ORIGIN OF NEOPROTEROZOIC BANDED IRON FORMATIONS FROM EGYPT: GEOCHRONOLOGICAL, PETROLOGICAL AND GEOCHEMICAL CONSTRAINTS

Banded iron formations (BIFs) occur in 13 localities in the eastern desert of Egypt as rhythmically layered bands intercalated with siltstones, calcareous mudstones, tuffs and lava flows constituting part of the ophiolitic – island arc Pan-African succession. These units are locally intruded by multiple generations of basic, intermediate, and acidic sills, dikes, and plugs. The BIFs, host rocks, and some intrusions were subjected to as many as 3 deformational events, and were regionally metamorphosed under peak conditions of 460 ± 50°C, 5 ± 2 kbar. All BIF deposits are dominated by oxide and silicate facies with minor carbonates and rare sulfides. Most deposits have unusually high but variable Fe/Si; and Fe³⁺/Fe²⁺, allowing for their subdivision into “fresh” (Fe²⁺/Fe³⁺ <0.1) and altered (Fe²⁺/Fe³⁺> 0.1) BIFs. All deposits are characterized by NASC – normalized, heavy REE – enriched patterns.

CL imaging of 16 zircons from 4 samples from Um Nar, Gebel Hadeed (fresh) and Wadi Kareim (altered) BIFs shows complex oscillatory zoning with some hydrothermal/ metamorphic overgrowths. LA-ICP-MS isotopic analyses of 3 crystals from Um Nar metasediments yield 721 ± 35 Ma for cores and 696 ± 9.5 Ma for rims. Analyses of a crystal from an acidic intrusive yield a mean age of 563 ± 9.6 Ma. Multiple points from 1 crystal from a metapyroclastic from Gebel Hadeed yield an age of 718 ± 10 Ma. Analyses obtained on 10 crystals from a metapyroclastic from Wadi Kareim yield 3 age populations of 817 ± 5, 770 ± 5, and 721 ± 4.5 Ma. A single crystal from the same rock yielded an age population of 1767 ± 10 Ma. These results suggest that the BIFs formed by hydrothermal activity from off axis submarine vents in two pulses (770 and 720 Ma) during a protracted event (850 – 700 Ma) of oceanic crust generation in a back-arc basin. Concomitant arc volcanism supplied the basin with pyroclastic material that imposed suboxic conditions by inhibiting growth of photosynthetic organisms. Hydrothermal alteration likely occurred at ~ 690 Ma, resulting in Ca metasomatism and/or Fe-enrichment. During the Pan-African orogeny, the ophiolitic material was emplaced onto the margin, leading to deformation and metamorphism of the ore bodies. Intrusion of syntectonic granites and gabbros led to localized contact metamorphism and more hydrothermal alteration ~ 640 – 590 Ma.