El Seboah peralkaline A-type magmatism, South Western Desert, Egypt: evidences for the HFSE and REE enrichment

Abstract

El Seboah peralkaline A-type magmatism is one of the numerous alkaline plutonic and volcanic (trachyte, rhyolite, and basalt)/subvolcanic rocks, which are occurring in the South West of Egypt along the Guinean-Nubian lineament. El Seboah magmatic rocks include fresh peralkaline granite and rhyolite dikes (pantellerite). The fresh peralkaline granite intruded the Late Cretaceous Nubia sandstones, and furthermore, the pantelleritic dikes cut it. The fresh peralkaline granite and the pantelleritic dikes show common features of the A-type granites. They contain alkalic mafic minerals such as aegirine and riebeckite in the peralkaline granite or aegirine in the pantelleritic dikes; they also have high alkalis and HFSE (e.g., Zr, Nb, REE, Th, and U) with high ratios of FeOt (total iron)/MgO and Ga/Al, and strong depletions in Ba, Sr, Eu, Ti, and P. Both of the fresh peralkaline granite and the pantelleritic dikes have peralkaline, ferroan, and fertile character, and belong to subgroup A1-type magmas. Additionally, these rocks were formed as separated batches, emplaced within the African plate along the volcanic zone of the Guinean-Nubian lineament, and the source for both rock types is suggested as the ocean island basaltic magmas. Moreover, the pantelleritic magma displays more enrichment in the HFSE than the alkaline granitic magma, suggesting highly fractionated and alkalinity association. The peralkaline granite was subjected to different degrees of hydrothermal alterations along the fault zones. The hydrothermally altered peralkaline granite shows a strong enrichment in the HFSE that occur in minerals such as zircon, bastnaesite-(Ce), monazite-(Ce), percleveite-(Ce), pyrochlore, fergusonite-(Ce), columbite-(Mn), thorite, uranothorite, and amorphous secondary uranium. In addition, it shows a strong increase in the FeOt and CaO and a depletion in Al2O3, MgO, Na2O, and TiO2 with a slight increase in K2O and a slight decrease in SiO2 compared to the fresh peralkaline granite. Consequently, I suggest that the hydrothermally altered peralkaline A-type granite within the El Seboah area as a potential source for the rare metal deposits, but further detailed studies are needed.