PETROGRAPHIC ANALYSIS OF SAMPLES FROM THE URANIUM DEPOSIT AT OKLO, REPUBLIC OF GABON

This paper summarizes the results of a petrographic (transmitted and r eflected light) and electron microprobe analytical study of samples co llected from the uranium ore deposit at Oklo. This study is preliminar y to detailed x-ray diffraction analysis and additional electron micro probe analyses. Twenty samples were examined; seven from reactor zone 9 (RZ-9), ten from RZ-10, two from RZ-13 and one from RZ-16. This suit e of samples includes examples from reactor cores and their associated hydrothermal alteration haloes. The most striking characteristic of t he Oklo samples is their heterogeneity. In particular, the samples var y with respect to texture, mineralogy, uranium mineral content, and ev idence for deformation. Deformation features provide evidence for both shear and extensional stresses and include brecciation and a variety of mineralized fractures. Opaque phases include organic matter, uranin ite, coffinite, and sulfides. In many cases, the opaque assemblage is concentrated along fractures. Considerable textural evidence, viz., em bayed grain margins and fractures with nonparallel margins, suggests p artial dissolution of uraninite in the Oklo ores. Uraninite with embay ed margins is commonly accompanied by organic matter. Oklo uraninites are, in many cases, altered to produce coffinite (USiO4 . n H2O) and a re associated with varying quantities of galena at grain boundaries, w ithin fractures, and within individual grains at intracrystalline loca tions. Textural evidence suggests multiple periods of uraninite format ion. Electron microprobe data are presented for uraninites from RZ-13. Analytical data are discussed for Oklo reactor zones 2, 9, 10, 13, an d 16 and for the reactor at Bangombe. Uraninites are compositionally s imilar with respect to ph for RZ-2, RZ-9, RZ-13, RZ-16 and for the Ban gombe reactor (average content of PbO is 5.92 weight %). Lead contents for uraninites from RZ-10 are generally higher and more variable than for the other reactor zones (e. g., PbO content for RZ-10 uraninites ranges from approximately 12 to 18 weight %).