Chemistry of columbite-tantalite minerals in rare metal granitoids, Eastern Desert, Egypt

Paragenetic, textural, and chemical characteristics of columbite-tantalite minerals are examined as steps towards identifying the metallogenetic proce sses of their host granitoids. Columbite-tantalite-bearing granitoids of th e Eastern Desert province of Egypt can be categorized into: (i) metaluminou s alkali granites; (ii) peraluminous Li-albite granites; and (iii) metasoma tized biotite and/or muscovite granite (i.e. apogranites). Columbite of the alkali granite is of FeNb2O6 composition and associated wi th annite. The low F and Li contents of the associated mica precludes the i mportant role of these volatile elements during the late stage of evolution of the alkali granites, thus delaying fractionation of Mn over Fe and Ta o ver Nb. Compositionally, columbite-tantalite of the Li-albite granites is constrain ed 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-tant alite with topaz, fluorite and lithian micas (in the series zinnwaldite-whi te 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 FeNb2 O6 and MnNb2O6. They are characterized by high Ti and U, and low Ta content s (the Ta/(Nb+Ta)(atom) ranges between 0.01 and 0.15), indicating depositio n from alkaline (K+, Na+ -rich), and relatively high-temperature interactin g fluids. However, the Mn-enriched columbites are commonly encountered in t he apical parts of the apogranites and formed in response to high mu(KF); a nd mu(LiF) required for stabilizing the associated Li-siderophyllite or zin nwaldite. Columbites of the apogranites commonly exhibit progressive (eithe r normal or reverse) zoning which can be attributed to the disequilibrium c onditions (e.g. sudden change in the pH) between the growing crystal and th e solutions.