Mineralogy of Platinum-Group Elements and Gold in the Ophiolite-Related Placer of the River Bolshoy Khailyk , Western Sayans , Russia

We describe assemblages of platinum-group minerals (PGM) and associated PGE–Au phases found in alluvium along the River Bolshoy Khailyk, in the western Sayans, Russia. The river drains the Aktovrakskiy ophiolitic complex, part of the Kurtushibinskiy belt, as does the Zolotaya River ~15 km away, the site of other placer deposits. Three groups of alloy minerals are described: (1) Os–Ir–Ru compositions, which predominate, (2) Pt–Fe compositions of a Pt3Fe stoichiometry, and (3) Pt–Au–Cu alloys, which likely crystallized in the sequence from Au–(Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu2, to PtAu4Cu5. The general trends of crystallization of PGM appear to be: [Os–Ir–Ru alloys]→ Pt3Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt–Fe phase) → Pt–Au–Cu alloys. We infer that Rh and Co mutually substitute for Fe, not Ni, and are incorporated into the pentlandite structure via a coupled mechanism of substitution: [Rh3+ + Co3+ + → 3Fe2+]. Many of the Os–Ir–Ru and Pt–Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt–Ir–Ni-rich alloys, and rarer phases like Cu-rich bowieite and a Se-rich sulfarsenide of Pt. The accompanying pyroxene, chromian spinel and serpentine are highly magnesian, consistent with a primitive ultramafic source-rock. Whereas the alloy phases indicate a highly reducing environment, late assemblages indicate an oxygenated local environment leading to Fe-bearing Ru–Os oxide (zoned) and seleniferous accessory phases.


Introduction
It was L.A. Yachevskiy who first found grains of platinum-group minerals (PGM) identified as "osmian iridium" and "platinum" in a placer in 1910 in the Usinskiy area of the western Sayans [1].These grains, enriched in platinum-group elements (PGE), were recognized in a heavy-mineral concentrate provided by Chirkov, a gold miner.Later, B.M. Porvatov, M.K. Korovin, and N.K. Vysotskiy [1][2][3] described the geology and occurrences of Au-PGM-bearing placers in the Usinskiy area and other regions of the western Sayans.Krivenko et al. [4] conducted large-scale investigations of associations of PGM in Au-PGE-bearing zones of placers of the Altai-Sayan folded region.Tolstykh et al. [5] reported the occurrence of a highly unusual association of PGM in a placer deposit of the Zolotaya River ("Golden River") watershed, located ca. 15 km from the Bolshoy Khailyk placer.There, arsenotellurides and telluroarsenides of Ir-Os-Ru and an arsenoselenosulfide of Ir occur with a Pt-Au-Cu alloy phase in association with grains of Os-Ir-Ru and Pt-Fe alloys.These placer occurrences were attributed to lode serpentinite rocks of the Kurtushibinskiy ophiolite belt [5].In addition, G.I. Shvedov and V.V. Nekos [6] found grains of placer PGM at the River Bolshoy Khailyk.
In the Aktovrakskiy complex [7], which forms part of the Kurtushibinskiy belt in Krasnoyarskiy kray (Figure 1), serpentitite bodies are fairly abundant at Bolshoy Khailyk, the site of our investigation.The area largely consists of volcanogenic and sedimentary rocks of the Chinginskaya suite of Lower Cambrian age, discordantly overlain by Ordovician "terrigenous" rocks.Dikes and intrusive bodies of Devonian rocks (granite, diorite, and metagabbroic rocks) are considered responsible for placer gold in the area.
Minerals 2018, 8, x FOR PEER REVIEW 2 of 25 of the Zolotaya River ("Golden River") watershed, located ca. 15 km from the Bolshoy Khailyk placer.There, arsenotellurides and telluroarsenides of Ir-Os-Ru and an arsenoselenosulfide of Ir occur with a Pt-Au-Cu alloy phase in association with grains of Os-Ir-Ru and Pt-Fe alloys.These placer occurrences were attributed to lode serpentinite rocks of the Kurtushibinskiy ophiolite belt [5].In addition, G.I. Shvedov and V.V. Nekos [6] found grains of placer PGM at the River Bolshoy Khailyk.
In the Aktovrakskiy complex [7], which forms part of the Kurtushibinskiy belt in Krasnoyarskiy kray (Figure 1), serpentitite bodies are fairly abundant at Bolshoy Khailyk, the site of our investigation.The area largely consists of volcanogenic and sedimentary rocks of the Chinginskaya suite of Lower Cambrian age, discordantly overlain by Ordovician "terrigenous" rocks.Dikes and intrusive bodies of Devonian rocks (granite, diorite, and metagabbroic rocks) are considered responsible for placer gold in the area.Our aims in the present article are to document the occurrences and mineralogical characteristics of assemblages of PGM and PGE-Au-rich phases in the alluvial placer associated with the River Bolshoy Khailyk (Figure 1), a tributary of the Urbun (Urgun) River, which flows into the River Yenisei.Our results and observations provide genetic implications and insights into the mineral-forming environments and trends of crystallization of the ophiolite-related PGE-Au mineralization at Bolshoy Khailyk.

Materials and Methods
We have examined numerous PGM grains and micrometric inclusions found in heavy-mineral concentrates collected at thirteen prospecting dug pits at a smooth bend along the riverbed of Bolshoy Khailyk, within the mining concession outlined (Figure 1).The concentrates are invariably enriched in grains of chromian spinel (~15-75 vol %), magnetite (up to ~30%), clinopyroxene and amphiboles (up to ~20% each), with subordinate amounts (<5%) of epidote, zoisite, and grossular-rich garnet.Our aims in the present article are to document the occurrences and mineralogical characteristics of assemblages of PGM and PGE-Au-rich phases in the alluvial placer associated with the River Bolshoy Khailyk (Figure 1), a tributary of the Urbun (Urgun) River, which flows into the River Yenisei.Our results and observations provide genetic implications and insights into the mineral-forming environments and trends of crystallization of the ophiolite-related PGE-Au mineralization at Bolshoy Khailyk.
Among the detrital grains of PGM, the Os-Ir-Ru alloy minerals strongly predominate (~80 vol %).Grains of these alloys vary in size from <0.5 to ~2 mm across.Commonly, they display a well-preserved hexagonal outline (Figure 2) and are composed of intergrowths of crystals of varying compositions (e.g., Figure 3b,f).Variously rounded grains also are present, however (Figure 3e).Among the detrital grains of PGM, the Os-Ir-Ru alloy minerals strongly predominate (~80 vol %).Grains of these alloys vary in size from <0.5 to ~2 mm across.Commonly, they display a well-preserved hexagonal outline (Figure 2) and are composed of intergrowths of crystals of varying compositions (e.g., Figure 3b,f).Variously rounded grains also are present, however (Figure 3e).Among the detrital grains of PGM, the Os-Ir-Ru alloy minerals strongly predominate (~80 vol %).Grains of these alloys vary in size from <0.5 to ~2 mm across.Commonly, they display a well-preserved hexagonal outline (Figure 2) and are composed of intergrowths of crystals of varying compositions (e.g., Figure 3b,f).Variously rounded grains also are present, however (Figure 3e).Grains of Pt-Fe alloy with a Pt 3 Fe-type stoichiometry (i.e., isoferroplatinum or ferroan platinum) account for up to 20 vol % in the samples examined; the prefix "ferroan" is used here to conform to the historically used name, without implication as to the valence of iron.The size of the Pt-Fe alloy grains, 2.5-3 mm across, generally exceeds that of the associated Os-Ir-Ru alloys.Commonly, these grains are anhedral.Nevertheless, subhedral to euhedral grains of Pt-Fe alloy are also encountered occasionally.The grain shown in Figure 3d has a perfectly developed rhombohedral cleavage, with rhombic patterns or voids.Lamellae of Ru-dominant alloy are hosted by the Pt-Fe alloy matrix (Figure 4b); this alloy phase of Ru can also display a rhombic shape, which conforms to the observed cleavage (Figure 5a), and is likely pseudomorphically formed as a result of late replacement of the Pt 3 Fe host.Grains of Pt-Fe alloy with a Pt3Fe-type stoichiometry (i.e., isoferroplatinum or ferroan platinum) account for up to 20 vol % in the samples examined; the prefix "ferroan" is used here to conform to the historically used name, without implication as to the valence of iron.The size of the Pt-Fe alloy grains, 2.5-3 mm across, generally exceeds that of the associated Os-Ir-Ru alloys.Commonly, these grains are anhedral.Nevertheless, subhedral to euhedral grains of Pt-Fe alloy are also encountered occasionally.The grain shown in Figure 3d has a perfectly developed rhombohedral cleavage, with rhombic patterns or voids.Lamellae of Ru-dominant alloy are hosted by the Pt-Fe alloy matrix (Figure 4b); this alloy phase of Ru can also display a rhombic shape, which conforms to the observed cleavage (Figure 5a), and is likely pseudomorphically formed as a result of late replacement of the Pt3Fe host.Grains of Pt-Fe alloy with a Pt3Fe-type stoichiometry (i.e., isoferroplatinum or ferroan platinum) account for up to 20 vol % in the samples examined; the prefix "ferroan" is used here to conform to the historically used name, without implication as to the valence of iron.The size of the Pt-Fe alloy grains, 2.5-3 mm across, generally exceeds that of the associated Os-Ir-Ru alloys.Commonly, these grains are anhedral.Nevertheless, subhedral to euhedral grains of Pt-Fe alloy are also encountered occasionally.The grain shown in Figure 3d has a perfectly developed rhombohedral cleavage, with rhombic patterns or voids.Lamellae of Ru-dominant alloy are hosted by the Pt-Fe alloy matrix (Figure 4b); this alloy phase of Ru can also display a rhombic shape, which conforms to the observed cleavage (Figure 5a), and is likely pseudomorphically formed as a result of late replacement of the Pt3Fe host.Many of placer grains of Os-Ir-Ru and Pt-Fe alloys have a porous (Figure 3e), abundantly fractured or altered rim (Figure 6b,c), associated with the development of secondary phases of PGE sulfide, arsenide, sulfarsenide, native gold and a Pt-Ir-Ni-rich alloy.Such zones of metasomatic alteration (labeled AZ: Figures 3c and 5b) are developed close to grain margins; they are isolated and surrounded by an outer rim of sperrylite.The rare compounds formed in these zones include Cu-rich bowieite and a Se-rich sulfarsenide of Pt (Figure 5b).Many of placer grains of Os-Ir-Ru and Pt-Fe alloys have a porous (Figure 3e), abundantly fractured or altered rim (Figure 6b,c), associated with the development of secondary phases of PGE sulfide, arsenide, sulfarsenide, native gold and a Pt-Ir-Ni-rich alloy.Such zones of metasomatic alteration (labeled AZ: Figures 3c and 5b) are developed close to grain margins; they are isolated and surrounded by an outer rim of sperrylite.The rare compounds formed in these zones include Cu-rich bowieite and a Se-rich sulfarsenide of Pt (Figure 5b).In addition, our materials contain micrometric inclusions of clinopyroxene, chromian spinel, amphiboles, serpentine and base-metal sulfides.We also examined compositional variations in one unusual grain ~2 mm across, composed of the phase PtAu4Cu5 with inclusions of other intermetallic compounds of the system Pt-Cu-Au, of an unnamed Pt-Cu stannide and a Co-Rh-rich pentlandite (Figure 7a,b).In addition, our materials contain micrometric inclusions of clinopyroxene, chromian spinel, amphiboles, serpentine and base-metal sulfides.We also examined compositional variations in one unusual grain ~2 mm across, composed of the phase PtAu 4 Cu 5 with inclusions of other intermetallic compounds of the system Pt-Cu-Au, of an unnamed Pt-Cu stannide and a Co-Rh-rich pentlandite (Figure 7a,b).In this study, we used essentially the same approach and analytical facilities as in the complementary project, devoted also to associations of PGM of the Sayan region [8].Compositions of various PGE alloys, PGM, PGE-and Au-rich phases, silicate minerals and hydrous silicates were investigated with wavelength-dispersive analysis (WDS) using a Camebax-micro electron microprobe (CAMECA SAS, Gennevilliers Cedex, France) at the Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Novosibirsk, Russia.The analytical conditions used for PGE-rich minerals were the following: 20 kV and 60 nA; the Lα line was used for Ir, Rh, Ru, Pt, Pd, and As; the Mα line was used for Os and Au, and the Kα line was used for S, Fe, Ni, Cu, and Co.We used as standards pure metals (for the PGE and Au), CuFeS2 (for Fe, Cu, and S), synthetic FeNiCo (for Ni and Co), and arsenopyrite (for As).The minimum detection limit is ≤0.1 wt % for results of the WDS analyses.The WDS analyses of chromite, clinopyroxene, and amphiboles were acquired at 20 kV and 40 nA, using Kα lines, and the following standards: chromite (for Fe, Mg, Al, and Cr), ilmenite (Ti), manganiferous garnet (for Mn), and synthetic V2O5 (for V).The amphibole analyses were done using diopside (for Ca), albite (for Na), orthoclase (for K), pyrope (Mg, Fe, Al, and Si), a glass of diopside composition doped with 2 wt % TiO2 for Ti, and manganiferous and chromiferous garnets (for Mn and Cr) as standards.For serpentine, we used an olivine standard (Mg, Si, Fe, and Ni), as well as diopside (Ca), and, as noted, garnets for Mn and Cr.
We employed scanning-electron microscopy (SEM) and energy-dispersive analysis (EDS) for the analysis of phases whose grain size is on the order of ≤2-5 μm, or larger.These phases were analyzed at 20 kV and 1.2 nA using a Tescan Vega 3 SBH facility combined with an Oxford X-Act spectrometer (Oxford Instruments, Abingdon, UK) at the Siberian Federal University, Krasnoyarsk, Russia.Pure elements (for the PGE, Fe, Cu), as well as FeS2 (for S), InAs (for As), were used as standards.The Lα line was used for As and the PGE, except for Pt and Au (Mα line); the Kα line was used for Fe, Cu, Ni, Co, and S.

3.1.Grains of Os-Ir-Ru, Pt-Fe Alloys and Exsolution-Induced Phases
As noted, the Os-Ir-Ru and Pt-Fe alloy minerals account for ~80% and ~20 vol %, respectively, of the detrital PGM.In contrast to the Os-Ir-Ru alloys at Sisim, eastern Sayans [8], we detect no zonation in these grains of alloy at Bolshoy Khailyk.However, there are significant grain-to-grain variations (Table 1), reflected in differences in contrast in back-scattered electron (BSE) images of mutually intergrown grains (Figure 3f).Inclusions of Pt-Fe alloy are present in an Os-Ir-Ru alloy matrix, and a Ru-Ir-Os alloy occurs as lamellae hosted in a Pt-Fe alloy (Figures 4a,b and 5a).In this study, we used essentially the same approach and analytical facilities as in the complementary project, devoted also to associations of PGM of the Sayan region [8].Compositions of various PGE alloys, PGM, PGE-and Au-rich phases, silicate minerals and hydrous silicates were investigated with wavelength-dispersive analysis (WDS) using a Camebax-micro electron microprobe (CAMECA SAS, Gennevilliers Cedex, France) at the Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Novosibirsk, Russia.The analytical conditions used for PGE-rich minerals were the following: 20 kV and 60 nA; the Lα line was used for Ir, Rh, Ru, Pt, Pd, and As; the Mα line was used for Os and Au, and the Kα line was used for S, Fe, Ni, Cu, and Co.We used as standards pure metals (for the PGE and Au), CuFeS 2 (for Fe, Cu, and S), synthetic FeNiCo (for Ni and Co), and arsenopyrite (for As).The minimum detection limit is ≤0.1 wt % for results of the WDS analyses.The WDS analyses of chromite, clinopyroxene, and amphiboles were acquired at 20 kV and 40 nA, using Kα lines, and the following standards: chromite (for Fe, Mg, Al, and Cr), ilmenite (Ti), manganiferous garnet (for Mn), and synthetic V 2 O 5 (for V).The amphibole analyses were done using diopside (for Ca), albite (for Na), orthoclase (for K), pyrope (Mg, Fe, Al, and Si), a glass of diopside composition doped with 2 wt % TiO 2 for Ti, and manganiferous and chromiferous garnets (for Mn and Cr) as standards.For serpentine, we used an olivine standard (Mg, Si, Fe, and Ni), as well as diopside (Ca), and, as noted, garnets for Mn and Cr.
We employed scanning-electron microscopy (SEM) and energy-dispersive analysis (EDS) for the analysis of phases whose grain size is on the order of ≤2-5 µm, or larger.These phases were analyzed at 20 kV and 1.2 nA using a Tescan Vega 3 SBH facility combined with an Oxford X-Act spectrometer (Oxford Instruments, Abingdon, UK) at the Siberian Federal University, Krasnoyarsk, Russia.Pure elements (for the PGE, Fe, Cu), as well as FeS 2 (for S), InAs (for As), were used as standards.The Lα line was used for As and the PGE, except for Pt and Au (Mα line); the Kα line was used for Fe, Cu, Ni, Co, and S.

Grains of Os-Ir-Ru, Pt-Fe Alloys and Exsolution-Induced Phases
As noted, the Os-Ir-Ru and Pt-Fe alloy minerals account for ~80% and ~20 vol %, respectively, of the detrital PGM.In contrast to the Os-Ir-Ru alloys at Sisim, eastern Sayans [8], we detect no zonation in these grains of alloy at Bolshoy Khailyk.However, there are significant grain-to-grain variations (Table 1), reflected in differences in contrast in back-scattered electron (BSE) images of mutually intergrown grains (Figure 3f).Inclusions of Pt-Fe alloy are present in an Os-Ir-Ru alloy matrix, and a Ru-Ir-Os alloy occurs as lamellae hosted in a Pt-Fe alloy (Figure 4a,b and Figure 5a).Extremely narrow and linear "lamellae" of Os-Ir-Ru alloy cut the associated inclusions of Pt-Fe alloy and occupy the cleavage plane of some of the Ir-Os-Ru placer grains, thus implying subsolvus conditions of their deposition.Some Ru-dominant phases exhibit a rhombohedral shape (Figure 5a); these are likely pseudomorphic and reproduce the characteristic forms of the rhombohedral cleavage in the matrix of Pt 3 Fe-type phase.A wire-like inclusion of Ir-Os alloy is noteworthy (Figure 6d).
A strong enrichment in Ru is detected in lamellar phases, e.g., those in Figure 4b  Alloy phases of the tulameenite-ferronickelplatinum series (≤0.1 mm across) occur as inclusions in the matrix of a ternary Pt-Au-Cu phase (Figure 7a), and show strong variations of composition owing to Ni-for-Cu substitution (Table 2).An intermediate member contains ~0.5 Ni atoms per formula unit (hereafter a.p.f.u.) in this series.In addition, partly subhedral inclusions (≤20 µm in size) of a ferronickelplatinum-type alloy (#1-3, Table 3) were observed as inclusions in grains of Os-Ir-Ru alloy.We document the existence at Bolshoy Khailyk of extensive fields of solid solution in the system Os-Ru-Ir (Figure 8).The general enrichment in Ru is recognized in the coexisting pairs of matrix and exsolution-induced inclusions.As shown for comparison in Figure 8, the Os-Ir-Ru alloy grains from the neighboring placer at the River Zolotaya [5] also display the characteristic pattern of enrichment in Ru.We document the existence at Bolshoy Khailyk of extensive fields of solid solution in the system Os-Ru-Ir (Figure 8).The general enrichment in Ru is recognized in the coexisting pairs of matrix and exsolution-induced inclusions.As shown for comparison in Figure 8, the Os-Ir-Ru alloy grains from the neighboring placer at the River Zolotaya [5] also display the characteristic pattern of enrichment in Ru.

Fe-Ni-Enriched Rims and Unnamed Alloys of the Series (Pt,Ir)(Ni,Fe,Cu)3-x-(Ir,Pt)(Ni,Fe,Cu)3-x
The porous and fractured rim on grains of Ir-Os-Ru alloy (Figure 3e) is notably enriched in Fe and Ni relative to the adjacent phases analyzed beyond the rim.For example, the rim shown in Figure 6b 3).These alloy phases are Ni-dominant and contain essential Pt, which could be important to stabilize the structure.Indeed, the IrNi3 phase has not been encountered in synthesis experiments, whereas the ordered phase PtNi3 crystallizes at 580 °C in the system Pt-Ni [10].The porous and fractured rim on grains of Ir-Os-Ru alloy (Figure 3e) is notably enriched in Fe and Ni relative to the adjacent phases analyzed beyond the rim.For example, the rim shown in Figure 6b 3).These alloy phases are Ni-dominant and contain essential Pt, which could be important to stabilize the structure.Indeed, the IrNi 3 phase has not been encountered in synthesis experiments, whereas the ordered phase PtNi 3 crystallizes at 580 • C in the system Pt-Ni [10].Its structure remains unknown; it is Ni-dominant and, thus, probably corresponds to garutiite (Ni,Fe,Ir), the hexagonal polymorph of native nickel cf.[11], or to its cubic polymorph, nickel.This phase is devoid of Pt, and differs from alloys of the series (Pt,Ir)(Ni,Fe,Cu) 3-x -(Ir,Pt)(Ni,Fe,Cu) 3-x by a lower value of (Ni + Fe)/PGE (1.5).Nevertheless, we cannot exclude that it belongs to the latter solid solution.
Micrometric and droplet-shaped inclusions of base-metal sulfides ≤10 µm in diameter occur close to margin in this grain of Ir-dominant alloy.Three types of phases are recognized on the basis of their compositions.The first is a monosulfide-type phase that contains: Ni 24.6, Fe 24.1, Cu 12.8, S 35.2, total 96.7 wt %, corresponding to the formula (Fe 0.40 Ni 0.39 Cu 0.19 ) Σ0.98 S 1.02 calculated for a total of 2 a.p.f.u.A bornite-like phase is with the composition: Cu 52.1, Fe 16.5, S 28.8, total 97.4 wt %; its formula is (Cu 4.06 Fe 1.47 ) Σ5.5 S 4.5 (for 10 a.p.f.u.).And the third is represented by a godlevskite-like phase: Ni 70.2, S 30.2, total 100.4 wt %, or Ni 9.5 S 7.5 (for 17 a.p.f.u.).

PGE Sulfides, Unnamed Ni[Ir(Co,Cu,Fe)] 2 S 4 , and Sperrylite
We analyzed several species of PGE-based sulfides found as inclusions (up to ~50 µm) hosted by grains of PGE alloys or as components of the rim or overgrowth on these grains.Results of analyses of ten grains (WDS) indicate that members of the laurite-erlichmanite series are relatively poor in Ir (up to ~10-15 wt %, Table 4).The exceptions are minute grains of Ir-rich laurite (≤5 µm across) deposited within a narrow rim that is developed around a grain of Ir-Os alloy; these are associated with members of the tolovkite-irarsite series and native gold [Au 74.1-91.1 Ag 0.9-19.5 Cu 6.4-8.1 ].Note that the Au-rich alloy is enriched in Cu, and its composition ranges up to high-purity gold.The Ir-rich laurite is devoid of Os; it has the following composition: Ru 46.1, Ir 19.7, S 35.5, total 101.3 wt %, corresponding to (Ru 0.82 Ir 0.18 ) Σ1.00 S 1.99 .The phase could possibly imply conditions of metastable crystallization at a low temperature as a representative of a late-stage assemblage at the rim.Compositions of five grains of cooperite are close to being stoichiometric (#3, 4, Table 4).A grain of bowieite in the zone of deuteric alteration (Figure 5b) is anomalously rich in Cu (#5, Table 4).Note.Numbers 1-4 pertain to results of WDS analyses, and #5-9 are SEM/EDS; "bdl" indicates that amounts of elements are below detection limits.The total of an.#6 includes 4.05 wt % Sb, which corresponds to 0.15 Sb a.p.f.u.The atomic proportions were calculated on the basis of a total of 3 a.p.f.u.(for laurite), 2 a.p.f.u.(cooperite), 5 a.p.f.u.(bowieite), and 7 a.p.f.u. for the unnamed phase related to thiospinels (UN Tsp).
We document the existence of an unknown sulfide that presumably is related to thiospinels, as it displays a Me 3 S 4 stoichiometry, cf.[14], where Me represents the total content of metals (i.e., Ir, Co, Fe and Cu).This phase occurs in peripheral portions of irregular grains (≤50 µm), enclosed within a grain of Ir-Os alloy.Its composition corresponds to the formula Ni[Ir(Co,Cu,Fe)] 2 S 4 (#6-9, Table 4).It seems likely that this phase is related to the synthetic thiospinel NiIr 2 S 4 [15]; one of the two Ir atoms is presumably replaced by Co at Bolshoy Khailyk.The mixed-valence character of Ir is recognized in PGE thiospinels ( [14] and references therein).
A total of 17 data-points (WDS) done on representative grains and inclusions of sperrylite yield normal levels of incorporation of Ir and Rh, up to ~3 wt % each, and up to 1.8 wt % S.

Zones of Deuteric Alteration
Zones of deuteric alteration (AZ: Figure 3c

Zoned Oxide of PGE-Fe
Herein we describe an unusual grain of zoned oxide (or oxides), which consists of a core-like zone (marked with the letter C: Figure 6a), a rim-like zone (R) and a veinlet (V); these phases are rich in Ru, and, to a lesser extent, in Os and Ir, with substantial Fe and minor V (Table 6).The following observations are noteworthy: (1) The compositions are generally consistent with a simplified formula Ru6Fe 3+ 2O15, in which the atomic Me:O ratio is close to 1:2; this grain could have arisen by

Zoned Oxide of PGE-Fe
Herein we describe an unusual grain of zoned oxide (or oxides), which consists of a core-like zone (marked with the letter C: Figure 6a), a rim-like zone (R) and a veinlet (V); these phases are rich in Ru, and, to a lesser extent, in Os and Ir, with substantial Fe and minor V (Table 6).The following observations are noteworthy: (1) The compositions are generally consistent with a simplified formula Ru 6 Fe 3+ 2 O 15 , in which the atomic Me:O ratio is close to 1:2; this grain could have arisen by desulfurization and oxidation of laurite (zoned), in which the Me:S proportion also is 1:2.If so, Fe and V were introduced externally via the oxidizing fluid.(2) Compositional variations (Table 6) indicate that levels of Os and Ir decreased with a relative increase in Fe and V from core to rim.
(3) A further change is expressed inward from the rim, with a decrease in the total PGE with increase in contents of Fe and V; the low total suggests that the veinlet phase likely contains some water or a hydroxyl component.(4) The cavity observed in the center (Figure 6a) could represent the channelway of the oxidizing fluid.In various settings worldwide, grains of PGE-based oxides typically display similar features, e.g., [20][21][22][23].These compounds remain poorly understood; in most occurrences, they could well represent cryptic mixtures rather than be single phases.Note that no bonding likely exists between platinum and oxygen in a Pt-Fe oxide grain studied by X-ray absorption spectroscopy [24].The Pd-based oxide grains appear to be more diverse in their compositions than grains of Ru-Os-Ir-rich oxides.Indeed, the Pd-rich grains contain a variety of elements, e.g., Sb, Bi, Pb, and Tl [25][26][27], which could well reflect diverse precursor grains.

Pt-Au-Cu Alloys and Unnamed (Pt,Pd) 3 Cu 2 Sn
Four groups of clustered or individual data-points of Pt-Au-Cu alloys are recognized at Bolshoy Khailyk (#1 to 4 in Figure 10) (Table 7).Field #1 pertains to the phase PtAu 4 Cu 5 [or Cu(Au,Pt)], the main portion of this grain (Figure 7a).The atomic proportions observed coincide with those of related intermetallic compounds reported from the Tulameen complex, Canada [28], an alluvial deposit at Sotajarvi, Finland [29], cf.[30], from the River Zolotaya [5], and from the Kondyor complex of Russia [31].Note that the Pt-Au-Cu phases reported from Kondyor form a series of compositions toward CuAu, i.e., tetra-auricupride, first discovered in China [32].Thus, the analyzed phase at Bolshoy Khailyk could represent a member in the solid-solution series of platiniferous tetra-auricupride, i.e., Cu(Au,Pt).
The compositional field #2 is clustered near the stoichiometry PtAuCu 2 (Figure 10).The composition #3 plots near PtCu, i.e., hongshiite [33].Note that the auriferous hongshiite (Figure 10) is fairly close in composition to the phase reported from an alluvial occurrence at the River Durance, France [30].The composition #4 plots close to PtAu, which is known from synthesis in nanometer-sized particles [34].Field #5 likely reflects a subordinate extent of solid solution of Au in a Cu-bearing platinum alloy (Figure 10, Table 7).The likely trend of crystallization of the Pt-Au-Cu alloys is shown in Figure 10.In addition, inclusions of highly pure platinum (100.8 wt % Pt; EMP data) were encountered in this grain of the Pt-Au-Cu alloy.
An unnamed stannide of Pt and Cu with a subordinate content of Pd occurs as small inclusions (≤10 µm across) in the matrix of the Pt-Au-Cu alloy.Compositional data (#11-13, Table 7) indicate a (Pt + Pd + Cu):Sn proportion of 5:1, which points to an unnamed species of PGM.In the system Pd 3 Sn-Cu 3 Sn [35], several ternary phases are known to form via solid-state transformations at low temperatures ≤550 • C, with a general increase in Cu with a drop in temperature in the system, e.g., Pd 5 CuSn 2 [or (Pd 2.5 Cu 0.5 ) Σ3.0 Sn], Pd 9 Cu 3 Sn 4 [Pd 2.25 Cu 0.75 ) Σ3.0 Sn] and Pd 2 CuSn, which are likely the synthetic equivalents of "stannopalladinite", taimyrite and cabriite [36], respectively.The Pt-for-Pd substitution is known in the taimyrite series, the Pt-dominant analogue of which is tatyanaite [37,38].related intermetallic compounds reported from the Tulameen complex, Canada [28], an alluvial deposit at Sotajarvi, Finland [29], cf.[30], from the River Zolotaya [5], and from the Kondyor complex of Russia [31].Note that the Pt-Au-Cu phases reported from Kondyor form a series of compositions toward CuAu, i.e., tetra-auricupride, first discovered in China [32].Thus, the analyzed phase at Bolshoy Khailyk could represent a member in the solid-solution series of platiniferous tetra-auricupride, i.e., Cu(Au,Pt).For comparison, compositions of Cu-Pt-Au phases are plotted from the Zolotaya River placer, western Sayans [5], the Tulameen complex, Canada [28], the Sotajarvi area, Finland [29], the Durance River area, France [30], and the Kondyor complex, Russian Far East [31].The numbers 1 to 5, shown on this plot, pertain to compositional groups discussed in the text.For comparison, compositions of Cu-Pt-Au phases are plotted from the Zolotaya River placer, western Sayans [5], the Tulameen complex, Canada [28], the Sotajarvi area, Finland [29], the Durance River area, France [30], and the Kondyor complex, Russian Far East [31].The numbers 1 to 5, shown on this plot, pertain to compositional groups discussed in the text.

Rh-Co-Rich Pentlandite
It is known that pentlandite incorporates essential Rh or Co (e.g., [8,[39][40][41][42][43]).In contrast to other occurrences, compositions of pentlandite at Bolshoy Khailyk display covariations of Co and Rh, both present in significant amounts in solid solution (Table 8).Thus, our results provide useful implications to establish a likely mechanism of the Rh incorporation.A total of 19 grains were analyzed, all of them subhedral to anhedral inclusions (≤20 µm across) hosted by the Pt-Au-Cu alloy (Figure 7a,b).The pair Co-Fe reveals a strongly negative correlation, whereas the pair Co-Ni is uncorrelated (Figure 11a,b).Interestingly, contents of Co display a significant and sympathetic correlation with Rh, a coefficient of correlation R of 0.8 (Figure 11c).The pair Fe-Rh is negatively correlated (R = −0.8).There are no definite correlations involving Ni with Rh or Fe (Figure 11e,f).

Micrometer-Sized Inclusions in Grains of PGE Alloys
In general, placer grains of Os-Ir-Ru alloys contain inclusions more commonly than the Pt-Fe alloys.The analyzed inclusions of diopside (Table 9) are highly magnesian [Wo48.5Fs2.6Ae0.4-0.7;Mg# 96.9-97.9].Inclusions of chromian spinel correspond to magnesiochromite; these are also highly magnesian, with values of Mg# up to 71.In addition, the chromian spinel displays unusually high values of Cr# (77-83.5:Table 9).Some grains (Figure 4d) possibly represent broken fragments of subhedral spinel from the lode rock.

Micrometer-Sized Inclusions in Grains of PGE Alloys
In general, placer grains of Os-Ir-Ru alloys contain inclusions more commonly than the Pt-Fe alloys.
The analyzed inclusions of diopside (Table 9) are highly magnesian [Wo 48.3-48.6En 48.4-48.5 Fs 2.6 Ae 0.4-0.7 ; Mg# 96.9-97.9].Inclusions of chromian spinel correspond to magnesiochromite; these are also highly magnesian, with values of Mg# up to 71.In addition, the chromian spinel displays unusually high values of Cr# (77-83.5:Table 9).Some grains (Figure 4d) possibly represent broken fragments of subhedral spinel from the lode rock.The amphibole inclusions are relatively abundant; they correspond to actinolite, magnesio-hornblende, and barroisite.Some of the inclusions of amphibole have Mg-rich compositions, with values of Mg# up to 93 (Table 9).Some other inclusions crystallized from much more fractionated silicate melts.Among these are grains of nearly end-member albite (Ab >90 ), hosted by a placer grain of Ru-dominant alloy [Ru 56.1 Os 39.8 Ir 4.1 ], and inclusions of quartz (up to ~50 µm) hosted by a grain of a ternary alloy, Ir 35.5 Os 32.6 Ru 31.9 .Euhedral grains of quartz were noted in inclusions hosted by Ir-dominant alloy from a placer deposit in British Columbia, Canada [44].Plagioclase inclusions [Ab 58 An 42 Or 0.6 ] were found in the Pt-Au-Cu alloy grain at Bolshoy Khailyk.Serpentine (Table 9) and calcite are locally present in inclusions.A micrometric inclusion of grossular(?), with ~0.5% Cr 2 O 3 and 3.7% MgO, displays a notable departure from normal stoichiometry, and likely is not single-phase.Compositions of five inclusions of magnetite (Figure 7b; WDS data) are relatively pure, with up to 0.52% MgO, 0.30% Cr 2 O 3 , and 0.31 wt % MnO.

Provenance and Genetic Implications
We have pointed out the close association of the placer deposit with abundant bodies of ophiolitic serpentinite (Figure 1).In addition, we document a high extent of Ru-enrichment in the association of PGM at Bolshoy Khailyk.These characteristics clearly point to an ophiolitic source of the placer PGM, which is presumably related to the Aktovrakskiy complex, belonging to the Kurtushibinskiy belt of the western Sayans.The faceted morphology of the placer grains (Figure 3b,d,f) is consistent with a short distance of transport from the source rocks.The Aktovrakskiy complex was previously inferred to be the primary source for the associated PGM placers at the Zolotaya River, western Sayans [5].We thus contend that this suite and related complexes of potentially mineralized ophiolites are the targeted PGM-producing source in other areas of the Altai-Sayan folded region.
We document the very high extents of Mg-enrichment in the inclusions of clinopyroxene (Mg# 96.9-97.9)and chromian spinel (Mg# 71) hosted by PGE alloys.In addition, serpentine is present, indicating the former presence of olivine in the system.Some of the amphibole inclusions also are highly magnesian (Mg# > 90), consistent with their crystallization in a primitive ultramafic rock.Therefore, the bulk of the investigated Os-Ir-Ru and Pt-Fe alloy minerals (with abundant exsolution of a Ru-dominant alloy) seem to have crystallized in a high-Mg chromitite or in a dunitic rock rich in magnesiochromite, with a subordinate role of pyroxenitic rocks inferred on the basis of the presence of quartz inclusions hosted by the PGE alloy.
We infer that subhedral grains of Os-Ir-Ru alloys crystallized first among the PGM and after the associated Mg-rich magnesiochromite and olivine, followed by subhedral grains of the isoferroplatinum-type alloy (Figure 3b,f); lamellar phases of Ru-dominant alloy (Figures 3b and 5a) formed largely by exsolution from the matrix phase upon cooling.They reflect a general enrichment in Ru existing in the lode ophiolite, and imply accumulation of levels of Ru during progressive crystallization of the Pt-Fe alloy.The presence of inclusions of amphiboles with lower values of Mg# (73-78; Table 9), and, especially, of Ab-rich plagioclase and quartz, imply that high degrees of fractionation were locally achieved in the ore-forming system.
The inferred incompatible behavior of minor constituents like S, As, Sb, Se, Sn, Au and base-metals (Cu, Ni, Co, Fe) was accompanied during the crystallization of Os-Ir-Ru-Pt alloys by a gradual buildup in the fugacity of S 2 and As 2 .Consequently, a variety of sulfide, arsenide, antimonide, and intermetallic phases crystallized at this point, at a lower temperature, possibly at the expense of droplets of residual melt as micrometric inclusions, or as phases coexisting in altered zones near the rims.These implications are consistent with observations made at other placer deposits e.g., [8,18], and references therein.In addition, the porous texture of the Fe-enriched rims (Figure 3e) may reflect the abundance of volatile species; they are similar to rims developed in grains of Ir-Ru-Os alloys from a placer associated with the Trinity ophiolite complex in California [45,46].
The zones of metasomatic modification (Figures 3c and 5b) formed, presumably, as a consequence of (1) the expected buildup in levels of f S 2 and f As 2 , (2) an accumulation of levels of incompatible constituents, such as Cu and Se, in the isolated portions at the margins during the crystallization of the Pt-Fe alloy (the external rim of sperrylite is developed around the grain), and (3) the late deposition of the S-As-Se-bearing assemblage as a result of subsolidus reactions and deuteric alteration of the early formed Pt-Fe alloy phase.

PGE-Ni-Fe-Cu Alloy, PGE-Fe Oxide and Potential Mineral-Forming Reactions
Corresponding redox reactions likely proceeded at a low temperature and led to the deposition of the alloys of (Pt,Ir)(Ni,Fe,Cu) 3-x type and PGE oxide (Figure 6a-c).These alloys occur as veinlets developed in porous and fractured zones at margins of placer grains.We suggest that they crystallized under reducing conditions at the expense of pre-existing pentlandite via a schematic reaction: [(Ni,Fe) 9 S 8 + 3Pt + 8H 2 → 3Pt(Ni,Fe) 3 + 8H 2 S].The Pt component was likely introduced and remobilized by a volatile phase from the associated PGE alloy.The other reactant, hydrogen, was likely a product of the serpentinization of olivine upon oxidation of Fe 2+ in primary silicates to Fe 3+ by a reaction involving H 2 O ( [47], and references therein).
The zoned grain of PGE-Fe oxide could form by combined oxidation and desulfurization of zoned grains of Ru-Os disulfide of the laurite-erlichmanite series.As noted, the observed zonal texture (Figure 6a) is likely a reflection of primary compositional zoning with respect to Ru-Os-(Ir), which is common in grains of laurite-erlichmanite worldwide.The likely schematic reaction at Bolshoy Khailyk is: [6RuS 2 + Fe 2 O 3 + 18O 2 → Ru 6 Fe 2 O 15 + 12SO 2 ], which involves a hematite component introduced by an oxidizing fluid phase.

Origin of Au-(Cu-Pt)-Rich Mineralization
The occurrence of Au-(Cu-Pt) alloys, i.e., PtAu 4 Cu 5 , PtAuCu 2 , Pt(Cu,Au), PtAu, and Au-bearing platinum alloy (Figure 10) appears to be related genetically with the Os-Ir-Ru-Pt mineralization at Bolshoy Khailyk.Thus, the inferred ophiolitic system was likely able to produce locally a high extent of Au-enrichment as a result of geochemical evolution and deposition from a fractionated Au-Cu-rich melt, which likely gained high levels of incompatible Cu + Au with subordinate Pt in a remaining volume during the bulk crystallization of Os-Ir-Ru alloy phases, after the preceding crystallization of chromian spinel (magnesiochromite) and olivine.The latter are presumably the main minerals in the inferred lode rocks (chromitite or magnesiochromite-rich dunite).As noted, PGE alloy grains hosting the quartz inclusions imply a pyroxenitic rock, which could be relevant to some other PGM grains containing inclusions of Ab-rich plagioclase and Fe-rich amphibole.
This suggestion is corroborated by the occurrence of an elevated Au content (up to 7.5 wt %) in the Fe-bearing Ir-Ru-Os alloy phases found associated with the Trinity ophiolite complex, California [46].In addition, an Au-Ag alloy, ranging up to almost pure gold (Au 99 ), precipitated pseudomorphously by a subsolidus reaction between a fluid phase or a residual Au-Ag-rich melt and exsolution-induced inclusions of the Pt-Fe alloy phases [46].Also, a residual melt enriched in Au likely appeared during the progressive crystallization of Pt-Fe alloy, leading to the formation of a rim of gold (Au 80 ) around placer grains of Pt-Fe alloy at Florence Creek, Yukon, Canada [48].We presume that, at Bolshoy Khailyk, this mechanism was likely significant to produce the Au-rich phase precipitated as a narrow rim (Au 75 ) on the Pt-Fe alloy grain (Figure 6d).Note that the gold phase is deposited within the inner portion of the composite rim, being thus separated by the outer rim of cooperite (Figure 6d).Therefore, the gold alloy phase could not be formed by a surficial process in the placer environment.
In addition, the close association observed between Au and PGE is not unusual (e.g., in the Norilsk deposits, Russia; [37,38]; and references therein), because they are geochemically related and belong to the same group of highly siderophile elements, e.g., [49].

Mechanism of Incorporation of Rh and Co into the Structure of Pentlandite
Occurrences of Rh-Co-enriched grains of pentlandite hosted by the Pt-Au-Cu alloy (Figure 7b) have important crystallochemical implications.We infer, based on our results (Figure 11, Table 8), that Rh and Co display a strong sympathetic relationship.They mutually substitute for Fe, not Ni, and are

Figure 1 .
Figure 1.Regional geology of the Bolshoy Khailyk area (a) based on [6], with minor modifications, and a map (b) showing the location of the placer area in the Russian Federation.The red square (a) shows the sampling area (this study).

Figure 1 .
Figure 1.Regional geology of the Bolshoy Khailyk area (a) based on [6], with minor modifications, and a map (b) showing the location of the placer area in the Russian Federation.The red square (a) shows the sampling area (this study).

Figure 2 .
Figure 2. Photo of a concentrate of platinum-group minerals collected in the Bolshoy Khailyk placer deposit.The brownish film present on the surface of some grains represents iron hydroxides.

Figure 3 .
Figure 3. Back-scattered electron (BSE) images showing characteristic textures of placer grains of platinum-group minerals from the Bolshoy Khailyk placer.(a) A partly rounded grain of Ir-Os alloy contains inclusions of Pt-Fe alloy.(b,f) Intergrowths of subhedral crystals of Os-Ir-Ru alloys.(c) A placer grain of Pt-Fe alloy hosting abundant lamellae of Ru-rich alloy (gray) is surrounded by a rim of sperrylite, Spy.Note the existence of a zone of metasomatic alteration (labeled AZ).(d) A subhedral crystal of Pt-Fe alloy displays a perfectly developed rhombohedral cleavage, accompanied by voids.(e) A rounded grain of Os-Ir alloy consists of a rim of Fe-enriched alloy phase of Os-Ir-Fe; note that this rim is very porous and fractured.

Figure 2 .
Figure 2. Photo of a concentrate of platinum-group minerals collected in the Bolshoy Khailyk placer deposit.The brownish film present on the surface of some grains represents iron hydroxides.

Figure 2 .
Figure 2. Photo of a concentrate of platinum-group minerals collected in the Bolshoy Khailyk placer deposit.The brownish film present on the surface of some grains represents iron hydroxides.

Figure 3 .
Figure 3. Back-scattered electron (BSE) images showing characteristic textures of placer grains of platinum-group minerals from the Bolshoy Khailyk placer.(a) A partly rounded grain of Ir-Os alloy contains inclusions of Pt-Fe alloy.(b,f) Intergrowths of subhedral crystals of Os-Ir-Ru alloys.(c) A placer grain of Pt-Fe alloy hosting abundant lamellae of Ru-rich alloy (gray) is surrounded by a rim of sperrylite, Spy.Note the existence of a zone of metasomatic alteration (labeled AZ).(d) A subhedral crystal of Pt-Fe alloy displays a perfectly developed rhombohedral cleavage, accompanied by voids.(e) A rounded grain of Os-Ir alloy consists of a rim of Fe-enriched alloy phase of Os-Ir-Fe; note that this rim is very porous and fractured.

Figure 3 .
Figure 3. Back-scattered electron (BSE) images showing characteristic textures of placer grains of platinum-group minerals from the Bolshoy Khailyk placer.(a) A partly rounded grain of Ir-Os alloy contains inclusions of Pt-Fe alloy.(b,f) Intergrowths of subhedral crystals of Os-Ir-Ru alloys.(c) A placer grain of Pt-Fe alloy hosting abundant lamellae of Ru-rich alloy (gray) is surrounded by a rim of sperrylite, Spy.Note the existence of a zone of metasomatic alteration (labeled AZ).(d) A subhedral crystal of Pt-Fe alloy displays a perfectly developed rhombohedral cleavage, accompanied by voids.(e) A rounded grain of Os-Ir alloy consists of a rim of Fe-enriched alloy phase of Os-Ir-Fe; note that this rim is very porous and fractured.

Figure 4 .
Figure 4. (a) BSE image showing exsolution phases of Pt-Fe alloy hosted by a grain of Ir-Os alloy.Note the presence of an Or-Ir alloy of late generation, cutting the exsolution lamellae of Pt-Fe alloy; this late alloy filled the cleavage space at a subsolidus stage.(b) BSE image of lamellar inclusions of a Ru-dominant alloy, labeled Ru-Ir, which has the composition [Ru 65.6 Ir 12.5 Os 9.6 Rh 6.7 Pt 5.7 ], and is hosted by an isoferroplatinum-type alloy of Pt-Fe: [Pt 63.4 Fe 24.8 Rh 6.0 Cu 3.6 Pd 2.2 ].(c) BSE image of inclusion of serpentine (Srp) hosted by a Pt-Fe alloy, [Pt 68.6 Fe 23.9 Cu 3.7 Pd 2.8 Rh 1.0 ].(d) Inclusion of a chromian spinel, labeled Chr, which corresponds to magnesiochromite, and is hosted by a placer grain of Os-Ir alloy [Os 45.0 Ir 39.7 Ru 15.3 ]; BSE image.

Figure 6 .
Figure 6.(a) BSE image of a zoned grain of Fe-bearing Ru-Os oxide, labeled Ru-Os-O, which is hosted by Os-Ir alloy [Os 45.9 Ir 44.2 Ru 9.9 ].The letter C refers to the point analysis of the core zone, the letter R to that of the rim, and the letter V to the veinlet.(b) BSE image showing a porous and fractured rim (labeled Ir-Os-Fe) relatively enriched in Fe and Ni, that is developed around a placer grain of Ir-Os alloy poor in Fe (0.5 wt % Fe), with the following composition: [Ir 60.3 Os 32.4 Ru 3.8 Rh 1.7 Fe 1.7 Pt 0.1 ].The rim contains 5.3 wt % Fe, 0.88 wt % Ni, and corresponds to [Ir 48.6 Os 29.8 Fe 15.6 Ru 3.4 Ni 2.5 ].(c) BSE image of veinlets composed of unnamed alloys, labeled UN [(Ir,Pt)(Ni,Fe,Cu) 3 -(Pt,Ir)(Ni,Fe,Cu) 3 ], which are hosted by an Ir-Os-Fe alloy rich in Fe (8.0 wt %) and Ni (1.8 wt %) [Ir 48.4 Os 24.7 Fe 22.3 Ni 4.7 ]; the latter alloy phase forms the rim (b).(d) Optical photograph showing the development of a composite rim, composed of cooperite (Cp) [(Pt 0.93 Pd 0.02 )S 1.05 ] and Au-Ag alloy, labeled Au [Au 75.3 Ag 22.5 Cu 2.2 ].Note the presence of a staple-like phase of Ir-Os alloy [Ir 50.7 Os 26.3 Ru 8.5 Fe 6.5 Pt 3.6 Ni 2.9 Rh 1.4 ].These phases occur in the narrow rim around a grain of Pt-Fe alloy [Pt 65.4 Fe 25.5 Rh 3.2 Ir 3.1 Pd 2.5 Os 0.2 Ni 0.1 ].

Figure 7 .
Figure 7. (a) Optical photograph showing inclusions of the tulameenite-ferronickelplatinum series (Tul) and of pentlandite (Pn), which are hosted by a placer grain of Pt-Au-Cu alloy.(b) BSE image of subhedral inclusions of Co-Rh-rich pentlandite (Pn) enclosed within the Pt-Au-Cu alloy.Mag is magnetite.

Figure 7 .
Figure 7. (a) Optical photograph showing inclusions of the tulameenite-ferronickelplatinum series (Tul) and of pentlandite (Pn), which are hosted by a placer grain of Pt-Au-Cu alloy.(b) BSE image of subhedral inclusions of Co-Rh-rich pentlandite (Pn) enclosed within the Pt-Au-Cu alloy.Mag is magnetite.

Figure 8 .
Figure 8. Compositional variations of grains of Os-Ir-Ru alloys from the Bolshoy Khailyk placer (this study), in comparison with alloys from the Zolotaya River deposit, western Sayans [5], in terms of the Os-Ir-Ru diagram (at %).The miscibility gap and nomenclature are on the basis of [9].

Figure 8 .
Figure 8. Compositional variations of grains of Os-Ir-Ru alloys from the Bolshoy Khailyk placer (this study), in comparison with alloys from the Zolotaya River deposit, western Sayans [5], in terms of the Os-Ir-Ru diagram (at %).The miscibility gap and nomenclature are on the basis of [9].

Figure 10 .
Figure 10.Compositions of alloys of Cu, Pt, and Au from the Bolshoy Khailyk placer (this study) on the diagram Cu-Pt-Au (at %).For comparison, compositions of Cu-Pt-Au phases are plotted from the Zolotaya River placer, western Sayans[5], the Tulameen complex, Canada[28], the Sotajarvi area, Finland[29], the Durance River area, France[30], and the Kondyor complex, Russian Far East[31].The numbers 1 to 5, shown on this plot, pertain to compositional groups discussed in the text.

Figure 10 .
Figure 10.Compositions of alloys of Cu, Pt, and Au from the Bolshoy Khailyk placer (this study) on the diagram Cu-Pt-Au (at %).For comparison, compositions of Cu-Pt-Au phases are plotted from the Zolotaya River placer, western Sayans[5], the Tulameen complex, Canada[28], the Sotajarvi area, Finland[29], the Durance River area, France[30], and the Kondyor complex, Russian Far East[31].The numbers 1 to 5, shown on this plot, pertain to compositional groups discussed in the text.

Figure 11 .
Figure 11.Variations and correlations observed in compositions of Rh-Co-rich pentlandite from the Bolshoy Khailyk placer in terms of binary plots Co vs. Fe (a), Co vs. Ni (b), Co vs. Rh (c), Fe vs. Rh (d), Ni vs. Rh (e), and Ni vs. Fe (f), with values expressed in values of atoms per formula unit, a.p.f.u., calculated for a total of 17 a.p.f.u.Values of the correlation coefficient R are shown, along with an equation of linear regression in (a).

Figure 11 .
Figure 11.Variations and correlations observed in compositions of Rh-Co-rich pentlandite from the Bolshoy Khailyk placer in terms of binary plots Co vs. Fe (a), Co vs. Ni (b), Co vs. Rh (c), Fe vs. Rh (d), Ni vs. Rh (e), and Ni vs. Fe (f), with values expressed in values of atoms per formula unit, a.p.f.u., calculated for a total of 17 a.p.f.u.Values of the correlation coefficient R are shown, along with an equation of linear regression in (a).

Table 1 .
Compositions of grains of Ru-Os-Ir alloy minerals from the Bolshoy Khailyk placer deposit.

Table 2 .
Compositions of placer grains of isoferroplatinum and inclusions of minerals of the tulameenite-ferronickelplatinum series hosted by a Pt-Cu-Au alloy grain from Bolshoy Khailyk.

Atomic proportions (per a total of 4 a.p.f.u.)
Note.Numbers 1-10 pertain to results of WDS analyses, #11-20 to EDS analyses.The label Isf means isoferroplatinum-type alloy; Tul-Fnp are members of the tulameenite-ferronickelplatinum series; "bdl" indicates that amounts of elements are below detection limits.

Table 3 .
Compositions of Ni-enriched alloys of Pt-Ir-Ni-Fe from the Bolshoy Khailyk deposit.

Table 4 .
Compositions of various PGE sulfides from the Bolshoy Khailyk deposit.

Table 5 .
Compositions of the tolovkite-irarsite-hollingworthite solid solutions from the Bolshoy Khailyk deposit.Number 1-18 are results of SEM/EDS analyses, and #19-21 are WDS analyses; "bdl" indicates that amounts of elements are below detection limits.

Table 6 .
Compositions of PGE-Fe oxide phases in a zoned inclusion hosted by an Os-Ir-(Ru) alloy grain from the Bolshoy Khailyk placer deposit.
Note. Results of SEM/EDS analyses are quoted in weight %; "bdl" stands for not detected.

Table 7 .
Compositions of Pt-Au-Cu-(Sn) alloys from the Bolshoy Khailyk deposit.Analyses # 1-10 are compositions of Pt-Cu-Au alloy phases, which are listed in the order of increasing Pt content.Analyses #1 and 2 correspond to PtCu 5 Au 4 [or Cu(Au,Pt)].#11-13 pertain to unnamed (Pt,Pd) 3 Cu 2 Sn.An.#2 and #11-13 are results of WDS analyses; #1 and #3-10 are results of SEM/EDS; "bdl" indicates that amounts of elements are below detection limits.The atomic proportions are expressed per a total of 100 at %, except for #11-13 recalculated on the basis of a total of 6 a.p.f.u. Note.

Atomic proportions (per a total of 17 a.p.f.u.)
Number 1-10 are results of WDS analyses.#11-19 are SEM/EDS analyses; "bdl" indicates that amounts of elements are below detection limits. Note.

Table 9 .
Compositions of inclusions of silicate minerals and chromian spinel hosted by placer grains of PGM from the Bolshoy Khailyk placer deposit.