Chemistry of Zircon in Rare Metal Granitoids and Associated Rocks, Eastern Desert, Egypt

摘要

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) peral-kaline, 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 > 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 + - W-enriched 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-fluoride 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 reflect a roofward increasing in mu_(KF), mu_(NaF), and mu_(HF) of the exsolved fluids. Columbite, xenotime, thorite, cassiterite, beryl and fluorite 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, reflecting disequilibrium conditions (e.g. sudden change in P, T, salinity, and pH) between the growing crystals and the exsolved fluids.