Geochemistry and Radioactivity of Mineralized Pegmatite from Abu Rusheid Area, South Eastern Desert, Egypt

Abstract

The mineralized pegmatite of Abu-Rusheid area, S. Eastern Desert, has quartz core and feldspars and mica margins. This pegamtite was analyzed by the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) to determine the content of the major and trace elements including the rare earth elements (REE). The mineralized pegmatite vein trends NNW-SSE with dip of about 10- 30° due WSW. It is emplaced parallel to foliation and banding of the cataclastic country rocks. The studied pegmatite is classified as rare metals-enriched and shows a zonal distribution from the barren core to the mineralized wall-zone in the metals; Nb, Ta, Zr, Hf, Th, U, Y and Ga. The main carriers of the trace metals are zircon, rutile, columbitetantalite, samarskite and other REE and Th-U minerals. Cassiterite, chlorite, deep violet fluorite, calcite, goethite, hematite, pyrite, magnetite, zinnwaldite, and phlogobite, are the main accessory minerals. The studied pegmatite is highly differentiated, NYF-type and characterized by high FeO/MgO ratio with high concentration of Nb, Y, Zr, Th, U, REE (except Eu), and Ga. It is derived from Fenriched proterozoic crustal source depleted in Cs, B and P. The rock is peraluminous and highly fractionated and had been affected by hydrothermal alteration. The pegmatite displays clear M-type tetrad effect of the REE. The calculated tetrad effect in the pegmatite of Abu-Rusheid demonstrates clear tendency towards the ratios of the common isovalents such as Zr/Hf, Nb/Ta and Y/Ho. However, the tetrad effect seems not to be mutual with the content of Na2O and Rb/Sr. The absence of coherence between tetrad effect and soda content may indicate insignificant role of the Na-metasomatism in developing the tetrad effect. The pegmatite has a prominent negative Eu anomaly and excessively low K/Rb ratio. The radioactivity of the studied pegmatite is related to a) uranium mineral (autunite), b) thorium minerals (thorite, thorianite and uranothorite) and c)