MECHANISMS OF LEAD RELEASE FROM URANINITE IN THE NATURAL FISSION REACTORS IN GABON

Twenty-four samples of uranium ore from the natural fission reactors i n Gabon were studied by detailed electron microprobe analysis and back scattered electron imaging in order to determine the behavior of radio genic Pb and fissiongenic nuclides. Lead content in uraninite varies f rom 19 wt% PbO in relicts of pristine uraninite, which were found only in reactor zone 10, to less than 5 wt% in altered uraninites. Differe nt mechanisms of Pb loss from uraninite prevailed in different reactor zones and included leaching, grain boundary diffusion, exsolution via continuous precipitation, and volume diffusion. As a result of these processes, Pb content in uraninites from all the reactor zones, except for reactor zone 10, are similar and vary around a mean value of 5.2 wt% PbO. All of these processes were thermally activated and episodic. The predominance of any single mechanism in a particular reactor zone was controlled by the accessibility of solutions to the uranium ore. The thermal event which caused Pb mobilization in the deposits resulte d from regional igneous activity in the Franceville Basin more than 11 00 Ma after the reactors sustained spontaneous fission reactions. Redu cing conditions prevented the long distance migration of Pb, as well a s of fissiongenic Mo and Ru.