*5.2. Gold Occurrence and Concentration in Sulfides from Proximal Alteration* 5.2.1. Gold Concentration

Data on the gold content in pyrite and arsenopyrite from the alteration of the studied sediment-hosted deposits are shown in Table 4 and Figure 15. Minimum mean values of Au were obtained for Py grains from the Malo–Taryn deposit (Au—5.1 ppm); the maximum mean values of Au are determined for arsenopyrite from the Badran deposit (66.9 ppm Au). For the proximal alteration of the V'yun and Shumnyi intrusion-hosted deposits, there are obvious differences between the gold contents in sulfides from clastic rocks compared with dykes. In alteration rocks from dykes in the V'yun deposit, the mean value of Au is 5.0 ppm, whereas, in Py from alterations in clastic rocks, the mean value of Au is 35.6 ppm. In two Apy samples from alterations in sandstones, the following values were obtained: 28.9 and 58.4 ppm Au. The Shumnyi deposit is characterized by the inverse value of variations in the Au content in Py. Thus, in Py from dykes, the mean value of Au is 28.8 ppm, and in Py from sandstones, the mean value of Au is 13.2 ppm.

In general, for all the studied deposits, gold contents in sulfides from proximal alterations were determined from fractions to be 168.5 ppm (Badran deposit, Apy). The highest gold content is found in Py, with 159.5 ppm (V'yun deposit) (Figure 15). Our results are comparable with data on the gold content in sulfides from alterations in many gold deposits in NE Russia, but they are noticeably lower compared with some large deposits with disseminated ores (Natalka—140–482.6 ppm Au, Mayskoe—300–1975 ppm Au [121], Nezhdaninskoe—up to 1400 ppm [28,112], Kyuchus—11.5–440 ppm Au [122]). Nevertheless, these results show the high economic potential of the disseminated mineralization in the studied deposits. For example, for the Khangalas deposit, it was shown that with a minimum gold content in alterations of 0.5 ppm, a length of 1.4 km, a thickness of 50 m, and a depth of 100 m for ore zones, reserves can be increased by 9.1 t Au [12].

#### 5.2.2. Gold in Pyrite3

Gold in sulfides can occur in an isomorphic structurally bound form and in the form of native nano- and microinclusions [31,119,123–125]. The problem of invisible gold has been studied in the most detail in pyrite, which is related to the discovery of a large Carlin deposit in Nevada, where gold is closely associated with arsenian pyrite [126]. However, it is known that pyrite and arsenopyrite with invisible gold occur in deposits of various types (e.g., orogenic, epithermal, intrusion-related, porphyry-Cu, iron-oxide copper-gold, etc.) [32,119]. Over the past 15–20 years, with the advent of new analytical techniques and technologies, a lot of information has been obtained about invisible gold and its form of occurrence in sulfides [31,32,124,127]. It was shown that invisible gold is mainly concentrated in pyrite with a high solid-solution As content—arsenian pyrite [128]. Arsenic can replace either Fe or S in pyrite; at the same time, the highest concentration of As is 8–11% [105,126]. It is assumed that the Au<sup>+</sup> ionic gold replaces Fe by entering distorted octahedral positions, and As replaces S in tetrahedral positions [129], but, nevertheless, the nature of invisible gold occurrence in sulfides is still debated [119]. A negative correlation between Fe and Au in pyrite may indicate the presence of Au in the lattice via the isomorphic substitution of Fe [130]. However, the ionic radius of Au<sup>+</sup> differs from the ionic radius of Fe2+, which makes it impossible to replace Au<sup>+</sup> with Fe2+ [131,132]. Chouinard et al. [133] proposed a mechanism of conjugate substitution for the Au3+ + Cu<sup>+</sup> <sup>↔</sup> 2Fe2+ type. Gold nanoparticles in arsenian pyrite can be localized at the boundaries of block structures, both as a surface formation and in defects in the crystal lattice [122,129].

It is important to identify [31], for epithermal and Carlin-type deposits, an increase in the solubility of Au in the pyrite structure with an increase in As content and to determine the saturation line of Au on an As vs. Au graph. Based on the data from EMPA, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), secondary-ion mass spectrometry (SIMS), and particle induced X-ray emission with a microfocused beam (micro-PIXE) analyses, Deditius A.P. et al. [32] studied the origin of the inclusion of Au and As and the solubility of gold in pyrite based on Cu-porphyry, Cu-Au, orogenic (OGDs),

volcanogenic–massive sulfide (VMS), iron-oxide copper–gold (IOCG), Au Witwatersrand, and coal deposits. They showed that Au1+ was the dominant form of Au in arsenian pyrite of the studied deposits, and the authors defined the empirical solid solubility of Au in As-pyrite as CAu = 0.004 <sup>×</sup> <sup>C</sup>As + 2 <sup>×</sup> <sup>10</sup>−<sup>7</sup> for a range of temperature between ~150–250 ◦C (Figure 22). Our recent studies of gold-bearing arsenian pyrite and arsenopyrite from the proximal alteration of the Khangalas deposit have also shown the predominance of structurally related forms of gold Au<sup>+</sup> in them [12]. These results were confirmed by rather low Au contents in the analyzed Py3. In most samples, Au does not exceed 2.5 ppm [12]. According to [124], the content of the structural form of Au in pyrite does not exceed ~ 5 ppm, and according to [32], it is less than 100 ppm Au. Higher concentrations are mainly related to the presence of nano- and microparticles [134]. The presence of native surface-bound (nano- or micro-) Au<sup>0</sup> in alteration sulfides is registered in deposits of various types [31,119,122,124].

Table 6 shows the mean arsenic content according to EPMA data, gold, and silver according to AAS data and the Ag/Au ratio in pyrite from the deposits discussed in this study. We used these results to determine the form of gold in sulfides. In the diagrams of the As/Au (Figure 22A) and Ag/Au (Figure 22B) ratios, the mean values of As, Au, and Ag in pyrite from the studied deposits of both types (sediment-hosted and intrusion-hosted) fall into the field of orogenic deposits of different ages and locations, belonging to the Au-As association of the Earth [119].

The Ag/Au ratio in Py3 does not differ significantly in the studied deposits; in the mean, it ranges from 0.07 (1:13.4) to 1.06 (1:0.9) (Table 6). Figure 22B shows that the Ag/Au ratio in the Py3 of the studied deposits is in good conformity with the results [119], which determined that, for deposits of the Au-As association, the disseminated arsenian pyrite has small values for the Ag/Au ratio up to 1, and the sedimentary pyrite has a higher, up to 1000, Ag/Au ratio. Large R. and Maslennikov V. [119] showed that the value of the Ag/Au ratio is a useful criterion to help distinguish disseminated sedimentary pyrite from hydrothermalalteration-disseminated pyrite in sedimentary rocks. The studied deposits are located in the field of orogenic deposits (Figure 22). The studied orogenic gold in sediment-hosted (Malo–Taryn, Badran, and Khangalas) and intrusion-hosted deposits (V'yun, Shumnyi) have similar ionic structurally bound Au<sup>+</sup> gold contents in Py3 (Figure 22A).


**Table 6.** Mean values of As, Au, and Ag in pyrite from the studied OGDs, central YKMB.

\* Mean values/number of analyses.

#### 5.2.3. Gold in Arsenopyrite1

In arsenopyrite, as in pyrite, gold can be in a native or isomorphic form. The inverse correlation between As and S in Apy (Figure 23) may reflect their conjugate isomorphic substitution in the process of formation [120,135]. Arsenic in arsenopyrite plays an important role as an indicator of the mineralization of Au. The atomic ratio As/S is mainly sensitive to temperature in the sulfur–buffer group, which leads to a higher ratio with an increasing temperature [135–137]. For the arsenopyrite1 of the Vorontsovskoye deposit, rich with sulfur and depleted of gold, it was determined that the ratio is As/S < 1, and for arsenopyrite2, depleted of sulfur and rich with gold, the ratio is As/S >1 [137]. It has been

0.89 (Table 7).

is no correlation (r = −0.18).

shown [137] that Apy1 crystallizes at a higher temperature and sulfur volatility, whereas for Apy2, these parameters are significantly lower. Our results do not align with these data. The atomic ratio of As/S for the gold-bearing Apy1 of all studied deposits shows that arsenopyrite is rich in sulfur, As/S < 1: Malo–Taryn—from 0.84 to 0.98 (only in two samples: 1.03), mean 0.94; Badran—from 0.77 to 0.99, mean 0.86; Khangalas—from 0.63 to 0.93, mean 0.83; V'yun—from 0.77 to 0.89, mean 0.85; and Shumnyi—from 0.75 to 0.94, mean 0.89 (Table 7). V'yun, dyke 15,850/53 5.0/6 1.3/6 0.26 V'yun, sandstone 15,850/53 35.6/8 5.6/8 0.16 Shumnyi, dyke 9970/23 28.8/4 2.1/4 0.07 Shumnyi, sandstone 9970/23 13.2/4 1.1/4 0.08 \* Mean values/number of analyses.

*Minerals* **2023**, *13*, x 31 of 41

**Table 6.** Mean values of As, Au, and Ag in pyrite from the studied OGDs, central YKMB.

**Deposit As, ppm (EPMA) Au, ppm (AAA) Ag, ppm (AAA) Ag/Au**

Khangalas 10,140/47 11.2/13 6.2/13 0.55

**Figure 22.** Plot of Py3. (**A**) Au vs. As, the lines of gold saturation after [32,49]; (**B**) Au vs. Ag of the studied OGDs, central YKMB. The demarcated fields are after [119]. **Figure 22.** Plot of Py3. (**A**) Au vs. As, the lines of gold saturation after [32,49]; (**B**) Au vs. Ag of the studied OGDs, central YKMB. The demarcated fields are after [119]. Shumnyi, dyke 44,230/21 – – – 0.89 \* Mean values/number of analyses.

ples: 1.03), mean 0.94; Badran—from 0.77 to 0.99, mean 0.86; Khangalas—from 0.63 to 0.93, mean 0.83; V'yun—from 0.77 to 0.89, mean 0.85; and Shumnyi—from 0.75 to 0.94, mean **Figure 23.** Au‐As plot for mean values of Apy1 in the studied OGDs, central YKMB. The lines of gold saturation after [32,49]. **Figure 23.** Au-As plot for mean values of Apy1 in the studied OGDs, central YKMB. The lines of gold saturation after [32,49].

The relationship between Au and Fe may show the form of gold occurrence in arse‐ nopyrite [136,137]. In our recent study [12], an inverse dependence of Au and Fe in the The qualitative linear scanning of an Apy1 grain from the Khangalas deposit showed that the concentrations of invisible Au, Pb, and Cu tend to decrease from the center to the The relationship between Au and Fe may show the form of gold occurrence in arsenopyrite [136,137]. In our recent study [12], an inverse dependence of Au and Fe in the Apy1 of the Khangalas deposit was shown, a strong inverse correlation (r = −0.9) with

rim, whereas the Co and Ni contents are noticeably higher in the central zones of the crys‐

studied deposits is a solid solution of Au+ in their crystal lattices. Micro‐ and nanoparticles

Thus, the predominant form of invisible Au in the arsenopyrite1 and pyrite3 of the

There are different ideas about potential sources of OGD fluids and metals, such as

Studies of stable isotopes such as δ34S provide important insights into the fluid source

and source(s) of S, which is important for the genetic interpretation of ore deposit for‐

in [43] and references therein. Their exhaustive review is given in [138]. The formation of OGDs from ore fluids with very diverse sources is discussed: some deposits have mag‐ matic water components [28,139–141], but some or most of the ore fluid is supplied from metamorphic waters [23,29,54,142,143] and juvenile sources; for example, in [11,43] and references therein. There are data on the source of some metals in the ore‐forming system from the host rocks [144]. Based on the example of deposits in the Juneau Gold Belt [145] and, later on, the a comprehensive analysis of the example of Jiaodong Province, China [43], it was shown that sulfur reached ore‐forming fluids during the metamorphic trans‐ formation of pyrite into pyrrhotite. The sulfur source was supposed to be dispersed synge‐ netic/diagenetic pyrite in terranes, devolatilized at a depth from the sediment wedge

Apy1 of the Khangalas deposit was shown, a strong inverse correlation (r = −0.9) with

studied deposits, a correlation diagram with the saturation line Au was used according to

To determine the form of gold occurrence in arsenopyrite from the alteration of the

of native gold are present in a subordinate amount.

*5.3. Sources of Components Based on Stable Isotopes S*

above a subduction zone [138].

later fluids rich in the Au‐polysulfide association.

thermal event.

**6. Conclusions**

increased contents Au of > 2 ppm was established, and with Au contents of <1 ppm, there is no correlation (r = −0.18).

To determine the form of gold occurrence in arsenopyrite from the alteration of the studied deposits, a correlation diagram with the saturation line Au was used according to [31]. In the diagram, the mean values of As and Au fall into the field of structurally bound Au<sup>+</sup> in Apy1.

**Table 7.** Mean values of As, Au, and Ag in the arsenopyrite1 of the studied OGDs, central YKMB.


\* Mean values/number of analyses.

**Figure 24.** Summary of δ34S values for orogenic gold deposits in the Yana–Kolyma metallogenic belt and the Nezhdaninskoe deposit, NE Russia. For comparison, materials according to [11,12,21,23,28,39]. Ranges of sulfur isotopic composition in various geological reservoirs according to [146]. In sudfigure: the left border of the line shows the minimum value; the right one shows the maximum value; vertical lines in the boxes denote the median, the X marks in the boxes denote the mean, and the left and right of each box denote the first and third quartiles, respectively. **Figure 24.** Summary of δ <sup>34</sup>S values for orogenic gold deposits in the Yana–Kolyma metallogenic belt and the Nezhdaninskoe deposit, NE Russia. For comparison, materials according to [11,12,21,23,28,39]. Ranges of sulfur isotopic composition in various geological reservoirs according to [146]. In sudfigure: the left border of the line shows the minimum value; the right one shows the maximum value; vertical lines in the boxes denote the median, the X marks in the boxes denote the mean, and the left and right of each box denote the first and third quartiles, respectively.

sediment‐hosted orogenic gold deposits, indicate subcrustal and metamorphic sources for the Au‐bearing fluid and sulfur. The slightly heavier sulfur isotopic composition of sul‐ fides from the Shumnyi and V'yun intrusion‐hosted deposits may indicate a mixture of subcrustal, metamorphic, partially magmatic, and sedimentary sources (Figure 24). A similar isotopic composition of sulfur arsenopyrite and pyrite quartz in the vein ore [23] and disseminated ore may indicate their formation during a single homogeneous hydro‐

The plots show that δ34S in sulfides in the V'yun, Shumnyi, Malo–Taryn, and Badran

This investigation of the chemical composition of disseminated sulfide mineraliza‐ tion from the proximal alteration of sediment‐hosted (Malo–Taryn, Badran, Khangalas) and intrusion‐hosted (V'yun, Shumnyi) orogenic Au deposits from the central part of the Yana–Kolyma metallogenic belt revealed that the gold endowment of these deposits is associated with pyrite3 and arsenopyrite1, and these minerals are an economically im‐ portant source of gold. The determination of the precise site of invisible gold within Py3 and Apy1 showed the possible prevalence of solid solution Au+ in their crystal lattices. Py3 from clastic and igneous rocks has a consistent association of As, Co, Ni, Cu, and Sb, as well as, less often, Pb, and has high conductivity, Co/Ni, and Ag/Au ratio characteristics

The qualitative linear scanning of an Apy1 grain from the Khangalas deposit showed that the concentrations of invisible Au, Pb, and Cu tend to decrease from the center to the rim, whereas the Co and Ni contents are noticeably higher in the central zones of the crystal [12]. It has been suggested that the increase in Au, Pb, and Cu is related to a portion of later fluids rich in the Au-polysulfide association.

Thus, the predominant form of invisible Au in the arsenopyrite1 and pyrite3 of the studied deposits is a solid solution of Au<sup>+</sup> in their crystal lattices. Micro- and nanoparticles of native gold are present in a subordinate amount.

### *5.3. Sources of Components Based on Stable Isotopes S*

There are different ideas about potential sources of OGD fluids and metals, such as in [43] and references therein. Their exhaustive review is given in [138]. The formation of OGDs from ore fluids with very diverse sources is discussed: some deposits have magmatic water components [28,139–141], but some or most of the ore fluid is supplied from metamorphic waters [23,29,54,142,143] and juvenile sources; for example, in [11,43] and references therein. There are data on the source of some metals in the ore-forming system from the host rocks [144]. Based on the example of deposits in the Juneau Gold Belt [145] and, later on, the a comprehensive analysis of the example of Jiaodong Province, China [43], it was shown that sulfur reached ore-forming fluids during the metamorphic transformation of pyrite into pyrrhotite. The sulfur source was supposed to be dispersed syngenetic/diagenetic pyrite in terranes, devolatilized at a depth from the sediment wedge above a subduction zone [138].

Studies of stable isotopes such as δ <sup>34</sup>S provide important insights into the fluid source and source(s) of S, which is important for the genetic interpretation of ore deposit formations; see [43,53,54,140,146] and references therein. In general, a large range of δ <sup>34</sup>S values in sulfides from −20.0 to +25.0‰ was obtained for the orogenic gold deposits [43]. The variation of δ <sup>34</sup>S is related to the involvement of various reservoirs in the formation of ores and variations in physical–chemical parameters during the evolution of ore-forming systems [147]. The values of sulfur sulfide isotopes in metamorphic terranes inside or near transcrustal faults of the Earth's crust according to [2] range between 0 and 10‰, but both higher and lower values have been observed. In addition, the dependence of the isotopic sulfur composition of the Phanerozoic deposits on the age of the host rock should be emphasized [43,51] (Figure 24).

Most of the orogenic gold deposits of the Verkhoyansk–Kolyma orogen have values of δ <sup>34</sup>S ranging from <sup>−</sup>12 to +10‰ (Figure 24) (e.g., [11,15,21,23,39,148]). The mean values of the sulfur isotopes of the arsenopyrite, pyrite, and polysulfide associations from the ore veins of these deposits, as well as from clastic rocks at a distance from the ore-bearing structures, were in a range of −5.0–+6.0‰ (Figure 24). In some deposits, a heavier δ <sup>34</sup>S was identified due to the influence of sedimentary diagenetic sulfides from the host rocks [39].

Recently, Gamyanin G.N. et al. [23] have performed a lot of work in studying the conditions of the formation of precious metal mineralization in the Adycha–Taryn metallogenic zone, located in the central sector of the YKMB. In particular, the values of δ <sup>34</sup>S in sulfides from the proximal alteration and veins allow the sources of sulfur to be constrained. Thus, for OGD veins, a rather narrow range of values of δ <sup>34</sup>S, close to 0, can be set: for arsenopyrite, it is from −2.1 to +2.4‰ (mean +0.4‰); for pyrite, it is from −6.6 to +5.4‰ (mean of −0.3‰) (Figure 24) [23]. In our recent work, similar values of δ <sup>34</sup>S were obtained for pyrite and arsenopyrite from the alteration of the Badran deposit [11] and arsenopyrite from vein ores and the alteration of the Khangalas deposit [12] (Figure 24). The isotopic composition of sulfide sulfur from the alteration of the Badran deposit studied using the local method showed the value of δ <sup>34</sup>S in pyrite to be slightly heavier than in arsenopyrite [11] (Figure 24). Two crystal populations were established in pyrite based on the δ <sup>34</sup>S variations. The first population includes isotope-zonal pyrite with sulfur weighing from the center to the rim of the grains (mean in the center of the grains, 0.2‰; mean at the rim of the grains, 1.5‰). The difference in the value of δ <sup>34</sup>S is up to 2.1‰. Similar zoning was noted earlier at the Sukhoi

Log deposit, where it is related to the evolution of fluid composition [148]. This can also be caused by variations in the fO2–pH conditions of ore-forming processes [149]. The second population, taking into account any analysis errors, is isotopically homogeneous pyrite; the values of δ <sup>34</sup>S in the center of the crystals differ slightly (0.01–0.33‰) from their rim. It was found that arsenopyrite with values of δ <sup>34</sup>S close to zero is more gold-bearing and indicates that sulfur minerals in sediment-hosted orogenic gold deposits were probably derived from the reduction of seawater sulfate [51].

New data on the isotopic composition of sulfide sulfur from the studied sedimenthosted and intrusion-hosted deposits, shown in Table 5 and in Figure 24, have a narrow range of δ <sup>34</sup>S values, from <sup>−</sup>6.4 to +5.6‰ (mean value of about 0‰). These data are consistent with the results of Gamyanin G.N. et al. [23] for OGD vein ores in the Adycha–Taryn metallogenic zone and our recent studies of disseminated sulfide mineralization [11,12]. Such values of δ <sup>34</sup>S for the gold deposits in the Yilgarn craton (Australia) are estimated to have a magmatic or mantle source of ore-forming fluid [52]. Similar results were obtained for a number of orogenic gold deposits in Kazakhstan (δ <sup>34</sup>S = 0.0 . . . <sup>−</sup>3.3‰), the source of the ore substance of which was determined to be mantle with partial borrowing from crustal sulfur [150,151], and the Nezhdaninskoe OGD, for which the values of δ <sup>34</sup>S of pyrite and arsenopyrite from the vein and disseminated ores prevail from −6 up to +0.7‰ (a mean of about +0.6‰) [28].

The plots show that δ <sup>34</sup>S in sulfides in the V'yun, Shumnyi, Malo–Taryn, and Badran deposits does not depend on the age of the host rocks and does not correlate with the seawater sulfate curve through the geological time. Near zero mean values of δ <sup>34</sup>S, together with the mantle signatures for sulfides and native gold grains [11] from the YKMB sedimenthosted orogenic gold deposits, indicate subcrustal and metamorphic sources for the Aubearing fluid and sulfur. The slightly heavier sulfur isotopic composition of sulfides from the Shumnyi and V'yun intrusion-hosted deposits may indicate a mixture of subcrustal, metamorphic, partially magmatic, and sedimentary sources (Figure 24). A similar isotopic composition of sulfur arsenopyrite and pyrite quartz in the vein ore [23] and disseminated ore may indicate their formation during a single homogeneous hydrothermal event.

#### **6. Conclusions**

This investigation of the chemical composition of disseminated sulfide mineralization from the proximal alteration of sediment-hosted (Malo–Taryn, Badran, Khangalas) and intrusion-hosted (V'yun, Shumnyi) orogenic Au deposits from the central part of the Yana– Kolyma metallogenic belt revealed that the gold endowment of these deposits is associated with pyrite3 and arsenopyrite1, and these minerals are an economically important source of gold. The determination of the precise site of invisible gold within Py3 and Apy1 showed the possible prevalence of solid solution Au<sup>+</sup> in their crystal lattices. Py3 from clastic and igneous rocks has a consistent association of As, Co, Ni, Cu, and Sb, as well as, less often, Pb, and has high conductivity, Co/Ni, and Ag/Au ratio characteristics with respect to hydrothermal pyrites. Both Py3 and Apy1 exhibit a distinct statistical correlation between Au and As and Au and Co contents. In the intrusion-hosted orogenic gold deposits, elevated concentrations of Co and Ni in Py3 were registered, suggesting that the ore fluid reacted with altered volumes of basic dykes. In Apy1, Co, Ni, Cu, and Sb were identified.

A synthesis of the geological, geochemical, and stable isotope data suggests that the invisible gold in the disseminated arsenian pyrite3 and in the arsenopyrite1 is most likely formed from subcrustal and metamorphic hydrothermal systems in the Verkhoyansk– Kolyma orogen.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/min13030394/s1, Table S1: Compositions (wt%) for pyrite in the proximal alteration rock OGD Central YKMB, analyzed by EPMA.; Table S2: Compositions (wt%) for arsenopyrite in the proximal rock OGD Central YKMB, analyzed by EPMA.

**Author Contributions:** Conceptualization: V.Y.F.; methodology, V.Y.F., L.I.P. and M.V.K.; validation, V.Y.F., L.I.P. and M.V.K.; writing—original draft preparation, V.Y.F., L.I.P. and M.V.K., writing—review and editing, V.Y.F., L.I.P. and M.V.K.; visualization, M.V.K.; supervision, V.Y.F.; project administration, V.Y.F.; funding acquisition, V.Y.F. and M.V.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Diamond and Precious Metals Geology Institute, Siberian Branch of the Russian Academy of Sciences (DPMGI SB RAS).

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are grateful to all engineers for their accurate, timely, and prompt analytical work and, especially, Tarasov Ya.A. for his help in the preparation and selection of the samples. We are sincerely grateful to Shulyak A.P. for translating the text and Dolgopolova A.V. for text editing and valuable comments, which contributed to the improvement of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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