A NON-GLACIOGENIC ORIGIN FOR NEOPROTEROZOIC BANDED IRON FORMATIONS FROM EGYPT

Thirteen small bodies of Banded iron formations (BIFs) occur in the central eastern desert of Egypt as rhythmic bands intercalated with siltstones, calcareous mudstones, tuffs and agglomerates of the ophiolitic – island arc succession of the Arabo-Nubian Shield. All BIFs are dominated by oxide and silicate facies with minor carbonates, and are characterized by NASC – normalized, heavy REE – enriched patterns, similar to those of modern oceanic waters. Most deposits have unusually high Fe/Si and Fe³⁺/Fe²⁺. All BIFs and their host rocks were regionally metamorphosed, with peak conditions ranging from 580 ± 50°C, 5 ± 2 kbar at Um Nar in the south to greenschist facies conditions to the north. Multiple generations of basic to acidic magmas intruded these units.

CL images of 44 zircons from 7 samples from Um Nar, G Hadeed, Um Ghamis, and Wadi Kareim BIFs show complex oscillatory zoning with some hydrothermal/ metamorphic overgrowths. LA-ICP-MS isotopic analyses of 36 crystals from tuffaceous metasediments from Um Nar and Wadi Kareim yield several age populations of 1.1 – 2.2 Ga, 820 – 850 Ma, 721 ± 35 Ma, 630 Ma, 600 Ma, and 550 Ma, and a distinct age population of 670 – 696 Ma for some rim overgrowths. Two acidic intrusives yield ages of 645 ± 7 and 606 ± 24 Ma. These results suggest that the BIFs formed by hydrothermal activity from off axis submarine vents in several pulses during a protracted event (850 – 700 Ma) of oceanic crust generation in an inter-arc basin. Concomitant arc volcanism supplied the basin with pyroclastic material that included zircon xenocrysts from an Archean crust. This debris imposed suboxic conditions on the basin by inhibiting growth of photosynthetic organisms, leading to increased concentrations of soluble Fe²⁺. Recovery of biota during periods of arc quiescence led to oxidation of Fe²⁺ and deposition of several layers of BIF. Hydrothermal alteration related to serpentinzation of lithospheric mantle rocks likely occurred at ~ 690 Ma, resulting in Ca-metasomatism and/or Fe-enrichment of crustal rocks. During the Pan-African orogeny, the BIFs, tuffs and oceanic lithosphere were thrust northward onto the continental margin as ophiolitic material and were regionally metamorphosed. Intrusion of syntectonic granites and gabbros led to localized contact metamorphism and more hydrothermal alteration ~ 640 – 550 Ma.