Mineral Chemistry of Albite-Enriched Granitoids at Um Ara, Southeastern Desert, Egypt

Abstract

Mineral chemistry and typomorphic characteristics are used to monitor the physicochemical evolution of late-magmatic to postmagmatic alteration processes that resulted in the development of a radioactive and albite-enriched microgranite stock. The mineral paragenesis indicates that postmagmatic fluids were enriched in Nb, Zn, Mn, U., Th, Zr, and Y, in addition to Rb, Li, and F Manganocolumbite with extremely high Nb/(Nb+Ta) (0.99), Mn/(Mn+Fe) (0.82), and zircon with high Zr/(Zr+Hf) (0.97) indicate crystallization under alkaline, relatively high-temperature conditions (>425° C). The close association of manganocolumbite, Nb-Mn-Zn- rich ilmenite (with 1.2 to 14.5 wt% ZnO), spessartine garnet (with 68.2-89.4 mol% spessartine), zircon, xenotime, zinnwaldite mica (up to 5.98 wt% F), and fluorite indicates the strong affinity of the elements of Nb, Y., Zr, Mn, and Zn for stable complexing by K⁺, Na⁺, Li⁺, and F⁻ rich supercritical fluids during the course of extraction and transportation. The enrichment of the interacting fluid in U and Th is depicted by the presence of up to 1.6% UO₂ in manganocolumbite and Hf-bearing zircon, and up to 10.5% ThO₂ in monazite, in addition to locally abundant thorite and uranophane. It is suggested that the uranium mineralization, mainly as fracture fillings, formed during the waning stage of hydrothermal activity.