PETROGRAPHY AND PARAGENESIS OF ORGANIC-MATTER ASSOCIATED WITH THE NATURAL FISSION REACTORS AT OKLO, REPUBLIC OF GABON - A PRELIMINARY-REPORT

Sixteen known uranium-rich pockets in the sediment-hosted uranium ore deposits in the Oklo area became nuclear fission reactors 1968+/-50 Ma ago and operated as such for up to 1 Ma. Nuclear criticality was caus ed by unique and fortuitous geological environments and events. These included the localized high concentrations of uranium in small pockets in the ore bodies, the fact that the relative abundance of the fissil e U-235 isotope was five times greater in uranium similar to 2 Ga ago than present, the presence of water which acted as a moderator in the natural reactors and the absence of neutron poisons, which are element s that can prevent nuclear chain reactions. Organic matter is present in ah of the Oklo natural reactors, but it is abundant only in reactor s 7 to 16. Organic matter has been studied in natural reactors 7 to 9 and at locations at various distances from these reactors. Bitumen in the Oklo reactors is the predominant organic phase. It is now solid bi tumen containing dispersed cryptocrystalline graphite in crystal domai ns smaller than optical microscopic size. Its immediate precursor was liquid bitumen, generated from syngenetic kerogen in organic-rich sedi mentary rocks in the Francevillian Basin of Gabon and in the natural r eactors themselves by hydrous reaction mechanisms prior to, during, an d after nuclear criticality. A preliminary paragenesis of organic matt er at Oklo may be defined through the following sequence of events. (a ) Protokerogen and kerogen evolve from abundant cyanobacteria in the t idal and deltaic sediments in the basal FA formation of the Francevill ian Series, during sedimentation and burial, respectively. (b) During subsequent subsidence of the basin the first generation of bitumen and petroleum occurred. (c) During uplift the sedimentary strata were fra ctured, and bitumen and petroleum migrated through fractures. Followin g this, oxidizing aqueous solutions carrying uranyl ions migrated thro ugh the highly fractured rocks, met and were reduced by organic matter and precipitated pitchblende or uraninite at the present sites of the Oklo uranium ore deposits and natural reactors. The natural reactors reached nuclear criticality, they were heated and produced the second generation of bitumen through hydrous reactions from the by now solid first-generation bitumen and remaining kerogen. Finally, hydrothermal effects associated with the dolerite dike swarm intrusion 750+/-150 Ma ago produced yet another localized generation of liquid bitumen from solid organic matter. Possibly other bitumen generations also occurred during and following this time interval. The organic matter in the na tural reactors soon became a solid, and was able to prevent the loss o f uraninite grains enclosed in it. These uraninite grains trapped and then immobilized fission products until, and to an extent even after, a major tectonic-igneous event, the intrusion of the dolerite dike swa rm.