A NEODYMIUM PERSPECTIVE FOR THE NEOPROTEROZOIC BANDED IRON FORMATIONS FROM THE ARABIAN NUBIAN SHIELD – A SNOWBALL EARTH MANIFESTATION?

Neoproterozoic Banded Iron Formations (BIFs) of the Arabian Nubian Shield (ANS) are found in the northwestern Saudi Arabia and in the central eastern desert of Egypt. We focused our present study at the Sawawin district of Saudi Arabia and at the Wadi Kareim deposit of Egypt. In both the areas the sediments which host the BIFs are immature clastics that rest conformably on volcanic rocks; based on this association ANS-BIF is considered as Algoma type. These sediments also contain a diamictite which contains far-travelled clasts and is likely glaciogenic. ANS-BIF are oxide facies iron formations. SEM studies show that iron-rich layers are millimeter to centimeter thick bands of hematite and magnetite whereas the silica-rich layers are mostly chert with minor hematite. Apatite is a dispersed accessory mineral. Sawawin samples were collected from a 150m long adit whereas Kareim samples were collected from exposures. Based upon the cross cutting relationship the age of the ANS iron formations coincided approximately with the ~ 700 Ma Sturtian glaciation. Considering a “snowball earth” stage when continental input stopped, Fe inputs would be limited to seafloor hydrothermal vents. We analyzed the iron formation samples for the Nd isotopic compositions in order to identify the source of the iron in the BIFs. Considering the samples to be 700 Ma old the ε Nd values for 5 samples range between +2.8 and +3.4. The positive ε Nd value is consistent with a mostly hydrothermal source for the iron. The characteristics ε Nd value for the Arabian Nubian Shield at that time was +4 to +6 and the slightly lower ε Nd allows a significant input of Nd and presumably iron from the older continental crust into the marine tectonic framework from which the iron formations precipitated.