Abstract
Numerous unzoned and zoned pegmatite pockets are the most important types from the radioactive point of view. They occur at the marginal parts and higher topographic level of G. Abu Furad younger granites. Zoned pegmatites are composed of extremely coarse-grained milky quartz core, intermediate mica zone and wall zone of feldspars. An alteration zone is found in contact between quartz core and intermediate zone. This zone records the highest radioactive values due to its mineral composition, as a result of alteration processes associated with radioactive minerals. The late magmatic alteration process (hematitization, kaolin-itization and fluoritization) of the pegmatite resulted in the formation of martite, fluorite, clay minerals and carbonates (calcite) in the alteration zone. The circulating fluids played an important role in remobilization and concentrating uranium. Fluid inclusion studies by microthermometry were carried out on authigenic minerals (such as quartz and fluorite) in the alteration zones. This study revealed that, at least two stages of the late-magmatic hydrothermal alteration were involved. The first stage is of high temperature low saline fluids characteristic of hematization that resulted from fluid-rock interaction with late magmatic fluids that very probably mixed with external low salinity fluids along brittle structures. The second stage is of low temperature high saline fluids that characterize the fluoritized alteration due to consequent reaction with wallrocks and mixing with meteoric water. Mixing of low salinity meteoric water with hot ascending saline hydrothermal solution leads to pH change and continuous interaction with wall rock. The change in pH plays the main role in remobil-ization and precipitation of many rare metals such as U, Th, Nb, Zr, Y and REEs in mineral assemblages such as euxenite, zircon, carnotite, thorianite, allanite, bastnesite and fluorite. Heating of slightly saline meteoric water by convection acting on the pegmatites along shallow shear zone causes the formation of fluorite and uranium mineralization