A significant part of zoned clinopyroxenite-dunite massifs is located in Russia (the Urals, Eastern Siberia, and the Far East). They are characterized by the presence of dunite cores, clinopyroxenite rims and placer platinum deposits. The most significant placer platinum deposits are spatially linked to the Nizhny Tagil, Svetly Bor, and Veresovy Bor clinopyroxenite-dunite massifs within the Middle Urals [1, 2]. These zoned-type massifs form part of the Platinum Belt of the Urals, which is located along the 60th meridian for more than 900 km. The most common platinum-group minerals (PGM) from bedrocks and placer platinum deposits are Pt–Fe minerals, among which high–temperature ferroan platinum (Pt2Fe) and isoferroplatinum (Pt3Fe) dominate over subordinate Os–Ir alloys, Ru–Os sulfides of the laurite–erlichmanite series (RuS2–OsS2), Ir–Rh sulfides of the kashinite–bowite series (Ir2S3–Rh2S3) and Ir–Rh thioshpinels of the cuproiridsite–cuprorhodsite–ferrorhodsite series (CuIr2S4–CuRh2S4–FeRh2S4) [3–6]. Secondary low-temperature PGM assemblage, associated with serpentinization [4, 7, 8], is represented by the tetraferroplatinum (PtFe)–tulameenite (PtFe0.5Cu0.5)– ferronickelplatinum (PtFe0.5Ni0.5) solid solutions series and Pt–Cu minerals.

Understanding the main events of platinum-metal ore formation is impossible without analyzing the sources and behavior of the main ore-forming components, namely platinum, osmium, sulfur and copper. In contrast to the Re–Os isotope data [9], which previously allowed to characterize various sources of platinum-group elements (PGE) and the multi-stage nature of PGE mineralization, the isotopic systematics of platinum, sulfur and copper for PGM from this type of geological settings still remains unexplored.

The main purpose of this work is to study the Cu-isotopic composition of Pt–Fe minerals from the bedrock deposits of the platinum-bearing Nizhny Tagil massif in the Middle Urals, an international standard of the zoned Ural-type complexes [3, 7, 10, 11].

GEOLOGICAL BACKGROUND AND SAMPLES STUDIED

The Nizhny Tagil massif is located in the southern part of the Platinum Belt of the Urals, 150 km northwest of the city Yekaterinburg (Fig. 1). The massif forms a pear-shaped body elongated in the submeridional direction with a length of about 14 km, and a width of outcrops up to 6 km (Fig. 1). The central part of the massif is composed of dunite of about 29 km2 (Fig. 1). Most of the chromite-platinum occurrences are localized along the western and southwestern slopes and foothills of the Solov’yev Mountain [3, 10]. The largest bedrock platinum deposits (Gosshakhta, Krutoy Log, Alexandrovsk Log) have been identified in this part of the massif; they mark the transition zone between the coarse-grained dunite of the central part of the massif and medium- to fine-grained dunites of the periphery. Typically, chromitite samples are characterized by high concentrations of PGE (1480.7–2682 ppb [13]), with a predominance of platinum (1297–2434 ppb) over other PGE. Due to the small size of chromite-platinum ore segregations they have no industrial significance.

Fig. 1.
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Schemes of location and geological structure of the platinum-bearing Nizhny Tagil clinopyroxenite-dunite massif after [12], with indication of sampling sites for bedrock PGM. (a): 1—sedimentary shelf deposits (S2-D2); 2—continental-slope sedimentary deposits (O1-2); 3—basalt, andesite-basalt, and green shale (O2-3?); 4—metabasaltic rocks (O3-S1): (a) small-grained amphibole-plagioclase rock (kytlymite), (b) amphibolite; 5–7—The Nizhny Tagil massif (5—dunite, 6—clinopyroxenite and wehrlite, 7—tylaite); 8—gabbro-pyroxenite complex; 9—geological boundaries: (a) interformational, (b) intraformational, (c) tectonic; 10—bedding elements: (a) banding and mineral flatness, (b) foliation, (c) linearity; 11—generalized orientation of planar structures; 12—location of chromitite samples from the Alexandrovsk Log and Krutoy Log PGE deposits. (b): 1—complexes of paleocontinental sector, 2—island-arc complexes of the Tagil megazone, 3—ophiolite-type complexes, 4—zoned-type massifs of the Platinum Belt of the Urals. MUF—Main Uralian Fault, NT—Nizhny Tagil massif.

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Pt–Fe minerals selected for this study were obtained from vein-disseminated chromitite segregations confined to the marginal areas of coarse-grained dunite in the central part of the massif, namely in the area of the Krutoy Log and Alexandrovsk Log deposits (Figs. 1, 2a, 2b). In total, 15 samples of Pt–Fe minerals from primary (7 samples) and secondary (8 samples) PGM assemblages were analyzed.

Fig. 2.
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Morphological features of: (a, b) vein-disseminated chromitites and (c) Pt–Fe minerals associated with chromite (Crt) and olivine (Ol) from the Krutoy Log deposit of the Nizhny Tagil massif. Features of the internal structure of PGM from chromitites of the Krutoy Log (d, e, g) and Alexandrovsk Log (f) PGE deposits. Pt2Fe—ferroan platinum, Lr—laurite, (Os, Ir)—iridian osmium, Pt(Fe, Cu)—minerals of the tetraferroplatinum–tulameenite series, PtFe0.5Cu0.5—tulameenite, Crt—chromite, Ol—olivine and Serp—serpentine. Back-scattered electron images (c–g); numbers 1–9 denote areas of electron microprobe analyses and correspond to those in Table 1.

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ANALYTICAL METHODS

Analytical studies were carried out at Common Use Centre “Geoanalyst” of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences (Yekaterinburg), the re-equipment and comprehensive development of which is funded by the grant of the Ministry of Science and Higher Education of the Russian Federation (Agreement no.: 075-15-2021-680). The morphology and chemical composition of PGM were studied by scanning electron microscope JSM-6390L (with an energy dispersive device INCA Energy 450 X-Max 80), and an electron microprobe analyzer CAMECA SX 100 equipped with five wavelength dispersive spectrometers. Quantitative analyses were performed at 15 kV accelerating voltage and 20 nA beam current, with a beam diameter of about 1–2 µm. The following X-ray lines and standards have been used: RuLα, RhLα, PdLβ, OsMα, IrLα, PtLα, NiKα (all native element standards), FeKα, CuKα, SKα (chalcopyrite), and AsLα (InAs alloy). Corrections were performed for the interferences involving Ru–Rh, Ru–Pd, and Ir–Cu. A total of 190 analyses were performed.

The method for determining δ65Cu included the selective chromatographic separation of Cu from a solution of the studied Pt–Fe mineral sample, followed by determination of the 65Cu/63Cu isotope ratio using a Neptune Plus multiple-collector inductively coupled plasma mass-spectrometer (Thermo Fisher). A detailed description of the technique is presented in [14]; digestion and chromatographic separation were carried out in a clean room unit (class 1000, ISO 6) and laminar flow cabinets (class 100, ISO 5). The decomposition stage of Pt–Fe minerals (0.00n mg) included their dissolution in concentrated nitric acid. For chromatographic isolation of pure Cu fraction, AG MP-1 ion-exchange resin (Bio-Rad inc., USA) was used [15]; the analyte isolation scheme was detailed in [14]. The 65Cu/63Cu isotope ratios in the analytical copper fraction were measured by sample-standard bracketing technique using a Neptune Plus mass-spectrometer. Measurement sequence was as follows: blank experiment (3% HNO3 solution) → NIST SRM 976 Cu standard → the studied PGM sample (3% nitric acid solution of the mineral) → NIST SRM 976 Cu standard. Each single measurement of the Cu-isotopic composition consisted of 60 cycles collected at 8-second integrations followed by a baseline measurement for 30 s. The copper isotope composition of the sample was calculated as δ65Cu = [(65Cu/63Cu)sample/(65Cu/63Cu)standard – 1)] × 1000‰; the measurement accuracy was ±0.14‰ (2σ). To control the entire analytical procedure and evaluate the correctness of the determined δ65Cu value, international reference rock samples USGS AGV-2 and BHVO-2 were used; the measured δ65Cu values for these samples were 0.14 ± 0.04 (2 SD, n = 5) and 0.12 ± 0.04‰ (2 SD, n = 5), respectively, which satisfactorily agrees with the data presented in the GeoRem database.

CHEMICAL COMPOSITION OF PGM

The majority of PGM from the chromitites of the Krutoy Log and Alexandrovsk Log deposits are represented by Pt–Fe mineral grains with sizes ranging from 10 to 2000 microns (Fig. 2c); these minerals show significant compositional variations. The dominant PGM are Pt–Fe minerals, having compositions close to Pt2Fe (i.e., Pt equals to the sum of PGE, at %; Fe is the total of iron, copper and nickel contents, at %; Fig. 2d–2g, Table 1, an. 1–2, 4, 6), intermediate member of the tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) (Figs. 2e–2g, 3, Table 1, an. 3, 5, 7, 8), and tulameenite (PtFe0.5Cu0.5) (Fig. 2 g, 3, Table 1, an. 9). Laurite and iridian osmium are also present as minute inclusions in Pt–Fe minerals (Fig. 2d, 2e). Pt–Fe mineral with a chemical composition close to Pt2Fe is dominated by Pt (80–83 wt %) and Fe (11–13 wt %) (Table 1, Fig. 3), with notable concentrations of Ir (up to 4.6 wt % = 3.5 at. %), besides smaller quantities of Cu (0.4–1.4 wt %), Ni (0.3–0.4 wt %), Rh (0.8–1.1 wt %) and Os (0.3–0.4 wt %). Chemically, this mineral corresponds to ferroan platinum, which is characterized by disordered face-centered cubic structure Fm3m [16]. Copper content in minerals of the tetraferroplatinum (PtFe)–tulameenite (PtFe0.5Cu0.5) solid solution series varies within 6.8–11.3 wt %, reaching 12.3 wt % in tulameenite (Table 1, an. 9).

Table 1. Typical chemical composition of Pt–Fe minerals from chromitites of the Nizhny Tagil Massif
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Fig. 3.
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Chemical composition of Pt–Fe minerals from placers (a) and chromitites (b) of the Nizhny Tagil massif in the ternary diagram Pt + (Ir, Os, Rh, Pd)—Cu + Ni—Fe, at.%. Asterisks indicate minerals of the Pt–Fe system.

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Copper Isotopic Compositions

For samples of ferroan platinum, the values of δ65Cu vary from –0.37 to 0.31% (mean value 0.03%, n = 7, standard deviation 0.23%, Table 2, Fig. 4). Minerals of the tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) solid solution series are characterized by lighter Cu-isotopic compositions (δ65Cu values range from –1.15 to –0.72, mean value –1.01‰, n = 8, standard deviation 0.17, Table 2, Fig. 4).

Table 2. Copper isotope data for Pt–Fe minerals of the Nizhny Tagil Massif
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Fig. 4.
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Copper isotopic composition of Pt–Fe minerals from chromitites of the Krutoy Log and Alexandrovsky Log deposits (symbols of black and red color, respectively). Ferroan platinum (circles), minerals of the tetraferroplatinum–tulameenite solid solution series (squares). The measurement accuracy was ±0.14‰ (2σ).

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Following the publications ([17, 18] and references cited therein), several common features of Cu-isotopic compositions for various ore-forming systems can be noted: (1) δ65Cu values of Cu-bearing minerals are close to zero; (2) the range of δ65Cu values in most geological environments is more than 1‰, and (3) minerals that have experienced low-temperature redox processes have more variable δ65Cu values than minerals crystallized at high temperatures.

A common occurrence of inclusions of laurite and iridian osmium in Pt–Fe minerals at Nizhny Tagil and the equilibrium phase relations of Os-bearing alloys, deduced from the binary system Os–Ir [19], are indicative of their high-temperature origin. The presence of reaction rims composed of minerals of the tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) series, replacing high-temperature ferroan platinum, indicative of their secondary origin; it is assumed [7] that copper-bearing sulfides were the source of copper, whereas olivine was the source of Fe and Ni. Most researchers associate the formation of this low-temperature PGM assemblage with the serpentinization of ultramafic rocks [7, 8].

It has been shown [20] that redox reactions play an important role in the fractionation of Cu isotopes at low temperatures. For example, variations in the Cu-isotopic composition in primary and secondary Cu-bearing minerals may be due to fractionation between different complex species in solution [15] or associated with the influence of isotopically different fluids during hydrothermal processes [20]. In this context, the lighter Cu-isotopic composition in secondary Cu-bearing PGM compared to that in ferroan platinum (δ65Cu = –1.01 ± 0.17, n = 7 and δ65Cu = 0.03 ± 0.23, n = 8, respectively) is consistent with the secondary nature of isotopic variations, likely due to the evolved composition of the ore-forming fluid during the low-temperature formation of the tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) solid solution series. Thus, the nature of variations in the isotopic composition of copper can be used as an additional marker that allows characterizing in a new way the conditions for formation of Pt–Fe minerals.