Petrology and geochemistry of the banded iron formation (BIF) of Wadi Karim and Um Anab, Eastern Desert, Egypt: Implications for the origin of Neoproterozoic BIF

Abstract

Banded iron formation (BIF) is exposed among the Precambrian rocks in the Wadi Karim and Um Anab areas in the Eastern Desert of Egypt. The BIF conformably alternates with Neoproterozoic arc metavolcanic rocks, which comprise metabasalts and mafic to intermediate metapyroclastic rocks. The BIF of Wadi Karim belongs to the oxide and mixed carbonate-oxide facies, while that of Um Anab belongs to the oxide facies only. Iron bands are generally composed of iron-rich mesobands rhythmically alternating with jasper, chert or carbonate mesobands as in the Wadi Karim area, or chert mesobands as in the Um Anab area. The BIF of Wadi Karim is composed essentially of magnetite, hematite and microcrystalline quartz, in addition to ankerite in the carbonate-bearing bands, while that of Um Anab is composed of magnetite and microcrystalline quartz. The positive correlation between Al 2O 3 and TiO 2 in the studied BIFs indicates that these chemical sediments incorporate minor detrital components. The limited areal extent of the studied BIF and its association with metavolcanic rocks are features of Algoma-type BIF of Archean greenstone belts. The spatial and temporal association between the BIF and the arc metavolcanic rocks indicates a genetic relationship between volcanic activity and BIF. The REY patterns of the studied BIFs suggest their precipitation after mixing of reduced bottom water carrying hydrothermal component with oxidized seawater in variable proportions. The absence of strong positive Eu anomaly, which characterizes BIF of Archean greenstone belts, in the BIF of the Eastern Desert may be attributed to contribution of Fe and Si through low-temperature hydrothermal solutions and to a high oxygen level of the atmosphere in the Neoproterozoic. It is proposed that the deposition occurred in an oceanic island arc setting, probably intra-arc or back-arc basins. In contrast with the majority of the Neoproterozoic iron formations (IFs), the studied BIFs are intimately associated with volcanic rocks and show no direct evidence of forming in response to glaciation. Consequently we attribute their formation to volcanic activity accompanying the break up of Rodinia rather than to snowball Earth condition. Thus, the Neoproterozoic IFs can be subdivided into Algoma and Rapitan-types. The Algoma-type, of volcanic association, is represented by African and Arabian Shield IFs, whereas the Rapitan-type, of glacial association, comprises deposits from all continents. The Wadi Karim and Um Anab BIFs constitute an additional evidence for the widespread return of IFs in the Cryogenian and Ediacaran after disappearance for ∼1 Ga reflecting the recurrence of anoxic ferruginous conditions in the Neoproterozoic deep sea.