This mine near the Mouana Mine is the site of over a dozen precambrian natural nuclear reactors. Secondary enrichment of uranium oxides in cm to meter sized veins moderated by ground water consumed six tons of U235 and elevated temperatures to a few hundred Celsius. Remarkably the non-volatile fission products moved only a few cm in the veins during the last 1.5 billion years!
Refs.:
Dymkov, Y., Holliger, P., Pagel, M., Gorshkov, A., and Artyukhina A. (1997): Characterization of a La-Ce-Sr-Ca aluminous hydroxy phosphate in nuclear zone 13 in the Oklo uranium deposit (Gabon). Mineralium Deposita, 32, 617-620.
http://www.curtin.edu.au/curtin/centre/waisrc/OKLO/Where/Where.html
