Geochemical characterizations and rare metal exploration of post-orogenic and anorogenic A-type granites from the Gabal El Dob area, Eastern Desert, Egypt.

Abstract

HE orogenic older granitoids of Gabal El Dob area were intruded by post-tectonic younger granitoids, which predominated by alkali feldspar granite and alkali riebeckite granite. These granites occur as two separated and elongated bodies emplaced along a shear zone trending NNE. They are discriminated according to their petrographical studies into subsolvus and hypersolvus granites. Both granitic types consist essentially of perthites and quartz with little amount of plagioclase. The accessory assemblages along the two granitic phases are differing in their quantity and constituents. The hypersolvus granite contains arfvedsonite and riebeckite, which dominantly enclose variable amount of zircon and allanite. Overall, the studied granites are silica and alkali-rich with low contents of MgO, CaO and TiO 2 . Comparative geochemical studies indicated that the hypersolvus granite is enriched in total alkalis, Ga, Zr, Nb, Y and ΣREE relative to subsolvus granite. The two granitic phases display REE patterns, characterized by moderately fractionated LREE and nearly flat HREE with a moderate negative Eu anomaly. The subsolvus granite has transitional characteristics between A-type and fractionated I-type granites, but the hypersolvus granite has the most characteristics of A-type granites. The high concentrations of ΣREE in the hypersolus granite (av. 530ppm) are interpreted to the high abundance of REE-bearing allanite. The high contents of ΣREE and Zr in the hypersolvus granite along with the elevated background values of U and Th in both granitic phases could be used as an indicator for rare-metal exploration in Gabal El Dob area.