Geologija (Jun 2013)
Mineral chemistry and genesis of Zr, Th, U, Nb, Pb, P, Ce and F enriched peralkaline granites of El-Sibai shear zone, Central Eastern Desert, Egypt
Abstract
El-Sibai mineralized shear zone trending NNE-SSW is located at the northern segment of Gabal El-Sibai (500m in length and 0.5 to 1.5 m in width). Rocks along the shear zone show different types of alterations such ashematization, kaolinitization, fluoritization, and silicification. These alterations are good traps for rare metals ofthorite, ferrocolumbite, pyroclore, plumbopyroclore, fluorite, cerite-(Ce), zircon, Th-rich zircon, zirconolite (mixtureof zircon & columbite), fluorapatite, titanite, and monazite minerals.The detailed mineralogical study of the El-Sibai shear zone revealed its enrichment in Th, Zr, Nb, Pb, U, F, P,LREE (Ce), especially concerning the hematization processes. The close correlation of ferruginated (hematitized)samples with high radioactivity is related to the high ability of iron oxides for adsorption of radioactive elementsfrom their solutions. The rare-metal minerals found in altered peralkaline granites (shear zone) are associated withhematitization, albititization, chloritization, fluoritization, and pyritization. Electron probe microanalysis (EPMA)provides an indication of a range of solid solution between thorite and zircon, in which intermediate phases, such asTh-rich zircon were formed. These phases have higher sum of all cations per formula (2.0 to 2.09 atoms per formulaunit, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantialamount of interstitial cations such as U, Y, Ca, and Al in these phases. Altered zircon enriched in Th and U (Th-richzircon) preferentially involves coupled substitution Ca2+ + (Th,U)4+ ↔ 2Zr4+ + 2Si4+, implying that significant amountof U and Th may enter the Zr and Si position in zircon.Thorite and Th-rich zircon are related to hydrothermal fluid. Also the genesis of the studied zircon is related tometasomatic hydrothermal zircon (MHZ). The abundantly detected zircon, Th-rich zircon, Th-bearing minerals andfluorite of demonstrably hydrothermal origin can be attributed to the role of fluorine-rich fluids. Although Zr andTh are generally considered as highly immobile elements, yet the occurrence of zircon indicates that their significantconcentrations can be transported under specified F-rich fluids. The hydrothermal origin could be accepted forthe thorite, huttonite monazite, zircon, Th-rich zircon, ferrocolumbite, pyroclore, plumbopyrochlore, zirconolite,fluorite, cerite-(Ce), fluorapatite within the El-Sibai altered peralkaline granites (shear zone).
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