Petrogenesis of Chatoyant Green Nephrite from Serpentinite-Related Deposits, Ospinsk, Russia: Insights from Mineralogy and Geochemistry

Abstract

Ospinsk is an area in Russia well-known for mining the highest quality green nephrite in the world. However, the chatoyant green nephrite found here has not been studied to date. This study explores the mineralogy, geochemistry, and petrogenesis of chatoyant green nephrite collected from Ospinsk using polarizing microscope back-scattered electron images, scanning electron microscopy, Fourier transform infrared spectrometry, laser Raman spectroscopy, electron microprobe analysis, and laser ablation inductively coupled plasma mass spectrometry, and compares them with S-type green nephrite from other regions of the world. Tremolite is the main mineral constituent, and chromite, chlorite, graphite, and magnetite are accessory minerals in the samples. The chatoyant green nephrite from Ospinsk is serpentinite-related green nephrite. The Ti content of the chatoyant green nephrite from Ospinsk is significantly higher than that of green nephrite from other places. The chatoyant green nephrite deposit in Ospinsk is a contact metasomatic deposit related to ultramafic rocks. The ultramafic rocks first altered to serpentinite and later converted to tremolite after repeated thermal contact-based metasomatism. During the metasomatism of serpentinite into green nephrite, unilateral, compressive, and shear stresses caused by obduction forced directional growth of tremolite, resulting in chatoyancy.

1. Introduction

Nephrite is a mineral aggregate composed mainly of minerals of the tremolite [Ca₂(Mg_5.0–4.5Fe²⁺_0.0–0.5)Si₈O₂₂(OH)₂]–ferro-actinolite[Ca₂(Mg_4.5–2.5Fe²⁺_0.5–2.5)Si₈O₂₂(OH)₂] isomorphic series. The “verdant to green” colored variety is called green nephrite. According to Harlow and Sorensen (2005), nephrite may have a metasomatic or metamorphic origin [1]. Serpentinite-related (S-type) nephrites normally have a metasomatic origin, whereas dolomite-related (D-type) nephrites have metamorphic origins [2]. S-type nephrite is often formed as a result of the interaction of serpentinite with gabbro dikes, which were then transformed into rodingites. D-type nephrite also forms at the contact of dolomites with amphibolites, for example in the Vitim region. In other cases, D-type jade is formed at the contact of limestones with diabases—for example in the Dahua deposit in Guangxi Zhuang autonomous region [3] and Luodian in Guizhou province [4]. In this study, green nephrite was generally interpreted as S-type. The geochemical properties of S-type green nephrite are significantly different from those of D-type green nephrite due to differences in their genesis. The Cr, Ni, Co, and Fe/Mg contents in the nephrite or whole rock can help identify the nephrite genetic type [1,5,6]. Green nephrite has been detected in several regions in Russia, such as Eastern Sayan [7,8,9,10,11,12,13,14,15], the Polar Urals [16], Western Sayan Mountains [16], Southern Urals [17], and the Middle Urals [18]. It has also been reported from many other countries, for example in Asia (Pakistan [19,20,21] and Afghanistan [22]; major deposits in China are from Manas [23,24,25,26], Qiemo [27], Hetian [5,6,28], and Ruoqiang Aqikule in Xinjiang province [29], Golmud in Qinghai [30], Hualien in Taiwan [31,32,33], and Shimian in Sichuan [34,35,36]); Europe (Jordanów, Slaski, Poland, [37,38,39,40]); North America (Kutcho [1,41,42,43], Cassiar [44,45], and Polar [46] Canada); and Oceania (Northwest Otago, New Zealand [47,48,49,50,51], and New South Wales, Australia [52,53]). Among them, only the green nephrites from Hualien [54] and Shimian [34,35] in China and from the Ospinsk mine in Russia show chatoyancy.

The green nephrite deposit in Russia is located in the southwest of the Siberian platform in East Sayan. Currently, there are more than 10 mines in the ophiolite belt of this area, among which the most famous ones are the Ospo, Gorlykgol, Ulankhoda, and Arahushun mines [12,55]. The Ospinsk mine in the Eastern Sayan part of the Siberian platform is a famous area for mining green nephrites. It produces the best quality of green nephrite in the world. Previous studies on the Ospinsk mine have mostly focused on the geodynamic setting of East Sayan [7,8,9,10,56,57,58,59], and rarely examined its petrogenesis. Mineral chemistry, spectroscopy, geochemistry, and petrogenetic studies of Ospinsk green nephrite are scarce, with disparate data [7,9,10,12,60]; furthermore, the genesis of the chatoyancy-bearing Ospinsk green nephrite has not been studied.

This study systematically examined the mineralogy, geochemistry, and petrogenesis of chatoyant green nephrite from the Ospinsk mine using polarizing microscope BSE images, SEM, FTIR, laser Raman spectroscopy, EPMA, and LA–ICP–MS. The results provide an insight into the structural and geological causes of the chatoyant green nephrite in the mine. We also compared the results with other deposits of S-type green nephrite throughout the world.

2. Geological Setting and Sample Description

Most nephrite deposits in Russia exist in the fold belt located on the southern margin of the Siberian craton, which can be conditionally divided into the Dzhida, Vitim, West Sayan, and East Sayan blocks. The Ospinsko–Kitoisky and Kharanursky ophiolite complexes are located in the East Sayan block. Green nephrite is mainly found in East Sayan, southwest of Lake Baikal (Figure 1a), with more than 10 nephrite mines. We selected the Ospinsk mine, located in the center of the Ospa–Kitoi block in East Sayan, south of Siberia (+51°50′, +102°13′; Figure 1b). It covers an area of 5 km² and contains fifteen nephrite veins distributed across three areas.

During the early Cambrian, the Salair foldbelt caused the southward accretion of the Siberian platform to form a prominent orogenic belt from Altai–Sayan through Central Mongolia to Erguna. East Sayan is one of the largest ancient crustal fragments in the Altai–Sayan tectonic belt. Alkaline rocks formed in the Caledonian belt during the middle (400–370 Ma) to late (290–268 Ma) Paleozoic [61]. Ophiolites of 1020 Ma age occur in the southeastern Sayan area due to the Caledonian orogeny [62]. The East Sayan range is at the boundary between the Archean and lower Proterozoic Siberian platforms, and is a nappe structure composed of thrusts [10]. Green nephrite is found in the ophiolite belts, while most of the main nappe is denuded. The rocks underlying the nappe have a thrust contact with the ophiolites. They usually consist of conglomerate-like formations, made up of rounded, elongated, and lensoid boudins of limestone, enclosed in a carbonate or carbonate–chlorite matrix. Few dikes in the study area have undergone metamorphism to form carbonated and chloritized schists.

For this study, 10 representative samples of chatoyant green nephrite from Ospinsk were collected (Figure 2), namely, yellow-green colored RC1, RC2, RC3, and RC4, green colored RC5 and RC6, and verdant to green colored RC7, RC8, RC9, and RC10. All samples were translucent–opaque and fine-grained schists.

3. Methods

Thin sections (0.03 mm thick) were cut from the samples, which were later powdered up to 300 mesh in a grinding chamber from China University of Geosciences (Beijing) to analyze their mineral composition and microstructures. The samples were observed using a polarized light microscope (OLYMPUS BX-51) (OLYMPUS, Tokyo, Japan), and corresponding BSE images were obtained using a field-emission scanning electron microscope coupled to an X-ray energy spectrometer (SHIMADZU, Kyoto, Japan). The sample chips for thin-section preparation were broken from a small fresh section of the rocks, and the thin sections were coated with carbon. The sections were then scanned using a field-emission scanning electron microscope (FE-SEM; SUPRATM 55) with a voltage of 15 kV (ZEISS, Jena, Germany). The studies were carried out in the FE-SEM laboratory of China University of Geosciences, Beijing.

Both the transmission infrared (BRUKER, Ettlingen, Germany) and Raman studies (HORIBA, Kyoto, Japan) were carried out in the Gem Research Laboratory at the School of Gemology, China University of Geosciences. The sample powders were mixed with potassium bromide (KBr) powder in a ratio of 1:100 and pressed to a pellet. A spectrum was obtained using a Fourier transform infrared spectrometer (Bruker TENSOR27). The power supply voltage was 85–265 V at 47–65 Hz, scanning range was 4000–400 cm⁻¹, scanning time was 16 s, and resolution was 4 cm⁻¹. Microscopic Raman spectrometers (HR-Evolution) were used at room temperature and pressure with an excitation source at a wavelength of 532 nm and excitation time of 3 s. The samples were divided into two batches, 1 and 2, with spectral ranges of 100–1800 cm⁻¹ and 100–2000 cm⁻¹, respectively, at a resolution of 4 cm⁻¹ and spot size of 5 μm.

Mineral chemical compositions were obtained using a JXA-8100 (JEOL, Akishima, Japan) (Electron Probe Laboratory, Institute of Earth Sciences, Chinese Academy of Sciences). The instrument used an acceleration voltage of 15 kV, beam current of 20 nA, and beam spot diameter of 5 μm. The trace element and rare earth element (REE) compositions were obtained using an LA–Q–ICP–MS (7500 a) (AGILENT, Santa Clara, CA, USA) from Agilent at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, located in the MC–ICP–MS laboratory. The sections for the mineral chemical studies were 3 mm × 3 mm × 1 mm in size and polished using a laser with a wavelength of 193 nm, diameter of 60 μm, and denudation frequency of 80 Hz. International standard NIST610 was used as the external standard, Si was the internal standard, and NIST612 was the monitored blind sample.

4. Results

4.1. Mineralogy

Tremolite is the main mineral in the chatoyant green nephrite from Ospinsk, and chromite, chlorite, graphite, and magnetite are accessory phases.

4.1.1. Tremolite

The tremolite grains were felted, fibrous, interlaced, oriented fibroblastic, foliated, and vein-like textures when viewed under a microscope (Figure 3). Green nephrite with a felted–fibrous interlaced texture is of a better quality than the other forms. Because it showed a delicate and compact appearance and was very hard, it was not easy to break with a hammer knock and the broken part was granular, which are the characteristics of high-quality green nephrite. Fibroblastic tremolite fibers were slender and ranged in size from 2 × 100 μm to 10 × 190 μm. The fibers were delicate and compact and aligned along two directions that intersect to form a network; the intersection angle of the directions were the typical conjugate shear angles of 60° and 120°. The foliated blastic texture was composed of irregular, small, and flaky tremolite grains that did not show any orientation, rendering the nephrite coarse and reducing its toughness. Vein-like tremolite occurred among the fibrous tremolite, which had formed at an earlier stage. The alignment was cross-vein; vein tremolite was directionally oriented with the same extinction position for all grains and the grains were nearly perpendicular to the lengthening direction of the vein. Interpenetrated tremolite grains had a fibroblastic texture with a weak directional alignment.

The petrography was accompanied by BSE imaging. Above a cavity (Figure 4a) the tremolite grains were small and more fibrous and directionally oriented. The tremolite shown in Figure 4b was finer than that shown in Figure 4a. Coarse-to-fine tremolite grain boundary diffusion was observed. The particle size was within the range of 5 × 6 μm~1 × 70 μm, and the particle size was small, which made the overall texture of the mineral more delicate.

The results of SEM (Figure 5) showed that the chatoyant green nephrite was microstructurally different from ordinary green nephrite. Tremolite grains were commonly fibroblastic and directionally oriented.

4.1.2. Subordinate Minerals

A small amount of magnetite was present in the studied green nephrite, not visible in the hand specimen, which appeared as euhedral crystals under a microscope. The section was almost square (Figure 6b) and the crystal was octahedral (Figure 6a). Small euhedral-magnetite inclusions were observed in the tremolite, showing their complete crystal shape, with relatively straight edges and no alteration trace. They appeared off-white under reflected light, with a strong metallic shine (Figure 6a). Combined with the subsequent Raman spectroscopy results, this area was determined to be magnetite. The BSE image (Figure 6b) shows magnetite in high brightness in silver white, and a clear boundary to tremolite.

Under plane-polarized light, graphite was black and opaque in the thin sheet and rotating it 360° led to full extinction. It appeared gray brown under reflected light and it strongly reflected light from a specific angle, then showing a bright gray–white, semi-metallic shine. In a high magnification, it can be detected as a group of completely separate aggregates. Combined with the subsequent Raman spectroscopy results, this area was determined to be graphite. Graphite occurred as altered hypidiomorphic flaky aggregates or as inclusions in the green nephrite (Figure 6c).

Chromite was iron black–brown black in the thin sheet; the sections were diamond-shaped or nearly square, almost opaque, only some edge parts were slightly transparent maroon red. Under reflected light it appeared black brown–gray and its metallic shine was not strong. Chlorite was brown, translucent, weakly polychromatic, light green–light brown-green, with low protrusion under cross-polarized light. Chlorite exhibited a first-level, gray–yellow white interference color under cross-polarized light. In the studied samples, chlorite had a high degree of alteration, occurred in the interstitial spaces between tremolite grains, and its complete crystal shape was not visible (Figure 6d). Part of the chlorite was split along the chromite particles (Figure 6e), and chlorite coated with chromite is visible in Figure 6f.

4.2. Spectroscopy

4.2.1. Tremolite

The infrared spectra of the studied chatoyant green nephrite (Figure 7a) were consistent with the standard infrared spectrum of tremolite [63]. In the infrared spectra of minerals of the tremolite–ferroactinolite isomorphic series, the wavenumbers of bands A–D were 3675 cm⁻¹, 3660 cm⁻¹, 3643 cm⁻¹, and 3625 cm⁻¹, respectively. With the increase in Fe²⁺ content, the peak value of 3643 cm⁻¹ became more prominent [63]. The test results (Figure 7b) showed two strong absorption peaks at 3674 cm⁻¹ and 3659 cm⁻¹. These absorption bands, namely the OH (MgMgMg) and OH (MgMgFe²⁺) absorption bands, occurred due to reverse stretching vibration of OH [24]. The results indicated the presence of Fe²⁺ in the studied chatoyant green nephrite.

Raman spectroscopy results showed that the matrix of the chatoyant green nephrite with the microcrystalline texture contained only the spectra of tremolite (Figure 7c). The Raman absorption peaks for tremolite were in the frequency band 100–420 cm⁻¹, and several peaks were attributed to vibrations of the (SiO₄)⁴⁻ lattice. There were three weak absorption peaks in the range of 420–500 cm⁻¹ due to the bending vibrations of Si-O, and a strong absorption peak occurred at 659 cm⁻¹. This strong peak was sharp and had a high intensity, and its position was attributed to symmetric stretching vibration of Si-O-Si. Three weak absorption peaks were noted in the range of 900–1100 cm⁻¹, and their positions were attributed to stretching vibrations of Si-O [64].

4.2.2. Magnetite

Characteristic shifts of 537 cm⁻¹ and 665 cm⁻¹ were observed in the Raman spectrum of magnetite (Figure 7d). According to the comparison of characteristic peaks, the peak corresponding to the vibrations of A_1g and T_2g of magnetite in the standard atlases [65], and the two characteristic absorption peaks observed here were characteristic of magnetite.

4.2.3. Graphite

In the Raman spectrum of sample RC7, the characteristic absorption peaks of graphite were superimposed at 1352 cm⁻¹ and 1582 cm⁻¹ above the characteristic lines of tremolite (Figure 7e). The Raman spectrum is very sensitive to the ordered state of the graphite structure and directly reflects the layered structure of the studied samples [66]. There were two prominent regions of vibration in the shift range of 1000–2000 cm⁻¹, peak D in the shift range of 1330–1380 cm⁻¹, and peak G in the shift range of 1550–1600 cm⁻¹. The morphology of these peaks reflected the difference in the degree of their ordering. Peak G was attributed to E_2g vibrations in the aromatic plane, which reflected the degree of ordering of the carbon atoms in the sample. With higher intensity of peak G, the aromatic structure of the carbon atoms became more ordered. Peak D was attributed to A_1g vibrations, which were caused by defects in the plane or by the existence of heteroatoms. The higher the intensity of peak D, the higher was the content of aliphatic, cycloaliphatic, and side chains in the structure of the sample. The order degree of graphite could be distinguished by the morphology of the D and G peaks [67,68]. D1 had a relatively narrow band and the intensity was also relatively lower, indicating that the content of aliphatic, cycloaliphatic, and side chains in the structure of the sample were relatively lower. The vibration peak around 1500 cm⁻¹ was absent, the G band became sharper and the intensity was stronger, indicating that graphite had a high degree of order, at the same time, D2 was very weak in the “right shoulder” of G, but could be distinguished from G. (Figure 7f).

4.3. Mineral Chemistry

Tremolite was mainly composed of SiO₂, MgO, and CaO, with minor quantities of FeO, MnO, Al₂O₃, and Cr₂O₃ (

Table 1. Electron probe microanalysis of tremolite in the selected samples (wt.%).

Sample	RC-1	RC-2	RC-3	RC-4	RC-5	RC-6	RC-7	RC-8	RC-9	RC-10
SiO2	56.65	58.17	57.78	56.34	55.64	56.32	56.19	56.05	56.68	57.63
Na2O	0.06	0.07	0.06	0.03	0.08	0.08	0.07	0.09	0.06	0.06
Cr2O3	0.01	0.03	0.04	0.03	0.19	0.13	0.14	0.03	0.31	0.11
K2O	0.06	0.04	0.07	0.20	0.13	0.04	0.02	0.05	0.02	0.04
MgO	21.75	22.30	22.27	21.91	21.54	21.41	21.36	21.43	21.68	22.34
MnO	0.11	0.12	0.11	0.12	0.10	0.11	0.12	0.13	0.10	0.12
FeO	3.90	3.45	3.79	3.71	4.03	3.97	3.78	3.91	3.49	3.67
Al2O3	0.31	0.30	0.27	0.56	0.65	0.38	0.34	0.37	0.43	0.29
NiO	0.08	0.16	0.12	0.10	0.13	0.14	0.13	0.11	0.16	0.11
CaO	12.17	12.30	12.16	11.82	12.13	12.17	12.27	11.59	12.47	12.10
TiO2	0.01	0.01	0.03	0.02	0.02	0.01	0.04	0.04	0.02	bdl
Cl	bdl	0.01	0.02	0.01	bdl	bdl	0.01	0.02	0.01	bdl
Total	95.10	96.95	96.71	94.85	94.64	94.75	94.46	93.81	95.41	96.46
Si	8.00	8.03	8.01	7.97	7.92	7.99	8.00	8.02	7.98	8.01
Na	0.02	0.02	0.02	0.01	0.02	0.02	0.02	0.02	0.02	0.02
Cr	bdl	bdl	0.01	bdl	0.02	0.01	0.02	bdl	0.03	0.01
K	0.01	0.01	0.01	0.04	0.02	0.01	bdl	0.01	bdl	0.01
Mg	4.58	4.59	4.61	4.62	4.57	4.53	4.53	4.57	4.55	4.63
Mn	0.01	0.01	0.01	0.01	0.01	0.01	0.01	0.02	0.01	0.01
Fe	0.46	0.40	0.44	0.44	0.48	0.48	0.45	0.47	0.41	0.43
Al	0.05	0.05	0.04	0.09	0.11	0.06	0.06	0.06	0.07	0.05
Ni	0.01	0.02	0.01	0.01	0.02	0.02	0.01	0.01	0.02	0.01
Ca	1.84	1.82	1.81	1.79	1.85	1.85	1.87	1.78	1.88	1.80
Ti	bdl	bdl	bdl	bdl	bdl	bdl	bdl	0.01	bdl	bdl
Cl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl	bdl
Sum	14.98	14.95	14.98	15.00	15.03	14.98	14.98	14.97	14.98	14.97
Mg + Fe	5.04	4.99	5.04	5.06	5.05	5.00	4.98	5.04	4.96	5.05
Mg/(Mg + Fe)	0.91	0.92	0.91	0.91	0.90	0.91	0.91	0.91	0.92	0.92
Fe/(Mg + Fe)	0.09	0.08	0.09	0.09	0.10	0.09	0.09	0.09	0.08	0.08

Note: bdl: below detection limit (0.01 wt.%); FeO = FeO + Fe₂O₃.

). The contents of SiO₂, CaO, and MgO in the chatoyant green nephrite samples were 55.64–58.17 wt.%, 11.59–12.29 wt.%, and 21.36–22.34 wt.%, respectively, which were slightly lower than their standard values (59.17 wt.%, 13.8 wt.%, and 24.81 wt.%, respectively).

In amphiboles, each cation can form a plasma-like substitution within the crystal structure. The ionic radius of Al³⁺ (0.053 nm) is similar to that of Si⁴⁺ (0.04 nm), which occupies the central position in the Si-O backbone tetrahedron in the amphibole. The ionic radius of Fe²⁺ (0.078 nm), occupying positions M1, M2, and M3 (the gaps between the chains of a [SiO₄] tetrahedron with opposite vertices) is similar to that of the off-chain cation Mg²⁺ (0.072 nm) on the Si-O backbone. The ionic radius of Na⁺ (0.102 nm), occupying position M4 is similar to that of the off-chain cation Ca²⁺ (0.100 nm) on the Si-O backbone. Ionic substitution can easily occur in the crystalline structure, resulting in a decrease in SiO₂, CaO, and MgO contents.

According to Tang et al. (2002), Russian green nephrite was formed by the alteration of ultramafic rock [25]. The content of SiO₂ in ultramafic rock was less than 45%, and the content of Si in the sample was high. This indicated that the contents of Si in the host ultramafic rock increased during nephritization, thereby indicating that they were supplied by hydrothermal fluid. Mg contents of the tremolite decreased, indicating that it was supplied by ultramafic rock during the formation of green nephrite. Al contents of the serpentinized ultramafic rock were relatively low, providing an Al-poor environment for the formation of green nephrite.

The Fe²⁺ ions in amphiboles were usually isomorphically substituted by Mg cations. Calculation results are presented in Table 1 (according to the estimation method, the value of Fe³⁺ is negative, so the estimation result was abandoned and all iron was regarded as bivalent). The Mg/(Mg + Fe²⁺) value of the Ospinsk samples was >0.9, with tremolite as the single component. The cation number (Table 1) was calculated using the cationic method, and the average crystallo-chemical formula of tremolite in the studied green nephrite samples was calculated as Ca_1.83(Mg_4.58Fe_0.44Al_0.06Mn_0.01)[Si_7.99O₂₂](OH)₂.

4.4. Geochemistry

In general, the content of Cr (900–2812 ppm), Ni (958.7–1898 ppm), and Co (42–207 ppm) in S-type nephrite is high, while the content of Cr (2–179 ppm), Ni (0.05–471 ppm), and Co (0.5–10 ppm) in D-type nephrite is low [23]. The analysis of trace elements (

Table 2. Trace element data of the selected samples (ppm).

Sample	RC1			RC2			RC3			RC5
	01	02	03	01	02	03	01	02	03	01	02	03
Li	0.26	0.32	0.28	0.60	0.47	0.44	0.13	0.16	0.14	0.49	0.55	0.51
Be	0.30	0.33	0.20	0.49	0.65	0.35	0.23	0.35	0.25	0.51	0.49	0.37
Sc	0.43	0.65	0.46	1.75	1.62	1.77	0.47	0.56	0.55	2.44	2.62	2.16
Ti	100.40	130.51	107.13	156.15	140.65	148.25	86.47	93.79	95.65	134.93	141.64	115.80
V	4.38	5.69	4.55	8.60	7.95	8.05	4.90	5.24	5.21	9.75	10.41	8.45
Cr	2.30	1.84	2.76	196.73	174.51	167.93	15.57	17.81	19.30	893.38	967.12	778.95
Mn	472.26	619.23	504.52	724.43	664.68	692.81	372.91	407.63	413.32	614.44	642.69	521.54
Ni	514.73	672.90	550.83	1190.88	1105.36	1155.08	584.88	633.07	636.04	1014.43	1056.54	865.86
Co	28.26	36.58	29.95	44.43	39.83	41.06	23.89	26.12	25.63	38.19	40.80	32.92
Cu	0.13	bdl	0.19	0.17	0.12	0.53	0.04	0.12	0.10	0.28	0.01	0.27
Zn	68.30	90.72	72.90	119.80	107.91	111.88	57.46	62.26	62.34	93.96	96.83	78.96
Ga	0.66	0.86	0.61	0.86	0.91	1.02	0.60	0.63	0.69	0.89	0.98	0.87
Rb	0.57	0.94	0.55	0.73	0.76	0.95	0.63	0.69	0.67	0.99	1.09	0.96
Sr	2.65	3.44	2.91	3.79	3.74	3.83	2.26	2.40	2.46	3.11	3.52	2.81
Zr	0.06	0.20	0.15	0.18	0.21	0.21	0.25	0.22	0.28	0.26	0.18	0.15
Nb	0.01	0.02	0.01	0.04	0.05	0.04	0.02	0.02	0.03	0.03	0.07	0.02
Ba	1.82	2.37	1.94	1.82	2.18	2.97	1.71	2.01	2.19	2.60	3.24	2.89
Hf	0.02	0.03	0.03	0.01	0.01	0.05	0.03	0.03	0.03	0.05	0.03	bdl
Ta	bdl	0.01	0.01	0.02	0.01	0.01	0.01	bdl	bdl	0.01	0.03	bdl
Pb	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41
Th	0.02	0.01	0.02	0.03	0.01	0.02	0.01	0.02	0.01	0.02	bdl	bdl
U	0.01	0.01	bdl	0.03	0.02	0.02	0.01	0.02	0.02	bdl	0.02	0.02
Sample	RC6			RC8			RC9			RC10
	01	02	03	01	02	03	01	02	03	01	02	03
Li	0.67	0.58	0.70	0.48	0.47	0.38	3.92	1.41	1.54	0.57	0.64	0.31
Be	0.50	0.21	0.76	0.75	0.22	0.38	1.52	0.58	0.19	0.50	0.04	0.35
Sc	2.46	2.33	2.90	1.42	1.31	1.37	14.38	5.32	6.23	1.83	1.94	1.03
Ti	144.68	141.06	156.00	172.04	156.95	145.79	500.06	202.33	230.21	159.93	168.53	92.22
V	10.03	9.52	11.51	8.57	7.59	7.19	32.51	12.16	13.38	11.47	11.52	6.52
Cr	697.53	640.51	747.27	68.67	61.51	59.92	5749.78	2119.58	2398.10	330.61	320.36	158.56
Mn	687.16	679.60	792.57	933.49	838.70	789.74	1692.79	744.78	798.29	692.58	718.40	402.83
Ni	1070.42	1040.15	1148.30	1131.03	1019.83	956.90	4204.46	1758.10	1973.65	987.73	1063.48	563.60
Co	40.75	40.16	44.69	48.33	43.32	41.70	128.94	53.63	61.25	44.06	46.75	24.98
Cu	0.10	0.14	0.29	0.02	0.16	0.14	0.11	0.33	0.22	0.32	0.13	1.80
Zn	105.87	106.29	118.73	139.38	125.25	116.72	288.88	120.35	130.71	112.57	112.17	75.17
Ga	1.00	0.84	1.13	1.24	1.15	1.15	3.29	1.48	1.78	11.06	0.97	0.67
Rb	1.30	1.78	1.26	1.07	0.82	0.90	4.15	1.94	2.39	0.79	1.17	0.37
Sr	3.31	3.16	3.71	4.88	4.30	3.96	12.39	5.28	6.10	4.95	3.52	1.78
Zr	0.31	0.25	0.31	0.16	0.15	0.20	0.30	0.23	0.41	0.11	0.06	0.06
Nb	0.06	0.05	0.06	0.08	0.04	0.09	0.29	0.10	0.07	0.04	0.04	0.04
Ba	2.91	3.07	3.69	3.78	3.48	3.09	11.47	5.14	5.22	51.93	2.57	2.39
Hf	0.02	0.08	0.02	bdl	0.08	0.06	0.12	0.04	0.01	0.03	0.02	0.01
Ta	0.01	0.02	0.01	0.03	0.02	0.01	0.02	0.03	0.02	0.02	0.01	bdl
Pb	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41	0.41
Th	0.03	0.03	0.05	0.04	0.01	0.01	0.04	0.03	0.03	0.01	0.03	0.01
U	0.02	0.02	0.03	0.03	0.03	0.01	0.06	0.02	0.04	0.02	0.03	0.01

Note: bdl: below detection limit (0.01 ppm).

) showed that the average Cr, Ni, and Co content in the studied green nephrite was within the range of S-type nephrite: Cr (691.3 ppm), Ni (1120.8 ppm), and Co (42.8 ppm). Indicated by a large span of colors in a single sample, the Cr content (1.84–5749.78 ppm) fluctuated greatly. Fe²⁺/(Mg + Fe²⁺) values were >0.06, with a mean of 0.089. These two observations indicated that the studied S-type green nephrite was an altered product from ultramafic rock. Co and Ni are iron-like elements present in high concentrations in ultramafic rocks. Their contents in the studied samples were similar to those in the host rock, indicating that they were supplied by the ultramafic rock.

The trace elements and REE of the samples were normalized using chondrites and a primitive mantle [69]. The samples were depleted of trace elements compared to chondrites. Zr, Ho, and Nb were the most depleted, while Yb, Tb, U, Ta, and Ba showed relatively high concentrations (Figure 8a). Large ion lithophile elements (Sr, Ba, and Pb) were enriched, while high-field-strength elements were depleted.

The ∑REE (

Table 3. Data on rare earth elements in the selected samples (ppm).

Sample	RC1-1	RC2-1	RC3-1	RC5-1	RC6-1	RC8-1	RC9-1	RC10-1
La	0.14	0.09	0.13	0.08	0.05	0.12	0.07	0.06
Ce	0.43	0.19	0.37	0.26	0.17	0.19	0.23	0.15
Pr	0.07	0.04	0.05	0.04	0.05	0.05	0.05	0.04
Nd	0.41	0.27	0.26	0.34	0.25	0.23	0.29	0.25
Sm	0.27	0.26	0.27	0.25	0.31	0.24	0.26	0.25
Eu	0.07	0.06	0.08	0.07	0.06	0.09	0.07	0.07
Gd	0.27	0.29	0.35	0.30	0.35	0.28	0.28	0.27
Tb	0.04	0.04	0.05	0.04	0.04	0.04	0.05	0.05
Dy	0.17	0.17	0.17	0.18	0.18	0.16	0.19	0.18
Ho	0.04	0.03	0.05	0.04	0.04	0.05	0.04	0.05
Er	0.11	0.11	0.10	0.10	0.13	0.14	0.11	0.12
Tm	0.03	0.04	0.04	0.03	0.05	0.04	0.05	0.04
Yb	0.14	0.16	0.17	0.20	0.17	0.21	0.20	0.20
Lu	0.05	0.04	0.04	0.05	0.04	0.04	0.05	0.04
Y	0.21	0.26	0.25	0.10	0.15	0.24	0.16	0.23
ΣREE	2.24	1.79	2.11	1.97	1.88	1.86	1.93	1.79
LREE	1.39	0.91	1.15	1.03	0.88	0.91	0.97	0.83
HREE	0.84	0.87	0.96	0.94	1.00	0.95	0.96	0.96
LREE/HREE	1.66	1.04	1.19	1.10	0.89	0.96	1.01	0.87
LaN/YbN	0.74	0.42	0.52	0.30	0.22	0.40	0.27	0.22
δEu	0.83	0.71	0.77	0.81	0.56	1.05	0.77	0.85
δCe	1.11	0.76	1.19	1.05	0.78	0.60	0.91	0.70

) of the studied chatoyant green nephrite was low (average 1.9 ppm), indicating their origin from ultramafic rock. The average ratio of light REE (LREE) to heavy REE (HREE) was 1.09 (0.87–1.66), and the average ratio of (La/Yb)_N was 0.39 (0.22–0.74). The chondrite-normalized REE patterns of the selected samples (Figure 8b) had a smaller slope. The average ratio of light REE (LREE) to heavy REE (HREE) of 1.09 showed that samples were depleted of HREE with slight enrichment of LREE. Eu showed a negative anomaly (Eu* = 0.6 − 1), which could be associated with the large amount of plagioclase in the host rock that led to negative Eu in the residual melt. This suggests that the nephrite-forming fluids might have originated from highly fractionated rocks.

5. Discussion

5.1. Comparison of Geochemical Characteristics

In this study, the authors selected S-type green nephrite samples from Golmud, Qinghai [30]; Hetian, Xinjiang [70]; Manas, Xinjiang [26]; Qiemo, Xinjiang [27]; Hualian, Taiwan [33]; Sichuan, China [71]; Kutcho, Canada [43]; Gorlikgol, Russia [11]; Cassiar, Canada [45]; Polar, Canada [46]; Ruoqiang, Aqikekule, Xinijang [29]; Rium, New Zealand [72]; South Westland, New Zealand [72]; and Pakistan [23]. The selected samples were compared with the Ospinsk chatoyant green nephrite for clarifying the mineral composition and geochemical characteristics of Ospinsk chatoyant green nephrite (

Table A1. Some major elements of samples of green nephrite from different deposits (ppm).

	SiO2	Al2O3	MgO	CaO	FeO	Si	Al	Mg	Ca + K + Na	Fe2+ + Fe3+	Fe2+	Mg/(Mg + Fe2+)	Fe2+/(Mg + Fe2+)
RC1	56.65	0.31	21.75	12.17	3.90	8.35	0.05	4.78	1.95	0.48	0.48	0.91	0.09
RC2	58.17	0.30	22.30	12.30	3.45	8.38	0.05	4.79	1.92	0.42	0.42	0.92	0.08
RC3	57.78	0.27	22.27	12.16	3.79	8.36	0.05	4.81	1.91	0.46	0.46	0.91	0.09
RC4	56.34	0.56	21.91	11.82	3.71	8.32	0.10	4.82	1.92	0.46	0.46	0.91	0.09
RC5	55.64	0.65	21.54	12.13	4.03	8.27	0.11	4.77	1.98	0.50	0.50	0.91	0.10
RC6	56.32	0.38	21.41	12.17	3.97	8.34	0.07	4.73	1.96	0.49	0.49	0.91	0.09
RC7	56.19	0.34	21.36	12.27	3.78	8.34	0.06	4.73	1.98	0.47	0.47	0.91	0.09
RC8	56.05	0.37	21.43	11.59	3.91	8.37	0.07	4.77	1.89	0.49	0.49	0.91	0.09
RC9	56.68	0.43	21.68	12.47	3.49	8.33	0.07	4.75	1.98	0.43	0.43	0.92	0.08
RC10	57.63	0.29	22.34	12.10	3.67	8.36	0.05	4.83	1.91	0.45	0.45	0.92	0.08
HT-1	57.56	0.81	22.35	12.79	3.09	7.95	0.13	4.60	1.93	0.36	0.36	0.93	0.07
HT-2	57.71	0.72	21.46	12.49	3.58	8.07	0.12	4.47	1.90	0.42	0.42	0.91	0.09
GM-1	56.91	0.96	21.45	13.30	5.45	7.61	0.15	4.46	1.93	0.52	0.52	0.90	0.10
GM-2	57.09	0.65	21.61	13.06	4.48	7.68	0.10	4.49	1.91	0.48	0.48	0.91	0.09
GM-3	56.37	1.33	21.72	13.49	3.22	7.83	0.17	4.56	1.95	0.40	0.40	0.94	0.07
MNS-1-2	55.87	0.85	21.68	12.50	4.53	7.86	0.14	4.55	1.91	0.53	0.53	0.90	0.10
MNS-3-5	58.58	0.49	20.70	12.76	4.32	8.17	0.08	4.30	1.94	0.50	0.50	0.90	0.10
MNS-3-7	58.14	0.05	21.11	12.89	3.62	8.18	0.01	4.43	1.96	0.43	0.43	0.91	0.09
MNS-4-2	60.11	0.51	18.74	12.17	6.69	8.38	0.08	3.90	1.85	0.78	0.78	0.83	0.17
MNS-5-1	56.62	0.10	22.71	13.36	3.77	7.85	0.02	4.70	1.99	0.44	0.44	0.91	0.09
QM-1-2	56.40	0.30	20.90	12.50	4.70	8.00	0.05	4.42	1.92	0.56	0.56	0.89	0.11
QM-3-2	56.60	0.74	21.50	12.60	5.14	7.86	0.12	4.45	1.90	0.60	0.60	0.88	0.12
QM-5-3	57.00	0.86	20.30	13.30	6.59	7.87	0.14	4.18	1.98	0.76	0.76	0.85	0.15
QM-5-4	56.80	0.77	19.40	13.00	6.65	7.98	0.13	4.07	1.97	0.78	0.78	0.84	0.16
HL-1-1	55.96	0.49	20.21	12.59	7.75	7.84	0.08	4.22	1.96	0.91	0.78	0.84	0.16
HL-1-2	55.59	0.53	19.56	12.85	8.37	7.84	0.09	4.11	2.01	0.99	0.85	0.83	0.17
HL-4-1	56.96	0.32	22.56	12.88	4.24	7.92	0.03	4.75	1.87	0.40	0.29	0.94	0.06
HL-4-2	57.58	0.19	23.18	12.72	3.49	7.87	0.05	4.65	1.94	0.15	0.34	0.93	0.07
SC2	59.02	0.28	20.01	5.74	5.74	8.15	0.04	4.12	1.97	0.66	0.66	0.86	0.14
SC3	58.89	0.55	19.09	5.97	5.97	8.17	0.09	3.95	2.03	0.69	0.69	0.85	0.15
SC4	58.41	0.48	19.45	6.35	6.35	8.03	0.08	3.99	2.07	0.73	0.73	0.85	0.15
KC1-2	57.96	0.27	11.99	11.99	4.24	8.04	0.04	4.58	1.79	0.49	0.49	0.90	0.10
KC2-4	57.24	0.24	20.89	12.79	4.46	8.05	0.04	4.38	1.95	0.49	0.49	0.90	0.10
KC3-2	55.47	0.80	19.74	12.21	6.08	7.93	0.14	4.21	1.92	0.67	0.67	0.86	0.14
KC4-1	57.04	0.76	20.35	12.40	6.00	7.97	0.13	4.24	1.89	0.67	0.67	0.86	0.14
KC5-4	56.41	0.24	21.05	12.83	4.85	7.95	0.04	4.42	1.97	0.57	0.57	0.89	0.11
GG-2-1	56.80	0.27	19.78	13.44	6.49	7.93	0.04	4.12	2.09	0.76	0.62	0.87	0.13
GG-3-1	56.93	bdl	20.48	13.32	6.25	7.91	0.08	4.24	2.02	0.73	0.58	0.88	0.12
GG-4-1	58.27	0.05	22.49	12.86	3.31	8.01	0.03	4.61	1.93	0.38	0.38	0.92	0.08
GG-5-1	57.22	0.07	21.89	13.49	3.81	7.92	0.06	4.52	2.03	0.44	0.31	0.94	0.06
GG-6-1	56.61	0.05	19.21	13.38	8.20	7.94	0.02	4.02	2.04	0.96	0.85	0.83	0.18
GG-6-2	58.59	0.05	21.68	12.85	3.61	8.08	0.02	4.46	1.91	0.42	0.41	0.92	0.08
GG-7-2	56.63	0.10	21.61	13.17	5.40	7.86	0.02	4.47	1.98	0.63	0.36	0.93	0.08
GG-8-1	56.99	0.15	21.26	13.30	4.77	7.95	0.02	4.42	2.01	0.56	0.47	0.90	0.10
GG-8-2	56.70	0.03	20.87	13.50	5.97	7.90	0.01	4.33	2.04	0.70	0.49	0.90	0.10
GG-10-1	57.10	0.13	21.43	13.48	4.85	7.90	0.04	4.42	2.02	0.56	0.41	0.92	0.08
GG-10-2	55.61	0.28	17.89	13.56	9.83	7.88	0.04	3.78	2.09	1.16	0.96	0.80	0.20
CC1-1	57.54	0.58	21.07	12.92	4.34	8.00	0.10	4.37	1.98	0.50	0.50	0.90	0.10
CC1-2	57.01	0.79	20.87	13.29	4.43	7.93	0.13	4.33	2.03	0.52	0.52	0.89	0.11
CC2-1	57.60	0.63	20.75	12.97	4.85	7.99	0.10	4.29	1.99	0.56	0.56	0.88	0.12
PC-1-1	56.69	0.15	22.21	13.04	4.69	7.82	0.02	4.57	2.00	0.54	0.54	0.89	0.11
PC-2-1	57.03	0.21	22.51	12.99	3.81	7.85	0.03	4.62	2.00	0.44	0.44	0.91	0.09
PC-3-1	58.11	0.04	22.94	13.34	3.95	7.87	0.01	4.63	1.99	0.45	0.45	0.91	0.09
PC-4-1	57.47	0.16	22.83	13.20	3.68	7.63	0.03	4.52	1.95	0.41	0.41	0.92	0.08
RQ-1	58.07	0.29	23.17	12.69	4.30	7.86	0.05	4.68	1.89	0.49	0.49	0.91	0.09
RQ-2	56.32	0.27	26.45	12.85	3.82	7.45	0.04	5.21	1.88	0.42	0.42	0.93	0.07
RQ-3	56.89	0.29	26.74	12.19	4.18	7.49	0.04	5.25	1.75	0.46	0.46	0.92	0.08
RQ-4	57.43	0.39	25.15	13.03	3.73	7.62	0.06	4.98	1.91	0.41	0.41	0.92	0.08
Rium	56.74	0.39	21.45	12.88	5.64	7.83	0.06	4.41	1.99	0.65	0.65	0.87	0.13
SW-2	56.84	0.07	23.03	12.12	4.15	7.81	0.01	4.72	1.93	0.48	0.48	0.91	0.09
SW-3	57.68	0.04	22.86	12.22	4.36	7.88	0.01	4.66	1.93	0.50	0.50	0.93	0.07
Vitim-3	57.40	1.10	24.90	12.40	0.66	7.83	0.18	5.07	1.84	0.08	0.08	0.99	0.01
PAK-1	58.18	0.15	21.92	13.37	3.55	8.02	0.02	4.50	2.01	0.43	0.41	0.92	0.08
PAK-2	58.22	0.19	21.49	13.37	3.45	8.07	0.03	4.44	2.02	0.43	0.40	0.92	0.08
PAK-3	57.60	0.29	21.51	13.23	3.24	8.01	0.05	4.46	2.03	0.44	0.38	0.92	0.08

Note: bdl: below detection limit; RC: Ospinsk, Russia; GM: Golmud, Qinghai [30]; HT: Hetian, Xinjiang [70]; MNS: Manas, Xinjiang [26]; QM: Qiemo, Xinjiang [27]; HL: Hualian, Taiwan [33]; SC: Shimian, Sichuan [71]; KC: Kutcho, Canada [43]; GG: Gorlikgol, Russia [11]; CC: Cassiar, Canada [45]; PC: Polar, Canada [46]; RQ: Ruoqiang, Aqikekule, Xinijang [29]; Rium: Rium, New Zealand [72]; SW: South Westland, New Zealand [72]; PAK: Pakistan [23].

and

Table A2. Some trace elements of samples of green nephrite from different deposits (ppm).

	Ti	V	Cr	Mn	Co	Ni	Cu	Sr/Ba	Zr/Hf	Nb/Ta
RC-1	100.40	4.38	2.30	472.26	28.26	514.73	0.13	1.45	11.33	1bdl
RC-2	156.15	8.60	196.73	724.43	44.43	1190.88	0.17	2.09	36.80	2.32
RC-3	86.47	4.90	15.57	372.91	23.89	584.88	0.04	1.32	7.72	1.38
RC-5	134.93	9.75	893.38	614.44	38.19	1014.43	0.28	1.20	4.96	2.72
RC-6	144.68	10.03	697.53	687.16	40.75	1070.42	0.10	1.13	14.76	7.00
RC-8	172.04	8.57	68.67	933.49	48.33	1131.03	0.02	1.29	40.50	2.96
RC-9	500.06	32.51	5749.78	1692.79	128.94	4204.46	0.11	1.08	2.43	14.30
RC-10	159.93	11.47	330.61	692.58	44.06	987.73	0.32	1.92	3.53	2.47
CP-1	53.82	34.97	549.86	847.82	48.32	675.36	0.47	-	-	-
CP-2	68.69	22.17	106.03	1112.58	25.25	363.06	0.34	-	-	-
CP-3	12.88	9.55	322.94	1965.42	41.47	417.02	0.49	-	-	-
CP-4	49.90	29.55	780.68	703.64	51.23	939.17	0.67	-	-	-
KC-1	30.18	24.57	942.02	3797.89	58.49	1763.75	0.46	0.36	1.39	3.00
KC-2	29.90	24.72	720.36	3908.12	57.45	1610.58	0.54	0.33	1.22	3.33
KC-3	30.40	25.72	1269.59	5006.82	50.55	1678.26	0.54	0.56	6.04	2.25
KC-4	38.02	22.19	1268.99	1235.53	50.98	1531.30	0.58	0.33	5.53	1.43
KC-5	80.82	30.04	1241.58	4501.67	65.08	1973.64	0.68	0.66	1.85	2.14
GM-1	69.76	663.81	1869.31	889.80	74.49	1053.08	3.42	0.83	45.91	12.00
GM-2	65.96	648.00	848.00	886.00	41.03	511.00	5.00	0.86	41.20	12.00
GM-3	159.78	466.42	1169.97	1016.00	42.79	490.99	3.50	0.56	46.53	14.33
MNS-1	50.54	28.37	2305.00	5.82	83.78	1674.00	2.76	11.07	49.04	13.06
MNS-2	94.10	23.18	2189.00	9.63	81.79	1726.00	5.06	10.16	36.87	15.50
MNS-3	10.26	15.94	1315.00	4.26	57.80	123bdl	2.51	3.47	77.67	27.00
MNS-4	43.15	19.99	143bdl	6.31	48.03	1008.00	4.16	10.44	45.30	9.93
Rium-1	32.89	8.27	475.19	1049.64	56.74	800.54	0.16	4.32	7.28	7.15
Rium-2	10.97	10.08	744.40	413.60	50.64	711.86	0.26	0.68	14.12	8.00
SW2	31.78	20.72	1131.24	943.57	87.91	1489.08	0.23	7.80	1.28	2.94
SW3	32.31	13.55	72.32	643.65	32.77	284.27	0.23	5.40	8.68	1.94

Note: RC: Ospinsk, Russia; CP: Polar, Canada [46]; KC: Kutcho, Canada [43]; GM: Golmud, Qinghai [30]; MNS: Manas, Xinjiang [26]; Rium: Rium, New Zealand [72]; SW: South Westland, New Zealand [72].

).

5.1.1. Comparison of Characteristics of Major Elements

Mg is mainly controlled by the protolith composition, and its content in green nephrite from different origin types varies significantly. The compositions of green nephrite obtained from multiple sites were divided according to the amphibole classification scheme (Figure 9). The main mineral of the studied green nephrite was tremolite, similar to the S-type green nephrite samples from Hetian in Xinjiang, Golmud in Qinghai, and Pakistan. The green nephrites from Manas in Xinjiang, Hualien in Taiwan, Gorlikgol in Russia, Polar in Canada, and New Zealand contained both actinolite and tremolite. The green nephrite from Sichuan and Qiemo in Xinjiang, and Kutcho and Cassiar in Canada, contained only actinolite. It is generally believed that a higher iron content yields the increased ferroactinolite component.

The Fe²⁺ and Fe³⁺ content of amphiboles of different origins also varies greatly [74,75,76]. Ca²⁺, Na⁺, and K⁺ usually occupy the M4 position (the gap between the chains of a [SiO₄] tetrahedron with opposing bottoms). Changes in the type and number of cations occupying the vacant M4 site change the crystal system, type of symmetry, and space group of the species. A ternary plot of c(Mg), c(Ca + Na + K), and c(Fe²⁺ + Fe³⁺) is often employed to discriminate between amphiboles [77,78]. The cation values c(Mg), c(Ca + Na + K), and c(Fe²⁺ + Fe³⁺) in the tremolite crystal structure calculated from the electronic probe test data were projected into the images of classified amphiboles. According to this plot (Figure 10a), green nephrite deposits from all the selected areas, including the Ospinsk mine, fall under contact metasomatic deposits.

Previous studies identified the types of nephrite deposits using the Fe²⁺/(Mg + Fe²⁺⁾ ratio because iron in S-type nephrite mainly comes from iron-rich serpentinite. The Fe²⁺/(Mg + Fe²⁺) ratios of green nephrite from different areas were all higher than 0.06 (Figure 10b, the horizontal axes have no practical significance and areas are listed in order), indicating that they were formed from ultramafic rocks (S-type).

5.1.2. Trace Elements

Figure 11a–c show that the Ti contents in the studied samples were significantly higher than in S-type green nephrite of different origin, but the fluctuation in content was low. Golmud and Manas green nephrite occasionally contain high Ti content, but Green nephrite from Golmud and Manas contained >2 ppm Cu, while the samples from other areas contained ≤1 ppm Cu. The V content of the studied green nephrite samples was lower than in those from other areas, while the V content of Golmud green nephrite was extremely high, which can be used as an identifying feature. The Cr in the studied samples was relatively lower compared to samples from other regions, and the difference in Cr content was more obvious for different samples. The green nephrite from Polar, Canada, also contained little Cr, but it contained little Ti and Ni compared to the studied green nephrite samples, while that from Kutcho, Canada, was abundant in Ni and contained more Mn than samples from other areas. The Mn content in green nephrite from Manas was lower than that in the green nephrite from other areas. It can be concluded that in comparison to the S-type green nephrite of different origin, that from Ospinsk has a high Ti content (86.47–500 ppm), a low V content (4.38–32.51 ppm), and a low Cr content (1.84–5749 ppm), which can be used as the identification characteristics of its origin.

The Cr–Co–Ni content of S-type green nephrite from all areas was high, (Figure 11d). Cr, Ni, and Co can also be used to identify the petrogenetic characteristics of nephrite [1]. S-type nephrite deposits contain large amounts of Cr, Co, Ni, and Fe, indicating that greater amounts are retained from the S-type nephrite protolith than the D-type nephrite protolith [79]. The contents of transition metal elements, such as Cr (900–2812 ppm), Ni (958.7–1898 ppm), and Co (42–207 ppm), are high in S-type nephrite, but low in D-type nephrite (Cr = 2–179 ppm, Ni = 0.05–471 ppm, Co = 0.5–10 ppm) [50,79,80].

A comparison of possible coloring elements contained in the trace elements of the chatoyant green nephrite from Ospinsk, is shown in Figure 12; the test results for each trace element showed their contents in sample RC9 were higher than in the other samples. Excluding this set of data, Cr and Ni in the studied samples were relatively enriched. With the change of sample color, the difference of Cr content (1.84–5749 ppm) is the most obvious, and the Ni content is the second highest (514.7–4204 ppm). When the samples were ordered by their green color from deep to shallow, the content of Cr showed an obvious decrease, the content of Ni changed following no obvious rule, and the contents of the elements Co, Ti, and V did not change significantly. When the content of elemental Cr was 400–2000 ppm, green nephrite appeared emerald green with a high saturation and more uniform color; it had a light green color when the content was below 50 ppm. It can be concluded that the depth of bluish-green of the chatoyant green nephrite from Ospinsk is mainly related to the amount of Cr content.

Zr/Hf and Nb/Ta are a pair of geochemical twins that have the same ionic charges and highly similar ionic radii. They gradually increase in amount during mineralization in highly alkaline environments [81,82,83]. The Sr/Ba ratio gradually increases from the coastal region towards the deeper part of the ocean, and is an important indicator for the identification of sedimentary environments [84,85,86]. Therefore, the Zr/Hf, Nb/Ta, and Sr/Ba ratios can be used to analyze variations in the alkalinity of mineralization environments. Increasing order of alkalinity of mineralized environments from which the nephrite originated, was Kutcho–Golmud–Ospinsk–New Zealand–Manas (Figure 13). A Sr/Ba ratio > 1 represents saline water, indicating an alkaline metallogenic environment. The Sr/Ba ratios of the studied chatoyant green nephrite ranged between 1 and 2, which were higher than those of the green nephrite from Kutcho, Canada and Golmud, Qinghai, but lower than those from Manas, Xinjiang, and New Zealand.

5.2. Formation Temperature of Graphite

Carbonaceous materials are widely found in metamorphic rocks, especially in those with a sedimentary protolith [87]. Unstable carbonaceous materials gradually change to stable graphite with burial and metamorphism. The degree of graphitization is associated with the peak metamorphic temperature, and is not affected by retrogression [88]. Therefore, the peak metamorphic temperature can be quantified by measuring the degree of graphitization of the carbonaceous materials.

The Raman spectral features of graphite can be used as an indicator of the metamorphic grade [89,90]. In our sample, the D peaks of graphite in the Raman spectrum were divided into D1, D2, and D3 (Figure 7f). D1, or the defect band, was associated with planar defects and heteroatoms. D3 appeared only in poorly crystalline graphite, and was usually related to outer defects in the aromatic layer. D2 was also associated with lattice defects [91]. D1 and D2 became weaker with an increase in the metamorphic grade. In low-temperature metamorphic facies, D1 had a wider band and greater strength, whereas in high-temperature metamorphic facies, it became narrower and weaker. With increasing metamorphic grade, the G band in graphite became wider in graphite with reduced crystallinity, and was located in the shift range of 1583–1589 cm⁻¹, becoming indistinguishable from D2. In crystalline graphite, the G band was narrow and located at 1577–1580 cm⁻¹. Although D2 was weak, it could still be distinguished from G. In low-temperature metamorphic facies, poorly crystalline graphite showed a Raman shift peak at 1500 cm⁻¹. However, it did not appear in high-temperature metamorphic facies [92]. The D peak of graphite in the sample was weak and narrow, and no peak appeared at a shift of 1500 cm⁻¹. The G peak was wide and occurred at a shift of 1582.07 cm⁻¹, close to the range of 1583–1589 cm⁻¹. However, the G band was narrow. The D2 and G peaks were distinguishable, indicating that the graphite in the sample was purely crystalline.

The Raman spectrum of the first sequence was broken down by resolving the overlapping peaks to obtain the D1/G intensity (R1). The ratio (R2) of the peak area of D1/(D1 + D2 + G) and the full wave at half maximum of the G peak (FWHM) were obtained using the Lorentz function. Based on these quantitative parameters, the metamorphic conditions of graphite inclusions in the studied green nephrite were further investigated (

Table 4. Peak position parameters in the Raman spectrum of graphite in the sample RC7.

	Peak Area	BeginPointX (cm−1)	EndPointX (cm−1)	FWHM (cm−1)	Raman Shift (cm−1)	Intensity (Counts)
D1	6585.20	1288.44	1402.74	44.35	1352.14	134.34
G	10,289.37	1523.25	1619.01	20.75	1582.07	362.34
D2	698.97	1619.01	1646.60	11.80	1619.01	57.20

).

The degree of graphite crystallinity can be determined using the FWHM of the G band, which became sharper with an increase in the degree of crystallization and vice versa. For example, in the high-temperature metamorphic lithofacies sample TML of Sangpiyu nephrite from Liaoning province, graphite was highly crystalline, with an average FWHM(G) of 11.61; in the low-temperature metamorphic lithofacies sample TSL, its crystallinity was poor, and the average FWHM(G) was 63.32 [92]. In the studied sample, the FWHM(G) value of sample RC7 was 20.87, indicating a high degree of crystallinity of graphite.

The graphitization process at different temperatures and pressures (HT-LP, LT-HP) has been extensively studied, and is said to be a gradual and continuous process that is mainly influenced by metamorphic temperature [85]. The natural temperature at which carbon is converted into graphite is approximately 450–500 °C [93]. The linear regression equation of the graphite Raman thermometer [85] is

T (°C) = −445 R2 + 641 (330 < T < 650 °C).

(1)

The values of R1 and R2 for graphite in sample RC7 were 0.371 and 0.375, respectively. The metamorphic temperature of graphite in the studied chatoyant green nephrite was ~474 ± 50 °C [87]. This equation only gives the formation temperature of graphite and cannot directly measure that of tremolite. The highest formation temperature of S-type nephrite is 300–350 °C [1], indicating the formation of graphite happens earlier than that of S-type green nephrite. Therefore, the calculated temperature of graphite metamorphism can only be used as an indicator for the early mineralization stage. The obduction process probably occurred in a medium-to-high-temperature environment during hydrothermal fluid activity and metamorphism of the studied samples.

5.3. Deposit-Forming Process of Ospinsk Chatoyant Green Nephrite

Considerable work has been performed on the origin of S-type green nephrite [1,10,31,32,47,94]. Harlow and Sorensen (2005) argued that S-type nephrite is formed by contact metasomatism of serpentine [1].

Green nephrite deposits occur in the East Sayan, southwest of the Siberian platform, Russia, which is a part of the klippe–ophiolite suite remaining in the East Sayan nappe. In the make-up of the ophiolite association of East Sayan, we may recognize ultramafic, mafic, and volcanogenic–sedimentary complexes. The ultramafic complexes contain mostly serpentinized peridotites. The underlying rocks of the autochthon (the underlying rocks have been tectonized into carbonate–chlorite schists) were tectonized near the contacts with the ophiolites, and the ultramafic rock underwent large-scale serpentinization. At the base and around the margins of the ultramafic sheets, there are, as a rule, zones of a serpentinite mélange. The mélange consists of intensely cataclased chrysotile–lizardite serpentinite, plagiogranite, diorite, gabbro, and other blocks are distributed in the zones of serpentinite mélange belt in a chaotic fashion. These blocks are usually surrounded by antigorite serpentinite. Nephrite veins occur at the immediate contact between the blocks of aluminosilicate rocks and the antigorite serpentinite [10]. The deposit-forming process includes the following three stages:

5.3.1. Serpentinization of Ultramafic Rock

Chromite is frequently found in the chatoyant green nephrites from the Ospinsk mine. Chromite forms during the earliest stage of mantle melting and rises to the upper crust during its obduction. In the studied green nephrite, chromite was mostly granular and broken, with chlorite along the fractures. This suggests that chlorite formed during deformation of the green nephrite at a later stage. Chlorite is widely distributed in low-grade metamorphic rocks and is an altered ferromagnesian mineral. It is a characteristic mineral in case of low metamorphism. Chromite forms in high-temperature magmatic environments, and its crystalline habit is usually is free of alterations. The low hydrothermal temperature and weak metasomatism occurred during the late hydrothermal metasomatism.

During the ophiolites’ obduction, the base of ultramafic rock contacts the underlying rock mass (green schist) and ultramafic rocks undergo serpentinization due to interaction with fluids. Surrounding rocks, such as dunite or harzburgite provide sufficient materials, such as Mg and Fe, for the formation of serpentinite, and large-scale serpentine formation of the ultramafite rocks. At the base and around the margins of the ultramafic sheets, zones of a serpentinite mélange form.

Magnetite in the studied green nephrite also formed during this stage as a product of the serpentinite-forming reaction. It formed during a phase with a high degree of serpentinization [95]. In the initial stage of serpentinization, only Fe-rich serpentinite and Fe-rich brucite were formed (Equation (2)) while magnetite formed during further serpentinization (Equations (3) and (4)) [96].

$$2{(\mathrm{Mg},\mathrm{Fe})}_2{\mathrm{SiO}}_4(\mathrm{peridotite})+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Mg}}_3{\mathrm{Si}}_2{\mathrm{O}}_5{(\mathrm{OH})}_4(\mathrm{serpentine})+(\mathrm{Mg},\mathrm{Fe}){\left(\mathrm{OH}\right)}_2(\mathrm{Fe}-\mathrm{richbrucite})$$

(2)

$$3{\mathrm{Fe}\left(\mathrm{OH}\right)}_2(\mathrm{ferrobrucite})\to {\mathrm{Fe}}_3{\mathrm{O}}_4(\mathrm{magnetite})+2{\mathrm{H}}_2\mathrm{O}$$

(3)

$$6{\mathrm{Fe}\left(\mathrm{OH}\right)}_2(\mathrm{ferrobrucite})+2{\mathrm{O}}_2\to 2{\mathrm{Fe}}_3{\mathrm{O}}_4(\mathrm{magnetite})+6{\mathrm{H}}_2\mathrm{O}$$

(4)

Graphite formed during medium- to high-temperature metamorphism of carbonates in the Siberian platform when the magma cooled slowly. The metamorphic temperature of graphite in the chatoyant green nephrite from Ospinsk was 474 ± 50 °C. Magnetite can form within the temperature range of 300–400 °C. This evidence suggests that graphite in the sample formed before magnetite.

The Ospinsk chatoyant green nephrite was derived from serpentinite, and olivine was the main Fe²⁺-bearing mineral in such ultramafic rock. Iron in chromite was also in the form of Fe²⁺ and had completely replaced Mg²⁺. During serpentinization, Fe²⁺ was oxidized to Fe³⁺ with the simultaneous formation of magnetite. This indicates that magnetite formed after chromite.

5.3.2. Nephritization of Serpentinite

Nephrite veins occur at the immediate contact between the blocks of aluminosilicate rocks (plagiogranite, quartz diorite, etc.) and the antigorite serpentinite. Nephrite is located between the boundary zones of both massive lizardite–chrysotile serpentinite and plagiogranite and tectonic blocks of plagiogranite and quartz diorite (with a diopside–clinozoisite–tremolite–albite–quartz rock interpreted as rodingite). “Rodingite” is interpreted as alteration of gabbro (diorite), the Ca in rodingitizing fluids is probably derived from serpentinization. At present, most models for nephrite formation have proposed a metasomatic interaction of serpentinite or serpentinizing peridotite with more silicic rocks. This interaction is thought to be promoted by a Ca-rich hydrous fluid flow along contacts, structural boundaries, fractures, and/or faults, to produce pods or layers of nephrite between the two contrasting rock types. Coleman (1966, 1967, 1977) noted that aqueous fluids flowing through bodies of serpentinizing ultramafic rock should become saturated with Ca²⁺ as a result of clinopyroxene breakdown during serpentinization [94,97,98]. O’Hanley (1996) argued strongly that to form nephrite (in contrast to rodingite) Ca must enter from outside the serpentinite after serpentinization and rodingite formation are essentially complete [99]. Karpov et al. (1989) proposed an external source for nephrite-forming components that would enter the host rocks from a “hypothetical source”; this external fluid was “ready” to form nephrite [100]. Adams et al. (2007) pointed out through isotope studies that Ca in nephrite comes from siliceous sedimentary rocks rather than serpentine rocks, which verified the arguments of Karpov and O’Hanley [49].

In this stage, first of all, hydrous fluids flowing through bodies of serpentinizing ultramafic rock become saturated with Ca²⁺ as a result of clinopyroxene breakdown. After metasomatism with the silicic rock, rodingite and Si-rich fluid form. After the serpentine is complete, the external Ca-rich fluid flows into the Si-rich and Ca-rich fluids, the serpentinite experiences contact metasomatism due to influx of Si-rich and Ca-rich hydrothermal fluids, and finally tremolite crystallizes (Equation (5)).

$$5{(\mathrm{Mg},\mathrm{Fe})}_3{\mathrm{Si}}_2{\mathrm{O}}_5{(\mathrm{OH})}_4\left(\mathrm{serpentine}\right)+14{\mathrm{SiO}}_2+6\mathrm{CaO}\to {\mathrm{Ca}}_2{(\mathrm{Mg},\mathrm{Fe})}_5[{\mathrm{Si}}_8{\mathrm{O}}_{22}]{(\mathrm{OH})}_2\left(\mathrm{tremolite}\right)+7{\mathrm{H}}_2\mathrm{O}$$

(5)

The content of impure minerals in the samples was low, which indicates a stable environment during mineralization and metasomatism.

5.3.3. Alteration

Tremolite grains formed in the early stages were coarse and large. They subsequently metasomatized into small fibrous tremolite during later alteration processes [101]. Late-stage alteration changed the form and habit of the tremolite grains from coarse to fine or even cryptocrystalline, and from columnar and radial to bundle-like and fibrous. Chlorite is a common accessory of the chatoyant green nephrite from the Ospinsk mine [11,28,102], and its formation is accompanied by mineralization and subsequent alteration (Equation (6)). As the tremolite cooled slowly, chromite was altered at low temperatures to form chlorite surrounding the chromite.

$$\begin{array}{l}(1.5-2.5){\mathrm{Al}}^{3+}+1.5{(\mathrm{Mg},\mathrm{Fe})}_3{\mathrm{Si}}_2{\mathrm{O}}_5{\left(\mathrm{OH}\right)}_4\left(\mathrm{serpentine}\right)+{\mathrm{H}}_2\mathrm{O}\to \\ {(\mathrm{Mg},\mathrm{Fe},\mathrm{Al})}_{5-6}{(\mathrm{Si},\mathrm{Al})}_4{\mathrm{O}}_{10}{\left(\mathrm{OH}\right)}_8\left(\mathrm{chlorite}\right)+2.5{\mathrm{H}}^{+}\end{array}$$

(6)

We drew a sequential diagram of mineral generation according to the above process and the relationship between minerals in the studied green nephrite (Figure 14).

5.4. Reasons for Chatoyancy

The mineral composition of the Sichuan chatoyant nephrite was relatively pure, mainly comprising tremolite, with chlorite and magnetite as accessories [34]. Micro-fibroblastic texture was dominant in the Sichuan chatoyant nephrite, accounting for ~30% of the mineral, and was the main reason for the chatoyancy [35]. Meanwhile, the Taiwan chatoyant nephrite was composed mainly of fine, long, sub-parallel fibrous tremolite, with minor chlorite. The habit of tremolite gives the mineral aggregate its chatoyancy [48]. Similarly, the tremolite grains in the Ospinsk chatoyant green nephrite were approximately parallel along the c-axis. Due to deformation processes, the needle-like and columnar tremolite grains in the chatoyant green nephrite easily split along the cleavage {110}, reducing its toughness.

During serpentinization and nephritization of the green nephrite in the East Sayan ophiolite belt, the deformation was most intense at the bottom of the overthrust fault (the hybrid zone), accompanied by the over-coverage of the ophiolite nappe [10]. Owing to the structural interaction between rocks with different physical and mechanical properties, strong frictional forces caused a rise in temperature, and mylonite and cataclasite were formed in the contact zone. During nephritization of serpentinite, the unilateral, compressive, and shear stresses caused by the obduction of the nappe forced the directional growth of tremolite. This finally resulted in chatoyancy from a macroscopic perspective (Figure 15).

6. Conclusions

Tremolite is the main mineral of the chatoyant green nephrite from the Ospinsk mine in Russia, and is poor in iron. The accessory minerals are chromite, chlorite, graphite, and magnetite. The chatoyant green nephrite deposit in the Ospinsk mine is a contact metasomatic deposit associated with ultramafic rocks that belongs to the East Sayan ophiolite complex. Its Sr/Ba ratio is between 1 and 2, which makes its mineralization environment relatively alkaline. The ultramafic rocks underwent serpentinization and then metamorphosed due to contact metasomatism, resulting in the formation of the green nephrite deposit. During metamorphism of serpentinite into nephrite, the nephrites and associated metasomatites underwent intense tectonic effects, the unilateral, compressive, and shear stresses caused by the obduction of the nappe forced the directional alignment of the fibrous tremolite, leading to chatoyancy from a macroscopic perspective. The formation temperature of graphite in the chatoyant green nephrite was 474 ± 50 °C and could be used as an indicator for the early stage of mineralization, suggesting that the studied deposit probably formed at medium- to high-temperature hydrothermal fluid activity. The Ti content of the chatoyant green nephrite from the Ospinsk mine was high, while its Cr content was low. Geochemical characteristics of the studied samples were used to distinguish them from other deposits around the world.