Mineralogy and geochemistry of metasomatized mantle peridotites from the Eastern Desert of Egypt: The role of granite-related hydrothermal fluids in gold mineralizations

Abstract

Metasomatized peridotites from three areas (Barramiya, Fawakhir and Um Halham) in the Eastern Desert of Egypt are geologically, petrographically and geochemically investigated. These metasomatized peridotites are members of supra-subduction zone (SSZ) ophiolites and are localized along shear zones in contact with late intrusive granitoids at Barramiya and Fawakhir and form lenses without structural control at Um Halham. The intensity of alteration, rock assemblage and mineralogical composition in the three areas are basically similar. A wide-range of SiO2, MgO and CaO and significant enrichments in S and As characterize samples from Barramiya and Fawakhir. Gold contents of the metasomatized samples from Barramiya and Fawakhir (up to 3.922 ppm and 0.093 ppm respectively) are higher than Mariana forearc serpentinites. The formation of gold mineralizations along shear zones between serpentinites and granitoids appears to be highly dependent upon three variables: a) the initial gold content of the peridotite protolith, b) the source and composition of the hydrothermal fluids and c) the intensity and duration of fluid-rock interaction. The metasomatic alteration of mantle peridotites before or during ophiolite obduction into the continental crust could not be assigned to significant gold mineralization. Gold deposits occur only where metasomatic alteration of mantle peridotites is associated with percolation of As-S-rich hydrothermal fluids at shallow crustal levels.