Geological and Mineralogical Studies on the Microbially-mediated Ironstone Facies, El Bahariya Depression, Western Desert, Egypt

Abstract

The present study deals with the sedimentology, mineralogy and geochemistry of the Lutetian ironstones of the northeastern part of the Bahariya Depression, Western Desert, Egypt. This study also focuses on the microbial role in the precipitation of these ironstone deposits. The ironstone succession, under consideration, rests unconformably on three latertitized and structurally-controlled Cenomanian submarine paleohighs (e.g. El Gedida, El Harra and Ghorabi mine areas).

The ironstone succession is composed of autochthonous/para-autochthonous facies rich in ferriferous ooids and oncoids and ferruginized skeletal particles. The facies assemblages are stacked in two main shallowing-upward ironstone sequences, separated by an intra-Lutetian unconformity. The lower ironstone sequence shows great lateral variations in both facies and thicknesses. Each sequence starts with lagoonal bioturbated black hematitic mud-ironstone or tidal flat yellow goethitic stromatolitic ironstones that represent deposition from suspension in low energy water conditions. These facies grade upward into shoals/megarippled pack-to grain-ironstones, representing deposition during agitated tidal conditions and/or storm hydrodynamic forces. Lateritic iron ore was formed as a result of subaerial weathering processes and meteoric water alterations, and demarcates the upper surfaces of the ironstone sequences.

Comprehensive mineralogical analyses and interpretations, based on XRD, SEM and

-Raman spectroscopy, are carried out. The ferriferous ooids and oncoids consist mainly of amorphous iron oxyhydroxides and goethite. The iron oxyhydroxides appear to be precipitated at circum-neutral pH as ferric gel on the EPS surfaces of the bacterial and cyanobacterial cells and filaments. During the early diagenesis, goethite recrystallized from the ferric gel precursor. Early diagenetic apatite forms discrete bands with the cortical laminae. Goethite and its precursor iron oxyhydroxides precipitated in oxidizing conditions as indicated by the abundance of benthic fauna at the sediment­ water interface and the iron isotope ratios. In this oxic environment, most of the organic materials are degraded and few amounts are preserved, probably adsorbed or hidden within the iron oxyhydroxides. Varieties of organic matter are detected by Raman spectroscopy like carbohydrates, lipid, proteins and carotenoids. The original