Using of uranium and thorium isotopes in age framework determination for different stages of alterations and mineralizations in Missikat altered granites, Central Eastern Desert, Egypt

Abstract

El Missikat area represents one of the most important uranium occurrences in the Eastern Desert of Egypt. Ten samples from altered granite and jasper vein are collected and uranium-thorium isotopes analyzed radiochemically by alpha spectrometry. Different thorium/uranium isotopic activity ratios were calculated in the collected samples which were below the chondritic ratio due to enrichment in uranium or removal of thorium, suggesting that other processes rather than charge and radius play the important role in mobilization of these elements. All the specific activities of 238U and 232Th in the studied rocks have values higher than the average permissible limits. 234U/238U activity ratio >1, 230Th/234U and 230Th/238U < 1 in the studied rocks indicated the prevailing of reduction conditions during uranium deposition and these rocks are considered as a site for uranium deposition. 238U/235U and 234U/235U activity ratios were slightly positively and negatively deviated from the natural, indicating the effect of alteration processes. Estimation the age of uranium deposition clarified that the age of uranium deposition of El Missikat silicified granites vary between 12.59 ± 0.46 and 40.07 ± 0.66 ka with 27.99 ± 0.53 ka as an average, while the age of uranium deposition in the desilicified granite of El Missikat ranged between 22.76 ± 0.53 and 48.2 ± 0.50 ka with an average 34.06 ± 0.51 ka. Age of uranium deposition in jasper vein is 28.23 ± 0.52 ka. The recorded age of uranium deposition in El Missikat granites and silica vein indicate the desilicification is the oldest alteration process and the silicification is the youngest.