NANO- AND MICRO-SCALE ALTERATION OF URANINITE AT THE OKLO-OKÉLOBONDO NATURAL FISSION REACTORS, GABON

High spatial resolution (10-30 mm) in situ U-Pb and oxygen isotopic analyses by secondary ion mass spectrometry (SIMS) coupled with high-resolution transmission electron microscopy (HRTEM) were used to characterize two alteration events associated with the Oklo-Okélobondo natural fission reactors. Reactor zones that occur in near surface environments have low d¹⁸O values and nanotextures that are consistent with interaction with ground water. These low d¹⁸O values (-14.4‰ to -8.5‰) suggest that the minerals exchanged with groundwater that was of meteoric origin. U-Pb ages of uraninite from these reactor zones suggest that fluid-mineral interaction occurred with present-day meteoric fluids. Some reactor zones are extensively altered to Si-rich uraninite and coffinite. Uraninites from these reactor zones give U-Pb ages that are consistent with emplacement of a local dike swarm at 860 Ma. In contrast, reactor zones that occur at depth have largely retained both their U-Pb ages and original oxygen isotopic composition (-10.2‰ to -5.6‰), and uraninites are well crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ isotopic analyses by SIMS, it is possible to characterize and determine the timing of post depositional alteration of micro-scale uranium-bearing phases.