*Article* **The Gold–Palladium Ozernoe Occurrence (Polar Urals, Russia): Mineralogy, Conditions of Formation, Sources of Ore Matter and Fluid**

**Valery Murzin <sup>1</sup> , Galina Palyanova 2,\* , Tatiana Mayorova <sup>3</sup> and Tatiana Beliaeva <sup>2</sup>**


**Abstract:** We studied the mineralization and sulfur isotopic composition of sulfides of gold–palladium ores in olivine clinopyroxenites from the Dzelyatyshor massif made up of a continuous layered series of rocks: olivine-free clinopyroxenite–olivine clinopyroxenite–wehrlite. The primary igneous layering of rocks, manifested as different quantitative ratios of clinopyroxene and olivine in them, controls the local trends of variability in the chemistry of mineral-forming medium and the concentrations of ore components, including noble metals, and sulfur in each separate layer during its cooling. The replacement of primary rock-forming minerals by secondary minerals, when the temperature decreases, is a characteristic trend for pyroxenites: (a) olivine → serpentine, secondary magnetite, and (b) clinopyroxene → amphibole, secondary magnetite → chlorite. The deposition of native gold in parageneses with PGM and sulfides at the Ozernoe occurrence took place during the replacement of earlier rock-forming minerals by chlorite. This process completed mineral formation at the deposit and took place at temperatures 150–250 ◦C and at the high activity of S, Te, Sb, and As of fluid. The variability of mineral formation conditions during chloritization is reflected in the change of native-sulfide forms of Pd by arsenide-antimonide forms and the sulfur isotopic composition of sulfides. The Pd content in native gold increases in the series—Au-Ag solid solution (<1.5 wt.% Pd)—Au-Cu intermetallides (to 6 wt.% Pd)—Cu-Au-Pd solid solutions (16.2–16.9 wt.% Pd). The sulfur isotopic composition of pyrite, chalcopyrite, and bornite varies from −2.1 to −2.9‰. It is assumed that a deep-seated magmatic basic melt was the source of fluid, ore components, and sulfur.

**Keywords:** clinopyroxenites; palladium gold; platinum group minerals; chlorite geothermometer; sulfur isotopic composition

### **1. Introduction**

The object of this study is the gold–palladium Ozernoe occurrence at the Dzelyatyshor wehrlite–pyroxenite massif in the Polar Urals (Russia). The massif is situated 80 km southwest of the town of Labytnangi in the upper reaches of the Dzelyatyshor stream, left inflow of the Malaya Kharamatalou river. In its geologic, petrographic, and mineralogic-geochemical features, this occurrence is most similar to the so-called "Baronsky" type of gold–palladium mineralization identified among the Uralian deposits [1]. The typical properties of this type of mineralization are: (1) occurrence in olivine pyroxenites of differentiated clinopyroxenite-gabbro massifs with titanomagnetite mineralization; (2) uneven dispersed impregnation of noble metals in rocks at extremely low contents of associated sulfides and (3) absence of visible changes in the appearance or composition of rocks of ore zones, which are outlined only on the basis of sampling results. In

**Citation:** Murzin, V.; Palyanova, G.; Mayorova, T.; Beliaeva, T. The Gold–Palladium Ozernoe Occurrence (Polar Urals, Russia): Mineralogy, Conditions of Formation, Sources of Ore Matter and Fluid. *Minerals* **2022**, *12*, 765. https://doi.org/10.3390/ min12060765

Academic Editor: Liqiang Yang

Received: 29 April 2022 Accepted: 14 June 2022 Published: 16 June 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the world literature, similar mineralization in the rocks of basic-ultrabasic composition is described as poorly sulfidic with a high content of PGE—«low-S-high platinum-group element deposits» [2–4].

A typical object of "Baronsky" type is the Baron–Klyuevo occurrence at the Volkovsky massif in the Middle Urals (Russia). It occurs in the upper differentiated magmatic sections of this massif in the rocks of dunite-clinopyroxenite-gabbro association. Zones with impregnated mineralization (Pd > Au > Pt, 0.02–0.04 wt.% S) are localized in olivinites, clinopyroxenites, and olivine–anorthite gabbro that smoothly pass into each other in the southern margin of the massif [1,5,6]. Gold–palladium mineralization at the Baronsky occurrence consists of two spatially separated mineral associations: (1) sulfide, with predominant vysotskite PdS, and (2) arsenide–antimonide, with predominant isomertieite Pd11Sb2As<sup>2</sup> [6,7].

The geologic setting of the Ozernoe occurrence at the Dzelyatyshor massif, petrographic and petrochemical characteristics, and mineralogic-geochemical features of rocks and gold–palladium mineralization were reported in previous studies [8–11]. In this study, we conducted a detailed analysis of the mineral composition and mineral parageneses of gold–palladium ores from the Ozernoe occurrence and obtained the first data on the deposition temperature of noble metal minerals, using the chlorite geothermometer, studied the sulfur isotopic composition of sulfides, determined the sources of ore matter and fluid, and revealed the features of the distribution of Pd and Cu in native gold.

#### **2. Brief Description of the Study Area**

In the geological aspect, the Dzelyatyshor massif is surrounded by the Voykar–Synya dunite–harzburgite ophiolite massif, which occurs in the polar part of the Main Uralian fault zone (Figure 1). The rocks of the northeastern framing of this massif belong to the Karshor peripheral striped dunite–wehrlite–clinopyroxenite–gabbro complex, whose age is determined as Late Ordovician [12]. In the southeast, the Karshor complex borders granitoids of the Sobsky granodiorite–tonalite complex (D1–2).

**Figure 1.** (**a**) Geographic location of the Urals on the Europe–Asia border; (**b**) position of Baronsky type objects in the tectonic structures of the Urals (modified from [13]) (on the right); (**c**) geologic structure and setting of gold–palladium mineralization at the Dzelyatyshor massif.

Clinopyroxenite bodies hosting gold–palladium mineralization of the Ozernoe occurrence were initially attributed to the Karshor complex. In terms of the most modern concepts [10], these clinopyroxenites are identified in the Dzelyatyshor wehrlite– clinopyroxenite massif which is isolated from the Karshor complex. In the petrochemical

characteristics, the Dzelyatyshor massif is similar to the wehrlite–clinopyroxenite series of the Platinum-bearing Belt of the Urals and possesses the features of layering displayed in the replacement of olivine by non-olivine rocks from bottom to top. Its contacts with hosting gabbroids of the Karshor complex are tectonic. The Dzelyatyshor massif is composed of a continuous layered series of rocks—non-olivine clinopyroxenite–olivine clinopyroxenite wehrlite [10]. Mineralization belongs to the poor sulfide stratiform type and tends to sheet-like bodies of olivine clinopyroxenites in the upper part of the massif. The bodies consist of 13 ore bodies from 2 to 30 m in thickness [10]. The content of metals in the ores, determined using atomic absorption spectrometry, are: 1.5 wt.% Cu, 2.9 ppm Au, 3.7 ppm Pd, and 1.0 ppm Pt [9,10].

Noble metal mineralization is localized at two levels within the massif. The first lower level occurs in the near-contact part of the massif. The second upper ore level occurs on the horizon of high-olivine pyroxenites with layers of wehrlite. The ore-bearing rocks contain impregnation of magnetite (to 5–7%) and hypogene sulfides, among which chalcopyrite, bornite, pyrrhotite, and pentlandite are predominant. The content of sulfides is typically less than 0.5%, to 1–3% in some parts. Among ores, we identified primary late magmatic minerals, forming segregations of interstitial, and secondary minerals in the form of veinlets and segregations of various shapes, associated with secondary silicates, mainly with serpentine [10].

Noble metal minerals, which amount to 30, are commonly associated with sulfides. The contents of Au and Ag directly correlate with copper content. Geochemical and spatial distribution of the early platinum–metal and later gold–copper mineralization is observed [10].

#### **3. Samples and Research Methods**

We studied the samples of olivine-bearing clinopyroxenites with elevated contents of Au and Pd (1–2 ppm), collected from near-surface mine workings (ditches 101, 108, and 126), driven while prospecting for gold by the JSC Yamal Mining Company in 2008. The rocks consist of coarse-grained varieties with olivine content from 1 to 10–15% and with grain sizes of main minerals from 1 to 15 mm. Ore minerals in clinopyroxenites are represented by dispersed impregnations of fine (less than 0.1 mm) and enlarged (to 2–3 mm) magnetite, and sulfides with grain sizes less than 0.5 mm. The content of magnetite in all samples is commonly no more than 2–3%, and that of sulfides is less than 0.5%. In rare cases, the amount of sulfides was 10%, and their grain sizes were more than 2 mm.

The studied samples of clinopyroxenites were separated into three groups according to the content of olivine and sulfides as well as to predominant minerals in sulfide parageneses (Table 1, Figure 2). Sulfides in clinopyroxenites I–III were attributed by us to pyrite–pyrrhotite–chalcopyrite, bornite–chalcopyrite and pyrrhotite–chalcopyrite–pyrite parageneses, respectively. Clinopyroxenites I and III are similar in the specific composition of sulfides, but differ in the content and quantitative ratios of the main sulfide minerals. Clinopyroxenites II and III are characterized by an association of sulfides with magnetite, the crystals of which are enclosed in sulfides or form rims on sulfide segregations (Figure 2c,d).

**Table 1.** Features of studied clinopyroxenite samples from Ozernoe occurrence.


Note. Minerals: Py—pyrite, Pyh—pyrrhotite, Ccp—chalcopyrite, Bn—bornite. Abbreviations of minerals are given following the recommendation of IMA [14].

**Figure 2.** Relationships of main ore minerals in the samples of clinopyroxenites I (**a**), II (**b**), and III (**c**,**d**). Bn—bornite, Ccp—chalcopyrite, Cpx—clinopyroxene, Cu-S—secondary copper sulfide, Mag—magnetite, Py—pyrite, Pyh—pyrrhotite.

Microprobe analyses of minerals of clinopyroxenites I and II were carried out with a CAMECA SX-100 equipped with five WDS spectrometers and a Bruker energy dispersive spectrometer system at the Common Use Center "Geoanalyst" of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences (Ekaterinburg, Russia). Quantitative WDS analyses were performed at 25 kV accelerating voltage and 20 nA sample current, with a beam diameter of about 1 µm.

Analysis of the polished sections of clinopyroxenites III was performed on the scanning electron microscope Tescan Vega 3LMN (Tescan, Czech Republic) with an energy dispersive spectrometer X-Max 50 (Oxford Instruments, Oxford, UK) in the Institute of Geology KomiSC UB of the RAS. Analytical conditions: accelerating voltage was 20 kV, beam current was 15 nA, with the beam diameter up to 1 µm.

Isotopic mass-spectrometric analysis of sulfide sulfur of clinopyroxene II was conducted in the Laboratory of Stable Isotopes in the FEGI FEB RAS. The samples were prepared by laser method using the NWR Femto femtosecond laser ablation [15,16]. The ratio of sulfur isotopes was measured on the MAT-253 mass-spectrometer (Thermo Fisher Scientific, Dreieich, Germany) relative to the laboratory working standard calibrated according to international standards IAEA-S-1, IAEA-S-2 и IAEA-S-3. Results of measurements of δ <sup>34</sup>S are reported relative to the international standard VCDT and are given in ppm (‰). The accuracy of analysis was ±0.2‰ (2σ).

The classification of amphiboles is based on the scheme from [17] and that of pyroxenes, on the classical scheme from [18]. The temperature regime of deposition of noble metal minerals was performed using data for chlorite geothermometer, based on the amount of tetrahedral aluminum (AlIV) and molar fraction of iron XFe by the formula T, ◦C = 17.5 + 106.2 <sup>×</sup> (AlIV <sup>−</sup> 0.88 <sup>×</sup> [XFe <sup>−</sup> 0.34]) [19].

### **4. Results**

*4.1. Mineralogy, Mineral Association, and Sequence of Mineral Formation at the Ozernoe Occurrence*

4.1.1. Primary (Early) Rock-Forming Minerals of Clinopyroxenites

	- Primary clinopyroxene with grain sizes from 0.1 to 10 mm in the samples of all groups of clinopyroxenites belongs to the diopside-hedenbergite with a varying ferrosilite mineral (Figure 3a). In clinopyroxenites I and II, it corresponds to diopside with an elevated content of FeO (4.47–7.04 wt.%) and Al2O<sup>3</sup> (0.79–2.74 wt.%) (Table 2). It contains minor Ti, Mn, Cr, and Na in amounts less than 1 wt.%. Clinopyroxene in clinopyroxenite III is also represented by diopside but with lower contents of FeO (0.66–2.17 wt.%), Al2O<sup>3</sup> (less than 1.7 wt.%), and other trace elements. This clinopyroxene also contains the thinnest plates of iron oxide, supposedly magnetite, which develop along cleavage cracks (Figure 4a). These plates seem to be the result of the decomposition of the solid solution or oxidation of Fe2+ .

**Figure 3.** Diagrams of composition of clinopyroxenes (**a**) and contents of FeO-Al2O<sup>3</sup> (**b**) in primary and secondary clinopyroxenites. Primary clinopyroxene from clinopyroxenites I (1), II (2), and III (3). 4—secondary clinopyroxene from clinopyroxenite I.

**Figure 4.** Clinopyroxene in samples of clinopyroxenites III (**a**) and I (**b**). (**a**) Thin plates of Fe-O phase in primary clinopyroxene (Cpx); (**b**) secondary clinopyroxene (Cpx 2) in intergrowths with chlorite (Chl), pyrite (Py), and chalcopyrite (Ccp) in primary clinopyroxene (Cpx 1).

• In spite of considerable variations in the composition of primary clinopyroxene, clinopyroxenites I–III have a direct correlation with the contents of FeO and Al2O<sup>3</sup> (Figure 3b). An inverse correlation dependence was revealed only for clinopyroxene grains with the highest iron content (Figure 3b, sign 4) (Table 2, No. 9–12). We attributed this clinopyroxene to a later generation. Along with sulfides and chlorite, it fills interstices in the aggregates of early primary pyroxene grains (Figure 4b).


**Table 2.** Chemical composition of clinopyroxenes in clinopyroxenites I–III (in wt.%).

Note. Primary clinopyroxene from samples of pyroxenites I (No 1–8), II (No 13–18), and III (No 19–15). No 9–12—secondary clinopyroxene. Here and below in the tables: FeO \*—calculated content from measured Fe; definitions with the values of element concentrations below 2θ (standard error of analysis) are highlighted in italics; «b.d.»—below the detection limit.

#### 2. Olivine

Olivine in the rocks under study is characterized by ferruginous fayalite component x(Fa) within 0.13–0.30. The most ferruginous olivine with x(Fa) = 0.25–0.30 is present in the samples of clinopyroxenite I and II, whereas olivine in clinopyroxenite III contains less iron x(Fa) = 0.13–0.15 (Table 3). A constant trace element in olivine is manganese (0.37–0.53 wt.%).

**Table 3.** Chemical composition of olivine from clinopyroxenites I–III (in wt.%).


• Magnetite

Xenomorphic segregations of primary magnetite to 2–3 in size fill interstices in the aggregates of clinopyroxene grains. Interstitial magnetite contains chains of small spinel inclusions, oriented in several directions, and rare plates of ilmenite and in some cases is rimmed by secondary silicates—amphibole (magnesio-hornblende) or chlorite. Interstitial magnetite is represented by the Cr-Ti-V-bearing variety. The content of Cr2O<sup>3</sup> in it attains 2.9 wt.%, TiO2—2.5 wt.%, V2O3—1.1 wt.% (Table 4, # 1–6). Tabular ilmenite in primary magnetite contains Mg (1.66 wt.% MgO), Mn (3.9 wt.% MnO), and V, Cr, Al in amounts less than 0.2 wt.%.


**Table 4.** Chemical composition of primary interstitial magnetite in clinopyroxenites I and II (in wt.%).

4.1.2. Secondary (Late) Minerals in Clinopyroxenites

Secondary clinopyroxenite minerals are chlorite, serpentine, magnetite, and sulfides, as well as accessory clinopyroxene, titanite, and noble metal minerals. These minerals are present in small quantities (no more than 5–10% in total) and are evenly distributed in the rock mass. Secondary minerals occur in the most permeable zones of granular aggregates of rock-forming minerals. They develop along the cleavage cracks of clinopyroxene, fill microcracks in olivine or clinopyroxene, and are localized in the interstices in olivine– clinopyroxene aggregates.

• Secondary clinopyroxene

Secondary clinopyroxene was observed in clinopyroxene I in single cases. It occurs in association with sulfides (pyrite, chalcopyrite) and chlorite, which fill interstices in the aggregates of primary clinopyroxene grains (Figure 4b). Secondary clinopyroxene in comparison with primary clinopyroxene has a higher iron content (9.6–14.5 wt.% FeO) and a low alumina concentration (less than 0.4 wt.% Al2O3) (Figure 3b; Table 2, No 9–12).

• Amphibole

In the samples of clinopyroxentites I and II, amphiboles belong to the group of Ca-amphiboles. They consist of actinolite (No 1–6), magnesio-ferri-hornblende (No 7), magnesio-hornblende (No 8–14), and tremolite (No 15, 16) (Table 5). All amphiboles, except tremolite, contain minor amounts of sodium (from 0.4 to 1.5 wt.% Na2O) and titanium (to 0.5 wt.% TiO2) (Table 5). Tremolite contains the least amount of Al2O<sup>3</sup> и FeO among amphiboles.

Actinolite is developed mainly in the samples of clinopyroxenite I in the form of angular interstitial segregations in the aggregate of clinopyroxene grains (Figure 5a). It is accompanied by small (less than 10 µm) grains of impurity-free secondary magnetite. We have also detected a grain of magnesio-ferri-hornblende virtually completely replaced by chlorite (Figure 6c). Magnesio-hornblende is developed predominantly in clinopyroxenite II in which it, together with chlorite, forms rims on the segregations of primary magnetite and a lattice-like chalcopyrite–bornite aggregate in the interstices of clinopyroxene (Figure 5b). Tremolite is present in all clinopyroxenite samples. It is developed along with the contacts of large clinopyroxene and olivine grains. In the contact zone of these minerals, clinopyroxene is replaced by tremolite, and olivine, by serpentine. Along with tremolite and serpentine, secondary magnetite is formed as symplectic intergrowths with tremolite and larger porous

segregations in serpentine (Figure 5c,d). Tremolite and serpentine in clinopyroxene II replace marginal parts of lattice-like bornite–chalcopyrite segregations.

**Table 5.** Chemical composition of amphiboles in clinopyroxenites I (No 1–5, 7, 14, 16) and II (No 6, 8–13, 15) (in wt.%).


**Figure 5.** Amphibole in the samples of clinopyroxenite I (**a**,**c**), II (**b**), and III (**d**). (**a**) Interstitial segregations of actinolite (Act) in clinopyroxene (Cpx) and associated grains of secondary magnetite (Mag 2); (**b**) magnesio–hornblende (Mhbl) and chlorite (Chl) in the rim of primary magnetite segregations (Mag 1); aggregate of bornite (Bn), chalcopyrite (Ccp), and secondary copper sulfide (Cu-S) in clinopyroxene matrix (Cpx); (**c**,**d**) development of tremolite (Tr), serpentine (Spr) and secondary magnetite (Mag 2) in the contact zone of clinopyroxene (Cpx) and olivine (Ol).

• Chlorite

In clinopyroxenites I and II, chlorite is developed in the interstices of clinopyroxene (Figure 6a) or along cutting microcracks (Figure 6b). Chlorite replaces primary clinopyroxene, magnesio-ferri-hornblende (Figure 6c), and magnetite (Figure 6d). Tabular chlorite in clinopyroxenite II together with tremolite and serpentine are developed in the marginal parts of segregations of bornite–chalcopyrite lattice-like aggregate (Figure 5b). A typical feature of chlorite in clinopyroxene I is the presence of titanite (Figure 6c,d). In clinopyroxenite III, chlorite is distributed mainly along the cleavage cracks of clinopyroxene.

In the chemical composition, chlorite corresponds to pennine and picnochlorite and is characterized by wide variations in iron content (Figure 7). Chlorite in clinopyroxenite I has the highest iron concentration XFe = 0.1–0.43 compared to chlorite in clinopyroxenite II and III (XFe < 0.1) (Table 6). Chlorite with the lowest iron concentration (XFe < 0.05) was found in clinopyroxenite III. Only in these samples chlorite (pennine) contains minor amounts of potassium (0.5–2 wt.% K2O).

**Figure 6.** Chlorite in the samples of clinopyroxene I. (**a**) Interstitial chlorite (Chl) in clinopyroxene (Cpx). Chlorite also contains titanite (Ttn) and magnetite (Mag) in clinopyroxene; (**b**) veinlets of chlorite (Chl) with inclusions of titanite (Ttn) in clinopyroxene; (**c**) segregation of magnesio-ferrihornblende) (Mfhbl) in clinopyroxene. Amphibole is replaced by chlorite with titanite inclusions (Ttn); (**d**) magnetite crystal (Mag) replaced by chlorite (Chl) in clinopyroxene.

**Figure 7.** Chlorite composition on the classification diagram from [20]. Analyses of chlorite from clinopyroxenites I (1), II (2), and III (3). Numbers show the formation temperatures calculated with a chlorite geothermometer [19] (Table 6).


Note. In calculating crystallochemical formula, Al(IV) supplements Si position to 8 cations, and Al(VI) accounts for the rest part of the total calculated amount of Al.
