The lower Proterozoic uranium ore deposits in the Franceville basin (Gabon) are the oldest high-grade uranium accumulations known. They are unique in that they contain evidence for natural nuclear fission reactors.Sedimentologic, tectonic, petrographic, and geochemical studies have been performed in order to reconstruct the geologic conditions in which uranium mineralization took place. Uranium deposits are located in deltaic sediments overlying fluviatile deposits of coarse sandstone and conglomerates which are the source rocks for uranium. Deltaic sediments are overlain by marine black shales (the FB formation). Petrographic observations, electron microscope studies, and geochemical and carbon isotope data indicate that these FB black shales are source rocks for petroleum trapped in the uranium deposits.Tectonic studies show that all the uranium deposits are in tectonic structures that served as traps for both petroleum and uranium. Uranium mineralization occurs in this setting when an oxidized uranium-bearing fluid has mixed with a reduced petroleum-bearing fluid. The uranium ores are affected by hydrofracturing which forms a good pathway for the oxidized uranium-bearing fluids and the reduced fluids. Hydrofracturing may be initiated by over-pressured fluids coming from undercompacted zones in the FB black shales.