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The maximum value of Mg# [= 100Mg/(Mg + Fe2+ + Mn)] in chromium-bearing spinel-group minerals (Chr) in the Ultrabasic Core Zone (UCZ) of the Lyavaraka orthopyroxenite – harzburgite – dunite complex of the Serpentinite Belt in the Kola Peninsula is 54.5–67.5. Such highly magnesian compositions of spinel are associated with notable enrichments of ferric iron (Fe3+# 58–63). There are two generations of accessory Chr in the UCZ unit. The first generation occurs as inclusions in olivine that is not unusually magnesian (Mg# 90.3), and the second is closely associated with serpentine. The compositional series of Chr at Lyavaraka attains more aluminous compositions than was observed in nearby intrusive bodies. The anomalously high level of Mg in Chr, also manifest in ilmenite, is mainly a result of the high intrinsic fugacity of oxygen attained locally in the melt. A progressive buildup in H2O and increase in fO2 likely resulted from efficient vesiculation and selective loss of H2 from the Al-undepleted komatiitic magma crystallizing in a shallow setting. The chromian spinel forming in such a modified magma is virtually unzoned in Mn, and a minor quantity of Mn is also present in olivine and orthopyroxene. In contrast, zinc is strongly partitioned in the core of Chr, as it is relatively incompatible in the coexisting olivine and orthopyroxene at that stage. Zinc efficiently partitioned into the H2O-enriched melt, which crystallized as the pegmatitic orthopyroxenite near the contacts at Lyavaraka. A high potential of oxidation appears to be characteristic of all orthopyroxenite – harzburgite – dunite suites of the Serpentinite Belt formed from a primitive melt of komatiitic composition.

––We have investigated the compositional variations of apatite (Ap) and rare-earth element (REE) minerals in the Monchepluton layered complex on the Kola Peninsula. On the basis of large sets of pertinent analytical data, we have estimated geochemical trends involving major, minor, and trace elements and studied their relation with the compositions of rock-forming silicate and oxide minerals. The variations observed in Ap differ considerably from trends reported for other layered intrusions. The composition fields of Ap are not consistent with the variations in the chemical composition of the bulk rocks and their constituent minerals, as determined along the representative cross sections of the entire complex. The compositional variations of Ap are fairly similar in all units of the complex. Chlorapatite (>6 wt.% Cl) is invariably abundant. There is no relationship between the Cl content of Ap and the degree of magnesium enrichment of the coexisting early magmatic silicates. In the F–Cl–OH diagram, broad fields of ternary solid solution are observed. There are no compositions along the Cl–F axis. The compositions of Ap are notably poor in Cl in the marginal series (the Nyud massif) and correspond to hydroxylapatite with a high content of fluorapatite component. Two composition fields of Ap are recognized in the Monchepluton complex: ≤3 wt.% and >6 wt.% Cl; there are, however, extensive overlaps. Two generations of apatite are thus implied. The first nucleated at the early stage of crystallization of H2O-bearing intercumulus melt as a result of substantial increase in the contents of P, F, Cl, and other incompatible components. The following stage of degassing of the crystallizing melt caused a decoupling of Cl and F. Fluorine remained mostly in the melt; in contrast, Cl was partitioned efficiently into an H2O-bearing fluid phase. At the early stage, the apatite incorporated combinations of hydroxylapatite and fluorapatite, with a low content of Cl. At the late stage, chlorapatite crystallized from a Cl-rich fluid, and ferrochloropargasite (4.1 wt.% Cl) formed in the Poaz massif as a result of autometasomatic alteration via reactions of this fluid with plagioclase and pyroxene. The apatite has high Sr contents (up to 4.1 wt.% SrO) in the highly magnesian cumulates of the Dunite block and the massifs of mounts Kumuzh’ya, Nittis, and Travyanaya. This enrichment illustrates the accumulation of Sr in the intercumulus melt, in which Ap was the only Sr-bearing phase in the absence or scarcity of intercumulus plagioclase. The REE contents also increased in the intercumulus melt and led to the formation of monazite-(Ce), REE-bearing Ap, and allanite-(Ce) in the remaining microvolumes of melt. Loveringite and Ap crystallized as coexisting phases in Mt. Sopcha. For the first time in a layered intrusion, an extensive range of compositions is documented in the Ce–La–Nd diagram for the REE-bearing phosphates (monazite and REE-rich apatite), which display a predominant La ↔ Nd substitution at the constant contents of Ce.

—The paper focuses on compositional variations of olivine and chromian spinel in the Monchepluton and Pados-Tundra layered intrusions, which host significant chromitite mineralization. Ore-bearing dunite (with up to 25–30 vol.% Mcr) in the Sopcheozerskoe chromite deposit from the Monchepluton complex, Kola Peninsula, Russia, bears an assemblage of phases with exceptionally high magnesium contents: Fo96 + augite (Mg# = 94) + magnesiochromite, Mcr (Mg# ≈ 65); Mg# = 100·Mg/(Mg + Fe2+ + Mn). However, olivine in the host dunite has normal maximum values of Mg# comparable to those in cumulus olivine from layered intrusions worldwide (Fo≤91–92). The Fo96 phase in the Sopcheozerskoe deposit shows the most primitive composition ever reported from any layered intrusion. Magnesiochromite occurs as unzoned homogeneous euhedral crystals unaffected by subsolidus exchange or metasomatic effects. Olivine in ore-bearing dunite (20–25 vol.% magnesian chromite) from the Pados-Tundra complex attains Fo93, with the Mg# value notably higher than the range (Fo85.5–90.6) in olivine from orthopyroxenite, harzburgite, and dunite within the intrusion. Olivine and chromian spinel in the two complexes behave coherently, with covarying patterns of Mg# and Ni contents in olivine at R = 0.75 (n = 160) and positive correlation between Mg# in coexisting chromian spinel and olivine grains at R = 0.8 (n = 150). This behavior indicates that the two phases attained equilibrium during crystallization. It appears unlikely that the extremely high Mg enrichment in olivine (Fo96), as well as in all associated phases of the Monchepluton complex, would result from a subsolidus reaction between olivine and chromian spinel or low-temperature alteration of olivine. We suggest a more realistic explanation that the olivine (+ high-Mg augite)–chromian spinel assemblage crystallized from komatiitic magma under the conditions of progressively increasing oxygen fugacity (fO2). The high Mg# in the Mcr-chromite-enriched system, above the maximum values common in cumulus olivine from layered intrusions (up to Fo96 against Fo≤91–92), may be caused by shortage of ferrous iron.

Author(s): Barkov, AY; Bindi, L; Tamura, N; Martin, RF; Ma, C; Winkler, B; Shvedov, GI; Morgenroth, W | Abstract: Fleetite, Cu2RhIrSb2, a new species of platinum-group mineral (PGM), was discovered intergrown with an Os–Ir–Ru alloy in the Miass Placer Zone (Au–PGE), southern Urals, Russia. A single grain 50 lm across was found. Osmium, ruthenium, and iridium are the main associated minerals; also present are Pt–Fe alloys, laurite, Sb-rich irarsite, Rh-rich tolovkite, kashinite, anduoite, ferronickelplatinum, heazlewoodite, PGE-bearing pentlandite and digenite, as well as micrometric inclusions of forsterite (Fo93.7), chromite–magnesiochromite, and Mg-rich edenite. In reflected light, fleetite is light gray; it is opaque, isotropic, non-pleochroic, and non-bireflectant. We report reflectance values measured in air. A mean of seven point-analyses (wavelength-dispersive spectrometry) gave Cu 13.93, Ni 8.60, Fe 0.10, Ir 28.07, Rh 7.91, Ru 1.96, Sb 39.28, total 99.85 wt.%, corresponding to (Cu1.41Ni0.58Fe0.01)R2.00(Rh0.49Ni0.36Ru0.12)R0.97Ir0.95Sb2.08 on the basis of six atoms per formula unit, taking into account the structural results. The calculated density is 10.83 g/cm3. Single-crystal X-ray studies show that fleetite is cubic, space group Fd3m (#227), a = 11.6682(8) A, V = 1588.59(19) A3, and Z = 16. A least-squares refinement of X-ray powder-diffraction data gave a = 11.6575(5) A and V = 1584.22(19) A3. The strongest five reflections in the powder pattern [d in A(I)(hkl)] are: 6.70(75)(111), 4.13(100)(220), 3.52(30)(311), 2.380(50)(422), 2.064(40)(440). Results of synchrotron micro-Laue diffraction experiments are consistent [a ¼ 11.66(2) A]. The crystal structure of fleetite was solved and refined to R ¼ 0.0340 based upon 153 reflections with Fo . 4r(Fo). It is isotypic with Pd11Bi2Se2 and best described as intermetallic, with all metal atoms in 12-fold coordination. Fleetite and other late exotic phases were formed by reaction of the associated alloy phases with a fluid phase enriched in Sb, As, and S in circulation in the cooling ophiolite source-rock. The mineral is named after Michael E. Fleet (1938–2017) in recognition of his significant contributions to the Earth Sciences.

For the first time, corona-type textures are described in ultrabasic rocks in three complexes of the Serpentinite Belt on the Kola Peninsula in the northeastern Fennoscandian Shield. Three variants of the corona texture formed at different stages during the crystallization of a komatiitic, Al-undepleted melt emplaced in a subvolcanic setting. The first type crystallized at an early stage (Mg# Ol = 87) in a fine-grained harzburgite of the Chapesvara-I sill, with the following order in the corona: Ol → Opx → Cpx → Pl → Amp (aluminous sodic-calcic). The second type displays the sequence Opx → Cpx → Amp → Pl → Qz, which is observed in the orthopyroxenite zone in the Lotmvara-I sill. The third type involves a symplectitic corona in a plagioclase-bearing orthopyroxenite in the Lyavaraka complex, in which the inferred order is: Cpx → Amp (aluminous hornblende) + symplectitic Qz, formed in direct contact with grains of Pl. The corona-type textures occur in fresh rocks and are not related to regional metamorphism. They likely formed as consequences of two important factors: (1) rapid cooling, leading to unsteady conditions of crystallization in a shallow setting; and (2) an intrinsic enrichment in H2O and other volatiles in the parental magma, giving rise to fluid-saturated environments at advanced stages of crystallization. This was followed by a deuteric deposition of Amp rims as a result of the accumulation of H2O and reaction of H2O-bearing fluid with early grains of pyroxene and late plagioclase. The likely existence of a close relationship is suggested by the drusites of the Belomorian complex, which are coeval. In addition, unusual occurrences of lamellar inclusions of phlogopite and Al2SiO5 are documented, hosted by interstitial grains of plagioclase in the orthopyroxenite zone of the Lotmvara-I sill. These are attributed to crystallization from late portions of remaining melt enriched in Al, K, Na, H2O, and Cl, which is indicated by the recorded occurrence of chlorapatite in this association. Thus, our findings indicate the presence and abundance of intrinsic volatiles, Cl, F, CO2, and especially magmatic H2O, which were important to lower the liquidus, decrease the density and viscosity of the highly magnesian melt of Al-undepleted komatiite, thus enabling its transport from the mantle to a shallow level in the crust.

Ferrotorryweiserite, Rh5Fe10S16, occurs as small grains (≤20 µm) among droplet-like inclusions (up to 50 μm in diameter) of platinum-group minerals (PGM), in association with oberthürite or Rh-bearing pentlandite, laurite, and a Pt-Pd-Fe alloy (likely isoferroplatinum and Fe-Pd-enriched platinum), hosted by placer grains of Os-Ir alloy (≤0.5 mm) in the River Ko deposit. The latter is a part of the Sisim placer zone, which is likely derived from ultramafic units of the Lysanskiy layered complex, southern Krasnoyarskiy kray, Russia. The mineral is opaque, gray to brownish gray in reflected light, very weakly bireflectant, not pleochroic to weakly pleochroic (grayish to light brown tints), and weakly anisotropic. The calculated density is 5.93 g·cm–3. Mean results (and ranges) of four WDS analyses are: Ir 18.68 (15.55–21.96), Rh 18.34 (16.32–20.32), Pt 0.64 (0.19–1.14), Ru 0.03 (0.00–0.13), Os 0.07 (0.02–0.17), Fe 14.14 (13.63–14.64), Ni 13.63 (12.58–14.66), Cu 4.97 (3.42–6.41), Co 0.09 (0.07–0.11), S 29.06 (28.48–29.44), and total 99.66 wt.%. They correspond to the following formula calculated for a total of 31 atoms per formula unit: (Rh3.16Ir1.72Pt0.06Ru0.01Os0.01)Σ4.95(Fe4.48Ni4.11Cu1.38Co0.03)Σ10.00S16.05. The results of synchrotron micro-Laue diffraction studies indicate that ferrotorryweiserite is trigonal; its probable space group is R3¯m (#166) based on its Ni-analog, torryweiserite. The unit-cell parameters refined from 177 reflections are a = 7.069 (2) Å, c = 34.286 (11) Å, V = 1484 (1) Å3, and Z = 3. The c:a ratio is 4.8502. The strongest eight peaks in the X-ray diffraction pattern derived from results of micro-Laue diffraction study [d in Å(hkil)(I)] are 2.7950 (202¯5) (100); 5.7143 (0006) (60); 1.7671 (224¯0) (44.4); 3.0486 (202¯1) (39.4); 5.7650 (101¯2) (38.6); 2.5956 (202¯7) (37.8); 3.0058 (112¯6) (36.5); and 1.5029 (42¯ 2¯12) (35.3). Ferrotorryweiserite and the associated PGM crystallized from microvolumes of residual melt at late stages of crystallization of grains of Os- and Ir-dominant alloys occurred in lode zones of chromitites of the Lysanskiy layered complex. In a particular case, the residual melt is disposed peripherally around a core containing a disequilibrium association of magnesian olivine (Fo72.9–75.6) and albite (Ab81.6–86.4), with the development of skeletal crystals of titaniferous augite: Wo40.8–43.2En26.5–29.3Fs20.3–22.6Aeg6.9–9.5 (2.82–3.12 wt.% TiO2). Ferrotorryweiserite represents the Fe-dominant analog of torryweiserite. We also report occurrences of ferrotorryweiserite in the Marathon deposit, Coldwell Complex, Ontario, Canada, and infer the existence of the torryweiserite–ferrotorryweiserite solid solution in other deposits and complexes.

We describe a potentially new species of a platinum cupride–stannide mineral (PCSM) of composition Pt(Cu0.67Sn0.33). It occurs in a placer deposit in the River Bolshoy Khailyk, southern Krasnoyarskiy kray, Russia. A synthetic equivalent of PCSM was obtained and characterized. The PCSM occurs as anhedral or subhedral grains up to 15 μm × 30 μm in association with various platinum-group minerals, Rh–Co-rich pentlandite and magnetite, all hosted by a placer grain of Cu–Au–Pt alloy. Synchrotron micro-Laue diffraction studies indicate that the PCSM mineral is tetragonal and belongs to the inferred space-group P4/mmm (#123). Its unit-cell parameters are a = 2.838 (3) Å, c = 3.650 (4) Å, and V = 29.40 (10) Å3, and Z = 1. The c:a ratio calculated from the unit-cell parameters is 1.286. These characteristics are in good agreement with those obtained for specimens of synthetic Pt(Cu0.67Sn0.33). A review on related minerals and unnamed phases is provided to outline compositional variations and extents of solid solutions in the relevant systems PtNi–PtFe–PtCu, PdCu–PdHg–PdAu, PdHg–PtHg, and AuCu–PtCu. The PCSM-bearing mineralization appears to be related genetically with an ophiolitic source-rock of the Aktovrakskiy complex of the western Sayans. The unnamed phase likely crystallized from microvolumes of a highly fractionated melt rich in Cu and Sn.

We describe the new species ognitite, NiBiTe, and a Co-rich variety of maucherite, hitherto unreported; both were discovered in the Ognit ultramafic complex of Neoproterozoic age in Eastern Sayans, Russia. The mean composition of ognitite (n = 7) is: Ni 17.05, Fe 0.07, Cu 0.14, Pd 0.14, Te 32.53, Bi 49.64, total 99.57 wt.%, corresponding to: (Ni1.11Cu0.008Fe0.005Pd0.005)Σ1.13Bi0.90Te0.97 (Σ atoms = 3 apfu). Ognitite is trigonal, space group P3m1 [R1 = 0.0276 for 81 reflections with Fo > 4σ(Fo)]. The unit-cell parameters derived from the single-crystal X-ray diffraction data are: a = 3.928(1) Å, c = 5.385(1) Å and V = 71.95(4) Å3, with Z = 1. The c:a ratio is 1.37. The powder X-ray diffraction data obtained on the same fragment used for the single-crystal study are: a = 3.9332(4) Å, c = 5.3920(6) Å and V = 72.24(1) Å3. Ognitite exhibits the brucite-type structure with edge-sharing NiTe3Bi3 octahedra forming sheets parallel to (0001). It is related to melonite, but is distinct compositionally by the extreme Bi-enrichment (melonite and its synthetic analogue contain At more than 9 wt.% Co, or ~2 apfu Co, the core of Co-rich maucherite [(Ni,Co)11As8] in a zoned crystal, which is surrounded by Co-depleted orcelite, far surpasses the norm (≤1 and up to 3.9 wt.% Co). The unit-cell parameters of the Co-rich maucherite are: a = 6.85(2) and c = 21.83(5) Å, which are based on results of synchrotron micro-Laue diffraction.The host rock consists of serpentine, clinochlore (Mg# 95–97) and skeletal chromite. We favour the metastable crystallisation of fluid-saturated globules of a sulfide–arsenide melt to explain the anomalous compositions of ore minerals at Ognit. These anomalies seem consistent with rapid cooling in a fluid-enriched system, possibly related to late-stage degassing of the magma, as reflected in a prominent metasomatic aureole at the contact with the enclosing gneissic rocks.

Chromitite zones associated with ultramafic units of the Lysanskiy layered complex of dunite–peridotite–gabbro composition could well represent the primary source for the placers bearing platinum-group minerals (PGM) of the entire drainage of the River Sisim and its tributaries, the rivers Ko and Seyba, eastern Sayans. Alluvial gold present in the placers of River Seyba, as elsewhere in the Sisim Placer Zone, reflects mineralisation during a recent period of tectonic activity. We focus on the PGM in the Seyba suite, and in particular on the attributes of pentlandite enriched in platinum-group-elements (PGE) and the compositionally similar and recently defined ferhodsite, which were trapped in host grains of Os–Ir–Ru alloy. Both minerals formed from small volumes of fractionated Fe–Ni–Cu melt considerably enriched in the PGE. In the Seyba suite, as in several others, the amounts of PGE in ferhodsite exceeds that in pentlandite, which results in a greater proportion of vacancies than in pentlandite.

Tamuraite, ideally Ir5Fe10S16, occurs as discrete phases (≤20 μm) in composite inclusions hosted by grains of osmium (≤0.5 mm across) rich in Ir, in association with other platinum-group minerals in the River Ko deposit of the Sisim Placer Zone, southern Krasnoyarskiy Kray, Russia. In droplet-like inclusions, tamuraite is typically intergrown with Rh-rich pentlandite and Ir-bearing members of the laurite–erlichmanite series (up to ~20 mol.% “IrS2”). Tamuraite is gray to brownish gray in reflected light. It is opaque, with a metallic luster. Its bireflectance is very weak to absent. It is nonpleochroic to slightly pleochroic (grayish to light brown tints). It appears to be very weakly anisotropic. The calculated density is 6.30 g·cm−3. The results of six WDS analyses are Ir 29.30 (27.75–30.68), Rh 9.57 (8.46–10.71), Pt 1.85 (1.43–2.10), Ru 0.05 (0.02–0.07), Os 0.06 (0.03–0.13), Fe 13.09 (12.38–13.74), Ni 12.18 (11.78–13.12), Cu 6.30 (6.06–6.56), Co 0.06 (0.04–0.07), S 27.23 (26.14–27.89), for a total of 99.69 wt %. This composition corresponds to (Ir2.87Rh1.75Pt0.18Ru0.01Os0.01)Σ4.82(Fe4.41Ni3.90Cu1.87Co0.02)Σ10.20S15.98, calculated based on a total of 31 atoms per formula unit. The general formula is (Ir,Rh)5(Fe,Ni,Cu)10S16. Results of synchrotron micro-Laue diffraction studies indicate that tamuraite is trigonal. Its probable space group is R–3m (#166), and the unit-cell parameters are a = 7.073(1) Å, c = 34.277(8) Å, V = 1485(1) Å3, and Z = 3. The c:a ratio is 4.8462. The strongest eight peaks in the X-ray diffraction pattern [d in Å(hkl)(I)] are: 3.0106(26)(100), 1.7699(40)(71), 1.7583(2016)(65), 2.7994(205)(56), 2.9963(1010)(50), 5.7740(10)(45), 3.0534(20)(43) and 2.4948(208)(38). The crystal structure is derivative of pentlandite and related to that of oberthürite and torryweiserite. Tamuraite crystallized from a residual melt enriched in S, Fe, Ni, Cu, and Rh, these elements were incompatible in the Os–Ir alloy that nucleated in lode zones of chromitites in the Lysanskiy layered complex, Eastern Sayans, Russia. The name honors Nobumichi Tamura, senior scientist at the Advanced Light Source of the Lawrence Berkeley National Laboratory, Berkeley, California.

The lopolithic Pados-Tundra layered complex, the largest member of the Serpentinite belt&ndash, Tulppio belt (SB&ndash, TB) megastructure in the Fennoscandian Shield, is characterized by (1) highly magnesian compositions of comagmatic dunite&ndash, harzburgite&ndash, orthopyroxenite, with primitive levels of high-field-strength elements, (2) maximum values of Mg# in olivine (Ol, 93.3) and chromian spinel (Chr, 57.0) in the Dunite block (DB), which exceed those in Ol (91.7) and Chr (42.5) in the sills at Chapesvara, and (3) the presence of major contact-style chromite&ndash, IPGE-enriched zones hosted by the DB. A single batch of primitive, Al-undepleted komatiitic magma crystallized normally as dunite close to the outer contact, then toward the center. A similar magma gave rise to Chapesvara and other suites of the SB&ndash, TB megastructure. Crystallization proceeded from the early Ol + Chr cumulates to the later Ol&ndash, Opx and Opx cumulates with accessory Chr in the Orthopyroxenite zone. The accumulation of Chr resulted from efficient cooling along boundaries of the Dunite block. The inferred front of crystallization advanced along a path traced by vectors of Ol and Chr compositions. Grains and aggregates of Chr were mainly deposited early after the massive crystallization of olivine. Chromium, Al, Zn and H2O, all incompatible in Ol, accumulated to produce podiform segregations or veins of chromitites. This occurred episodically along the moving front of crystallization. Crystallization occurred rapidly owing to heat loss at the contact and to a shallow level of emplacement. The Chr layers are not continuous but rather heterogeneously distributed pods or veins of Chr&ndash, Ol&ndash, clinochlore segregations. Isolated portions of melt enriched in H2O and ore constituents accumulated during crystallization of Ol. Levels of fO2 in the melt and, consequently, the content of ferric iron in Chr, increased progressively, as in other intrusions of the SB&ndash, TB megastructure. The komatiitic magma vesiculated intensely, which led to a progressive loss of H2 and buildup in fO2. In turn, this led to the appearance of anomalous Chr&ndash, Ilm parageneses. Diffuse rims of Chr grains, abundant in the DB, contain elevated levels of Fe3+ and enrichments in Ni and Mn. In contrast, Zn is preferentially partitioned into the core, leading to a decoupling of Zn from Mn, also known at Chapesvara. The sulfide species display a pronounced Ni-(Co) enrichment in assemblages of cobaltiferous pentlandite, millerite (and heazlewoodite at Khanlauta), deposited at &le, 630 &deg, C. The oxidizing conditions have promoted the formation of sulfoselenide phases of Ru in the chromitites. The attainment of high degrees of oxidation during crystallization of a primitive parental komatiitic magma accounts for the key characteristics of Pados-Tundra and related suites of the SB&ndash, TB megastructure.

We describe occurrences of platinum-group minerals (PGM) and an uncommon mineral enriched in Cl, and provide a brief review of Cl-bearing minerals associated with basic–ultrabasic complexes. An unusual phosphohedyphane-like phase (~30 µm), close to CaPb4(PO4)3Cl, occurs in one of the PGM-bearing veins of massive sulfides in the Monchepluton layered complex, Kola Peninsula, Russia. These veins consist of varying amounts of pyrrhotite, pentlandite, chalcopyrite, pyrite and accessory grains of galena; they are fairly abundant in the heavy-mineral concentrate, as are small (

Solid state sciences 105, 106282 (2020). doi:10.1016/j.solidstatesciences.2020.106282, Pt(Cu$_{0.67}$Sn$_{0.33}$) has recently been found in a natural sample. In order to be able to characterize this new ternary compound, we synthesized it from the elements. Samples were characterized by X-ray powder diffraction, differential scanning calorimetry, thermal relaxation calorimetry, and scanning electron microscopy studies. Density functional theory-based model calculations complemented the experimental studies. Pt(Cu$_{0.67}$Sn$_{0.33}$) was already formed at a relatively low temperature of 773 K. Rietveld refinement of Pt(Cu$_{0.67}$Sn$_{0.33}$) has been carried out in CuAu-type or L1$_0$-type structure, space group , with Pt on 0,0,0 and disordered Cu and Sn on $\frac{1}{2},\frac{1}{2},\frac{1}{2}$ and Z = 1. The lattice parameters are = 2.823(1) Å, = 3.64(1) Å, and = 29.00(4) Å ; which are in good agreement with values obtained earlier on the natural sample and with the results of DFT calculations. The vibrational entropy for Pt(Cu$_{0.67}$Sn$_{0.33}$) is $S^{vib}_{298.15}$ = 79.9(7) J mol$^{−1}$ K$^{−1}$. The pressure dependence up to 36(2) GPa of the unit-cell volume and the lattice parameters and unit-cell volume have been obtained by synchrotron based powder diffraction using a diamond anvil cell. A fit of a 3-order Birch–Murnaghan equation of state to the Pt(Cu$_{0.67}$Sn$_{0.33}$) (p, V)-data results in a bulk modulus of B$_0$= 215(27) GPa and B´= 5(2)., Published by Elsevier, Amsterdam [u.a.]

Highly atypical mineralization involving Pd-Pt, Au-Ag, REE, Y, Zr, U, Th, and Cl-F-enriched minerals is found in zones with base metal sulfides (BMS, ~5 vol.% to 20 vol.%) in the eastern portion of the Oktyabrsky deposit in the Norilsk complex (Russia). The overall variations in Mg# index, 100 Mg/(Mg + Fe2+ + Mn), in host-rock minerals are 79.8 → 74.1 in olivine, 77.7 → 65.3 in orthopyroxene, 79.9 → 9.2 in clinopyroxene, and An79.0&nbsp, → An3.7. The span of clinopyroxene and plagioclase compositions reflects their protracted crystallization from early magmatic to late interstitial associations. The magnesian chromite (Mg# 43.9) trends towards Cr-bearing magnetite with progressive buildups in oxygen fugacity, ilmenite varies from early Mg-rich to late Mn-rich variants. The main BMS are chalcopyrite, pyrrhotite, troilite, and Co-bearing pentlandite, with less abundant cubanite (or isocubanite), rare bornite, Co-bearing pyrite, Cd-bearing sphalerite (or wurtzite), altaite, members of the galena-clausthalite series and nickeline. A full series of Au-Ag alloy compositions is found with minor hessite, acanthite and argentopentlandite. The uncommon assemblage includes monazite-(Ce), thorite-coffinite, thorianite, uraninite, zirconolite, baddeleyite, zircon, bastnäsite-(La), and an unnamed metamict Y-dominant zirconolite-related mineral. About 20 species of PGM (platinum group minerals) were analyzed, including Pd-Pt tellurides, bismuthotellurides, bismuthides and stannides, Pd antimonides and plumbides, a Pd-Ag telluride, a Pt arsenide, a Pd-Ni arsenide, and unnamed Pd stannide-arsenide, Pd germanide-arsenide and Pt-Cu arseno-oxysulfide. The atypical assemblages are associated with Cl-rich annite with up to 7.54 wt.% Cl, Cl-rich hastingsite with up 4.06 wt.% Cl, ferro-hornblende (2.53 wt.% Cl), chlorapatite (&gt, 6 wt.% Cl) and extensive solid solutions of chlorapatite, fluorapatite and hydroxylapatite, Cl-bearing members of the chlorite group (chamosite, up to 0.96 wt.% Cl), and a Cl-bearing serpentine (up to 0.79 wt.% Cl). A decoupling of Cl and F in the geochemically evolved system is evident. The complex assemblages formed late from Cl-enriched fluids under subsolidus conditions of crystallization following extensive magmatic differentiation in the ore-bearing sequences.

This is the first report of occurrences of platinum-group minerals (PGM) and unnamed phases of platinum-group elements (PGE) from the Pados-Tundra ultramafic complex of Paleoproterozoic age, Kola Peninsula. The PGM occur as individual inclusions or intergrowths (≤1–10 μm) hosted by cores of zoned grains of chromite–magnesiochromite (Chr) in chromitite of the Dunite block in the eastern portion of the complex. Osmium-poor laurite is abundant, commonly as intimate intergrowths with clinochlore (>80% laurite grains), followed by Os- and Ir-dominant alloys (i.e., the minerals osmium and iridium, respectively); unnamed phases of ruthenium selenide [RuSe 2 ] and rhodium telluride [RhTe] are rare. Selenium-rich laurite, not reported previously, likely belongs to an inferred RuS 2 –RuSe 2 series. Native Ru forms micro-(nano)-spherules in framboid-type aggregates, in association with relict laurite, which are surrounded by a skeletal grain of clinochlore; the host is Chr. The framboidal texture of nanophases of ruthenium, hitherto unreported in PGE alloys, likely formed as a result of deposition from a H 2 -bearing fluid, involving a reaction of desulfurisation and reduction of laurite. A highly S-deficient environment is indicated; a low sulfur fugacity, presumably below the Os–OsS 2 buffer, caused the observed coexistence of a low-Os variety of laurite with Os-dominant alloy. An ultimate loss of total S is implied to have decreased the S/Se ratio, which resulted in the precipitation of Ru-dominant sulfoselenide–selenide phases, likely from an oxidizing fluid at a late stage of mineralization. We also infer that the laurite–clinochlore intergrowths crystallized relatively late, from microvolumes of an H 2 O-bearing fluid of contrasting composition, enriched in Ru, S, and lithophile elements.

New Pd-Tl-(Bi,As)-rich compounds are described from the Anomal'nyi Cu-PGE deposit, Kondyor alkaline ultramafic complex, northeastern Russia. They occur in vein-type settings associated with bodies of 'kosvite' (magnetite-rich clinopyroxenite) in the dunite-(peridotite) core, in pegmatiticand micaceous rocks. The ore zones are substantially enriched in phlogopite; they consist of diopside, disseminated titaniferous magnetite and fluorapatite (Sr-bearing). The observed assemblages of ore minerals include sulfides of the chalcopyrite-bornite-(secondary chalcocite) associationand various species of platinum-group minerals (PGM) [isomertieite or arsenopalladinite rich in Sb, mertieite-II, mertieite-I, sobolevskite, kotulskite, merenskyite, zvyagintsevite, palarstanide, paolovite, sperrylite, maslovite or moncheite, hollingworthite and unnamed species of PGM] anda Ag-Au alloy. The oxides [Pd4(Bi,Te,Tl)O6 and Pd4(Tl,Bi,Te)O6] probably formed in situ by oxidation reactions at the expense of the associated PGM intergrowths. These involve palladium bismuthide-thallide phases, Pd5(Tl,As,Bi)and Pd5(As,Tl,Bi), which are documented here for the first time. A fairly evolved environment, enriched in Cu, Pd, Te, Bi, Pb and volatile components, is indicated for the vein-type deposit at Anomal'nyi.

We describe occurrences of palladoan melonite in intimate intergrowths with cobaltite-gersdorffite from the Neoproterozoic dunite-wehrlite-gabbro complexes of Kingash and Kuskanak, Eastern Sayans, Russia. The observed compositional trends of melonite are consistent with the overallvariations examined on the basis of numerous literature sources. The levels of Bi in NiTe2are normally limited to ≤0.25 Bi atoms per formula unit (apfu), under natural conditions. Greater levels (≤0.5 Bi apfu) are associated with the (Pd + Pt) enrichment in the palladoanvarieties. The telluride–sulfarsenide intergrowths probably formed at Kingash and Kuskanak late in the crystallization history of the ore zones, from microdroplets of residual melt rich in semimetals (Te, Bi, As) and noble metals (Pd and Ag), below the solidus of the enclosing gabbroicrocks and within a narrow range of temperatures (500–550°C). On the basis of our observations made on specimens of melonite and synthetic Ni(Te,Bi)2–x(x = 0.6), we infer that the limit of incorporation of Bi into a melonitetype phase is ≤0.5 Bi apfu.

Dunite&ndash, harzburgite&ndash, olivine-bearing orthopyroxenite successions in the subvolcanic Chapesvara-I and Chapesvara-II intrusions in the Serpentinite Belt, western Kola Peninsula, are notably magnesian. The mean Mg# value (whole-rock) is 86.6, and the olivine is Fo84&minus, 89. The upper contact facies (UCF) displays a lower Mg# (81.6). It consists of grains of Fo92 and abundant chromian spinel, implying rapid crystallization of an almost unfractionated melt. On average, the whole-rock Al2O3/TiO2 value is 22.45, close to 22.9 (UCF) and to the primitive mantle, ~22. The rise of primitive ultramafic magma presumably occurred in a special tectonic setting at the boundary of the Paleoproterozoic Lapland Granulite Terrane and the Belomorian Composite Terrane of Archean age. The Chapesvara suite resembles examples of the Al-undepleted komatiites in the Barberton Belt, South Africa, with magmas of up to 30&ndash, 35% MgO. The UCF rock yields an anomalously low molar MgO/SiO2 value, close to that of dunitic rocks located at the center of the Chapesvara-II body. This rock is the most primitive, as indicated by the maximum Fo content of olivine, the lowest value of (Gd/Yb)N, 0.52, and the lowest abundances of middle to heavy rare-earth elements (REE) in the chondrite-normalized spectrum. The crystallization of the Chapesvara-II sill-like intrusion likely proceeded in two stages, which are evident from the olivine compositions varying from the maximum Fo92 (UCF) to Fo&le, 89.5 (the central dunite zone). At Stage 1, the UCF rock (Fo92) crystallized first, close to the upper contact. The area of crystallization then shifted to a central portion of the Chapesvara-II body, in which the dunitic zone (Fo89.5) formed in situ (Stage 2). The compositional variations in chromian spinel are consistent with this suggestion. Two crystallization trends were recognized. The type-1 trend displays a relative maximum or minimum close to the center, and then diverges into two linear subtrends directed upward and downward. This pattern is manifested in the variations of Mg# in olivine and chromian spinel, the whole-rock contents of Al and Ca, and in levels of incompatible elements: Ti, V, Zr, Y, and Hf. The type-2 trend decreases or increases uniformly from top to bottom. Variations in amount of Ni in olivine, the Fe3+# index in chromian spinel, and in values of Mg# (rocks), follow a type-2 trend. Variations in total amounts of REE, Nb, and Th, which gradually increase downward, are also related to a type-2 trend. Thus, a contrasting development and possible interference of the two types of evolutionary trends were observed in the crystallization history of the Chapesvara-II sill-like body. A double-front crystallization, hitherto unreported, involved two fronts moving upward and downward, respectively. The upward subtrend appeared to be of subordinate importance, whereas the extent of fractional crystallization of the downward front was much greater. Crystallization proceeded from the top to the bottom, presumably because of the preferential loss of heat at the roof. Variations in the Fe3+# index indicate that the level of fO2 also increased downward with progressive crystallization. Convection cells were presumably the key mechanism of accumulation of the crystallizing olivine grains to form the central dunite zone close to the center of the sill-like intrusion. The observed characteristics of the Chapesvara complex indicate the existence of a primitive-mantle source and imply a highly magnesian composition of intruding magma not only for Chapesvara, but also for the Pados-Tundra layered complex and associated suites of the Serpentinite Belt in the Kola Peninsula.

A new high entropy alloy, Pt2AuCuNiSn, has been obtained at comparatively low temperatures by a solid state reaction. A Rietveld refinement gave a satisfactory description of X-ray diffraction data obtained at ambient temperature. SEM/EDXmeasurements gave no indication for chemical inhomogeneities on a micron length scale. Temperature dependent in situ diffraction measurements using hard (lambda = 0.1204 A) radiation up to 875 K show several transient phases upon heating a mixture of the elements, consistent with results of DSC measurements. The parameter describing the variability of the radii of the constituent atoms, d_r, is uncommonly large (d_r = 8.7 %) and may be the origin of the instability of this alloy at high temperatures.

Journal of solid state chemistry 294, 121837 (2021). doi:10.1016/j.jssc.2020.121837, A new single-phase noble metal high entropy alloy, Pt$_2$AuCuNiSn, has been obtained at comparatively low temperatures by a solid-state reaction. A Rietveld refinement gave a satisfactory description of the X-ray diffraction data, showing that the compound crystallizes with space group $Fm\bar{3}m$, where only one Wyckoff site is occupied. In bulk samples, SEM/EDX measurements gave no indication for chemical inhomogeneities on a ��m length scale. Temperature dependent in situ diffraction measurements using high energy ($��$ ���= ���0.1204 �����) radiation up to 875 ���K show several transient phases upon heating a mixture of the elements, consistent with results of DSC measurements. The parameter describing the variability of the radii of the constituent atoms, , is uncommonly large ($\delta_\tau$ ���= ���8.7%) and maybe the origin of the instability of this alloy at high temperatures., Published by Academic Press, Orlando, Fla.

We document a new and unusual occurrence of patterns of protruding spheroidal weathering developed in a dunitic rock of the Pados-Tundra mafic-ultramafic complex of Early Proterozoic age, Kola Peninsula, Russia. It provides an example similar to that reported recently from a mineralized harzburgite in the Monchepluton layered complex in the same region. These patterns are genetically different from common results of “normal spheroidal weathering” sensu stricto. The spheroidally weathered dunite at Pados-Tundra consists of a high-Fo olivine, Ol (Fo 87. 5), which is, in fact, not altered. Accessory grains of aluminous chromite are present. Relief spheroids (1.5 to 4 cm in diameter; up to ~5 vol. %) are distributed sparsely and heterogeneously. They are hosted by the olivine matrix and composed of talc, Tlc, and tremolite, Tr, (Mg# = 95-96) formed presumably at the expense of orthopyroxene, Opx, (i.e., pre-existing oikocrysts) during a deuteric (autometasomatic) alteration. In contrast, oikocrystic Opx (En 86.0) is quite fresh in related spheroids at Monchepluton, in which only minor deuteric alteration (Tlc + Tr) are observed. We infer that (1) the ball-shaped morphology of the weathered surface is a reflection of the presence of oikocrysts of Opx, which crystallized after Ol at the magmatic stage; they were entirely replaced by the deuterically induced Tlc + Tr at Pados-Tundra. (2) Differential rates of weathering are implied for rock-forming minerals in these ultramafic rocks, with a higher resistance of Opx vs. Fo-rich Ol, and Tlc + Tr vs. Fo-rich Ol. (3) The ball-like shape of the large spheroids, produced by magmatic processes, may likely represent an additional factor of their higher stability to weathering in the superficial environment. Similar patterns can be expected in other mafic-ultramafic complexes, especially in layered intrusions.

We examine overall compositional ranges and substitutions of elements in intermetallic compounds of Pd, Pt, Cu, and Sn (cabriite, taimyrite, “stannopalladinite”, and tatyanaite), on the basis of (1) new results based on electron microprobe analyses of these stannides from Noril9sk (Krasnoyarskiy kray, Russia) and Lukkulaisvaara (northern Karelia, Russia), and (2) data published in the literature on natural and synthetic stannides. These results are consistent with the existence of a single and extensive field of solid solutions of the stannides of Pd-Pt-Cu, related to cabriite–taimyrite and tatyanaite, with generalized formulae (Pd,Pt) 2+ x Cu 1– x (Sn,Sb) – (Pt,Pd) 2+ x Cu 1– x (Sn,Sb), where 0 x 2+ x (Fe,Cu) 1– x (Sb,Sn). No miscibility gap is evident. An unlimited substitution of Pt-for-Pd appears to exist; in addition, Pd partly substitutes for Cu, whereas there is no evidence for the existence of a Pt-for-Cu substitution in these solid solutions. The stannides of Pd-Pt-Cu were likely deposited relatively late in the crystallization history of mineral associations, at low temperatures, likely as a result of a significant drop in temperature and increase in activity of Cu and Sn in a fractionated sulfide melt or a fluid phase. An atypical combination of relatively low levels of fugacities of O 2 and S 2 could account for the appearance of these rare compounds instead of common oxide or sulfide minerals, and could explain the observed scarcity of phases of the Pd-Pt-Cu stannides in nature.

The ∼3.5 km thick Kivakka complex (2.45 Ga) in northern Karelia hosts a zone of disseminated Pd-Pt-Ni-Cu mineralization near its base. The zone is related to a special or “critical horizon” and displays relative maxima in bulk-rock concentrations of Cu, Ni, Co, and Cr, in the ratios Cu:Ni, Cu:Zr, and 100Fe 2 O 3 /(FeO + Fe 2 O 3 ), and elevated levels of Cr in ortho- and clinopyroxene, along with higher levels of incompatible Ti, V, and Zr. Early crystallizing minerals (highly magnesian pyroxenes and An-rich plagioclase) are associated in this zone with 1–10% (commonly 3 to 5%) base-metal sulfides (BMS) and interstitial phases [nearly pure Ab, chlorapatite, zircon, Fe-(Ti) oxides, etc .]. Apatite is characterized by extensive variations in Cl (both intragranular and intergranular), which attain the composition of chlorapatite (>6 wt.% Cl), not observed below the zone. The compositional trend of the apatite series is extensive; the content of F generally increases upward, whereas Cl decreases, with the observed maximum (3.3% F) near the top of the intrusion. The PGE-rich zone is inferred to have formed at the “critical level” as a consequence of a combination of several factors. Factor 1 represents an abrupt change in the sequence and character of cumulus phases; a massive crystallization of cumulate An 75–81 is accompanied by a sharp decrease in the amount of orthopyroxene with cessation of crystallization of olivine. As a likely result, the remaining melt became substantially enriched in Cr. Alternatively, the Cr enrichment is related to an influx of a new portion of magma. Factor 2 represents a rise in levels of oxygen fugacity, f (O 2 ). Factor 3 is displayed by the sharp increase in Cl in the intercumulus apatite; the fractionated melt well above this zone appears to be depleted in Cl. The sudden drop in Cl is attributed to degassing of the melt, from which a Cl-rich volatile phase exsolved, causing fractionation of Cl from F. Factor 4 involves an increase in levels of sulfur fugacity, f (S 2 ), resulting in the deposition of the disseminated BMS in association with tiny grains of Pd-Pt bismuthotellurides and sperrylite. Thus, the formation of PGE mineralization appears to have proceeded in Kivakka as a sequence of processes and events, rather than as a result of a single factor or process during fractional crystallization of the complex.

We describe assemblages of platinum-group minerals (PGM) and associated PGE&ndash, 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&ndash, Ir&ndash, Ru compositions, which predominate, (2) Pt&ndash, Fe compositions of a Pt3Fe stoichiometry, and (3) Pt&ndash, Au&ndash, Cu alloys, which likely crystallized in the sequence from Au&ndash, (Cu)-bearing platinum, Pt(Au,Cu), Pt(Cu,Au), and PtAuCu2, to PtAu4Cu5. The general trends of crystallization of PGM appear to be: [Os&ndash, Ru alloys] &rarr, Pt3Fe-type alloy (with inclusions of Ru-dominant alloy formed by exsolution or via replacement of the host Pt&ndash, Fe phase) &rarr, Pt&ndash, 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+ + □ &rarr, 3Fe2+]. Many of the Os&ndash, Ru and Pt&ndash, Fe grains have porous, fractured or altered rims that contain secondary PGE sulfide, arsenide, sulfarsenide, sulfoantimonide, gold, Pt&ndash, 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&ndash, Os oxide (zoned) and seleniferous accessory phases.

We report the results of a mineralogical investigation of placer samples from the upper reaches of the Sisim watershed, near Krasnoyarsk, in Eastern Sayans, Russia. The placer grains are predominantly Os–Ir–(Ru) alloys (80%) that host various inclusions (i.e., platinum-group elements (PGE)-rich monosulfide, PGE-rich pentlandite, Ni–Fe–(As)-rich laurite, etc.) and subordinate amounts of Pt–Fe alloys. Analytical data (wavelength- and energy-dispersive X-ray spectroscopy) are presented for all the alloy minerals and the suite of micrometer-sized inclusions that they contain, as well as associated grains of chromian spinel. The assemblage was likely derived from chromitite units of the Lysanskiy mafic–ultramafic complex, noted for its Ti–(V) mineralization. In the Os–Ir–(Ru) alloys, the ratio Ru/Ir is ≤1, Ir largely substitutes for Os, and compositional variations indicate the scheme [Ir + Ru] → 2Os. In contrast, in the laurite–erlichmanite series, Ir and Os are strongly and positively correlated, whereas Ir and Ru are negatively correlated; Ru and Os are inversely correlated. These compositions point to the scheme [Os2+ + 2Ir3+ + □] → 4Ru2+ or alternatively, to Os2+ + Ir2+ → 2Ru2+. We deduce a potential sequence of crystallization in the parental rock and address the effects of decreasing temperature and increasing fugacity of sulfur and arsenic on the assemblage. Inclusions of Ti-rich minerals in the alloy grains are consistent with the Lysanskiy setting; the complete spectrum of chromite–magnesiochromite compositions indicates that an important part of that complex was eroded. A localized fluid-dominated micro-environment produced the unique association of laurite with monazite-(Ce), again considered a reflection of the special attributes of the Lysanskiy complex.

Microcrystals of platinum-group element (PGE)-bearing cobaltite occur in the Gulcari A deposit of vanadiferous titanomagnetite in the lower zone of the Rio Jacaré mafic-ultramafic layered intrusion, Brazil. Aggregates of cobaltite and sperrylite are cluster-like and developed generally along the boundary of Fe-Ti oxide grains with deuteric silicates. Our observations of cryptic zoning, compositional variability and interelement correlations are based on 37 analytical points (wavelength-dispersion spectrometry mode) of cobaltite, and indicate that Ir and Rh behave uniformly with Ni and antipathetically with Co which, in turn, correlates directly with S content. Iridium, Rh and Ni apparently substitute for Co in the As-enriched grain core, and the substitution mechanism invokes solid solution with a cattierite-type molecule: (Ni + Ir + Rh) + (AsS) = Co + (S2). The PGE-bearing cobaltite probably crystallized as a primary phase at 500 to 300°C, from microvolumes of a late fluid phase. The observed enrichment in S and decrease in the As:S ratio at the cobaltite grain margins is a reflection of the increase in sulfur fugacity (fS2) with decrease in temperature of crystallization.

We document unusual zones of Cr enrichment (up to 0.9 wt.% Cr 2 O 3 ) in sector-zoned clinopyroxene crystals from gabbro sampled near the outer contact of the Mont Royal intrusive complex, of Early Cretaceous age, located in the city of Montreal. Mont Royal is one of the Monteregian Hills, but by no means the best-known mineralogically. We describe the compositional variation in macrocrysts of Ti-rich clinopyroxene by means of element-distribution maps and a detailed 0.37 mm traverse across the contact between a relatively homogeneous core and a mantle showing sector and oscillatory zoning. A Cr- and Mg-enriched zone appears in that interval and is also recorded in the coeval ferri-kaersutite. Both elements, being highly compatible, are not expected to build up in a boundary layer. After a period of slow growth in a hotter part of the reservoir, the macrocrysts seem to have migrated near the outer contact, where more rapid loss of heat and degassing contributed to the development of sector and oscillatory zoning. Such zoning was accompanied by an overall rise in f (O 2 ). The anomalous Cr-Mg-enriched zones seem more likely attributable to local cooling and oxidation of the magma than to an episode of replenishment in the magma reservoir.

New results (compositional data and reflectance values) are reported for some rare sulfides enriched in K, Tl and Pb, which are related to djerfisherite, thalfenisite, bartonite, a “Cl-bearing bartonite”, or chlorbartonite, and also for shadlunite, from the Noril'sk and Salmagorsky complexes, Russia. Our observations and comparisons with relevant data in the literature imply that: (1) bartonite is probably a S-dominant (or Cl-free) analogue of djerfisherite; and a “Cl-bearing bartonite” and chlorbartonite are probably compositional variants of the djerfisherite–bartonite series. (2) The most probable formulae of bartonite and djerfisherite are (K,Me2+)6(Fe,Cu,Ni)25–xS26(S,Cl) and (K,Me2+)6(Fe,Cu,Ni)25–xS26(Cl,S), where 0 ≤ x ≤ 5, respectively. (3) Two independent substitution mechanisms probably operate in the natural series. A coupled substitution [Me2+ + S2– ↔ K+ + Cl–] is reflected by an observed deficit in K, accompanied by the incorporation of Me2+(Pb, Fe, or Ni) in the K site. Another mechanism is inferred to be [2Fe3+ + 〈 ↔ 3Fe2+], which assumes the existence of vacancy-type defects at the Me site. Thus, the second mechanism could possibly control the existing variations of Σ(Fe, Cu, Ni) in the range of ∼21 to 25 a.p.f.u., documented in djerfisherite- and bartonite-type minerals. The minerals analysed from Noril'sk, which are free of Cl and related to bartonite and to a Tl-dominant analogue of bartonite (unnamed species), probably crystallized from microvolumes of late fluid rich in K and Tl, under conditions of relatively low oxygen fugacity in the environment. Uniform contentss of Fe and Cu, observed in coexisting phases of normal (Cl-bearing) djerfisherite and bartonite (or Cl-free analogue of djerfisherite) at Salmagorsky imply that they reached equilibrium with regard to the distribution of these elements during crystallization. These phases probably formed as a result of fluctuations in the ratios of sulfur and chlorine fugacity in a fluid at a postmagmatic hydrothermal stage.

The platinum-group minerals (PGM) consist of a group of accessory minerals that concentrate the six platinum-group elements (PGE): osmium (Os), iridium (Ir), ruthenium (Ru), rhodium (Rh), platinum (Pt), and palladium (Pd) [...]

Tetra-auricupride, ideally AuCu, represents the only species showing the coexistence of Au with an elevated level of Pt, as in the case of a detrital grain studied structurally for the first time, from an ophiolite-associated placer at Bolshoy Khailyk, western Sayans, Russia. We infer that tetra-auricupride can incorporate as much as ~30 mol. % of a &ldquo, PtCu&rdquo, component, apparently without significant modification of the unit cell. The unit-cell parameters of platiniferous tetra-auricupride are: a 2.790(1) &Aring, c 3.641(4) &Aring, with c/a = 1.305, which are close to those reported for ordered AuCu(I) in the system Au&ndash, Cu, and close also to the cell parameters of tetraferroplatinum (PtFe), which both appear to crystallize in the same space group, P4/mmm. These intermetallic compounds and natural alloys are thus isostructural. The closeness of their structures presumably allows Pt to replace Au atoms so readily. The high extent of Cu + Au enrichment is considered to be a reflection of geochemical evolution and buildup in levels of the incompatible Cu and Au with subordinate Pt in a remaining volume of melt at low levels of fO2 and fS2 in the system.

Compositional variations of major and minor elements were examined in Pt–Fe alloys from various geological settings and types of deposits, both lode and placer occurrences. They included representatives of layered intrusions, Alaskan-Uralian-(Aldan)-type and alkaline gabbroic complexes, ophiolitic chromitites, and numerous placers from Canada, USA, Russia, and other localities worldwide. Pt–Fe alloy grains in detrital occurrences are notably larger in size, and these are considered to be the result of a special conditions during crystallization such as temperature, pressure, geochemistry or time. In addition, the number of available statistical observations is much greater for the placer occurrences, since they represent the end-product of, in some cases, the weathering of many millions of tonnes of sparsely mineralized bedrock. Typically, platinum-group elements (PGE) present in admixtures (Ir, Rh, and Pd) and minor Cu, Ni are incorporated into a compositional series (Pt, PGE)2–3(Fe, Cu, Ni) in the lode occurrences. Relative Cu enrichment in alloys poor in Pt implies crystallization from relatively fractionated melts at a lower temperature. In contrast to the lode deposits, the distribution of Ir, Rh, and Pd is fairly chaotic in placer Pt–Fe grains. There is no relationship between levels of Ir, Rh, and Pd with the ratio Σ(Pt + PGE):(Fe + Cu + Ni). The compositional series (Pt, PGE)2–3(Fe, Cu, Ni) is not as common in the placer occurrences; nevertheless, minor Cu and Ni show their maximums in members of this series in the placer grains. Global-scale datasets yield a bimodal pattern of distribution in the Pt–Fe diagram, which is likely a reflection of the miscibility gap between the ordered Pt3Fe structure (isoferroplatinum) and the disordered structure of native or ferroan platinum. In the plot Pt versus Fe, there is a linear boundary due to ideal Pt ↔ Fe substitution. Two solid solution series are based on the Ir-for-Pt and Pd-for-Pt substitutions. The incorporation of Ir is not restricted to Pt3Fe–Ir3Fe substitution (isoferroplatinum and chengdeite, plus their disordered modifications). Besides, Ir0 appears to replace Pt0 in the disordered variants of (Pt–Ir)–Fe alloys. There is a good potential for the discovery of a new species with a Pd-dominant composition, (Pd, Pt)3Fe, most likely in association with the alkaline mafic-ultramafic or gabbroic complexes, or the mafic units of layered intrusions. The “field of complicated substitutions” is recognized as a likely reflection of the crystallochemical differences of Pd and Ir, extending along the Ir-Pd axis of the Ir–Pd–Rh diagram. The inferred solid solution extends approximately along the line Ir–(Pd:Rh = 2:3). Minor Pd presumably enters the solid solution via a coupled substitution in combination with the Rh. An Ir-enrichment trend in Pt–Fe alloys typically occurs in the Alaskan-type complexes. The large size of the Pt–Fe nuggets associated with some of these complexes is considered to be related to an ultramafic-mafic pegmatite facies, whereas significant Pd-enrichment is characteristic of gabbroic source-rocks (e.g., Coldwell Complex), resulting in a markedly different trend for the Pt versus Fe (wt.%). However, based on our examination of a large dataset of Pt–Fe alloys from numerous origins, we conclude that they exhibit compositional overlaps that are too large to be useful as reliable index-minerals.

We report results of electron-microprobe analyses for detrital grains of chromian spinel ( n ≈ 800) and associated minerals recovered from modern fluvial and buried paleochannel (Pt–)Au placer deposits ( n = 30) of mostly Holocene age distributed throughout British Columbia. The majority of analyzed spinel-group minerals are members of the chromite–magnesiochromite solid-solution series, and extend to Fe 3+ -rich compositions of chromian spinel. The values of mg# [100Mg/(Mg + Fe 2+ )] and cr# [100Cr/(Cr + Al)] in grains from all 30 placer samples occupy a relatively narrow range, 58–82 and 68–92, respectively, consistent with a mafic to ultramafic source. Four types of core-to-rim patterns of zoning have been identified: 1) a weak decrease in Cr coupled with a strong decrease in mg# at typically constant cr# and fe 3+ #, as a result of limited chromian spinel – liquid fractionation and subsolidus Fe 2+ –Mg exchange with ferromagnesian silicates; 2) a sharp decrease in Cr and cr# and increase in fe 3+ # [100Fe 3+ /(Al + Cr + Fe 3+ )] with limited decrease in mg#, reflecting magmatic fractionation; 3) a significant increase in Cr and cr# with weak to strong decrease in mg#, reflecting late-stage interaction with a Cr-rich magma followed by variable enrichment in Fe 2+ during cooling; and 4) a strong decrease in Cr and cr# with a small increase in mg# and fe 3+ #, likely due to interaction with a more oxidized melt and subsolidus re-equilibration with an aluminous phase (Al-rich residual liquid or amphibole). Values of mg# in placer silicate grains (olivine, clinopyroxene, either Na–Al-poor or Na–Al–Ti-rich, rare aluminous orthopyroxene, edenitic amphibole, among others) and in micro-inclusions in chromian spinel attain 90–95. We evaluated the provenance of chromian spinel grains in the placers using the global spinel database and a new one focusing on Alaskan-type intrusions. Chromian spinel in the Atlin area and a majority of Dease Lake placers exhibits a pronounced Cr–Al trend indicative of an ophiolitic affinity (Slide Mountain and Cache Creek oceanic terranes). In certain placers in the Tulameen area, the chromian spinel defines a Cr–Fe 3+ trend indicating derivation from the Tulameen Alaskan-type intrusion in the Quesnellia island-arc terrane. Placer deposits in central British Columbia (Manson Creek – Cariboo region) display a mixed heritage involving both ophiolitic and Alaskan-type sources. The recognition of an Alaskan-type mafic to ultramafic source for some of the Cr–PGE-bearing placers in central British Columbia underscores the potential for bedrock mineralization in an area where such intrusions are presently poorly represented.

Five placer grains of Pt–Fe alloy, ca. 0.5 to 2 mm, roundish in shape, and hitherto unconfirmed from this locality, were recovered from a placer deposit in Burwash Creek, Kluane area, Yukon. Our results of multiple electron-microprobe analyses (EMP) display the presence of elevated levels of Pd (0.49–5.84, mean 2.38 wt.%), Rh (1.05–1.94, mean 1.50 wt.%), Ir ( 45.7–47.5 Mg 37.5–48.0 Fe 4.9–16.8 , mg # = 100Mg/(Mg + Fe 2+ ) in the range 69.0–90.7], plagioclase (Ab 57.5–60 An 35.1–39.9 Or 2.6–4.8 ), and sodic–calcic amphiboles (0.6 and 1.1 Na apfu ), some of which are rich in Cl (0.72 and 1.17 wt.%, or 0.17 and 0.36 Cl apfu , respectively). The analyzed inclusions of PGM are members of the bowieite Rh 2 S 3 – kashinite Ir 2 S 3 solid-solution series, miassite (Rh,Pt,Pd) 17 S 15 , cooperite PtS, isoferroplatinum Pt 3 Fe (or Fe-rich platinum), vasilite (Pd,Cu,Pt) 16 S 7 , keithconnite Pd 20 Te 7 , an unusual Pt–Pd–Ni–Cu alloy ( i.e. , Pd-rich platinum: Pt 47.7–52.8 Pd 29.3–37.9 Ni 6.3–8.0 Cu 6.1–7.5 ), and a sulfotelluride of Pd, Pd 11 Te 2 S 2 (?), which may represent a new isomertieite-type compound. The trace-element composition of the silicate melt inclusions determined using laser-ablation ICP–MS shows a notable enrichment in large-ion lithophile elements. We infer that the reported association of PGM and the trace-element composition of silicate melt inclusions observed at Burwash Creek probably correspond to an Alaskan–Uralian-type mineralization; considered less likely is a relation with a Triassic ultramafic–mafic complex (sill-like) of so-called Kluane-type, also developed in the placer area.

Patterns of compositional zoning are documented in nine grains of magnesiochromite–chromite (Mgc–Chr), 0.3–0.4 mm in size, subhedral or subrounded, recovered from six placer deposits in British Columbia. Their core zones correspond to Mgc or magnesian Chr, mantled by zones of Chr. A rim of Zn-rich chromite (9.1% ZnO) occurs in one grain. In most of these grains, Fe 2+ increases and Mg decreases toward the margin. In contrast, an “anomalous” rim, slightly richer in Mg than the core, is developed in one grain, in which an increase in Mg (≤2% MgO) is accompanied by a corresponding increase in Fe 3+ , Al, minor Ti, and by a decrease in Fe 2+ and Cr. In the other zoned grains, contrasting trends are documented for Fe 3+ and Cr, which decrease or increase during crystallization toward the margin. Aluminum covaries with Fe 3+ and Cr, as a reflection of different mechanisms of coupled substitution. In general, various factors may have exerted a control over the observed zoning, such as 1) loss of Mg during subsolidus re-equilibration between the Mgc–Chr and coexisting olivine, 2) the regime and variations in levels of oxygen fugacity, f (O 2 ), 3) a re-equilibration between the Mgc–Chr and a late melt enriched in Cr, and (4) accumulation of Al in a remaining melt or loss of Al during a re-equilibration at a lower temperature. The Zn was probably introduced into the rim by a reaction involving a late-stage fluid phase. The rim richer in Mg may reflect a diffusion-controlled mechanism of growth resulted from a late-stage reaction with an isolated portion of the more oxygenated melt rich in Fe 3+ , Al and Ti and relatively depleted in Cr. Two types of potential source-rocks, ophiolitic and Alaskan-type, are evaluated for these placer grains of zoned Mgc–Chr.

We describe an unusual pattern of zoning in an amalgam of Au and Ag; it occurs as peripheral zones in a 0.3 × 0.15 mm grain in a placer deposit associated with the Tulameen – Similkameen river system, British Columbia. This amalgam formed at the expense of primary Au–Ag alloy, which is present as relics in the core: Au 54–55 Ag 45–46 (Hg-free; not zoned). Abundances of Au gradually increase outward and complement decreasing levels of Ag in the amalgam. Values of Au/Ag increase from 1.1–1.2 in the core to 1.8 at the margin of the grain. The observed range of compositions, (Au 1.50–1.87 Ag 1.06–1.40 ) ∑2.81–3.00 Hg 1.00–1.19 , is limited. Two possibilities are discussed to explain the formation of the zoned amalgam. (1) This phase could be natural, formed by reaction between primary Au–Ag alloy and a late melt or oxidizing fluid, which may have remobilized and transported Hg as a result of destabilization and oxidation of cinnabar. If so, the observed zonation would reflect an increasing activity of Au species (and decreasing activity of Ag) during crystallization. (2) This phase may have formed under supergene conditions, as a result of anthropogenic introduction of Hg into placer gravels during historic mining operations in this region; we favor this hypothesis. Zoning in the amalgam may have been controlled principally by electrochemical factors related to the process of “self-electrorefining”. Gold is more “noble” than silver, so that ions of Au probably migrated and were preferentially deposited closer to the margin of the grain. This mechanism is similar to that responsible for the “secondary” rim rich in Au (poor in Ag) in placer grains of Au–Ag alloy, as reported from other localities worldwide.

We report results of multiple electron-microprobe analyses of nine grains of alloy minerals, 2–5 mm in size, rich in the platinum-group elements (PGE), from a unique, very old collection (~1890s) of placer material from Trinity Co., California. Osmium, iridium, ruthenium, and rutheniridosmine are the principal alloy species poor in Fe (typically

We report results of detailed electron-microprobe analyses (EMP) of fourteen grains of Pt–Fe alloy, ca. 0.4 to 1 mm, principally droplet-shaped or roundish, from Florence Creek, Yukon. We also describe new occurrences of Pt–(Pd)–Fe alloy from Arch Creek (up to 11 wt.% Pd) and Canadian Creek, Yukon. An extensive compositional series is observed at Florence, in which values of ∑PGE/(Fe + Cu + Ni) vary from 2.2 to 5.4. Minor Ir and Os display a contrasting behavior in this series. The observed levels of Ir are generally greater than those of Os. The Ir correlates negatively with Pt (the correlation coefficient, R, is −0.81; 140 EMP point-analyses); in contrast, the correlation between Os and Pt is slightly positive (R = 0.64). Probably, at a higher temperature of crystallization, Ir readily replaces Pt in the structure of a Pt–Fe alloy. As a result, Ir did not attain its maximum, and the observed inclusions of exsolution-induced Ir-dominant alloy, hosted by the Pt–Fe alloy, are moderately enriched in Ir (52–54 at.%). An inclusion of erlichmanite (4.2 wt.% Rh) and a partial rim of Au–Ag alloy (Au 0.80 Ag 0.18 ) were observed. Droplet-like inclusions of highly aluminous silicate minerals, hosted by grains of Pt–Fe alloy, are rich in Na, K and in Cl (up to 2 wt.%). Veinlets and micrograins of an Os-rich alloy (86–92 wt.% Os) are intimately associated with micrograins of nearly pure albite, Ab 97.8 An 1.7 Or 0.5 , and with an intermediate member of the chamosite–clinochlore series. The observed association chamosite + albite + native osmium, infilling a cavity in a host grain of Pt–Fe alloy, was likely deposited from a late Na–H 2 O–(Cl)-bearing fluid phase at a postmagmatic–hydrothermal stage, at a temperature less than 700°C. The Os and Na were possibly mobilized and transported as Na–(Os)-rich complexes related to the sodium hydroxy- or oxo-osmates. The precipitation of the metallic Os phase could be related to a change in physicochemical conditions, e.g., in fluid pressure or redox potential, leading to instability of these Na–(Os)-rich hydroxy- or oxo-osmate complexes. We suggest that a mineralized zone rich in chromite–magnesiochromite and hosted by an Alaskan–Uralian-type complex is the likely provenance for grains of Pt–Fe alloy recovered in Florence Creek.

We document the first natural occurrence of oscillatory zoning in hematite, due to cryptic variations in tin content. Our studies bear on a placer grain ( ca. 0.5 × 0.7 mm) from a deposit located in Canadian Creek, Yukon, characterized by electron-microprobe analysis (EMP) and Raman spectroscopy. This grain was found in association with ferberite and hubnerite, which form an extensive and nearly continuous series (with up to 10–12 mol.% MgWO4 in ferberite), bismuthinite, daubreeite (?) [BiO(OH)], and tetradymite (Bi2Te1.9S1.1), which displays a minor deviation from stoichiometry owing to S-for-Te substitution. The compositional data, obtained via EMP traverses oriented across oscillatory zones (up to 10 μm thick, diffuse, and “bright” in back-scattered electron imaging) indicate that this zoning is a reflection of covariations in Fe, Sn, and, to a lesser degree, Al. The “bright” oscillatory zones are enriched in Sn, up to 2.26% SnO2, and in Al, up to 0.42% Al2O3, and are correspondingly poorer in Fe. The Sn-enrichment observed in hematite could be related to a solid-solution series from α-Fe2O3 toward a mixed-valence compound Sn2O3, yielding a mechanism (Sn4+ + Sn2+) ↔ 2 Fe3+. However, a regime of low values of oxygen fugacity would be required to stabilize Sn2+, which seems unlikely in this system. Instead, a series could extend from α-Fe2O3 toward SnO2 (cassiterite). Presumably, Sn4+ enters octahedral sites; a substitution Sn4+-for-Fe3+ is combined with a reduction of Fe3+ to Fe2+ in order to maintain charge balance. We prefer this mechanism, and suggest that the incorporation of minor Al3+ may occur to minimize lattice strain due to the incorporation of Sn4+ and Fe2+. This grain of Sn-bearing hematite may have been derived, along with the W- and Bi-rich minerals, from a mineralized zone associated with a fluid-rich environment, probably a hydrothermal deposit (possibly Au-bearing), which is related to Cretaceous felsic magmatism developed in the placer area.

We have conducted electron microprobe (EMP) analysis of 158 grains of platinum-group minerals (PGM; 0.1–1 mm in size) from 11 placer samples collected from Holocene fluvial placers and buried paleochannel placers at various localities in British Columbia. These grains principally comprise Pt-Fe-(Cu) alloy minerals: Fe-rich platinum [ΣPGE:(Fe + Cu + Ni) = 3.6–7.6], Pt3Fe-type alloy (isoferroplatinum or Fe-rich platinum), subordinate “Pt2Fe”-type alloy (probably, a compositional variant of Fe-rich platinum) and the tulameenite-tetraferroplatinum series. Less-abundant are iridium [Ir-dominant Ir-Os-(Pt) alloy] and osmium [Os-dominant Os-Ir-(Pt) alloy]. Ruthenium [Ru-dominant Ru-Ir-Os alloy] occurs as a single grain. One of these Pt-Fe alloy grains is unusually zoned; its core zone is: Pt74.0Fe20.4Cu1.9Ir1.5Rh1.1Pd1.0Os0.08Ru0.01Ni0.01 (in at%) [ΣPGE:(Fe + Cu + Ni) = 3.5], and its rim zone is: Pt78.5Fe15.5Cu1.7Ir1.5Rh1.4 Pd1.2Ni0.15Os0.06Ru

We report new results of multiple EMP analyses of sorosite from the Baimka Au–PGE placer deposit, western Chukotka, northeastern Russia. It occurs as (1) relatively large (0.2–0.4 mm) euhedral, cryptically zoned hexagonal grains, with Fe–Nirich zones in the core, (2) large irregular or skeletal grains, and (3) elongate subhedral to euhedral (hexagonal), or anhedral micrograins (≤20 m). They all are hosted by native tin (Sb-bearing), and are associated with stistaite Sn1+xSb and herzenbergite SnS. The Fe content of these zoned grains decreases from the center (3 wt.%) toward the edge (≤0.05 wt.%). The Ni content also decreases from the core (1.2 wt.%) to the periphery ( 415°–227°C, the latter being the eutectic temperature in the binary system Cu–Sn. A mineralized mafi c rock (gabbro, gabbro–diorite, or clinopyroxenite) associated with the Yegdegkychsky gabbro – diorite – syenite – monzonite complex in the placer area could be a potential primary source for the sorosite-bearing mineralization.

An extensive, nearly continuous and hitherto unreported solid-solution series is observed in Sr-Na-REE titanates in a fenitized megaxenolith, Khibina alkaline complex, Kola Peninsula, Russia. These titanate minerals, related to crichtonite, landauite and davidite-(Ce,La), may represent potentially new members of the crichtonite group. They display compositional zoning, and are associated with diverse oxide minerals, albite and alkali feldspar. Our compositions (EMP) suggest that the “large cations” (Sr, Na, K, and the rare-earth elements, REE), along with minor Ca, occupy fully the A ( M 0 ) site in their structure; in contrast, Ca enters dominantly the B ( M 1 ) site, probably via a coupled substitution of the type [(Ca 2+ +Ti 4+ ) = (Zr 4+ +Fe 2+ )]. The extensive Na + -for-Sr 2+ substitution observed at the A site is coupled with a Ti 4+ -for-Fe 2+ substitution at the C ( M 3 - M 4 - M 5 ) site. The tetrahedral T ( M 2 ) site is dominated by Fe. Among the REE, only the light REE are present in substantial amounts (up to 7.37 wt.% REE 2 O 3 ); they are positively intercorrelated, indicating an ordered distribution at the A site. The incorporation of the REE 3+ , which probably replace Sr 2+ and K + , is controlled by a corresponding decrease in Zr 4+ (and by relative increase in divalent Mn) at the B site in order to maintain charge balance. The observed presence of up to 1.87 wt.% Cr 2 O 3 in the Sr-Na-REE titanate minerals at Khibina indicates that rocks of mafic affinity were the protolith for the mineralized megaxenolith. The high Na contents of these minerals are clearly related to the geochemical environment ( i.e. , Na-metasomatism). The contrasting association of Cr and Nb (up to 1.06 wt.% Nb 2 O 5 ) in these minerals undoubtedly involves derivation from two different sources. We suggest that the Ti-(Fe)-Nb-REE oxide mineralization formed in the megaxenolith as a result of interaction of a pre-existing mafic rock(s), probably Proterozoic rocks of the Imandra-Varzuga Supergroup, with metasomatizing oxidizing fluids of alkaline affinity.

A shell-like polycrystalline grain (ca. 1 mm) of W-(Mo)-bearing Os-Ir alloy (11.4–18.6 wt% W; up to 1.5% Mo) is present in a very old collection (probably the 1890s) of tiny nuggets from Trinity Co., California. An extensive compositional series [(Os0.43–0.80Ir0.28–0.05) W0.12–0.18], and inverse Ir-Os correlation, are observed; the mean composition [Os0.676W0.153Ir0.124Fe0.021Mo0.015Ru0.011; Σatoms = 1], based on results of 50 electron-microprobe analyses, displays a ratio (Os + Ir):W of 5:1. The observed variations and element correlations suggest that (W + Mo) contents are controlled by Ir, and incorporated via the following substitution scheme: [(W + Mo) + Ir] ↔ Os. The X-ray diffraction data indicate that the W-rich alloy has a hexagonal close-packed structure, related to that of osmium and allargentum, with a = 2.7297(4) A, c = 4.3377(6) A, and V = 27.99(1) A3 ; the c:a ratio is 1.59. The probable space-group is P63/mmc, and Z = 2; the calculated density is 21.86(1) g/cm 3 . The W-rich alloy is associated with an Os-Ru-Ir alloy rich in Fe (7.0–9.7 wt%), which exhibits atomic Fe ↔ [Os + Ru] and Ir ↔ [Os + Ru] mechanisms of substitution. We suggest that these W-(Mo)- and Fe-rich alloys formed by metasomatic alteration of a primary Os-Ir-Ru alloy, associated with mineralized ultramafi c-mafi c rocks of ophiolite affi nity. A fl uid phase may well have remobilized and transported W, Mo, and Fe. The W-rich alloy likely crystallized from a reducing fl uid under conditions of low fugacities of O2 and S2, thus promoting the observed siderophile behavior of W and Mo. These unusual W-(Mo)- and Fe-rich alloy grains were likely derived, as a placer material, from the Trinity ophiolite complex of northern California.

Various platinum-group minerals (PGM), phases rich in the platinum-group elements (PGE), and unusual species occur in the Kirakkajuppura area, Penikat layered complex, Finland. The PGM are intimately associated with actinolite and clinochlore. The base-metal sulfides [chalcopyrite, millerite, bornite, chalcocite, ferroan chalcocite(?), digenite or roxbyite: Cu9S5] are in minor or trace amounts. The observed PGM and rare species are: vysotskite–braggite series [PdS–(Pt,Pd)S], zvyagintsevite [Pd3Pb], a zvyagintsevite-like phase [(Pd,Pt)3(Pb,Cu)], unnamed Pd–Pb oxide [Pd7PbO8 or Pd3.5Pb0.5O4], various Cu–Fe–PGE thiospinels (large grains and unusual micro-aggregates), laflammeite [Pd3Pb2S2], laurite–erlichmanite series [(Ru,Os,Ir)(S,As)2 –(Os,Ru,Ir)(S,As)2], irarsite–hollingworthite series [(Ir,Rh,Ru)As1− x S1+ x – (Rh,Ir,Ru)As1− x S1+ x ; x ≤ 0.2], konderite-like Pb–(Pd)-rich sulfides and a Fe-dominant analogue of konderite [(Cu,Ni,Fe)3+ x (Fe,Pb)(Rh,Ir,Pd)8− x S16; x < 0.5], unnamed Rh1− x (Ni,Fe,Cu)2+ x S3 (0 ≤ x ≤ 0.1), a Pd–Pt–Cu alloy [Pt-rich, Cu-rich palladium: Pd89.42–91.22Pt5.94–6.16Cu2.62–4.64], skaergaardite–hongshiite [(Pd0.92 Pt0.10)∑ 1.02Cu0.98], keithconnite [Pd20(Te,Bi)7], plumbian keithconnite [Pd20(Te6Pb)], unnamed Pd5(As,Te,Sn,Pb)2, Pd-rich, Cu-rich gold [Au76.1–81.1Pd13.7–16.0Cu6.1–6.3Ag1.2–3.4], Zn-rich copper (natural?), unnamed Pb4O(VO4)2, and an undefined Pb-rich iron silicate (hydrous?). In addition, unnamed (Th,Ca)(V5+,Si,P)O4 and associated baddeleyite, barite, and a phosphatian huttonite or thorite occur in a pegmatitic gabbro from Kirakkajuppura. The Kirakkajuppura PGE mineralization probably formed by efficient hydrothermal remobilization and transport of PGE from their “primary” zones [Pd–Pt zones associated with the low-sulfide SJ reef and Ru–Os-mineralized chromitite zones], resulting in the local deposition of the extremely S-poor, PGE-rich assemblage, which is intimately associated with hydrous silicates and is located near the northern end of the Penikat complex.

Platinum-group minerals (PGM) from a selected suite of gold- and platinum-group-element (PGE) placer deposits in British Columbia have been analyzed using the electron-microprobe (EMP). The PGM placer grains ( n = 70, 0.1–1.5 mm in size) principally comprise various Pt–Fe–(Cu) alloys: “Pt 3 Fe”-type alloys [Fe-rich platinum (formerly, ferroan platinum: Bayliss et al. 2005) or isoferroplatinum], Fe-rich Pt with an atomic ∑PGE:(Fe + Cu + Ni) ratio of 3.6–5.6, “(Pt,Ir) 2 Fe”-type alloy, members of the tulameenite–tetraferroplatinum solid-solution series extending from Pt 2 Fe(Cu,Ni) toward PGE 1+ x (Fe,Cu,Ni) 1− x , less common Ir-dominant Ir–Os–(Ru–Pt) alloys, subordinate Os-dominant alloys, and minor Ru-rich alloys and rutheniridosmine, the latter with an atomic Ir:Os:Ru proportion close to 1:1:1. Trace amounts of PGE sulfides and sulfarsenides: cooperite PtS, Ni-rich cuproiridsite (Cu,Ni,Fe)(Ir,Rh,Pt) 2 S 4 , unusual sperrylite-type [(Pt,Rh,Fe)(As,S) 2− x ] and platarsite-type [PtAs 1− x S 1+ x , or unnamed Pt(S,As) 2− x ] phases, and unnamed (Ir,Rh,Pt)S (?) crystallized at a late stage in low-S environments. Some PGM grains contain micro-inclusions of diopside, augite, ferro-edenite, a potassian sodic-calcic amphibole (richterite?), talc, clinochlore and euhedral quartz. High values of mg# [100Mg/(Mg + Fe)] of the ferromagnesian minerals in these inclusions suggest highly magnesian source-rocks. Textural and compositional data, in particular the zoned intergrowths of Pt–Fe–(Cu) alloy grains, which broadly resemble the zoned Pt–Pd–Cu stannides from the Noril’sk complex, indicate the following sequence of crystallization: (Pt,Ir,Rh) 3 Fe → (Pt,Ir,Rh) 1+ x (Fe,Cu) 1− x or Pt 2 Fe(Cu,Ni). The zoned Pt–Fe–Cu alloys likely formed by fractional crystallization of primary solid-solutions under closed-system conditions as a result of increase in the activity of Cu in the residual liquid after an early-stage (magmatic) crystallization of the Cu-poor core. The Cu-rich Pt–Fe alloys formed around these core zones and at their peripheries during a significant drop in temperature at a late stage of crystallization of the composite alloy grains. The compositions of micro-inclusions and exsolution lamellae of Os- and Ir-dominant alloys in Pt–Fe alloys imply uniform temperatures of equilibration within the range 750–800°C. The compositions of Cu-rich Pt–Fe alloys, PGE sulfides and sulfarsenides, and exsolution lamellae of Os- and Ir-dominant alloys, observed in intimate association with Pt–Fe alloys, are likely related to the crystallization of the coexisting Pt–Fe alloys. Narrow zones of Pt–Fe alloys richer in Pt and poor in Fe, observed at the boundary of placer Pt–Fe alloy grains, provide evidence for a removal of Fe and corresponding addition of Pt as a result of interaction with a low-temperature fluid. The terrane affinities, compositions and associations of the placer PGM examined appear consistent with two types of potential source-rocks, associated with Alaskan-type intrusions and the Atlin ophiolite complex. The preservation of faceted morphologies on many of the placer PGM grains implies a relatively short distance of transport from their source.

L'oulankaite, stannosulfotellurure complexe de metaux de base et de metaux precieux provenant du gisement de Nadezhda, complexe stratiforme de Lukkulaisvaara, dans le nord de la Karelie, secteur nord-ouest de laRussie, fait preuve d'une variation importante en composition. A la localite type, cette espece unique parmi les mineraux du groupe du platine (MGP) est associee a plusieurs sulfures de metaux de base et de MGP dans des lentilles de gabbronorite a grains grossiers, voire pegmatitiques, montrant les effets d'une alteration hydrothermale, le tout a l'interieur d'un filon-couche de microgabbronorite. Nos documentons des covariations en teneurs de (Pd + Pt) et de (Cu + Fe + Ag) dans l'oulankaite a faible teneur en Ag (

Nous decrivons un sulfure meconnu de Fe, Pb, Cu, Rh, Pd, et Ir [ci-apres: "Pb-(Fe)+Pd Kdr"], potentiellement une nouvelle espece affiliee a la konderite trouvee en association etroite avec un thiospinelle enrichi en elements du groupe du platine (EGP), de la serie cuprorhodsite - ferrorhodsite - cuproiridsite (Cpr) dans le gisement a EGP a faible teneur en soufre de Kirakkajuppura, que renferme le complexe stratiforme de Penikat, en Finlande. Cette phase, presente surtout en deux grains, un plus gros (0.15 mm) et l'autre en bordure d'un grain de Cpr, fait preuve de proportions de cations rappelant un thiospinelle: (Cu + Fe + Pb + Ni):(Rh + Ir + Pd + Pt + Co):S egal a 1:2:4, mais s'en distingue par son apparence, sa texture rubannee et ses proprietes optiques, et aussi par la presence de Pb (jusqu'a 9.7%, poids) et Pd (jusqu'a 6.4%), dont la correlation entre teneurs est positive. Une relation entre la phase "Pb-(Fe)+Pd Kdr" et la konderite decouverte dans le complexe de Kondyor, de type Ourale-Alaska [dont la composition est (Cu,Ni,Fe) 3 Pb(Rh,Pt,Ir) 8 S 1 6 : phase hexagonale et derivee du thiospinelle) parait vraisemblable. Les resultats d'analyses effectuees a la microsonde electronique semblent indiquer des substitutions couplees de Fe-par-Pb et Rh-par-Pd; les compositions riches en fer de ce sulfure laissent prevoir l'existence d'un analogue de la konderite a dominance de Fe. Le schema de substitution dans le plus gros des grains peut se resumer ainsi: [Cu + + 2 (Pb + Ni) 2 + ] + 2 Pd 2 + = 2 Fe 3 + + Rh 3 + ]. De plus, des variations en S pourraient s'averer essentielles pour maintenir l'electroneutralite. En revanche, le Cu ne semble pas implique dans la substitution dans le moins gros des grains. En fait, le Ni serait plus important que le Cu dans ce cas. Le plus gros des grains et le thiospinelle (Cpr), observes en intercroissance, contiennent des quantites equivalentes d'iridium, et seraient donc equilibres, mais tout le Pb est reparti selectivement dans le "Pb-(Fe)+Pd Kdr". La presence ici de ces chalcogenures de type thiospinelle et konderite, qui ne sont pas caracteristiques de massifs stratiformes, traduit la pauvrete en soufre des roches du gisement de Kirakkajuppura, qui ressemble de ce point de vue les complexes de type Ourale-Alaska.