Integration of remote sensing and geochemical data to characterize mineralized A-type granites, Egypt: implications for origin and concentration of rare metals

Neoproterozoic mineralized granites from the Umm Naggat and Homrit Waggat areas in the Central Eastern Desert (CED) of Egypt, are parts of the Neoproterozoic Nubian Shield. On the basis of textural and chemical characteristics, they resemble highly fractionated ferroan peraluminous A-type granites. Decorrelation stretch (DS) and band ratio (BR) techniques of Sentinel-2 and Landsat-9 data were used for the spectral identification of lithological units, alteration and mineralized zones in A-type granites. Spatial and spectral extent of the hydrothermal mineralized alteration zones (e.g., sericitization, carbonatization, kaolinitization, ferrous silicates and hydroxyl) related to the rare metal-bearing granitic plutons can be discriminated by processed ASTER data. Some structural features have been identified by Sentinel-1enhanced Soble directional filter images. The NW–SE Najd fault system is conjugated with N–S and NE–SW faults, which structurally control the distribution of both mineralized alteration zones and rare metal-bearing granites in the CED of Egypt. The studied mineralized granites comprise syenogranite and alkali feldspar granite. Essential minerals are quartz, K-feldspar (Or_94-99), plagioclase (An_0-7) and biotite, with subordinate amounts of chlorite, muscovite and fluorite. Zircon, Fe-Ti oxides, rutile, apatite, epidote, titanite, columbite and thorite are main accessory phases. Average zircon saturation temperature (T_Zr) of the studied granites ranges from 780 °C to 880 °C at pressures of 0.7–3.0 kbars and depth < 8 km. These granites are highly evolved (SiO₂ = 73–78 wt. %), and show characteristics of high-K calc-alkaline peraluminous rocks (A/CNK = 1–1.13). They are enriched in Rb, Nb, Y, Ta, Hf, Ga, Zr and rare-earth elements (ΣREEs: up to 558 ppm) and show pronounced negative Eu anomalies (Eu/Eu* = 0.01–0.29), similar to post-collisional rare metal-bearing A-type granites either in Egypt or elsewhere in the world. These A-type granites more likely crystallized from highly fractionated I-type tonalite-granodiorite magmas, followed by extensive fractional crystallization in the upper crust during and just after lithospheric delamination. Rare-metal minerals such as zircon, rutile, xenotime, thorite, cerite-(Ce), apatite, parisite, uranothorite, columbite, ishikawaite and bastnaesite crystallized under both magmatic and hydrothermal conditions. Remote sensing and geochemical data enabled us to characterize mineralized zones in A-type granites and indicated that albitization is accompanied by higher concentrations of REEs (544 ppm), Zr (up to 378 ppm), Y (142 ppm), Nb (127 ppm) and Th (26 ppm) than other alteration types, suggesting stabilization of these elements by Na-, F- and Cl-rich fluids during Na-metasomatism.