Uranium-series disequilibrium dating of secondary uranium ore from the south Eastern Desert of Egypt

The secondary uranium ore of Um Ara mining area, south Eastern Desert of Egypt, is composed mainly of uranophane and beta -uranophane, They occur in the oxidized zone as idiomorphic crystals filling cavities and as coatings on the fracture surfaces of the alkali-feldspar and albitized granites. The activity ratios U-234/U-238 and Th-230/U-234 of the secondary uranium ore and host rocks indicate that there were two main phases of uranium mobility in recent geological time. An earlier precipitation of uranium from solutions forming the secondary ore and a subsequent mobilization resulted in adsorption of uranium to the host granitic rocks. The Th-230/U-234 age of secondary uranium ore varies from (50 +/- 10) to (159 + 69/-45) ka whereas, the age of uranium adsorption to the rocks ranges from (18 +/- 6) to (38 +/- 5) ka. The time of secondary uranium ore precipitation can be attributed to the Saharan II pluvial period which prevailed in Egypt during humid oxygen isotope climatic stage 5. The later uranium mobility and subsequent adsorption by the rocks took place during Kubbaniyan and Nabtian pluvial periods that coincided with oxygen isotope stages 3 and 1, respectively. (C) 2001 Elsevier Science Ltd. All rights reserved.