CLAYS AT THE NATURAL NUCLEAR-REACTOR AT BANGOMBE, GABON - MIGRATION OF ACTINIDES

Clay minerals were examined from the natural nuclear reactor at Bangom be. The clay mineralogy of the reactor facies is: illite, kaolinite an d minor chlorite (FB formation); illite with chlorite and kaolinite in hydrothermal veins (argile de pile); illite and chlorite (reactor cor e); and i) chlorite and kaolinite +/- illite +/- smectite or ii) illit e in the underlying FA formation. Illite crystallinity values are in g eneral highest in rocks that experienced the highest temperatures duri ng reactor operation. Chlorite chemistry varies with respect to positi on around the reactor: both di-tri and trioctahedral chlorites are pre sent in hydrothermal veins within the argile de pile, dioctahedral chl orites are present in the FA formation within cm of the reactor core, trioctahedral chlorites are present within a meter of the reactor. Che mical data for the argile de pile and the FB formation suggest that fo rmation of the former unit occurred as a result of thermal metamorphis m of the FB pelites, as opposed to large-scale dissolution of the sand stones of the FA formation. Autoradiography is used to study the sorpt ive characteristics of the clays for actinides. The results show that actinide sorption varies in the order: chlorite > illite congruent to kaolinite.