PETROGENSIS AND MINERALOGICAL CHARACTERIZATION OF PEGMATITES FROM KHOUR ABALEA, SOUTH EASTERN DESERT, EGYPT

Abstract

Khour Abalea in Abu Rusheid area is located at the midst part of the cataclastic rocks having U-shapedIt was formed as a result of deep strike-slip faults, some forming shear zone in some trends. Two forms ofpegmatites have been distinguished (pockets and veins) occurring within the cataclastic rocks as hostedrocks. Petrographically, they consist of quartz, orthoclase, microcline and micas.The identified minerals in the studied pegmatites can be grouped into primary uranium minerals(uraninite), secondary uranium minerals (kasolite, uranophane and meta-autunite), thorium minerals(thorite and uranothorite), the niobium-tantalum minerals (columbite, ferro-columbite, samarskite, euxeniteand fergusonite), the accessories (zircon, monazite, cassiterite, pyrite and fluorite) and mica (muscovite,phlogopite and fluor-phlogopite) as well as the opaques (hematite and goethite).Geochemically, the studied pegmatite (S-type) are calc–alkaline and peraluminous in nature. It is highlyfractionated possibly due to hydrothermal alterations. The REEs display clear tetrad effect of M-typeaccompanied with relative enrichment of the HREE over the LREE and marked negative Eu anomaly(Eu/Eu* =0.03 to 0.36). The calculated tetrad effect in the pegmatite demonstrates clear tendency towardsthe ratios of the common isovalents such as Zr/Hf, Nb/Ta and Y/Ho. However, the tetrad effect seems notto be mutual with the content of Na2O and Rb/Sr. The absence of coherence between tetrad effect and sodaor potassic contents may indicate insignificant role of the Na- or K-metasomatism in developing the tetradeffect.Detailed spectrometric survey of the study pegmatite revealed their enrichment of eU (49-306 ppm)and eTh (76-634 ppm) and the potential anomalous radioactivity is attributed to both magmatic andhydrothermal processes.