Chemistry of Micas in Rare-Metal Granitoids and Associated Rocks, Eastern Desert, Egypt
- 1 October 1999
- journal article
- research article
- Published by Taylor & Francis Ltd in International Geology Review
- Vol. 41 (10), 932-948
- https://doi.org/10.1080/00206819909465180
Abstract
Paragenetic, textural, and chemical characteristics of micas from 10 rare-metal granitic stocks and the associated greisens were examined in order to identify the metallogenetic processes of the host granitoids. The investigated 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) metasomatized, Nb » Ta + Sn + Zr + Y + U ± Be ± W-enriched apogranites (e.g., Um Ara, Abu Rusheid, Mueilha, and Homr Akarem). Mica of the alkali granite is of the annite-siderophyllite series, and is characterized 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-siderophyllite-bearing magmas. The micas closely resemble those of the anorogenic pegmatites and A-type granites. Primary mica of the Li-albite granites is compositionally constrained between zinnwaldite in the lower zones, and white mica in the apical, more evolved zone, and is associated with columbite-tantalite, topaz, and fluorite. The occurrence of zinnwaldite with high contents of Mn and F indicates its stabilization 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 zones can be attributed to either volatile degassing or to the beginning of topaz crystallization. These two factors brought about an evolutionary trend for micas, which contrasts with the documented trends of Li-micas in other 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 microclinized and albitized zones; this systematic compositional change appears to reflect roofward increasing in μKF and μLiF of the exsolved fluids. Columbite, cassiterite, zircon, xenotime, beryl, and fluorite are common associates of the zinnwaldites. However, white micas from the greisenized apogranite and endogreisen veins have diminishing Li contents. The subsolidus formation of zinnwaldite and Li-siderophyllite in the apogranites, and white mica in the associated greisens, represent transitions from magmatic to hydrothermal environments under the influence of decreasing P, T, salinity, and alkalinity of the exsolved fluids.Keywords
This publication has 29 references indexed in Scilit:
- On Li-bearing micas: estimating Li from electron microprobe analyses and an improved diagram for graphical representationMineralogical Magazine, 1997
- Mineral Chemistry of Albite-Enriched Granitoids at Um Ara, Southeastern Desert, EgyptInternational Geology Review, 1994
- Geochemistry of the Wadi Hawashia Granite Complex, northern Egyptian ShieldJournal of African Earth Sciences, 1994
- Fertile granites of Precambrian rare-element pegmatite fields: is geochemistry controlled by tectonic setting or source lithologies?Precambrian Research, 1991
- Vapor-undersaturated experiments with Macusani glass+H2O at 200 MPa, and the internal differentiation of granitic pegmatitesBeiträge zur Mineralogie und Petrographie, 1989
- Compositions of trioctahedral micas in the Cornubian batholithMineralogical Magazine, 1988
- Evolution of the Miscibility Gap Between Muscovite and Biotite Solid Solutions with Increasing Lithium Content: An Experimental Study in the System K2O-Li2O-MgO-FeO-Al2O3-SiO2-H2O-HF at 600°C, 2 kbar PH2O: Comparison with Natural Lithium MicasMineralogical Magazine, 1986
- Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processesGeology, 1983
- Geochemistry and petrogenesis of a peralkaline granite complex from the Midian Mountains, Saudi ArabiaLithos, 1980
- Phengite micas; synthesis, stability, and natural occurrenceAmerican Journal of Science, 1965