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Paragenetic, textural, and chemical characteristics of columbite-tantalite minerals are examined as steps towards identifying the metallogenetic processes of their host granitoids. Columbite-tantalite-bearing granitoids of the Eastern Desert province of Egypt can be categorized into: (i) metaluminous alkali granites; (ii) peraluminous Li-albite granites; and (iii) metasomatized biotite and/or muscovite granite (i.e. apogranites).Columbite of the alkali granite is of FeNb2O6 composition and associated with annite. The low F and Li contents of the associated mica precludes the important role of these volatile elements during the late stage of evolution of the alkali granites, thus delaying fractionation of Mn over Fe and Ta over Nb.Compositionally, columbite-tantalite of the Li-albite granites is constrained between MnNb2O6 and MnTa2O6 (the Ta/(Nb+Ta)atom. ratio ranges between 0.10 and 0.80). This low to high ratio and the association of columbite-tantalite with topaz, fluorite and lithian micas (in the series zinnwaldite-white mica) indicate a higher solubility for Ta-fluoride complex compounds and their more stabilized state at lower temperatures in Li- and F-rich sodic melts. The columbite-tantalite commonly exhibits a mottled or patchy zoned texture with the rims consistently higher in Ta than the cores, reflecting the later effect of a corrosive supercritical vapour phase.The columbites of metasomatized granites range in composition between FeNb2O6 and MnNb2O6. They are characterized by high Ti and U, and low Ta contents (the Ta/(Nb+Ta)atom. ranges between 0.01 and 0.15), indicating deposition from alkaline (K+ Na+ -rich), and relatively high-temperature interacting fluids. However, the Mn-enriched columbites are commonly encountered in the apical parts of the apogranites and formed in response to high μKF and μLiF required for stabilizing the associated Li-siderophyllite or zinnwaldite. Columbites of the apogranites commonly exhibit progressive (either normal or reverse) zoning which can be attributed to the disequilibrium conditions (e.g. sudden change in the pH) between the growing crystal and the solutions.

Typological study, including paragenic, morphological, textural, and chemical characteristics of zircon from nine rare metal granitic stocks and associated greisens, was carried out in order to identify the metallogenic processes of their host granitoids. The investigated zircon-bearing granitoids and type occurrences can be categorized into magmatically and metasomatically specialized types. The magmatic type includes: (i) peralkaline, Zr + Nb-enriched, A 1 -granite (e.g. Um Hibal); (ii) metaluminous, Nb + Zr + Y-enriched, A 2 -type alkali granite (e.g. Hawashia and Ineigi); and (iii) peraluminous, Ta ³ Nb + Sn + Be ± W-enriched, Li-albite granite (e.g. Nuweibi, Igla and Abu Dabbab). The metasomatized granites are Nb>>Ta + Sn + Zr + Y + U ± Be ± Wenriched and hydrothermally altered alkali feldspar granite (i.e. apogranite; e.g. Um Ara, Abu Rusheid, and Um Naggat). Zircon of peralkaline granite is characteristically equant with well-developed pyramidal faces and short prisms (i.e. pseudo-octahedral form) with length/width ratios in the range of 2:1 -1:1. It is of Zr 0.990 Hf 0.007 SiO 4 composition and is associated with hypersolvus assemblage consisting of alkali feldspar, quartz, aegirine and minor reibeckite. Zircon of metaluminous alkali granites is of Zr 0.99 Hf 0.01 SiO 4 composition and is associated with sub-to transolvus assemblage of K-feldspar, quartz, plagioclase and annite-siderophyllite mica. It is prismatic with length/width ratios in the range of 5:1 -3:1, doubly terminated with small pyramidal faces. Compositionally, zircon of Li-albite granite ranges between Zr 0.925 Hf 0.075 SiO 4 and Zr 0.705 Hf 0.295 SiO 4 . It is idiomorphic with a simple combination of prism and bipyramidal terminations with a length/width ratio of 3:1 -2:1. This zircon commonly exhibits a normal zoning with rims consistently higher in Hf than cores. The higher Hf content, of this zircon coupled with its association with topaz, tantalite and lithian micas (e.g. zinnwaldite and Li-white mica), indicates a higher solubility of Hf-fl uoride complexes and their more stabilized state at lower temperature in Li-and F-rich sodic melts. Zircon of apogranite association ranges in composition between Zr 0.967 Hf 0.013 SiO 4 in the lower unaltered alkali feldspar granite zone and Zr 0.805 Hf 0.064 (Y, U, Th, heavy rare-earth elements) [HREE]) 0.125 SiO 4 in the apical metasomatized (i.e. microclinized, albitized, and greisenized) apogranite zones. This compositional change appears to refl ect a roofward increasing in KF , NaF , and HF of the exsolved fl uids. Columbite, xenotime, thorite, cassiterite, beryl and fl uorite are common associates of this zircon. This zircon is of bipyramidal to typical octahedral form with complete absence of prism concurrently with conspicuous development of pyramid, thus the zircon crystals have a length/width ratio of 1:1 -0.5:1. The neoformed metasomatic zircon commonly exhibits either normal or reverse zoning with rims consistently different in Hf, U, Y, and HREE than cores, refl ecting dise...