Nusab El Balgum mass complex represents one of peralkaline volcanic activity phenomena in the south Western Desert of Egypt, which is typical for within-plate event, which formed in Mesozoic period. It consist s of acidic volcanic (rhyo lite and their pyroclastics) and sub-volcanic granitic rocks (incomplete ring, small stock and dyke of a peralkaline apha nites) as well as dykes (trachyte, bostonite, rhyodacite, rhyolite and porphyritic rhyolite) variable in thickness and the most of run in NNE-SSW trend. The peralkaline granitic rocks, especially th ose located at the southwestern part of this mass are characterized by extreme enrichments in HFSE (rare metals such as Zr, Nb, U and Th and REEs) which are the highest concentrations (e.g., >1% Zr, 0.5% Nb and 2.6% total REEs, Y up to 1%, eU up to 300 ppm and eTh up to 1100 ppm). The rare metal bearing minerals are thorite, uranothorite, autunite, amorphous secondary uranium, zircon and ferrocolumbite, while the REEs bearing minerals are bastnaesite, monazite and xenotime. The positive relations in all of the binary diagrams of Zr versus Nb, Y, eU and eTh, Nb versus Y, eU and eTh, Y versus eU and eTh in post magmatic intensely hematised peralkaline granites indicated that, this process is responsible for the enrichment in these HFSE. The chondrite-normalized pattern of high-altered peralkaline granites indicates: 1) higher LREEs enriched pattern (La/Gd = 11.34 and 12.25) means the alteration processes t a k ing place under open system and these rocks evolved from magma of lithospheric rifting , 2) ΔCe < 1 anomaly, means that the alteration fluids were slightly oxic and 3) strong negative—ΔEu < 1. This indicates the nature of residual peralkaline melt: a) it was extremely rich in fluorine, H 2 O, and thus very low viscosity, despite its low temperature (<650°C); b) it was strong depleted in feldspar-compatible ele ments , as indicated by strong negative Eu anomalies; and c) it had abundances of HFSE cations. Redistribution of ele ments took place by post magmatic hydrothermal solutions .
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