Podiform Chromitites and PGE Mineralization in the Ulan-Sar’dag Ophiolite (East Sayan, Russia)

: In this paper, we present the first detailed study on the chromitites and platinum-group element mineralization (PGM) of the Ulan-Sar’dag ophiolite (USO), located in the Central Asian Fold Belt (East Sayan). Three groups of chrome spinels, differing in their chemical features and physical–chemical parameters, under equilibrium conditions of the mantle mineral association, have been distinguished. The temperature and log oxygen fugacity values are, for the chrome spinels I, from 820 to 920 ° С and from ( − 0.7) to ( − 1.5); for chrome spinels II, 891 to 1003 °C and ( − 1.1) to ( − 4.4); and for chrome spinels III, 738 to 846 °C and ( − 1.1) to ( − 4.4), respectively. Chrome spinels I were formed through the interaction of peridotites with mid-ocean ridge basalt (MORB)-type melts, and chrome spinels II were formed through the interaction of peridotites with boninite melts. Chrome spinels III were probably formed through the interaction of andesitic melts with rocks of an overlying mantle wedge. Chromitites demonstrate the fractionated form of the distribution of the platinum-group elements (PGE), which indicates a high degree of partial melting at 20–24% of the mantle source. Two assemblages of PGM have been distinguished: The primary PGE assemblage of Os-Ir-Ru alloys-I, (Os,Ru)S 2 , and IrAsS, and the secondary PGM assemblage of Os-Ir-Ru all о ys-II, Os 0 , Ru 0 , RuS 2 , OsS 2 , IrAsS, RhNiAs with Ni, Fe, and Cu sulfides. The formation of the secondary phases of PGE occurred upon exposure to a reduced fluid, with a temperature range of 300–700 °C, log sulfur fugacity of ( − 20), and pressure of 0.5 kbar. We have proposed a scheme for the sequence of the formation and transformation of the PGMs at various stages of the evolution of the Ulan-Sar’dag ophiolite. the of of the

The ophiolites are localized in the form of extended branches in the south (Il'chir) and north (Holbin-Hairhan). The features of these ophiolites are as follows: rock associations, geochemical and mineralogical characteristics, a geological structure resembling individual "massifs", geodynamic conditions and a formation time, which are actively studied for a considerable time. Dobretsov and Zhmodik et al. obtained data on the heterogeneity of ophiolites of the south-eastern part of Eastern Sayan (SEPES) [32,33]. They indicate that the ophiolites of the southern branch were formed in a midocean ridges setting, and ophiolites of the northern branch were formed in an island arc setting [32][33][34][35][36][37][38]. The Ulan-Sar'dag ophiolite (USO) occupies a special structural position in the Eastern Sayan and Central Asian Fold Belt (Dunzhugur island arc). The USO is a tectonic plate, which is located between ophiolites of the southern and northern branches of the Dunzhugur island arc, near the contact zone of the Gargan "block" gneisses, with granites of the Sumsunur complex. The USO is underlaid by volcanogenic and sedimentary rocks of the Ilchir suite and limestones of the Irkut suite. Ophiolites include mantle restites (dunites, harzburgites), podiform chromitites, cumulates (metawehrlite, metapyroxenite, metagabbro), a basic dike and a volcanic complex (Ilchir suite) (Figures 2 and 3). USO has a lenticular body that is elongated in the east-west direction and is 1.5 × 5 km 2 . It is composed of dunites and harzburgites. Harzburgites predominate in the central part, dunites and serpentinites prevail in the margin, and the latter lie in contact with the volcanogenic-sedimentary sequence of the Ilchir suite. The rocks of the cumulative series are metamorphosed under the conditions of epidoteamphibolite and amphibolite facies. The volcanogenic-sedimentary sequence is composed of volcanic, volcanogenic-sedimentary, and metasedimentary rocks (black schists and marbles). Volcanogenicsedimentary rocks are sulfurized, and sulfide mineralization is confined to the schistosity zones. High-Ti basalts (MORB tholeiites), boninites, and island-arc andesites are distinguished in the volcanic complex. All rocks in the ophiolite nappe bottom are intensively deformed and mark the thrust zone. They include numerous crushing zones, signs of shear displacement and gliding planes.
In the contact zone of serpentinites and rocks of the Ilchir suite, the talcum powder zones and zones of actinolite-tremolite composition are widely manifested. The types of schlieren, lenticular, and vein-like chromite bodies are localized in dunites and serpentinites ( Figure 4). The length of schlieren and massive chromitite bodies ranges from a few centimeters to several meters, and their width varies from 5 cm to 0.5 m. The predominant structuraltextural type is a massive chromitite. In some schlieren chromitites, the tectonic flow structures (Figure 4e) and the "snowball" type ( Figure 4f) are observed.

Podiform Chromitites
Massive chromitites are the predominant petrographic variety of rocks in the mantle peridotites of the USO; disseminated, schlieren, and rhythmically-banded ores are less common. Chrome spinels make up 80-95 vol. % of massive chromitites. The intergranular space is filled with olivine and secondary silicates: serpentine, chlorite, and, rarely, talc. The structure of massive chromitites varies from fine-grained to coarse-grained. The crystal bodies vary from subidiomorphic grains (0.1-0.5 cm) to allotrimorphic aggregates of grains of up to 1.5 cm in size. Chrome spinels have a cataclastic texture, and they are partially fragmented. The grains are dissected by cracks filled with secondary silicates. In the central part, chrome spinels are unchanged, and in cracks and on the grain rims, they are often replaced by chrome magnetite. Some grains contain olivine inclusions. A wide range of minerals show accessory mineralization in chromite ores: PGMs, sulfides, sulfarsenides, and sulfosalts of base metals ( Figure 5).

Olivine
Olivine in chromitites occupies the intergranular space of chrome spinels; it is often replaced by serpentine or chlorite. The data, which correspond to chrysotile and forsterite, are shown in Table 2. The MgO content is 49-54 wt. %, and the NiO variations are insignificant and amount to 0.37-0.42 wt. %, which corresponds to olivines from depleted peridotites. The fraction of the Fo component is 92-97.

Geochemistry of Platinum-Group Elements
The content of PGE in mantle peridotites and podiform chromitites was determined. The content of PGE in dunites and serpentinites is 94-180 ppb; in chromitites, it ranges from 242 to 992 ppb ( Table  3). The PGE content increases with an increase in the volume percentage of chromite in the rock. In addition, depending on the proportion of the chromite and silicate components in the rock, the IPGE/PPGE ratio changes (Figure 7a,b) (IPGE: Os, Ir, and Ru; PPGE: Rh, Pt, and Pd). For example, in massive ores (85-95 vol. % of chromite), IPGE > PPGE, and IPGE/PPGE = 0. 86-2.15. In schlieren lenticular chromitites and chromitites with deformational textures, the PPGE proportion increases and amounts to IPGE/PPGE = 0.03-0.67. In chromitites I, the IPGE/PPGE is higher than in chromitites II (Table 3). Table 1. Representative chemical composition of Cr spinels and the calculated parental melt composition (wt. %) of Cr spinels. Thermometric and oxybarometric data for Cr spinels. No.

Mineralogy of Platinum-Group Elements
The first data on PGE mineralization (PGM) in chromitites of USO have been obtained. The chemical composition of the minerals is presented in Table 4, and classification diagrams are presented in Figure 8a,b. Primary and secondary PGMs have been distinguished. The most common platinumgroup minerals in the chromitites of the USO are PGE sulfides: laurite-erlichmanite (Ru,Os)S2, with different Ru/(Ru + Os) ratios. Other PGE phases are represented by high-temperature Os-Ir-Ru alloys-I, phases of variable-composition Os-Ir-Ru alloys-II, native Os, Ru, sulfarsenides of (Os, Ir, Ru), and zaccarinite RhNiAs. The primary and secondary PGMs will be described separately.

Primary Platinum-Group Minerals
High-temperature Os-Ir-Ru alloys-I are in the form of idiomorphic inclusions in chrome spinels, and xenomorphic grains are intergrown with laurite. Single grains contain inclusions of (Ru,Os)S2 of less than 10 µm in size. The alloys are enriched with Os, and its content varies from 71 to 79 wt. %; the content of Ir ranges from 20 to 28 wt. %; and the content of Ru is very low and ranges from 2 to 5 wt. %. There are some impurities of Fe and Ni. In the classification diagram, the compositions of the primary alloys correspond to osmium with low contents of Ir and Ru ( Figure 8a). Dissolution microstructures are observed in some grains ( Figure 9).   Table 4.

Laurite-Erlichmanite (Ru,Os)S2
Sulfides can be divided into two groups, according to their composition and microstructural features ( Table 4). The first group, laurite I, contains laurite-erlichmanite with a homogeneous composition.
Laurite-erlichmanite occurs as an inclusion in chrome spinels (10-15 µm), and in some cases, it can be intergrown with amphibole (magnesio-hastingsite hornblende) in chrome spinel or be found as individual grains (Figure 9c,d). The ratio, Ru´ = Ru/(Ru + Os), is 0.61-0.78. The content of Os is 20-33 wt. %, and Ru is 27-40 wt. %. In the classification diagram, they are in the laurite field ( Figure 8b). The second group, laurite II, contains laurites of a heterogeneous composition, containing micro inclusions of (Os-Ir-Ru) II alloys and laurites, which are replaced by PGE sulfarsenides (Figure 9e

Secondary Platinum-Group Minerals
The secondary PGMs are presented by (Os,Ru)S2-laurite III, native Os and Ru, Os-Ir-Ru alloys-II of a variable composition, irarsite IrAsS, zaccarinite RhNiAs, and (Ru,Ni,Os,Ir,Rh)(As,S) sulfarsenides of a non-stoichiometric composition. The secondary phases are localized in the chloritized silicate intergranular space of chrome spinels. Some grains (micro particles of osmium and other phases) are very small (less than 2-3 µm), and their chemical composition can be determined only semiquantitatively. In some cases, micro particles of a variable composition (Ru,Ir,Os,Cu,Te,Ni,Ba,S,O) can be found with secondary PGMs. Further, due to their very small size and partial coincidence with the elemental composition of the host mineral, qualitative analysis can also be hampered.

Os-Ir-Ru Alloys-II
Micro particles of Os-Ir-Ru alloys-II are the common phases and are found in a secondary mineral assemblage. They are localized mainly in laurite II, or they are a part of polyphase aggregates with Ni, Cu sulfides, sulfarsenides of Os, Ir, and Ru, zaccarinite RhNiAs, and laurite III (Figure 10a Table 4.

Laurite III
The occurrence forms of secondary laurite III are very diverse. It is mainly localized in Crcontaining chlorite, where it forms polyphase aggregates with PGE-containing chalcocite (Cu2S) (Figure 10a), recrystallized PGM aggregates with native Os (Figure 10b), micro particles in association with Ni3S2, NiS and phases of a non-stoichiometric composition (Ru,Ir,Os,Cu,Te,Ni,Ba,S,O) ( Figure  10c). The following features are characteristic of laurite III: a) A porous structure, with Cr-bearing minerals of the chlorite group in the voids, and b) a very small grain size (micro particles). The chemical composition corresponds to the end member of the laurite-erlichmanite solid solution: RuS2-OsS2 (Figure 8b). Laurite III is characterized by very low Os and Ir contents and the absence of Rh. Sometimes it contains Ni impurities. The value of Ru´ varies insignificantly and amounts to 0.93-1 (Table 4); in turn, OsS2 has low contents of Ru and Ir and is located in a recrystallized aggregate with laurite III and native Os (Figure 10b).
The (Ir,Os,Ru)AsS in the secondary association is in the form of polyphase aggregates. It replaces laurite. It contains Os and Ru in insignificant amounts. Secondary irarsite is presented as very small microparticles (less than 5 µm) in chlorite in association with Ni3S2 and laurite III.
(Ru,Ni,Os,Ir,Rh)(As,S) is found in the form of micro particles (7 µm) in a polyphase aggregate, consisting of laurite II and NiS (Figure 9f). There is a deficit of S and As in this phase.
Zaccarinite RhNiAs is found in polyphase aggregates in association with Ni3S2, secondary (Os-Ir-Ru) II alloys and native Ru (Figure 10d Table 5). Heazlewoodite is often found in polyphase aggregates with secondary PGE minerals. It has a homogeneous composition and is identical in individual grains and in the intergrowth with platinum metal phases.

Chromitite Formation: Composition of Parental Melts
Chromite bodies in ophiolites are formed due to the partial melting of rocks of the upper mantle. The interaction of the melt with mantle peridotites plays an important role in the formation of podiform chromitites. In the channel filled with molten mantle, the ascending olivine-chromitecotectic melt mixes with the silica-enriched melt, formed through the harzburgite-melt reaction. The formation of surrounding dunites along the host harzburgites is the result of a combination of olivine precipitation from "older" magma and the destruction of orthopyroxene in harzburgite, interacting with the magma channel. Thin chromite schlieren and streaks can be formed by separating the chromite from the cotectic olivine-chromite melt [9,[42][43][44][45][46].
In the Pt/Pt*-Pd/Ir diagram (Figure 11), dunites and chromitites are within the partial melting trend. The late metamorphic and hydrothermal processes, during which Pd enrichment occurred, probably cause the high Pd/Ir ratio in some Ulan-Sar'dag chromitites. In the OSMA diagram, most of the ore chrome spinels are in the olivine-spinel equilibrium field, and some of the chrome spinels are beyond this field. This can be explained by the distortion of the magmatic system closedness, changes in the compositions during the tectonic processes and metamorphism of chrome spinels. Chromitites of the USO were formed at the 28-35% degree of partial melting ( Figure 12). The data on the dunites, harzburgites, and chromitites of the northern and southern branches of the Ospa-Kitoy ophiolite are presented for comparison ( Figure 12). The Mg´(Ol)-Cr´(Sp) ratio in dunites and harzburgites corresponds to the 30-40% degree of partial melting; in ore chrome spinels from the northern branch, it corresponds to 30-40%; in ore chrome spinels of the southern branch, it corresponds to 35%. The composition of the chrome spinels of the USO varies significantly, which may reflect the interaction of mantle peridotites with the melts of various compositions. The joint presence of high-Cr´ and high-Al´ chrome spinels is common in many ophiolite belts, but as a rule, chrome spinels with different compositions are found in different ophiolite nappes [7,9,[52][53][54][55][56][57][58][59]. Their joint occurrence in one ophiolite nappe is less common; in this case, the ophiolite nappe, as a rule, contains peridotites depleted to different degrees [45,[60][61][62][63]. The composition of chrome spinels of podiform chromitites that includes, as the main components, FeO, MgO, Al2O3, and TiO2 is a function of the composition of the parental melts [9,15,16,18,21,64]. We have calculated the contents of the Al2O3, TiO2 and FeO-MgO ratios in the parental melts, which were in equilibrium with the podiform chromitites (Table 6), according to Equations (1)-(3) [15]. Table 6 shows, for comparison, the values of these parameters for the chrome spinels of ophiolites from around the world.
Al2O3Sp(wt. %) = 0.035 × (Al2O3)melt 2.42 ,  The chemical composition of the chrome spinels I group (Crsp I) and the composition of the parental melt are similar in these parameters for medium-aluminous chrome spinels of the Ospa-Kitoy ophiolite ( Table 6). The (Al2O3)melt value of Crsp I is 13-18 wt. %, and the Al2O3Sp / Al2O3melt ratio corresponds to the spreading trend and abyssal peridotites (Figure 13a). Despite the low TiO2 content, Crsp I has the TiO2Sp/TiO2melt trend, which is calculated for spinels from the MORB-type peridotites (Figure 13b).
The chemical composition of the chrome spinels II group (Crsp II) and the composition of the parental melt are similar in these parameters for the low-Al´ chrome spinels of the Ospa-Kitoy ophiolite ( Table 6). The (Al2O3)melt values for the chrome spinels of groups II and III are 10-12 and 10-13 (wt. %), and the (FeO/MgO)melt ratio is 0.4-0.85 and 0.7-1, respectively. The values of the Al2O3Sp/Al2O3melt ratios in chrome spinels II and III correspond to the trend of the chrome spinels from the island-arc boninites and chromitites of the Ural-Alaska complexes (Figure 13c). The (TiO2)melt values for the chrome spinels I, II, and III groups overlap because of the low TiO2 content (0-0.2 wt. %). In general, the values of (Al2O3)melt/(FeO/MgO) are similar to the high-Cr´ chrome spinels from the Troodos and Zetford Mine ophiolites (Figure 13d), which were formed in a suprasubduction setting. In the discrimination relationship diagrams, the [Al2O3/Fe 2+ /Fe 3+ ], [Mg´/Cr´], and [Al2O3/TiO2] (Figure 14a-c) of the chrome spinels I group are in the field of the MORB-type peridotites. Chromitites were formed during the interaction of harzburgites with primitive MORB-like melts at deep levels of the upper mantle. Through the interaction of MORB-type melts, which are in equilibrium with abyssal dunites [65,66], precipitation of chrome spinels with Cr´ 0.4-0.6, as a product of the melt-peridotite reaction, is possible [43,67]. Among the volcanic rocks of the USO, metabasalts, with enriched mid-ocean ridge basalt (E-MORB) geochemical characteristics, are available [68].   [72][73][74], Troodos boninites [75,76], Thetford boninites [77] and MORB [78][79][80] are shown for comparison.
The chrome spinels II group is localized in the boninite field. The TiO2 and Al2O3 content corresponds to the chrome spinels of the suprasubduction peridotites and overlaps with the chrome spinels from the New Caledonia island arc (Figure 14c). The chrome spinels II are formed during the reaction of the peridotites with the island-arc boninite melt in the subduction zone. The chrome spinels III group lies on the boundary of the boninite fields and magmatic complexes of the Ural-Alaskan type. Three mechanisms can be suggested for the formation of the third type of chrome spinels. The first is the interaction with high-iron low-titanium melts [81]. The second mechanism is through plastic deformations under mantle or crust-mantle conditions (low fO2, Table 1), because of the reactions of the Mg-Fe exchange with olivine. High-iron chrome spinels are found in the structural types of chromite, including chromitites with deformation structures. When compared with chrome spinels of the Alaskan type chromitites, the chrome spinels III group from the chromitites of the USO have low TiO2 contents, high Cr2O3 contents and low (mantle) fO2 values (Table 1). Variations in TiO2 contents, from 0 to 0.22 wt. %, may indicate a reaction with TiO2containing melts [82]. The third mechanism is through the partial melting of the fluid-metasomatized mantle during the interaction of andesitic melts with rocks of an overlying mantle wedge [71,83,84]. Several processes condition the composition of chrome spinels: partial melting, cooling of mantle peridotites and plastic deformations in the upper mantle. Many researchers have shown that, during ultramafite cooling, regardless of their formation (mantle or cumulative), (Mg ↔ Fe 2+ ) exchange reactions take place between coexisting olivines and chrome spinels, as a result of which the coefficient of these elements' distribution increases in favor of chrome spinels, and consequently, the calculated temperatures of the olivine-spinel equilibrium decrease [85][86][87]. During plastic deformations, exchange processes and rebalancing between olivine and chrome spinels also occur, and schlieren and lenticular segregations of chromitites transform into massive chromite bodies (Figure 4c-f). Based on this, it is assumed that the obtained values for the temperature, pressure and oxygen fugacity correspond not to the formation of ultramafites and chromitites, but to the stages of the formation and transformation of these bodies: the tectonic flow of the upper mantle rocks, the effect of metasomatizing fluids and other processes.
We have calculated and estimated the P-T parameters with the help of an olivine-spinel (Ol-Sp) geothermometer, provided in [88]. The oxygen fugacity was determined using an Ol-Sp oxybarometer, provided in [89], in accordance with oxybarometers [90,91]. For the chrome spinels I group (high Al´), the calculated temperatures of the Ol-Sp equilibrium are TOl-Sp = 1020-920 °C, and fO2 = (−0.7)-(−1.5) ( Table 1) 3). The temperature values overlap, but for the chrome spinels II group, higher temperatures are noted. The values of fO2 are discrete. It is assumed that a more reducing environment and higher solid-phase reaction temperatures are required for the formation of the chrome spinels II group, in comparison with the chrome spinels groups III and I. The content of impurities (Al2O3) in olivine was used as an alternative geothermometer. For our objects, this method was limited by the small amount of preserved olivine. We were interested in the chromitites containing PGE-and PGE-free mineralization. Using the geothermometer of De Hoog and Gall for the Al content in olivine [25], the temperature was estimated by Equation (4). For the chromitites containing PGM, the temperature, according to the olivine thermometer, was 893-1332 °C, and for the PGM-free chromitites, it was 1073-1225 °C (Table 1).
An assessment of the temperature using an olivine thermometer and inclusions of primary hightemperature (Os-Ir-Ru)-I alloys gives the values of 1200-1300 °C, which is more consistent with the expected temperatures of the chrome spinel formation from the melt.

Distribution of PGE in Mantle Peridotites and Chromitites
The form of PGE distribution in the mantle peridotites and chromitites of ophiolites reflects the mantle conditions of chromitite formation and PGE mineralization. In the process of partially melting the mantle source, the PGEs fractionate. The melt is enriched in Rh, Pt, and Pd (PPGE), since PPGE is incompatible with Os, Ir, and Ru (IPGE). The mantle restite will be enriched in IPGE [17,92,93]. This process is confirmed by the form of PGE distribution in the Ulan-Sar'dag peridotites ( Figure  15a,b), for which ∑IPGE> ∑PPGE. In the case of a low degree of partial melting, there are no obvious differences between the PPGE and IPGE contents, and the PGE distribution is flat. At a high degree of partial melting, PPGE is depleted in restite mantle rocks, relative to IPGE, and the total PGE contents become lower. In this case, the distributions have a negative slope. The PGE distribution in the mantle peridotite of Ulan-Sar'dag demonstrates: (1) a positive Ru and Pd picks; (2) a flat type of distribution, with a negative slope towards Pd; (3) a positive slope of Ir, Rh, and Pt and a negative slope of Ru (Figure 15b). Podiform chromitites are characterized by a fractionated form of PGE distribution (Figure 15c), which indicates a high degree of partial melting (about 20%-24%) of the mantle source. Three types of PGE distribution are observed in Ulan-Sar'dag chromitites (Figure 15d): (1) a negative slope from Os to Ir and Ru to Pt and a positive slope from Ir to Ru and Pt to Pd; (2) a negative slope of the distribution curve from Os to Pt and a positive slope from Pt to Pd; (3) low contents of Os, Ir and Ru and an enrichment in Rh (Pd is uncharacteristic of chromitites). Chondrite-normalized PGE relations in chromitites of the USO are similar to those of chromitites of ophiolites (Figure 15c,d) formed in a suprasubduction environment from all over the world [7,11,57,63,[96][97][98]. The pronounced positive Ru anomaly is due to the predominant laurite phase in the chromitites of the USO. It is known that Ru has a maximum affinity with S, and the predominance of IPGE sulfides, in turn, indicates a high fugacity of sulfur in the melt, in contrast to the parental melts for chromitites of the northern and southern branches of the Ospa-Kitoy ophiolite (Figure 16a-c). The experimental data show that Os, Ir, and Ru are concentrated by trapping submicroscopic clusters of these elements in the metallic state during chromite crystallization [99][100][101]. Tsoupas [49] believes that an IPGE enrichment in chromitites can be associated with post-magmatic processes during a long period of deformations, beginning from plastic deformations in the asthenospheric mantle and ending with brittle deformations in the crust. This is confirmed by wide variations in the contents of IPGE, PPGE and the IPGE/PPGE ratio from 0.03 to 2.15 (Table 3) in the studied chromitites. Negative Pt anomalies in chromitites are closely related to the unique properties of Pt itself. The distribution coefficients of the alloy/sulfide liquid at 1000 °C are 1-2 for Pd and more than 1000 for Pt. Thus, the distribution coefficient of the alloy/sulfide melt for Pt is 1000 times greater than it is for Pd [102]. This is confirmed by the distribution diagrams, where a Pt negative anomaly is clearly visible (Figure 15c,d). An extreme Pd enrichment in one of the chromitites is most likely associated with late magmatic processes and exposure to the reduced fluid. Chrome spinels has high FeO contents and low fO2 values. There are no Pd phases in this chromite. We believe that PGM is concentrated in recrystallized dunite, which requires further study. Detailed studies on the metaperidotites, metagabbros, and metavolcanogenic sedimentary rocks of the USO showed that some rocks have signs of significant exposure to a high-temperature fluid phase, which leads to rock metasomatism [68,103].

Sequence of the Formation and Transformation of the Platinum-Group Mineral Assemblage
Based on the chemical and textural features of PGM and associations with the magmatic and hydrothermal minerals of chromitites, several stages of PGE mineralization were distinguished ( Figure 17). •

I-Magmatic Stage
At the magmatic stage, under the upper mantle conditions, euhedral-subhedral hightemperature Os-Ir-Ru I alloys and laurite I are formed (Figures 8 and 9a-d), which are captured by chromite grains [5,11,22,[104][105][106]. The high osmium content in the primary (Os-Ir-Ru) I alloys is caused by an early crystallization of laurite-erlichmanite (Figure 9c). According to the experimental data, laurite without an Os impurity crystallizes from the melt at a high temperature (T = 1200-1300 °C), P = 5-10 kbar and low log sulfur fugacity (fS2), from (−0.39) to 0.07 [24,[106][107][108][109]. A decrease in temperature and increase in fS2 leads to a replacement of Ru by Os, and as a result, laurite rich in Os is formed. The predominance of laurite-erlichmanite over solid (Os-Ir-Ru) solutions in the chromitites of the USO is a distinctive feature, in comparison with the PGE mineralization of the northern and southern branches of the Ospa-Kitoy ophiolite (Figure 16a-c). In combination with the high and medium Al´ chrome spinels, this indicates the formation of chromitites and PGE mineralization of the USO because of the interaction between the initially S-saturated tholeiite magma and depleted harzburgites. Sulfarsenides and arsenides of Ru and Ir are formed from the residual fluid phase at the late magmatic stage (Figure 10g,h). With magmatic system cooling, volatile components, such as S and As, accumulate with the formation of the residual fluid phase. There is a partial replacement of laurite by irarsite with the formation of laurite II. •

II-Stage of Serpentinization and Exposure of Fluid
The PGMs in chromitites demonstrate signs of PGE remobilization (Figure 10a-d). The most intense changes of PGMs occur at the stage of the serpentinization of ultramafites. A fluid-rock interaction occurs with the participation of reduced gases (H2, CH4) and the H2O of mantle origin. Dehydrating rocks of the subducting slab, as well as mantle-reduced fluids penetrating along the fault zones in tectonically weakened sectors, serve as the fluid source. At this stage, the following platinum-metal phases are formed: native Os and Ru, (Ir-Ru) alloys, phases of a variable (Os-Ir-Ru) composition, and newly formed laurite III, IrAsS, RhNiAs and RuAs (Figures 9f and 10a-d).
Remobilized secondary PGMs form polyphase aggregates in the serpentine-chlorite matrix ( Figure  10c) in association with Ni sulfides, sulfarsenides, and arsenides. The joint occurrence of PGMs with Ni sulfides and the presence of such elements as Ni, Fe, Te, Cu, Co, As, and Sb in the platinum phases indicate PGE mobility in a fluid-saturated medium. The processes of the redistribution and concentration of PGE, including refractory Os, Ir, and Ru, occur at relatively low temperatures, reducing conditions corresponding to the formation of nickel sulfides, sulfarsenides, and arsenides, and low-temperature PGE-bearing intermetallides [110]. At the initial stage, the penetration of fluid through the permeable zones into laurite-erlichmanite led to the desulfurization of sulfides, a deviation from stoichiometry, the appearance of microdefects in the crystal lattice, and the formation of nanopores on the grain surface. These processes led to the formation of microporous structures and the separation of native Os and Ru (Figure 9e,f) [108,109,111,112]. According to the experimental data, congruent RuS2 → native Ru decomposition occurs at T = 300 °C, log fS2 = (−20), and P = 0.5 kbar [113][114][115][116][117][118]. The (Os-Ir-Ru) phases of a variable composition are probably the products of changes in (Os,Ir,Ru)AsS, since Ir has a maximum affinity with As, and irarsite therefore survives for the longest. At the same stage, secondary (newly formed) laurite III can be formed. They grow over primary laurite-erlichmanite or are confined to chloritization zones, and as a rule, they are in association with nickel and copper sulfides and sulfarsenides (Figure 10a). Irarsites IrAsS can be formed during serpentinization. In this case, irarsites are in close association with Ni3S2, forming joint aggregates. The microstructural features of such aggregates indicate their simultaneous formation (Figure 10c). The physical-chemical modeling of the forms of PGE transport in the fluid systems indicates the formation of carbonyl, chloride, hydrosulfide, and bisulfide complexes, in the form of which they are transported, and the formation of secondary PGM occurs. When PGEs are transported by bisulfide complexes, and As and Sb appear in the system, PGE solubility decreases, the composition of the solution changes, and the system deviates from equilibrium. Sulfur released from bisulfide complexes reacts with Ni to form Ni3S2. • III-Stage of Ophiolite Obduction. Regional Metamorphism.
As the ophiolite rises to the surface, the rocks undergo repeated processes of serpentinization under the influence of metamorphogenic fluids with an increased activity of O2, As, and Sb [48]. In chromitites, chrome spinels change into chrome magnetites or magnetites. There are no clear criteria for distinguishing remobilized PGMs under the crustal conditions (under the influence of metamorphogenic fluids). We believe that non-stoichiometric platinum-metal phases, containing Cu, Te, As, Sb, and O, could be formed under the crustal conditions. These elements can be transported by aqueous solutions, with a subsequent re-deposition [119,120]. These events are mainly controlled by the Eh-pH conditions, and these minerals can be formed directly in the supergenic environment. Under the conditions of a changing temperature, varying Eh-pH in the Os-S-O-H system, low f(S2) and exposure to an oxidizing high-temperature fluid at a temperature of about 500 °C [121], Os becomes more mobile than other PGEs, which leads to the further redistribution and redeposition of osmium. At this stage, the following PGE-containing phases can be formed: (Ru,Ir,Te,Ni,Ba,S,O), (Os,Ir,Ru,As,S,O), and (Ir,Ni,Cu,Ru,Os,Cl). Most often, such phases are localized in the micro voids of early PGMs (laurite, irarsite, etc.). Another process of PGM changing is the enlargement and agglomeration of PGE nanoparticles to the micro level. During progressive metamorphism (epidoteamphibolite and amphibolite facies) and/or a thermal event (introduction of granite intrusion), the changed P-T conditions affect the stability of nanoparticles. As the temperature rises to 590-650 °C, the nanoparticles in the sulfide matrix become unstable [122], which leads to their coalescence ( Figure  10b) and enlargement (to micron sizes). The accessory mineralization of the crust-metamorphogenic stage is represented by the products of the changes in Ni, Fe, and Cu sulfides and sulfarsenides, with the appearance of oxygen-containing phases of a non-stoichiometric composition.

Conclusions
(1) High-and medium-Al´ chrome spinels were formed through the interaction of mantle peridotites with tholeiite melts in a spreading setting. High-Cr chrome spinels were formed during the interaction of mantle peridotites with boninite melts in suprasubduction environments. The predominance of PGE sulfides over high-temperature Os-Ir-Ru alloys indicates their formation from S-saturated magma, which is typical of tholeiite melts.
(2) The formation temperatures of magmatic PGM-chromite association are estimated at 1000-1200 °C. The temperatures of olivine-spinel equilibrium, reflecting the formation of chromitites and tectonic deformation processes, range from 1000 to 740 °С, and the log oxygen fugacity f(O2) is low, ranging from (−0.76) to (−4.4), which indicates the upper mantle conditions, as well as the effect of reduced mantle fluids.
(3) Platinum-group mineralization in the Ulan-Sar'dag chromitites reflects a long history of formation and transformation, a change in the fluid conditions from magmatic to metamorphic ones. Primary PGMs (Os-Ir-Ru alloys-I, laurite I) were formed under the condition of a high fugacity of sulfur. The physical-chemical conditions were as follows: Т < 1200-1300 °C, and log ƒ(S2) > (−2)/(−1). Under the influence of reduced fluids on chromitites, the desulfurization of laurite and the formation of secondary PGMs (native Os and Ru, Os-Ir-Ru alloys-II, laurite III, IrAsS, and RhNiAs) occur in association with serpentine, chlorite, nickel sulfides and arsenides. This association can be formed at T = 300-700 °C, log f(S2) = (−20), and P = 0.5 kbar.
(4) At the stage of ophiolite obduction and exposure to a metamorphogenic fluid, especially osmium, a further redistribution of PGE occurs. New phases of a non-stoichiometric composition (PGE + Cu, Te, Ba, As, Sb, O, and Cl) are formed. During progressive metamorphism, an enlargement and agglomeration of PGE nanoparticles to the microlevel occur.

Conflicts of Interest:
The authors declare no conflicts of interest.