エジプトのアブ・ルシェイドおよびシカイト花崗岩における多金属鉱化作用に関するリモートセンシングと地球化学的制約

The post-tectonic granites and pegmatites of the Abu Rusheid-Sikait area, in the South Eastern Desert of Egypt, represent highly mineralized plutons within the Arabian Nubian Shield (ANS). This study integrates remote sensing datasets, field work, petrographic, and geochemical analyses to investigate these plutons. Machine learning algorithms (MLAs), including Support Vector Machine (SVM) and Random Forest (RF), applied to Minimum Noise Fraction (MNF)-enhanced PRISMA data, successfully discriminated lithological units with an overall accuracy of up to 89%. Spectral analysis identified four main hydrothermal alteration zones (phyllic, argillic, propylitic, and ferrugination), which were validated by field and laboratory data. Geochemically, the post-collisional granites show geochemical characteristics of highly evolved peraluminous A-type granites generated in a within-plate environment similar to other A-type granites in the ANS. All the studied granites display negative Eu/Eu* (0.02-0.19) anomalies, reflecting plagioclase fractionation. The geochemical similarities between the pegmatites and the surrounding granites support their genetic relation. Zinnwaldite-muscovite and garnet-muscovite granites, along with associated pegmatites, are highly enriched in rare earth and high field strength elements (e.g., ΣREEs up to 1310 ppm, Zr up to 4477 ppm, Nb up to 1500 ppm, Ta up to 216 ppm, U up to 411 ppm). This mineralization occurs as both disseminated accessory minerals within host granitoids and concentrated along structurally controlled zones that are affected by the Nugrus shear zone, faults (NW-SE, N-S, and NE-SW), and fractures, as confirmed from automatic surface structure lineament extraction maps, field work, and petrography. The enrichment is primarily controlled by extreme magmatic fractionation, which produced primary rare metal-bearing minerals (zircon, columbite, xenotime, monazite). This magmatic signature was subsequently overprinted by hydrothermal alteration, which redistributed and further concentrated metals along structurally controlled pathways concentrated metals along brittle fracture zones formed by the late-stage reactivation of the Najd Fault System, forming secondary minerals like kasolite and galena. Ferrugination is considered the main alteration related to uranium and REEs remobilization and concentration in specific locations, sometimes alongside iron oxides (hematite, goethite). This work presents an integrated model that links magmatic processes, structural controls, and hydrothermal alteration, providing a valuable framework for rare metal exploration in the ANS.