carbon isotopic and trace element compositions

Abstract

Field relationships, textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances (CS) occur in the black shales, uranium ores and fossil nuclear fission reactors of Oklo. The categories include kerogens of the Mikouloungou ‘coal’ in the FA Formation, the C1 pelites of the FA Formation, the black shale of the FB Formation, and bitumens present in veinlets in the basal black shale of the FB Formation, in the FA Formation, in normal ore and in hydraulic fractures, in and near the fossil reactors, and in late dolerite dikes. Bitumen migration occurred at Oklo during at least three intervals: (1) ca. 2.0 Ga ago when petroleum liquids, derivatives of which are preserved as solid bitumen, were generated by thermal maturation of black shale kerogen; (2) ca. 1.968±50 Ma ago when 15 or more pockets of uranium ore achieved criticality; and (3) 977 to 981±27 Ma ago when dolerite dikes intruded. Radionuclide migration occurred during intervals 2 and 3. Bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between ‘heat affected’ and ‘normally matured’ CS at Oklo. In and near Oklo reactors, where several types of solid bitumen commonly occur in close association, carbon of the youngest (low reflectance) bitumen is isotopically heavier than that of older (high reflectance) bitumen due to thermal cracking. This process, together with enhanced radiolysis of bitumen in, and near the reactors, have combined to increase its δ 13C (avg.=−25.8‰) compared to that of bitumen samples in normal ore (avg.=−35.5‰), distant from the reactors. Graphitized bitumen inhibited migration of 235U and some fission products in uraninite of the natural reactors. However, alkaline earth elements such as Ba, Sr and Mo, and alkaline elements Rb and Cs, not compatible in the UO 2 crystalline structure, have migrated in hydrothermal solutions and in the once fluid Oklo bitumens. In situ analysis of solid bitumens by laser ablation ICP-MS detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends.