Mineralogical and whole-rock geochemical characterization of cherts from Gebel EL-Khashab (Oligocene), East Cairo, Egypt: evidence for silica sources

Chert, a primarily silica-bearing sedimentary rock, is often found in limited layers within the Earth's lithosphere. Optical microscopy, X-ray diffraction, major and trace element analyses were used to study chert samples from Gebel El-Khashab—an Oligocene formation consisting of fluvial sands and chert gravels together with silicified wood fragments and tree trunks in east Cairo, Egypt. The chert samples are quartz ± goethite ± moganite, with no evidence of an opal precursor. They are characterized by a variety of silica fabrics, comprising microcrystalline quartz, megaquartz, and chalcedonic quartz. The microcrystalline quartz is most common and likely formed inorganically by direct precipitation from solution. Several trace components are recognized in microcrystalline fabrics, including: (1) calcite, (2) detrital quartz and zircon, (3) echinoderms and other unidentified fossils, (4) organic textures, and (5) organic-rich structures that are potential fossil morphologies. The preservation of echinoderm fossils and organic morphologies point to primary deposition of silica. The chert samples studied here averaged 95 wt.% SiO 2 , 2 wt.% Fe 2 O 3T , 0.5 wt.% CaO, 0.11 wt.% MnO, 0.1 wt.% Al 2 O 3 , 0.03 wt.% MgO, 0.02 wt.% TiO 2 , 0.02 wt.% K 2 O, and 0.03 wt.% Na 2 O. The low total rare earth element content (3 ppm), high Eu anomalies, low Y/Ho values and the discrimination diagrams Al–Fe–Mn, Fe/Ti–Al/(Al + Fe + Mn), and Al 2 O 3 /TiO 2 –Al/(Al + Fe + Mn) indicate the hydrothermal origin of silica in these cherts. The hydrothermal silica-derived cherts were deposited close to the mid-oceanic ridge, as indicated by the Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3T ) and the Fe 2 O 3T /TiO 2 –Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3T ) association diagram. The chert deposits were transported in Oligocene to their current setting. The positive Ce anomalies of some chert samples point to an influence of a terrestrial material source.