Mechanism of formation, mineralogy and geochemistry of the ooidal ironstone of Djebel Had, northeast Algeria

摘要: Abstract The Djebel Had Ironstone (DHIS), an 8 m thick stratiform sedimentary iron formation, forms part of the important mining district south Tebessa, in northeastern Algeria. Stratigraphic, lithological, structural and metallogenic similarities, suggest DHIS may extend further into southwestern Tunisia. We show that mineralization occurs as layers ooidal ironstones inter-laminated marl within mid-Eocene gypsiferous marls. more or less rounded 0.1–2.0 mm brownish-blackish ooids, are composed goethite, limonite, hematite, with traces magnetite piemontite. grains display a smooth outer surface bound by argilo-ferruginous layer embedded siliceous-calcite cement. They unusually friable, crumbling at slightest shock. A high total (FeT) content 50.12%, is dominated up to 71.06% hydroxide (FeO(OH). Much present common feature iron-rich ooids North African origin. However, lack prominent chlorite minerals not detrital Instead, negligible Ti Al oxide concentration chemical provenance for DHIS. data ferruginous conditions developed potentially restricted/semi-restricted continental shelf margin where seafloor redox was sensitive alternating cycles sea level change. propose new mechanism formation ironstones, associated water eutrophication, bottom anoxia promoted rise weathering phosphate-rich rocks. Phosphorus cerium enrichment, coupled reconstructed depositional sediment mineralogy, intense biomass production stimulated deoxygenation waters deposition beneath column.