Alkaline igneous rocks, a potential source of rare metals and radioactive minerals: Case study at Amreit area, south Eastern Desert, Egypt

Alkaline igneous rocks represent one of the most economically important resources of radioactive minerals and rare metals. New field observations and petrographic studies are integrated with whole-rock geochemical analyses and Gamma ray spectroscopy data of alkaline rocks associated with the Amreit complex. The fieldwork was achieved by the collection of more than forty samples from alkaline granites and alkaline syenites. The youngest rocks cropping out in the study area are the cogenetic alkaline rocks, ranging from alkaline granite to alkaline syenite. These alkaline rocks are composed essentially of K-feldspar, alkali amphiboles (arfvedsonite), and sodic pyroxene, with accessories such as zircon, apatite, and ilmenite. Mineral characterization of the highly radioactive zones in both alkaline granite and alkaline syenite displays enrichment in monazite, thorite, zircon, ferro-columbite, xenotime, and allanite minerals. Geochemical analyses indicate that the Amreit rocks are alkaline with peralkaline affinity and have high concentrations of total alkalis (K₂O + Na₂O), large ion lithophile elements (LILEs; Ba and Rb), high field strength elements (HFSEs; Y, Zr and Nb), rare earth elements (REEs) and significantly depleted in K, Sr, P, Ti, and Eu, typically of post-collision A-type granites. Typically, the Amreit alkaline igneous rocks are classified as within plate granites and display A2 subtype characteristics. The fractionation of K-feldspars played a distinctive role during the magmatic evolution of these alkaline rocks. The geochemical characteristics indicate that the studied alkaline igneous rocks which were originated by fractional crystallization of alkaline magmas were responsible for the enrichment of the REE and rare metals in the residual melt. The high radioactivity is essentially related to accessory minerals, such as zircon, allanite, and monazite. The alkaline granite is the most U- and Th-rich rock, where radioactivity level reaches up to 14.7 ppm (181.55 Bq/kg) eU, 40.6 ppm (164.84 Bq/kg) eTh, whereas in alkaline syenite radioactivity level is 8.5 ppm (104.96 Bq/kg) eU, 30.2 ppm (122.61 Bq/kg) eTh. These observations suppose that these alkaline rocks may be important targets for REEs and radioactive mineral exploration.