Assessment of magmatic versus post-magmatic processes in the Mueilha rare-metal granite, Eastern Desert of Egypt, Arabian-Nubian Shield

Abstract

The Mueilha rare-metal granite, exposed in the central Eastern Desert of Egypt, is a post-collisional intrusion that formed in the final magmatic stage of the evolution of the Arabian-Nubian Shield. The Mueilha intrusion was emplaced as a high-level magmatic cupola into metamorphic country rocks. It consists of two cogenetic intrusive bodies: an early phase emplaced at shallow depth and now penetratively altered to white albite granite and a later phase of red granites emplaced at greater depth that better preserve magmatic features. The albite granite is less common and represents the upper margin of the Mueilha intrusion, the apex of the magmatic cupola. The red granites are volumetrically dominant and appears to have crystallized from the margins inward, forming a composite pluton zoned from muscovite granite to alkali feldspar granite. All parts of the Mueilha pluton appear to have been emplaced within a short time interval, before complete crystallization of the earliest phase. The geochemistry of the Mueilha granites is typical of rare-metal granites, characterized by high SiO2, Na2O + K2O, Nb, Rb, Ta, Y, U, Th, Sn, and W with depletion in P, Mg, Ti, Sr and Ba. They are weakly peraluminous and highly fractionated with A-type character. The chondrite-normalized REE patterns have strongly negative Eu anomalies, typical of highly differentiated granites that evolved through a transitional magmatic–hydrothermal stage. The primary magma feeding the Mueilha intrusion was generated by partial melting of the juvenile crust of the Arabian-Nubian Shield; it subsequently underwent extensive fractional crystallization and metasomatism by late- to post-magmatic fluids. Separation of fluids from the oversaturated melt promoted both diffuse greisenization and focused segregation of pegmatite and fluorite and quartz veins. Alkalis liberated from feldspars consumed by greisenization were redeposited during albitization in the uppermost part of the magma chamber. Despite penetration of the intrusion boundary by discrete dikes, veins, and aphophyses, diffuse alteration of the metamorphic country rocks is not apparent. Primary columbite-series minerals crystallized from the melt and were later partly replaced by secondary Nb and Ta minerals (fluorcalciomicrolite and wodginite) during hydrothermal alteration.