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

Elena Sorokina,Anatoliy Nikolaev,Maria Anosova,Sergey Nikandrov,Alina Somsikova,Stefanos Karampelas,Yuriy Kostitsyn,Mikhail Rassomakhin,Nataliya Kononkova,Vasiliy Kotlyarov

doi:10.3390/min9010036

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.

Highlights

Sapphire is referred to the blue color variety of corundum (Al2 O3 ) with Fe and Ti as main chromophores
Suggested Petrogenesis of Sapphire in the Meta-Ultramafic Host Rocks of the Ilmen Mountains studied blue sapphires surrounded by spinel coronas embedded
The studied blue sapphires surrounded by spinel coronas embedded in chlorite‐muscovite likely formed after metasomatic alteration of orthopyroxenites; i.e., containing matrix present metasomatites that are found within meta‐ultramafic host rocks

Summary

Introduction

Sapphire is referred to the blue color variety of corundum (Al2 O3 ) with Fe and Ti as main chromophores. Of various colors and qualities, occurs in numerous localities in the Ilmen Mountains (Figure 1a). The most studied gem quality sapphire, of the three, is associated with alkaline syenite pegmatites (Figure 1b, mines 298 and 349), first reported by Barbot de Marni (1828) [1]. These sapphires are genetically linked with the intrusions of miascites (a leucocratic variety of nepheline syenite discovered close to Miass in the Ilmen Mountains and named after the place of discovery)

Methods

Results

Discussion

Conclusion