Microscopic study of uraniferous ores from Oklo

Three categories of ore can be distinguished at Oklo: (A) Types with carbonaceous matter, observable in sandstone or in conglomeratic rocks with disseminations of pitchblende (colloform UO2) as very small globules in polymerized asphaltic material; (B) Types with uraninite (euhedral UO2) in a clay matrix (chiefly chlorite), the whole being reconstituted or scattered in a microbreccia of a galena almost entirely of radiogenic lead; (C) Intermediate types, containing uraninite and globular pitchblende together, the pitchblende globules being larger than in the case of category A and appearing to have been formed at the expense of the uraninite - the matrix contains phyllites and accessory quartz, sometimes with disseminated carbonaceous materials (asphaltic). Coffinite is often present in the case of categories A and C, but it may have derived from recent processes. Deficiencies in 235U have been observed only in the case of category B. No difference from a mineralogical point of view is observable between ''normal'' massive uraninite ores and those from Oklo, with modified isotopic spectra. In particular, no evidence of significant thermal or mechanical modification has been found