Secondary uranium in phosphorites and its relation to groundwater in Egypt

Abstract

The stratiform phosphorite deposits of Egypt, which are found in an east-west band between 24° and 27° north latitude, contain from 25 to 100 ppm uranium, a significant portion of which can be shown to have been secondarily adsorbed from groundwater. This fraction is readily leachable in 0.2 N K₂CO₃ and in the Red Sea area has the same ²³⁴U/²³⁸U activity ratio, 1.3, as the U dissolved in the associated groundwater. Both the excess of ²³⁴U and the deficiency of ²³⁰Th relative to ²³⁸U shows that the leachable U was adsorbed within the last 250 ky. Secondary U with the same ²³⁴U/²³⁸U activity ratio also occurs in the Western Desert phosphorite, indicating that it too has been adsorbed relatively recently, even though the groundwater table today is well below the phosphatic strata. On the other hand, the leachable U in the intervening Nile Valley deposits has an activity ratio near 1.0, suggesting that those strata have been above the groundwater table for the last million years. The adsorption of U may not be entirely associated with the principal phosphate mineral, carbonate-fluorapatite (francolite), Ca₅(PO₄, CO₃)(F, OH). Iron, organic and clay phases are also present in the phosphorite ore.