MINERALOGY, GEOCHEMISTRY AND GENESIS OF BANDED IRON-FORMATIONS, EASTERN DESERT, EGYPT

Banded iron formations (BIF) are intercalated with Neoproterozoic volcanosedimentary units in 13 localities in the Egyptian Central Eastern Desert. The BIF and host rocks were strongly deformed and metamorphosed under greenschist to epidote amphibolite facies conditions. The BIF exhibits rhythmic banding in which magnetite and hematite-rich layers alternate with quartz-rich layers with chlorite, epidote, ± garnet, calcite, stilpnomelane, actinolite. Magnetite is usually the dominant ore mineral.

Geochemically, Egyptian BIFs have higher Fe/Si compared to Algoma, Superior, or Rapitan BIF types, with altered samples showing highest Fe/Si values. All 13 BIFs have rare earth element patterns similar to modern day oceanic water, with a few displaying a weak positive Eu anomaly. Some deposits are also characterized by high Al and P, with SiO₂/Al₂O₃ < 20, all of which suggest a hydrogenous origin with a detrital component. Host metavolcanic and metavolcaniclastic rocks show chemical signatures indicative of an immature oceanic arc setting with MORB affinities for the southern bodies and back- or fore-arc basin affinities for the northern localities.

Although the Neoproterozoic age of the Egyptian BIF is similar to Rapitan type BIFs, their size and general characteristics indicate a volcanogenic Algoma type origin before being thrust onto the continental crust during the Pan African Orogeny. The nature and geochemical composition of the Egyptian BIF deposits and their host rocks suggest formation in back- and fore-arc basins surrounding an immature island arc. Restricted circulation of hydrothermal fluids in fore- and back-arc basins concomitant with arc volcanism increased Fe⁺² and Si in solution. During periods of arc quiescence, oxidation of Fe²⁺ led to deposition of iron hydroxide or hydromagnetite. The formation of authigenic magnetite instead of hematite indicates subaqueous precipitation under relatively reducing conditions, likely in sloped or terraced silled basins along the deep edges of fore- and back- arc basin shelves. Submarine diagenesis and subsequent metamorphism formed porphyroblastic magnetite. Martitization of magnetite was for the most part post-metamorphic, and likely related to hydrothermal alteration that post-dated ophiolite and BIF emplacement on the island arc.