Rare metal paragenesis in the serpentinites of Wadi Nugrus-Wadi Sikait area, South Eastern Desert, Egypt: Mineralogical and tectonic implications

Abstract

In the south of the Eastern Desert in Egypt, lies The Wadi Nugrus - Wadi Sikait region in the (ANS) Northern Arabian-Nubian Shield, at 90 km southwest of Marsa Alam City on the Red Sea coastline. This area comprises arc assemblages and ophiolite rocks, interspersed with ophiolitic mélange formations. The ultramafic rocks are extensively serpentinized, with some areas transformed into talc-carbonate rocks. These serpentinites locally include base metals (primary base metals as cuprite, cassiterite, bismite, native Ni, nickeline, heazlewoodite, pyrite, bravoite), precious metals (gold and argentite), and secondary metals willemite, wolframite and mimetite), of anomalous concentration.

Geochemically, the serpentinite rocks exhibit low SiO₂ content, high MgO content, and very low Al₂O₃ content. Furthermore, when plotted on the SiO₂–Fe₂O₃–MgO triangular diagram, it is evident that all ultramafic rocks from W. Sikait and Nugrus display compositions characteristic of harzburgite. They are depleted in Rb, Ba, Zr, Y, Sr, and Ti, while the Rb, Nb, K, and P elements in Nugrus, show concentrations around unity relative to primitive mantle. Conversely, Cs, U, Th, and Pb display enrichment relative to primitive mantle.

The heightened levels of chromium (Cr) and nickel (Ni) within serpentinite rocks are indicative of their ultramafic origins. Furthermore, the notable abundance of (LILE) large ion lithophile elements is evidenced by Ba/Th ratios varying from 21.9 to 26.7 in W. Sikait and W. Nugrus, respectively.