Characterization of the Sündikendağı deposit of moganite-rich, blue chalcedony nodules, Mayıslar–Sarıcakaya (Eskişehir), Turkey

• Gem blue chalcedony in the Sündikendağı deposit, Sarıcakaya-Eskişehir region, Turkey.
• Unbanded chalcedony formed as epigenetic concretions during diagenesis of sandstone.
• Nodule-containing sandstone produces as much as 3 million tons of rough chalcedony.
• The mining and exporting of unbanded blue chalcedony nodules began in Roman times.

Blue chalcedony nodules have been mined from the Sündikendağı deposit in the Mayıslar–Sarıcakaya (Eskişehir) region of north-central Turkey since ancient times; however, no modern geological study of this deposit has yet been published. Although ancient and current mining production have both taken place in an area of complex geology, our study and analyses of the deposit suggests a simple model of sedimentary deposition for its origin. The repeated episodes of tectonic activity, accompanied by brittle deformation, metamorphism, and hydrothermal activity, which characterize this part of the Anatolian Peninsula with its complex junction of tectonic plates, appear to have had little influence on the blue chalcedony nodules that make the deposit valuable other than perhaps to influence their trace-element composition.The physical nature of the nodules as revealed by polarized-light microscopy and XRD—they are composed only of fibrous length-fast quartz (chalcedony) and fibrous length-slow quartz (moganite), but contain neither platy opal-CT nor opal-C—is consistent with a sedimentary origin as are their overall shape and strata-bound occurrence in a sandstone (arkose).The relatively high concentrations of some trace elements in the nodules revealed by ICP-AES, suggest involvement of hydrothermal fluids during the direct epigenetic formation of chalcedony concretions during diagenesis of the enclosing sandstone or by alteration of diagenetic concretions of another composition. Sources could include upwardly moving hydrothermal fluids entering the sedimentary basin from underlying older Sarıcakaya intrusive rocks or sea-floor hydrothermal vents in the vicinity during diagenesis in the Palaeocene and Eocene (65–37.8 Ma) periods.Oxygen isotope analyses (SMOW) (using EA-IRMS) of both the blue chalcedony nodules (δ18O = + 28.2‰ to + 30.8‰) and the enclosing sandstone (δ18O = + 11.3‰ to + 13.2‰) suggest that the nodules formed during diagenesis at a low temperature of around 55 °C, although they are encased in sandstone whose grains came from rocks that formed at significantly higher temperature, perhaps above 100 °C.The unbanded Sündikendağı chalcedony nodules are similar in occurrence to the banded Fairburn agates of South Dakota, USA and the Dryhead agates of Montana, USA, which formed in Palaeozoic limestones, except that the blue chalcedony is hosted in sandstone. Other sedimentary agates are generally believed to have formed by the alteration of diagenetic concretions from the outside, inward. No other agates or chalcedonies hosted in sandstone are known for comparison with this deposit. Thus, the deposit appears to be unique. It is possible that the Sündikendağı unbanded blue chalcedony formed as epigenetic concretions during diagenesis of the sandstone—a mechanism previously shown for large crystals of other minerals found in sandstones.