Mechanism of Formation, Mineralogy and Geochemistry of the Ooidal Ironstone of Djebel Had, Northeast Algeria

The Djebel Had Ironstone (DHIS), an 8 m thick stratiform sedimentary iron formation, Stratigraphic, lithological, structural and metallogenic similarities, suggest the DHIS may extend further into southwestern Tunisia. We show that mi- neralization occurs as layers of ooidal ironstones and inter-laminated iron marl within mid-Eocene gypsiferous marls. The grains display a smooth outer surface bound by an argilo-ferruginous layer embedded in siliceous-calcite cement. They are unusually friable, crumbling at the slightest shock. A high total iron (FeT) content of 50.12%, is dominated by up to 71.06% iron hydroXide (FeO(OH). Much of the iron is present as goethite, a common feature of iron-rich ooids of North African origin. However, the lack of prominent chlorite minerals suggest the DHIS is not of a detrital origin. Instead, a negligible Ti and Al oXide concentration suggest a chemical provenance for the DHIS. The data suggest that ferruginous conditions developed in a potentially restricted/semi-restricted continental shelf margin where seafloor redoX was sensitive to the alternating cycles of sea level change. We propose a new mechanism for the formation of ooidal ironstones, associated with shelf surface water eutrophication, bottom water anoXia promoted by sea level rise and the weathering of iron phosphate-rich rocks.