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.

1. Introduction

Sapphire is referred to the blue color variety of corundum (Al₂O₃) with Fe and Ti as main chromophores. Other colored sapphires are called fancy and need the color prefix (e.g., pink sapphire, colorless sapphire), apart from the red variety, which is called ruby.

Corundum, of various colors and qualities, occurs in numerous localities in the Ilmen Mountains (Figure 1a). However, gem quality sapphires are found in solely to three of them, all in situ within the primary bearing rocks (See Figure 1b, mines 298, 349, and 418) [1,2,3,4,5,6,7]. Occurrences are located inside of the state’s Ilmen Nature Reserve, though their commercial exploitation is forbidden; samples can be collected just for research purposes in close collaboration with Nature Reserve’s researchers. 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).

Gem-quality sapphire mineralization also occurs within the Kyshtymsky stratum of the Ilmenogorsky complex [4]. This stratum is included in the Saitovsky series [5], one of the structural units of the Vishnevogorsky–Ilmenogorsky polymetamorphic zone. This zone is a fragment of regional post-collision shear, in which the Ilmenogorsky complex is the southern unit (see more on Regional geology of Ilmenogorsky complex in [6]) (Figure 1a,b). The series is composed of meta-terrigenous quartzite-schist strata, which include garnet-feldspar-biotite, quartz-mica, and garnet-feldspar-amphibole schists; quartzites and quartzite-gneisses; and, units with mafic–ultramafic affinities. Lenses and lenticular bodies within this association are made up of olivine-enstatite, enstatite, talc-anthophyllite, talc-carbonate, tremolite-anthophyllite rocks, and various amphibolites [7]. SHRIMP U-Pb zircon dating from these rocks reflects the complex evolution of the series: ~1.3 Ga (relict cores of terrigenous zircon); from about 460 to ~420 Ma (stage of metamorphic evolution), ~320 Ma, and ~280 Ma (stages of collisional deformation), for more information see [5]. The dimensions of the mafic–ultramafic units, which host the sapphires, the thickness of the schists and quartzites, and their structural relationships vary widely (Figure 1c).

Gem quality sapphires were found as aggregates within chlorite-phlogopite; hosted by meta-ultramafic rocks. They were discovered at a depth of approximately 300 m in a drill core situated at the southern part of the Ilmen State Reserve [2]. In 2003, one of the authors (Sergey N. Nikandrov) found gem-quality sapphire in the similar micaceous (commonly muscovite) association, located between the lakes Bolshoy Tatkul and Bolshoe Miassovo (mine 418, see Figure 1b). Pink sapphires and rubies were previously found in mafic–ultramafic rocks, but this is the first occurrence of sapphires within such a spinel–chlorite–muscovite metasomatic rock.

Sapphires of gem quality are rare and they are principally found in secondary placer deposits; these are found seldom in situ and only in a few types of primary host rocks [3]. Research on gem quality sapphires found in situ in primary rocks, e.g., as those recently discovered in spinel–chlorite–muscovite rock within meta-ultramafites near the Ilmenogorsky alkaline complex in the Ilmen Mountains of the South Urals, may provide clues to the unknown petrogenesis of those from secondary placer occurrences. The present article is the first complete characterization of these sapphires, focusing on their genesis. For this, local geology and petrography were studied as well as the mineralogy, chemistry, and isotopy of corundum–spinel–chlorite–mica rocks. The results are also compared to those that were acquired on other gem-quality sapphires found in the Ilmen Mountains (syenite pegmatites) and checked their possible linkage as well as with those from other deposits for better understanding the gem corundum genesis.

2. Materials and Methods

2.1. Sapphires

Five transparent to translucent colorless to blue corundum of gem quality and their solid inclusions from mine 418 were analyzed by the electron microprobe analyses (EMPA) at GEOKHI RAS using Cameca SX100 (CAMECA, Gennevilliers, France) and wavelength-dispersive (WDS) detection mode with 15 kV acceleration voltage, 30 nA beam current, and 3 µm beam size. Four studied sapphire samples present pronounced colorless and blue zones and one sample was completely colorless (Figure 2b,c and Table S1). A set of natural and synthetic reference materials was used for calibration and instrument-stability monitoring. An overlap correction was applied for V > Ti, Cr > V, Sc > Ta, and Nd > Ce. The detection limits for the measured elements varied for different analytical sessions and were less than 0.01 wt. %. Raman spectra were acquired to identify the inclusions inside the sapphires at the Gemological Institute of America laboratory (Carlsbad, CA, USA) using a Renishaw inVia Raman spectrometer combined with optical microscope. The system was coupled with Ar⁺ laser emitting at 514 nm with a power of 10 mW on the sample. All of the measurements were from 200 to 2000 cm⁻¹, with a 60 s acquisition time, three cycles, at a resolution of about 1.5 cm⁻¹, and under 50× magnification at a room temperature. Rayleigh scattering was blocked using a holographic notch filter. Backscattered light was dispersed using a grating with 1800 grooves/mm. The spectrometer was calibrated at 520.7 cm⁻¹ using Si as a reference.

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) was performed on all five corundum samples that were previously analyzed by wavelength-dispersive electron microprobe analyses (WDS EMPA). Twenty-seven spots were measured on both blue and colorless zones (three spots on blue-colored zones of each four sapphire samples, one sapphire sample is completely colorless). Analysis took place in GEOKHI RAS using a New Wave Research UP-213 Nd:YAG laser (New Wave Research, Inc., Fremont, CA, USA) combined with Element-XR (Thermo Finnigan, Waltham, MA, USA) ICP-MS. Trace-element compositions were determined by monitoring ⁶Li, ⁹Be, ²⁴Mg, ²⁷Al, ⁴⁴Ca, ⁴⁷Ti, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ⁶⁶Zn, ⁷¹Ga, ⁸⁸Sr, ⁸⁹Y, ⁹¹Zr, ⁹³Nb, ¹⁸¹Ta, ²⁰⁸Pb, and ablating a material with a spot size of 55 µm at a repetition rate of 4 Hz and an energy density of about 21.17 J/cm². Warm up/background time was 90 s, dwell time was 100 s, and wash out time was 120 s. NIST SRM 610 and NIST SRM 612 glasses (National Institute of Standards and Technology; U.S. Department of Commerce: Gaithersburg, MD) were used as the reference materials and ATHO as an unknown for quality control (QCM). The time-resolved signal was processed in Glitter commercial software using ²⁷Al as the internal standard applying the theoretical value of 52.93 wt. % of Al in pure crystalline α-Al₂O₃ for analyzing the corundum unknowns. The measured concentrations of reference material and QCM agree for all elements within 10% and 15%, respectively, of preferable values by [9]. This larger discrepancy between the measured and preferable values can be attributed to isobaric interferences that cannot be resolved with the instrumentation used [10].

Absorption spectra on randomly oriented blue colored zones of sample No1 and No2 were recorded at Kazan Federal University at room temperature by using SHIMADZU UV-3600 spectrometer (Shimadzu Corp, Kyoto, Japan). Spectra were acquired from 185 to 3300 nm, with a data interval and spectral bandwidth of 1 nm and a scan rate of 300 nm/min.

2.2. Host Rocks

Ten samples extracted from different rock types at mine 418, identified macroscopically, of about 100–150 g, were selected for host rock analyses (see Figure 3a and Table S2). Whole-rock major element analyses of these samples were performed at GEOKHI RAS using Energy Dispersive X-ray Fluorescence spectrometer (EDXRF) AXIOS Advanced (PANalytical B.V., Almelo, The Netherlands). The equipment provides the determination the quantitative concentrations of elements from oxygen to uranium from about 10⁻⁴ to 100 wt. %. The whole-rock trace element measurements were done in the Institute of Mineralogy Ural Brach Russian Academy of Sciences by using Inductively Coupled Plasma-Mass Spectrometer (ICP-MS) Agilent 7700×. Agilent and GSO provided reference multi-element solutions for ICP-MS on all measured trace elements; BCR-2 basalt glass was used as the internal standard. The values of the relative standard deviations during the measurement did not exceed the values established for this analytical equipment.

For Rb-Sr and Sm-Nd isotope studies, powder from two samples containing gem quality sapphire (K418-12 and K418-13, Table S2) was produced. The samples 12-1, 12-2, 13-1, and 13-2 were taken from the 100–150 g of samples 418-12 and 418-13 used previously for EDXRF and ICP-MS. Another 2 samples (rock1 and rock2) from a bigger portion of about 1 kg of sample 418-13 with sapphire, spinel, feldspar, and muscovite in minerals association, as well, as two muscovites (mica1 and mica2) extracted from appropriate samples rock1 and rock2 were chosen to constrain the Rb-Sr age. Sample dissolution was carried out in a mixture of hydrofluoric and nitric acids (5:1) on a shaker under incandescent lamps during three days. After evaporation, 1 mL of concentrated HCl acid was added three times to a dry residue. Rb, Sr, and REE were collected using fluoroplastic chromatographic columns with DowexW 50 × 8 synthetic ion-exchange resin. The collection was done by stepwise elution with 2.2 acid normality (n) of HCl (for Rb and Sr) and 4.0n HCl (for the Sm + Nd). From the REE mixture, Sm and Nd were collected by stepwise elution with 0.15n HCl, 0.3n HCl, and 0.7n HCl using polyethylene columns with synthetic ion-exchange resin Ln-spec. The isotopic studies took place on a Finnigan™ Triton multi-collector thermal ionization mass-spectrometer (TIMS) by ThermoFisher Scientific (Waltham, MA, USA) at GEOKHI RAS while using a two-tape (Re-Re) ion source for Rb, Sm, and Nd and a single-tape (Re) ion source for Sr. Measurements were performed in static mode with simultaneous recordings of ion currents for different isotopes. For the elimination of mass-dependent interference, normalization was carried out according to the exponential law for ⁸⁶Sr/⁸⁸Sr = 0.1194 and ¹⁴⁸Nd/¹⁴⁴Nd = 0.241572. The analyses of the international standards Sr-SRM-987 and Nd-LaJolla monitored the reproducibility and correctness of isotope measurements for Sr and Nd. The average value of ⁸⁷Sr/⁸⁶Sr = 0.710236 ± 0.000009 (2σ) and ¹⁴³Nd/¹⁴⁴Nd = 0.512095 ± 0.000006 (2σ). Concentrations of Rb, Sr, Sm, and Nd were determined by isotopic dilution using ⁸⁵Rb-⁸⁴Sr and ¹⁴⁹Sm-¹⁵⁰Nd tracers. The data-reduction was performed using in-lab software. Isoplot4.13 for Excel [11] was used for isochrone diagram with MSWD = 4.1.

Eight petrographic thin-sections of visually monolithic host rocks (samples 418-3, 418-4, 418-6, 418-7, 418-9. 418-10, 418-11, and 418-14), 24 randomly chosen corundum-spinel-chlorite-mica (sample 418-12) and one corundum-spinel (sample 418-13) wafers were studied for petrography (see Table S2). Ten of these wafers with different corundum morphology were used to perform the chemical composition of minerals found in association with corundum by EMPA at the Institute of Mineralogy Ural Branch Russian Academy of Sciences (Miass, Russia) using the electron microscope REMMA—202 M coupled to energy-dispersive spectrometer LZ-5. The operating conditions are 20–30 kV acceleration voltage and 4–6 nA beam current. The beam sizes varied from 1 to 2 µm. A set of natural and synthetic reference materials was used for calibration and instrument-stability monitoring.

3. Results

3.1. Geology and Petrology of Sapphire-Bearing Rocks and Meta-Ultramafic Host Rocks

Sapphire occurrence is a plate-shaped symmetrical-zonal body with traces of considerable deformation, with a dip azimuth of 330°; dip angle is 40°; and, its thickness varies from 1.0 to 1.3 m (Figure 3a). The mine 418 with sapphire-spinel-chlorite-muscovite rock was opened from the surface by a trench measuring about 4 × 9 m (Figure 3b). The block of meta-ultramafites hosting corundum-spinel-chlorite-muscovite rock occupied the first tens of meters and up to about 35 m along the drainage ditch (Figure 3a). The meta-ultramafic host rocks consist of purely mono-mineral coarse-grained enstatite up to 4–5 cm in length with minor olivine and clinopyroxene; in some areas, they are replaced by talc and chrysotile-asbestos. Besides, there is a completely replaced pyrite by goethite in the rock. In the surface subsoil layer, there are chalcedony-quartz concretions up to 10 cm in length, while limonite and supergenic carbonate developed along the cracks of host rocks.

The following zones are distinguished by the structure of host and corundum-bearing rocks (from the selvage to the center, Figure 3b): 1. Meta-ultramafic host rock, which is macroscopically composed of orthopyroxene and clinopyroxene, and minor olivine, have clearly visible brittle deformations (sample 418-7, Table S2); 2. The ferruginated and carbonated meta-ultramafic host rock of the near-selvage zone has a common brownish-orange color; its minerals are almost completely replaced by quartz and carbonate; however, relics of olivine were still detectable (sample 418-6); 3. Talc zoning in the meta-ultramafic host rock (sample 418-4) alternates towards the center on a dense green-colored rock (sample 418-3). The transition to green-colored rock is smooth from almost mono-mineral talc to mono-mineral amphibole. In the amphibole zone, in addition to the monoclinic amphibole (actinolite-tremolite), rhombic anthophyllite was also detected; 4. The central zone is strongly affected by both weathering and deformation processes and it consists of lenticular lenses of various compositions (mica -sample 418-11-, actinolite, actinolite-mica, micaceous pockets with corundum, spinel pockets). In this zone, actinolite-tremolite rock is replaced by shallow rot; only small fragments remain in individual lenses (samples 418-9 and 418-10). The corundum, spinel, and other minerals that accompany them (Figure 2a–c) came from this zone during mineralogical studies described below (samples 418-12, 13, and 14).

3.2. Mineralogy of Sapphire-Hosted Micaceous Lenses

Two sapphire associations were identified, accompanied by spinel. In one, sapphire forms transparent to translucent crystals within the spinel corona. In the other, one lens is an assembly of small (sized about 1 cm) transparent to translucent corundum crystals with almost black spinel, not forming a corona around corundum crystals.

More than 20 major, minor, and accessory minerals are associated with sapphires. Some minerals are common (spinel, clinochlore etc.) and others represent rare or single findings (Ni-bearing sulfide mineralization and baddeleyite) (Table S3. Their texture relationships in regard of sapphire are shown in Table S4. Most minerals were identified under the optic microscope and by Raman Spectroscopy; chemical compositions of minerals were analyzed by EMPA and are presented in Tables S5–S12.

3.2.1. Major Minerals

Minerals of spinel- MgAl₂O₄ -gahnite ZnAl₂O₄ series were common detected in 3 mm thick dark green to black-colored corona around sapphire crystals, as well as in inclusions (Figure 4a). The dark-green to black color is likely due to Fe; Co traces (up to 0.28 wt. %) may also contribute to the coloration. Induction surfaces (i.e., surfaces of synchronous growth) that were detected between spinel and sapphire suggest their synchronous growth. However, spinel most likely started crystallizing at a slightly later stage and when sapphire was already growing in the system. Gahnite appears in spinel areas with higher ZnO contents (up to 22 wt. %). In zonal grains, ZnO values varied from 2.46 to 7.47 wt. % (Table S5). Muscovite KAl₂(AlSi₃O₁₀)(OH)₂ formed micaceous lenses with embedded corundum, as well as micro-inclusions (up to 0.2 mm) within chlorite group minerals (Figure 4b). Muscovite is characterized by a higher MgO content from 0.5 to 2.5 wt. % in comparison to stoichiometric muscovite (Table S6). Clinochlore Mg₅Al(AlSi₃O₁₀)(OH)₈ and amesite Mg₂Al(AlSiO₅)(OH)₄ form inclusions and rather large ingrowths (up to 5 mm) around spinel, inclusions within spinel, and in fractures along the border of corundum and spinel (Figure 4b). These minerals were also found in the rims around the uraninite-brockite and monazite-(Ce) (for the chemical composition, Table S6). Anorthite CaAl₂Si₂O₈ (An₁₀₀) is found in the “spinel lenses” forming crystals that are sized up to 3 mm in the intergrowth with amphibole and chlorite group minerals (see chemical composition in Table S7). All identified major minerals are syngenetic with sapphire.

3.2.2. Minor Minerals

Apatite group minerals Ca₅(PO₄)₃(OH,F,Cl) were found within corundum fractures and chlorite group minerals as prismatic epigenetic inclusions up to 1 mm in length (Figure 4c). Its composition varies from chlorapatite to fluorapatite to hydroxylapatite (Table S8). Zircon ZrSiO₄ forms small zonal epigenetic inclusions (up to 0.1 mm in length) within the fractures of corundum, chlorite group minerals, and spinel. Besides zircon, the amphibole group minerals and apatite (Figure 4c) also fill out these fractures. Zircons contain up to 3.16–4.73 wt. % of HfO₂ (Table S9). Protogenetic diaspore AlO(OH), along with clinochlore forms rims around inclusions of uraninite, intergrowth with clinochlore within corundum (Figure 4d), and platy aggregates in the fractures of barium feldspar. Diaspore contain up to 0.1 wt. % of SiO₂ and 0.34 wt. % of FeO_tot. Millerite NiS forms micro-inclusions (up to 15 µm) within corundum, spinel, and chlorite group minerals; the mineral is found in association with maucherite, heazlewoodite, and galena (Figure 5a). The FeO_tot content of millerite varies from 2 to 20 wt. % (Table S10). Maucherite Ni₁₁As₈ was detected in the intergrowth with heazlewoodite and millerite in crystals sized to 15 µm (for chemical composition, Table S10). Microlite group minerals (Na,Ca)₂Ta₂O₆(O,OH,F) were found in small (up to 0.1 mm) grains and cube-octahedral crystals within chlorite group minerals (Figure 5b). Minerals characterized by the high UO₂ content (up to 11.34 wt. %); values of WO₃ and Nb₂O₅ both vary up to 3.15 wt. % and 3.57 wt. %; ThO₂ is 1.2 wt. % and Y₂O₃ 1.61 wt. %. Nickeline NiAs is found as inclusions within clinochlore and dravite; the size of mineral varied up to 20 µm (Figure 5c and Table S9). Uraninite UO₂ forms zonal micro-inclusions that are sized up to 150 µm (Figure 5d). The atomic number (pfu) is up to 0.41 in the mineral. Allanite-(Ce) (CaCe)(Al₂Fe²⁺)(Si₂O₇)(SiO₄)O(OH)—dissakisite-(Ce) (CaCe)(Al₂Mg)(Si₂O₇)(SiO₄)O(OH) form zonal-sectorial and mosaic-like crystals up to 0.2 mm in length (Figure 5f); the oscillatory zonation is due to the different concentrations of CaO and REE (Table S11). Monazite-(Ce) Ce(PO₄) has been found along the cracks of dissakisite-(Ce) grains, forming crystals of up to 20 μm in length (Figure 5f). CaO content ranges from 0.38 to 8.25 wt. % in the mineral, Ce₂O₃ values vary from 23.11 to 37.53 wt. %, and La₂O₃ is 10–22 wt. %; in some analyses, additional SiO₂ and ThO₂ were detected (Table S9). Ni-bearing and REE-mineralization are likely syngenetic in origin.

3.2.3. Accessory Minerals

Vigezzite or fersmite (Ca,Ce)(Nb,Ta,Ti)₂O₆ forms likely syngenetic elongate-prismatic inclusions up to 70 µm long within clinochlore (Figure 5e). The content of Nd₂O₃ is about 3.5 wt. % in the mineral; La₂O₃ values vary up to 2.16 wt. %, Pr₂O₃ is up to 1.15 wt. %; and, FeO_tot up to 1 wt. %.

Heazlewoodite Ni₃S₂ is associated with corundum, spinel, clinochlore, and millerite. The size of the mineral is less than 0.3 mm. It contains up to 1.22 wt. % of FeO_tot [13]. Galena PbS is found in intergrowth with millerite in the form of crystals sized up to 10 µm (Figure 5a and Table S10). Gersdorffite NiAsS is identified within the inner part of a zonal aggregate composed of heazlewoodite and galena in contact between chlorite group minerals and corundum. CoO content in the mineral varies up to 13 wt. % (Table S10). The Ni- and Pb-bearing mineralization is likely syngenetic in origin. Dravite Na(Mg₃)Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH) forms brownish-colored syngenetic or protogenetic crystals up to 3 cm in length. It lies in micaceous lenses in association with corundum and in “spinel lenses”. The mineral contains lower FeO and CaO values in comparison with stoichiometric dravite; two values are up to 1.6 wt. % and 0.5 wt. %. Rutile TiO₂ forms syngenetic micro-inclusions within spinel sized up to 50 µm. The mineral is characterized by high tantalum content (up to 31.79 wt. % of Ta₂O₅) and Nb₂O₅ values of up to 1.46 wt. %. Hyalophane (K,Ba)Al(Si,Al)₃O₈ forms likely epigenetic micro-inclusions along the fractures within the chlorite group minerals; mineral size is up to 60 µm. BaO content in hyalophane is about 6 wt. % (Table S7). Celsian Ba(Al₂Si₂O₈) forms lamellar epigenetic crystals and aggregates in fractures along a boundary between corundum and amesite; mineral size varies from 5 µm to 30 µm. BaO values in mineral are up to 33.67 wt. % (Table S7). Marialite (Na,Ca)₄[Al₃Si₉O₂₄]Cl forms syngenetic inclusions within chlorite group minerals sized up to 20 µm; Cl content in the mineral is about 1.7 wt.%, CaO is 11 wt. %, Na₂O is about 7 wt. % (Table S7). Pargasite NaCa₂(Mg₄Al)Si₆Al₂O₂₂(OH)₂ and tschermakite ☐(Ca₂)(Mg₃Al₂)(Al₂Si₆O₂₂)(OH)₂ form syngeneitc micro-inclusions (up to 150 µm) within clinochlore of “spinel lenses” (Table S12). Baddeleyite ZrO₂ is found as an epigenetic micro-inclusion within zircon sized up to 5 µm.

3.3. Sapphire Mineralogy, Geochemistry, and UV-Vis-NIR Spectroscopy

Sapphires practically always form single crystals, with colorless translucent to blue transparent zones, sized up to 4 cm in length. Only small, less than 1ct, sapphires of gem quality can be faceted. A dark-green to black colored spinel-gahnite corona practically always surrounds sapphire crystals (Figure 2a–c). The shape of sapphire-spinel aggregates is often irregular and rounded due to numerous outgrowths of muscovite and clinoclore. Less common (in one or two lenses), there are short-prismatic and barrel-shaped sapphire crystals; however, without flat faces that are caused by intergrowth with the muscovite and spinel.

LA-ICP-MS measurements from different zones of sapphires are presented in

Table 1. The trace-elements composition of sapphire colored zones by LA-ICP-MS ¹.

No	Spot	Color	24Mg	47Ti	51V	53Cr	57Fe	71Ga	Ga/Mg	Fe/Mg	Cr/Ga	Fe/Ti	Mg × 100	Ti × 10
1	1	Cl	35.18	Bdl	1.05	56.17	1039.18	29.03	0.83	29.54	1.94	-	3518	-
	2	Cl	75.09	Bdl	Bdl	43.46	1121.67	32.05	0.43	14.94	1.36	-	7509	-
	3	Cl	46.83	Bdl	0.94	98.21	1072.46	35.79	0.76	22.9	2.74	-	4683	-
	1	Bl	13.09	279.22	1.23	81.71	973.14	28.99	2.21	74.34	2.82	3.49	1309	2792.2
	2	Bl	22.16	218.01	Bdl	Bdl	820	27.21	1.23	37.00	-	3.76	2216	2180.1
	3	Bl	21.93	189.74	Bdl	70.60	979.65	31.00	1.41	44.67	2.28	5.16	2193	1897.4
2	1	Cl	16.03	49.25	2.46	Bdl	793.43	29.13	1.82	49.50	-	16.11	1603	492.5
	2	Cl	17.68	267.84	1.11	40.05	915.65	31.11	1.76	51.79	1.29	3.42	1768	2678.4
	3	Cl	16.31	206.99	1.19	80.09	931.05	28.62	1.75	57.08	2.80	4.50	1631	2069.9
	1	Bl	22.01	167.50	0.76	520.29	981.77	30.45	1.38	44.61	-	5.86	2201	1675
	2	Bl	22.63	109.67	Bdl	63.15	931.31	28.85	1.28	41.15	2.19	8.49	2263	1096.7
	3	Bl	11.69	129.5	Bdl	58.41	788.16	31.82	2.72	67.42	1.83	6.09	1169	1295
3	1	Bl	15.22	215.82	1.34	87.16	1002.05	9.22	0.61	65.84	9.45	4.64	1522	2158.2
	2	Bl	19.98	174.27	Bdl	62.49	1021.17	27.56	1.38	51.11	2.27	5.86	1998	1742.7
	3	Bl	21.19	265.80	1.42	Bdl	853.12	26.88	1.27	40.27	-	3.21	2119	2658
	1	Cl	20.34	Bdl	0.92	95.93	1128.79	33.43	1.64	55.50	2.87	-	2034	-
	2	Cl	106.21	Bdl	1.56	114.49	767.40	32.59	0.31	7.23	3.51	-	10,621	-
	3	Cl	36.18	Bdl	1.19	Bdl	892.02	32.41	0.90	24.66	-	-	3618	-
4	1	Cl	16.86	185.12	1.32	Bdl	686.05	23.21	1.38	40.69	-	3.71	1686	1851.2
	2	Cl	15.53	127.12	1.31	Bdl	700.49	22.8	1.47	45.11	-	5.51	1553	1271.2
	3	Cl	11.91	90.24	Bdl	Bdl	801.94	24.81	2.08	67.33	-	8.88	1191	902.4
5	1	Cl	53.17	Bdl	Bdl	Bdl	949.86	26.94	0.51	17.86	-	-	5317	-
	2	Cl	34.99	Bdl	1.1	Bdl	934.3	30.5	0.87	26.70	-	-	3499	-
	3	Cl	19.22	Bdl	Bdl	Bdl	830.66	30.31	1.58	43.22	-	-	1922	-
	1	Bl	18.58	247.54	Bdl	39.46	800.69	26.99	1.45	43.09	1.46	3.23	1858	2475.4
	2	Bl	11.22	117.5	1.12	Bdl	764.53	27.66	2.47	68.14	-	6.5	1122	1175
	3	Bl	12.05	92.4	1.28	Bdl	732.68	27.34	2.27	60.80	-	7.93	1205	924

¹ Cl, BL—colorless and blue sapphire zones on a Figure 3b,c. ⁶⁰Ni was measured but was below the detection limit.

. The position of analyzed spots is shown in Figure 3b,c. Mg, Ti, V, Cr, Fe, and Ga, elements commonly used to trace the geological origin were the only detected element of this method [14,15]. Si may also be present as a trace of sapphires; however, it cannot be measured by LA-ICP-MS due to high interference of ²⁷AlH⁺ with ²⁸Si⁺ and ²⁸SiH with ²⁹N-N [16]. In the blue-colored spots, Mg ranges from 11 to 23 parts per million weight (ppmw), Ti ranges from 92 to 279 ppmw, V from b.d.l. to 1 ppmw and Cr values ranges from b.d.l. to 87 ppmw (except one analyses detected 520 ppmw likely due to the ablation of diaspore inclusion; its spot was excluded from the plots provided below), relatively low Fe content (from 732 to 1021 ppmw), and fairly low Ga values (from 9 to 31 ppmw). The absorption spectra were comparable with those that were observed on sapphires of metamorphic or metasomatic origin and similar to those from Ilmen Mountains linked with syenite pegmatites [6,17]. Values of trace elements, such as Mg, Ti, Fe, Cr, Ga, and their ratios—Ga/Mg, Fe/Ti, Fe/Mg, and Cr/Ga have been used for the determination of the origin of “magmatic” versus “metamorphic” blue sapphires [14,15]. LA-ICP-MS trace element measurements of the studied sapphires showed Ga/Mg < 2.7, Fe/Mg < 74, Cr/Ga > 1.5, and Fe/Ti < 9, which is typical of sapphires of metamorphic origin [14,15]. On a Fe versus Ga/Mg diagram and a Fe-Ti × 10-Mg × 100 ternary plot [14], all measured corundum samples plotted in the overlapping field of “metamorphic” and “plumasitic” sapphires (Figure 6 and Figure 7). Using the FeO–Cr₂O₃–MgO–V₂O₃ versus FeO + TiO₂ + Ga₂O₃ discriminant diagram of [18], the studied sapphires plot slightly off the bottom of the overlapping area with rubies found in “mafic–ultramafic rocks” and more in the field of “metasomatic” sapphires and sapphires in “alkali basalts” (Figure 8a,b). In the discriminant factor diagram by [18], the Ilmen sapphires within meta-ultramafites are plotted to the “plumasitic” field and they are close to those from skarns, which is mostly caused by their enrichment with Ti (Figure 9).

3.4. Whole-Rock Geochemistry

EDXRF measurements of hosted meta-ultramafic and corundum-bearing rocks are presented in Table S13. According to the powder EDXRF values, meta-ultramafic host rocks have high SiO₂ content up to 50–61 wt. %, MgO was about 20–32 wt. %, CaO was 0.29–17.24 wt. %, Fe₂O₃ was 2.9–8.2 wt. %, Al₂O₃ was 0.11–3.97 wt. %, and low alkaline elements concentrations were also detected (in total, about 0.1–0.2 wt. %). Micaceous lenses with corundum and spinel, however, showed high MgO and Al₂O₃ contents (the two are about 16–25 wt. % and 16.26–38.52 wt. %, respectively), combined with variable SiO₂ (25–80 wt. %), 3.44–9.5 wt. % of Fe₂O₃, 0.26–1.39 wt. % of CaO, and low concentration of alkaline elements (Na₂O + K₂O ≤ 1 wt. %) [20]. Thus, sapphire-bearing rocks show higher Al₂O₃ and K₂O, however, lower SiO₂ values than those detected in meta-ultramafites, but similar concentrations of Na₂O, MgO, CaO, and TiO₂ (Figure 10). As for numerous metasomatic deposits, there are difficulties in the calculation of mass balance for the newly discovered sapphire-spinel-chlorite-muscovite rock, its formation occurred with significant mass-transfer. The aluminum that is required for the formation of corundum is commonly passive in weakly acidic and neutral conditions; the transfer of aluminum is possible in alkaline and strongly acidic conditions [21]. The mobility of aluminum in a complex with sodium was previously discussed [22]. In the case of this sapphire occurrence, Al was particularly mobile and it was most likely transported by fluids that are linked to miascites (Table S13 and the later discussion). Additionally, sapphire-bearing rocks were enriched with LIL-elements, such as Ba (Ba-felspars), P (apatite), and Sr (as a trace-element in feldspar and muscovite), which came from miascite-generated fluids as well. However, the sources of Mg and some siderophyle elements (Ni) are linked likely to meta-ultramafic host rocks; their parts were re-worked by metasomatic fluids.

ICP-MS measurements are shown in Table S13. The distribution of REE in micaceous lenses has the general trend—an enrichment with LREE as compared to HREE (Figure 11a). This phenomenon is possibly due to the inclusions of LREE-bearing minerals, e.g., monazite-(Ce), allanite-(Ce) or others. In the meta-ultramafic host rocks, this trend is less pronounced or it disappears: REE contents normalized to chondrite are almost the same for both LREE and HREE. Nearly all of the studied rocks show negative Eu anomalies. The REE trend of these corundum-bearing rocks is close to that observed for Ilmen corundum-bearing syenite pegmatites (Sorokina et al., unpublished data) and to that of corundum syenite pegmatites hosted by ultramafic rocks in the French Pyrénées [23]. Enrichment in HSFE was also noted (mine 418); commonly observed for alkaline-ultramafic associations of the platform type (ultramafic rocks, syenites–carbonatites etc.; [24]), and similar to those of miaskite (nepheline syenite) of the Ilmenogorsky complex. On a trace element diagram normalized to primitive mantle (Figure 11b), all studied rocks present distinct K and Ti negative anomalies. However, the sapphire-bearing rocks also show positive Ta and Gd anomalies, but with different Sr anomalies.

3.5. Rb-Sr and Sm-Nd Isotope Measurements

Rb-Sr and Sm-Nd isotope measurements of sapphire-bearing rocks 418-12 and 418-13 previously analyzed by EDXRF and ICP-MS are presented in Table S14. As well, two samples of 418-13 rock containing feldspar and two samples of mica extracted from appropriated rock sample 418-13 associated with sapphire mineralization were analyzed in order to constrain the Rb-Sr age. Obtained values of Rb, Sr, Sm, and Nd by TIMS are closely similar or comparable to those that were received by ICP-MS (Tables S12 and S13). ⁸⁷Sr/⁸⁶Sr for samples 418-12 and 418-13 are varied from 0.708764 ± 0.000010 (2σ) to 0.710587 ± 0.000006 (2σ). For mica samples, these values are higher and vary from 0.711371 ± 0.000008 to 0.714131 ± 0.000016. The calculated Rb-Sr isochrone age for three rock samples and mica-2 sample is 289.3 ± 8.7 Ma (2σ), MSWD is 4.1 (Figure 12). Note, the calculated Rb-Sr age for another mica sample (mica-1) with three rock samples gave much higher error (352 ± 35 Ma) and higher MSWD (72), which was likely due to the secondary alteration of muscovite samples mica-1. (⁸⁷Sr/⁸⁶Sr)₂₈₉ of all samples recalculated on 289 Ma are about 0.707351 to 0.707919. ¹⁴³Nd/¹⁴⁴Nd for the samples 418-12 and 418-13 are 0.5122 ± 0.000008 (2σ) and 0.5122 ± 0.000012 (2σ), εNd_{(289 Ma)} is −7.8. The obtained high ⁸⁷Sr/⁸⁶Sr ratios and low εNd correspond to the formation of the newly discovered corundum-bearing rocks under the significant influence of crustal fluids.

4. Discussion

4.1. Suggested Petrogenesis of Sapphire in the Meta-Ultramafic Host Rocks of the Ilmen Mountains

The studied blue sapphires surrounded by spinel coronas embedded in chlorite-muscovite matrix present metasomatites that are found within meta-ultramafic host rocks. These host rocks are most likely formed after metasomatic alteration (serpentinization) of orthopyroxenites; i.e., containing orthopyroxene with minor olivine and clinopyroxene. While the orthopyroxenites were formed at about 1.3 Ga and are related to the Saitovsky series of Kyshtimsky strata of the Ilmenogorsky complex, Rb-Sr and Sm-Nd isotopic data indicate their linkage with enriched mantle domains of types EM1 and EM2 [27].

The metasomatism required for the formation of these rocks with blue sapphires took place around 289.3 ± 8.7 Ma (measured by Rb-Sr dating of sapphire-bearing rocks and mono-mineral muscovite associated with sapphires). This age represents the cooling age after the metamorphic peak (transpression) that occurred in the Ilmenogorsky complex at about 330–320 Ma during the Uralian orogeny [28]. The metasomatic fluids formed after the partial melting of miascites occurred in the region (440–420 Ma). These fluids were also the source of HFSE, LILE, and alumina (remobilized under alkaline conditions [21,22]) needed for corundum formation, and transported these chemical elements at a minimum distance of about 4 km (calculated from the eastern flank of Ilmenogorsky complex) reaching the meta-ultramafic rocks. In Figure 11, similar REE trends between miascites and corundum-bearing rocks in mine 418 are presented (i.e., enrichment of LREE as compared to HREE), which is an important evidence of the metasomatic fluid source. Partial melting of miascites is associated with the formation of migmatities with more acidic composition from albitites to even granites. In the Ilmenogorsky miascite massif, alumina remobilization can also be associated with the formation of zeolitic veins and zeolitizated zones of pegmatites, filling late-forming zones with aluminum hydroxides (gibbsite, nordstrandite, boehmite, diaspore).

Desilication also took place during the contact of fluids with ultramafic rocks and sapphire formation. Desilication is commonly observed during corundum crystallization in numerous deposits worldwide ([1,29,30], etc.). The importance of buffering capacity of ultramafic host rocks with respect to silica in the Greenland ruby deposit has been previously shown [31]. In this deposit, the meta-ultramafic host rocks show similar buffering capacity in respect to the desilication of Al-bearing alkaline fluids that are linked to miascites. Initial ultramafic rocks are also most likely the source of Mg required for spinel and chlorite formation found in metasomatites linked with the corona around the sapphires, as well as the source of Ni for nickel-bearing minerals.

Sapphire crystallization was likely caused by the breakdown of diaspore possibly by the following reaction: 2diaspore → corundum + H₂O. Note, diaspore formed in meta-ultramafic host rock likely due to a previous metasomatic event that is linked with the partial melting of miascites. A previous metasomatic event was likely a significant time before the deposition of sapphire-bearing metasomatites. Chlorite formation occurred during the metasomatic reworking of meta-ultramafic host rocks. Meanwhile, the spinel corona most likely formed as a consequence of metasomatic fluid-chlorite interactions with the corundum as a reaction rim. Under decreasing pressure and temperature, muscovite crystallized during the breakdown of corundum: corundum + orthoclase + H₂O → muscovite; previously observed for a corundum-mica-tourmaline rock hosted by ultamafites of New Zealand [32].

Experiments have shown that blue sapphires (incorporation of Ti and Fe into the corundum lattice) can be formed at around 700 °C after muscovite and biotite, along with a peraluminous silicic melt at 2 kbar (water pressure) [33]. Corundum and hercynite spinel also coexist with orthoclase-rich feldspar and biotite at 750 °C [33]. Besides, other experiments have shown that muscovite can still be stable, slightly above 700 °C and at about 1.8 kbar [34] (Figure 13a). Thus, it seems that sapphire-spinel-clinochlore-muscovite metasomatites formed at a temperature around 700–750 °C and pressure about 1.8–3.5 kbar; at the eutectic point of corundum + orthoclase + H₂O ↔ muscovite; where the three minerals are in equilibrium (Figure 13b).

4.2. Comparison with Sapphire and Ruby from Metasomatites

To date, only pink sapphires to rubies (probably none of them of gem quality) were found in metasomatites within the ultramafic host rocks; e.g., they are known in the eastern Transvaal [38] and in the Barberton greenstone belt [39] of South Africa, in Zimbabwe [40], and in Westland, New Zealand [32]. Ruby-fuchsite rock, ‘verdite’ from Zimbabwe is enriched in Ni as represented by gersdorfitte (NiAsS), which is also the case of the described occurrence in the Ilmen Mountains, in which andalusite, chlorite, margarite, tourmaline, diaspore, rutile, and native Bi are present as well. However, in Zimbabwe, ‘verdite’, which occurs in bodies a few kilometers length, was formed as the result of post-volcanic, pre-metamorphic exhalative alteration of komatiitic lavas [41]. Another approach to the genesis of ‘verdite’ suggests an alteration of ultramafic rocks in a high-temperature low-pH hydrothermal solution carrying LILE [42]. Ruby-bearing boulders of metasomatic origin from Westland in New Zealand, as is the case of the described occurrence, are found embedded in micaceous matrix associated, however, with fuchsite, margarite, and tourmaline [32]. In New Zealand, similarly to the described Ilmen sapphire occurrence, the crystallization of minerals in association with corundum required significant mass-transfer during the metasomatic process, possibly 50–90% [32]. Regionally metamorphosed pink sapphire to ruby deposits within an ultramafic protolith in Paranesti in Greece were also reported [43]. In common with the Ilmen sapphires, the Paranesti rubies are characterized by spinel-picotite inclusions. Spinel inclusions and Ni-bearing sulfides were also found within sapphires from some Australian occurrences [44]. However, the spinel inclusions in these sapphires are more Co-rich than Zn-rich, as is the case in the present Ilmen sapphires (Table S5). An inclusion of Zn-rich spinel within sapphire is also reported from a placer in Kedrovka, in the Russian Far East [45].

4.3. Comparison of Chemical Data with Sapphires from Other Geological Environments

In a Fe versus Ga/Mg diagram [14], all measured corundum samples were plotted to “metamorphic” and “plumasitic” population fields (Figure 6), clearly away from the “magmatic” blue sapphires that are found within syenite pegmatites of Ilmen Mountains (see again [6,17]). However, these trace-element diagrams should be used with the caution and possibly revised, since it was found that some of the sapphires from magmatic deposits could be plotted in the “metamorphic field” [46,47]. It is probably important to refine these plots using solely data of gem-quality samples and to group them by color (e.g., plot sapphire and rubies separately), taking into account that most of trace elements used for these are corundum chromophores (e.g., Fe, Ti, Mg, Cr, V). For instance, as shown in Figure S1, the population field is more scattered when plotting colorless opaque sapphires together with those of blue color.

On the FeO–Cr₂O₃–MgO–V₂O₃ versus FeO + TiO₂+ Ga₂O₃ discriminant diagram (Figure 8), the plot is slightly away from the area with rubies in “mafic–ultramafic rocks” and is mostly in the field with “metasomatic” sapphires and sapphires in “alkali basalts”. The UV-Vis-NIR spectra of studied sapphires were comparable to those that were observed on sapphires of metamorphic or metasomatic origin.

5. Conclusions

Most gem-quality sapphires are mined from placers of unclear primary and possibly diverse origins. Studies of gem-quality sapphires that are found in situ within host rocks may provide clues to the petrogenesis of those found in secondary occurrences.

Gem-quality sapphire is known from syenite pegmatites in the Ilmen Mountains of the South Urals. An occurrence of gem-quality sapphires from these mountains was also discovered within meta-ultramafites where sapphires form concentric aggregates within spinel-gahnite coronas. Sapphires form irregular-shaped translucent to transparent crystals with colorless and blue zones. The concentric sapphire aggregates with spinel coronas are embedded in a clinochlore-muscovite matrix. In this occurrence, corundum(sapphire)–spinel(gahnite)–clinochlore-muscovite aggregates associate with anorthite (An₁₀₀), apatite, zircon, diaspore, microlite, allanite-dissakisite-(Ce), monazite-(Ce), dravite, Ba-feldspars, and various Ni-bearing minerals. The mantle-generated ultramafic host rocks (orthopyroxenites) that are composed of enstatite with minor olivine have undergone significant serpentinization. Altered meta-ultramafic host rocks have high contents of SiO₂ (approximately 50–61 wt. %) and MgO (approximately 20–32 wt. %), but low Al₂O₃ (0.11–3.97 wt. %), as well as alkaline elements concentrations (in total, about 0.1–0.2 wt. %), whereas the sapphire-bearing rocks showed high MgO (16–25 wt. %) and high Al₂O₃ (16.26–38.52 wt. %) contents combined with variable SiO₂ (25–80 wt. %), but low concentrations of alkaline element (Na₂O + K₂O ≤ 1 wt. %). Most of the sapphire-bearing rocks—in contrast with the meta-ultramafic host rocks—are enriched in LREE and depleted in HREE with negative Eu anomalies. Spider REE diagrams of the studied samples are similar to those of sapphires in the syenite pegmatites, linked to intrusions of miascites (nepheline syenite) in the Ilmen Mountains. Sapphire-spinel-clinochlore-muscovite rock formed after metasomatic alteration of orthopyroxenites at a temperature around 700–750 °C and pressure about 1.8–3.5 kbar. The metasomatic fluids enriched in HFSE, LILE, and alumina formed after the partial melting of miascites (nepheline syenites), while Mg and some siderophile elements (Ni) came from mantle-generated meta-ultramafic host rocks. Formation of this metasomatic rock occurred at about 289 ± 9 Ma (Rb-Sr dating of sapphire-bearing rocks and mono-mineral muscovite associated with sapphires), representing the cooling ages after the peak metamorphism at about 330–320 Ma during the Uralian orogeny.

Sapphire crystallized after the breakdown of diaspore by the reaction: 2diaspore → corundum + H₂O. Chlorite occurred during the metasomatic reworking of meta-ultramafic host rocks. However, the spinel corona surrounding sapphire aggregates most likely formed as a consequence of metasomatic fluid-chlorite interactions with the corundum. Under decreasing pressure and temperature, muscovite crystallized during the breakdown of corundum by the reaction: corundum + orthoclase + H₂O → muscovite.

Despite on obvious metasomatic genesis, LA-ICP-MS analyses of blue sapphires showed Ga/Mg < 2.7, Fe/Mg < 74, Cr/Ga > 1.5, and Fe/Ti < 9, in the range of “metamorphic” sapphires. The Fe versus Ga/Mg diagram and Fe-Ti × 10-Mg × 100 ternary plot should be used with the caution and possibly revised using solely data of gem-quality samples and to group them by color.