• Noble metal minerals

Gold, silver, palladium, and platinum minerals are represented by particles to 5 µm, rarely to 10–15 µm in size. They occur predominantly in cleavage cracks in clinopyroxene, less frequently in chlorite or in the interstices of clinopyroxene (Figures 10–12). As the sizes of most particles of noble metal minerals are very small, their chemical composition (Table 10, No 3–14) was determined by ruling out the influence of X-ray excitation of matrix mineral and bringing the sum of measured contents of main minerals to 100%.

Samples of clinopyroxenite 1 with pyrite-chalcopyrite paragenesis (sample 1546) were found to contain particles of palladium antimonides of less than 10 µm, which occur in cleavage cracks in pyroxene (Figure 10a) and chlorite veinlets in it (Figure 10b,c). In chemical composition, palladium antimonides correspond to mertieite I (Pd11(Sb,As)<sup>4</sup> (Pd/(Sb + As) = 2.72–2.74) (Table 10, No 1,2) and stibiopalladinite (Pd5Sb2) (Pd/(Sb + As) = 2.33–2.52) (Table 10, No 3–5). The largest mertieite I grains contain tiny platinum sulfide inclusions 1–2 µm in size (Table 10, No 6).

**Figure 10.** Noble metal minerals in clinopyroxene I. (**a**) Stibiopalladinite (Stpdn) in the cleavage crack in clinopyroxene (Cpx); (**b**) mertieite I (Met) grain in clinopyroxene (Cpx) and chlorite (Chl) veinlet; (**c**) stibiopalladinite (Stpdn) grain enclosed in chlorite. Chlorite is associated with pyrite (Py) and chalcopyrite (Ccp), which are partially replaced by hypergene iron and copper oxides (Fe-Cu-O); (**d**) veinlets of platinum sulfoarsenide (Pt-As-S) cutting clinopyroxene and chlorite.

**Figure 11.** Noble metal minerals in clinopyroxenite II: (**a**) native copper–palladium phases (Cu-Pd) in cleavage cracks in clinopyroxene (Cpx); (**b**) interstitial bornite (Bn) segregation in intergrowth with native copper–palladium phases (Cu-Pd); (**c**) polyphase intergrowth of native Cu-Pd phases (Cu-Pd) (Table 10, No 10), palladium sulfide (Pd-Cu-S) (Table 10, No 12), phases Pd<sup>3</sup> (Cu,Ni) (Table 10, No 11) and pyrrhotite (Pyh) in clinopyroxene (Cpx); (**d**) Au,Ag particle (Table 10, No 13) in secondary copper sulfide (Cu-S) developed on bornite (Bn).

**Figure 12.** Noble metal minerals in clinopyroxenite III. (**a**) Crystals of palladium antimonide (Pd-Sb) on the surface of pyrite grain (Py); (**b**) Ag-Au particle (Au,Ag) (Table 10, No 14) in chalcopyrite (Ccp) at the contact of two pyrite (Py) grains.

**Table 10.** Chemical composition of noble metal minerals in clinopyroxenites (in wt.%).


Note. Samples of clinopyroxenites I (No 1–8), II (No 9–13), and III (No 14).

Clinopyroxene I also contains the thinnest microveinlets of sulfoarsenide Pt with composition intermediate between sperrylite (PtAs2) and platarsite (PtAsS) (Table 10, No 7, 8). These microveinlets cut clinopyroxene and are developed along the cleavage cracks of chlorite (Figure 10d).

In clinopyroxenite II with bornite-chalcopyrite sulfide paragenesis, particles of noble metal minerals 3 µm and less in size are also developed in the interstices of clinopyroxene and along its cleavage cracks. They are represented by native Pd-Cu and Pd-Cu-Ni phases (Figure 11,a,b), native gold (kustelite) (Figure 11d), and copper–palladium sulfide. Native and sulfide phases can form polyphase aggregates of grains (Figure 11c). The composition of native Cu-Pd phases is described by the generalized formula Pd0.38–0.48Cu0.43Fe0.04–0.13 Au0.02–0.05Pt0.01–0.02 (Table 10, No 9, 10), corresponding in stoichiometry to skaergaardite (PdCu). Phase Cu-Pd-Ni (Table 10, No 11) is calculated for the stoichiometry of Pd3(Cu,Ni) phase which has not been described in literature. The composition of Ag-Au particles (Ag0.76–0.90Au0.1–0.24) corresponds to fineness 170–240‰. The composition of copper– palladium sulfide is calculated for the formula of (Pd,Cu,Fe)2S not described in literature (Table 10, No 12).

In clinopyroxenite III with pyrrhotite–chalcopyrite–pyrite paragenesis, noble metal minerals are rare, their sizes do not exceed 2–3 µm. Among these minerals, we detected Pd-Sb phases, which in the ratios of components are similar to stibiopalladinite and mertieite, and native gold (see Table 10, No 14). Pd-Sb phases are localized on the surface of large pyrite grains in contact with chlorite developed on clinopyroxene (Figure 12a). A particle of native gold (fineness ~720‰) is enclosed in chalcopyrite at the contact of two pyrite grains (Figure 12b).

#### *4.2. Sulfur Isotopic Composition of Sulfides*

Sulfur of pyrite, bornite, and chalcopyrite from clinopyroxenite II and III samples of the Ozernoe occurrence has a homogenous isotopic composition δ <sup>34</sup>S= <sup>−</sup>2.1 . . . <sup>−</sup>2.9 (Table 11). The δ <sup>34</sup>S values of bornite and chalcopyrite, which compose lattice-like aggregates and replace copper sulfides (spionkopite–yarrowite), virtually do not differ (Figure 13a,b). Table 11 shows the data on the sulfur isotopic composition of sulfides from the Baronsky occurrence—the typical object of gold–palladium mineralization in the Urals. The δ <sup>34</sup>S = <sup>−</sup>1.2 . . . <sup>−</sup>2.6‰ values of chalcopyrite from this occurrence (Figure 13e,f) are identical to the sulfur isotopic composition of clinopyroxenite II and III samples from the Ozernoe occurrence.

**Table 11.** Isotopic composition of sulfur (δ <sup>34</sup>S) sulfides from the Ozernoe and Baronsky deposits.


**Figure 13.** Sulfur isotopic composition of sulfides from Ozernoe (**a**–**d**) and Baronsky (**e**,**f**) deposits. Sites of analysis are shown by circles with a diameter 100 µm, numbers show δ <sup>34</sup>S values in ‰. Minerals: Py—pyrite, Ccp—chalcopyrite, Bn—bornite, Cu-S—copper sulfide replacing bornite.

#### **5. Discussion**

#### *5.1. Sequence of Mineral Formation at Ozernoe Occurrence*

Our data show that the primary minerals of the magmatic stage include the main minerals of studied rocks—clinopyroxene, olivine, and Cr-Ti-V-bearing magnetite (with platelets of ilmenite). Interstitial amphibole (magnesio-ferri-hornblende), which was later replaced by chlorite, could have also been deposited from residual fluid-enriched melt (Figure 6c). Further hydrothermal-metasomatic mineral formation proceeded in the interstitial space and along cleavage cracks of clinopyroxene, contacts of clinopyroxene and olivine and microcracks that cut olivine and clinopyroxene. It started from the deposition of amphibole (actinolite and magnesio-hornblende) and bornite-chalcopyrite solid solutions (iss), which decomposed later. As the temperature decreased and water activity increased, the earlier deposited minerals were replaced by serpentine (on olivine), amphibole (tremolite), secondary magnetite, and chlorite with synchronous titanite. At the contact zone of olivine and clinopyroxene, symplectic aggregates of magnetite with tremolite serpentine were deposited.

The bornite–chalcopyrite solid solution (iss) in clinopyroxene II is the earliest sulfide. This is supported by its further decomposition into a lattice-like aggregate of bornite and chalcopyrite and partial replacement by tremolite (see Figure 8c). The process of mineral formation in clinopyroxenites of all groups led to the formation of chlorite, sulfide parageneses with pyrite, pyrrhotite, and chalcopyrite, as well as platinum group minerals and Au-Ag minerals. Geothermometry of chlorite composition shows that the hydrothermal system by that moment had cooled to 250 ◦C and continued cooling in parallel with a decrease in the iron content of chlorite (Figure 7).

#### *5.2. Specific Features of the Evolution of Mineral Formation in Clinopyroxenites I–III*

A common feature of ore-forming clinopyroxenites I–III is the same specific composition of primary minerals represented by diopside, olivine, and Cr-Ti-V-bearing magnetite. All pyroxenites have the same general tendency of replacement of primary rock-forming minerals by secondary with decreasing temperature: a) olivine → serpentine, secondary magnetite, and b) clinopyroxene → amphibole, secondary magnetite → chlorite.

The determined different ratios of clinopyroxene and olivine contents and different chemical compositions of these minerals indicate variability of the composition of initial melts for clinopyroxenites I–III. This variability reflects the primary magmatic stratification of rocks of the Dzelyatyshor massif, which is, according to [10], manifested as alteration of rocks differing in the composition of clinopyroxene and olivine. A direct dependence of the contents of FeO and Al2O<sup>3</sup> in primary pyroxene revealed for clinopyroxenites under study (Figure 3b) is typical of the clinopyroxenite–gabbro trend of Uralian massifs and is due to cotectic olivine and clinopyroxene fractionation [21].

Postmagmatic clinopyroxene in clinopyroxenite I has an inverse dependence of FeO and Al2O<sup>3</sup> contents (Figure 3b). The lowest iron content of clinopyroxene in clinopyroxenite III is most likely related to the postmagmatic "discharge" of part of iron into the oxide phase in the form of thinnest platelets along cleavage cracks (Figure 4a).

The identified higher contents of the main indicator elements (Cr, Ti, V, Mn, Al) in primary magmatic magnetite, compared to secondary hydrothermal magnetite, reflect the ratio of concentrations of these trace elements in the magmatic melt and hydrothermal fluid, which was determined for various ore deposits [22,23].

The revealed differences in the mineral composition of clinopyroxenites I–III and the chemical composition of primary minerals are typical of secondary minerals (Table 12). In particular, clinopyroxenites I–III differ in the set of sulfide minerals and their quantitative ratios (Table 1). In clinopyroxenite I with the lowest content of sulfides (less than 0.1%), chalcopyrite drastically prevails over pyrrhotite and pyrite. The content of sulfides in clinopyroxenite II can reach 0.5%, and intermediate chalcopyrite–bornite solid solution among them prevails over chalcopyrite. In clinopyroxenite III, the content of sulfides

in uncommonly high (to 10%). Their main sulfide mineral pyrite predominates over chalcopyrite and pyrrhotite.

**Minerals Clinopyroxenite I Clinopyroxenite II Clinopyroxenite III** *Primary*: Clinopyroxene 1 Diopside (4.5–7 wt.% FeO) Diopside (5–6.8 wt.% FeO) Diopside (0.7–2.2 wt.% FeO) Olivine x(Fa) = 0.25–0.26 x(Fa) = 0.28–0.30 x(Fa) = 0.13–0.15 Magnetite 1 Cr-Ti-V-bearing Cr-Ti-V-bearing Cr-Ti-V-bearing *Secondary*: Clinopyroxene 2 Diopside (9.6–14.5 wt.% FeO) Amphibole Actinolite, magnesio-ferri-hornblende, tremolite Magnesio-hornblende, tremolite Chlorite Pennine, picnochlorite Pennine Pennine Serpentine Serpentine A Serpentine A Serpentine B Magnetite 2 0.n wt.% TiO<sup>2</sup> и MgO <1 wt.% Cr2O<sup>3</sup> , MgO, V2O<sup>3</sup> 0.n wt.% MnO Sulfides Ccp >> Po > Py (Bn-Ccp)ss >Ccp; <sup>δ</sup> <sup>34</sup>S = <sup>−</sup>2.1 . . . <sup>−</sup>2.7‰ Py > Ccp >> Po; <sup>δ</sup> <sup>34</sup>S = <sup>−</sup>2.1...−2.9‰ Pyrrhotite NFeS = 0.96–0.97 NFeS = 0.94–0.95 Noble metal minerals Antimonides Pd (mertieite, stibiopalladinite), sulfo-arsenide Pt (sperrylite-platarsite) Native Pd-Cu and Pd-Cu-Ni phases, native silver (170–240‰). Mertieite, stibiopalladinite, argentopentlandite, native gold (720 ‰)

**Table 12.** Comparative characteristics of primary and secondary minerals of various groups of clinopyroxenites.

Noble metal minerals in clinopyroxenes under study were deposited simultaneously with chlorite. Only platinum sulfoarsenide in clinopyroxene I developed later than chlorite, being deposited in the cleavage cracks of this mineral (see Figure 10d). At the same time, whereas clinopyroxenite II contains native Pd, clinopyroxenites I and III include Pd antimonides. Au-Ag particles of solid solutions detected in clinopyroxenites II and III do not contain palladium (Table 10).

#### *5.3. Association of Noble Metal Minerals at Ozernoe Occurrence*

Our study is based on the limited number of samples with gold–palladium mineralization, as a result of which we detected a limited number of noble metal minerals. It was reported in previous works [8,10] that in rocks with a low content of sulfides (no more than 1%), chalcopyrite–bornite segregations similar to those described in clinopyroxenites II are most common. At the same time, some samples contain cubanite, platelets of which can be considered exsolution products of intermediate cubanite–chalcopyrite solid solution. As we did not observe the joint occurrence of bornite–chalcopyrite and cubanite-chalcopyrite segregations, two ore mineral associations were identified—cubanitepentlandite–pyrrhotite–chalcopyrite and bornite–chalcopyrite [10]. The sets of noble metal minerals in the composition of these mineral associations are virtually the same and consist of native metals, sulfides, antimonides, arsenides, arsenoantimonides, and tellurides (Table 13).

#### *5.4. Features of Native Gold Composition*

It was reported in previous research [8,10], which is not in conflict with our data (Table 10), that the composition of native gold in the studied occurrence demonstrates considerable variability. The chemical composition of native gold reported by these authors was recalculated and is presented in Table 14. Some analyses indicate low total contents of elements, which can be due to the small sizes of gold grains and X-ray excitation of matrix minerals. Nevertheless, we present these analyses in the table to demonstrate the ratios of main components and minimum levels of palladium contents in native gold. From the analyses reported in [10] we excluded those with high contents (more than 1 wt.%) of As, Te, and S elements that are not typical of native gold. In Table 14, analyses of gold grains are grouped based on their association with copper sulfides (bornite, chalcopyrite) and arsenides, antimonides, and tellurides of noble metals.

**Table 13.** Noble metal minerals at Ozernoe occurrence from [8,10].


Note. \*—minerals described in this work in the composition of bornite–chalcopyrite paragenesis.

**Table 14.** Chemical composition of native gold at Ozernoe occurrence from [8,10].



**Table 14.** *Cont.*

Note. ss—solid solution.

In general, native gold at the Ozernoe occurrence is represented by Cu-bearing Au-Ag solid solutions (Cu to 6.5 wt.%) as well as Au-Cu solid solutions and intermetallides (tetraauricupride and auricupride) with wide variations in fineness (150–750‰). According to the composition of components, it belongs to Au-Cu, Au-Cu-Pd, Au-Ag-Cu, Au-Ag, Au-Ag-Pd, and Au-Ag-Cu-Pd. Gold grains containing no palladium are quantitatively predominant (21 grains of the 35 studied). The absence of palladium in the composition of both Au-Ag, and Au-Cu phases was reported in [10] for sample 509019, in which palladium minerals were not detected and native gold is associated with chalcopyrite and bornite. In the samples, which contained sulfides, tellurides, antimonides, and palladium arsenides, native gold is predominantly palladium-bearing. On the whole, there is a tendency to increase the palladium contents in the series—Au-Ag solid solutions (no more than 1.5 wt.% Pd) and Au-Cu intermetallides—Au-Cu solid solutions (to 6 wt.% Pd) and Cu,Pd,Au solid solutions (16.2–16.9 wt.% Pd) (Figure 14).

**Figure 14.** Chemical composition and palladium and copper content of native gold from Ozernoe occurrence on Au-Ag-Cu diagram.

#### *5.5. Reconstruction of Physico-Chemical Parameters of Ore Formation*

Following the identified sequence of mineral formation at the Ozernoe occurrence, we determined the stability fields of the main mineral associations and reconstructed the values of

sulfur fugacity in the ore-forming system. Log *f*S2—T diagram (modified from [24–26]) shows the stability boundaries of iron, copper, gold, silver, and platinum minerals depending on temperature and sulfur fugacity (Figure 15). Early bornite–chalcopyrite solid solution (iss) in clinopyroxenite II is evidence of high temperatures of formation (350–520 ◦C). As clinopyroxenite II contains magnetite together with bornite–chalcopyrite solid solution (iss), this allows estimation of temperatures and sulfur fugacity in the system. The stability lines of magnetite (Py+Hem)/Mag and solid solution (Bn+Ccp)/iss intersect at 480 ◦C and *f* S<sup>2</sup> = 10−<sup>3</sup> , therefore, the stability field of solid solution does not exceed the specified values (Figure 15, field 1). Further in the course of mineral formation, sulfide parageneses with pyrite, pyrrhotite, chalcopyrite, platinum group minerals and Au-Ag minerals were deposited at temperatures 150–250 ◦C (see Table 6). Chalcopyrite, pyrite, pyrrhotite in clinopyroxenite I together with Pt sulfide in the given range of temperatures are stable at log *f* S<sup>2</sup> = −23 . . . −11.5 (Figure 15, field 2). In clinopyroxenite II, native silver (170–240‰) occurs in association with bornite and chalcopyrite, and their stability field is limited by the equilibrium of Uyt/Ag0.75Au0.25 and Ccp/(Bn+Pyh) (Figure 15, field 3).

**Figure 15.** Sulfur fugacity dependence on temperature for Fe-S-O, Cu-Fe-S, Au–Ag–S, and Pt-S systems, and log *f* S<sup>2</sup> and T estimations for different associations at Ozernoe occurrence. Minerals: Bn bornite, Ccp—chalcopyrite, iss—bornite-chalcopyrite solid solution, Py—pyrite, Pyh—pyrrhotite, Mag—magnetite, Hem—hematite, Pn—pentlandite, Apn—argentopentlandite, Pvk—petrovskaite; Uyt—uytenbogaardtite; Aca—acanthite; Cc—chalcocite; Cv—covellite.

In clinopyroxenite III, chalcopyrite grains contain pentlandite and argentopentlandite inclusions. According to the diagram (Figure 15), the stability fields of Ni-bearing phases are located in the higher temperature range. Maximum sulfur fugacity values for this association correspond to the stability line of argentopentlandite (log *f* S<sup>2</sup> < −11), minimum values were estimated from the equilibrium Ccp/(Bn+Pyh) (log *f* S<sup>2</sup> > −21) (Figure 15, field 4). The association native gold (720 ‰) with chalcopyrite, pyrite and pyrrhotite in clinopyroxenite III is stable at log *f* S<sup>2</sup> = −30 . . . −7.5 (Figure 15, field 5).

To estimate the temperatures and values of S<sup>2</sup> fugacity, using a pyrite-pyrrhotite buffer, we analyzed pyrrhotite grains from pyrrhotite-chalcopyrite-pyrite parageneses (clinopyroxenite III). Pyrrhotite composition in this association reaches NFeS = 0.94–0.95. The calculations with the help of equations [27,28] showed that during the formation of

pyrrhotite of the given composition, the temperature reaches 196 ◦C and sulfur fugacity log ƒS2 = −22,7, which does not go beyond the bounds of the above-estimated boundary values ƒS2. The results obtained indicate that mineral formation at the Ozernoe occurrence took place at decreasing temperatures and increased sulfur fugacity in the system.

#### *5.6. Model of Formation of Gold–Palladium Mineralization at Ozernoe Occurrence*

The earlier proposed model of formation of gold-palladium Ozernoe occurrence suggests that mineralization is polychronous and polygenic, and formed in three stages [10]. The first, magmatogenic, stage is the concentration of PGE in sulfides and olivine-rich rocks. The second, hydrothermally-metasomatic, stage is a transformation of platinoids into sulfide forms and the enrichment of rocks with gold and copper under the influence of Karshor and Soba plutonites. The third, "epithermal", stage was marked by the deposition of arsenide, antimonide, and bismuthide forms of platinoids mainly at low-temperature serpentinization and redistribution of gold in the linear zones of concordant tectonic contacts of the clinopyroxenite massif.

In this manuscript we do not discuss the question of the primary magmatic origin of Fe-Cu sulfide melts enriched with native metals in olivine clinopyroxenites of the Dzelyatyshor massif, accepted for the first stage in the model [10], as we did not detect relics of magmatic sulfides in studied samples. The problem of the influence of As, Bi, and Te on the fractionation of Cu, Fe, Ni, and PGE during crystallization of sulfide magma is also beyond [29,30]). At the same time, the role of low-temperature remobilization of non-ferrous and noble metals as well as S, As, Sb, Te, and Bi from magmatic sulfides by postcumulus fluids, reported in some publications, is undoubtedly important [3,4,31,32]. The most large-scale enrichment of post-cumulus fluids with copper, palladium, and gold takes place on the cooling of melt of gabbro composition. Fluid discharge leads to the formation of high-sulfide mineralization in olivine gabbro, which belongs to the "Volkovsky" type in the Urals [33]. This type of mineralization is represented by large bornite-chalcopyrite bodies with native gold and tellurides of Pd [34].

The conducted research allows us to suggest a simpler model of deposition of goldpalladium mineralization, in which the only source of ore matter and fluid were cooling portions of basic melt. The absence of another external source of substance and fluid is supported by the homogeneity of the sulfur isotopic composition of sulfides and its closeness to the magmatic zero mark. A slight lightening of the sulfur isotopic composition relative to the magmatic zero mark is most likely related to the depletion of heavy isotope during the separation of fluid from basic melt and kinetics of isotope exchange between fluid and sulfides at a further decrease in temperature.

We think that the primary magmatic layering of rocks manifested in different quantitative ratios of clinopyroxene and olivine in them further controls the local trends in the variability of the chemistry of mineral-forming medium and the concentration of ore components, including noble metals, and sulfur in each layer on its cooling.

Copper and iron sulfides as well as noble metal minerals, including native gold, were deposited among the latest and low-temperature minerals. Mineral forms of metals in each portion (layer) of cooled melt were determined by the variability of the activity of post-magmatic fluid components separated from melt. Most likely, these components that bind native metals into their own minerals, when their activity increases, are S, Sb, As, Te, Se, and Bi. The variations in the activity of these components could be due to the change in redox properties of the fluid and concentrations of metal in it, related to the hydration (serpentinization) of olivine. Absorption of water during serpentinization of olivine provided the residual metal-bearing fluid with reducing properties and stability of native forms of metals.

Within the frames of our model, the sets of sulfide minerals and noble metal minerals attributed to earlier distinguished cubanite–pentlandite–pyrrhotite–chalcopyrite and bornite–chalcopyrite mineral associations [10] could also be a result of the evolution of different compositions of the initial melt.

### **6. Conclusions**


**Author Contributions:** Conceptualization, V.M. and G.P.; methodology, V.M.; validation, G.P.; investigation, V.M. and T.M.; writing—original draft preparation, V.M.; writing—review and editing, G.P.; visualization, V.M., T.B. and T.M.; Formal analysis, T.B.; supervision, V.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The studies are supported by the Russian Foundation for Basic Research (project No. 20-05-00393a) and are carried out as a part of the IGG UB RAS State assignment (state registration No. 122022600107-1), using the «Geoanalitik» shared research facilities of the IGG UB RAS. The re-equipment and comprehensive development of the «Geoanalitik» shared research facilities of the IGG UB RAS is financially supported by the grant of the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2021-680). The studies are carried out within the framework of the state assignment of the Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences and of the FRC Institute of Geology Komi SC UB RAS State assignment (state registration No. 1021062211108-5-1.5.2) financed by the Ministry of Science and Higher Education of the Russian Federation.

**Data Availability Statement:** The data presented in this study are mainly contained within the article and available in the references listed. To a minor degree, the data presented are not publicly available due to privacy and available on request from the corresponding author.

**Acknowledgments:** We are very grateful to the reviewers for their comments and suggestions.

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

### **References**

