Search

The ultramafic-mafic Talnakh intrusion in the Norilsk province (Russia) hosts one of the world’s major platinum group element (PGE)-Cu-Ni sulfide deposits. This study employed a multitechnique approach, including in situ Hf-O isotope analyses of zircon combined with whole-rock Nd isotope data, in order to gain new insights into genesis of the Talnakh economic intrusion. Zircons from gabbrodiorite, gabbroic rocks of the layered series, and ultramafic rocks have similar mantle-like mean δ18O values (5.39 ± 0.49‰, n = 27; 5.64 ± 0.48‰, n = 34; and 5.28 ± 0.34‰, n = 7, respectively), consistent with a mantle-derived origin for the primary magma(s) parental to the Talnakh intrusion. In contrast, a sulfide-bearing taxitic-textured troctolite from the basal part of intrusion has high δ18O (mean of 6.50‰, n = 3), indicating the possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks. The Hf isotope compositions of zircon from different rocks of the Talnakh intrusion show significant variations, with ɛHf(t) values ranging from –3.2 to 9.8 for gabbrodiorite, from –4.3 to 11.6 for unmineralized layered-sequence gabbroic rocks, from 2.3 to 12 for mineralized ultramafic rocks, and from –3.5 to 8.8 for mineralized taxitic-textured rocks at the base of the intrusion. The significant range in the initial 176Hf/177Hf values is ascribed to interaction of distinct magma sources during formation of the Talnakh intrusion. These include (1) a juvenile source equivalent to the depleted mantle, (2) a subcontinental lithospheric source, and (3) a minor crustal component. Initial whole-rock Nd isotope compositions of the mineralized taxitic-textured rocks from the base of the intrusion (mean ɛNd(t) = –1.5 ± 1.8) differ from the other rocks, which have relatively restricted ranges in initial ɛNd (mean ɛNd = 0.9 ± 0.2). The major set of ɛNd values around 1.0 at Talnakh is attributed to limited crustal contamination, presumably in deep magma chambers, whereas the smaller set of negative ɛNd values in taxitic-textured rocks is consistent with greater involvement of a crustal component and reflects an interaction with the wall rocks during emplacement.

This study utilized SHRIMP, LA MC–ICP–MS and ID-TIMS analytical techniques and presents for the first time U–Pb age, Lu-Hf-isotope features of zircon and the Nd-isotope composition of variously mineralized rocks from the ore-bearing Dyumtaley intrusion, one of the most promising targets for platinum-group element (PGE)-Cu–Ni ores within the Taimyr metallogenic province. 206Pb/238U ages of zircon from monzodiorite and gabbro of the Dyumtaley intrusion (250.9 ± 3.1 to 253 ± 1 Ma, respectively) indicate their close temporal relationship to a Permo-Triassic boundary and synchronism with a tholeiite-basaltic volcanism of the Siberian platform. Hafnium-neodymium isotope signatures (eHf(t) 6.9–8.5, eNd(t) 2.9–4.2) imply the significant role of the depleted mantle material in magma generation of the Dyumtaley intrusion.

Limited data on the isotopic composition of sulfur of platinum-group minerals (PGM) are reported in the literature. The LA-ICP-MS method was used to study the S-isotopic composition of Ru-Os-Ir sulfides and sulfoarsenides forming secondary corrosion and superimposed PGM assemblages from placer deposit of the East Shishim river occurring within the Verkh-Neivinsky dunite-harzburgite massif. Sulfides from corrosion PGM assemblage (laurite, As-containing laurite) replace the grains of primary native Os and Ru minerals, forming rims on them. PGM from superimposed assemblage (laurite, irarsite, tolovkite, etc.) originated on the surface of laurite rims that form part of corrosion PGM assemblage. Variations in 34S values of secondary PGM (from –4.6 to 7.6 ‰) are more pronounced than those for laurite and erlichmanite from primary PGM assemblage (i. e., 0.2—2.3 ‰). The results are consistent with a model of the ultramafic massif ascending to the surface and subsequent water metamorphism during the tectonic flow and crust-mantle interaction. At the initial stage of metamorphism, mantle sulfur is present, isotopically lightened under oxidation conditions and temperature reduction. The final stage is marked by sulfur of the metamorphic fluid formed by the involvement of isotopically-heavy sulfur derived from the host sedimentary rocks.

This study presents the first results of oxygen isotope analyses (δ18O) collected on zircons from the Talnakh economic intrusion within the Noril’sk province. Zircons from gabbro-diorite, gabbroic rocks of the layered series and plagioclase-bearing wehrlite have similar mantle-like mean δ18O values (5.39 ± 0.48‰; 5.63 ± 0.48‰ and of 5.28 ± 0.34‰, respectively), which differ from δ18O in zircons from sulfide-bearing melanocratic troctolite with taxitic texture in the lower part of the intrusion (mean δ18O = 6.50 ± 0.98‰). These new oxygen isotope data support (i) the mantle-derived origin of the primary magma(s), parental to the Talnakh intrusion, and (ii) possible involvement of a crustal component during the formation of sulfide-bearing taxitic-textured rocks.

This study presents the first data set of sulfurisotope compositions of primary Ru-Os sulfides, represented by laurite (RuS2) - erlichmanite (OsS2) series, within a primary platinum-group mineral (PGM) assemblage derived from the Verkh-Neivinsky dunite-harzburgite massif, a typical example of the mantle ophiolite association at the Middle Urals. The S-isotope signatures of Ru-Os sulfides studied are consistent with derivation of the ore material from a mantle source for Ru-Os sulfides.

This study evaluates for the first time Hf-isotope characteristics of zircon from dunite of the Kondyor massif, which is closely related to an economic platinum placer deposit. The significant range in eHf(t) values (from –8.4 ± 0.8 to 10.5 ± 1.3) in Mesozoic zircons indicates the interaction of a “juvenile” mantle source (eHf(t) = +15) with distinct magma sources, equivalent to a subcontinental lithospheric mantle and/or a continental crust. The peculiarities of the Hf-isotopic composition of Precambrian zircons (eHf(t) from –2.3 to +0.3) is consistent with their xenogenic origin and inheritance from basement rocks of the Siberian Craton.

This study presents new chemical and isotope data for Ru–Os sulfides and coexisting platinum-group minerals (PGMs) within primary assemblage derived from the Verkh-Neivinsky dunite-harzburgite massif. PGMs were studied using electron microprobe analysis, laser ablation and multiple collector-inductively coupled plasma-mass spectrometry. The obtained results are consistent with: (i) high-temperature formation of Ru–Os sulfides and coexisting Os-bearing alloys, (ii) presence of two contrasting sources in the ore formation (i.e., subchondritic and suprachondritic), (iii) resemblance of the initial Os-isotope composition in coexisting laurite and Os-bearing alloy, (iv) significant Os-isotope variations in individual PGMs from the primary assemblage, and (v) potential of using model 187Os/188Os ages of PGMs for geodynamic paleoreconstructions.

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.

Intrusions of the Kruglogorsky type are an integral part of magmatic formations in the Noril’sk area. The marginal portions of these intrusions are composed of microdolerite, dolerite, and contact gabbrodolerite. The central parts of the intrusions consist of leucogabbro and of olivine-free, olivine-bearing, and olivine gabbro-dolerite. Leucogabbro is a characteristic rock of this type of intrusions and sometimes composes up to half of the thicknesses of the rock units. The rocks with plagioporphyritic textures are widespread. Olivine-free, olivine-bearing, and olivine gabbro-dolerite occur as horizons with indistinct boundaries, which are unevenly distributed over the vertical sections of the lithological units. The olivine is the most magnesian (Fo89-64) and richest in Ni (up to 0.23 wt % NiO) in the olivine gabbro-dolerite. The clinopyroxene is represented by augite (Fs12-29). The rock-forming minerals are typically zoned. The Sr isotopic composition of the rocks (calculated for an age of 250 Ma) varies within a considerable range (87Sr/86Sr = 0.705972–0.708006), due to metasomatic alterations. The variations in the Nd and Sr isotopic composition of the Kruglogorsky intrusion are close to those in rocks of the Noril’sk-type ore-bearing intrusions. The olivine-bearing and taxitic gabbro-dolerite host Pt–Cu–Ni ore mineralization, which are of economic value for disseminated ores of the Talnakh area.

This work presents new data on the Hf-isotope systematics of baddeleyite and the Nd-isotope compositions of calcite carbonatites of the Guli massif, located within the Maimecha–Kotui province. The Hf–Nd isotope signatures of carbonatites (eHf ~ 10.4, eNd ~ 5.8) indicate that depleted mantle material was involved during magma generation. The application of the Hf–Nd isotope systematics provided an essential tracer for the depleted mantle contribution into the source of carbonatites from the Guli Massif.

This paper reports chemical, geochronological, and Hf–Nd–Sr isotopic-geochemical data on granite, leucogabbro, and microgabbro porphyrite vein bodies in the gabbro of the Volkovsky massif. It was proved that the vein granite and leucogabbro are genetically related to the leucogabbro–anorthosite–plagiogranite (anorthosite–granite) series of the Urals Platinum Belt. The granite was dated by U-Pb laser ablation inductively coupled plasma mass spectrometric method at 409.0 ± 2.3 Ma. The rock has 87Sr/86Sr(409 Ма) = 0.70358, high eNd(409 Ма) = 6.4–6.5, and eHf(409 Ма) ≥ 10.8. Similar values of 87Sr/86Sr(409 Ма) = 0.70370 and eNd(409 Ма) = 5.9 were obtained for the vein leucogabbro. The isotopic-geochemical data are consistent with existing concept of the formation of the leucogabbro–anorthosite–plagiogranite (anorthosite–granite) series through partial melting of the olivine gabbro. The measured 143Nd/144Nd = 0.512939 value obtained for the microgabbro porphyrite reflects their more radiogenic composition and likely a mantle source. The granite, associated leucogabbro, and microgabbro porphyrite were emplaced at the final magmatic stage in the massif evolution. This event marks the upper age boundary of the Au–Pd mineralization related to the gabbroic rocks. The vein rocks lack any signs of the mineralization. However, it is highly probable that they were sources of energy and fluid for reworking of the earlier olivine gabbro and redeposition of ore components in this rock.

Chromitites enclosed within metasomatised Finero phlogopite peridotite (FPP) contain accessory platinum-group minerals, base metal (BM) sulfides, baddeleyite, zircon, zirconolite, uraninite and thorianite. To provide new insights into mantle-crustal interaction in the Finero lithosphere this study evaluates (1) the mineral chemistry and Os-isotope composition of laurite, (2) the crystal morphology, internal structure, in-situ U-Pb, trace-element and Hf-isotope data of zircon from two chromitite localities at Alpe Polunia and Rio Creves. The osmium isotope results reveal a resticted range of ‘unradiogenic’ 187Os/188Os values for laurite at Alpe Polunia (0.1247–0.1251, mean 0.1249 ± 0.0001). Re-Os model ages (TRD) of laurite reflect an Early Paleozoic partial melting event (ca 450 Ma or older), presumably before the Variscan orogeny. The Os isotopic composition of laurite/chromitite probably preserves their mantle signature and was not affected by later metasomatic processes. U-Pb and Hf-isotope data show that the Finero chromitites have distinct zircon populations with peculiar morphology, internal cathodoluminescence textures, trace-element composition and an overall U-Pb age span from ∼310 Ma to 190 Ma. Three age peaks at Rio Creves (220 ± 4 Ma, 234.2 ± 4.5 Ma and 277.5 ± 3.2 Ma) are consistent with a prolonged formation and multistage zircon growth, in contrast to the common assumption of a single metasomatic event during chromitite formation. The trace-element signatures of zircons are comparable with those of mantle-derived zircons from alkaline ultramafic rocks, supporting the carbonatitic nature of the metasomatism. Hf-isotope compositions of the Finero zircons, with eHf values ranging mainly from −3 to +1, are consistent with crustal input during metasomatism and could indicate that the parental melts/fluids were derived from a relatively old source; the minimum estimates for Hf model ages are 0.8–1.0 Ga. Our findings imply that mantle rocks and metasomatic events at Finero have a far more complex geological history than is commonly assumed.

This study presents compositional features of platinum-group element mineralization derived from the Late Archaean placers in the eastern part of the Witwatersrand basin. The significant presence of platinum-group minerals, formed by polycomponent solid-solution series in the system Ru–Os–Ir–Pt(±Fe), was determined using an electron microprobe analysis. Compositional data indicate that the source for polycomponent solid solutions of the Ru–Os–Ir–Pt–Fe system was the Archaean mantle of the Earth, slightly differentiated with respect to platinum-group elements.

Features of the chemical composition of platinum-group mineral assemblages from chromitites of the zonal Svetly Bor, Veresovy Bor, and Nizhny Tagil clinopyroxenite–dunite massifs of the Middle Urals are compared for the first time.

The compositional and isotope–geochemical features of zircons from wehrlite of the Feklistov massif, which formed platinum coastal placers, are discussed in this paper for the first time. Zircons from wehrlite of the Feklistov massif, similarly to worldwide zoned clinopyroxenite–dunite massifs, are characterized by different morphology, composition and a wide spectrum of ages (from 2.717 to 0.373 Ga). The Late Devonian age (373.2 ± 7.5 Ma) of zircons allows us to characterize the timing of the formation of wehrlite from the Feklistov massif and to correlate its emplacement with a significant superplume event, which covered the Siberia and Laurussia continents. The geological meaning of this dating refers to limiting the lower age boundary for emplacement of the Feklistov clinopyroxenite–dunite massif into the Earth’s crust, which does not contradict geological observations.

The new data for the geology and mineralogy of the platinum group element (PGE) mineralization related to the chromite–platinum ore zones within the dunite of the Svetly Bor and Veresovy Bor massifs in the Middle Urals are discussed. The geological setting of the chromite–platinum ore zones, their platinum content, compositional and morphological features of the platinum group minerals (PGM) are compared to those within the Nizhny Tagil massif, the world standard of the zonal complexes in the Platinum Ural belt. The chromite–platinum orebodies are spatially related to the contacts between differently granular dunites. Majority of PGM are formed by Pt–Fe alloys that are close in terms of stoichiometry to isoferroplatinum (Pt3Fe), and associated with Os–Ir alloys, Ru–Os and Ir–Rh sulfides, and Ir–Rh thiospinels of the cuproiridsite–cuprorhodsite–ferrorhodsite solid solution. The tetraferroplatinum (PtFe)–tulameenite (PtFe0.5Cu0.5) solid solution and Pt–Cu alloys belong to the later PGM assemblage. The established features of the chromite–platinum ore zones testify to the highly probable identification of the PGE mineralization within the dunite of the Svetly Bor and Vesesovy Bor massifs and could be used in prospecting and exploration for platinum.

Economic copper, nickel, and platinum-group element (PGE) deposits are associated with the Talnakh ultramafic–mafic intrusion of the Noril’sk Province in the northwestern part of the Siberian Platform. The results of the chemical Th–U–Pb dating of monazite from the ore-bearing melanotroctolites of the Talnakh intrusion are presented. The isotope-geochemical features of monazite are compared with zircon, sulfides, and baddeleyite.

This study presents the first highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance and Re–Os isotopic data for primary Pt–Fe minerals and Os-rich alloys from the Kondyor zoned-type ultramafic massif located in the southeastern part of the Siberian Craton, Russia. The Kondyor massif is composed of three zones: (A) the oldest dunite core with associated chromitites, with an age of emplacement of ~250 Ma, (B) a younger rim represented by metadunite, wehrlite, clinopyroxenite, and melanocratic gabbro that has transitional contacts with the dunite core, and (C) the youngest apatite-phlogopite-magnetite-rich clinopyroxenite stockwork exposed in the southwestern part of the massif. It has intrusive contacts with the dunite core and formed at ~125 Ma. Chromitites from zone (A) and clinopyroxenites from zone (C) analyzed in this study are characterized by distinct platinum-group mineral (PGM) assemblages dominated by Pt–Fe alloys in the former and cooperite with sperrylite and Pt–Fe alloys in the latter, with subordinate amounts of Os–Ir alloys present in both assemblages. The Re–Os isotope results identify a narrow range of initial 187Os/188Os values for Os–Ir alloys from the zone A chromitites (0.1249–0.1254) and the chromitites themselves (0.12466 ± 0.00005). Such chondritic initial 187Os/188Os values (γ187Os(T =250 Ma) ranging between −0.5 and + 0.1) indicate that the HSE budget of the chromitites and PGM from zone (A) was largely controlled by that of the mantle domain that evolved with long-term near-chondritic Re/Os, a notion that is also supported by the recent data of Luguet et al. (2019). In contrast, Os alloys from the zone C clinopyroxenites have initial 187Os/188Os values ranging between 0.1302 and 0.1308 (initial γ187Os(T =125 Ma) = +3.2 to +3.7), indicative of a suprachondritic time-integrated Re/Os in the source of these rocks. The mineralogical and isotope-geochemical data point to a high-temperature origin of the studied PGM and at least two distinct sources of HSE in the ultramafic rocks of the Kondyor massif.

This study evaluates in detail the mineral chemistry, wholerock and mineral separate Os-isotope compositions of distinct platinumgroup mineral (PGM) assemblages in an isolated chromitite pod at Harold's Grave which occurs in mantle tectonite in the Shetland Ophiolite Complex (SOC), Scotland. This was the first ophiolite sequence worldwide that was shown to contain ppm levels of all six platinum-group elements (PGE) in podiform chromitite, including the contrasting type localities found here and at Cliff. At Harold's Grave the primary PGM assemblage is composed mainly of laurite and/or Os-rich iridium and formed early together with chromite, whereas the secondary PGM assemblage dominated by laurite, Osrich laurite, irarsite, native osmium and Ru-bearing pentlandite is likely to reflect processes including in-situ serpentinization, alteration during emplacement and regional greenschist metamorphism. The osmium isotope data define a restricted range of 'unradiogenic' 187Os/188Os values for coexisting laurite and Os-rich alloy pairs from 'primary' PGM assemblage (0.12473-0.12488) and similar 'unradiogenic' 187Os/188Os values for both 'primary' and 'secondary' PGM assemblages (0.1242±0.0008 and 0.1245±0.0006, respectively), which closely match the bulk 187Os/188Os value of their host chromitite (0.1240±0.0006). The unprecedented isotopic similarity between primary or secondary PGM assemblages and chromitite we report suggests that the osmium isotope budget of chromitite is largely controlled by the contained laurite and Os-rich alloy. This demonstrates that closed system behaviour of the Re- Os isotope system is possible, even during complex postmagmatic hydrothermal and/or metamorphic events. The preserved mantle Os-isotope signatures provide further support for an Enstatite Chondrite Reservoir (ECR) model for the convective upper mantle and are consistent with origin of the complex as a Caledonian ophiolite formed in a suprasubduction zone setting shortly before obduction.

This study evaluates combined U-Pb, Hf-Nd-Sr-Os radiogenic and Cu-S stable isotope data for the ultramafic-mafic intrusions of the Noril’sk and Taimyr Provinces to provide new insights on the timing and sources of materials involved in the generation of variously mineralized lithologies and associated Ni-Cu-PGE sulfide ores. New isotopic-geochemical evidence indicates a complex geological history for the economic intrusions and associated Ni-Cu-PGE sulfide deposits. We propose that an extended period of concentration of the ore components in staging chambers was a key factor in the formation of economic Ni-Cu-PGE sulfide deposits. This requires a reevaluation of models for the origin of the Norilsk-type intrusions and their relationship to the Siberian flood basalts. A set of key geophysical and isotope-geochemical fingerprints is suggested to guide exploration for Ni-Cu-PGE sulfide ores.

Combined Cu–S isotope data for Ni-Cu-PGE sufides from the economic deposits hosted in the Noril’sk-1, Talnakh and Kharaelakh intrusions, from the subeconomic deposits in the Chernogorsk, Zub-Marksheider, and Vologochan intrusions, and from the non-economic occurrences in the Nizhny Talnakh intrusion in the Noril’sk Province (Russia), and the J-M Reef in the Stillwater Complex (USA), are presented for the first time. Most sulfide samples including the ores from the Talnakh intrusion and the Stillwater Complex have a restricted range of δ 65 Cu values from − 1.1‰ to 0‰, whereas those from the Kharaelakh and Noril’sk-1 intrusions have more variable δ 65 Cu values of − 2.3‰ to 1.0‰. Samples from the three economic deposits in the Noril’sk Province have distinct mean δ 65 Cu values of –1.56 ± 0.27‰ for Kharaelakh, –0.55 ± 0.41‰ for Talnakh and 0.23 ± 0.28‰ for Noril’sk-1, consistent with those for carbonaceous chondrite and iron meteorites. The variation of δ 65 Cu values is interpreted to represent a primary signature of the ores, although a magmatic fractionation of Cu isotopes and/or assimilation from an external source cannot be ruled out, particularly in the case of the Kharaelakh ores. Collectively the eight investigated intrusions have an overall sulfur δ 34 S range from − 0.7‰ to 13.8‰. Disseminated and massive ores of the same intrusion have similar δ 34 S values. The disseminated and massive ores of the Khraelakh intrusion have a mean δ 34 S value of 12.57 ± 0.54‰ and 12.76 ± 0.42‰, respectively, which are distinctly different from those of the disseminated (10.97 ± 0.44‰) and massive ores (10.88 ± 0.77‰) of the Talnakh intrusion. The different δ 34 S values of the intrusions may be attributed to sulfur fractionation in deep-seated magma chambers rather than at shallow levels or the present emplacement site. The mean δ 34 S value of the disseminated ores from the subeconomic Ni-Cu-PGE deposits and non-economic occurrences spans over 10.5‰, averaging 0.39‰ with a standard deviation of 1.55‰ for the highly contaminated Zub-Marksheider rocks, 6.38 ± 1.89‰ for Nizhny Talnakh, 6.90 ± 1.17‰ for Vologochan and 10.86 ± 0.43‰ for Chernogorsk. Samples from the Zub-Marksheider intrusion have mantle-like S isotope signatures, consistent with a deep-seated origin. Although individual mineralized instrusions have a restricted range the overall trend they define of falling δ 34 S with rising δ 65 Cu values might be a useful indicator of the economic potential of Ni-Cu-PGE sulfide deposits.

This paper compares the geological, geophysical, and isotopic geochemical data on the Paleoproterozoic East Scandinavian Pd-Pt province in the Baltic Shield and the Late Paleozoic Noril’sk Pd-Pt province in the Siberian Craton. Both provinces contain large magmatic PGE deposits: low-sulfide in the Baltic Shield and high-sulfide in the Siberian Craton. Multidisciplinary evidence shows that the East Scandinavian mafic large igneous province, which has a plume nature, is intracratonic and was not subjected to the crucial effect of subduction-related and other contamination processes, whereas the Noril’sk province is pericratonic with substantial crustal contamination of the intrusive processes. Low-sulfide Pd-Pt deposits dominate in the East Scandinavian province, while high-sulfide Ni-Cu-PGE deposits play the leading role in the Noril’sk province. The U-Pb, Sm-Nd, and Rb-Sr isotopic data indicate multistage and long-term (tens of millions of years) geological history of mafic large igneous provinces. The plume magmatism with specific geochemistry and metallogeny is probably related to lower mantle sources.

The ultramafic–mafic Noril'sk-1 intrusion in Polar Siberia (Russia) hosts one of the world's major platinum-group-element (PGE)-Cu Ni sulphide deposits. In situ Hf-isotope analyses of zircon and baddeleyite, combined with whole-rock Nd-isotope results, identify three distinct clusters of Hf Nd isotope values restricted to different lithological units (e.g., gabbro-diorite, unmineralised layered sequence gabbros, and mineralised portions, represented by ultramafic and taxitic-textured rocks). Hf-isotope signatures of zircon and baddeleyite from unmineralized mafic rocks (e Hf from 7.3 ± 1.1 to 11.4 ± 0.3) reflect the dominant role of mantle-derived magmas and suggest that a juvenile mantle was one of the main sources for the ultramafic–mafic Noril'sk-1 intrusion. The less radiogenic Hf-isotope values for zircons from mineralised rocks (e Hf from 4.9 ± 1.4 to 6.4 ± 1.2) and gabbro-diorite (e Hf − 1.2 ± 1.9) are indicative of involvement of distinct source components, possibly equivalent to a subcontinental lithospheric mantle and a continental crust, respectively. The significant range in the initial 176 Hf/ 177 Hf values of unmineralised rocks and mineralised lithologies indicates interaction of distinct magma sources during formation of the Noril'sk-1 intrusion. Our new findings, in conjunction with whole-rock Nd-isotope results, imply that economic intrusions hosting PGE–Cu Ni deposits of the Noril'sk area have a far more complex magmatic history than is commonly assumed.

In this paper we present textural and mineral chemistry data for a PGM inclusion assemblage and whole-rock platinum-group element (PGE) concentrations of chromitite from Harold’s Grave, which occurrs in a dunite pod in a mantle tectonite at Unst in the Shetland Ophiolite Complex (SOC), Scotland. The study utilized a number of analytical techniques, including acid digestion and isotope dilution (ID) ICP-MS, hydroseparation and electron microprobe analysis. The chromitite contains a pronounced enrichment of refractory PGE (IPGE: Os, Ir and Ru) over less refractory PGE (PPGE: Rh, Pt and Pd), typical of mantle hosted ‘ophiolitic’ chromitites. A ‘primary’ magmatic PGM assemblage is represented by euhedrally shaped (up to 60 μm in size) single and composite inclusions in chromite. Polyphase PGM grains are dominated by laurite and osmian iridium, with subordinate laurite + osmian iridium + iridian osmium and rare laurite + Ir-Rh alloy + Rh-rich sulphide (possibly prassoite). The compositional variability of associated laurite and Os-rich alloys at Harold’s Grave fit the predicted compositions of experiment W-1200-0.37 of Andrews and Brenan (Can Mineral 40: 1705–1716, 2002) providing unequivocal information on conditions of their genesis, with the upper thermal stability of laurite in equilibrium with Os-rich alloys estimated at 1200–1250 °C and f(S2) of 10−0.39–10−0.07.

The chemical compositions of magnetite-orthopyroxene symplectites (MOS) and rock-forming minerals—olivine (Ol), clinopyroxene (Cpx), and magnetite(Mt)—have been studied in 20 samples of olivine-bearing rocks in the Urals, including troctolite, olivine gabbro, and gabbronorite. MOS are orthopyroxene (Opx) monocrystals up to 500 μm in size containing myrmekite-like magnetite intergrowths up to 20–30 μm in width. According to the microprobe examination, the dark-colored minerals are characterized by a high Fe mole fraction F = Fe/(Fe + Mg) = 0.20–0.50, whereas F = 0.33–0.65 is typical of the bulk rock compositions. The plagioclase varies in composition from An90 to An50. No significant compositional difference has been established between the MOS and rock-forming minerals. The F opx and F Ol are closely correlated (linear trend, r = 0.97); F Ol/F Opx is ∼1.2. Similarly, a positive correlation between F Opx and F Cpx is noted (linear trend, r = 0.90); F Opx/F Cpx is ∼1.2. The crystallization temperature of the Ol-Opx-Cpx assemblage is roughly estimated at 700–800°C. A high positive correlation (r = 0.95) is established between the TiO2 contents in the magnetites from the MOS (Mt1) and host rock (Mt2). The Mt1/Mt2 ratio reaches ∼0.8, implying that Mt1 contains somewhat less TiO2 than Mt2. Hence, the rock-forming and MOS minerals make up an equilibrium assemblage. As follows from the structural pattern, symplectites were formed as products of the reaction between olivine and oxygen in the solid state with the entire volume of the rock involved in the oxidation; i.e., the distance of the diffusion was significant. Free oxygen appeared as a product of the dissociation of the water penetrating into the hot gabbro and ultramafic rocks at the initial stage of the tectonic extension and high-temperature hydration. According to the redox state of dunite coexisting with gabbro, the oxygen fugacity is estimated at +2.7 log units of fO2 relative to the QFM buffer. The structure and products of the olivine oxidation were eventually obliterated in the course of the hydration.

The material and isotope-geochemical peculiarities of zircons from dunites of the Kondyor massif (Aldan Province, Southeast Siberian Craton) have been described for the first time. This massif is associated with an economic platinum placer deposit. On the basis of detailed study of the morphology, internal structure, and geochemical characteristics, zircons have been subdivided into two types. The first type is represented by crystals of oval and rounded shapes, characterized by a bimodal distribution of “ancient” (2477 ± 18 and 1885 ± 52 Ma) U-Pb (SHRIMP-II) ages. The second type of zircons forms idiomorphic crystals and aggregates of prismatic habitus, forming two “young” age clusters (176 ± 1.2 and 143 ± 2.0 Ma). The obtained results allow us to make a conclusion about the long evolution of platinum-bearing dunites. The first type of zircons formed by metamictic cores and homogeneous peripheral rims indicates the time of metamorphism at the Archean-Proterozoic boundary (∼2.5 Ga), implying an older age of porous zircon cores. Such zircons characterize the minimal age of the initial mantle substance or a close time when the platinum-bearing dunite was generated. Based on their formation time, the second type of zircons can be referred to the epoch of tectonic-magmatic activation of the Aldan Shield. The new geochronological data, along with the material identity of dunites from the Urals and the Aldan Province, argue for a common genetic origin of platinum-bearing dunites of the fold belts and ancient cratons.

We present for the first time the mineralogical and isotope-geochemical particularities of zircon and baddeleyite from various rocks of the economic ore-bearing Noril’sk-1 intrusion located in the northwestern part of the Siberian platform. The ultramafic-mafic Noril’sk-1 intrusion hosts one of the world’s major economic platinum-group-element (PGE)-Cu-Ni sulphide deposits. A detailed study of crystal morphology and internal structure identify four zircon populations, characterized by different U-Pb (SHRIMP-II) ages. The U-Pb ages of baddeleyite and the defined zircon populations cover a significant time span (from 290 ± 2.8 to 226.7 ± 0.9 Ma). The established U-Pb ages imply that crystallization of baddeleyite and zircon populations corresponds to several stages of protracted evolution of the ore-forming magmatic system (290 ± 2.8, 261.3 ± 1.6, 245.7 ± 1.1, 236.5 ± 1.8, and 226.7 ± 0.9 Ma, respectively) that served as the favorable factor for accumulation of magmas and ores of unique scales and concentrations.

The ultramafic–mafic Noril’sk-type intrusions in the northern part of Siberia, in Russia, host some of the world’s major economic platinum-group-element (PGE)–Cu–Ni sulfide deposits. We present the results of Re–Os and S isotope systematics of PGE–Cu–Ni sulfide ores associated with the economic Talnakh and Kharaelakh intrusions within the Talnakh ore junction, Noril’sk Province. At Kharaelakh, Re–Os isotope analyses of disseminated and massive PGE–Cu–Ni sulfide ores identify distinct sources of sulfide mineralization, with Re–Os isochron ages that cluster around 247 Ma. Different initial Os isotope compositions of massive (187Os/188Os = 0.1283 ± 0.0054, γOs(t) = 1.1) and disseminated (187Os/188Os = 0.1331 ± 0.0052, γOs(t) = 4.9) sulfide ores at Kharaelakh indicate either a less contaminated nature of the massive orebody with respect to sulfide disseminations or distinct isotope sources involved in their generation. At Talnakh, Re–Os isotope systematics of massive and disseminated ores show broadly similar patterns ( e.g., 187Os/188Os = 0.1366 ± 0.0037, γOs(t) = 7.6 and 187Os/188Os = 0.1350 ± 0.0036, γOs(t) = 6.4, respectively), indicating their similar sources, likely of mantle origin. Combined with new geochronological and isotope-geochemical data on zircon, the Re–Os isotope systematics of sulfide ores are consistent with protracted evolution of ultramafic–mafic magmas at deep-seated staging chambers in the Noril’sk region, which served as the favorable factor for accumulation of ores of unique scales and concentrations. Sulfur isotope values (δ34S) of disseminated and massive PGE–Cu–Ni sulfide ores at Kharaelakh (12.2 ± 0.5‰ and 12.7 ± 0.1‰, respectively) are notably different from those at Talnakh (10.8 ± 0.1‰ and 10.9 ± 0.1‰, respectively), indicative of distinct involvement of heavy sulfur during formation of PGE–Cu–Ni sulfide ores. The restricted range of δ34S values can be employed as useful fingerprint in the assessment of the potential of prospective ore deposits.

The Nizhny Tagil and Guli clinopyroxenite-dunite massifs, located in the Middle Urals and Maimecha-Kotui Province, respectively, are associated with world-class platinum-group elements (PGE) placer deposits. Both massifs contain small bodies of schlieren to massive chromitite associated with dunite. The predominance of Pt-Fe alloys at Nizhny Tagil is consistnt with the whole-rock “M”-shaped mantle-normalized PGE pattern of the chromitite. In contrast, the preponderance of laurite and Os-Ir alloys at Guli is consistent with a negatively sloped PGE pattern, the latter being characteristic of ophiolite-type podiform chromitites. The ‘unradiogenic’ 187Os/188Os values obtained for both platinum-group minerals (PGM) and chromitite are indicative of a common near-to-chondritic source for the PGE and implies that the osmium isotope budget of chromitite is largely controlled by laurite and Os-rich alloy. Average model 187Os/188Os ages calculated for the Nizhny Tagil and Guli massifs correspond to the late Riphean (e.g., 862 ± 48 Ma and 616 ± 8 Ma, respectively). The compositional and isotope-geochemical results provide new constraints on the temporal evolution of ultramafic rocks of the Uralian Platinum Belt and northern segment of the Siberian Platform.

This contribution firstly presents particularities of mineral chemistry of platinum-group elements (PGE) mineralization from placer deposits linked to the Bor-Uryakh massif of the Maimecha-Kotui Province, northern part of the Siberian Craton. The chemical composition of PGE mineralization has been studied by electron microprobe analysis. At Bor-Uryakh, main platinum-group minerals (PGM) comprise Os-Ir and Pt-Fe alloys represented by individual crystals, and polyphase PGM assemblages. The majority (e.g., 12 out of 19) of the Os-rich nuggets are iridian osmium, with subordinate amounts of native osmium (Os) and chengdeite (Ir3Fe). Pt-Fe alloys have a stoichiometric composition close to Pt2Fe. According to the nomen-clature by L. Cabri and C. Feather [1975] these minerals correspond to ferroan platinum. Based on geological position and geochemical features of investigated PGE mineralization the particular rock sources have been established. This study has demonstrated the similarity of chemical characteristics of Os-Ir and Pt-Fe alloys of the Bor-Uryakh massif to those of PGM from the Guli massif (Maimecha-Kotui Province), platiniferous zoned-type ultramafic massifs (e.g., Kondyor, Inagli and Chad) of the Aldan Province and Platinum belt of the Urals (Nizhny Tagil, Kytlym, etc.).

This study was partly supported by Russian Foundation for Basic Research (grant 09-05-01242).

This study firstly presents chemical and initial Os-isotopic compositions of Os-Ir-Ru minerals of two ultramafic formations of Polar Siberia, which are exemplified by Guli clinopyroxene-dunite massif of the Maimecha-Kotui Province and the Kunar dunite-harzburgite massif from the Chelyuskin ultramafic belt of the Taimyr Peninsula. The study employed a range of methods, including electron microprobe analysis, negative thermal ionization mass spectrometry (N-TIMS) and laser ablation attached to an inductively coupled plasma mass spectrometry (LA MC-ICP-MS). The majority of platinum-group minerals (PGM) from the Guli massif are Os-(Ir-Ru) solid solutions or Os-rich minerals. At Kunar, minerals of Ru-Os-Ir system (i.e., osmium, ruthenium, iridium and rutheniridosmine) dominate the PGM assemblage. The ruthenium trend in the mineral compositions is due to the formation of these minerals under high pressures and temperatures at considerable depths. The 187Os/188Os values of Os-rich minerals from the Guli massif range from 0.12309 ± 0.00002 to 0.12606 ± 0.00003 (n = 168). The initial Os-isotopic composition of PGM from the central block of the Guli massif is characterized by the 187Os/188Os values, varying in the range 0.12404–0.12606. Osmiumrich minerals from the southwestern block of the Guli massif are characterized by the least “radiogenic” 187Os/188Os values (i.e., 0.12309–0.12341). Low relative to the chondritic universal reservoir (CHUR) 187Os/188Os values are indicative of a near-to-chondritic source of platinum-group elements (PGE). The most “productive” stage of PGM formation at Guli (n = 121) is recorded in the time interval of 545–615 Ma. The older model 187Os/188Os ages of osmium minerals are characteristic of the southwestern block of the Guli massif (e.g., 745–760 Ma). The results of the initial Os-isotopic composition for Os-rich alloys are consistent with a model, in which PGM were formed during multi-stage melt depletion events in the mantle. This agrees well with the suggestion that the Guli massif consists of heterogeneous blocks of ultramafic rocks. The 187Os/188Os ratio in the investigated PGM from the Kunar massif varies in a wider range (0.1094–0.1241, n = 28). For the dominant set of PGM samples (n = 25), regardless of their chemical composition, four groups of the initial osmium isotopic compositions can be estimated, with average 187Os/188Os values of 0.1217 ± 0.0002 (n = 7), 0.1223 ± 0.0002 (n = 7), 0.1230 ± 0.0002 (n = 6) and 0.1238 ± 0.0003 (n = 6), respectively. The average model Re-Os ages for the defined groups of the Kunar massif are consistent with Late Riphean age interval (e.g., 975 ± 42 Ma, 892 ± 42 Ma, 791 ± 28 Ma and 681 ± 42 Ma, respectively). Significant variations in the 187Os/188Os values and model ages for Ru-Os-Ir alloys at Kunar are close to those from other duniteharzburgite massifs of the Earth, pointing out for their prolonged multi-stage evolution within the upper mantle.

The composition and isotopic-geochemical peculiarities of zircons from the orebearing ultra� mafic-mafic intrusions of western Taimyr that are promising for finding Pt-Cu-Ni mineralization were characterized for the first time. The similar U-Pb age of zircons from the Binyuda and Dyumaltei intrusions (248.3 ± 13 and 244.4 ± 2.4 Ma, respectively) indicates that the intrusions were formed almost synchronously with tholeiitic basalts of the Siberian Platform. The age and Nd-Sr data of intrusions of western Taimyr are distinct from those of economic orebearing intrusions of the Noril'sk province characterized by longterm magmatic evolution of the rocks and different material sources.

The ultramafic–mafic Kharaelakh intrusion in the northwestern part of the Siberian Craton (Russia) hosts major economic platinum-group-element (PGE)–Cu–Ni sulphide deposits. In situ U–Pb, REE and Hf-isotope analyses of zircon from these rocks, combined with detailed study of crystal morphology and internal structure, identify four zircon populations. U–Pb ages of these populations cover a significant time span (from 347 ± 16 to 235.7 ± 6.1 Ma) suggesting multiple magmatic events that cluster around 350 and 250 Ma, being consistent with two recognised stages of active tectonism in the development of the Siberian Craton. The oldest zircon population, however, represents previously unknown stage of magmatic activity in the Noril’sk area. Epsilon-Hf values of +2.3 to +16.3 in the analysed zircons reflect a dominant role of mantle-derived magmas and suggest that juvenile mantle material was the main source for the ultramafic–mafic Kharaelakh intrusion. A significant range in initial 176Hf/177Hf values, found in zircons that cluster around 250 Ma, indicate mixing between mantle and crustal magma sources. Our findings imply that economic intrusions hosting PGE–Cu–Ni deposits of the Noril’sk area have a far more complex geological history than is commonly assumed.

Material identity of dunite from zoned-type massifs of the Aldan shield (Kondyor, Chad and others) and dunite “cores” from the Uralian Platinum Belt (i.e., UPB) allows considering them as blocks of ancient material, which have the platform nature, integrated into complex structure of the UPB. In order to provide more clarity in the proposed hypothesis, grains of zircon from dunite of the Nizhny Tagil massif, an undisputable example of a zoned Uralian-type clinopyroxenite-dunite complex, genetically not connected to gabbro, have been studied by methods of isotope geology. Grains of zircon are represented by crystals and their fragments in the size range from 70 up to 170 microns long with length to width ratios from 1.3 up to 1.6. Parameters of U-Th-Pb isotope system were measured using sensitive high-resolution ion microprobe SHRIMP-II at 11 local areas of 8 crystals. Results for studied zircons in concordia diagram 207Pb/235U-206Pb/238U define three statistically different age clusters, i.e., 585 ± 29, 1608 ± 56 and about 2655 million years, accordingly. We presume that the Late Archean age cluster characterize time of dunite generation in a subcontinental mantle. Presence of three discrete age clusters, along with morphological variety of zircons and 232Th/238U values in an interval 0.071–1.692, together with already existing geological facts, reflect prolonged evolution of the dunite in complex geochemical and geodynamic conditions, supporting the conclusion about autonomy of the dunite “cores” in the structure of the UPB.

This study presents the first extensive data set of Os isotopic compositions of primary Ru–Os–Ir alloys and Ru–Os sulfides derived from two Neoproterozoic ophiolite-type ultramafic massifs (Kunar in Northern Taimyr, Russia, and Kraubath in Eastern Alps, Austria). The study employed a multi-technique approach and utilized a number of analytical techniques, including hydroseparation, electron microprobe analysis, negative thermal ionization mass-spectrometry (N-TIMS) and laser ablation attached to multiple collector-inductively coupled plasma-mass spectrometry (LA MC-ICP-MS). The results identify two different Os isotope sources (subchondritic and suprachondritic) for bedrock Ru–Os sulfides at Kraubath, whereas detrital Ru–Os–Ir alloys at Kunar are characterized by ‘unradiogenic’ 187Os/188Os values, indicative of a chondritic or subchondritic mantle source of platinum-group elements (PGE). The ‘unradiogenic’ 187Os/188Os values yield a very wide range of 187Os/188Os (0.1094±0.0003 to 0.1241±0.0004, n=19, N-TIMS data), which is almost identical to the Os isotope composition of bedrock laurite, (Ru,Os)S2, in podiform chromitite within an ophiolitic mantle section at Kraubath (0.11249±0.00062 to 0.12437±0.00051, n=17, N-TIMS and LA MC-ICP-MS data). These results are consistent with a model in which a prolonged history of melting events of parent ultramafic source rocks took place in the oceanic upper mantle. In contrast, erlichmanite and laurite from banded chromitite within the transition zone at Kraubath show high 187Os/188Os (0.13080 to 0.13212) values, indicative of a suprachondritic source of PGE. This signature might be interpreted as either evidence of an enriched mantle source or an indication of a ‘radiogenic’ crustal component, which was introduced during a subduction-related event. The Os isotope results obtained support the conclusion that the Re–Os system, represented by PGM, has remained unchanged from the time of formation of the PGM until now, despite later thermal events, which occurred in the vicinity of ophiolite-type complexes.

The compositionally diverse platinum-group minerals (PGM) from the Evander Goldfield, in the eastern part of the Witwatersrand Basin, South Africa, have been studied for the first time by a number of modern techniques. The characteristic feature of PGM from Evander is an extensive presence of Ru-rich alloys ( i.e. , Ru–Os–Ir, Ru–Os–Ir–Pt, Ru–Ir–Pt, Ru–Pt), which prevail over osmium, iridium, rutheniridosmine, Pt–Fe, Pt–Ru–Fe and Pt–Ir–Os alloys and other PGM. The 187Os/188Os value, measured by N–TIMS in PGM that contain Os in the range 18–53 wt.% ( i.e., Pt–Ir–Os alloy, ruthenium, rutheniridosmine and osmium), varies from 0.0987 to 0.1068, revealing the lowest three 187Os/188Os values (0.0987–0.1024) reported so far in terrestrial PGM. The 187Os/188Os value measured by LA MC–ICP–MS in PGM with Os contents between 2 and 10 wt.% (Pt–Fe, Ru–Ir–Pt and Ru–Pt alloys) was found to range from 0.1053 to 0.1095. The model 187Os/188Os ages obtained for the main set of PGM (4104–3020 Ma, n = 12) imply that the PGM are detrital and were thus not deposited by later hydrothermal fluids. They also favor a scenario in which the majority of PGM were incorporated into the Witwatersrand basin by their release during weathering of ultramafic or mafic source-rocks.

The chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite–erlichmanite (RuS 2–OsS2), ruarsite–osarsite (RuAsS–OsAsS) series and Os–Ir alloy (Os,Ir)] from variably altered podiform chromitites of the Kraubath and Hochgrossen dunite–harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics in the Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-group element (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt, Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100*Cr/(Cr + Al)] of 74 to 87 and a Mg# [100*Mg/(Mg + Fe 2+ )] of 44 to 61, values typical of podiform chromitites from the mantle section of an ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and Hochgrossen, respectively). Sperrylite, PtAs 2, is the most abundant PGM (61%) in the altered chromitite, whereas minerals of laurite–erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os–Ir alloy ) occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrossen, laurite and Os–Ir alloy are present as solita ry grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation

We report highly unusual platinum-group mineral (PGM) assemblages from geologically distinct chromitites (banded and podiform) of the Kraubath massif, the largest dismembered mantle relict in the Eastern Alps. The banded chromitite has a pronounced enrichment of Pt and Pd relative to the more refractory platinum-group elements (PGEs) of the IPGE group (Os, Ir, Ru), similar to crustal sections of ophiolites. On the contrary, the podiform chromitite displays a negatively sloping chondrite-normalised PGE pattern typical of ophiolitic podiform chromitite. The chemical composition of chromite varies from Cr# 73–77 in the banded type to 81–86 in the podiform chromitite. Thirteen different PGMs and one gold-rich mineral are first observed in the banded chromitite. The dominant PGM is sperrylite (53% of all PGMs), which occurs in polyphase assemblages with an unnamed Pt-base metal (BM) alloy and Pd-rich minerals such as stibiopalladinite, mayakite, mertieite II, unnamed Pd–Rh–As and Pd(Pt)–(As,Sb) minerals. This banded type also contains PGE sulphides (about 7%) represented by a wide compositional range of the laurite–erlichmanite series and irarsite (8%). Os–Ir alloy, geversite, an unnamed Pt–Pd–Bi–Cu phase and tetrauricupride are present in minor amounts. By contrast, the podiform chromitite, which yielded 21 different PGMs, is dominated by laurite (43% of all PGMs) which occurs in complex polyphase assemblages with PGE alloys (Ir–Os, Os–Ir, Pt–Fe), PGE sulphides (kashinite, bowieite, cuproiridsite, cuprorhodsite, unnamed (Fe,Cu)(Ir,Rh)2S4, braggite, unnamed BM–Ir and BM–Rh sulphides) and Pd telluride (keithconnite). A variety of PGE sulpharsenides (33%) including irarsite, hollingworthite, platarsite, ruarsite and a number of intermediate species have been identified, whereas sperrylite and stibiopalladinite are subordinate (2%). The occurrence of such a wide variety of PGMs from only two, 2.5-kg chromitite samples is highly unusual for an ophiolitic environment. Our novel sample treatment allowed to identify primary PGM assemblages containing all six PGEs in both laurite-dominated podiform chromitite as well as in uncommon sperrylite-dominated banded chromitite. We suggest that the geologically, geochemically and mineralogically distinct banded chromitite from Kraubath characterises the transition zone of an ophiolite, closely above the mantle section hosting podiform chromitite, rather than being representative of the crustal cumulate pile.