Secondary rare metal enrichment following biotite alteration in the Umm Naggat granitic pluton, Central Eastern Desert, Egypt

Abstract

Biotite is an essential mineral in regulating NbTa fractionation during magma evolution. Post-magmatic, metasomatic biotite alteration may form secondary rare metal (RM)-bearing phases. Nevertheless, the mechanism of biotite alteration remains incompletely elucidated. The Umm Naggat granitic pluton (UMNG) in Egypt comprises RM-bearing amphibole-biotite granite (SBTG) in its southern sector, with biotite being significantly more abundant than amphibole. Amphibole and biotite in the SBTG crystallized from Nb-enriched magma and represent primary high-field strength elements (HFSE) reservoirs. Amphibole contains Nb (280 ppm), Ti (5760 ppm), Y (506 ppm), Zr (87 ppm), and Mn (2801 ppm), while biotite contains significantly higher Nb (1630 ppm), Ta (83 ppm), Sn (392 ppm), Ti (9268 ppm), Rb (2846 ppm), and Li (6331 ppm). The Nb enrichment in biotite mainly depended on its composition (Ti and Fe), enhanced by high formation temperatures.

In the SBTG, hydrothermal alteration is divided into the metasomatic and aqueous stages. The metasomatic fluid was enriched in Na-F-Nb-Ta-Zr, causing albitization and precipitation of metasomatic fluorite, columbite-(Fe), and zircon. Biotite is extensively altered during the aqueous stage, mainly to muscovite and chlorite, through a coupled dissolution-precipitation mechanism. The replacement textures of primary biotite imply that the hydrothermal fluids were acidic and fluorine-bearing, with considerable Y and REE amounts. During the alteration, Ti, Fe, Sn, Ta, and Nb were liberated from the biotite into the fluid as Ta-Nb-F complexes. The continuous consumption of H⁺ and F⁻ resulted in their depletion, making the Ta-Nb-F complexes unstable, reducing their solubility, and leading to the precipitation of secondary Ta-Nb-Sn-Y-REE-Fe-Ti-bearing oxides near or within muscovite and chlorite. The former alteration-precipitated oxides, including niobian rutile, Nb-ilmenite, columbite-(Fe), and euxenite-(Y), are significantly enriched in Nb, Ta, and Sn compared to their primary equivalents (primary niobian rutile, Nb-ilmenite, columbite-(Fe), and fergusonite-(Y)). The spatial distribution of biotite alteration byproducts within the SBTG reveals distinct variations where the northern sector of the SBTG includes muscovite, niobian rutile, columbite-(Fe), and euxenite-(Y), while the southern sector includes muscovite, chlorite, niobian rutile, columbite-(Fe), and Nb-ilmenite with the dominancy of Fe cations in the fluid increasing over Ti cations toward the south. The hydrothermal fluid likely evolved into a more acidic character as the fluid temperature dropped with the alteration advancing toward the south of the SBTG. Considering its distinctive petrography, mineralogy, and textural variations, the alteration-formed Nb-Ta-Sn-enriched mineralization may delineate a new potential style and source for RMs in Egypt and worldwide.