Mineralogical and geochemical investigations of the Middle Eocene ironstones, El Bahariya Depression, Western Desert, Egypt

The Middle Eocene ironstone succession is located in the northeastern part of El Bahariya Depression, Western Desert, Egypt This succession is subdivided into lower and upper sequences and consists of two main shallow marine ironstone facies associations. The first is a lagoonal manganiferous mud and fossiliferous ironstone facies association and consists mainly of goethite and hematite, detrital minerals (quartz, rutile, and feldspars), manganese minerals (todorokite, psilomelane, pyrolusite, birnessite, aurorite and manjiroite), and authigenic clay minerals (kaolinite and illite). The second is a peritidal microbially mediated stromatolitic and nummuliticooidal-oncoidal ironstone facies association consists of goethite, apatite, and secondary minerals that include quartz, jarosite, psilomelane, and pyrolusite. Organic materials such as proteinaceous compounds, lipids, cellulose, and carotenoids were detected in the cortices of the ferruginous ooids and oncoids. The marine ironstone facies were exposed to subaerial weathering and subsurface alteration processes. The weathering resulted in the formation of lateritic iron ores and paleosols during humid climatic periods. The lateritic iron ores consist mainly of colloform goethite, hematite and psilomelane. The identification of cavity-filling sulfate, nitrate, carbonate and silicate minerals in the marine ironstones and the lateritic iron ore may indicate more recent alteration under arid climatic conditions. The subsurface alteration is attributed to the oxidation of sulfides, primarily pyrite, and weathering of glauconitic clastic rocks in the underlying Cenomanian Bahariya Formation during the interaction with acidic heated groundwater. The formation of ferrous and ferric sulfate, and silicate minerals, and mobilization of trace metals are the products of the alteration process. Enrichments in Ba, Co, K. Pb and Sr are correlated with manganese oxides, whereas anomalous P, V, Cr, Ni, Zn, As, Mo, and U are correlated with iron oxyhydroxides. Crown Copyright (C) 2011 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. All rights reserved.