Not all Neoproterozoic iron formations are glaciogenic: Sturtian-aged non-Rapitan exhalative iron formations from the Arabian-Nubian Shield

Neoproterozoic iron formations are exposed in the Wadi Hamama area (Egypt) in the northwestern part of the Arabian-Nubian Shield. Mafic and felsic volcanic and volcaniclastic rocks of an intra-oceanic island-arc setting host multiple, thin iron-formation units. Major element compositions of the iron formation confirm a low detrital input, whereas the rare-earth elements and Y data suggest deposition related to an influx of low-temperature hydrothermal fluids. Unlike most Neoproterozoic banded iron formations, but similar to other iron-formation occurrences from the Arabian-Nubian Shield, the Nd isotopic compositions of the Wadi Hamama iron formations are predominantly mantle-like. SIMS U-Pb zircon ages of the host volcaniclastic units indicate that the age of iron-formation deposition is ca. 695 Ma, which is within the Sturtian epoch that is presumed to be a glacial event of global extent. Nevertheless, there is no robust evidence of any influence of Sturtian glaciation in the Arabian-Nubian Shield. Our results rather suggest that the iron formations in the area may have formed as low-temperature exhalites on the floor of an island-arc basin. The iron formations were deposited during periods of volcanic quiescence, with metals having been derived during low-temperature pervasive hydrothermal alteration of volcanic and volcaniclastic rocks exposed at the seafloor-seawater interface. Precipitation took place due to mixing of metal-bearing hydrothermal fluids and cold, oxygenated seawater. There is no need to invoke possible effects of global glaciation to explain the origin of the Sturtian-aged iron formations in the shield. Our study thus suggests that not all Neoproterozoic iron formations are necessarily linked to glacial events as the Hamama deposit represents a non-Rapitan exhalative iron formation.