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

Field relationships, textural and reflectance characteristics and carbon is otopic compositions indicate that at least eight different categories of ca rbonaceous substances (CS) occur in the black shales, uranium ores and foss il nuclear fission reactors of Oklo. The categories include kerogens of the Mikouloungou 'coal' in the FA Formation, the Cl pelites of the FA Formatio n, 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 lat e 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 bla ck shale kerogen; (2) ca. 1.968 +/- 50 Ma ago when 15 or more pockets of ur anium ore achieved criticality; and (3) 977 to 981 +/- 27 Ma ago when doler ite dikes intruded. Radionuclide migration occurred during intervals 2 and 3. Bitumenization trends established for organic matter which has undergone thermal maturation fair to discriminate between 'heat affected' and 'norma lly 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 r eflectance) bitumen due to thermal cracking. This process, together with en hanced radiolysis of bitumen in, and near the reactors, have combined to in crease its delta C-13 (avg. = -25.8(0)/(00)) compared to that of bitumen sa mples in normal ore (avg. = - 35.5(0)/(00)), distant from the reactors. Gra phitized bitumen inhibited migration of U-235 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 UO2 crystalline structure, have migrated in hydrothermal solutions and in the o nce fluid Oklo bitumens. In situ analysis of solid bitumens by laser ablati on ICP-MS detects differences in trace element composition developed as a r esult of their varying geological histories and evolutionary trends. (C) 20 01 Elsevier Science B.V. All rights reserved.