Occurrence of dendritic agate from Dereyalak village (Eskişehir) – NW of Turkey and its relationship to sepiolite nodules in the region

•The Dereyalak dendritic agate nodules occur in the Pliocene polymictic conglomerates.•The main mineral constituents of the agates is alpha quartz, opal-CT, opal-C and moganite.•The agates could have formed by sepiolite replacement by low-T silica-rich hydrothermal solutions.

This study presents the geology and mineralogy of the agate nodules that occur in the Pliocene polymictic conglomerates, northwest of Dereyalak village, Eskişehir, northwest Turkey, and their relationship to sepiolite nodules. The host-rock of the agates is a layer 5–25 m thick with an outcrop length of more than 1.5 km. This host rock consists of well-rounded pebbles, mainly composed of opal, ultramafics, sandstones, limestones and volcanics. The conglomerates unconformable overlie an Upper Cretaceous ophiolite complex. This tectonic contact, which trends E–W, is accompanied by another NE–SW-trending major fault. Most of the agate nodules are present along these fault zones.The mainly white and black agate nodules range from 5 to 30 cm in diameter and show a zoned macrostructure. The transparent core is surrounded by a translucent white zone, which, in turn, is surrounded by a zone rich in black dendrites. An opal-rich zone is present nearest the outermost rim. XRD analyses revealed alpha quartz (chalcedony), opal-CT (pseudocrystalline cristobalite), opal-C (pseudocrystalline tridymite) and moganite as the main mineral constituents of the agates. Under polarised light, the predominance of microcrystalline, fibrous quartz (“chalcedony”) is observed. The black colour and fabric of the third, outer zone is due to the presence of black pyrolusite and manganite dendrites. SEM images also show different textures in agates that confirm the presence of a zoned structure in the dendritic agates.Nodular sepiolite (Meerschaum) is well known and has been exploited for centuries in Eskişehir region. Major sepiolite nodule deposits are mainly observed northeast and east of the Dereyalak region. The field observations and the mineralogical data, including comparison between the internal texture of agates and sepiolite nodules and additional geochemical analyses, suggest that the Dereyalak dendritic agates could have formed by sepiolite replacement by low-T silica-rich hydrothermal solutions that most likely circulated along the fracture systems.