Remote sensing and geochemical constraints on younger granites from Eastern Desert of Egypt: New insights into the genesis of rare metals

Abstract The present study deals with integration of remote sensing and geochemical data for exploration and evaluation of some rare metals-bearing granitic plutons in the Eastern Desert of Egypt. These Neoproterozoic granites are a part of Nubian Shield and resemble in textural and chemical characters to A-type granites. The granitoid rocks range in compositions from tonalite to alkali feldspar granites based on CIPW norm and point counting of thin sections. They have studied by Scanning Electron Microscopy (SEM)-Energy Dispersive X-Ray Spectroscopy (EDS) for detecting rare metals, elemental mapping and identification of other oxides and silicate minerals. The main minerals of younger granites are quartz, albite, orthoclase, and microcline with secondary minerals such as biotite, muscovite, amphiboles, chamosite, serecite, kaolinite and calcite. Their accessory minerals are zircon, garnet, rutile, pseudorutile, ilmenite, magnetite, titanomagnetite, goethite, allanite, titanite, apatite, xenotime, monazite and ferrocolumbite. Electron Probe Micro-analyzer (EPMA) and Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS), respectively, were used to determine major oxides and in-situ trace elements of these minerals. The investigated younger granites are alkaline to calc- alkaline post-collisional with higher SiO2, Al2O3, Na2O, K2O, Y, Rb, REE, Zr/Hf, Nb/Ta, Th/U, suggesting peraluminous A-type granites. However, the studied granites are relatively high in Ba due to occurrence of some plagioclase grains and slightly low Fe2O3t, CaO, TiO2, MnO, MgO Sr, Ti and P relative to younger granites in Egypt. The investigated tonalite and granodiorites are peraluminous to metaluminous with high Al2O3, Fe2O3, MgO, CaO, Sr, Ba, Co, Ni and Zn and low in total alkalis, Rb, Nb, V and Y compared with the younger granites.The studied granites were suffered from various tectonic episodes accompanied with different types of hydrothermal solutions, which have various compositions and formed different alteration types (eg., albitization, kaolinitization, serecitization, silicification, argilization and chloritization) as well as precipitated high concentration of REE and LILE (eg., Ba, Pb, K). The rare metals (eg., xenotime and monazite) are mainly concentrated along zircon, garnet and biotite grains due to substitute of REE with Fe, Mg, Zr and K of these host minerals. Some younger granites are enriched in garnet minerals especially spessartine and almandine that confirm enriched parental melts of A-type granites with Al, Fe and Mn and they are magmatic-hydrothermal in origin. Geochemical features of these granites show strong linear positive correlation of Ta versus Nb and Zr versus Hf that indicate the high enrichment of these elements due to magmatic origin. These rocks have been successfully discriminated on Landsat 8 images including; False Color Optimum Index Factor (OIF; 7, 6, 2 as RGB), Minimum Noise Fraction (MNF; 4, 3, 2), Principal Component Analysis (PCA; PC4, PC3, PC1), and band ratios (b4/b2, b5/b6, b6/b7). The results of remote sensing and geochemical studies of these areas can be used to predict the concentration of rare metals-bearing granitic plutons in similar other areas in Eastern Desert of Egypt