Abstract
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 consists of acidic volcanic (rhyolite and their pyroclastics) and sub-volcanic granitic rocks (incomplete ring, small stock and dyke of a peralkaline aphanites) 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 those 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 taking place under open system and these rocks evolved from magma of lithospheric rifting, 2) ΔCe 2O, and thus very low viscosity, despite its low temperature (ments, as indicated by strong negative Eu anomalies; and c) it had abundances of HFSE cations. Redistribution of elements took place by post magmatic hydrothermal solutions.
Highlights
The peralkaline granitic rocks were emplaced as dykes and small stocks, characterized by economical to subeconomical deposits of high-field strength elements (HFSE, i.e., REE, Zr, Y, Nb, etc.) as stated by [1,2]
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
This indicates the nature of residual peralkaline melt: a) it was extremely rich in fluorine, H2O, and very low viscosity, despite its low temperature (
Summary
The peralkaline granitic rocks were emplaced as dykes and small stocks, characterized by economical to subeconomical deposits of high-field strength elements (HFSE, i.e., REE, Zr, Y, Nb, etc.) as stated by [1,2]. The association of extreme HFSE with the hydrothermal alteration processes as in case peralkaline granite of Lake Strange and other plutons, suggests these processes may play a role in HFSE concentrations [7]. The extreme enrichment of HFSE in The Lake Strange found in intensity-hematised zone and concentrated by hydrothermal alterations [8]. The comparison between slightly altered granite and highaltered one occur in the study area, indicate the role of post magmatic hydrothermal alterations in redistributions and the concentrations of HFSE in these rocks. While the Chondrite-normalized REEs pattern indicate physio-chemical conditions, which are prevailing during alterations processes
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