[en] The geology of the Franceville basin (Gabon) is examined: stratigraphy, tectonics and geodynamics. The mobile zone of the Ogooue is specially studied: lithology, metamorphism and tectonics, isotopic geochronologic data are given. The different uranium deposits are described. A whole chapter is devoted to the study of Oklo natural nuclear reactor. A metallogenic model is proposed evidencing conditions required for deposit genesis. Tectonics, microstructures sedimentology, organic matter, diagenesis and uraniferous mineralizations are examined

[en] Nuclear fission reactions occurred 2000 Ma ago in the uraninites of the Oklo-Okelobondo and Bangombe uranium deposits (Gabon). These uraninites contain actinides and fission products and therefore are considered as natural analogues for artificial nuclear spent fuel of PWR type nuclear plant. Mineralogical, chemical and oxygen isotopic analyses have been performed in order to determine the stability of the uraninites in weathering conditions. Results show that the alteration of uraninites by meteoric waters affects the cell parameters and the chemistry of the uraninites but not the oxygen isotopic composition. Radiolysis of water is invoked as a possible mechanism for the very low values of δ¹⁸O (-22.0 0/00 SMOW) measured in the uraninites. (authors)

[en] The comparative study of different uranium deposits from the eastern border of the inlier of Mounana reveals that the four deposits at present known, i.e. those of Boyindzi, Mounana, Oklo and Okelobondo, are situated on the same flexural zone affected by sliding. The two first, at the contact of the basement, are subject to break deformations, while the two others are implicated in extensive flexible structures resulting from the same sliding tectonics. (author)

[en] Since the Libreville symposium in 1975 knowledge of the geological environment of the reactors has advanced as a result of a more extensive study of the Francevillian uranium deposits. In the Oklo deposit a detailed stratigraphy of the Cl bed (uraniferous mineralized bed) has been established, making it possible to re-establish stratigraphically the position of the natural reactors. A tectonic analysis of the Oklo deposit has revealed the special features of the Oklo structure and the reaction zones situated in the shear troughs. Petrographic studies have revealed the presence of two types of ore with distinct modes of formation. In the first case, the role played by organic materials seems predominant, while in the second case migrations of oxidizing solutions are the main source of the reconcentrations. Finally, a geochemical study made of samples from Oklo and Okelobondo points to the existence of an ''isolated'' geochemical phase containing uranium and a certain number of trace elements. This phase is associated with the organic material. This study also deals with the migration of lead at Oklo and Mounana. (author)

[en] Two billion years ago, the increase of oxygen in atmosphere and the high ²³⁵U/²³⁸U uranium ratio (> 3%) made possible the occurrence of natural nuclear reactors on Earth. These reactors are considered to be a good natural analogue for nuclear waste disposal. Their preservation during such a long period of time is mainly due to the geological stability of the site, the occurrence of clays surrounding the reactors and acting as an impermeable shield, and the occurrence of organic matter that maintained the environment in reducing conditions, favourable for the stability of uraninite. Hydrogeochemical studies and modelling have shown the complexity of the geochemical system at Oklo and Bangombe (Gabon) and the lack of precise data about uranium and fission products retention and migration mechanisms in geological environments. (author)

[en] New data about fluid inclusions associated to a siliceous diagenesis show that a deformation phase in the first stage of catagenesis disturbed their microthermometric behaviour. Nevertheless, temperature and pressure of fluids associated to the uraniferous paragenesis and contemporary with the Oklo natural reactors are estimated at 140-160⁰C and 250-500 bar

[en] The uranium deposits in the basin of Franceville (Gabon) host the only natural fission reactors known in the world. Unique geologic conditions favored a natural fission reaction 2 Ga ago. This was detected by anomalous isotopic compositions of rare earth elements (fission products) and uranium. In total 16 reactor zones were found. Most of the them are mined out. A current research project of the European Commission concentrates on the reactor zone of Bagombe, which is only 10-11 m below the surface. This reactor zone has been influenced by surface weathering processes. Six drill cores have been sampled at the site of the reactor zone of Bagombe during the course of the project. Only one drill core (BAX 08) hit the core of the reactor which is approximately 10 cm thick, 2-3 m wide and 4-6 m long. The migration of fission products can be traced by the anomalous isotope ratios of REE due to the fisson process. The normal and constant ratio of ¹⁴⁹Sm/¹⁴⁷Sm is 0.92. The isotope ratio of ¹⁴⁹Sm/¹⁴⁷Sm close to the reactor zone is as low as 0.28 due to the intense neutron capture of ¹⁴⁹Sm and subsequent decay. Similar changes in isotopic patterns are detectable on other rare earth elements (REE). The isotope ratios of Sm and other REE of whole rock and fracture samples surrounding the reactor indicate that fission products migrated only a few centimeters above and mainly below the reactor zone. Organic matter (bitumen, kerogen) seems to act as a trap for fission products. REE-patterns show a less intense weathering with depth in the log profile. (orig.)

[en] A group of scientists argue the case for not mining the last known nuclear fission reactor located in Oklo, Gabon. Studies of its nuclear fission reactions, which occurred 2 billion years ago, are essential to our understanding of how fission products and actinides could migrate in nuclear waste repositories. Such a resource is almost irreplacable and its commercially viable uranium contents low, adding strength to the argument for conservation. (UK)

[en] Uranium deposits of Alligator River in Australia and of North Saskatchewan are compared. Common points are stratigraphic position, regional metamorphism, associated tectonic structure and importance of hydrothermalism

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