GEOCHEMISTRY OF THE BANDED IRON FORMATIONS AND THEIR HOST ROCKS IN THE EASTERN DESERT OF EGYPT

Thirteen deposits of banded iron formations occur in an area extending over 30,000 km² in the central Eastern Desert of Egypt. The deposits most resemble Algoma-type iron formations, but have higher Fe/Si rations that vary from one deposit to another and are of Neoproterozoic age, whereas most Algoma type banded iron formations are Archean. Vatiations in the Fe/Si and Fe²⁺/Fe³⁺ from one deposit to another allow for their subdivision into “fresh” (Fe²⁺/Fe³⁺ > 0.1; Fe/Si > 3) and altered (Fe²⁺/Fe³⁺ < 0.1) BIFs. The banded iron formations are also interbedded with metasediments, as well as mafic to acidic metapyroclastics and metavolcanics, suggesting proximity to an active arc. All deposits are dominated by oxide and silicate facies; carbonates are minor, whereas sulfides are rare. Some deposits are characterized by the assemblage andradite-rich garnet-epidote-quartz-magnetite-hematite-calcite. Textures suggest that fine-grained Si-bearing magnetite ± hematite dust formed during diagenesis. The presence of abundant andraditic garnets and epidote suggests that the deposits underwent calcium-metasomatism. Both magnetite and hematite underwent grain coarsening/recrystallization during metamorphism. Abundant late veins of epidote and calcite suggest a second stage of calcium-metasomatism after the metamorphism. Supergene alteration resulted in the formation of goethite and lepidocrocite and an overall enrichment of Fe.

Whole rock geochemical data show that all the BIFs have REE patterns similar to that of ocean water. Major and trace element discriminant diagrams show that the host metavolcanics and metapyroclastics are calc-alkaline to tholeiitic, with N-MORB to island arc signatures. These data suggest that the BIFs were deposited by hydrothermal vent activity in some type of ocean basin close to an island or volcanic arc. Suboxic conditions necessary for BIF formation were maintained through the delivery of ash and dust to the small ocean basin. During the Pan-African orogeny accretion and obdution led to ophiolite emplacement and subsequent regional metamorphism under epidote-amphibolite facies conditions.