International Geological Journal - Official Journal of the Carpathian-Balkan Geological Association

Mineralogy and genesis of sapphire in corundum-bearing xenoliths from the Miocene andesites in the Záhradné, Hubošovce and Vechec quarries in the Slanské vrchy Mountains (Slovakia)

Published: Apr 2024

Pages: 117 - 131



Abstract: Sapphires crystals were identified in the corundum-bearing xenoliths in Miocene (Upper to Middle Sarmathian) andesites of the Slanské vrchy Mountains, eastern Slovakia at the Záhradné, Hubošovce and Vechec localities. The sapphire crystals occur in (1) micaceous xenoliths, which are built mostly of dark mica from annite–phlogopite series (biotite), K-feldspars, plagioclase with abundant inclusions of hercynite, ilmenite and Ti-rich magnetite, locally with pyroxenes of enstatite-ferosilite series at the localities Záhradné and Hubošovce. Less abundant are sapphires occurring in cordieritic xenoliths (2), dominantly consisting of cordierite, plagioclase and sillimanite with minor hercynite and ilmenite inclusions discovered in the Vechec quarry. Sapphires are dark blue to light blue with vitreous to diamond lustre and no visible pleochroic colour change forming mostly pseudohexagonal tabular, locally more complex euhedral to subhedral crystals up to 2.0 mm in size with triangular-shaped patterns on crystal faces. Raman spectroscopy showed characteristic corundum peaks at 419 cm−1 A1g mode and 384 cm−1 Eg mode with other peaks assigned to Al2O3 crystal vibrations. Chemical composition (EPMA, LA-ICP-MS) shows typical content of 98.12–99.60 wt. % Al2O3 with increased concentrations of Fe, Ti, Cr, V, Mg and Ga, locally also Na, Ca, K, B and Li. Genesis of corundum-bearing xenoliths interpreted from paragenetic observations and geochemical data shows clear metamorphic trend. Formation of sapphires was caused by incorporation of Al-rich precursor metasediments depleted in silica into the magmatic reservoir, which caused thermal overprint of the precursor mineral assemblage and led to the formation of high temperature mineral association suitable for corundum crystallization. Furthermore, sapphire crystals from the Hubošovce quarry exhibit spinel coronas, which are typically developed from destabilization of corundum during their retrograde development.

Keywords: Slanské vrchy Mts., Záhradné, Hubošovce, Vechec, xenolith, corundum, sapphire

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