[en] Until recently, very few radiometric ages were available for the Precambrian of Gabon. Recent age determinations have considerably enhanced the knowledge of the geological history of Gabon

[en] The paper recounts the principal stages in uranium prospecting in Gabon and reviews the methods, results and future prospects. (author)

[en] The structure of Africa became established at the end of the Precambrian era (500-600 million years ago). It is the result of a series of relatively brief paroxysmal events, which constitute good chronological markers, and of long periods of relative stability. A complete succession of events - erosion, transport, sedimentation and folding - constitutes an orogeny or cycle; the final, paroxysmal phase is called an ''orogenesis''. As regards Africa, authors distinguish between four major orogeneses: Precambrian A (500-600 to 900-1200 million years ago), Precambrian B (900-1200 to 1800-2000 million years ago), Precambrian C (1800-2000 to 2500 million years ago), Precambrian D (before 2500 million years ago). Africa is conventionally considered to be made up of four consolidated and granitized cratons: the West African (or guineo-eburnean), Congolese, Kalahari and nilotic cratons. With these cratons are associated internal, ''intracratonic'' basins with relatively shallow detritic sedimentation and intercratonic zones with their own deep sedimentation; the latter, located at the periphery of the cratons, on the fold axes, are called ''mobile belts''. Gabon lies in the north-west part of the Congolese craton. The Franceville basin is one of the intracratonic basins of the Congolese craton. The age of its sediments has been estimated at 1740+-20 million years. The Franceville basin can thus be assigned to the Precambrian B orogenesis

No abstract available

[en] All mining and milling activities in Gabon have ceased. Gabon's total production of uranium up to the end of 1996 amounted to 26 109 t. The Mounana production centre operated from 1988 until 1997, and the Mikolongou production centre operated from 1991 until 1999, after which all mine production ceased

[en] After a brief presentation of the Francevillian series, the radiochronological data obtained some years ago are discussed, account being taken of possible connections of the series with the intrusive N'Goutou complex on one hand and the metamorphic Ogooue system on the other. If the syenitic N'Goutou intrusion is contemporary with the deposition of the sediments, the age given by the Rb-Sr isochrone of the Franceville basin (1740 or 1800 million years depending on the constants used), which cannot be distinguished from that of the syenites, agrees - within the margin of the analytical errors - with the age of the sedimentation. If, on the other hand, the syenitic intrusion is later, a greater age - possibly of the order of 2000 million years - can be envisaged for the deposition of the sediments. This would be confirmed if it were proved that the Franceville basin merges without any major discordance into the metamorphic Ogooue system, acquiring increasing metamorphism towards the north-west, and if it were demonstrated that a substantial magmatic event subsequently modified the apparent age of the series. The new measurements presented in the paper were performed at Strasbourg by the K-Ar method. The following samples were analysed: veined dolerites which cut through the Franceville basin and the base; a syenite from an intrusive complex in the Chaillu Massif; pelites from the Oklo deposit; a pelite sample obtained by drilling from the extreme base of the series. The dolerites have rather widely differing apparent ages which probably reflect a deposition about 850 million years ago and, in some cases, a more or less complete loss of radiogenic argon due to rearrangement of the fractures in the course of Mayombian folding about 500 million years ago. The Chaillu syenite has an apparent age of 2200 million years, which may be regarded as the latest date for its actual deposition. Thus, these different intrusions are not related in age with the syenitic N'Goutou complex. Whatever their stratigraphic level, the pelitic samples from the Franceville basin have apparent ages consistent with those found previously (1700-1800 million years). On the other hand, two samples from the wall of one of the reaction zones have lost (almost completely in one case) radiogenic argon. This is due to recent effects on the reaction zones of meteoric water circulation in the cracks

[en] After having described the main steps in the treatment of uranium ores, the author describes the treament activities for these ores, as they are organized in France and in the African countries having made cooperation agreements with France in this field

[en] In the Haut-Ogooue region of eastern Gabon the Francevillian (middle Precambrian) formations constitute a detritic series with two superposed cycles in which uranium is most often to be found in the upper part of the first-cycle sandstones. At present there are five known uranium deposits there. The control of the mineralization is physico-chemical, stratigraphic (most frequently), sedimentological or tectonic; many of these factors can be involved simultaneously. The uranium comes from the leaching of the surrounding basement masses and the vitroclastic sediments from the top of the series. Tectonics played a decisive role at the time that the formations were deposited and the deposits constituted. (author)

[en] The Oklo uranium deposit, situated 1500 m to the south of the Mounana deposit, is close to a basal rise which influenced sedimentation and conditioned the tectonics. The deposit is a sedimentary one in a sandstone layer 4-10 m thick and forms the extreme upper layer of the sandstone basal formation of the Francevillian series. It is overlaid by predominantly pelitic facies. The layer has an eastward monoclinal dip complicated locally by north-south tectonic features. The maximum size of the mineralized zone is 900 m by 600 m. The variations in the degree of mineralization are due to sedimentological phenomena on which are superposed later tectonic features (local enrichment factors)

[en] The long history of the Compagnie des mines d'Uranium de Franceville (Comuf) is linked to the history of the young Gabonese Republic. The psychological and social role of Comuf in the development of Gabon exceeds its strictly economic impact, which in itself is far from negligible. Over the last thirty years Comuf has produced and delivered almost 20 000 tonnes U. Its proven reserves ensure that its activities can continue at the present level for at least 20 years and may permit it to remain open for over half a century. This means it can continue to work, as it always has done, for the long term: an essential condition for it to make a worthwhile contribution to the development of the country. This paper concentrates on the impact of Comuf's activities on its environment, its contribution to the local communities and its role in the country's economy. (author)