Composition and origin of Ti-Nb-Ta-Zr bearing minerals in the Abu Diab highly evolved granite from the Central Eastern Desert of Egypt
The central Eastern Desert (CED) of Egypt is well-known for its granite-related Nb-Ta mineralization. The garnet-bearing muscovite granite (GMG) of the Abu-Diab composite pluton in the CED consists mainly of quartz, K-feldspar (Or88-98), albite (An0-4) and muscovite, with accessory minerals including garnet, zircon, columbite, ilmenite, Ti-rich hematite, rutile, ilmenorutile, thorite, apatite, xenotime and chlorite. The GMG is weakly peraluminous and has low Nb/Ta (9.6-15.4) and Zr/Hf (16-31) with discernible REEs tetrad effect (TE1-3 = 1.11-1.35), typical of highly evolved granites. Zircon contains high concentrations of U and Th typical of late-magmatic zircon and similar to zircon type from highly evolved granite. The homogenous and weak zoned columbites are classified as manganocolumbite. The formation of Ta-rich rim in the columbite may indicate that later fluids were from the GMG granite itself at advanced fractionation into exsolving fluids, and not from an external source. Ilmenite is greatly enriched in MnO, which indicates the significant pyrophanite (up to 29 mol %) molecules in ilmenite through simple substitution of Mn for Fe2+ with increasing oxygen fugacity under magmatic-hydrothermal conditions. Xenotimes show low analytical totals, suggesting probably hydration during their post-magmatic alteration, while apatite is small-grained associated with tiny zircons, suggesting late-crystalized phases. During late magmatic differentiation stage, the interaction of granitic melt with F-rich late magmatic fluids could be resulted in the formation of Ti-Nb-Ta-Zr minerals. Overall, the Abu Diab GMG possesses mineralogical and geochemical features that make it a potential target for Ti-Nb-Ta-Zr minerals.