gems, zircon, Saxony, Germany, locality, species

Zircons from Tertiary alkali basalts in SE Saxony, Germany, were studied using electron microprobe, morphology, and inclusion analysis to determine their origin, revealing two primary alkaline-related types from the Seufzergründel placer: (I) gem-quality, honey-coloured, drop-like zircons with low trace element content, and (II) red-brown to grey, non-transparent zircons with chaotic zoning and high trace element content. A third zircon type, rich in trace elements and with clear zoning, likely derives from calc-alkaline granite. Inclusions of SO₃-rich apatite and magnesiokatophoritic amphibole support a syenitic or nepheline-syenitic source for the alkaline types. Thorite and uraninite inclusions are interpreted as exsolutions from metastable zircon–thorite solid solutions, with Th–U–Pb chemical ages yielding an “exsolution age” of 16 ± 8 to 39 ± 4 Ma. Ti-in-zircon thermometry indicates formation temperatures around 800 ± 50 °C. Zircons from related Upper Lusatian basaltic and phonolitic rocks display similar characteristics but more compositional variability. The majority of these zircons are attributed to Tertiary rift-related alkaline magmatism centered in the Eger (Ohře) Graben of North Bohemia.