Differentiation of ironstone types by using rare earth elements and yttrium geochemistry – A case study from the Bahariya region, Egypt

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This paper deals with the geological and geochemical characterization of ironstone deposits encountered in two different sedimentary successions (upper Cretaceous and lower Cenozoic) in northern Bahariya, Egypt. The ironstones occur as uneconomic thin bands, lenses and concretionary beds in Cenomanian clastic rocks of the Bahariya Formation and as economic iron ore associated with Eocene carbonate depositional units. The ironstones contain similar iron-bearing minerals, mainly goethite and hematite, which display a variety of fabrics, i.e. concretionary, massive, stromatolitic-like, oolitic, pisolitic, reniform aggregates, boxwork, liesegang, geode-like and brecciated. The iron-rich minerals preferentially replaced and/or cemented the primary and diagenetic (mainly dolomite) carbonates. Preservation of fabrics, sedimentary structures and thickness of the precursor carbonates is conspicuous. Whole-rock composition of the Cretaceous ironstones shows lower Fe2O3 and MnO contents and relative enrichment in detrital-derived elements, namely Al, Zr and Nb, when compared with the Eocene ironstones, the latter showing enrichment in Fe, Mn, Si, and Ba oxides as well as Cu, Zn, Ni and Sr trace elements. Total REE content in the Cretaceous ironstones ranges widely from 70 to 348 ppm whilst the Eocene ironstones show quite low content, mostly from 1.96 to 31 ppm. Post Archean Australian Shale (PAAS)-normalized patterns of the upper Cretaceous ironstones display flat REE patterns, with small positive Eu and Pr, slightly negative Y and Ce anomalies which are close to unity and intermediate Nd concentrations (between 10 and 100 ppm). In contrast, PAAS-normalized REE + Y patterns of the Eocene ironstones display LREE positive slope with enriched flat HREE trend as well as negative Ce anomaly, positive Y and Eu anomalies, and low Nd concentration (in general less than 10 ppm). Geochemical data along with sedimentary features of the upper Cretaceous and Eocene ironstones in Bahariya point to different origins in the two ironstone types, the former having been originated diagenetically whereas the latter were constrained by hydrothermal fluids. A syngenetic marine origin is ruled out for any of the studied ironstones. Higher amount of detrital derived elements as well as higher REE concentration in the upper Cretaceous ironstones than in the Eocene ironstones suggest some contribution from the associated clastic sediments. The negative Ce anomaly determined in both types indicates anoxic iron-rich solution that passed to oxidized surface. The presence of positive Y and Eu anomalies in the Eocene ironstones suggests that iron precipitation was favoured by hydrothermal reducing conditions, when slightly acidic fluids reached oxidizing alkaline waters.
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