Radiogenic Isotope Fractionation as an Indication for Uranium Mobility in the Granites of El Shallal Area, West Central Sinai, Egypt

Abstract

The granitic rocks at Wadi El Shallal area are categorized into metaluminous hornblende-biotite granites and peraluminous two- mica granites. The chemical analyses indicate that the two-mica granites are slightly altered and characterized by high uranium and thorium contents (averages 20.27 and 27.12 ppm, respectively). Compared to the hornblende-biotite granites, the 230 Th/ 234 U and 234 U/ 238 U activity ratios of the two-mica granites display values that depart significantly from equilibrium. The sample plots of the two granitic types are distributed in a trend reflecting more 234 U loss from the two-mica granites. Leaching experiments confirmed that the area around mineral grain boundaries are more enriched in 234 U relative to its parent 238 U as a result of α-recoil process that displaces the daughter nuclei outside the grain surface. This daughter ( 234 U) is ejected from the grain into the open spaces and therefore, subjected to leaching by fluids. The observed uranium series disequilibrium in the granitic rocks from El Shallal area reflects isotopic fractionation and a recent migration of uranium in the two-mica granites. The high uranium contents and U-Th isotopic composition of the two-mica granites fairly suggest that these rocks are probably the source for uranyl mineralization of the nearby Um Bogma Formation reflecting a history of leaching, mobilization and precipitation by meteoric and groundwater. However, more intensive investigations for uranium *Dept. Geology, Fac. Science, Ain Shams Univ., Cairo, Egypt.