Origin of Blue Sapphire in Newly Discovered Spinel–Chlorite–Muscovite Rocks within Meta-Ultramafites of Ilmen Mountains, South Urals of Russia: Evidence from Mineralogy, Geochemistry, Rb-Sr and Sm-Nd Isotopic Data

Abstract

Blue sapphire of gem quality was recently discovered in spinel–chlorite–muscovite rock within meta-ultramafites near the Ilmenogorsky alkaline complex in the Ilmen Mountains of the South Urals. More than 20 minerals were found in the assemblage with the blue sapphire. These sapphire-bearing rocks are enriched in LREE and depleted in HREE (with the negative Eu anomalies) with REE distribution similar to those in miascites (nepheline syenite) of the Ilmenogorsky alkaline complex. 87Sr/86Sr ratios in the sapphire-bearing rocks varied from 0.7088 ± 0.000004 (2σ) to 0.7106 ± 0.000006 (2σ): epsilon notation εNd is −7.8. The Rb-Sr isochrone age of 289 ± 9 Ma was yielded for the sapphire-bearing rocks and associated muscovite. The blue sapphires are translucent to transparent and they have substantial colorless zones. They occur in a matrix of clinochlore-muscovite as concentric aggregates within spinel-gahnite coronas. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses showed values with trace elements typical for “metamorphic” blue sapphires, with Ga/Mg < 2.7, Fe/Mg < 74, Cr/Ga > 1.5 (when Cr is detectable), and Fe/Ti < 9. Sapphires overlap “metasomatic” at “sapphires in alkali basalts” field on the FeO–Cr2O3–MgO–V2O3 versus FeO + TiO2 + Ga2O3 discriminant diagram. The sapphires formed together with the spinel-chlorite-muscovite rock during metasomatism at a contact of orthopyroxenites. Metasomatic fluids were enriched with Al, HSFE, and LILE and genetically linked to the miascite intrusions of Ilmenogorsky complex. The temperature required for the formation of sapphire–spinel–chlorite–muscovite rock was 700–750 °C and a pressure of 1.8–3.5 kbar.

Corundum Anorthosites-Kyshtymites from the South Urals, Russia: A Combined Mineralogical, Geochemical, and U-Pb Zircon Geochronological Study

“…The UV-Vis-NIR spectra of the studied sapphires are comparable to those observed on sapphires of metamorphic or metasomatic origin [33] and of the other two occurrences in the region linked with syenitic pegmatite [2] and metasomatites within meta-ultramafic host rocks [3].…”

“…Blue sapphires in kyshtymites (the 5th versta deposit and the larger occurrence called Borzovsky deposit [15][16][17]) are located at the western flank of Vishnevogorsky nepheline syenite (miascite)carbonatite alkaline complex of the South Urals with unique REE-mineralization [18] (Figure 1). These two deposits are accompanied by other primary sapphire occurrences in syenite pegmatites (mines 298 and 349) [2] and sapphires in metasomatites within meta-ultramafic host rocks (mine 418) ( Figure 1) [3]. Figure 1.…”

“…However, its blue gem-quality variety (i.e., sapphire) colored mainly by iron and titanium is rare and commonly found in secondary placers of debatable origin [1]. Recent studies of blue sapphires from different deposits worldwide demonstrate growing interest in their genesis due to findings of blue sapphires in situ within the primary rocks, for instance, those from alkali basaltic terrains [2][3][4][5][6][7][8].…”

Editorial for Special Issue “Mineralogy and Geochemistry of Gems”

“…A comparative study of ruby chemistry and inclusions between Myanmar and eastern Australia is presented by Sutherland et al [13] and clearly shows that although having formed in different parts of the world, at different times and under different tectonic settings, unusual Ga-rich rubies occur in both regions, indicating primary generation involving magmatic processes. Sorokina et al [14] focus on the genesis of gem-quality sapphires from the Ilmen Mountains (South Urals, Russia) found in situ within ultramafites. These sapphires were formed together with a spinel-chlorite-muscovite rock during the metasomatic alteration of orthopyroxenites at a temperature around 700-750 • C and pressure of about 1.8-3.5 kbar.…”

“…On the other hand, blue sapphires from the southern part of the Naxos and Ikaria Islands (both occurring in fissures within metabauxites hosted in marbles) display atypical magmatic signatures, indicating a likely hydrothermal origin. In these four papers [13][14][15][16], it is evident that some ratios used to separate gem corundum from different geological environments (e.g., Ga/Mg ratio [17]) should be applied with caution. They possibly need revision using solely data obtained from gem-quality samples, in addition, the data should be grouped carefully according to colour.…”