Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Isotopic age of zircon from the gemstone assemblage with sapphire from the Jizerka fluvial sediments in the Jizerské hory Mts., Czech Republic

 

Jiří Sláma, Josef Klomínský

Geoscience Research Reports 52, 2019, pages 85–87
Map sheets: Harrachov (03-23)

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Published online: 10 May 2019

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Abstract

The Jizerka alluvial placer in the Jizerské hory Mts. in North Bohemia is the best-known occurrence of gemstones in the Czech Republic, and the only one that bears significant amount of jewel quality sapphire. Besides the most common Mg-Fe3+-rich ilmenite (so called iserine), the less frequently occurring gemstones, namely zircon (jewel variety hyacinth), corundum (varieties sapphire and ruby) and spinel have been extracted in the past for jewellery purposes. The placer that has been exploited since 16th century is represented by Quaternary fluvial sediments of the Jizerka brook (summary in Rous et al. 2019). The primary host rock of the gemstones and iserine was not known till now but it is most probably represented by the recently identified volcanic diatreme structure of Pytlácká jáma located ca. 2 km from the Jizerka brook (Rous et al. 2019). In order to constrain the origin and age of the Jizerka fluvial assemblage, zircon has been studied using the techniques of LA-ICP-MS. Out of the 19 zircon grains that have been dated using the U-Pb in-situ method, 13 grains (ca. 68%) provided a uniform age with 206Pb/238U weighted mean of 5.64 ± 0.09 Ma (Fig. 1). The other six dated grains yielded various ages ranging between 24.8 and 46.4 Ma. The young zircons (ca. 5.64 Ma) have all well-developed magmatic oscillatory or sector zonation in CL (Fig. 2), occasionally with signs of crystal face resorption, diffusion or fir-tree textures. Such features together with indications of zircon surface magmatic corrosion point towards the presumed mantle origin of these zircon megacrysts, and the other gemstones of the placer. The late Miocene (Messinian) age of zircon megacrysts is unique within the North-Bohemian Cenozoic volcanic province as well as among the other sapphire-zircon occurrences in Europe. The origin of sapphires, accompanying gemstones and other heavy minerals in alluvial placer of Jizerka are most probably connected with very young volcanic event (approximately 5.64 Ma) - likely an extremely fast ascent of alkali basalts and formation of explosive diatreme maar structure with later deposition of material in colluvial and fluvial sediments in the nearby areas (Rous et al. 2019).
 

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