Abstract: Oklo Phenomenon: Geologic and Nuclear Aspects

Douglas G. Brookins

The Oklo uranium mine, near Franceville, Republic of Gabon, has attracted worldwide interest because part of the deposit properly can be described as a "fossil nuclear reactor." The bulk of the ore at Oklo is relatively low grade (0.2 to 0.5 percent U) and sporadically distributed in carbonaceous sandy and conglomeratic parts of the 1.8-b.y. old Francevillian Series. In places sandstone layers have been fractured during tilting; the fractures, infilled by shale, contain redistributed, very high-grade ore (to 75 percent U). This high-grade ore, formed close to 1.8 b.y. ago, is depleted in 235-U (i.e., 0.3 to 0.6 percent versus 0.7 percent for "normal" U). This discovery was made by the French A.E.C. in 1970 and the world was advised of "the Oklo phenomenon" in 1972. From t e time of discovery, the French A.E.C. investigators suspected that a fission reaction might have occurred, as the conditions for criticality--high U content, absence of initial poisons, favorable water: U ratio, etc.--were probably met as the ore formed. Confirmation of fission reactions at Oklo was documented by discovery of anomalous enrichments in many of the isotopes produced by fission in a conventional light-water reactor, although at Oklo some elements have migrated and others have remained in situ. Even those isotopes which have migrated at Oklo are thought to be redistributed locally and their presence masked by large amounts of normal elements introduced later. This unique deposit thus lends support to those advocating geologic sites for radioactive waste disposal, and is espe ially valuable for providing data for theoretical modeling in conjunction with hydrodynamic and other criteria for site selection.

The geologic setting of Oklo is in many ways similar to deposits of the Colorado Plateau. Fortunately, the sedimentary rocks in the 1.8 b.y. old Francevillian basin have not been disturbed significantly since their formation and thus the uranium deposits also are essentially undisturbed. As several other uranium deposits of a similar nature (except for the "reactor ore") are aligned along the same tectonic element(s) within the basin and echelon elements are present in unexplored basins close by, prospecting for sedimentary, sandstone-type uranium deposits (as opposed to the usual search for Precambrian quartz-pebble conglomerate deposits) in not only the Precambrian basins in Gabon (or nearby) but also worldwide should result in the finding of new uranium deposits.

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana