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

Sláma, Jiří; Klomínský, Josef

doi:10.3140/zpravy.geol.2019.20

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“…The high similarity of REE and trace element composition of the most frequent population of 5.6 Ma old Jizerka zircon xenocrysts [27] and the zircon inclusion in blue sapphire (Table 1) may indicate their similar lithospheric source within magmatic activity of the Jizerka area. Small or absent Eu anomalies may characterize mantle-derived 5.6 Ma old zircons and zircon inclusion in Jizerka sapphire [56].…”

Section: Discussionmentioning

confidence: 99%

“…The mineral xenocrysts found in the Jizerka gemstone placer were most likely transported to the surface by repeated explosions of alkali basalt magmas between ca 30 and 5.6 Ma [27]. This transport may be preferentially linked to the buried Pytlácká jáma Pit and the Bukovec volcanic diatremes near the Jizerka placer (Figure 2).…”

Section: Discussionmentioning

confidence: 99%

“…The largest inclusion of the magmatic melt, 46 µm in diameter, was located in the niobian rutile (Figure 4d). Such syngenetic inclusions demonstrate that the sapphire crystallised in an environment enriched with incompatible trace elements and volatiles [3,27,37,38].…”

Section: Jizerka Blue Sapphirementioning

confidence: 98%

“…The major group of zircons from Jizerka placer with ages between ca. 27 and 35 Ma corresponds to the age of Neovolcanic activity in the Lusatian Volcanic Field [23,27]. Because none of the minerals typical of the gem placer has been found within the volcanic rocks cropping out in the near vicinity of the Jizerská louka Meadow, the buried Pytlácká jáma diatreme is considered the most probable source of the sapphire mineral deposits in the Jizerka Rivulet [14,22].…”

Section: The Study Areamentioning

confidence: 99%

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

Klomínský

Sláma

2023

Minerals

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

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%