Geochemistry of banded iron formations and their host rocks from the Central Eastern Desert of Egypt: A working genetic model and tectonic implications

Abstract

Banded iron formations (BIF) occur intercalated with Neoproterozoic volcanosedimentary units in 13 localities in the Egyptian Central Eastern Desert (CED). These units are characterized by magnetite and hematite-rich layers alternating with quartz-rich layers containing andradite–rich garnet, epidote, ± calcite in the southern parts of CED, and chlorite ± stilpnomelane, and calcite in the north. Localized hydrothermal alteration affected most deposits and is manifested by secondary Ca–bearing minerals. All BIFs and their host rocks were strongly deformed and metamorphosed under greenschist to epidote amphibolite facies conditions during the collisional stage of the Pan African orogeny.Geochemically, CED BIFs have higher Fe/Si compared to Algoma, Superior, or Rapitan BIF types. All BIFs have rare earth element – Y patterns similar to those of modern day oceanic water, with a few samples displaying a weak positive Eu anomaly. All BIFs have high SiO2/Al2O3 and Fe/Ti, and low Al/(Al + Fe + Mn), which suggest a hydrogenous origin with hydrothermal contributions and low detrital component. Geochemical trends, Y/Ho, and Pr/Yb values suggest deposition of Wadi El Dabbah BIF closest to land and Um Nar and Wadi Kareim closest to hydrothermal vents. Host metavolcanic and metavolcaniclastic rocks show chemical signatures indicative of an immature oceanic arc setting with MORB affinities for the southern areas and back- or fore-arc basin affinities for the northern localities.These results lead to the conclusion that CED BIFs and their host rocks formed in small intra-arc basins and sloped or terraced silled basins in the back- and fore-arc areas surrounding an immature island arc. Restricted circulation of hydrothermal fluids in these basins concomitant with arc volcanism increased Fe+2 and Si in solution. During periods of arc quiescence, oxidation of Fe2+ led to deposition of Fe-oxyhydroxide. Diagenesis formed fine-grained magnetite, whereas subsequent hydrothermal alteration and metamorphism formed porphyroblastic magnetite and specularite.