Nb-Ta-Sn oxides as markers of magmatic fractionation and magmatic-hydrothermal evolution: The example of the Nuweibi granite intrusion, Eastern Desert, Egypt

The Nuweibi rare-metal granite is located in the Central Eastern Desert of Egypt and represents a highly evolved leucogranite pluton that intruded into Neoproterozoic basement rocks of the Arabian-Nubian Shield. It is separated by the NNE-SSW trending oblique sinistral Dabr fault into two principal granite facies: a medium-grained albite granite in the western block and a porphyritic albite granite in the eastern block. The major ore minerals among the disseminated Nb-Ta-Sn mineralization are columbite-group minerals (CGM), wodginite, microlite, and cassiterite, which follow a distinct crystallization sequence including partial dissolution and late recrystallization. These ore minerals are usually strongly zoned, exhibiting a wide range of chemical compositions. As peculiarity, cm-sized euhedral columbite crystals with thin Ta-rich rims are found in the massive quartz cap of the eastern block of the intrusion. On the basis of mineral textures and microanalysis of mineral chemistry of the Nb-Ta-Sn oxides, we propose a genetic model for the evolution of the Nuweibi granitic intrusion involving magmatic, magmatic-hydrothermal, and subsolidus ore-forming processes: 1) upward migration of a granitic melt to the level of emplacement and fractional crystallization resulting in enrichment of the residual melt in incompatible elements (e.g., Ta, Nb, Sn) and volatiles (H₂O and F); 2) incipient magmatic crystallization of early Nb-rich CGM-1, albite, and snowball quartz and Ta enrichment in the melt (more incompatible than Nb); 3) onset of fluid exsolution (fluid saturation in the melt at the magmatic-hydrothermal transition), magmatic crystallization of CGM-2 with increasingly higher #Ta with co-existing fluid; 4) solidification of the quartz cap due to undercooling saturation with precipitation of columbite under hydrothermal conditions from magmatic fluids; 5) late magmatic fluid circulation, dissolution of the CGM and re-precipitation of CGM-3, followed by late-stage wodginite and microlite; 6) oblique sinistral and normal faulting after solidification of the Nuweibi granite, leading to the western block of the intrusion to move diagonally upwards resulting in today's opposing levels of exposure. As a result, the mineral paragenesis, texture, and mineral chemistry of Nb-Ta-Sn minerals prove to be remarkable tracers of the evolution from magmatic to hydrothermal conditions in rare-metal granites.