Ironstone deposits hosted in Eocene carbonates from Bahariya (Egypt)—New perspective on cherty ironstone occurrences A.M. Afify a,b, ⁎, M.E. Sanz-Montero a , J.P. Calvo a a Department of Petrology and Geochemistry, Faculty of Geological Sciences, Complutense University, Madrid, C/ José Antonio Nováis, 2, 28040 Madrid, Spain b Department of Geology, Faculty of Science, Benha University, 13518 Benha, Egypt abstract article info Article history: Received 3 August 2015 Received in revised form 17 September 2015 Accepted 18 September 2015 Available online 26 September 2015 Editor: Dr. B. Jones Keywords: Cherty ironstone Dolomitization Tectonic constraints Hydrothermalism Eocene carbonates Egypt This paper gives new insight into the genesis of cherty ironstone deposits. The research was centered on well-ex- posed, unique cherty ironstone mineralization associated with Eocene carbonates from the northern part of the Bahariya Depression (Egypt). The economically important ironstones occur in the Naqb Formation (Early Eo- cene), which is mainly formed of shallow marine carbonate deposits. Periods of lowstand sea-level caused exten- sive early dissolution (karstification) of the depositional carbonates and dolomitization associated with mixing zones of fresh and marine pore-water. In faulted areas, the Eocene carbonate deposits were transformed into cherty ironstone with preservation of the precursor carbonate sedimentary features, i.e. skeletal and non-skeletal grain types, thickness, bedding, lateral and vertical sequential arrangement, and karst profiles. The ore deposits are composed of iron oxyhydroxides, mainly hematite and goethite, chert in the form of micro- to macro-quartz and chalcedony, various manganese minerals, barite, and a number of subordinate sulfate and clay minerals. De- tailed petrographic analysis shows that quartz and iron oxides were coetaneous and selectively replaced carbon- ates, the coarse dolomite crystals having been preferentially transformed into quartz whereas the micro- crystalline carbonates were replaced by the iron oxyhydroxides. A number of petrographic, sedimentological and structural features including the presence of hydrothermal-me- diated minerals (e.g., jacobsite), the geochemistry of the ore minerals as well as the structure-controlled location of the mineralization suggest a hydrothermal source for the ore-bearing fluids circulating through major faults and reflect their proximity to centers of magmatism. The proposed formation model can contribute to better un- derstanding of the genetic mechanisms of formation of banded iron formations (BIFs) that were abundant during the Precambrian. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The Eocene strata in northern Bahariya contain ironstone deposits of economic significance, some of them reaching a large size (Fig. 1). Despite significant research on these deposits (El Shazly, 1962; El Akkad and Issawi, 1963; Basta and Amer, 1969, and references therein), their origin is still a matter of debate. More recent publications show different and contrasting hypotheses for the source and mechanisms of formation of the Bahariya ironstones. Dabous (2002) concluded that the Bahariya iron- stone deposits are not lateritic and that their formation was related to mixing of warm ascending groundwater leaching iron from the underly- ing Nubia aquifers and descending water with iron leached from the over- lying Upper Eocene–Lower Oligocene glauconitic clays. Salama et al. (2013, 2014) concluded that the Bahariya ironstones were deposited pri- marily in a marine setting and their formation was enhanced by microbial activity. They related the formation of the ironstone to global warming during the early Paleogene, closely associated with eustatic sea-level changes. Recent work by Baioumy et al. (2013, 2014) supports sources and mechanisms of iron and manganese formations that are contradicto- ry: supergenetic ore deposits (most probably from the Naqb limestone host rock) or hydrogenous iron mixed with iron of hydrothermal origin (sea water precipitation to hydrothermal exhalite). New insight based on field, petrographic, mineralogical and geo- chemical studies of the ironstone deposits in the northern part of the Bahariya Depression (Fig. 1) is provided in this paper. The similarities between the ironstones and carbonate host rocks as well as the close re- lationship between the ore mineral body and the regional tectonic structure led us to revisit the models proposed for the formation of iron- stone deposits of Bahariya. Moreover, the scarcity of Phanerozoic, in particular Cenozoic cherty ironstone makes the Bahariya ore deposits an interesting case study to investigate some new perspectives about the formation constraints of these kinds of rocks. It can help also in un- derstanding the mechanisms of older iron-rich deposits where chert is an important constituent. In the study area, iron is paired with quartz formation, this resulting in sedimentary structures that resemble Sedimentary Geology 329 (2015) 81–97 ⁎ Corresponding author at: Department of Petrology and Geochemistry, Faculty of Geological Sciences, Complutense University, Madrid, C/ José Antonio Nováis, 2, 28040 Madrid, Spain. E-mail address: adelmady@ucm.es (A.M. Afify). http://dx.doi.org/10.1016/j.sedgeo.2015.09.010 0037-0738/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo