Introduction

Various degrees of mantle partial melting yield a diverse range of magma types and facilitate the (re-)mobilization of chalcophile and siderophile elements through the lithosphere, including critically important base metals (Cu, Ni), semi-metals (Te, As, Bi, Se), and precious metals (Au, PGE). Much of the existing literature focused on the petrological and geochemical aspects of metal fractionation, and concentration in magmatic sulfides implicitly assumes that mineralizing systems are mainly derived from high degrees (> 20%) of partial melting of an asthenospheric mantle source under “dry” conditions (Campbell and Barnes 1984; Keays 1995; Li et al. 2001; Li and Ripley 2005; Naldrett 2010, 2011). This approach relies on substantial melting to (1) generate sulfide-undersaturated primary silicate melts and (2) prevent the retention of chalcophile and siderophile metals in the source (Naldrett and Duke 1980).

According to existing models, magmatic sulfide deposits are believed to form primarily in areas of the crust where metal-rich mantle-derived magmas achieve sulfide saturation upon ascent, leading to the exsolution of immiscible base and precious metal-rich magmatic sulfide liquids (Naldrett 2004). However, in “dry” magmatic systems, the well-established relationship between sulfur solubility and pressure would typically prevent magmas from reaching sulfide saturation in the upper crust (Mavrogenes and O’Neill 1999). Therefore, a key requirement for ore genesis would be the thermo-mechanical assimilation of S-bearing crustal rocks located along the magmatic conduits, which would trigger localized sulfide saturation and subsequent storage of the precious and base metals into magmatic sulfides at various crustal levels (Barnes and Lightfoot 2005; Ripley and Li 2013). Nonetheless, some significant magmatic sulfide deposits (Seat et al. 2009; Wei et al. 2019) show no indications of sulfur addition, raising questions about alternative models to explain metal transfer through the lithosphere and the genesis of magmatic sulfide deposits.

Recent research, focused on the physical mechanisms of trapping and transporting dense sulfide liquids within ascending magmas, suggests that volatiles such as CO2 and H2O may play a crucial role in metal mobility (Blanks et al. 2020), shifting the model towards the idea that lower degrees of partial mantle melting under “wet” conditions could be conducive to fertile systems’ genesis (Wallace 2008; Gál et al. 2013; Mungall et al. 2015; Holwell et al. 2019). According to these models, primary melts may not necessarily need to be sulfide-undersaturated, as sulfide droplets could adhere to low-density volatile bubbles and ascend with the rising magmas, preventing metal loss to the restite (Yao et al. 2020).

This hypothesis was put forward following the commonly observed association of sulfide globules with enveloping volatile-rich phases, such as mica, amphibole, apatite, and carbonate (Prichard et al. 2004; Le Vaillant et al. 2017; Blanks et al. 2020; Iacono-Marziano et al. 2022). However, while the theory of volatile-assisted transport of sulfide liquid accounts for some aspects of mineralogical and textural variation in the sulfide-volatile phase association, it does not fully explain the complete range of observed variability. Additionally, it fails to address the presence of this association in mafic intrusions at various crustal levels, including deep magmatic systems where volatile exsolution is not necessarily expected.

In this study, we propose a novel hypothesis for how magmatic sulfide liquid is transported. We suggest that the volatile-rich association surrounding sulfides represents the coexistence and interaction of three immiscible melts—carbonate, sulfide, and silicate. The sulfide globules would be enveloped by carbonate melt, which would be relatively buoyant. To test this hypothesis, we studied sulfide globules in the Rudniy intrusion in NW Mongolia, documenting the composition and morphology of three different generations of sulfide globules. This well-preserved intrusion remains untouched by late secondary processes that might alter the initial metal distribution. The composition and mineralogy of sulfide globules and the volatile-rich halos resemble those in the larger Norilsk intrusion, albeit on a smaller scale. The processes observed in the Rudniy intrusion can be scaled up to provide valuable insights into the complex formation of magmatic sulfide systems.

Our findings suggest that the interaction between sulfide liquid and carbonate melt may have played a crucial role not only in physically transporting sulfides but also in determining metal distribution. Our new model complements existing ones and helps us better understand the common paragenetic association observed in various magmatic systems worldwide.

Geological setting

Mongolia lies in the center of the Central Asian Orogenic Belt, bordered by the Siberian craton in the north and the Tarim and Sino-Korean cratons in the south (Tomurtogoo 2002). The broad geodynamic framework displays a complex mosaic structure, where a relatively small Precambrian block located in the Khangai region is surrounded by different terranes (Badarch et al. 2002). The Rudniy intrusion lies in the Tsagaan-Shuvuut terrane, situated in the western periphery of the Tuva Through, a rift-type volcanic trough formed in the Early Devonian following the impingement of a mantle plume (Kuzmin et al. 2010; Vorontsov et al. 2010; Kruk et al. 2015). The Rudniy intrusion outcrops on a steep slope of the Tsagaan-Shuvuut ridge and was discovered by field geologists from the Sobolev Institute of Geology and Mineralogy in 2011. Although the age of the Rudniy intrusion gabbro is unknown, a similar intrusion with comparable petrographic and geochemical characteristics located 0.5 km away (Fig. 1b) was dated at 406.5 ± 7.1 Ma by SHRIMP-II (Izokh et al. 2011).

Fig. 1
figure 1

(a) Geographical location of the Rudniy intrusion (red star). (b) Schematic geological map of the eastern part of the Tsagaan-Shuvuut Ridge. Compiled according to the geological map M46-XIV (1: 200 000) in 1987 (compiled by: A. Dagvaochir, H. Endonpuntsog, Ts. Gansukh, Z. Noosai, Ts. Dembereldagva)

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According to age and structural-mineralogical characteristics, the similar gabbroid massifs located in the Tsagaan-Shuvuut ridge are correlated with gabbro-syenite and gabbro-diorite intrusions of the Torgalyk and the Bayan-Kol complexes (Krivenko 1965; Izokh et al. 2010; Karmysheva et al. 2019; Vetrov et al. 2022) emplaced within the Ureg Nuur volcanogenic sedimentary complex (Fig. 1b). Those numerous small mafic intrusions in NW Mongolia and the Tuva Region in Russia were formed as part of the Altai-Sayan large igneous province (Vorontsov et al. 2010). However, the Rudniy intrusion is the only one known to date to contain sulfide minerals among all the magmatic bodies in the region, which makes it a good natural laboratory to study the processes that could lead to sulfur saturation.

The Rudniy intrusion is a small (250 × 70 m) sill-like mafic–ultramafic body hosted in Devonian shale and siltstone with a carbonate-micaceous matrix (Vishnevskiy and Cherdantseva 2016). The siltstone is metamorphosed at greenschists facies. The location of the intrusion on the periphery of the Tuva depression and its emplacement within slightly metamorphosed volcano-sedimentary country rocks are consistent with upper crustal conditions of emplacement.

The Rudniy intrusion comprises two distinct layers—leucogabbro in the base and melanogabbro above it. The boundary between the two is demarcated by a troctolite layer. This troctolite contains scattered sulfide globules that can be classified into three different types that differ from each other in terms of size, shape, PGE content, and association with volatile-rich minerals. This mineralized zone is referred to as the ore horizon.

The ore horizon outcrops in two areas, at the southern and northern ends of the intrusion (locations 1 and 2 in Fig. 2a, respectively). Three textural varieties of ore-bearing troctolite were identified within the ore horizon each containing a different type of globules. To simplify the identification of various sulfide globules, they were classified based on their relative sizes as small (1–4 mm), medium (4–10 mm), and large (1–4 cm).

Fig. 2
figure 2

a Outcrop of Rudniy intrusion, with an overlapping simplified structure, showing the two main units (different grey tones) and their MgO contents (A to B, inset). The black thick line marks the border between the two units, and the yellow dashed line shows the position of the ore horizon at the base of the melanogabbro. The two white circles represent the outcrops sampled on the ore horizon and are shown schematically in the next panel. b Photographs of two samples—on the left—trachytoid troctolite from the Northern End (outcrop 2); on the right—the layered mineralized zone from the Southern End (outcrop 1). c Schematic illustration (not to scale) of the composition of the ore horizon and relationship of three textural varieties of troctolite comprising the ore horizon, each with different types of sulfide globules: (1) two layers in the Southern End: Upper Layer (large sulfide globules) and Lower Layer (medium sulfide globules); (2) Northern End of the ore horizon (small sulfide globules)

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The ore horizon at the southern end of the intrusion is layered and includes Lower and Upper Layers. The Lower Layer, directly in contact with the leucogabbro, comprises fine-grained dark-colored troctolite (Fig. 2b). The distinctive feature of this layer is the presence of long skeletal crystals of chromian spinel and olivine, which locally form subparallel clusters. This narrow layer is not uniform in width (< 7 cm), as it locally curves and pinches out. Troctolite from the Lower Layer hosts predominantly medium sulfide globules (Fig. 2c). In fact, some of the sulfide globules can vary in size, and Lower and Upper Layer can have sulfide globules that could have been classified as small or medium ones. However, here and after, for simplification, the size will characterize the predominant globules of the specific horizon. The Upper Layer is a medium-grained troctolite with phaneritic texture (Fig. 2b). The domain in the troctolite containing sulfide minerals is about 15-cm thick. The Upper Layer contains large sulfide globules (Fig. 2c).

At the northern end of the Rudniy intrusion, the strike extension of the ore horizon is outcropping (Fig. 2a, outcrop 2). The Northern End is not layered, and it mainly comprises troctolite with skeletal plagioclase laths elongated in one direction, with subparallel alignment perpendicular to the basal contact of the intrusion (Fig. 2b) and disseminated skeletal crystals of chromian spinel. The Northern End troctolite contains small sulfide globules (Fig. 2c). The horizon was clearly traced for an extension of about 10 m, with some intermittent distribution of sulfide minerals between outcrops 1 and 2 (Fig. 2a). Where the horizon was not obvious, the contact was traced by traverse sampling and confirmed by whole-rock geochemistry.

Materials and methods

This study involved an investigation of 36 specimens that were collected during field expeditions in Mongolia from 2011 to 2014. Most samples were obtained from two traverses perpendicular to the contacts between leucogabbro and melanogabbro within the Rudniy intrusion, with sampling intervals of 5–7 m. Figure 2a illustrates selected sample locations.

The methods employed to characterize these samples are briefly summarized here, with further details available in Electronic Supplementary Material 1 (ESM 1). This research entailed extensive mineralogical, petrological, and geochemical investigations. Concentrations of major and trace elements in bulk samples from the Rudniy intrusion were measured to assess the potential influence of crustal contamination on its formation. Sulfide and platinum group element (PGE) mineralogy were thoroughly examined using optical microscopy, scanning electron microscopy, and electron microprobe analysis. The mineral chemistry of sulfides, silicates, and oxides (such as baddeleyite, plagioclase, mica, amphibole, and clinopyroxene) was determined using a JEOL 8530F scanning electron microscope.

To address the source of sulfur, in situ analyses of 32S, 33S, and 34S isotopes were conducted on pentlandite, pyrrhotite, and chalcopyrite using the Cameca IMS-1280 large-geometry secondary ion mass spectrometry (SIMS). Selected samples were also examined using the XFM beamline at the Australian Synchrotron, ANSTO, to identify any chemical zonation within the mineral assemblage. Furthermore, to study the distribution of trace elements, PGEs, and other chalcophile elements in base metal sulfides, three polished mounts with sulfide globules were analyzed using laser ablation inductively coupled plasma mass spectrometry.

To assess the composition of the volatiles involved in the formation of the Rudniy intrusion, we analyzed bulk volatile components within medium-sized sulfide globules from the Lower Layer of the ore horizon using a pyrolysis-free coupled gas chromatography–mass spectrometry (GC–MS).

Results

Field and petro-chemical characteristics of the Rudniy intrusion

The contact of leucogabbro unit with the country rocks is marked by the presence of microgabbro (chilled margin) with a doleritic and locally variolitic texture (Fig. 3a). Above it is the ophitic and chemically uniform leucogabbro unit (average MgO—8.9 wt%, Table 1, ESM 2 Table A1), with a mode of plagioclase—60%, clinopyroxene—35%, and olivine—5% (Fig. 3b). The accessory minerals include biotite, chromian spinel, and hornblende. The melanogabbro unit displays a differentiated MgO trend, from higher concentrations at the bottom contact to lower MgO contents upwards (Fig. 2a, inset). The polarity of this differentiation pattern may be used to infer the magmatic stratigraphy, with the rock composition ranging from troctolite (olivine—45–55%, plagioclase—35–45%, clinopyroxene—5–10%, chromian spinel—1–5% (Fig. 3d–f)) at the base, through olivine gabbro to gabbro (olivine < 5%, plagioclase—55–60%, clinopyroxene—35–40% (Fig. 3c)) at the upper contact with the country rocks. Both units display evidence of modest but ubiquitous hydrous alteration, represented by serpentine veins crosscutting olivine grains and the formation of actinolite at the edges of clinopyroxene and hornblende.

Fig. 3
figure 3

Photomicrographs (PPL on the top, XPL on the bottom of the circles) of the rocks comprising the Rudniy intrusion. Top row: a Lower contact of the leucogabbro unit represented by gabbro with doleritic and variolitic textures made by plagioclase (Pl) lamellae with interstitial olivine (Ol) and clinopyroxene (Cpx). b Large-grained ophitic leucogabbro from leucogabbro unit made up of plagioclase lamellae and interstitial clinopyroxene and olivine. c Medium-grained melanogabbro with phaneritic texture comprised of plagioclase, olivine, poikilitic clinopyroxene, and minor biotite (Bt), and chromian spinel (CrSp). Bottom row: three textural varieties of troctolite hosting disseminated sulfide globules (Sulf): d troctolite from Lower Layer of the ore horizon with skeletal olivine crystals and interstitial plagioclase, clinopyroxene, chromian spinel, and biotite; this troctolite hosts medium sulfide globules (not present in the thin section). e Troctolite with phaneritic texture from Upper Layer of the ore horizon accommodating large sulfide globules (not present in the thin section). f Troctolite with skeletal plagioclase laths from the Northern End, containing small sulfide globules (3–4 mm in size)

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Table 1 Whole-rock geochemistry data for major elements distribution (wt%) for selected samples along a traverse crosscutting the Rudniy intrusion, line AB in Fig. 2a. Trace element compositions for all samples are included in ESM Table A1
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The geochemical composition of the leucogabbro shows slight LREE enrichment and a lack of any Eu anomaly on chondrite-normalized REE diagrams (Fig. 4a, ESM 1 Table A1). The trace element distribution is characterized by negative Th and Ta anomalies accompanied by positive Sr and Y anomalies (Fig. 4b). The geochemical characteristics of the melanogabbro unit have relatively flat REE patterns, no Eu anomaly, and slightly lower concentrations of REE than the leucogabbro (Fig. 4a). On the spider diagram (Fig. 4b), the trace element patterns of the melanogabbro are also very similar to those of the leucogabbro but with lower concentrations.

Fig. 4
figure 4

a Chondrite-normalized REE patterns and b primitive mantle-normalized trace element spidergram of two gabbroid units and the ore horizon in the Rudniy intrusion. Normalizing values for chondrite and for primitive mantle are from McDonough and Sun (1995). c In situ sulfur (δ34S) isotope composition for chalcopyrite (Ccp), pentlandite (Pn), and pyrrhotite (Po) from three types of sulfide globules from the Rudniy intrusion. n, number of analyses

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The geochemical composition of ore-bearing troctolite is characterized by relatively high MgO (25%, Sample A111-11 in Table 1 and ESM 2 Table A1) and flat REE distribution patterns with slight positive Eu anomalies (Fig. 4a). The trace element distribution is similar to those of the melanogabbro, with significantly negative Th and positive Sr anomalies (Fig. 4b).

All the sulfides from the three types of globules are characterized by similar sulfur isotopic compositions (δ34S ~  − 0.98–2.04‰; ESM 2 Table A2, Table 2, Fig. 4c).

Table 2 A range of sulfur isotope variations in base metal sulfides from three types of sulfide globules in Rudniy intrusion. The full table is available in ESM Table A2
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Back-scattered electron images and false color three-element distribution maps of three types of globules obtained using XFM beamline at Australian Synchrotron are represented in Fig. 5 accompanied by schematic illustrations of the typical textures of different sulfide globules. We discuss below the distinguishing features that characterize the three types of sulfide globules.

Fig. 5
figure 5

Images and schematic illustrations of three types of sulfide globules from the Rudniy intrusion. a–c Back-scattered electron images of a small sulfide globules with vermicular inclusions of chlorite (Chl), apatite (Ap), serpentine (Srp), and calcite (Ct). White dashed line outlines a patchy rim of clinopyroxene, amphibole, plagioclase, and magnetite. b Medium elongated sulfide globule with a clear coat around it (white dash line). c Large flattened sulfide globule with part of the globule enveloped in skeletal ilmenite crystal. The coat is less distinct in comparison with medium sulfide globules. d–f False-color XFM three-element maps from XFM Beamline at the Australian Synchrotron: d RGB map of the relative normalized distribution of Ca, Fe, and Cr of a small sulfide globule and a halo around it. e Distribution of Ca, Fe, and Ti in a polished thin section from the Lower Layer of the ore horizon with a spherical medium sulfide globule. The globule is enveloped by an elongated coat (b). f RGB image of abundances of Ca, Fe, and Cr in the sample with large sulfide globule (section perpendicular to the elongation). The halo is patchy. g–i Schematic illustrations of three types of sulfide globules highlighting the size, shape, and relationship with volatile-rich phases illustrated in figures a–f. g A small sulfide globule. e Medium sulfide globules with 2 types of textures: zoned and loop. d A large sulfide globule

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Small sulfide globules

The small sulfide globules (1–4 mm in diameter) were found in the trachytoid troctolite of the Northern End of the ore horizon (outcrop 2 in Fig. 2). They are mainly rounded or slightly elongated in shape in an orthogonal fashion relative to the elongation of the unit. The internal structure of the sulfide globules is clearly zoned perpendicular to the elongation, with different sulfides volumetrically occupying even proportions (Fig. 5a, d, g). The lower part of each globule, intended as the part of the globule closest to the contact with the underlying leucogabbro, is occupied by pyrrhotite with patches of pentlandite and chalcopyrite; the middle part consists of coarse-grained pentlandite, and the top part mainly comprises chalcopyrite. Cubanite is extremely rare and locally forms thin lamellae in chalcopyrite.

Small sulfide globules are associated with an abundance of volatile-bearing minerals, including calcite, apatite, chlorite, and serpentine, commonly forming vermicular textures within sulfides (Figs. 5a and g, 6a). Apatite associated with small sulfide globules is water-rich (from 2.75 to 3.05 wt%), with a moderate concentration of chlorine from 0.5 to 1.2 wt%, with no fluorine detected (Table 3). The microprobe data showing the composition of main phases associated with sulfides is presented in ESM 2 Table A3. Domains characterized by the presence of these vermicular textures occur within the majority of small sulfide globules, mostly hosted in pentlandite. Apart from vermicular textures, small sulfide globules display a patchy rim that comprises amphibole, clinopyroxene, plagioclase, and magnetite.

Fig. 6
figure 6

Back-scattered electron images of minerals and textures in the analyzed samples. a Vermicular textures formed by sulfide-calcite-serpentine-chlorite-apatite intergrowth from small sulfide globules. b Sulfide-calcite-chlorite-apatite vermicular intergrowth in the marginal part of a medium sulfide globule. c Chromian spinel skeletal grain on the margin of troctolite with the coat, with the shade of grey indicating Cr-Ti zonation. d Part of the coat with volatile-rich minerals surrounding a medium sulfide globule. e Spherical calcite inclusion surrounding chlorapatite in coarse-grained pentlandite from a large sulfide globule. f Relatively large euhedral crystal of apatite in association with chlorite in a large sulfide globule. Act, actinolite; An, anorthite; Ap, apatite; Aug. augite; Ccp, chalcopyrite; Chl, chlorite; Ct, calcite; CrSp-, chromian spinel; Hs, hastingsite; Krs, kaersutite; Mgt, magnetite; Ol, olivine; Pn, pentlandite; Po, pyrrhotite; Scp-, scapolite; Srp, serpentine; Tcr, tacharanite

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Table 3 Chemical compositions of selected apatite grains (mass %) associated with different sulfide globules in Rudniy intrusion. The full table is available in ESM Table A3
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Medium sulfide globules

The medium sulfide globules are hosted in the Lower Layer of the ore horizon and appear to be trapped between long skeletal olivine crystals. On average, they are sized between 3 and 6 mm, forming rounded or slightly elongated globules oriented orthogonal to the basal contact. Some of the elongated globules reach 7–15 mm in size (Fig. 5b, e, h). These sulfide globules have the most complex internal texture among all sulfide globules, combining attributes characteristic of both small and large sulfide globules.

Two texturally different types of medium globules were recognized: the ones that are clearly zoned (like small sulfide globules) and the ones displaying loop texture, in which pentlandite and/or chalcopyrite surround pyrrhotite monocrysts (like large sulfide globules). Mineralogically, pyrrhotite is the predominant phase in medium sulfide globules taking up to 60% of the volume, followed by pentlandite (15%) and chalcopyrite (25%). Within chalcopyrite grains, cubanite forms thin or wide lamellae that intersect one another at 120°. In some cases, relatively large areas of complex fine-grained sulfides occur in medium sulfide globules. Those zones contain small-scale, randomly mixed intergrowths of chalcopyrite, pyrrhotite, pentlandite, and cubanite, referred to as mixed sulfide domains. Oxide phases associated with sulfides are mostly represented by ilmenite and titanomagnetite. Ilmenite commonly forms skeletal crystals that pierce or rim sulfides as well as surround them. Titanomagnetite crystals are usually located at the bleb margins forming reticulated textures that comprise preserved lamellae of ilmenite intergrown with pyrrhotite and chalcopyrite. Volatile-bearing phases, especially apatite, chlorite and calcite, form vermicular textures similar to those in small sulfide globules (Fig. 6b). These textures generally occur in sulfide globules that are clearly zoned. Separate euhedral apatite grains or irregularly shaped calcite and chlorite inclusions locally occur in sulfides regardless of their textural type.

Volatile-rich halos

The distinctive feature of medium sulfide globules is that they are surrounded by an assemblage enriched in volatile-rich phases that is radically different from the host troctolite. These halos have a sharp rounded margin with troctolite marked by a chain of chromian spinel crystals. Whereas smaller sulfide globules are usually surrounded by larger volumes of volatile-rich mineral assemblages (Fig. 5e), bigger varieties of medium sulfide globules only have thinner counterparts, either enveloping or just tailing to one side (Fig. 5b). These assemblages of silicates, oxides, and volatile- and alkali-rich minerals are referred to as “coats.” The coats can have different shapes, from spherical to elongated, with a sulfide globule usually located on one side of the coat.

The mineralogical assemblage of the coats surrounding sulfide globules includes plagioclase, clinopyroxene (augite), amphibole (sodian kaersutite (Ca-Ti amphibole), apatite, phlogopite, chlorite, ilmenite, calcite, tacharanite (hydrous calcium silicate (Ca12Al2Si18O33(OH)36), and baddeleyite (± zircon), as shown in Figs. 6d and 7. From the rounded sharp contact of the coat with the host troctolite towards the center, euhedral clinopyroxene crystals crown the margins. Whereas the majority of crystals are confined to the margins, they can also partly be embedded in the troctolite. The clear Cr-Ti zonation of clinopyroxene can be observed in the false color (RGB) map in Fig. 7b. The Cr/Ti ratio decreases towards the center of the coat. Cr-Ti zonation is also characteristic of the chromian spinel crystals located just outside of the coat in the host troctolite (Fig. 6c).

Fig. 7
figure 7

Medium sulfide globule from the Lower Layer with a distinct elongated coat enveloping it. a Back-scattered electron image. b Combined RGB map with logged Cr in red, iron in green, and logged Ti in blue from XFM Beamline at the Australian Synchrotron. Amp, amphibole; Ap, apatite; Bdl, baddeleyite; Ccp, chalcopyrite; Cpx, clinopyroxene (augite); Ct, calcite; CrSp-, chromian spinel; Ilm, ilmenite; Ol, olivine; Phl, phlogopite; Pl, plagioclase; Pn, pentlandite; Po, pyrrhotite; Tcr, tacharanite; Ti-Mgt, titanomagnetite

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The inner edges of clinopyroxene are hydrated to amphibole (sodian kaersutite), which can also form separate euhedral grains randomly distributed within the coats commonly associated with phlogopite. Two other amphibole varieties were identified within the halos: hornblende forming at the edges of kaersutite and hastingsite forming fibrous aggregates at the periphery of sulfide globules (Fig. 6d). All amphiboles and phlogopites contain small amounts of fluorine (0.3 wt% on average) and chlorine (0.05 wt%). The chemical compositions of amphiboles are represented in ESM 2 Table A3. Apart from hastingsite, the area immediately outside of a sulfide globule is also characterized by numerous other hydrous minerals, such as phlogopite, chlorite, and tacharanite. Most of these minerals can also be concentrated close to the center of the coat. Most of the volume (about 50%) of the coats is occupied by plagioclase forming tabular grains of irregular size with polysynthetic twins, displaying various compositions (An8-An40), generally with less Ca in comparison with the plagioclase from the surrounding troctolite.

Another characteristic feature of the coat assemblage is the presence of ilmenite, apatite, calcite, and baddeleyite (± zircon). Ilmenite forms long skeletal crystals either scattered or in spatial association with sulfides, piercing them or embracing part of the globule. Apatite forms long needle-like euhedral crystals within sulfides (as inclusions), at contact with them, or scattered throughout the coat. Apatite from the coats is fluorine- and water-rich (with up to 2.3 wt% of fluorine, with an average of 1.2 wt% of water) with low contents of chlorine (0.5 wt% on average) (Table 3, ESM 2 Table A3). Rare irregular grains of calcite are in the coat, although they are more abundant as inclusions in sulfides. Accessory minerals are represented by small euhedral grains of baddeleyite and, more rarely, zircon.

Bulk compositions of volatiles in a coat, a medium sulfide globule and a troctolite sample from the lower layer of the ore horizon, were analyzed to constrain the relative % of fluids associated with the sulfide liquid during its formation. The results show that the predominant volatile associated with all three samples is water (60, 54, and 53 rel %, respectively), followed by nitrogen (except for the troctolite sample), oxygenated hydrocarbons, and aliphatic hydrocarbons (Fig. 8, Table 4, full table is included in ESM 2 Table A4). Notable amounts of CO2 (12 rel % and 4.1 rel %) were identified in troctolite and sulfide globules, whereas no detectable CO2 was documented in the coat.

Fig. 8
figure 8

Relative % proportions of volatiles extracted by mechanical shock destruction from a a medium sulfide globule, b a coat surrounding a medium sulfide globule, and c troctolite from the Lower Layer. H2O, water; NC, nitrogenated compounds; O HC, oxygenated hydrocarbons; A HC, aliphatic hydrocarbons; CO2, carbon dioxide. Based on the results of GC–MS analysis represented in Table 4 and ESM Table A4

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Table 4 Bulk composition of fluids (combined by compound types) extracted from a medium sulfide globule, surrounding coat, and host troctolite and by mechanical shock destruction. The extended table is available in ESM Table A4
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Large sulfide globules

The large sulfide globules (1–4 cm) are abundant in the Upper Layer of the ore horizon and occur in the first 7–20 cm of the basal part of the melanogabbro (outcrop 1 in Fig. 2). In addition to their size, they are characterized by morphologies elongated subparallel to the contact of the intrusion. Base metal sulfides are not clearly differentiated as in the other globule types but rather form loop textures similar to those described at Voisey’s Bay (Lightfoot et al. 2012) and Copper Cliff (Barnes et al. 2017), formed by pyrrhotite and coarse-grained pentlandite surrounded by chalcopyrite-cubanite intergrowth pentlandite. The mixed sulfide domains are common in large sulfide globules. Ilmenite and magnetite are generally associated with sulfide globules, with the former commonly forming skeletal crystals piercing or partially enveloping the sulfides.

Vermicular textures do not occur within large sulfide globules; however, the volatile-rich phases are still commonly associated with sulfides. For example, calcite forms relatively large (up to 300 µm in diameter) spherical inclusions in sulfide, preferably in pentlandite (Figs. 5f, 6e), which commonly embrace the fractions of sulfides associated with apatite. Calcite together with chlorite can also form irregular veins that crosscut base metal sulfides in the large sulfide globules. Apatite, which is commonly rich in fluorine (1.6–2.3 wt%, Table 3) with less chlorine (0.3–0.7 wt%), forms either small, elongated grains surrounding sulfide globules or relatively large euhedral crystals within sulfides (Fig. 6f).

The coat of large sulfide globules is usually narrower, less regular, and less distinctive than in other types. Usually, it comprises clinopyroxene, amphibole (sodic water-rich kaersutite), plagioclase, and apatite. However, in some cases, the coat forms elongated domains attached to one of the narrow ends of the globule. The mineralogical composition of these tailing coats will be similar to the ones described for medium sulfide globules. However, almost no chromite is detected at the contact between the coat and the troctolite. Olivine grains around the blebs are partially replaced by biotite, amphibole, and serpentine.

Platinum group minerals in the three types of sulfide globules

Platinum group minerals (PGMs) are ubiquitously associated with all sulfide globules, forming euhedral and anhedral grains hosted within the base metal sulfides (BMS) and laths elongated along cubanite lamellae in chalcopyrite. Some PGMs are found in silicates next to the edge of the sulfide globules. A total of 240 grains of PGMs in the three types of globules were documented and investigated using SEM with EDS and EPMA. Among them, 153 representative analyses were selected based on the totals with up to ± 3.5% deviation. Moreover, repeated analyses of the same grains were averaged to reduce the number of analyses used for the statistics. The size of each grain was measured, and the surface area was calculated to evaluate the predominant PGM in each sulfide phase and to estimate the allocation of PGMs between different types of sulfide globules (ESM 2 Table A5).

Only 10 euhedral PGMs were found and analyzed in small sulfide globules, including moncheite (PtTe2, 51%), merenskyite (PdTe2, 47%), and electrum ((Au,Ag), 2%), as shown in Fig. 9a. PGMs are predominantly hosted in pentlandite (58%), followed by pyrrhotite (23%) and chalcopyrite (19%) as shown in Fig. 9d.

Fig. 9
figure 9

Pie diagrams representing the proportion of PGM (a–c) and the proportion of hosting BMS (d–f) in the three types of sulfide globules in the ore horizon. n, number of PGM grains; n.o.s., number of samples; Pn, pentlandite; Cbn, cubanite; Ccp, chalcopyrite; Po, pyrrhotite; Mix, mixed sulfide domains

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PGMs are more abundant in the medium sulfide globules. In total, 46 PGMs of different sizes and shapes were analyzed (Fig. 9b). Most of the grains comprise merenskyite (43%), melonite (NiTe2, 35%), and moncheite (PtTe2, 19%). The rest is represented by sperrylite (PtAs2). The main host of PGMs in medium sulfide globules is chalcopyrite with cubanite lamellae (44%), followed by pyrrhotite (27%) and pentlandite (25%), as shown in Fig. 9e. The rest of the PGMs were found within silicates comprising the coat (3%) and in the mixed sulfide domains (1%).

Large globules have the most abundant and diverse PGM minerals among all types of globules. A total of 97 PGMs were found and analyzed. Similar to other types of globules, merenskyite, melonite, and moncheite are the most common PGMs (Fig. 9c). However, in addition to bismuth tellurides of Pt, Pd, and Ni, large sulfide globules also contain Ag-bearing phases (hessite Ag2Te—18%, sopcheite Ag4Pd3Te4—8%, electrum (Au,Ag)—2%, sperrylite—2%, empressite AgTe—2%, telargpalite—(Pd,Ag)3(Te,Bi)—0.06%) and Sn-bearing phases (stannopalladinite (Pd,Cu)3Sn2—1%, taimyrite (Pd,Cu,Pt)3Sn—0.01%, paolovite Pd2Sn—0.4%, and an unknown phase, whose formula was calculated as Pt3SnTe4 (UM2016-13-Te:PtSn)—1.4%).

PGMs from large sulfide globules are evenly distributed between pentlandite (35%), chalcopyrite (32%), and pyrrhotite (25%) as shown in Fig. 9f. Eight percent were also located within the mixed sulfide domains. Bismuth tellurides are mostly found in pentlandite, whereas Ag- and Sn-bearing phases are predominantly hosted in chalcopyrite-cubanite intergrowths. In some cases, PGMs are fragmented and scattered in silicates at the edges of sulfide globules. However, the grains were too small to get proper analyses; therefore, those points were not included in the statistics.

Trace element distribution in the three types of sulfide globules

To assess zonation or heterogeneity in the distribution of trace elements within BMS, we acquired LA-ICP-MS maps on three areas of interest (2 × 2 mm each) within the three types of sulfide globules. The most interesting results were obtained in an area from a medium sulfide globule with a loop texture, containing chalcopyrite-cubanite intergrowths in contact with coarse-grained pentlandite (the main concentrators of precious metals) and two individual anhedral grains of pyrrhotite (Fig. 10). The Pd, Ag, Sn, and Pt maps are consistent with the presence of nano-scale nuggets preferentially concentrated in chalcopyrite and cubanite, which cannot be observed in reflected light due to the limitation of the microscope (Fig. 10b).

Fig. 10
figure 10

Relative content maps (warm-cold color scale) showing the distribution patterns of Pd, Ag, Sn, Os, Pt, and Au among base metal sulfides in a representative area from a medium elongated sulfide globule. a A reflected light image of a medium elongated sulfide globule showing the section mapped. b Reflected light image of a 2 × 2 mm area analyzed. c The sketch of the mapped area. Black (and dark blue) is the background, light blue is the low relative contents, orange is the intermediate contents, and yellow is the highest relative contents

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Only a few dispersed Au nano-nuggets were identified in the mapped area. The Pd distribution map in pentlandite shows high concentrations of this metal in solid solution in the center of the pentlandite grain (red area on the upper right corner of the 108Pd map of Fig. 10) but dispersed as nano-nuggets towards the periphery of the grain. Elevated values of silver were also detected in solid solution in the central part of the pentlandite grain and within lamellae of cubanite. Relatively high concentrations of Sn were detected in solid solution within chalcopyrite. Conversely, the pyrrhotite grains did not seem to contain significant amounts of Pd or Pt, with only one of the grains containing Os in a solid solution (yellow area in the 189Os map of Fig. 10).

In situ LA-ICP-MS analyses of all base metal sulfides from the three types of globules were carried out to compare the concentrations of chalcophile and siderophile elements as well as chalcogens (Te, As, Bi, Sb, and Se) in the disseminated sulfides (average concentrations of each sulfide phase are provided in Table 5 and ESM Tables A6 to A9). To illustrate the differences in the distribution of precious metals and semimetals, the values were normalized to the primitive mantle (Lyubetskaya and Korenaga 2007). The normalized values were plotted and sorted by sulfide mineral (Fig. 11). Small sulfide globules are characterized by relatively high measured values of Pt and Au. Whereas Pt is mostly concentrated in chalcopyrite (1.33 ppm), Au was preferentially found in pentlandite (0.81 ppm, Fig. 11c), with smaller values in pyrrhotite (0.24 ppm, Fig. 11d) and chalcopyrite (0.13 ppm). Small globules are also characterized by relatively high values of Pd in pentlandite (31.85 ppm, Fig. 11c) and pyrrhotite (0.11 ppm, Fig. 11d); concentrations of Sn and Ag are low in all BMS in small sulfide globules (0.1–0.5 ppm).

Table 5 Average trace element composition (ppm) of different types of sulfide globules based on LA-ICP-MS analyses. Full tables are available in ESM Tables A6-A9. Po, pyrrhotite; Pn, pentlandite; Ccp, chalcopyrite; Cbn, cubanite
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Fig. 11
figure 11

Primitive mantle-normalized concentrations of precious metals and semimetals in different sulfide globules from the Rudniy intrusion. Data were obtained using LA-ICP-MS. Normalization values are from Lyubetskaya and Korenaga (2007) and references within

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In contrast, large sulfide globules are characterized by elevated concentrations of Ag (4.31 ppm in pentlandite, 1.53 ppm in chalcopyrite, and 5.65 ppm in cubanite) and Sn (9.65 ppm in cubanite). Concentrations of Pd, Ir, Pt, Au as well as Bi and Te are noticeably lower in large sulfide globules, especially in pyrrhotite (Fig. 11d) and cubanite (Fig. 11a). The two textural types of medium sulfide globules (zoned and with loop texture) showed some differences in trace element patterns. In general, the distribution of precious metals and semimetals in zoned medium sulfide globules has patterns similar to those of small sulfide globules, with lower abundances of Ag in pyrrhotite and pentlandite (< 1 ppm, Fig. 11c, d). In contrast, medium sulfide globules with loop textures have patterns similar to the large sulfide globules, characterized by higher Sn (16.97 ppm in chalcopyrite) and Ag (2.24 ppm in pentlandite, 1.28 ppm in chalcopyrite). The distribution patterns in pyrrhotite are unusual, whereby both types of medium sulfide globules have similar patterns to the small ones.

R-factor calculation

To estimate the degree of equilibration between sulfide liquid with silicate melt in the three types of globules, R-factor calculations were performed using the equation from Campbell and Naldrett (1979). Concentrations of the strongly chalcophile element Pd in three types of sulfide globules were used for the calculations. The Pd tenors (concentration of Pd in 100% sulfide) were calculated from previously reported whole-rock compositions of three types of troctolite from the ore horizon (Vishnevskiy and Cherdantseva 2016). An average Pd composition of 10 ppb for the initial silicate liquid was assumed on the basis of values from non-mineralized basalts from the Emeishan large igneous province (Song et al. 2009). The resulting R-factor values for large, medium, and small sulfide globules are 1180, 1406, and 1451, respectively.

Discussion

Understanding the geochemical behavior of metals in magmatic sulfide systems is challenging due to the complex tectono-magmatic histories of most large deposits (e.g., Arndt 2011; McDonald and Holwell 2011), involving multiple cycles of magma pulses, differentiation, fractionation, crustal contamination, fluid recirculation, degassing, and subsequent redistribution, loss, or enrichment of chalcophile elements across various sulfide phases (Mansur et al. 2021). Additionally, original magmatic textures can be obscured, and metal distributions can be altered by metamorphic, hydrothermal, and supergene processes. (Coghill and Wilson 1993; Liu et al. 2016; Beinlich et al. 2020). Therefore, the small gabbroic Rudniy intrusion, characterized by rapid crystallization and minimal metamorphic alteration, provides an ideal natural laboratory to gain crucial insights into the primary magmatic processes driving the formation of Cu-Ni-PGE minerals.

Emplacement and crystallization of the Rudniy intrusion

The presence of dendritic elongated crystals of olivine and plagioclase (Fig. 3d, f) at the basal contact of the melanogabbro unit may indicate crystallization under supercooling conditions (Tegner and Wilson 1995), signifying an interaction between hot, newly intruded magma forming the melanogabbro and partially or fully solidified underlying leucogabbro. However, if the underlying leucogabbro had been fully crystallized, we would have expected to observe glassier or at least finer-grained textures in the chilled margin. Therefore, the most likely scenario is that the magma that formed the melanogabbro could interact with a partially solidified leucogabbro, reflecting the dynamic nature of the plumbing system that formed the Rudniy intrusion and injection of magma along already existing conduits (e.g., Hayes et al. 2015).

Sulfide saturation of the Rudniy intrusion

The mineralized zone occurs very locally along the basal contact of the melanogabbro, which includes the chilled margin and 10–15 cm above it. In order to assess whether the magma was sulfide-saturated under the presumed emplacement conditions of 1250 °C and 3 kbar (determined based on the geological context and metamorphic effects on the intrusion and country rocks), we utilized the PELE software to model the crystallization path of the liquid (Boudreau 1999), employing the whole-rock composition of sample A111-11 (Table 1). The modeling results indicated the absence of any sulfide liquid stability under those conditions, supporting the notion that the magma, was initially emplaced as sulfide undersaturated. This finding was further substantiated by petrographic examinations, which revealed the absence of any sulfide inclusions within olivine. Consequently, the central question revolves around the mechanism responsible for inducing sulfide saturation within the melanogabbro.

Sulfur isotope characteristics of sulfides from the ore horizon reveal a predominantly mantle-like signature, with an average δ34S of 0.4‰ (Table 2, Fig. 4c). Combined with primitive and uncontaminated lithophile trace element signatures (Fig. 4a, b), the results effectively rule out substantial contamination from the Devonian siltstone country rocks. Based on previous observations regarding the contact between melanogabbro and leucogabbro, it is reasonable to infer that sulfide saturation primarily resulted from localized interaction between these two magmas.

We propose that the newly intruded hot magma (melanogabbro) locally eroded and assimilated the previously emplaced and partially crystallized intrusion (leucogabbro). This interaction triggered sulfide saturation, along with the incorporation of alkalis, phosphorous, titanium, and zirconium, which are anomalously elevated in the leucogabbro (Table 1). This process is schematically illustrated in Fig. 12. In response to this interaction and supercooling, the new silicate magma exsolved two immiscible liquids: the sulfide liquid and the liquid composing the enveloping coats (Fig. 7). This would explain their rounded shape and sharp margins of halos with the host troctolite.

Fig. 12
figure 12

Schematic illustration depicting the process of crystallization of the melanogabbro and its associated ore minerals. (a) The onset of mafic magma emplacement on top of the partially solidified leucogabbro due to reactivation of the plumbing system; the two insets show the places of mixing between hot mafic magma and partly solidified leucogabbro. (b) Illustration of the processes that led to the formation of the Southern End of the ore horizon (outcrop 1 in Fig. 2). The depicted zone shows the area of mixing of two magmas, which led to the segregation of immiscible sulfide and carbonate liquids. Magmatic turbulence (black arrow) results in the breaking down of sulfide globules into smaller ones: (1) large sulfide globule with carbonate liquid tail; (2) separation of large sulfide globules into smaller ones; (3) dynamic movement of a medium sulfide globule; and (4) separation of sulfide globule with carbonate coat and break up of sulfide liquid. (c) An illustration of the formation of the ore horizon at the Northern End. Crystallization starts from the lower contact of the intrusion. The mixing of magmas of different temperatures leads to convective currents that tear the sulfide globules apart. (d, e) Schematic illustration highlighting the main features of three types of sulfide globules representing the stages of crystallization of magmatic sulfide liquid. (d) Large sulfide globules that form at an early stage of sulfide and carbonate liquid segregation. (e) Medium sulfide globule that forms at an intermediate stage. (f) Small sulfide globules, formed due to breakup of large sulfide globules and separation of carbonate melt. Cbn, cubanite; Ccp, chalcopyrite; Ilm, ilmenite; Pn, pentlandite; Po, pyrrhotite; Mix, mixed sulfide domains

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The composition of the liquid within the halos can be inferred from their mineralogical assemblage (clinopyroxene, Ca-Na plagioclase, Ca-Na amphibole, phlogopite, ilmenite, magnetite, tacharanite, apatite, calcite, and baddeleyite). Accordingly, the original liquid would include the following components: SiO2, CaO, MgO, K2O, Na2O, FeO, TiO2, Al2O3 and ZrO2, H2O, fluorine, chlorine, CO2, N-bearing components, and hydrocarbons (Fig. 8, Table 4, ESM 2 Table A4). A liquid with this composition that is immiscible with both sulfide and silicate melts can be best represented by a silica-contaminated carbonate melt (Yaxley et al. 2022). This liquid tends to envelop sulfide globules due to its exceptional wetting capabilities (Yaxley et al. 2022). The coexistence of those three distinct liquids has been previously documented in melt inclusions hosted in perovskite from carbonated nepheline magma from the Kerimasi volcano, Tanzania (Guzmics et al. 2012), and was confirmed through experimental studies (Kogarko 1998; Kogarko et al. 2004; Weidendorfer and Asimow 2022).

Aird and Boudreau (2013) first indicated the possibility that the carbonate-sulfide assemblages observed in a wide range of PGE deposits worldwide may be primary magmatic in nature, with late fluid circulation leading to metal and sulfur redistribution (e.g., Maier et al. 2021). However, if the halos were formed due to late fluid circulation (Boudreau and Mccallum 1986), we would have expected to see more diffuse rims, without the sharp edges and sudden changes in mineralogy that characterize the coats in the Rudniy intrusion.

Numerous other studies emphasized the important role of volatiles in ore-forming processes (Fiorentini and Beresford 2008; Fiorentini et al. 2008). Some suggested that adherence between sulfide globules and gas bubbles may decrease the buoyancy of the compound aggregates and facilitate the physical transport of the metals (e.g., Mungall et al. 2015; Blanks et al. 2020; Schoneveld et al. 2020; Yao and Mungall 2022). Depending on the depth of emplacement, it was proposed that the gas bubbles either contained supercritical CO2 fluids at high pressures (Blanks et al. 2020) or water-rich fluids at shallower conditions (Le Vaillant et al. 2017). In both cases, gas bubbles would allegedly form hollow spaces attached to sulfides with chromian spinel crystals nucleated on the margins (Pleše et al. 2019). Eventually, those hollow spaces would be filled with late magmatic liquid due to pressure-driven infiltration mechanisms (Le Vaillant et al. 2017; Barnes et al. 2019).

However, the proposed supercritical carbon oxide- and water-dominated fluids do not account for the preserved mineralogical assemblage documented in the natural examples (for example, alkali-rich phases or carbonates), which would require components that cannot be borrowed from the coexisting silicate mafic/ultramafic magma, not even when highly differentiated. There are also unresolved inconsistencies related to the shape and morphology of the bubbles: firstly, their geological preservation would be severely compromised at the bottom of intrusions due to compression and differentiation; secondly, whereas in all experiments, sulfide globules are attached to the bubbles from the outer side (e.g., Mungall et al. 2015; Yao and Mungall 2020; Iacono-Marziano et al. 2022); in nature, sulfide globules are commonly fully surrounded by volatile-rich assemblages, unless extruded on the Earth’s surface (Beresford et al. 2000). This is the reason why we suggest referring to these assemblages as “coats” rather than “caps,” as they are usually termed in the literature (e.g., Le Vaillant et al. 2017). Instead, it is proposed that the geochemistry, mineralogy, and range of textures observed in nature (e.g., presence of calcite, apatite inclusions within sulfides, vermicular textures) can be best explained by the localized coexistence of carbonate, silicate, and sulfide liquids. This includes compound globules documented in upper-crustal Norilsk intrusion (Russia) and the deep-seated alkali-rich intrusions, for example, that make up the Valmaggia pipe in Italy (Blanks et al. 2020; Schoneveld et al. 2020).

Evolution of sulfide liquid and coexisting carbonate melt

Upon emplacement of the silicate magma, the temperature contrast between two magmas leads to thermo-mechanical erosion, exsolution of the sulfide and carbonate liquids, and fast crystallization at the contact of two units. Simultaneously with the growth of dendritic crystals of olivine and chromian spinel from the bottom, olivine and spinel crystals start crystallizing and accumulating from the rest of the intrusion (Fig. 12b). Precipitating crystals flatten newly exsolved sulfide globules parallel to the basal contact. In the narrow ore horizon, temperature differences lead to local convection and turbulence (Robertson et al. 2015), causing the breakup of larger sulfide globules into smaller ones with different degrees of internal differentiation depending on how much time each globule has to settle down and crystallize. It is thought that the large globules, due to their size, were most likely exsolved in situ and not transported, as shown through numerical modeling (Robertson et al. 2015). This inference is supported by the lowest R-factor number calculated for large sulfide globules in relation to other types of globules, reflecting lower interaction of coexisting silicate magma (Campbell and Naldrett 1979), and by the presence of loop textures and mixed sulfide domains, which is another indicator that the sulfide liquid did not have enough time to fractionate (Lightfoot et al. 2012). In contrast, medium and small sulfide globules are formed as a result of the break-up of larger globules. During local convection and break-up, they have more time to equilibrate with a bigger volume of silicate magma and differentiate.

Volatiles associated with sulfide and carbonate liquids

Both large and medium sulfide globules are surrounded by immiscible carbonate melt, whereas small sulfide globules have only patchy relict rims. Although joined together, the coats isolate the sulfide globules from interacting with the host silicate magma, preserving fluids and metals from escaping: only apatite from large and medium sulfide globules contains high concentrations of fluorine, which otherwise would have escaped due to its very high mobility (Xu et al. 2006). According to our observations, it is likely that during the evolution of the magmatic process, the coats eventually separate from their companion sulfide globules due to differences in relative density and viscosity (Kono et al. 2014).

Only insignificant amounts of CO2 gas were measured within the coats, which may reflect extensive CO2 degassing upon emplacement of the melanogabbro. The remaining CO2 would have been degassed due to the reaction of silica contamination of the carbonate liquid (Yaxley et al. 2022). We assume that once the immiscible carbonate liquid was exsolved from the silicate magma, it was only shortly in equilibrium with it. Eventually, the crystallization of olivine and clinopyroxene would have increased the silica activity of the residual magma, causing it to react with coexisting carbonate melt. The result of this reaction is the additional release of CO2 from the carbonate coats. This process is also ascribed to explain the absence of high-temperature carbonates such as dolomite and ankerite, which would have been expected in the coats (Aird and Boudreau 2013). However, it is suggested that some of the CO2 gas may be preserved in the sulfide liquid (Fig. 8, Table 4). In fact, it is possible that sulfide and carbonate liquids mix to some extent, either physically, chemically, or both. That is why we observe rounded and irregular carbonate inclusions in all types of sulfide globules. It is also possible that some of the incompatible components that make up the coats could have been chemically dissolved in sulfide liquid, as shown in the very similar textures generated in experiments by Wykes and Mavrogenes (2005). Upon cooling, the residual carbonate liquid and/or dissolved alkaline and volatile elements are exsolved in the form of vermicular textures, also known as myrmekitic textures (Mitchell 1997). In this case, they are mostly accommodated in the phase that crystallized last, which according to our observations, would be pentlandite that forms due to the peritectic reaction of monosulfide solid solution (mss) with Cu-rich residual sulfide liquid (Mansur et al. 2019).

It remains unclear what happens to the carbonate liquid after it separates from the sulfides. We can expect two possible scenarios. The first one is that globules of carbonate liquid with fragments of sulfides rise buoyantly to the top of the magmatic conduit, similar to what is observed in the Norilsk 1 intrusion (Schoneveld et al. 2020). The second scenario is that carbonate liquid is eventually destabilized in the mafic magma due to temperature decreases and silicate magma fractionation (Watson et al. 1982). In this case, the result would be a mineralized alkali-rich mafic rock, similar to the ones documented in Holwell et al. (2019).

Distribution of metals and semimetals during the evolution of the sulfide liquid

The three types of sulfide globules documented in the Rudniy intrusion provide snapshots of three distinct stages of crystallization of the sulfide liquid. We discuss here the evolution in the distribution of metals in sulfides as a function of the rate of crystallization and degree of fractionation, and we estimate the role of volatile components and carbonatite liquid in this process. The large sulfide globules, which represent sulfide liquid that was exsolved in situ and was not transported, are enriched in more soluble Ag and Sn and are relatively depleted in less soluble PGEs (Ir, Pt, Pd) and Au (Fig. 12d). In contrast, small sulfide globules, which are thought to crystallize last after the breakup of the larger globules, are relatively enriched in PGEs and Au and depleted in Sn and Ag (Fig. 12f). Variability in PGE concentrations is thought to depend upon R factor, whereby the smaller sulfide globules that resulted from stirring and breaking of bigger ones had the chance to interact with larger volumes of silicate melt.

The formation of PGM nuggets, especially the ones located at the edges of sulfide globules, still remains debatable. It was suggested that PGEs that partition into the sulfide liquid with temperature decrease tend to form separate PGE- and semimetal-rich liquid globules (Prichard et al. 2004; Barnes et al. 2006; McDonald and Holwell 2011). These residual metalloid-rich melts tend to escape the crystallizing sulfide liquid into the silicate matrix, which results in the appearance of PGMs in the narrow zones around sulfide. However, some other studies argue that PGEs form nuggets before they interact with sulfide liquid (Anenburg and Mavrogenes 2020). In our samples, PGMs preferably occur in coats surrounding large sulfide globules, with almost no nuggets found in coats from small sulfide globules.

The difference in concentration of Ag and Sn dissolved in base metal sulfides and represented as nuggets in different sulfide globules remains unclear. We suggest that the depletion of these elements could be caused by the interaction of volatile-rich carbonate liquid with sulfides and their subsequent separation. It is inferred that the aqueous phase associated with sulfide liquid could potentially accommodate some amounts of Sn and Ag. Because small sulfide globules are depleted in Ag and Sn, we argue that the segregation of carbonate globules could potentially facilitate the escape of those metals from a sulfide liquid.

The distributions of Os contents in pyrrhotite that was documented using LA-ICP-MS mapping techniques can be the very first documentation of a preserved intermediate state of monosulfide solid solution exsolution with formation of two monosulfide solid solutions (mss)—the first sulfur-rich monoclinic and second sulfur-poor hexagonal pyrrhotite, as mentioned in early experimental studies (Naldrett et al. 1967; Misra and Fleet 1973). These features and subtle differences that were inferred experimentally but never documented in nature may be visible for the first time at Rudniy due to the exceptional level of preservation and fast rate of crystallization of this magmatic system.

Conclusions

The Rudniy intrusion provides a natural laboratory to constrain the processes that lead to sulfide liquid segregation, as well as the transportation and enrichment in precious metals in large and economically important magmatic sulfide systems. The three types of documented sulfide globules provide snapshots of three distinct stages of crystallization of the sulfide liquid upon cooling and emplacement of the intrusion. All globules generally display mantle-dominated sulfur isotopic signatures but show variable metallogenic and mineralogical characteristics, as well as notably different sizes and morphologies reflecting variable cooling and crystallization regimes in different parts of the intrusion. The sulfide globules are surrounded and intergrown with volatile-rich (i.e., CO2-, H2O-, F-, and Cl) phases such as calcite, chlorite, mica, amphibole, and apatite. They are almost identical texturally and mineralogically to those documented in the Norilsk deposit (Le Vaillant et al. 2017; Barnes et al. 2019; Schoneveld et al. 2020). However, in this study, we interpret their origin differently, in that these assemblages represent the product of crystallization of volatile-rich carbonate melt that is immiscible with both silicate and sulfide liquids. We put forward the hypothesis that volatile-rich carbonate melt envelops sulfide droplets, facilitating their transport in magmatic conduits and that this process may be more widespread than commonly thought.