Redistribution of fissiogenic and non-fissiogenic REE, Th and U in and around natural fission reactors at Oklo and Bangombé, Gabon

Abstract Elemental abundances and isotopic compositions of rare earth elements (REE), Th and U of twenty eight samples from four kinds of natural fission reactors at the Oklo and Bangombe uranium ores were determined by mass spectrometry. Fissiogenic and non-fissiogenic REE components of each sample were calculated from the isotopic compositions. The geochemical distribution of both fissiogenic and non-fissiogenic REE differs among the four reactors. Each reactor shows the following features possibly associated with the local geochemical conditions. (1) Reactor 9 at the shallow depth of the Oklo deposit contains higher amounts of non-fissiogenic REE than the other reactors. The distribution of non-fissiogenic REE does not reflect the primordial REE abundance patterns, and might have been disturbed in and around the reactor due to supergene alteration. Our results suggest that significant amounts of fissiogenic lighter REE (LREE), such as La, Ce and Pr, were mobile in the reactor; (2) Reactors 10 and 13 located deep underground have much lower contents of non-fissiogenic REE than reactor 9. All fissiogenic REE show high retention in the reactors. Non-fissiogenic REE of reactor 10 inform of the primordial ore formation without any geochemical disturbance. Little redistribution of both fissiogenic and non-fissiogenic REE in the reactor 10 was observed; and (3) The Bangombe reactor located at shallow depth has apparently well preserved fissiogenic REE. Non-fissiogenic REE in the Bangombe reactor might have been disturbed, possibly by hydrothermal processes. The distribution behavior of fissiogenic REE is considered to be related with physico-chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor, but clear relationship between nuclear parameters and the REE distribution could not be obtained in this study.