Uranium/Thorium isotope evidence for ground-water history in the Eastern Desert of Egypt

The ground-water in the surficial aquifer of the central Eastern Desert of Egypt has a relatively long residence time and is essentially in chemical equilibrium with its host rocks. The content of major elements and uranium suggest that granites have been the principal sources of dissolved ions even where sedimentary rocks are present. A leaching study of uranium and thorium isotopes in seven granitic host rocks reveals a history of change in the aquifer. Lightly adsorbed U has a high234U/238U activity ratio similar to the contemporary associated ground-water. However, much of the uranium in the granites is present as acid-leachable adsorbates with a low activity ratio signature, indicative of an earlier period of aggressive leaching and uranium mobility. Using whole rock disequilibrium dating methods we estimate that the time of U mobility occurred 80,000 to 100,000 years ago during the Saharan II pluvial period.