Shear zone-hosted gold mineralization in the Late Proterozoic rocks of El Beida area, south Eastern Desert, Egypt

Abstract

The El Beida area of the Eastern Desert, Egypt, comprises a wide variety of Late Proterozoic rocks. It is occupied by ophiolitic serpentinites, metagabbros and pillow metabasalts, an island-arc assemblage of metasediments and metavolcanics, and syntectonic granitoids. The serpentinites occur as a small, NNW stretched slice obducted over the metavolcanics along a carbonatized shear zone. The obducted rocks are characterized by development of talc–carbonates, magnesite pockets and quartz–carbonate veinlets. The El Beida area is dissected by another northwest thrust fault that runs through the pillow metabasalts and metavolcanics. Quartz veins and hypogene alteration processes (i.e. silicification and chloritization) are well developed within the thrusting plane, forming a diverse shear zone. The El Beida gold deposit is localized in two shear zones (northwest–southeast) confined to the serpentinite and pillow metabasalts. Gold mineralization is associated mainly with the early (D1) deformation event in the area. The shear zones were affected by two stages of influx of hydrothermal fluids during plate destruction of the Late Proterozoic rocks. Mineralogically, the shear zones contain chromite, magnetite, hematite, ilmenite, pyrite, chalcopyrite, goethite, limonite, covellite, bornite and gold. The carbonatized shear zone (SiO2<50%) within the obducted area of the serpentinite rocks has a very low Au content (0.24–0.45 ppm). The silicified shear zone of the pillow metabasalt rocks, however, is richer in SiO2(>80%) and contains up to 45 ppm Au. Gold is observed as fine grains (ca 0.3 mm) and minute blebs (20-0.5 µm) intergrown with quartz or as fine, irregular specks associated with hematite and malachite. Gold is also found as minute, irregular specks (ca 10 µm) remobilized from the altered hematite. Analyses of gold specks in the quartz veins and hematite of the silicified shear zone gave compositions of 92 and 94% Au, respectively.