The rare metal deposits of the El Garra El Hamra syenites, South Western Desert, Egypt

Abstract

The El Garra El Hamra complex is a small high-level plug (with associated volcanics) that was emplaced along an ENE-WSW trending dextral strike-slip fault corresponding to the Guinean-Nubian lineament. Owing to continued movement along this fault, the intrusion split into two parts that were displaced horizontally by approximately 580 m. The intrusion consists mainly of syenites (fresh and altered) and granites (altered) that are locally overlain by alkaline trachyte and rhyolite and are hosted by Nubian sandstones. The fresh syenites are composed of perthitic K-feldspars, albite, alkali pyroxenes (aegirine-augite and aegirine) and secondary alkali amphiboles (arfvedsonite and riebeckite). Accessory minerals include opaques, apatite, zircon, xenotime-(Y) and bastnäsite-(Ce). Calcite and hematite are alteration products. Geochemically, the syenites are peralkaline; their CIPW norms classify them as ranging between syenite and quartz syenite in compositions. They were emplaced in a within-plate tectonic setting along the Guinean-Nubian lineament (Kalabsha), close to the extension of the Chephren-Kom Ombo trend (a branch of the Trans-African Shear Zone). They were evolved from a magma compositionally similar to Ocean island basalt (OIB). Numerous dextral and sinistral strike-slip faults cut the intrusion and may have facilitated the circulation of fluids that altered them hydrothermally. These faults developed during or shortly after emplacement of the intrusion or perhaps during subsequent reactivation of the Guinean-Nubian and Trans-African shear zones and were accompanied by dykes that range in compositions from intermediate to acidic composition; most of which strike ENE-WSW. Hydrothermal alteration of the syenites locally led to extreme enrichment in rare metals (e.g. LREE, HREE, Zr, Hf, Th, U, Nb and Be) as the minerals zircon, gittinsite, bastnäsite-(Ce), parisite-(Ce), monazite–(Ce), REE-rich fluorapatite, chevkinite-(Ce), xenotime-(Y), pyrochlore, columbite-(Fe), Nb-rutile and thorite. Fluorite and barite are important gangue minerals.