Chemistry of micas in rare-metal granitoids and associated rocks, Eastern Desert, Egypt

Paragenetic, textural, and chemical characteristics of micas from 10 rare-m etal granitic stocks and the associated greisens were examined in order to identify the metallogenetic processes of the host granitoids. The investiga ted granitoids and type occurrences can be categorized as: (1) metaluminous , Nb + Zr + Y-enriched alkali granite (e.g., Hawashia, Ineigi, and a stock northwest of Um Naggat); (2) peraluminous, Ta > Nb + Sn +/- W + Be-enriched Li-albite granites (e.g., Nuweibi, Igla, and Abu Dabbab); and (3) metasoma tized, Nb much greater than Ta + Sn + Zr + Y + U +/- Be +/- W-enriched apog ranites (e.g., Um Ara, Abu Rusheid, Mueilha, and Homr Akarem). Mica of the alkali granite is of the annite-siderophyllite series, and is c haracterized by an average FeO* of 28.14, low MgO of 0.05, a mean Fe*/(Fe* + Mg)(atom) value of 0.996, TiO2 of 0.69, enhanced Al2O3 of 14.91, MnO of 0 .58, Li2O of 0.26, and moderate to low F of 0.86. These characteristics are representative of the relatively highly evolved nature of the annite-sider aphyllite-bearing magmas. The micas closely resemble those of the anorogeni c pegmatites and A-type granites. Primary mica of the Li-albite granites is compositionally constrained betwe en zinnwaldite in the lower zones, and white mica in the apical, more evolv ed zone, and is associated with columbite-tantalite, topaz, and fluorite. T he occurrence of zinnwaldite with high contents of Mn and F indicates its s tabilization at rather low temperatures in Li- and F-rich sodic melts. The restriction of white mica with lower Mn, F, and Li contents to the apical z ones can be attributed to either volatile degassing or to the beginning of topaz crystallization. These two factors brought about an evolutionary tren d for micas, which contrasts with the documented trends of Li-micas in othe r Li-granites (i.e., from Li-siderophyllite or Li-muscovite to lepidolite). Micas range in composition between white mica in the lower unaltered zones of the apogranites and Li-siderophyllite-zinnwaldite in the apical microcli nized and albitized zones; this systematic compositional change appears to reflect roofward increasing in mu(KF). and mu(LiF) Of the exsolved fluids. Columbite, cassiterite zircon, xenotime, beryl, and fluorite are common ass ociates of the zinnwaldites. However, white micas from the greisenized apog ranite and endogreisen veins have diminishing Li contents. The subsolidus f ormation of zinnwaldite and Li-siderophyllite in the apogranites, and white mica in the associated greisens, represent transitions from magmatic to hy drothermal environments under the influence of decreasing P, T, salinity, a nd alkalinity of the exsolved fluids.