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Article

Microminerals as Complimentary Guides into Metallogeny and the Ore-Forming Potential of Igneous Rocks: Evidence from the Stanovoy Superterrane (Russian Far East)

by
Valeria Krutikova
,
Nikolai Berdnikov
and
Pavel Kepezhinskas
*
Institute of Tectonics and Geophysics, Russian Academy of Sciences, Khabarovsk 680000, Russia
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(5), 504; https://doi.org/10.3390/min15050504
Submission received: 2 April 2025 / Revised: 4 May 2025 / Accepted: 6 May 2025 / Published: 9 May 2025
(This article belongs to the Special Issue Igneous Rocks and Related Mineral Deposits)
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Abstract

:
Numerous mineral microinclusions discovered in the Triassic Ildeus mafic–ultramafic intrusion are dominated by base metal sulfides, gold, silver, and their alloys, as well as rare earth element (REE) minerals. These mineral microinclusions were formed through both the magmatic differentiation of the Ildeus intrusion and the multi-stage interaction of intrusive rocks with late-magmatic, post-magmatic and post-collisional fluids. A comparison of the results of our microinclusions study with ore mineralization discovered within the Ildeus intrusion suggests that microinclusion assemblages in igneous rocks are, in some cases, precursors of potentially economic mineralization. In the case of the Ildeus rocks, sulfide microinclusions correspond to potentially economic disseminated nickel–cobalt sulfide ores, while microinclusions of gold and its alloys correlate with intrusion-hosted, erratic gold mineralization. The occurrence of silver and rare earth element minerals in Ildeus plutonic rocks indicates the possible presence of silver and REE mineralization, which is supported by sub-economic whole-rock silver and REE grades in parts of the Ildeus intrusion. The results of our investigation suggest that studies of mineral microinclusions in magmatic rocks may be useful in the evaluation of their metallogenic specialization and ore-forming potential and could possibly be utilized as an additional prospecting tool in the regional exploration for precious, base, and rare metals.

Graphical Abstract

1. Introduction

Igneous rocks associated with ore deposits carry important information about the metal sources, the composition of mineralizing melts and fluids, as well as the conditions of fluid exsolution and the transport of critical ore metals into the upper crust [1,2,3,4,5,6,7,8,9,10]. Typical criteria for recognizing the ore-bearing potential of igneous rocks include their trace element and isotopic characteristics [11,12,13,14,15,16], chemical variations in rock-forming [17,18,19,20,21,22,23,24,25,26] and accessory [27,28,29,30,31] minerals, as well as the composition of the melt and fluid inclusions [32,33,34,35,36]. Some magmatic rocks display close geologic and (possibly) genetic affinities to certain ore deposits and are successfully used as exploration guides across different tectonic terranes and geodynamic environments [11,13,37,38,39,40]. The widely recognized examples include Phanerozoic adakites (high Sr/Y felsic rocks [41,42,43,44]); Precambrian komatiites [45,46,47,48,49]; potassic alkaline rocks, such as shoshonites [50,51,52,53], kimberlites [54,55,56], carbonatites [57,58,59,60], alkaline granites, and rhyolites [61,62,63,64,65,66]; and some other rock associations.
Some plutonic and volcanic rocks contain associations of metal and mineral microinclusions (usually up to 50 µm in size), hereafter termed microminerals, in rock-forming silicates, accessory minerals, and, in the case of volcanic rocks, groundmass glass [67,68,69,70,71]. Occasionally, these micromineral assemblages directly correlate with the mineralogy of ores hosted by the micromineral-bearing igneous rock formations. For example, platinum-group metal (PGM)- and Cu-Ag-Au alloy-bearing microinclusions in explosive volcanic rocks in the Lesser Khingan mineral province record the early stages of the formation of spatially associated PGM-Au-bearing iron–manganese ore deposits [72,73,74,75,76]. The iron titanium oxide–apatite–sulfide–sulfate (ITOASS) microinclusions in magmatic rocks from the Stanovoy superterrane are linked to the early magmatic and metasomatic stages of the formation of iron oxide–copper–gold (IOCG)/iron oxide–apatite (IOA) mineralization, providing a conceptual basis for the refined prospecting for these valuable mineral deposits throughout the Stanovoy cratonic superterrane [77,78]. Thus, comprehensive studies of micromineral assemblages in igneous rocks can provide new insights into regional metallogenic patterns in addition to the currently used economic geology and exploration approaches.
In this paper, we report new data on the micromineral assemblages in the Triassic Ildeus mafic–ultramafic intrusion from the Stanovoy superterrane at the southern edge of the Siberian craton. We document several associations of microminerals and compare them with ore-grade mineralization recently discovered within the Ildeus intrusion. We then further explore the potential usage of microminerals as a complimentary guide to the regional metallogeny and ore-forming potential of igneous rocks in accreted terranes and orogenic belts at plate edges.

2. Geologic and Petrologic Background

The Stanovoy cratonic superterrane is located at the southern edge of the Siberian craton (Figure 1) and is composed of Precambrian high-grade (granulite) metamorphic complexes, amphibolite facies and greenschist facies metavolcanic and metasedimentary rocks, and Mesozoic subduction (248–203 Ma)- and collision (145–127 Ma)-related mafic to felsic plutonic and volcanic rocks [79,80]. The Precambrian to Mesozoic metamorphic, magmatic, and sedimentary rock units are intruded by post-collisional Early Cretaceous (122–117 Ma) diorites, monzonites, and granites with distinct adakite geochemistry [80,81,82,83], as well as high-Nb lamprophyric dikes [84]. The Stanovoy superterrane records the protracted tectonic history of the Mesozoic subduction of the Mongol-Okhotsk oceanic basin along the southern active convergent margin of the Siberian craton, followed by the Jurassic collision of the Siberian craton with the Amur microcontinent and Early Cretaceous post-collisional rifting and granitic magmatism [80].
Precambrian granulite and amphibolite complexes (the Stanovoy series) are intruded on by several Triassic mafic–ultramafic bodies, which are typically viewed as arc root plutonic complexes of the Mesozoic active margin at the southern edge of the Siberian craton [80,81,82,84]. The Ildeus mafic–ultramafic massif is located within the eastern part of the Stanovoy superterrane (Figure 1) and is characterized by a core of coarsely layered plagioclase- and amphibole-bearing peridotites, olivine websterites, wehrlites, and gabbronorites, surrounded by a thin (<100 m) rim composed of two-pyroxene gabbros and gabbroic anorthosites [82,84,85]. Both peridotites and gabbros are frequently crosscut by numerous clinopyroxenite and orthopyroxenite dikes [84]. The Ildeus complex experienced several episodes of the later-stage ultramafic (serpentine + talc + chlorite ± carbonate) and felsic (quartz + albite + K-feldspar + biotite ± calcite) metasomatism and the emplacement of low- and high-sulfidation quartz–carbonate–adularia veins [81,82]. Abundant adakite veinlets, veins, and dikes intrude on all earlier plutonic and metasomatic rocks throughout the Ildeus intrusion [82,83,84].
Ultramafic rocks in the Ildeus intrusion’s core commonly display meso- and adcumulate textures composed of olivine, orthopyroxene, and minor clinopyroxene with inter-cumulus, high-Al amphibole and plagioclase (Figure 2a). Pyroxenites (mostly olivine websterites) in the core also have a cumulate texture, while pyroxenite veins and dikes are characterized by a granular texture [82,84]. Gabbroic rocks contain varying amounts of plagioclase, orthopyroxene, and clinopyroxene (Figure 2b), as well as amphibole with minor olivine and biotite, displaying a range of poikilitic, granular and hypidiomorphic textures [82]. Accessory minerals include Cr-Al-Fe spinel, magnetite (with varying amounts of Ti, V, and Cr), ilmenite, rutile, titanite, apatite, zircon, and baddeleyite.
Major and trace element variations in mafic–ultramafic rocks from the Ildeus intrusion indicate their subduction-related origin [81,82,84,85]. Peridotites, pyroxenites, and gabbros plot into the field of arc cumulates or the area of compositional overlap between the arc and mid-ocean ridge (MOR) cumulates in the AFM (Na2O+K2O–total FeO–MgO) diagram (Figure 2c). Although a substantial data point scatter is observed in the AFM space, the bulk of the Ildeus rock compositions tend to follow calc-alkaline differentiation trend defined by the gradual decrease in MgO and increase in total alkali contents (Figure 2c). The subduction-related character of the Ildeus intrusion is further confirmed by primitive mantle-normalized trace element patterns (Figure 2d). Ildeus mafic–ultramafic rocks exhibit an enrichment in large-ion lithophile and depletion in high-field strength elements, coupled with a moderate enrichment of light over heavy rare earth elements typical of convergent zone magmas [89].

3. Materials and Methods

Samples for this study were obtained during a 2022 exploration drilling of the Ildeus intrusion by the Oslo-based Khingan Minerals AS (Norway) [81,82]. All analytical work was carried out at the Khabarovosk Innovation-Analytical Center (KhIAC) of the Institute of Tectonics and Geophysics (Far East Branch of the Russian Academy of Science, Khabarovsk, Russian Federation).
A petrographic study of plutonic rocks from the Ildeus intrusion was carried out using the Imager A2m microscope (Carl Zeiss, Jena, Germany). Investigation of the morphology and chemical composition of the minerals, mineral microinclusions, native metals, and their alloys was conducted using a Vega 3 LMH (Tescan, Brno, Czech Republic) scanning electron microscope (SEM) equipped with an X-Max 80 (Oxford Instruments, Abingdon, UK) energy dispersive spectrometer (EDS) with the following operating conditions: accelerating voltage of 20 kV, beam current of ~500 nA, and beam diameter of 0.2 µm. An extensive database of reference samples for all of the chemical elements incorporated into the Aztec Version 4.0 software and the Co-standard Oxford Instruments № 6864-15 were used as standards during our SEM study. Accuracy of the EDS analysis was estimated to be ±0.1 wt.%. X-ray spectra of microinclusions of less than 5 µm in size, besides the spectral response from the inclusion itself, typically include some excitation spectra from a host mineral. The correction method proposed in [90] was used to interpret mixed spectra from ultra-small metal and mineral microinclusions. Selected SEM-EDS spectra are listed in Figure S1 (Supplementary Materials).

4. Results

Mafic to ultramafic rocks from the Ildeus intrusion contain abundant primary (magmatic) and secondary (metasomatic) metal and mineral microinclusions in silicate and oxide minerals [77,80,81,82,84,85,90,91]. We have previously used various assemblages of microinclusions to reconstruct magmatic–hydrothermal evolution and metal transport at arc plutonic roots [81,82,85]. Microinclusions in the Ildeus intrusion can be subdivided into the following principal groups: (1) sulfides, (2) Au and Au-bearing alloys, (3) Ag and Ag-bearing minerals and alloys, and (4) REE-bearing minerals.

4.1. Sulfide Microinclusions

Sulfide microinclusions are by far the most common and are observed in all studied samples of plutonic rocks from the Ildeus intrusion. Microinclusions of Co-Cu-Ni-Fe sulfides are frequently spatially associated with larger (>50 µm) sulfide grains (Figure 3a–c). Sulfide populations are typically dominated by primary magmatic pyrrhotite, Ni-bearing (Ni = 0.7–7.3 wt.%) pyrrhotite, pentlandite, Co-enriched (Co = 1.7–21.0 wt.%) pentlandite, and chalcopyrite included in cumulate clinopyroxene, orthopyroxene, and interstitial high-Al amphibole (Figure 3d–g, Figure S1a,b). Interstitial amphibole contains microinclusion of chalcocite and millerite (Figure 3h). A single microinclusion of composite Co-Ni-Fe-Zn sulfide is observed in primary magmatic orthopyroxene (Figure 3i, Figure S1c). In addition, microinclusions of chalcopyrite and galena are present in secondary chlorite and orthoclase [82].

4.2. Microinclusions of Au and Au-Bearing Alloys

Gold-bearing microinclusions are represented by native gold, two-component Ag-Au, and three-component Cu-Ag-Au alloys, as well as multi-component alloys of gold with some other chalcophile (Cu, Zn, Ag, etc.) and siderophile (Ni, Co) metals. Polycrystalline grains of almost pure gold are observed as microinclusions in clinopyroxene (Figure 4a, Figure S1d) and the magnetite–calcite matrix (Figure 4b). Ag-Au alloys are typically present in magmatic orthopyroxene (Figure 4c, Figure S1e) and secondary interstitial albite (Figure 4d). Clinopyroxene in ultramafic cumulates frequently contains microinclusions of Cu-Ag-Au alloys (Figure 4e, Figure S1f). A microinclusion of the multi-component Ni-Zn-Ag-Cu-Au alloy is observed in interstitial amphibole from the ultramafic Ildeus cumulate (Figure 4f, Figure S1g).

4.3. Microinclusions of Ag-Bearing Minerals

Silver microinclusions in Ildeus rocks occur as native metal, alloys with Cu, Zn, and Au, as well as chlorides and sulfides in association with various rock-forming silicate minerals. An elongated native silver particle is present in orthopyroxene (Figure 5a). Serpentine in Figure 5b contains an inclusion of a Ag-Au alloy (Figure S1h). Microinclusions of cupriferous silver (Figure 5c) and silver with minor zinc and copper (Figure 5d, Figure S1i) are observed in orthopyroxene. Serpentine in Figure 5e contains an inclusion of porous chlorargyrite (Figure S1k). Acanthite, frequently with an admixture of some chalcocite, is present in olivine from the Ildeus intrusion (Figure 5f, Figure S1l).

4.4. Microinclusions of REE-Bearing Minerals

Monazite and xenotime are the most common among the REE-bearing microminerals present in the intrusive rocks of the Ildeus intrusion. Monazite frequently forms monazite-orthopyroxene aggregates in association with apatite (Figure 6a). Individual micrograins of euhedral to subhedral and anhedral monazite are observed in albite (Figure 6b, Figure S1m), serpentine (Figure 6c), secondary orthoclase, and primary orthopyroxene [80]. Xenotime is frequently associated with titanite and albite (Figure 6d, Figure S1n). Microinclusions of REE-bearing oxides are observed in chlorite (Figure 6e, Figure S1o). Some sodic feldspars contain botryoidal aggregates of bastnaesite (Figure 6f, Figure S1p). Microinclusions of REE oxides and carbonates are also present in some olivines and orthopyroxenes from the Ildeus ultramafic rocks [80].

5. Discussion

Ore mineral microinclusions in igneous rocks crystallize as individual mineral phases during magmatic differentiation and are also formed during the later-stage metasomatic alteration of their magmatic hosts [2,26,92,93,94,95,96,97,98]. The results of the current study suggest that associations of ore microminerals can potentially be used as complementary exploration tools in a variety of mineralized terranes across different tectonic settings.

5.1. Base Metal Sulfides

Previous exploration within the Ildeus intrusion established the presence of disseminated nickel, cobalt, and copper mineralization, commonly associated with peridotites and pyroxenites in the Ildeus core (Table 1). This disseminated sulfide mineralization is principally composed of pentlandite, Co-enriched pentlandite, pyrrhotite (occasionally with some Co and Ni), chalcopyrite, and bornite, with minor (possibly late-stage) galena and sphalerite (Table 1 (see also Table 6 in [82])). Pentlandite and pyrrhotite, in most cases, occur as large anhedral-to-subhedral grains in variably altered olivine-rich cumulates (Figure 7). Occasionally, interstitial pentlandite forms a sulfide matrix, effectively “cementing” altered olivine grains (Figure 7b). In some cases, pentlandite is observed via inclusions in cumulate olivine (Figure 7a), orthopyroxene, and clinopyroxene (for example, Figure 14e in [82]), suggesting the presence of two generations of nickel–iron–sulfides in the Ildeus intrusion (Table 2; [82,84]). Surface grab samples yielded Ni values of up to 0.55 wt.%, while limited data from the 2022 drill core indicate Ni grades of 0.05 to 0.2 wt.% in core intervals with disseminated sulfide mineralization (Table 1). Although grades of up to 0.34% Cu and 0.1% Co were reported by previous Russian exploration parties [84], our limited core analysis resulted in substantially lower cobalt and copper grades (Table 1), suggesting that the drill-indicated sulfide mineralization is dominated by pentlandite and pyrrhotite. Moreover, pentlandite and pyrrhotite are not only the most abundant microinclusions in the Ildeus ultramafic rocks (Table 2); they also dominate the disseminated sulfide mineralization, emphasizing direct correlation between sulfide microinclusions and the macromineralization of the same composition within the Ildeus intrusion (Table 1). The assemblages of sulfide microinclusions, the mineralogy of ores, and the observed nickel, cobalt, and copper grades in the Ildeus intrusion are broadly similar to magmatic sulfide deposits in convergent margin settings such as the Late Paleozoic mafic–ultramafic intrusions in NW China (Kalatongke, Baishiquan, Pobei, Yueyawan, etc. [99,100,101]), the Mesoproterozoic Kabanga ultramafic massif in Tanzania [102,103], and the Carboniferous Aguablanca mafic plutonic complex in SW Spain [104,105].
Textural evidence suggests a magmatic origin for pentlandite + pyrrhotite mineralization, although some pyrrhotite, selected Cu-sulfides, and additional pyrite, sphalerite, and galena could have been formed during later-stage metasomatic transformations of ultramafic rocks from the Ildeus intrusion [82,84]. Sulfur isotope data for the Ildeus massif and its satellite Lucha intrusion (located to the north of Ildeus) (Figure 1b) reported in [106] support the case for the magmatic origins of nickel sulfide mineralization, which is consistent with the available textural, mineralogical, and geochemical data [82,84]. The Ildeus δ34S value of 4.7 substantially differs from the typical mid-ocean ridge basalt (MORB) δ34S range of −0.18 to −1.91 and trends towards the average δ34S of ~8 characteristic of the slab-derived sulfur (Figure 8).
Interestingly, the satellite Lucha intrusion exhibits mantle (e.g., MORB-like) δ34S, which is comparable to the sulfur isotopic variations in magmatic sulfide ores in non-convergent margin mafic–ultramafic intrusions such as Voisey’s Bay and Stillwater, as well as the convergent margin Alaskan–Uralian-type complexes exemplified in Figure 8 by the Turnagain intrusion in British Columbia. The latter shows some variations towards host pyrite-bearing graphite phyllite, suggesting possible contamination by sedimentary sulfide [111]. Norilsk sulfide ores exhibit strongly positive δ34S values, partially overlapping with host Permian evaporates (Figure 8). Abundant geological, petrological and geochemical data suggest that Norilsk sulfide-bearing intrusions have undergone extensive contamination by the Paleozoic anhydrite, rock salt, and dolostone [119,120]. The ultramafic lower crustal section of the Talkeetna arc in Alaska, mafic cumulates from the Lesser Antilles arc, and lavas from modern volcanic arcs display some enrichment in heavy sulfur isotopes, with a tendency towards slab-derived sulfates (Figure 8) indicating the potential recycling of marine sulfate-bearing sediments into arc magma sources [115,121,122]. The Ildeus δ34S value of +4.7 falls within the range of various arc igneous rocks, which suggests derivation from subduction-related magma sources, with the possible involvement of recycled oceanic sediments.

5.2. Gold-Bearing Minerals

Alongside the disseminated Ni-Fe-sulfide mineralization, mafic and ultramafic rocks from the Ildeus intrusion contain potentially economic gold mineralization with erratic Au grade distribution (Table 1 and Table 2). Gold grades of up to 0.19 g/t in the Ildeus intrusion were first reported in [106], together with some silver (up to 6 g/t) and palladium (up to 0.04 g/t). Some of the first surface grab samples collected by us along the western margin of the Ildeus intrusion returned some lode-type grades of 596 g/t, along with many samples in the range of 0.05 to 0.5 g/t [84,85]. Gold mineralization at the Ildeus intrusion is represented by flattened gold micronuggets (Figure 9a), typically accompanied by various sulfides and other metal alloys (Pd-Pt, Fe-Pt, Ni-Rd-Pt, Fe-W, Ti-Co-W, Pb-Sb, Sn-Zn-Cu, etc. [82,84,85,89,90]). Alongside gold, these micronuggets contain some minor nickel, silver, and copper (Figure 9b). In addition to the uniform flattened textural appearance (Figure 9a), these gold grains show uniform chemical compositions in a Cu-Ag-Au ternary diagram (Figure 9b), suggesting their formation under similar petrological conditions and, probably, through a uniform ore-forming process. The spatial association of gold with ultramafic cumulates in the Ildeus intrusion (Table 2 [84,85]) suggests potential magmatic origin, which is consistent with gold’s flattened appearance, possibly caused by their growing from residual metal-rich magmatic liquids in interstitial spaces between the expanding cumulate olivine, orthopyroxene, and clinopyroxene grains [89,90]. The constant presence of minor nickel admixture (Figure 9b) may also reflect crystallization from the Ni-rich parental melt that gave birth to the Ni-sulfides, which are frequently found together with gold micronuggets in the Ildeus intrusion. At the same time, we cannot exclude the re-deposition of primary magmatic gold matter as texturally similar flattened particles in microfractures and inter-granular spaces under the influence of the late- and/or post-magmatic fluids.
Alongside nickel, gold micronuggets occasionally contain small amounts of zinc (Figure 10a) and show some surface sculpturing features that might be attributable to later-stage interaction with metasomatic fluids (Figure 10b).
Abundance of gold micronuggets in individual rock samples from the Ildeus intrusion show a strong correlation with the Au contents in these rocks (Figure 10c), with the highest gold microparticle counts being associated with the lode-type Au grades of up to 596 g/t [85,86]. It is worth mentioning here that placer gold is being intermittently mined on a small scale by local alluvial miners from multiple creeks draining the Ildeus intrusion. The gold-bearing particle association is the second most abundant after the sulfide microinclusions in the studied ultramafic rocks (Table 2). The Au-bearing microinclusions in ultramafic cumulates and the later-stage pyroxenite dikes (Table 2) clearly indicate the existence of potentially economic gold mineralization associated with the Ildeus intrusion (Table 1). The true scale of this erratic bulk gold mineralization is a subject for future exploration efforts in this part of the Stanovoy superterrane.

5.3. Silver-Bearing Minerals

Silver-bearing microinclusions in the Ildeus intrusion are represented by native and cupriferous silver, chlorargyrite, and acanthite, with small amounts of copper (Figure 5). Based on the abundance of Ag-bearing microinclusions in certain Ildeus igneous lithologies (Table 2) and the relationships between microinclusion assemblages and visibly disseminated nickel sulfide and gold mineralization, as described above, one can expect that separate silver mineralization might exist within the Ildeus mafic–ultramafic intrusion [81,82]. Several drill holes within the northern part of the Ildeus intrusion contain intervals composed of various mafic–ultramafic rocks assayed at 0.1 to 1 g/t Ag (Table 1), which represent drill-supported silver geochemical anomalies. A single one-meter section of websterites at the bottom of one of the holes was assayed at 162.2 g/t Ag [81,82], which is comparable to the silver content in low-grade hydrothermal deposits [123]. In the volcanogenic massive sulfide (VMS) and sedimentary exhalative (SEDEX)-type deposits, where silver is a byproduct of polymetallic ores, its grades are around 30–50 g/t, while in the vein type, hydrothermal silver mineralization Ag grades in excess of 500 g/t are common [123]. With the exception of a single drill core sample with a Ag grade of 162.2 g/t, all of the silver contents detected in the Ildeus samples so far are interpreted as geochemically anomalous [81,82], and no silver ore has been discovered in this part of the Stanovoy superterrane as of yet. The abundance of silver-bearing microinclusions in the Ildeus mafic–ultramafic rocks (Table 2) suggests high exploration potential for large-scale, low-to-medium-grade silver mineralization at the Ildeus intrusion. The presence of microinclusions of silver chlorides (Figure 5e) and iodides (Figure 13c,d in [82]) may indicate the late-magmatic or, possibly, later-stage metasomatic nature (Figure 9b) of these silver mineral assemblages [124], possibly linked to the Lower Cretaceous post-collisional tectonic processes in the Stanovoy superterrane [81]. Alternatively, at least some silver could be of primary magmatic origin, as suggested by the presence of abundant microinclusions of native and cupriferous silver, Ag sulfides, and Ag chlorides in Quaternary lavas from the Kamchatka volcanic arc [125].

5.4. REE-Bearing Minerals

The recent discovery of rare earth element (REE) mineral microinclusions in the Ildeus intrusion has been reported in [80]. The REE microminerals include monazite (Figure 6a–c) and xenotime (Figure 6d), as well as REE oxides (Figure 6e) and carbonates (Figure 6f). Ultramafic rocks with abundant REE microinclusions (Table 2) are associated with total REE contents ranging from 330 to 890 ppm (Table 1 [80]). One microinclusion-rich sample yielded total REE content of 1938 ppm, which is comparable with the REE grades in some low-grade disseminated rare earth deposits (total REE ≥ 0.2% [126]). While monazite and xenotime are commonly considered as magmatic minerals in alkaline complexes and associated REE mineralization [126,127,128], REE-enriched carbonates such as bastnaesite are usually interpreted as precipitates from post-collisional metasomatic fluids exsolved from REE-enriched alkaline magmas and carbonatites [129,130,131]. In the case of the Ildeus intrusion, we interpret microinclusions of REE minerals as indicators of possible metasomatic rare earth element mineralization caused by the interaction with metal-rich trans-lithospheric fluids linked to the widespread post-collisional processes in the Stanovoy cratonic superterrane [80].

5.5. Relationship Between Microminerals and Ore Mineralization

The results of our investigation into the microinclusions in the Ildeus intrusive rocks indicate a link between various microinclusion assemblages and the formation of potentially economic mineralization within the Ildeus intrusion. For example, sulfide microinclusions correspond to the disseminated Ni-Co sulfide mineralization confirmed by drilling within the northern part of the Ildeus intrusion [82]. Microinclusions of gold and its alloys correlate with the erratic bulk gold mineralization in wehrlites, websterites, and pyroxenites (Table 2, Figure 9a [82]). The presence of silver- and REE-bearing microminerals in several Ildeus rocks suggests that the Ildeus intrusion may potentially host large-scale silver and REE mineralization, which is consistent with the sub-economic grades of these metals in some intrusive lithologies (Table 1).
The general hypothetical model for the formation of such complex polymetallic mineralization within the Ildeus intrusion is presented in Figure 11. Textural and mineralogical data support the formation of magmatic sulfide mineralization—possibly the earliest—in the Ildeus mafic–utramafic intrusion through crystal fractionation and accumulation (Figure 11a) under mantle and crustal conditions similar to those of continental layered igneous complexes worldwide [5,38,132,133]. The late magmatic crystallization processes and late- and post-magmatic, metal-rich fluids appear to be responsible for the gold mineralization and, possibly, for the not yet discovered bulk silver mineralization (Figure 11b). This later-stage precious metals deposition might be similar to the later-stage hydrothermal gold and platinum-group metal mineralization formed through a high-salinity hydrous fluid infiltration in such layered intrusions as the Bushveld complex in South Africa [134] and the Stillwater complex in Montana, USA [135,136]. Experimental data suggest the elevated mobility of gold and silver in hydrothermal fluids associated with mafic–ultramafic intrusive systems controlled by such physicochemical parameters as oxygen fugacity, acidity, and chlorinity [137].
This interpretation is consistent with presence of the abundant chlorine-bearing microinclusions associated with gold- and silver-bearing minerals in the Ildeus intrusive rocks (Figure 5e [82,84,85]). Finally, low-grade REE mineralization in some Ildeus intrusive lithologies can be linked to the metal-rich, post-collisional fluids associated with Early Cretaceous slab tear and break-off under the Stanovoy superterrane [80].
The exact role of microminerals in the formation of potentially economic macromineralization is yet to be fully understood and clarified. Based on the available petrological and geochemical information for the Ildeus intrusion and other igneous suites, it appears that the observed micromineral assemblages may reflect initial elevated contents of certain metals in primary magmas and their volcanic and plutonic differences [36,38,72,75,77,89,90,97]. Alternatively, microminerals can possibly be later reassembled and accumulated into metal macromineralization through the common processes of magma differentiation, fluid-induced precipitation, and fluid–rock interaction under the upper crustal conditions [2,4,8,12]. Oxygen fugacity, fluid composition (especially salinity), and sulfur activity will play additional and important roles in the transformation of micromineral assemblages in igneous rocks into related economic ore deposits and showings [19,30,34,44,47,48,132,137,138].

6. Conclusions

Mafic–ultramafic rocks from the subduction-related Triassic mafic–ultramafic Ildeus intrusion in the Stanovoy superterrane of the Russian Far East contain four distinct assemblages of mineral microinclusions, namely, (1) base metal sulfides, (2) Au and Au-bearing alloys, (3) Ag and Ag-bearing minerals, and (4) REE-bearing minerals. We interpret these microinclusions as records of the early stages of the development of the Ildeus crustal magmatic–hydrothermal system, linked to its general mineralizing potential. The Ildeus intrusion contains disseminated nickel–cobalt sulfide and gold mineralization, which is compositionally similar to the sulfide- and Au-rich microminerals observed in the Ildeus peridotites and pyroxenites. Silver-bearing and REE-bearing minerals have, so far, been found only as microinclusions, but some grab and drill core samples are characterized by anomalous silver and total REE contents comparable to metal grades in low-grade ore deposits.
Our results suggest that a genetic link may exist between assemblages of ore mineral microinclusions of certain compositions and the ore macromineralization present in magmatic rocks. If this is the case, future investigations into microminerals in igneous rocks can be used as complimentary guides for (1) deciphering of metallogenic evolution of their primary magmas and (2) prospecting and exploring the ore mineralization potentially associated with micromineral-bearing magmatic complexes.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15050504/s1, Figure S1. Selected SEM-EDA spectra for microminerals from intrusive rocks of the Ildeus intrusion. (a–p) BSE images: a—Figure 3e; b—Figure 3f; c—Figure 3i d—Figure 4a; e—Figure 4c; f—Figure 4e; g—Figure 4f; h—Figure 5b; i—Figure 5d; k—Figure 5e; l—Figure 5f; m—Figure 6b; n—Figure 6d; o—Figure 6e; p—Figure 6f.

Author Contributions

Conceptualization, V.K., N.B. and P.K.; methodology, V.K. and N.B.; software, V.K.; validation, V.K.; formal analysis, V.K.; investigation, V.K., N.B. and P.K.; resources, V.K. and N.B.; data curation, V.K. and N.B.; writing—original draft preparation, V.K., N.B. and P.K.; writing—review and editing, V.K., N.B. and P.K.; visualization, V.K. and N.B.; supervision, N.B.; project administration, N.B.; funding acquisition, N.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded through the state assignment of the Institute of Tectonics and Geophysics, Far Eastern Branch, Russian Academy of Sciences (topic no. 124042300007-3, youth laboratory), using scientific equipment of the Khabarovsk Innovation and Analytical Center.

Data Availability Statement

Data are available upon request from the corresponding author.

Acknowledgments

We greatly appreciate the highly constructive reviews from four anonymous reviewers that significantly improved this manuscript. This study would not have been possible without the long-term support from the personnel of Khingan Minerals AS (Oslo, Norway) and Khingan LLC (Khabarovsk, Russia). We specifically thank Oleg Alekseenko for his support during our fieldwork in the Stanovoy superterrane (Russian Far East).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Tectonic setting of the Stanovoy superterrane at the southern edge of the Siberian craton. (b) Simplified geologic map of the central part of the Stanovoy superterrane. (c) Simplified map of the Ildeus mafic–ultramafic intrusion. Modified after [77].
Figure 1. (a) Tectonic setting of the Stanovoy superterrane at the southern edge of the Siberian craton. (b) Simplified geologic map of the central part of the Stanovoy superterrane. (c) Simplified map of the Ildeus mafic–ultramafic intrusion. Modified after [77].
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Figure 2. Petrographic (a,b) and geochemical (c,d) characteristics of plutonic rocks from the Ildeus intrusion. (a) Amphibole-bearing peridotite mesocumulate from the ultramafic core. (b) Two-pyroxene gabbro from the mafic rim. (c) A (Na2O+K2O)–F (total FeO)–M (MgO) diagram for Ildeus mafic–ultramafic rocks (solid red dots) modified from [86]. (d) Primitive mantle-normalized trace element patterns for mafic–ultramafic plutonic rocks from the Ildeus intrusion. Normalizing values are from [87]. Ildeus data in (c,d) are from [77,82,84,85]. Mineral abbreviations: Cpx—clinopyroxene; Opx—orthopyroxene; Amp—amphibole; Pl—plagioclase; Mag—magnetite. Mineral abbreviations in this figure as well as all other figures with BSE images follow the nomenclature of [88].
Figure 2. Petrographic (a,b) and geochemical (c,d) characteristics of plutonic rocks from the Ildeus intrusion. (a) Amphibole-bearing peridotite mesocumulate from the ultramafic core. (b) Two-pyroxene gabbro from the mafic rim. (c) A (Na2O+K2O)–F (total FeO)–M (MgO) diagram for Ildeus mafic–ultramafic rocks (solid red dots) modified from [86]. (d) Primitive mantle-normalized trace element patterns for mafic–ultramafic plutonic rocks from the Ildeus intrusion. Normalizing values are from [87]. Ildeus data in (c,d) are from [77,82,84,85]. Mineral abbreviations: Cpx—clinopyroxene; Opx—orthopyroxene; Amp—amphibole; Pl—plagioclase; Mag—magnetite. Mineral abbreviations in this figure as well as all other figures with BSE images follow the nomenclature of [88].
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Figure 3. BSE images of sulfide microinclusions in rocks of the Ildeus intrusion. (a) Pyrrhotite and pentlandite in association with clinopyroxene and orthopyroxene. (b) Pyrrhotite inclusion in amphibole. (c) Pyrrhotite and pentlandite inclusions in orthopyroxene. (d) Pyrrhotite microinclusion in amphibole. (e) Ni-bearing pyrrhotite microinclusion in amphibole. (f) Cobalt-enriched pentlandite microinclusion in orthopyroxene. (g) Chalcopyrite inclusion in amphibole. (h) Intergrowth of millerite with chalcocite in amphibole. (i) Composite Co-Ni-Fe-Zn sulfide microinclusion (or intergrowth of several sulfides) in orthopyroxene. Mineral abbreviations: Cpx—clinopyroxene; Opx—orthopyroxene; Amp—amphibole; Po—pyrrhotite; Pn—pentlandite; Ni-Po—Ni-bearing pyrrhotite; Co-Pn—Co-enriched pentlandite; Ccp—chalcopyrite; Cct—chalcocite; Mlr—millerite.
Figure 3. BSE images of sulfide microinclusions in rocks of the Ildeus intrusion. (a) Pyrrhotite and pentlandite in association with clinopyroxene and orthopyroxene. (b) Pyrrhotite inclusion in amphibole. (c) Pyrrhotite and pentlandite inclusions in orthopyroxene. (d) Pyrrhotite microinclusion in amphibole. (e) Ni-bearing pyrrhotite microinclusion in amphibole. (f) Cobalt-enriched pentlandite microinclusion in orthopyroxene. (g) Chalcopyrite inclusion in amphibole. (h) Intergrowth of millerite with chalcocite in amphibole. (i) Composite Co-Ni-Fe-Zn sulfide microinclusion (or intergrowth of several sulfides) in orthopyroxene. Mineral abbreviations: Cpx—clinopyroxene; Opx—orthopyroxene; Amp—amphibole; Po—pyrrhotite; Pn—pentlandite; Ni-Po—Ni-bearing pyrrhotite; Co-Pn—Co-enriched pentlandite; Ccp—chalcopyrite; Cct—chalcocite; Mlr—millerite.
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Figure 4. BSE images of microinclusions of Au and Au-bearing alloys in rocks of the Ildeus intrusion. Native gold microinclusion in clinopyroxene (a) and in association with calcite and magnetite (b). (c,d) Microinclusion of Ag-Au alloy in orthopyroxene (c) and albite (d). (e) Microinclusion of Cu-Ag-Au alloys in clinopyroxene. (f) Microinclusion of Ni-Zn-Ag-Cu-Au alloy in amphibole. Mineral abbreviations: Ab—albite; Amp—amphibole; Cal—calcite; Cpx—clinopyroxene; Mag—magnetite; Opx—orthopyroxene.
Figure 4. BSE images of microinclusions of Au and Au-bearing alloys in rocks of the Ildeus intrusion. Native gold microinclusion in clinopyroxene (a) and in association with calcite and magnetite (b). (c,d) Microinclusion of Ag-Au alloy in orthopyroxene (c) and albite (d). (e) Microinclusion of Cu-Ag-Au alloys in clinopyroxene. (f) Microinclusion of Ni-Zn-Ag-Cu-Au alloy in amphibole. Mineral abbreviations: Ab—albite; Amp—amphibole; Cal—calcite; Cpx—clinopyroxene; Mag—magnetite; Opx—orthopyroxene.
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Figure 5. BSE images of microinclusions of Ag-bearing minerals in rocks of the Ildeus intrusion. (a) Native silver microinclusion in orthopyroxene. (b) Microinclusion of Au-Ag alloys in serpentine. (c) Cupriferous silver microinclusion in orthopyroxene. (d) Microinclusion of Zn-Cu-Ag alloy in orthopyroxene. (e) Chlorargyrite microinclusion in serpentine. (f) Microinclusion of acanthite with minor chalcocite in olivine. Mineral abbreviations: Serp—amphibole; Ol—olivine; Opx—orthopyroxene.
Figure 5. BSE images of microinclusions of Ag-bearing minerals in rocks of the Ildeus intrusion. (a) Native silver microinclusion in orthopyroxene. (b) Microinclusion of Au-Ag alloys in serpentine. (c) Cupriferous silver microinclusion in orthopyroxene. (d) Microinclusion of Zn-Cu-Ag alloy in orthopyroxene. (e) Chlorargyrite microinclusion in serpentine. (f) Microinclusion of acanthite with minor chalcocite in olivine. Mineral abbreviations: Serp—amphibole; Ol—olivine; Opx—orthopyroxene.
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Figure 6. BSE images of microinclusions of REE-bearing minerals in rocks of the Ildeus intrusion. (a) Monazite-orthopyroxene aggregate in association with apatite. (b,c) Monazite microinclusions in albite (b) and serpentine (c). (d) Xenotime microinclusion in association with titanite in albite. (e) Microinclusion of REE oxide in chlorite. (f) Bastnaesite microinclusion in albite. Mineral abbreviations: Ab—albite; Ap—apatite; Chl—chlorite; Mnz—monazite; Opx—orthopyroxene; Srp—serpentine; Ttn—titanite; Xtm—xenotime.
Figure 6. BSE images of microinclusions of REE-bearing minerals in rocks of the Ildeus intrusion. (a) Monazite-orthopyroxene aggregate in association with apatite. (b,c) Monazite microinclusions in albite (b) and serpentine (c). (d) Xenotime microinclusion in association with titanite in albite. (e) Microinclusion of REE oxide in chlorite. (f) Bastnaesite microinclusion in albite. Mineral abbreviations: Ab—albite; Ap—apatite; Chl—chlorite; Mnz—monazite; Opx—orthopyroxene; Srp—serpentine; Ttn—titanite; Xtm—xenotime.
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Figure 7. (a,b) Disseminated pentlandite mineralization in variably altered dunnites from the central part of the Ildeus intrusion. Mineral abbreviations: Ol—olivine; Srp—serpentine; Mag—magnetite; Hem—hematite; Pn—pentlandite.
Figure 7. (a,b) Disseminated pentlandite mineralization in variably altered dunnites from the central part of the Ildeus intrusion. Mineral abbreviations: Ol—olivine; Srp—serpentine; Mag—magnetite; Hem—hematite; Pn—pentlandite.
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Figure 8. Variations of sulfur isotopes in Ildeus and Lucha mafic–ultramafic intrusions from the Stanovoi superterrane (Russia) [106] compared with selected magmatic sulfide ores and associated sediments from Norilsk (Russia) [107], Voisey’s Bay (Canada) [108], Stillwater (USA) [109], Talkeetna ultramafic arc (Alaska, USA) [110], Turnagain Alaskan–Uralian-type plutonic complex and host graphite phyllite [111], arc cumulates [112], and arc magmas from the Marianas, Japan, and Indonesia [113,114,115]. Data for MORB are from [116], those for slab sulfur are from [117], those for sedimentary sulfides are from [110], and those for marine sulfates are from [118].
Figure 8. Variations of sulfur isotopes in Ildeus and Lucha mafic–ultramafic intrusions from the Stanovoi superterrane (Russia) [106] compared with selected magmatic sulfide ores and associated sediments from Norilsk (Russia) [107], Voisey’s Bay (Canada) [108], Stillwater (USA) [109], Talkeetna ultramafic arc (Alaska, USA) [110], Turnagain Alaskan–Uralian-type plutonic complex and host graphite phyllite [111], arc cumulates [112], and arc magmas from the Marianas, Japan, and Indonesia [113,114,115]. Data for MORB are from [116], those for slab sulfur are from [117], those for sedimentary sulfides are from [110], and those for marine sulfates are from [118].
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Figure 9. Flattened gold micronuggets from the Ildeus ultramafic rocks (a) and their composition (total of 46 grains) in the Cu-Au-Ag ternary plot (b). The inset shows the distribution of Au, Cu, Ag, and Ni contents in the individual micronuggets (a total of 29 grains) determined by the SEM-EDS.
Figure 9. Flattened gold micronuggets from the Ildeus ultramafic rocks (a) and their composition (total of 46 grains) in the Cu-Au-Ag ternary plot (b). The inset shows the distribution of Au, Cu, Ag, and Ni contents in the individual micronuggets (a total of 29 grains) determined by the SEM-EDS.
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Figure 10. (a,b) BSE images of large particles of an Au-bearing alloy extracted from the Ildeus intrusion. (c) Variation of bulk Au content in the Ildeus intrusion sorted according to the ascending Au values in samples #1 to #200, modified from [85].
Figure 10. (a,b) BSE images of large particles of an Au-bearing alloy extracted from the Ildeus intrusion. (c) Variation of bulk Au content in the Ildeus intrusion sorted according to the ascending Au values in samples #1 to #200, modified from [85].
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Figure 11. A schematic conceptual diagram showing possible models of formation of magmatic sulfide (a), gold and silver (b), and REE (c) mineralization in the Ildeus mafic–ultramafic intrusion.
Figure 11. A schematic conceptual diagram showing possible models of formation of magmatic sulfide (a), gold and silver (b), and REE (c) mineralization in the Ildeus mafic–ultramafic intrusion.
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Table 1. Principal associations of microminerals compared to the observed mineralization and metal grades in the Ildeus intrusion.
Table 1. Principal associations of microminerals compared to the observed mineralization and metal grades in the Ildeus intrusion.
Mineral
Associations		MicromineralsKnown Disseminated
Mineralization		Metal Grades (1)
SulfidesPentlandite, Co-enriched pentlandite, pyrrhotite, Ni-pyrrhotite, chalcopyrite, chalcocite, Co-Ni-Fe-Zn sulfidePentlandite, Co-enriched pentlandite, pyrrhotite, chalcopyrite, bornite with minor sphalerite and galenaUp to 0.55% Ni, 0.1% Co and 0.34% Cu in individual grab samples; typically, 0.05–0.2% Ni, 0.01–0.02% Co, and 0.005–0.08% Cu in the drill core
Au and Au alloysNative Au, Ag-Au and Cu-Ag-Au alloysMostly Cu-Ag-Au (±Ni and Zn) alloysUp to 596 g/t Au in individual grab samples; typically, 0.01–2.5 g/t in the drill core
Ag-bearing mineralsCupriferous silver, acanthite, silver halidesCurrently unknownUp to 163 g/t in select drill core samples; typically, 0.1–1 g/t over widths of several m in the core
REE-bearing mineralsMonazite, xenotyme, REE oxides and carbonatesCurrently unknownTotal REE content of 330–890 g/t in individual samples; one sample has a total REE content of 1938 g/t
(1) Refers to mineralized intervals as defined by the Russian exploration standards and based on core logging during the 2022 drilling campaign by Khingan Minerals AS. Data for the disseminated mineralization and metal grades in the Ildeus intrusion are from [80,81,82,84,85].
Table 2. Distribution of main associations of microinclusions in the principal types of plutonic lithologies in the Ildeus mafic–ultramafic intrusion.
Table 2. Distribution of main associations of microinclusions in the principal types of plutonic lithologies in the Ildeus mafic–ultramafic intrusion.
Principal Intrusive LithologiesBase Metal SulfidesAu-Bearing MicroinclusionsAg-Bearing MicroinclusionsREE-Bearing Microinclusions
Dunite, plagioclase-bearing duniteAbundantPresentPresentPresent
Harzburgite, lherzoliteAbundantPresentPresentAbsent
Wehrlite, websteriteAbundantAbundantAbundantPresent
Pyroxenite dikes and veinsPresentAbundantAbundantAbsent
Gabbro-norite and gabbro-anorthositePresent *AbsentPresentAbsent
Notes: Abundant: >10 microinclusions per SEM sample (~2 cm2 in area); Present: 1–10 microinclusions per SEM sample; Absent: Not detected by SEM-EDS. *—almost exclusively Cu-sulfides, sometimes as ITOASS (iron titanium oxide–apatite–sulfide–sulfate)-type segregations [77].
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Krutikova, V.; Berdnikov, N.; Kepezhinskas, P. Microminerals as Complimentary Guides into Metallogeny and the Ore-Forming Potential of Igneous Rocks: Evidence from the Stanovoy Superterrane (Russian Far East). Minerals 2025, 15, 504. https://doi.org/10.3390/min15050504

AMA Style

Krutikova V, Berdnikov N, Kepezhinskas P. Microminerals as Complimentary Guides into Metallogeny and the Ore-Forming Potential of Igneous Rocks: Evidence from the Stanovoy Superterrane (Russian Far East). Minerals. 2025; 15(5):504. https://doi.org/10.3390/min15050504

Chicago/Turabian Style

Krutikova, Valeria, Nikolai Berdnikov, and Pavel Kepezhinskas. 2025. "Microminerals as Complimentary Guides into Metallogeny and the Ore-Forming Potential of Igneous Rocks: Evidence from the Stanovoy Superterrane (Russian Far East)" Minerals 15, no. 5: 504. https://doi.org/10.3390/min15050504

APA Style

Krutikova, V., Berdnikov, N., & Kepezhinskas, P. (2025). Microminerals as Complimentary Guides into Metallogeny and the Ore-Forming Potential of Igneous Rocks: Evidence from the Stanovoy Superterrane (Russian Far East). Minerals, 15(5), 504. https://doi.org/10.3390/min15050504

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