Geochemical characteristics and genesis of niobium–yttrium–fluorine granite pegmatite from Hmrat El Sorwhyia area, North Eastern Desert, Egypt

Abstract

The granite pegmatites of Hmrat El Sorwhyia, situated in the northern Egyptian-Nubian Shield, are predominantly hosted within the central areas of syenogranitic bodies, with limited occurrences noted along the marginal sites. The Hmrat El Sorwhyia granitic pegmatites represent a notable example of rare-metal mineralisation. This study introduces novel bulk- rock compositions, including major, trace, and rare earth elements, to investigate and constrain the origin and mechanisms influencing the genesis of the Hmrat El Sorwhyia granitic pegmatites and their rare-metal mineralisation. The studied pegmatites exhibit diverse morphologies, featuring lenticular pockets that contain the highest radioelement concentrations. The maximum (equivalent uranium) eU value is 760 ppm while the maximum (equivalent thorium) eTh value is 5552 ppm. These pegmatites have granitic composition and peraluminous characteristics, indicative of A-type affinity associated with post-collisional tectonic settings, and are categorised as niobium–yttrium–fluorine-type pegmatites. The formation of granite pegmatites through fractional crystallisation during the late-stage magmatic processes involved the concentration of U, Th, rare earth elements, Zr, Y, and Nb. This concentration facilitated the crystallisation of thorite, uranothorite, columbite, cerianite, and zircon. Additionally, assimilation-fractional crystallisation mechanisms led to the exchange of accessory phases between successive injections of pegmatite and the surrounding wallrock, resulting in variable mineral concentrations. These open-system processes were responsible for the development of these pegmatites.