Spontaneous fission reactions occurred in several uranium deposits in SE Gabon approximately two billion years ago. The reactor zones, between 10 to 50 cm thick, are found in Proterozoic sandstones and consist of high-grade uranium ore mantled by illite and/or chlorite (Gauthier-Lafaye et al., 1989). During a mineralogical study of sandstones (quartz arenites) underlying a natural fission reactor at Bangombé (20 km south of the Oklo uranium deposit), we have found several grains of a uranous silicate, coffinite, with unusually high concentrations of phosphorous and enriched in light rare earth elements (LREE).
