Geochemistry of late fracture filling around the Okélobondo U deposit, Gabon: Evidence for complex circulation patterns.

Louis Raimbault (1), Paul-Louis Blanc (2) Uranium geochemistry, Nancy France - 13-16/04/2003

The uranium ores and nuclear reaction zones at Oklo, Okélobondo, and Bangombé have suffered three major fluid-circulation events leading to redistribution of elements: criticality, some 2 Ga ago; dolerite intrusion, about 800 Ma ago; and, at least in Bangombé, contemporaneous supergene alteration. However, although providing valuable information on element mobility in extreme conditions, such events are unlikely to occur during the lifetime of a nuclear-waste repository which should be designed to avoid such conditions. Therefore, they are relevant to catastrophic scenarii of repository evolution, but are irrelevant to normal operating conditions. We need however, for performance assessment studies, estimates of the effect of tiny circulations which occurred during "quiescent" periods, a possible analog to our projected repositories. Evidence for groundwater circulation without connection to the major hydrothermal events (at Okélobondo, supergene alteration is lacking) is provided by open-fracture filling material, which we use as a tracer of trace-element contents in circulating solutions, as a way to characterize those solutions and their interactions with host rocks. (1) CIG, Ecole des Mines, Fontainebleau, France (2) IRSN