Petrological and geochemical characteristics of Egyptian banded iron formations: review and new data from Wadi Kareim

Abstract

The banded iron formations in the eastern desert of Egypt are small, deformed, bodies intercalated with metamorphosed Neoproterozoic volcaniclastic rocks. Although the 13 banded iron deposits have their own mineralogical, chemical, and textural characteristics, they have many similarities, the most notable of which are the lack of sulphide and paucity of carbonate facies minerals, a higher abundance of magnetite over hematite in the oxide facies, and a well-developed banding/lamination. Compared to Algoma, Superior, and Rapitan type banded iron ores, the Egyptian deposits have very high Fe/Si ratios, high Al 2 O 3 content, and HREE-enriched patterns. The absence of wave-generated structures in most of the Egyptian deposits indicates sub-aqueous precipitation below wave base, whereas their intercalation with poorly sorted volcaniclastic rocks with angular clasts suggests a depositional environment proximal to epiclastic influx. The Egyptian deposits likely formed in small fore-arc and back-arc basins through the precipitation of Fe silicate gels under slightly euxinic conditions. Iron and silica were supplied through submarine hydrothermal vents, whereas the low oxidation states were likely maintained in these basins through inhibition of growth of photosynthetic organisms. Diagenetic changes formed magnetite, quartz and other silicates from the precipitated gels. During the Pan-African orogeny, the ore bodies were deformed, metamorphosed, and accreted to the African continent. Localized hydrothermal activity increased Fe/Si ratios.