Jizerka Gemstone Placer—Possible Links to the Timing of Cenozoic Alkali Basalt Volcanism in Jizera Mountains, Czech Republic

The Jizerka Quaternary alluvial placer in the Czech Republic has been a well-known source of gemstones since 16th century, and only one Europe that yielded significant amount jewel-quality sapphire. Besides Mg-rich ilmenite (“iserine”), which is most common heavy mineral at locality, some other minerals have mined for jewellery purposes. These are corundum (sapphire ruby varieties), zircon (“hyacinth” gemstone variety) spinel. Here, we present detailed petrological geochronological investigation enigmatic relationship between sapphires their supposed host rocks, supporting xenogenetic link. Our hypothesis based on thermal resetting U–Pb isotopic age inclusion found inside blue sapphire to estimated time anticipated alkaline basalt intrusion. rocks zircon) not yet known, but could be related Cenozoic volcanism located near gem (Bukovec diatreme volcano, Pytlácká jáma Pit Hruškovy skály pipe). transport sapphire, surface was connected with serial volcanic events, likely fast ascent alkali basalts formation multi-explosive maar structures later deposition volcanoclastic material eluvial sediments nearby areas. All xenocrysts usually show traces magmatic corrosion textures, indicating disequilibrium transporting magma. In order constrain provenance assemblage, associated phases (niobian rutile, baddeleyite silicate melts) studied using LA–ICP–MS (laser ablation–inductively coupled plasma–mass spectrometry) geochemistry situ dating. Modification into by exposure temperature above 1400 °C basaltic melt accompanied resetting. A dated 31.2 ± 0.4 Ma, its rim 31 16 Ma. composition inclusions incorporated niobian rutile suggests parental rock syenite. Eocene late Miocene (Messinian) ages new findings within Eger Graben structure, as well among sapphire–zircon occurrences European Variscides. indicate provenience this assemblage from different unknown levels upper crust or lithospheric mantle.