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Article

Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite

by
Evgeniy V. Kislov
Dobretsov Geological Institute of the Siberian Branch of the Russian Academy of Sciences, 670047 Ulan-Ude, Russia
Geosciences 2024, 14(11), 303; https://doi.org/10.3390/geosciences14110303
Submission received: 8 October 2024 / Revised: 8 November 2024 / Accepted: 9 November 2024 / Published: 10 November 2024
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Abstract

:
The Kavokta deposit of the dolomite type nephrite is located in the Middle Vitim mountain country, Russia (Russian Federation). The deposit area is composed of granite of the Late Paleozoic Vitimkan complex. The granite contains complex shape blocks of Lower Proterozoic rocks. They are represented by metasandstone, crystalline schist, amphibolite, and dolomite marble. The calcite–tremolite and epidote–tremolite skarns were formed on the contact of dolomite and amphibolite. Calcite–tremolite skarn contains nephrite bodies. The mineral composition of 16 core samples obtained during the geological exploration conducted by JSC “Transbaikal Mining Enterprise” within Vein 1 of Prozrachny site has been studied in thin sections using a petrographic microscope, and in polished sections using a scanning electron microscope, with an energy-dispersive microanalysis system. Twenty-five minerals have been identified. They have been attributed to relict, metasomatic associations of the pre-nephrite and nephrite stages and hydrothermal and secondary associations. The intensity of the nephrite’s green color is explained by the Fe admixture in tremolite, and the black color is explained by its transition to actinolite in the areas of contact with epidote–tremolite skarn after amphibolite. In the formation and alteration of nephrite, dolomite is replaced by diopside, diopside by tremolite, prismatic tremolite by tangled fibrous tremolite, and tremolite by chlorite. Granite provides heat for metasomatism. Participation of amphibolite in the nephrite formation determines the variety of nephrite colors. The role of metamorphism is reduced to tectonic fragmentation facilitating fluid penetration; stress provides a tangled fibrous cryptocrystalline texture.

1. Introduction

Nephrite is a highly marketable jewelry–ornamental stone, a dense aggregate of monoclinic amphibole of the tremolite–ferroactinolite series, mainly tremolite, with a characteristic tangled fibrous microstructure [1]. It is especially appreciated in China, New Zealand, and the Pacific coast of North America [2]. The most valuable nephrite is white or bright green and translucent, with a minimum amount of inclusions of ore minerals, black, with a “cat’s eye” effect, as well as alluvial pebbles with edges of staining [3].
Nephrite deposits belong to two endogenous geological and industrial types: serpentinite type (S-type) in ophiolites and dolomite type (D-type) in tremolite–calcite magnesian skarns [1,3]. Deposits of the first type are a source of mainly green to brown (tobacco, marsh) and black nephrite; deposits of the second type produce mainly light-colored nephrite—from white to light green (salad), brown (honey), and less often black [1,3]. Placers represent the exogenous geological and industrial type; alluvial ones are the most productive among them [3]. The Kavokta deposit belongs to the D-type [4,5].
The Russian state register of mineral reserves includes 27 nephrite deposits of both S- and D-type. Among them, 18 deposits are located on the territory of the Republic of Buryatia, Russian Federation. In 2023, six D-type nephrite deposits were mined in Buryatia, including Kavokta, Golyube, Nizhni Ollomi, and Khaita. Another D-type nephrite deposit—Voimakan (Buryatia)—was being prepared for mining, and Udokan (Zabaikalsky Krai) and Burom (Buryatia) were being explored. All deposits of D-type nephrite of Russia are located in the Vitim nephrite-bearing region [6].
The majority of D-type nephrite deposits are located in China [7], some of the largest of them in northwestern part of the country. The Hetian nephrite belt, which has probably been worked for six millennia, is located in the Xinjiang Uygur Autonomous Region. It includes both the primary deposits [8,9], with Alamas as one of the best studied [10,11,12], and placer deposits (along the Yurungkash—“white nephrite river”—and Karakash—“black nephrite river”) [13,14,15,16]. In the east, the Hetian belt is adjoined by the nephrite-bearing areas of Altyn Tagh [17,18,19,20,21,22]. Golmud and other deposits of Qinghai Province are located further to the east [23,24,25]. There are a number of deposits in Northeast China: Tieli in Heilongjiang Province [26,27,28], Panshi in Jilin Province [29], Xiuyan and Sangpiyu in Liaoning Province [30,31]. The Xiaomeiling deposit in Jiangsu Province [32,33] is located in Central Eastern China, the Luanchuan deposit in Henan Province [34] in Central China, the Dahua deposit in Guangxi Zhuang Autonomous Region [35,36,37,38], the Longxi deposit in Sichuan Province [39], and Luodian deposit in Guizhou Province [40,41] in Southern and Southwestern China.
In other countries, there are the following noteworthy deposits: Chuncheon in South Korea [42,43], Cowell on the Eyre Peninsula in South Australia [44,45,46], Alpe Mastabia (Val Malenko) in Lombardy, Italy [47], Zloty Stock in Lower Silesia, Poland [48,49,50].
The role of metamorphic and metasomatic processes in nephrite formation is still debatable. In most of the studied D-type nephrite deposits, nephrite bodies are localized at the contact of dolomite and granite. There are exceptions, for example, Dahua [36] and Lodian [41] deposits formed in the contact of diabase and limestone, or Luanchuan [35], formed in dolomite outside the contact with metagabbro. The deposits of the Vitim nephrite region are characterized by the formation of nephrite at the contact of dolomite and amphibolite, composed of xenoblocks in the granite of the Angara–Vitim batholith. The unique features of the association of nephrite with amphibolites determines the need to study them.
At the same time, the Russian deposits of D-type nephrite have not been studied enough. Research work on Russian nephrite deposits was carried out mainly in the 1980s. To some extent, studies of the nephrite of the Vitim region have been resumed only in recent years [4,5,51,52,53,54,55]. This is especially true for the study of the mineral composition, which helps to understand the formation conditions and the search criteria development. This work is devoted to the geological setting, mineral composition, and formation features of the nephrite of the Kavokta deposit using the example of Vein 1 of the Prozrachny site.

2. Materials and Methods

The sixteen core samples were obtained during the geological exploration conducted by JSC “Transbaikal Mining Enterprise” within Vein 1 of the Prozrachny site. The GIA color scale was used to determine the color and shades. The mineral composition of samples has been studied in thin sections using the Olympus Bx-51 petrographic microscope, Olympus NDT, USA, and in polished sections using the LEO-1430VP scanning electron microscope, Carl Zeiss, Germany, with the INCA Energy 350 energy-dispersive microanalysis system, Oxford Instruments, Great Britain, in Analytical Centre “Geospectr”, Dobretsov Geological Institute of the Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, by analysts E.A. Khromova and E.V. Khodyreva. Measurement conditions: accelerating voltage—20 kV, probe current—0.3–0.4 nA, probe size—<0.1 microns, measurement time—50 s (lifetime), analysis error reaches 2–4 wt.% depending on the surface quality of the sample and the characteristics of its composition. An interactive software package developed at the Dobretsov Geological Institute of the Siberian Branch of the Russian Academy of Sciences is used to process the research results. The program implements an original method for identifying mineral phases based on stoichiometry of minerals. The program’s result is a report in the form of a set of Excel tables containing concentrations of elements and components, atomic percentages, and formulas calculated taking into account the identification of minerals. For a number of minerals (chromite, epidote, garnet, magnetite, muscovite, pyroxene, spinel, ilmenite), the content of 2- and 3-valent iron is calculated by iterative adjustment to stoichiometry using the golden ratio search.

3. Results

3.1. Kavokta Deposit

The geological study of the Middle Vitim mountain country began in the middle of the XIX century due to the search for gold and the ways to supply mines. However, nephrite remained unknown in this region for a long time. In 1944, Y.K. Dzevanovsky found a pistachio-green nephrite boulder 35 × 20 × 7 cm in size in the lower reaches of the Kalar River (right tributary of the Vitim) 24 km from the mouth, in the Topor tract, and two years later V.P. Selivanov reported on a similar find in the lower reaches of the Tsipa River (left tributary of the Vitim) [56]. In 1975, M.I. Grudinin (Institute of the Earth’s Crust, Irkutsk) discovered numerous boulders and pebbles of light green nephrite in alluvial deposits while researching the middle reaches of the Vitim River [4]. In 1976, the “Baikalkvartssamotsvety” (“Baikal Quartz Gems”) enterprise established the Shaman (later Vitim) search party to verify M.I. Grudinin’s discovery. From 1976–1978, geologists of this party outlined the distribution area of nephrite boulders along the Vitim, Bambuyka, and Tsipa rivers and began searching for primary deposits using the pebble–boulder method [4].
In 1983, light-colored nephrite was discovered in the basin of the Kavokta River during the prospecting work conducted by the “Baikalkvartsamotsvety” enterprise in the area of the lower reaches of the Tsipa River [5]. The first alluvial nephrite boulder was found by Yu.S. Veprev. V.I. Strugov and V.Ya. Belyaev identified and outlined the distribution area of nephrite boulders in the riverbed deposits of the Kavokta River. In 1984, V.I. Strugov and N.V. Sekerina found the bedrock outcrops. From 1984–1993, the Kavokta primary deposit with Prozrachny and Medvezhy sites was explored in the upper reaches of the Kavokta River, which was transferred for mining in 1994 [5]. Since 2007, the family and ancestral Evenk community of “Dylacha” have been mining with operational exploration. Since 2014, exploration and operation have been carried out by JSC “Transbaikal Mining Enterprise”. In 2021, the reserves of the Levoberezhny site were included in the state register [5].
The Kavokta deposit is the largest deposit of D-type nephrite in Russia: as of 15 January 2024, its raw nephrite reserves are 4648.58 tons, and 367.2 tons were extracted in 2023 [6]. The deposit is distinguished by the high quality of nephrite—increased blackness, intense translucency, white and light green coloring, and brown edges of staining, which allow cutting multi-colored products. The geological position, structure, and composition of the Kavokta deposit and the features of nephrite localization are typical for the entire Vitim region, including Golyube, Nizhni Ollomi, Khaita, Voimakan, Udokan, and Burom deposits [4].
The deposit area is composed of granites and diorites of the 1st phase of the Late Paleozoic Vitimkan complex (γC2–3) (Figure 1). Small bodies of leucocratic granites of the 2nd phase of the Vitimkan complex are manifested to a lesser extent. The granites contain xenoblocks of complex shape, outliers in the roof sags of the rocks of the Talalinskaya strata (formerly the Suvanikha suit), as it is now believed, of the Lower Proterozoic (PR1). They are represented by metasandstone, crystalline schist, amphibolite, and dolomite marble (Figure 1). The degree of metamorphism corresponds to amphibolite and epidote–amphibolite facies. River sediments are developed along the Kavokta River (aQH).
Full metasomatic zonality: dolomite marble—calcifyre—calcite–tremolite skarn with nephrite—epidote–tremolite skarn—amphibolite or diorite. Reduced zoning options are more often observed.

3.2. Vein 1

There are three sites outlined in the deposit, consisting of six nephrite-bearing zones, including nephrite veins. The Prozrachny site in the northwest of the deposit includes nephrite-bearing zones 1 and 2. Nephrite-bearing zone 1 of the sublatitudinal strike is located on the southern flank of the site and unites veins 1, 4, and 9 (Figure 2).
In this work, the nephrite composition and genesis are considered on the example of vein 1 of the Prozrachny site. This is the most productive vein at the moment. At the same time, it is quite characteristic of the deposit in its geological position, structure, and composition (Figure 2).
Vein 1 is a nephrite body with tectonic contacts of complex morphology with clamps, inflations in the contact area of dolomite marbles, and epidotized amphibolites (Figure 3). The length of the vein is 15 m, the thickness is 0.2–2.8 m, and the fall is steep to the southwest at an angle of 60°–70°. It is open to a depth of 15 m by mining. At depth, a complex propeller-like morphology is revealed due to changes in the angles of incidence from 20° to 65° [5]. A small part of the vein was exposed by erosion (Figure 3). It was mainly traced by drilling wells (Figure 3); their core was used in this work and then opened by a quarry.
Dolomite marbles are composed of relatively small bodies of elongated and irregular shape. These are white, light gray, medium–coarse-grained rocks of massive, striped texture. They consist of dolomite (50–60%) and calcite. Up to 5% forsterite, diopside, tremolite, and serpentine are found in skarned marbles.
Amphibolites spatially gravitate towards the bodies of dolomite marbles. Externally, they represent striped and spotted rocks of greenish tones, composed of large prismatic grains of hornblende and irregularly tabular plagioclase. Magnetite, microcline, chlorite, and titanite are present in small amounts. Secondary changes are pronounced in the replacement of amphibole with an epidote–clinozoisite aggregate, chloritization, actinolitization, and sossuritization.
Granites of the Vitimkan complex containing xenoblocks are represented by porphyritic and coarse-grained gneiss-like differences.
Calcite–tremolite rocks, productive for nephrite, are represented by narrow zones with a capacity of up to 3–4 m. Relatively simple lens- and vein-shaped bodies of various lengths and capacities are common, as a rule, with clear, fairly straight-line contacts and sustained fall to depth. Calcite–tremolite skarns are massive white, striped, spotted cryptocrystalline rocks. They are composed of an aggregate of fine-grained, fine-flow calcite and microfiber tremolite.
The distribution of nephrite in the bodies of skarns is extremely uneven. The most characteristic is the veined and lenticular form of nephrite bodies with a thickness from the first mm to the first cm with gradual transitions to calcite–tremolite skarns. Larger nephrite bodies usually have tectonic contacts with calcite–tremolite rocks, often with slickensides along which a long-fiber tremolite develops. Two systems of steeply falling cracks of the sublatitudinal and northwestern strike break the nephrite and the host rocks into plane-parallel blocks with a thickness of 5–15 cm and a length of 70–80 cm.

3.3. Structure and Composition of Nephrite

The color of nephrite is grayish-white, light green, grayish-green to green and grayish-brown (Figure 4), rarely to black (Figure 5). Translucency is from 1 to 5 cm. The texture of nephrite in the sections is diverse, often with areas of different textures in one section: fibroblastic to granonematoblastic, microfiber to tangled microfiber, radially radiant (Figure 6), paniculate, relict to pseudomorphic. The structure is mottled, heterogeneous, disordered, less often massive, shale or relict lattice-like.
Twenty five minerals have been identified using scanning electron microscopy. Tremolite is significantly predominant (Table 1, Figure 7a–e). Heterogeneity is observed—stripes and spots of different colors are visible on an electron microscope, while differences in iron content (Figure 7b) and its absence (Figure 7c) are observed. Actinolite is found in black nephrite in association with tremolite (Table 1). There are interlayers of tremolite and calcite, less often phlogopite (Figure 7d), and aggregates of tremolite with phlogopite, calcite, and chlorite (Figure 7e).
Among the tremolites, rare resorbed grains of minerals characteristic of dolomites and amphibolites are observed: dolomite (Figure 7f,k) with MgO content 20.75–22.54 wt.% and CaO—29.21–29.31 wt.%—according to the results of the analysis of four grains in two samples; scapolite—zonal meionite—in which the central part of the grains contains more Cr (Table 1, Figure 7g); titanite (Table 1, Figure 7h), magnetite with 4.87 and 10.64 wt.% Cr2O3 (Figure 7f); zircon (Figure 7i), containing no impurities according to the results of five analyses of five samples.
In addition, rare relics of metasomatic minerals were recorded in nephrite: corroded grains of diopside (Table 1, Figure 7j); a forsterite aggregate (Figure 7k) with a content of 0.96–1.02 wt.% FeO according to the results of three analyses in one sample; angular, resorbed epidote grains, including cerium epidote, whose presence was indicated in the center of epidote aggregates (Table 1, Figure 7l).
In most samples, apatite forms relatively large and isometric grains (Figure 7g,m,n), sometimes with calcite inclusions. According to the results of seven analyses, no impurities were recorded in four samples, and the F content was 0–6.19 wt.%, but mainly 4–5 wt.%. In large and widespread calcite of the first generation (Figure 7d,e,k,m,n), there are inclusions of tremolite and fluorapatite, with MgO content up to 6.68 wt.%. Fluorophlogopite and less often phlogopite are also typical (Table 1, Figure 7d,e,s,x).
Sulfides form rare, small, mostly idiomorphic grains: galena (Figure 7p,q,t; 10 analyses in 8 samples), molybdenite (Figure 7s,t; 2 analyses in 2 samples), pyrite (Figure 7q,r; 4 analyses in 3 samples), sphalerite (Figure 7p; 10 analyses in 5 samples, in 1 case 1.46 wt.% Fe). With one exception, isomorphic impurities are not detected in sulfides. Other hydrothermal minerals include: barite (Figure 7u), according to four analyses in three samples 0–14.65 wt.% SrO), fluorite (Figure 7v,w, two analyses in one sample without impurities), scheelite (a single grain without impurities).
Minerals of secondary changes include: veins, aggregates of calcite of the 2nd generation, mostly without impurities, rarely with a small admixture of magnesium (Figure 7d,e); wavy layers of prehnite (Table 1, Figure 7w); films along cracks of manganese and barium minerals close in composition to romanèchite (Ba,H2O)2(Mn4+,Mn3+)5O10 (Figure 7w); serpentine-group mineral making cracks (Figure 7p, from five analyses in two samples, in one, 1.38 wt.% FeO); gypsum (Figure 7u, 1.74 wt.% MgO); spot-like aggregates of chlorite (Table 1, Figure 7e,w).

4. Discussion

Based on the spatial–temporal relationships, minerals can be classified into five parageneses:
-
Relict minerals of dolomites and amphibolites: dolomite, Cr magnetite, titanite, scapolite (meionite), zircon;
-
Metasomatic minerals of the pre-nephrite stage: diopside, forsterite, epidote;
-
Metasomatic minerals of the nephrite stage: apatite, calcite I, tremolite, phlogopite;
-
Minerals of the hydrothermal stage: barite, galena, molybdenite, pyrite, sphalerite, fluorite, scheelite. The absence of isomorphic impurities in sulfides may indicate their low-temperature origin;
-
Minerals of secondary stage: gypsum, calcite II, prehnite, romanèchite(?), serpentine, chlorite.
The widespread development of fluorophlogopite and fluorapatite explains the previously noted [4] high F contents in the D-type nephrite. Rare small grains of fluorite cannot provide this phenomenon. A.N. Suturin and co-authors [4] explained the increased content of fluorine in D-type nephrite to 1 wt.% with the participation of fluids of granite origin but did not show which minerals of nephrite contain fluorine. They noted the presence of fluorotremolite, but such a mineral was not found in the samples we studied.
The iron content in the nephrite tremolite is highly significant. The mineral composition of four transparent polished nephrite plates from the Kavokta deposit has been analyzed before [5]. The results include the analyses of 138 grains, 114 of them are analyses of tremolite. Tremolite of white nephrite contains 0–0.31 wt.% FeO, on average, 0.06 wt.% FeO. Tremolite of yellowish-greenish nephrite contains 0.11–0.43 wt.% FeO, on average, 0.30 wt.% FeO. Green nephrite contains 0.46–0.96 wt.% FeO, on average, 0.77 wt.% FeO. Dark green nephrite contains 0.66–1.11 wt.% FeO, on average, 0.88 wt.% FeO. This allows the conclusion that iron concentration affects the color of nephrite [5].
The nephrite from the Voimakan deposit (the Vitim region) has been analyzed as well. FeO content in the tremolite of white, gray, and brown nephrite is mainly below the detection limits, and in some samples, it reaches 0.53 wt.% [55]. FeO content in the tremolite of light green nephrite varies from 0 to 1.30 wt.%. FeO content in the tremolite of uneven green nephrite varies even more from 0.35 to 5.44 wt.%. In the black area of this sample, there is one grain with content of 16.48 wt.% FeO, that corresponds to actinolite. It is located in the tremolite with contents of 5.27 and 6.47 wt.% FeO [55].
The whole-rock analyses were conducted for other deposits of the Vitim region only [4]. Placer nephrite from the Vitim River contains 0.59 wt.% FeO and 0.76 wt.% Fe2O3 on average. Nephrite from the Burom deposit contains 1.09 wt.% FeO and 0.55 wt.% Fe2O3 on average. Nephrite from the Golyube deposit contains 0.34 wt.% FeO and 0.33 wt.% Fe2O3 on average [4].
Nephrite tremolite was analyzed at some deposits of the Hetian area. A content of 1.20–1.72 wt.% FeO was determined at the Saidikulam deposit [8]. Tremolite of white nephrite contains 0.00–0.05 wt.% FeO, tremolite of white-green nephrite contains 0–0.02 wt.% FeO, and tremolite of green nephrite contains 0.03–0.17 wt.% FeO at the Alamas deposit [10]. Tremolite of white nephrite from the Yurungkash River contains 0.11–0.72 wt.% FeO. The Karakash River alluvial nephrite is more complex in composition. Green to black nephrite contains both actinolite with 4.11–14.39 wt.% FeO and tremolite with 0.31–1.97 wt.% FeO [13,14]. At the same time, black nephrite with graphite inclusions contains tremolite with 0.16–0.63. wt.% FeO [16].
The results for the Altyn Tagh area are similar. Tremolite of nephrite from the Tashisayi deposit contains 0.16–0.50 wt.% FeO [17], tremolite of nephrite from the Yinggelike deposit contains 0.27–3.48 wt.% FeO [18], and tremolite of nephrite from the Mida deposit contains 0.09–0.24 wt.% FeO [21]. The black nephrite of the Margou deposit is different, containing tremolite with 0.17–4.29 wt.% FeO and actinolite with 4.66–6.29 wt.% FeO [22].
The content of FeO in the tremolite of nephrite from the Xiuyan deposit in Liaoning varies from 0.16 wt.% to 2.07 wt.% with an average value of 1.20 wt.%. In the bluish white sample, the content of FeO is the lowest with an average value of 0.30 wt.%. The content of FeO in the yellow-green sample varies greatly with an average of 1.67 wt.%. In the dark green sample, the content of FeO is more stable with an average of 1.52 wt.%; whereas in the deep green sample, the average FeO content is 1.32 wt.%. [30]. Tremolite of placer nephrite from this deposit contains 0.16–7.53 wt.% FeO [31].
Similar values were obtained by the analysis of tremolite of nephrite from other deposits in China: Tieli, Heilongjiang—0.33–0.76 wt.% FeO [26], 0.26–1.13 wt.% wt.% FeO [27]; Panshi, Jilin—0.01–0.25 wt.% FeO [29]; Xinyu, Jiangxi—0.21–0.42 wt.% FeO [33]; Luanchuan, Henan—0.69–4.15 wt.% FeO [34]; Longxi, Sichuang—0.18–0.42 wt.% FeO [39]. Black nephrite from the Dahua deposit, Guangxi, depending on the cause of coloring, contains either actinolite with 11.67–25.75 wt.% FeO [36] or tremolite with 0.53–0.95 wt.% FeO [37].
There is little information on deposits in other countries. Tremolite of nephrite from the Chuncheon deposit, South Korea, contains 0.32–1.74 wt.% Fe2O3 [43], 0.22–0.55 wt.% FeO [44]. Tremolite of nephrite from the Val Malenco deposit, Italy, contains 0–0.08 wt.% FeO [47]. Tremolite of nephrite from Złoty Stok, Poland, contains 1.70–5.68 wt.% FeO [48].
The iron content in the nephrite tremolite varies, depending on the geology of the deposit and the composition of the wall rocks. At the same time, it determines the color of nephrite in most cases.
FeO has not been found in 12 samples of tremolite of different colors (Table 1). In the rest, the iron content is: in grayish light green 550101—0–1.45%, 519703—0–4.64%, in grayish green 915902—0.78–1.24%, 916202—0.82–3.91%, in the black area of this sample, it is up to 14.23% (Figure 6). The green color is determined by the admixture of Fe in tremolite: as Fe content increases, the tone becomes richer, which is observed at the Kavokta deposit, and earlier [5]. The black color of nephrite in sample 916202 is explained by high Fe content due to a close contact with epidote–tremolite skarn after amphibolite—only this sample contains titanite and Cr magnetite. The black area is composed of tremolite and actinolite with an extremely uneven distribution of Fe (Table 1).
In previous papers, there is information on abnormally low oxygen isotope ratios of D-type nephrite of the Vitim region [51]. It was suggested that the nephrite-forming fluid had been of meteoric origin. In this case, granite is not a source of fluid, especially since it does not come into direct contact with nephrite bodies. Granite only provides regional heating, activating meteor fluids abnormally depleted by isotope 18O. As a result of the infiltration transfer of the heated fluid, metasomatic reactions begin in the contact of amphibolites and dolomites, leading to the formation of skarns—epidote–tremolite and calcite–tremolite with nephrite bodies.
Formation of nephrite, according to the relationships of minerals, takes place in several stages. The tremolite formula is used as an extreme member of an isomorphic series to simplify.
1. Initially, dolomite at the progressive stage is replaced by diopside during infiltration transfer by silica fluid from amphibolites:
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2.
At the regressive stage, the diopside is already replaced by a tremolite aggregate during the transfer of magnesium and silica from amphibolites by the fluid:
2CaMgSi2O6 + MgO + 4SiO2 + H2O + O2 → Ca2Mg5(Si4O11)2(OH)2.
In another variant, the diopside is replaced by calcite–tremolite skarn—CO2 from the first reaction:
5CaMgSi2O6 + H2O + 3CO2 + 4O2 → Ca2Mg5(Si4O11)2(OH)2 + 3CaCO3 + 6SiO2.
At the same time, calcite of the skarn can also be replaced by tremolite to form nephrite—silica from amphibolite, magnesium can be from both amphibolite and dolomite:
2CaCO3 + 5MgO + 8SiO2 + H2O → Ca2Mg5(Si4O11)2(OH)2 + 2CO2.
2. In some samples, relics of diopside or its pseudomorphs are not detected. In this case, it is assumed that the replacement of the diopside has been completed or the tremolite was formed directly from dolomite with infiltration of silica and magnesium from amphibolite:
4CaMg(CO3)2 + 8SiO2 + 6MgO + 2H2O + 7O2 → 2Ca2Mg5(Si4O11)2(OH)2 + 8CO2.
3. In one sample, a forsterite aggregate is observed—it could be formed after dolomite with infiltration of silica from amphibolite:
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3 + 2CO2.
And then forsterite could be replaced by tremolite, interacting with calcite during further silica input from amphibolite:
5Mg2SiO4 + 4CaCO3 + 11SiO2 + 2H2O + 2O2 → 2Ca2Mg5(Si4O11)2(OH)2 + 2CO2.
4. The initial prismatic tremolite is subsequently replaced by a tangled fibrous one—nephrite is formed as a result.
5. As the regressive process continues, tremolite is replaced with infiltration of alumina from amphibolite with chlorite and calcite.
Ca2Mg5[Si4O11]2(OH)2 + Al2O3 + 3H2O + 2CO2 → Mg5Al[Si3AlO10](OH)8 + 2CaCO3 + SiO2 + 4O2.
In this case, the role of granites is reduced to a regional increase in temperature, which provides metasomatic reactions. Direct participation of amphibolite, containing more Fe, not granite, in the metasomatism determines the variety of nephrite colors. The role of metamorphism is reduced to tectonic fragmentation, which facilitates the penetration of fluids, and stress, which provides a tangled fibrous cryptocrystalline texture of nephrite.

5. Conclusions

Sixteen nephrite samples from the core of Vein 1, Prozrachny site of the Kavokta D-type nephrite deposit, located in the Middle Vitim mountain country (Russia) have been studied and a total of twenty-five minerals have also been identified. Based on the spatial and temporal relationships of the minerals, they are classified into five parageneses: relict, metasomatic of the pre-nephrite and nephrite stages, hydrothermal and secondary. The high F content in D-type nephrite is explained by the widespread development of fluorapatite and fluorophlogopite. The intensity of the nephrite’s green color is explained by the admixture of Fe in tremolite, and the black color is explained by the development of actinolite in the contact with amphibolite. A model of nephrite formation has been presented: development of diopside along dolomite, replacement of diopside with tremolite, replacement of prismatic tremolite with tangled fibrous tremolite. In some cases, tremolite can develop directly along the dolomite or replace forsterite. Subsequently, tremolite is replaced by chlorite. Granite does not participate in the formation of nephrite directly, but heats up the meteor fluid, which is necessary for metasomatic reactions. Participation of amphibolite in the nephrite formation determines the variety of nephrite colors. Metamorphism causes tectonic fragmentation, which facilitates the penetration of fluids, and stress, which provides a tangled fibrous cryptocrystalline texture of nephrite.

Funding

The analyses were carried out at the expense of the Russian Science Foundation grant No. 22-27-20003, https://rscf.ru/project/22-27-20003.

Data Availability Statement

The data supporting reported results can be provided by the author upon request.

Acknowledgments

The author is grateful for the materials provided by JSC “Transbaikal Mining Enterprise”, personally to A.P. Suzdalnitsky, D.B. Sharakshinova, and E.V. Raevskaya, analysts E.A. Khromova and E.V. Khodyreva, interpreter V.Yu. Baryshnikova, for help with information—I.S. Goncharuk, for preparation of figures—V.F. Sotnikova, V.V. Vanteev, and A.V. Trofimov. Comments and suggestions of three reviewers contributed to a significant improvement in the manuscript. The work was carried out within the framework of the Dobretsov Geological Institute of the Siberian Branch of the Russian Academy of Sciences research project № AAAA-A21-121011390003-9. The study involved the facilities of Analytical Center “Geospectr”, Dobretsov Geological Institute of the Siberian Branch of the Russian Academy of Sciences, Ulan-Ude, Russia.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Scheme of the geological structure of Prozrachny site, Kavokta deposit based on the materials of JSC “Transbaikal Mining Enterprise”.
Figure 1. Scheme of the geological structure of Prozrachny site, Kavokta deposit based on the materials of JSC “Transbaikal Mining Enterprise”.
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Figure 2. Scheme of the geological structure of Prozrachny site, Kavokta deposit based on the materials of JSC “Transbaikal Mining Enterprise”.
Figure 2. Scheme of the geological structure of Prozrachny site, Kavokta deposit based on the materials of JSC “Transbaikal Mining Enterprise”.
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Figure 3. Geological plan of Vein 1, Prozrachny site based on the materials of JSC “Transbaikal Mining Enterprise”. The numbers of the samples used in this work are marked. They correspond to the numbers in the text and captions to the figures.
Figure 3. Geological plan of Vein 1, Prozrachny site based on the materials of JSC “Transbaikal Mining Enterprise”. The numbers of the samples used in this work are marked. They correspond to the numbers in the text and captions to the figures.
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Figure 4. Variety of nephrite colors, sample 464501.
Figure 4. Variety of nephrite colors, sample 464501.
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Figure 5. Green to black nephrite, sample 916202.
Figure 5. Green to black nephrite, sample 916202.
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Figure 6. Various-sized radial tremolite aggregates, crossed nicols, samples 464901 (a) and 550101 (b).
Figure 6. Various-sized radial tremolite aggregates, crossed nicols, samples 464901 (a) and 550101 (b).
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Figure 7. Mineral composition of nephrite. The images were taken using a LEO-1430VP scanning electron microscope: (a)—tremolite bundles, 917001; (b)—parallel-fibrous tremolite aggregate with different Fe content, 519703; (c)—light and dark tremolite strips of the same composition, 465401; (d)—secondary calcite cuts tremolite and interlayering of phlogopite, calcite, and tremolite, 519703; (e)—tremolite with 0 wt.% Fe with aggregate of tremolite with 1.13 wt.% FeO, phlogopite, calcite, chlorite, 519703/1; (f)—chromium magnetite, actinolite grains and veinlets, dolomite at tremolite, 916202; (g)—meionite at tremolite, 464901; (h)—titanite grains at tremolite, 916202; (i)—zircon grain at tremolite, 917001; (j)—corroded diopside grain at tremolite, 517601; (k)—forsterite aggregate with grains of dolomite, tremolite, apatite, 464501; (l)—epidote aggregate—in the center the epidote-Ce, along the periphery of epidote at tremolite, 516701; (m)—large segregation of calcite with inclusions of tremolite and fluorapatite at tremolite, 917001; (n)—apatite crystal with an elongated inclusion of calcite at tremolite, 917001; (o)—fluorapatite aggregate, 464501; (p)—elongated grains of galena and sphalerite at tremolite, serpentine is along a crack, 464501; (q)—pyrite grain with galena inclusions, 464501; (r)—pyrite at tremolite, 915902; (s)—molybdenite plate-like grains at fluorphlogopite, 915902; (t)—intergrowths of molybdenite and galena at tremolite, 916001; (u)—gypsum and barite intergrowth, 917001; (v)—fluorite aggregate at tremolite, 465401; (w)—interlayers of prehnite, fluorite, and chlorite at tremolite, 465401; (x)—tremolite with phlogopite cut by romaneshite? 464401. Act—actinolite, Ap—apatite, Brt—barite, Cal—calcite, Chl—chlorite, Cr-Mag—Cr-rich magnetite, Di—diopside, Dol—dolomite, Ep—epidote, Fl—fluorite, Fo—forsterite, Gn—galena, Gp—gypsum, Mei—meionite, Mn—Mn minerals, Mol—molybdenite, Phl—phlogopite, Prh—prehnite, Py—pyrite, Sp—sphalerite, Srp—serpentine, Tr—tremolite, Ttn—titanite, Zrn—zircon.
Figure 7. Mineral composition of nephrite. The images were taken using a LEO-1430VP scanning electron microscope: (a)—tremolite bundles, 917001; (b)—parallel-fibrous tremolite aggregate with different Fe content, 519703; (c)—light and dark tremolite strips of the same composition, 465401; (d)—secondary calcite cuts tremolite and interlayering of phlogopite, calcite, and tremolite, 519703; (e)—tremolite with 0 wt.% Fe with aggregate of tremolite with 1.13 wt.% FeO, phlogopite, calcite, chlorite, 519703/1; (f)—chromium magnetite, actinolite grains and veinlets, dolomite at tremolite, 916202; (g)—meionite at tremolite, 464901; (h)—titanite grains at tremolite, 916202; (i)—zircon grain at tremolite, 917001; (j)—corroded diopside grain at tremolite, 517601; (k)—forsterite aggregate with grains of dolomite, tremolite, apatite, 464501; (l)—epidote aggregate—in the center the epidote-Ce, along the periphery of epidote at tremolite, 516701; (m)—large segregation of calcite with inclusions of tremolite and fluorapatite at tremolite, 917001; (n)—apatite crystal with an elongated inclusion of calcite at tremolite, 917001; (o)—fluorapatite aggregate, 464501; (p)—elongated grains of galena and sphalerite at tremolite, serpentine is along a crack, 464501; (q)—pyrite grain with galena inclusions, 464501; (r)—pyrite at tremolite, 915902; (s)—molybdenite plate-like grains at fluorphlogopite, 915902; (t)—intergrowths of molybdenite and galena at tremolite, 916001; (u)—gypsum and barite intergrowth, 917001; (v)—fluorite aggregate at tremolite, 465401; (w)—interlayers of prehnite, fluorite, and chlorite at tremolite, 465401; (x)—tremolite with phlogopite cut by romaneshite? 464401. Act—actinolite, Ap—apatite, Brt—barite, Cal—calcite, Chl—chlorite, Cr-Mag—Cr-rich magnetite, Di—diopside, Dol—dolomite, Ep—epidote, Fl—fluorite, Fo—forsterite, Gn—galena, Gp—gypsum, Mei—meionite, Mn—Mn minerals, Mol—molybdenite, Phl—phlogopite, Prh—prehnite, Py—pyrite, Sp—sphalerite, Srp—serpentine, Tr—tremolite, Ttn—titanite, Zrn—zircon.
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Table 1. Chemical composition of minerals, wt.%.
Table 1. Chemical composition of minerals, wt.%.
TremoliteActinoliteDiopsidePhlogopiteChloriteEpidoteScapoliteTitanitePrenite
Mean n = 40Range Mean n = 9RangeMean n = 7RangeMean n = 3RangeMean n = 4Range
SiO258.4955.54–60.5953.7254.8352.69–56.1845.4542.79–49.7238.0034.12–45.6536.5834.02–40.8036.9336.7530.6141.27
TiO2000000000000037.010
Al2O30.190–1.721.210.880–1.899.358.01–10.6411.926.75–15.1023.8521.86–25.9621.5423.222.1722.64
Cr2O3000000000005.120.8500
FeO0.460–3.9214.20000.620–2.371.880–4.351.150–2.770000
Fe2O3---0.250–1.23----5.203.53–2.09----
MgO23.7521.82–27.0614.2317.4616.55–21.0127.0826.15–27.7929.5528.29–31.012.320–2.624.915.6700
CaO13.2510.72–15.2512.5224.7722.54–27.06002.651.05–5.0820.9317.78–24.5822.8224.6028.1025.83
K2O000009.878.75–11.26000000
Ce2O30000000003.830–9.4100
La2O30000000002.300–5.7400
F000002.080–4.17000000
Total96.14 95.8898.19 94.45 84.00 99.47 91.3191.0997.9089.73
Note: n—number of analyses.
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Kislov, E.V. Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite. Geosciences 2024, 14, 303. https://doi.org/10.3390/geosciences14110303

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Kislov EV. Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite. Geosciences. 2024; 14(11):303. https://doi.org/10.3390/geosciences14110303

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Kislov, Evgeniy V. 2024. "Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite" Geosciences 14, no. 11: 303. https://doi.org/10.3390/geosciences14110303

APA Style

Kislov, E. V. (2024). Kavokta Deposit, Middle Vitim Mountain Country, Russia: Composition and Genesis of Dolomite Type Nephrite. Geosciences, 14(11), 303. https://doi.org/10.3390/geosciences14110303

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