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Platinum-group elements (PGEs) occur in ophiolitic chromitite in the Dominican Republic as platinum-group minerals (PGMs) in spatial association with hydrothermal uvarovite and chromian clinochlore. Bulk-rock total PGE content in a single analyzed chromitite sample is of 6.54 g/t. Three main PGM types are distinguished: euhedral magmatic laurite completely encased in chromite, subhedral to euhedral Ru-Os-Fe-(Ir) compounds partially encased in chromite, and anhedral Pt-Fe–Ni-rich grains exclusively embedded in uvarovite or chromian clinochlore. The Ru-Os-Fe-(Ir) compounds are interpreted as magmatic Ru-Os sulfides that experienced desulfurization during hydrothermal alteration of the chromitites, whereas the Pt-Fe–Ni-rich grains are hydrothermal in origin. We propose a model in which the Pt-Fe–Ni-rich PGMs formed via the accumulation of nanoparticles directly precipitated from the hydrothermal fluids. An estimation of the temperature of crystallization of uvarovite and chromian clinochlore suggests hydrothermal alteration of the chromitite within the thermal range of 150–350 °C. Thermodynamic modeling shows that, within this range of temperature, Pt could be mobilized as aqueous bisulfide complexes (HS−) by S-poor, highly reducing hydrothermal fluids originated during serpentinization of the host chromitite rock. The crystallization of Ni sulfides in the chromitite would drop the S concentration of the fluid, causing the precipitation of Pt as native element. Ultimately, this process contributes to constrain the conditions for the genesis of hydrothermal PGE mineralizations in ophiolitic chromitites.

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.

Extremely abundant PGE-minerals (PGM) hosted in chromitites from the Veria ophiolite complex in Macedonia (N. Greece) may be unique among ophiolite complexes. This study focuses on differences between the low- and high-PGE chromitites. New textural, mineralogical and geochemical constraints from those ores are presented, aiming to define factors controlling the PGE enrichment in a supra subduction environment, in the light of post-magmatic processes. The whole ore analyses for mmajor and trace elements indicated an unusually high-IPGE content (up to 25 ​ppm) and higher Fe, Ca, Mn, Zn and V contents in high-PGE compared to low-PGE in massive chromitites. The wide compositional variation of chromite, even in the same polished section, the occurrence of very fine PGM (less than 20 ​μm) as inclusions within chromite and extremely large (>1000 ​μm), angular or fine-grained PGM aggregates ones within a matrix of highly fragmented chromite - Cr-garnet matrix, may indicate crystallization/recrystallization of chromite from more than one precursor phases. Laurite (RuS2) is very limited, occurring as remnants surrounding by Ru–Os–Ir oxides/hydroxides, of a wide compositional variation. Irarsite occurs as euhedral crystals up to 200 ​μm, surrounding by chromite, as anhedral exsolutions 1–200 ​μm within laurite, or creating segregates with platarsite and relics of (Ru, Pt, Rh, Os) sulfarsenides. Platinum–Ru–Rh–Pd-minerals occur commonly as relatively fine-grained assemblages, up to 50 ​μm, along with irarsite and other relics of (Ru, Pt, Rh, Os) sulfarsenides. Pt-alloys show a variation ranging from tetraferroplatinum to Pt–Ir–Fe–Ni alloys. The presence of laurite relics in large IPGM, awaruite, heazlewoodite, and carbon-bearing material reflecting a super-reducing environment, and the transformation of primary PGM into Os–Ir–Ru-alloys and oxides/hydroxides in association with Fe-chromite and Fe3+-bearing garnet (andradite-uvarovite solid-solution series) may reflect changes of the redox conditions from reducing to oxidizing. The relatively high Na content in hydrous mineral inclusions within high-PGE chromitites suggest a hydrous mantle source and provide the possibility for estimation of the P (average 3.0 ​kbar) and T (average 874 ​°C), indicating formation at a shallow mantle environment.

—Small grains of platinum group minerals (PGMs) have been found in chromitites of the Ergaksky and Kalninsky ultramafic massifs, which are marginal northeastern fragments of the Kurtushiba ophiolite belt in Western Sayan. Along with previously described PGMs, the Cu–Ru–Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, platinum group element (PGE) sulfides of the MeS and Me3S4 types, sulfoarsenide with the high Rh content (12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif were found for the first time. The PGM compositions indicate a high formation temperature of Ru–Os sulfide (1200–1250°C) in both massifs, as well as a high partial sulfur pressure during the formation of chromitites and syngenetic PGMs in the Ergaksky massif. The PGM assemblages were formed during several stages. The Os–Ir–Ru alloys and the laurite–erlichmanite series disulfides (predominantly partitioning Ru) are early; the Cu–Ru–Os alloys, sulfides (Ru,Fe)3S4, (Ru,Ni,Fe)S, and PGE sulfoarsenides are late and were crystallized under the influence of reductive fluids as a result of PGE remobilization and redeposition.

The Epilchik Ural-Alaskan type mafic-ultramafic zoned complex, located in the northernmost part of the Koryak-Kamchatka Platinum Belt (Russian Far East), contains lode platinum mineralization. The outcrops of the complex are crossed by the Snegovaya River, which hosts placer platinum-group minerals (PGM) occurrences. The primary PGM assemblage of the Epilchik lode chromitites is typical for Ural-Alaskan type complexes, dominated by Ir-rich isoferroplatinum which occurs as small euhedral inclusions within chromite. Secondary alloys include tetraferroplatinum (PtFe), tulameenite (Pt2FeCu), an unnamed Fe3Pt compound, and Pt-bearing native iron, which form alteration rims around isoferroplatinum or occupy fractures in chromite. Other secondary minerals from lode chromitites are hollingworthite (RhAsS) and Os-Fe alloy (“hexaferrum”). The PGM assemblages of the Snegovaya River placer occurrences are comparable with those of the lode chromitites with a predominance of Ir-rich isoferroplatinum and secondary alloys, including tetraferroplatinum, tulameenite, and an unnamed Fe3Pt compound. However, placer occurrences are much richer in regard to mineral diversity, comprising native osmium (Os,Ir), native iridium (Ir,Os), laurite (RuS2), erlichmanite (OsS2), cuprorhodsite (CuRh2S4), cuproiridsite (CuIr2S4), Co-malanite (Cu(Pt,Co)2S4), unnamed sulfides Rh3S4, (Rh,Pt)S, sperrylite (PtAs2), platarsite (PtAsS), irarsite (IrAsS), hollingworthite (RhAsS), osarsite (OsAsS), cooperite (PtS), native gold, unnamed telluride (Pt2Te), and an unnamed Fe-Ir-Rh alloy. Similarities between lode and placer mineralization indicate that the Epilchik complex was the dominant source for the placers. The larger diversity of PGM in placers may be attributed to the heterogeneity of samples which could be derived from unsampled or unexposed outcrops of the complex. Alteration of isoferroplatinum to secondary intermetallic compounds occurred via progressive development of reduced conditions as a result of serpentinization. Subsequent pervasive As-mineralization redistributed platinum-group elements, including Os and Ir. A final mineralizing pulse deposited sporadic native gold and Pt telluride minerals. A comparison of key mineralogical and physical features between the Epilchik complex and other Ural-Alaskan type complexes is used to determine the economic potential for placers along the Snegovaya River. Key criteria include (1) the size and degree of erosion of the outcrop area of the complex; (2) the predominance of isoferroplatinum over native platinum; (3) high Ir content in Pt-Fe alloys; and (4) the predominance of secondary intermetallic compounds over sulfides. Based on these criteria, the occurrences associated with the Epilchik complex are considered prospective for high volume and economic grade accumulations of PGE. This method is useful for assessing the economic potential of placer occurrences during indirect early-stage prospecting with a limited sample suite.

The mantle tectonite of the Kraka ophiolite contains several chromite deposits. Two of them consisting of high-Cr podiform chromitite—the Bolshoi Bashart located within harzburgite of the upper mantle transition zone and Prospect 33 located in the deep lherzolitic mantle—have been investigated. Both deposits are enveloped in dunite, and were formed by reaction between the mantle protolith and high-Mg, anhydrous magma, enriched in Al2O3, TiO2, and Na2O compared with boninite. The PGE mineralization is very poor (&lt, 100 ppb) in both deposits. Laurite (RuS2) is the most common PGM inclusion in chromite, although it is accompanied by erlichmanite (OsS2) and (Ir,Ni) sulfides in Prospect 33. Precipitation of PGM occurred at sulfur fugacity and temperatures of logƒS2 = (−3.0), 1300–1100 °C in Bolshoi Bashart, and logƒS2 = (−3.0/+1.0), 1100–800 °C in Prospect 33, respectively. The paucity of chromite-PGM mineralization compared with giant chromite deposits in the mantle tectonite in supra-subduction zones (SSZ) of the Urals (Ray-Iz, Kempirsai) is ascribed to the peculiar petrologic nature (low depleted lherzolite) and geodynamic setting (rifted continental margin?) of the Kraka ophiolite, which did not enable drainage of the upper mantle with a large volume of mafic magma.

The platinum-group element (PGE) contents of komatiites have been used to estimate the PGE contents of their mantle sources and investigate temporal variations in the PGE systematics of the mantle throughout Earth history. However, the use of mantle melts to investigate source characteristics is critically dependent on understanding the fractionation of the elements in question between mantle sources and their derived magmas. We present new PGE and Re abundance data, together with Re-Os isotope data for the pristine 1.9 Ga Winnipegosis komatiites, Canada. Whole rock Re-Os isotope data define a 1865 ± 40 Ma isochron, consistent with previous age estimates, with initial γOs of +0.9 ± 1.4. The PGE and Re variations in these komatiites were controlled by fractionation of olivine, chromite, Os-Ir alloys, a Ru-Os rich phase (possibly laurite) from the komatiite melt. The calculated parental melt composition at 23.6 ± 1.6 wt% MgO has 1.1 ± 0.1 ppb Os, 0.91 ± 0.08 ppb Ir, 4.0 ± 0.5 ppb Ru, 7.0 ± 0.8 ppb Pt, 7.2 ± 0.7 ppb Pd, and 0.53 ± 0.06 ppb Re. The calculated PGE contents are low relative to many komatiites; using existing methods for calculating the PGE contents of mantle sources suggests the Winnipegosis source had PGE contents from 25 ± 19 wt% to 57 ± 7 wt% of primitive mantle values. This surprising finding substantiates a growing ‘PGE paradox’, where almost all komatiites appear to have formed from PGE-depleted mantle sources. We suggest the simplest resolution to this paradox is that current methods systematically underestimate mantle source PGE contents. We test the assumptions implicit in these methods by comparing detailed models of PGE behaviour during mantle melting to a large database of parental melt compositions of high degree melts. Ruthenium behaviour in high degree melts can be adequately modelled using our current understanding of Ru partitioning. However, the Ru contents of high degree mantle melts are strongly dependent on the degree of melting, as Ru-rich melts formed after sulphide exhaustion are increasingly mixed into early-formed Ru-poor melts. Platinum and Pd behaviour during mantle melting cannot be successfully modelled under the widespread assumption that they are strongly incompatible following sulphide exhaustion, suggesting that these elements cannot be used to estimate PGE contents of the mantle sources of high degree melts. Parental melt Pt and Pd in many high degree lavas are correlated with Al2O3/TiO2, suggesting a pressure control on their partitioning. We attribute the relatively low PGE contents of the Winnipegosis komatiites to their moderate degrees and depth of melting compared to other komatiites, rather than invoking a PGE-depleted mantle source. More broadly, the complex controls (T, P, fO2, S content, prior depletion) on the PGE contents of komatiites mean that most komatiites can be reconciled with being derived from sources with primitive mantle-like PGE contents, and few komatiites can be confidently asserted to come from PGE-depleted mantle sources. Pending a better understanding of PGE behaviour during mantle melting, we recommend caution when using komatiite PGE systematics to infer the highly siderophile element evolution and accretionary history of Earth’s mantle.

For the first time data on platinum group minerals (PGM) from the deluvial placer of the Veresovka River are given; their root source are dunites of the northern part of the Veresovoborsky massif. Among minerals typical of this type of placers, Pt–Fe minerals, Os–Ir–(Ru) intermetallic compounds, kashinite, bowieite, laurite, erlichmanite, and Ir–Rh thiospinels have been identified. Irarsite, hollingworthite, zvyagintsevite, potarite, cooperite, ferhodsite, and some unnamed Pb–Те minerals were also revealed, which rare for platinum placers associated with zonal clinopyroxenite–dunite massifs, but are widely distributed in the studied placer. Xingzhongite was detected there for the first time in placers of the Ural Platinum Belt. The morphological features of all these minerals are characterized in detail, together with their chemical composition, determined by electron microprobe analysis. Their genetic relationships are characterized taking into account the aggregate data obtained. This comprehensive study of PGM individuals and aggregates is used as a basis to distinguish the parageneses in their association.

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.

The Neoproterozoic Bou Azzer ophiolite in the Moroccan Anti-Atlas Panafrican belt hosts numerous chromitite orebodies within the peridotite section of the oceanic mantle. The chromitites are strongly affected by serpentinization and metamorphism, although they still preserve igneous relicts amenable for petrogenetic interpretation. The major, minor and trace element composition of unaltered chromite cores reveal two compositional groups: intermediate-Cr (Cr# = 0.60 – 0.74) and high-Cr (Cr# = 0.79 – 0.84) and estimates of parental melt compositions suggest crystallization from pulses of fore-arc basalts (FAB) and boninitic melts, respectively, that infiltrated the oceanic supra-subduction zone (SSZ) mantle. A platinum group elements (PGE) mineralization dominated by Ir-Ru-Os is recognized in the chromitites, which has its mineralogical expression in abundant inclusions of Os-Ir alloys and coexisting magmatic laurite (RuS) and their products of metamorphic alteration. Unusual mineral phases in chromite, not previously reported in this ophiolite, include super-reduced and/or nominally ultra-high pressure minerals moissanite (SiC), native Cu and silicates (oriented clinopyroxene lamellae), but “exotic” zircon and diaspore have also been identified. We interpret that clinopyroxene lamellae have a magmatic origin, whereas super-reduced phases originated during serpentinization processes and diaspore is linked to late circulation of low-silica fluids related to rodingitization. Zircon grains, on the other hand, with apatite and serpentine inclusions, could either have formed after the interaction of chromitite with mantle-derived melts or could represent subducted detrital sediments later incorporated into the chromitites. We offer a comparison of the Bou Azzer chromitites with other Precambrian ophiolitic chromitites worldwide, which are rather scarce in the geological record. The studied chromitites are very similar to the Neoproterozoic chromitites reported in the Arabian-Nubian shield, which are also related to the Panafrican orogeny. Thus, we conclude that the Bou Azzer chromitites formed in a subduction-initiation geodynamic setting with two-stages of evolution, with formation of FAB-derived intermediate-Cr chromitites in the early stage and formation of boninite-derived high-Cr chromitites in the late stage., This research was financially supported by the Fondo Europeo de Desarrollo Regional (FEDER) Funds, the Spanish Projects CGL2015–65824, RTI2018-099157-A-I00, PID2019-105625RB-C21,and A.RNM.186.UGR18. Additional funding was provided by a FPU-PhD grant to NPS by the “Ministerio de Educación” of the Spanish Government, the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ, and the Mexican research programs CONACYT-Ciencia B ́asica A1-S-14574 and UNAM-PAPIIT grant IA-101419 to VC. The help extended by MANAGEM mining company is greatly acknowledged. We are also indebted to Thomas Aiglsperger, Lidia Butjosa and Cristina Villanova-de-Benavent for their participation during fieldwork, to Aleu Andreazini for his support during sample preparation, to Xavier Llovet from the Centres Científics i Tecnol`ogics of the Universitat of Barcelona (CCiTUB) for his careful help with EMPA, and to Carlos Ortega-Obreg ́on from the Laboratorio de Estudios Isot ́opicos of the Centro de Geociencias (UNAM, Mexico) for his help with the LA-ICP-MS analyses on chromite. Ben-Xun Su, one anonymous reviewer, and editors Ibrahim Uysal and Franco Pirajno are deeply acknowledged for their constructive criticism that has helped to improve the manuscript.

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.

Submicroscopic to micro-diamonds, silicates, wüstite, platinum group minerals (PGM), sulfides and ultra-high pressure (UHP) mineral inclusions have been recovered from podiform and banded chromitites of the Bulqiza ophiolite in Albania. The alignment of acicular silicate inclusions with crystallographic planes of magnesiochromite suggests an exsolution origin. Positive and negative-crystal faces of cubic silicate inclusions are consistent withs a high-temperature paragenesis. Micro-inclusions and nanoparticles (

Platinum group element (PGE) mineralization is associated with ultramafic and mafic rocks. There are three major types of PGE mineralization in Mongolia. The most common is (1) PGE mineralization related to ophiolites and includes podiform chromite, (2) PGE in layered gabbro intrusion, and (3) PGE-Au placer. PGE mineralization in Mongolia is mainly related to podiform chromite type (synonym Alpine type chromite), which forms pod-like chromite masses in ultramafic part of ophiolite complexes of Neoproterozoic-Early Cambrian age in northern and western Mongolia and Late Paleozoic in southern Mongolia. Geological environment of these ultramafic rocks is characterized by the position in accreted terranes as part of ophiolite assemblages. Rock types are tectonically deformed dunite and harzburgite, mainly serpentinized with chromite-magnetite-Ru-Os-Ir alloys and laurite. Gabbroid plutons are not well studied for PGE mineralization. One is the Nomgon pluton, a layered troctolite-anorthosite-leucogabbro intrusion. Sulfide mineralization is represented by chalcopyrite and bornite. In the sulfide-bearing zone Pt and Pd with As and Sb, sperrylite, Au, and Ag are present. Sperrylite contains rhodium up to 12.21%. PGE placers and heavy concentrate aureoles occur close to ultramafic plutons. In recent years, in southern and western Mongolia, PGE together with gold have been discovered in Cretaceous and Quaternary sedimentary deposits. Mineralization is associated with near-surface modification of ultramafic-mafic rocks. Economically important PGE deposits and occurrences are not yet discovered, but still there is the good possibility to find PGE mineralization in gabbro intrusions.

The Neoproterozoic Nubian Shield ophiolite rocks (~800 and 730 Ma), consisting mainly of mafic–ultramafic complexes and their metamorphic derivatives formed during multidimensional tectonic activities, are widespread in the Eastern Desert of Egypt. Therefore, they may represent a potential province of many magmatic and post-magmatic mineral deposits; among which three ore systems including Cu-Ni sulfide from Wadi Allaqi, garnierite-bearing serpentinite from Wadi Dubur as a marker of the nickel lateritization, and Cr deposits (chromitites) from the Central and Southern Eastern Desert are the subject of the present review. The Wadi Allaqi Cu–Ni deposit (2.8% Cu and 1.53% Ni on average) at Abu Swayel occurs as lenses extending NE-SW in tectonic zones of amphibole-rich rocks, which are enveloped by biotite-garnet schist that became pegmatitic at the shear zones. The host amphibole-rich rocks were most probably metamorphosed after ultramafic pyroxene-rich rocks, which were formed from the Allaqi forearc mantle-related Cu, Ni, and S-rich basaltic melt, and produced early syngenetic low-grade disseminated ores. Sulfur came in the mantle from the assimilated country sedimentary rocks in the subduction zone, as evidenced by REE enrichment of the Allaqi peridotite (LaN/LuN = 4.10–5.94). The ore concentrated due to epigenetic physical remobilization involving D2 regional thrusts, shear zones, and folding, being enhanced with the arc/arc collision of Gerf and Gabgaba terranes. Post-magmatic regional metamorphic fluids caused chemical remobilization and secondary sulfide and platinum group minerals precipitation with the metamorphic silicate minerals between 500 and 350 °C. The altered and weathered serpentinites (D-G) from Wadi Dubur are garnierite-bearing demonstrating a relic of early weathering horizon of Ni residual concentration (saprock-like) and overlie unweathered serpentinites (D-S) that represent the bedrock. Secondary serpentinization in D-G rocks most probably caused the early individualization of Ni and formation of Fe oxyhydroxides (with Raman spectra at 212–214, 270–275, and 290–293 cm−1). Garnierite occurring as vein fillings (Type I) formed by an early alteration of the serpentine species in the saprock, with a fracturing control in a low relief zone of stable tectonic terrain that promoted the high-standing of the water table. This caused in situ precipitation of Ni and Fe. The uplift of the previously formed regolith and the sequence lowering of the water table caused progressive deformation and syn-tectonic weathering, and hence the leaching of Ni with Si and Mg by the percolating acidic solutions downward to be captured by secondary silicate phases. This precipitated Si- and Mg-rich and Fe-poor neoformed nepouite/pecoraite-garnierite (Type II) in gouges and voids. The Ni lateritization, assigned by the formation of garnierite, in the Nubian Shield serpentinites in Wadi Dubur is an indication of humid weathering under tropical to subtropical conditions most probably during the Cretaceous. However, the absence of the uppermost part of the ferruginous zone of the lateritic crusts in the Wadi Dubur Ni mineralization argued for a post-lateritization erosion. Chromium deposits in the Neoproterozoic ophiolites, outcropping in the form of lensoidal pods, were recorded in several places from Central to Southern Eastern Desert (CED to SED) such as Gebel El-Rabshi, Wadi El-Lawi, Wadi El-Zarka, Wadi Ghadir, Um-Huitate, Wadi Bezah, Wadi Sayfayn, Um Salatit and El-Barramiyah (CED), Siwigat, Ashayer, Ras Shait, Gebel Arais, Balamhindit, El-Galala, Um-Thagar, Gebel Abu Dahr, and Malo Grim area (SED). The Cr deposits are variable in sizes from small masses to large lenses (50 m across) and are hosted by serpentinites after harzburgites and subordinate dunites. These host peridotites are refractory residues after high partial melting, mainly 25 to 40% melting. The Egyptian Cr deposits are classified into: Al-rich and Cr-rich varieties with the chromian number (Cr#) 0.75, respectively. Their chromite Cr-number [Cr# = Cr/(Cr + Al)], 0.5–0.85, with low TiO2 (

Laser ablation MC-ICP-MS was used to measure the Os-isotope compositions of single sulfide grains, including laurite (RuS2) and pentlandite [(Fe,Ni)9S8], from two chromitite bodies and host lherzolites from ophiolites of North Andaman (Indo-Burma-Sumatra subduction zone). The results show isotopic heterogeneity in both laurite (n = 24) and pentlandite (n = 37), similar to that observed in other chromitites and peridotites from the mantle sections of ophiolites. Rhenium-depletion model ages (TRD) of laurite and pentlandite reveal episodes of mantle magmatism and/or metasomatism in the Andaman mantle predating the formation of the ophiolite (and the host chromitites), mainly at &asymp, 0.5, 1.2, 1.8, 2.1 and 2.5 Ga. These ages match well with the main tectonothermal events that are documented in the continental crustal rocks of South India, suggesting that the Andaman mantle (or its protolith) had a volume of lithospheric mantle once underlaying this southern Indian continental crust. As observed in other oceanic lithospheres, blocks of ancient subcontinental lithospheric mantle (SCLM) could have contributed to the development of the subduction-related Andaman&ndash, Java volcanic arc. Major- and trace-element compositions of chromite indicate crystallization from melts akin to high-Mg IAT and boninites during the initial stages of development of this intra-oceanic subduction system.

The Mesoarchean Sukinda Massif in the Singhbhum craton is the largest chromite ore deposit in India that contains ≈95% of Indian Cr resources. The Sukinda Massif consists of an elongated layered ultramafic unit (≈25 km × 400 m) that occurs within the 3.5 Ga supracrustal sequences of the Tomka-Daitari-Mahagiri greenstone belt in eastern India. The ultramafic unit comprises serpentinized dunite, orthopyroxenite and chromitite. There are six chromitite seams which are present within the serpentinized dunite. All ultramafic rocks are extensively weathered and capped by laterites (≈30 m thick). In the southwestern part of the Sukinda Massif, in the Katpal area, chromitite seams and the host ultramafic rocks are fragmented, forming breccias cemented by the gabbroic to granodioritic rocks. Whole-rock major, trace and platinum-group elements (PGE) geochemistry indicates that the ultramafic unit and the gabbro, including the cementing matrix materials of the breccias, are cogenetic. The ultramafic rocks formed by fractional crystallization of a parental boninitic or high-Mg siliceous magma. The cementing gabbroic rocks formed from an evolved boninitic magma generated from fractionated boninite or high-Mg siliceous basalt. The positive correlation between MgO and Ni, and MgO and Cr is due to fractionation of early cumulus like olivine and chromite from the high-Mg parental magma. The correlation between Zr and Cu suggests that the parental boninitic magma was S-undersaturated. However, the matrix gabbro shows a flatter trend with increasing Zr and Cu, indicating sulfide saturation occurred later in the evolved boninitic magma. In Sukinda, the concentrations of PGE in the massive chromitites (≈176–875 ppb) from the main ultramafic unit and in chromitite fragments (≈61–279 ppb) from the breccias are higher than in serpentinite (≈19–71 ppb), orthopyroxenite (≈14–19 ppb) and gabbro (≈3–11 ppb). Three samples of the massive chromitites from the main ultramafic unit have significant PGE concentrations (PGEtotal ≈ 651–875 ppb) with IPGE (Ir, Os, Ru) ≈ 528–634 ppb, much higher than PPGE (Pd, Pt, Rh) ≈ 93–332 ppb. Detailed PGE mineralogical studies of these samples revealed presence of IPGE-bearing platinum-group minerals (PGM) dominated by Os-Ir-Ru alloys (containing minor Pt) enclosed in chromite grains. Other PGMs are laurite present as composite grain within chromite plus irarsite which are associated with cracks in chromite grains and a small sperrylite grain attached to an Os-Ir-Ru alloy. All the As-bearing PGM are associated with cracks in the chromite suggesting introduction of As during alteration of primary PGM to PGM-arsenides. The positive correlation between IPGE (Os, Ir, Ru) and MgO or Cr indicates they are fractionated during the early stage of magmatic differentiation from a S-undersaturated boninitic magma. The low Ir of the silicate rocks indicate prior removal of Ir-bearing alloys from the parental boninitic magma which would account for the occurrence of Os-Ir-Ru alloys in massive chromitites. The results suggest that Os-Ir-Ru alloys from the Sukinda chromitites crystallized at relatively high temperature and low fS2 condition within a S-undersaturated boninitic magma ascending from the upper mantle where they entrapped by the growing chromite crystals. Overall, the initial PGE analysis from the Sukinda breccias, presented herein, indicate similarities with the Nuasahi breccias in the Singhbhum craton, and highlight the potential for mineralization in this area.

Small scattered grains of PGE minerals have been found in chromitites of Ergaksky and Kalninsky ultramafic massifs, which are extreme northeastern fragments of the Kurtushibinsky Ophiolite Belt of Western Sayan. For the first time, together with previously known PGE minerals, there were found the Cu—Ru—Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, PGE sulfides of MeS and Me 3 S 4 types, sulfoarsenide with the high content of Rh (~12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif. Compositions of PGE minerals indicate high temperature of the Ru—Os sulfide formation (~1200—1250 oC) in both massifs, as well as high partial pressure of sulfur under the formation of chromitites and syngenetic PGE minerals in the Ergaksky massif. PGE mineral assemblages were formed in several stages. Os—Ir—Ru solid solutions and disulfides of the laurite—erlichmanite series (mainly fractionating ruthenium) formed earlier than Cu—Ru—Os solid solutions, sulfides (Ru,Fe) 3 S 4 , (Ru,Ni,Fe)S, and PGE sulfoarsenides, which crystallized under influence of reduced fluids as a result of remobilization and redeposition of PGE.

Laurite, ideally (Ru,Os)S2, is a common accessory mineral in podiform and stratiform chromitites and, to a lesser extent, it also occurs in placer deposits and is associated with Ni-Cu magmatic sulfides. In this paper, we report on the occurrence of zoned laurite found in the Merensky Reef of the Bushveld layered intrusion, South Africa. The zoned laurite forms relatively large crystals of up to more than 100 &micro, m, and occurs in contact between serpentine and sulfides, such as pyrrhotite, chalcopyrite, and pentlandite, that contain small phases containing Pb and Cl. Some zoned crystals of laurite show a slight enrichment in Os in the rim, as typical of laurite that crystallized at magmatic stage, under decreasing temperature and increasing sulfur fugacity, in a thermal range of about 1300&ndash, 1000 &deg, C. However, most of the laurite from the Merensky Reef are characterized by an unusual zoning that involves local enrichment of As, Pt, Ir, and Fe. Comparison in terms of Ru-Os-Ir of the Merensky Reef zoned laurite with those found in the layered chromitites of the Bushveld and podiform chromitites reveals that they are enriched in Ir. The Merensky Reef zoned laurite also contain high amount of As (up to 9.72 wt%), Pt (up to 9.72 wt%) and Fe (up to 14.19 wt%). On the basis of its textural position, composition, and zoning, we can suggest that the zoned laurite of the Merensky Reef is &ldquo, hydrothermal&rdquo, in origin, having crystallized in the presence of a Cl- and As-rich hydrous solution, at temperatures much lower than those typical of the precipitation of magmatic laurite. Although, it remains to be seen whether the &ldquo, laurite precipitated directly from the hydrothermal fluid, or it represents the alteration product of a pre-existing laurite reacting with the hydrothermal solution.

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.

In the alluvial deposits of the Prizhlimny Creek (southern part of the Koryak Highland), grains of platinum-group minerals are found along with gold. We have established that the grains are native platinum (Pt, Fe) containing Cu (up to 5 wt.%), Os (up to 8 wt.%), and Rh (up to 2 wt.%). Inclusions in the platinum are native osmium (the content of Ir impurity reaches 12 wt.%, the average content being 0.2–4 wt.%), an unnamed intermetallic compound of composition PtRh, sulfides and arsenides of PGE (cooperite, laurite, malanite, cuproiridsite, cuprorhodsite, sperrylite, hollingworthite, unnamed compounds PdS, (Ir,Ru)S2, (Ir,Pt)S2, Cu, and Fe (bornite, chalcopyrite), chromite, and Cr-magnetite. Replacement of native-osmium crystals by compound IrO2 is described. It has been established that this compound formed during oxidation accompanied by the replacement of isoferroplatinum by native platinum. The data obtained agree with the results of study of platinum-group mineral assemblages from placers localized in weakly eroded Ural–Alaskan-type massifs whose apical parts formed under high oxygen activity conditions. Clinopyroxenites of the Prizhimny massif are considered to be the potential source of PGE.

The Purang harzburgite massif in SW Tibet (China) hosts abundant chrome ore deposits. Ores consist of 20 to >95% modal chromian spinel (Cr-spinel) with mylonitic fabric in imbricate shaped pods. The composition of Cr-spinel in these ores ranges from Al-rich [Cr#Sp or Cr/(Cr + Al) × 100 = 47.60–57.56] to Cr-rich (Cr#Sp: 62.55–79.57). Bulk platinum-group element (PGE) contents of chromitites are also highly variable ranging from 17.5 ppb to ∼2.5 ppm. Both metallurgical and refractory chromitites show a general enrichment in the IPGE (Os, Ir and Ru) with respect to the PPGE (Rh, Pt and Pd), resulting mostly in right-sloping primitive mantle (PM)-normalized PGE profiles. The platinum-group mineral (PGM) assemblages of both chromitite types are dominated by heterogeneously distributed, euhedral Os-bearing laurite inclusions in Cr-spinel. The Purang chromitites have quite inhomogeneous 187Os/188Os ratios (0.12289–0.13194) that are within the range of those reported for mantle-hosted chromitites from other peridotite massifs. Geochemical calculations demonstrate that the parental melts of high-Cr chromitites were boninitic, whereas those of high-Al chromitites had an arc-type tholeiitic affinity. Chromite crystallization was most likely stimulated by changes in magma compositions due to melt-peridotite interaction, leading to the establishment of a heterogeneous physicochemical environment during the early crystallization of the PGM. The highly variable PGE contents, inhomogeneous Os-isotopic compositions and varying Cr#Sp ratios of these chromitites imply a polygenetic origin for them from spatially distinct melt inputs. The generally low γOs values (

We have first generalized geochemical and mineralogical data indicating the important role of crystallization of Os–Ir–Ru phases and fractionation of refractory iridium subgroup of PGE (IPGE) at the early stages of the evolution of parental magmas and primitive cumulates from the Yoko–Dovyren layered massif (northern Baikal area, Russia). The object of study was two types of plagioclase peridotites from the lower part of the intrusion, differing in the porosity of primary olivine cumulates: less melanocratic (but more primitive) type I and more melanocratic type II. Inclusions of refractory IPGE (Os, Ir, and Ru) discovered during LA–ICP-MS studies of aluminochromite from type I rocks are the first evidence for the presence of Os–Ir–Ru phases. Subsequent electron microscopy examinations revealed more than 25 grains of laurite and Ir-containing osmium in aluminochromite from plagioperidotites of both types. Attention is focused on the importance of the Ru/Cr2O3 and Ir/Cr2O3 ratios in rocks for the separation of IPGE at early and late fractionation stages. The conclusion is drawn that the higher Ru/Cr2O3 and Ir/Cr2O3 ratios in type I plagioperidotites indicate higher enrichment of aluminochromite in inclusions of refractory IPGE minerals. This is consistent with the fact that these rocks are assigned to the most primitive high-temperature ultramafites genetically related to the parental magma, which was in equilibrium with olivine Fo88 at ~1290 ºC. We have established that the parental Dovyren magma was already depleted in IPGE and rhodium before its entrance into a chamber. No signs of early sulfide–silicate immiscibility have been detected. © 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

The near-bottom part of the Yoko-Dovyren layered ultramafic-mafic intrusion host the Baikal deposit of Cu–Ni sulfide ores with Pt–Pd mineralization, whereas horizons and pockets of low sulfide ores with Pt–Pd mineralization occur at higher stratigraphic levels, including the boundary between strata of troctolite and gabbronorite, within these rocks, as well as in strata of peridotite at the lower part of the intrusion. This paper represents a new (for the Yoko-Dovyren intrusion) type of “refractory IPGE-mineralization” discovered in the lower peridotite ranging from two-pyroxene-plagioclase-bearing lherzolite. This mineralization occurs in thin intercalations of plagioclase lherzolite containing as much as 7% of alumochromite, up to 50 ppb Ru, 15 ppb Ir, and 60 ppb Pt. Crystals of cumulate alumochromite with 0.2–0.8 wt % TiO2 contain hexagonal plates of Ir-osmium up to 5 m in size. Crystals of cumulate alumochromite with 1.2–2.8 wt % TiO2 host pentagonal dodecahedrons of laurite up to 4 m in size. One of the alumochromite crystals with an inclusion of Os-poor laurite was found inside a crystal of cumulate olivine Fo86. Intergrowth of laurite and Ir-osmium enclosed in alumochromite with 1.1% TiO2 was observed in one case. Laurite from Yoko-Dovyren contains 93–66%, predominantly 92–82%, RuS2 endmember (n = 10); 3–20, predominantly 5–12%, OsS2 endmember; 4–5% IrS2 endmember; and up to 0.7% Pd and 0.5% Au. Ir-osmium is divided into two groups by composition. The first group is enriched in Os (58–73 wt %, on average 64 wt %) and Ru (3–8 wt %, on average 5 wt %), contains 24–34 wt % Ir (n = 4), up to 1.4 wt % Au, and no Pt. Compositions of the second group have 57–58 wt % Os, 27–30 wt % Ir, 1.5–5.5 wt % Ru, approximately 10 wt % Pt (n = 3), and up to 0.2 wt % Pd. The Cr# and Fe2+/(Fe2+ + Mg) values, which range within 58–69 and 61–72, respectively, are identical in alumochromite with both enclosed laurite and Ir-osmium. Alumochromite, relatively enriched in Ti, crystallized slightly later, suggesting later crystallization for hosted laurite. Occurrence of Ir-osmium seems to indicate a picritic magma undersaturated with sulfide sulfur during bulk crystallization of alumochromite Judging from the diagram from (Brennan and Andrews, 2001), intergrowths of laurite and Ir-osmium, evidence that their probable crystallization temperature did not exceed 1250°C. The presence of own minerals of Ru, Os, Ir in the rocks, containing the first ppb of these PGE shows startling degree of magmatic differentiation. In the matrix of plagioclase lherzolites, containing laurite and Ir-osmium, in association with phlogopite, pargasite, pentlandite, troilite and chalcopyrite there were found the smallest crystals of geversite, sperrilite, insizwaite, niggliite, naldrettite, zvyagintsevite, in association with serpentine and chlorite–native platinum, Pd-platinum, osarsite, irarsite, platarsite.

Inclusions of platinum-group minerals (PGM) within alluvial isoferroplatinum nuggets from the Freetown Peninsula, Sierra Leone, are aligned with their shape determined by the structure of their host. The edges of the majority of the inclusions lie at 0°, 45° or 90° to external crystal edges of the nugget which shows that the inclusions are not randomly oriented earlier minerals incorporated within their host. The inclusions are later infills, probably formed at the surface of the nugget during growth and subsequently enclosed by the growing nugget. PGM on the present surface of the nugget represent the last stage of this partnership. A single nugget containing abundant inclusions is described here but similar features are observed in other nuggets from the same area. The inclusions contain laurite (RuS2), irarsite–hollingworthite (IrAsS–RhAsS), Pd–Te–Bi–Sb phases, Ir-alloy, Os-alloy, Pd-bearing Au, an Rh–Te phase, Pd–Au alloy and Pd–Pt–Cu alloy. PGM found on the nugget surface include laurite, irarsite and cuprorhodsite (CuRh2S4). The Pd–Te–Bi–Sb phases may include Sb-rich keithconnite (Pd20S7) and compositions close to the kotulskite–sobolevskite solid-solution series (PdTe–BiTe). Textural evidence suggests that formation of the nuggets began with the isoferroplatinum host and the voids were filled starting intergrowths of laurite and irarsite–hollingworthite with both laurite and irarsite–hollingworthite often showing compositional zonation and each of them replacing the other. Filling of the voids probably continued with Pd-Cu-bearing gold, Sb-rich keithconnite (Pd,Pt)20.06(Te,Sb,Bi)6.94, keithconnite, telluropalladinite Pd9(Te,Bi)4, RhTe and finally Ir-alloy and then Os-alloy. The nuggets are thought to be neoform growths in the organic- and bacterial-rich soils of the tropical rain forest cover of the Freetown intrusion. The mineralogical assemblage in the layered gabbros of the intrusion has been previously shown to differ from the alluvial assemblage in the rivers and these inclusions, not seen in Pt3Fe in the unaltered rocks, add a further item to the catalogue of differences.

In the Ospin–Kitoi ultramafic massif of the Eastern Sayan, accessory and ore Cr-spinel are mainly represented by alumochromite and chromite. Copper–nickel mineralization hosted in serpentinized ultramafic rocks occurs as separate grains of pentlandite and pyrrhotite, as well as assemblages of (i) hexagonal pyrrhotite + pentlandite + chalcopyrite and (ii) monoclinal pyrrhotite + pentlandite + chalcopyrite. Copper mineralization in rodingite is presented by bornite, chalcopyrite, and covellite. Talc–breunnerite–quartz and muscovite–breunnerite–quartz listvenite contains abundant sulfide and sulfoarsenide mineralization: pyrite, gersdorffite, sphalerite, Ag–Bi and Bi-galena, millerite, and kuestelite. Noble metal mineralization is represented by Ru–Ir–Os alloy, sulfides, and sulfoarsenides of these metals, Au–Cu–Ag alloys in chromitite, laurite intergrowth, an unnamed mineral with a composition of Cu3Pt, orcelite in carbonized serpentinite, and sperrylite and electrum in serpentinite. Sulfide mineralization formed at the late magmatic stage of the origination of intrusion and due to fluid–metamorphic and retrograde metasomatism of primary rocks.

Podiform chromitites of the Vavdos ophiolite are associated with dunite bodies hosted within residual mantle harzburgite. The mantle source is highly depleted and apparently developed at fore-arc settings as inferred from the presence of boninitic melts. The ascendance of these melts through dunite pathways and their subsequent interaction with the depleted harzburgite instigated chromitite mineralization within dunite. Spinel occurs in a) massive accumulations (Mg# = 0.56–0.68 and Cr# = 0.69–0.77), b) in non-massive disseminated textures (Mg# = 0.52–0.61 and Cr# = 0.80–0.81) and c) in schlieren chromitite deposits within dunite bodies (Mg# = 0.43–0.48 and Cr# = 0.79–0.82). Olivine is the main silicate inclusion in chromitites, whereas the absence of hydrous mineral phases is linked with crystallization at relatively deeper parts of the mantle. Olivine inclusions in spinel are enriched in Fo, Ni and Ca compared to the interstitial olivine grains. No compositional gaps were noted between the mineral analyses of chromitites and dunites; the calculated parental melts are also homogeneous, reinforcing the view for production of melt batches by similar processes. Platinum-group elements (PGE) concentrations and platinum-group minerals (PGM) demonstrate high (Os + Ir + Ru)/ (Rh + Pt + Pd) ratios, typical of many Tethyan ophiolitic chromitites. The main PGM phase is laurite that forms an extended solid solution series with substitution of Ru for Os + Ir, linked to the cooling of the system. Massive chromitites contain Ru-rich laurite and Os-Ir alloys whereas non-massive ores contain Ru-poor laurite. The PGM were included in fresh magnesiochromite favoring their magmatic origin and initial entrapment at ~ 1200 °C and low sulfur fugacity.

Podiform chromitites occur in mantle peridotites of the Late Triassic Puerto Nuevo Ophiolite, Baja California Sur State, Mexico. These are high-Cr chromitites [Cr# (Cr/Cr + Al atomic ratio = 0.61-0.69)] that contain a range of minor- and trace-elements and show whole-rock enrichment in IPGE (Os, Ir, Ru). That are similar to those of high-Cr ophiolitic chromitites crystallised from melts similar to high-Mg island-arc tholeiites (IAT) and boninites in supra-subduction-zone mantle wedges. Crystallisation of these chromitites from S-undersaturated melts is consistent with the presence of abundant inclusions of platinum-group minerals (PGM) such as laurite (RuS2)-erlichmanite (OsS2), osmium and irarsite (IrAsS) in chromite, that yield TMA ≈ TRD model ages peaking at ~ 325 Ma. Thirty-three xenocrystic zircons recovered from mineral concentrates of these chromitites yield ages (2263 ± 44 Ma to 278 ± 4 Ma) and Hf-O compositions [ɛHf(t) = − 18.7 to + 9.1 and 18O values < 12.4 ] that broadly match those of zircons reported in nearby exposed crustal blocks of southwestern North America. We interpret these chromitite zircons as remnants of partly digested continental crust or continent-derived sediments on oceanic crust delivered into the mantle via subduction. They were captured by the parental melts of the chromitites when the latter formed in a supra-subduction zone mantle wedge polluted with crustal material. In addition, the Puerto Nuevo chromites have clinopyroxene lamellae with preferred crystallographic orientation, which we interpret as evidence that chromitites have experienced high-temperature and ultra high-pressure conditions (< 12 GPa and ~ 1600 °C). We propose a tectonic scenario that involves the formation of chromitite in the supra-subduction zone mantle wedge underlying the Vizcaino intra-oceanic arc ca. 250 Ma ago, deep-mantle recycling, and subsequent diapiric exhumation in the intra-oceanic basin (the San Hipólito marginal sea) generated during an extensional stage of the Vizcaino intra-oceanic arc ca. 221 Ma ago. The TRD ages at ~ 325 Ma record a partial melting event in the mantle prior to the construction of the Vizcaino intra-oceanic arc, which is probably related to the Permian continental subduction, dated at ~ 311 Ma.

The Bir Tuluha ophiolite is one of the most famous chromitite-bearing occurrences in the Arabian Shield of Saudi Arabia, where chromitite bodies are widely distributed as lensoidal pods of variable sizes surrounded by dunite envelopes, and are both enclosed within the harzburgite host. The bulk-rock geochemistry of harzburgites and dunites is predominately characterized by extreme depletion in compatible trace elements that are not fluid mobile (e.g., Sr, Nb, Ta, Hf, Zr and heavy REE), but variable enrichment in the fluid-mobile elements (Rb and Ba). Harzburgites and dunites are also enriched in elements that have strong affinity for Mg and Cr such as Ni, Co and V. Chromian spinels in all the studied chromitite pods are of high-Cr variety; Cr-ratio (Cr/(Cr + Al) atomic ratio) show restricted range between 0.73 and 0.81. Chromian spinels of the dunite envelopes also show high Cr-ratio, but slightly lower than those in the chromitite pods (0.73–0.78). Chromian spinels in the harzburgite host show fairly lower Cr-ratio (0.49–0.57) than those in dunites and chromitites. Platinum-group elements (PGE) in chromitite pods generally exhibit steep negative slopes of typical ophiolitic chromitite PGE patterns; showing enrichment in IPGE (Os, Ir and Ru), over PPGE (Rh, Pt and Pd). The Bir Tuluha ophiolite is a unimodal type in terms of the presence of Ru-rich laurite, as the sole primary platinum-group minerals (PGM) in chromitite pods. These petrological features indicates that the Bir Tuluha ophiolite was initially generated from a mid-ocean ridge environment that produced the moderately refractory harzburgite, thereafter covered by a widespread homogeneous boninitic melt above supra-subduction zone setting, that produced the high-Cr chromitites and associated dunite envelopes. The Bir Tuluha ophiolite belt is mostly similar to the mantle section of the Proterozoic and Phanerozoic ophiolites, but it is a “unimodal” type in terms of high-Cr chromitites and PGE-PGM distribution.

The platinum-group element (PGE) contents in magmatic ores and rocks are normally in the low μg/g (even in the ng/g) level, yet they form discrete platinum-group mineral (PGM) phases. IPGE (Os, Ir, Ru) + Rh form alloys, sulfides, and sulfarsenides while Pt and Pd form arsenides, tellurides, bismuthoids and antimonides. We experimentally investigate the behavior of Os, Ru, Ir and Rh in As-bearing sulfide system between 1300 and 1020 °C and show that the prominent mineralogical difference between IPGE (+Rh) and Pt and Pd reflects different chemical preference in the sulfide melt. At temperatures above 1200 °C, Os shows a tendency to form alloys. Ruthenium forms a sulfide (laurite RuS2) while Ir and Rh form sulfarsenides (irarsite IrAsS and hollingworthite RhAsS, respectively). The chemical preference of PGE is selective: IPGE + Rh form metal–metal, metal-S and metal-AsS complexes while Pt and Pd form semimetal complexes. Selective complexing followed by mechanical separation of IPGE (and Rh)-ligand from Pt- and Pd-ligand associations lead to PGE fractionation.

Although Earth’s continental crust is thought to derive from melting of the Earth’s mantle, how the crust has formed and the timing of its formation are not well understood. The main difficulty in understanding how the crust was extracted from the Earth’s mantle is that most isotope systems recorded in mantle rocks have been disturbed by crustal recycling, metasomatic activity and dilution of the signal by mantle convection. In this regard, important age constraints can be obtained from Re-Os model ages in platinum group minerals (PGM), as Re-poor and Os-rich PGM show evidence of melting events up to 4.1 Ga. To constrain the origin of the Re-Os fractionation and Os isotope systematics of natural PGM, we have investigated the linkage between sulfide and PGM grains of variable composition via a series of high-temperature experiments carried out at 1 bar. We show that with the exception of laurite, all experimentally-produced PGM, in particular Pt 3 Fe (isoferroplatinum) and Pt-Ir metal grains, are systematically richer in Re than their sulfide precursors and will develop radiogenic 187 Os / 188 Os signatures over time relative to their host base metal sulfides. Cooling of an PGM-saturated sulfide assemblage shows a tendency to amplify the extent of Re-Os fractionation between PGM and the different sulfide phases present during cooling. Conversely, laurite grains (RuS 2 ) are shown to accept little to no Re in them and their Os isotope composition changes little over time as a result. Laurite is therefore the PGM that provides the most robust Re-depletion ages in mantle lithologies. Our results are broadly consistent with observations made on natural PGM, where laurites are systematically less radiogenic than Pt-rich PGM. These experimental results highlight the need for the acquisition of large datasets for both mantle materials and ophiolite-derived detrital grains that include measurements of the Os isotope composition of minerals rich in highly siderophile elements at the grain scale (i.e. PGM and base metal sulfides). Only with such datasets is it possible to identify past episodes of mantle melting.

Accurate characterization of the platinum group mineral (PGM) assemblages for Cu-Ni-PGE deposits are typically constrained by sample size and the difficulty of finding statistically significant numbers of grains, which is expected given the low concentrations of platinum group elements ( 75 µm) can account for large fractions of total mass. Despite these limitations, an accurate survey of PGM from different deposit types would have significant value towards developing deposit models and respective exploration strategies. In this study, we present results for a comprehensive evaluation of PGM at four copper-PGE occurrences hosted within separate but co-genetic gabbro or troctolite intrusions in the Coldwell Alkaline Complex and confirm that accurate surveys are possible with sufficient sample material and efficient PGM concentration methods. The PGM concentration methods used include: (1) hydroseparation of sieved size fractions of pulverized material, and (2) panning of grain separates produced by electric pulse disaggregation of drill core specimens. A favourable comparison of the results has verified the reliability of each method and added confidence that the PGM assemblages identified at three of the four locations are fully characterized. Precious metal mineral (PMM) assemblages are determined for the Main zone and W Horizon at the Marathon deposit, and the main zones at each of the Geordie Lake deposit and Area 41 occurrence. A total of 10,824 PMM grains (PGE and Au-Ag) and 68 mineral species, including 16 unknown minerals, were identified, of which 768 grains and 31 species occur at the Main zone, 523 grains and 41 species at Area 41,9485 grains and 43 species at W horizon, and 56 grains and 12 species at Geordie Lake. The PMM are grouped as follows: Pd-Ge, PGE-S-As, Pt-Fe alloy, Pd-Cu-Pb-Au, Pd-Ni-S, Pd-Pt-Sn, Pt-As, Pd-As, Pd-Pt-Sb-As, Pd-Pt-Bi-Te, and Au-Ag. All of the deposits were found to contain similar proportions of Pd-Pt-Sb-As, Pd-Pt-Bi-Te and Au-Ag minerals. But the W Horizon and Area 41 are distinguished from the Marathon Main zone and Geordie Lake deposits by the presence of minerals in the PGE-S-As, Pt-Fe alloy, Pd ± Cu ± Pb ± Au and Pd-Ge groups. Taken together, the PMM assemblages for deposits in the Coldwell exhibit a strong correlation to PGE enrichment relative to the range for mantle Cu/Pd values (1000–10,000). And there is no relationship between the abundances of Pd-Pt-Bi-Te and Pd-Pt-Sb-As minerals that are commonly associated with hydrous phases, and the intensity of hydrothermal alteration. Thus minerals found only at the W Horizon and Area 41, where significant PGE upgrading has occurred, including Pt-Fe alloys, rustenburgite, marathonite, palladogermanide, unknown Rh-Ni-Fe-sulfide, Au-Pd-Cu alloy, braggite, coldwellite, laurite, zvyagintsevite, laflammeite, and unknown phases Pd 5 As 2 , Pd 3 As, Pd 3 (As,Pb,Bi) might be considered as index minerals for PGE enriched types of mineralization in the Coldwell.

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 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.

The Bushveld Complex contains the largest platinum-group element (PGE) deposits of the world that are represented by persistent stratiform reefs highly enriched with PGE with respect to underlying and overlying rocks. New Re-Os isotope and elemental LA MC-ICPMS data on platinum-group minerals (PGM) from the mineralized reefs are presented with implications to correlation between the different segments of the Bushveld Complex and a role of superimposed processes at the reef formation. We analyzed laurite (RuS 2 ), hollingworthite (RhAsS), sperrylite (PtAs 2 ) and Pt-Fe alloys from the Merensky Reef, Pseudoreef and the PGE reef of the Platreef. The measured 187 Os/ 188 Os value for Platreef laurite is 0.1751 ± 0.0004 whereas the ratios for sperrylite and hollingworthite range to slightly higher values (0.1713–0.1818 and 0.1744–0.1835 respectively). The observed textures of the analyzed PGM, such as Pt-Fe symplectites in base metal sulfides (BMS), laurite inclusions in chromite and sperrylite rims around sulfide-silicate aggregates, are interpreted as features of primary magmatic crystallization whereas hollingworthite overgrowths and exsolutions in sperrylite are likely to have originated from later solid state transformation or metasomatic processes. The Platreef is a composite sill-like body in the northern limb correlative to the Critical Zone in terms of stratigraphic position, whole-rock geochemistry and isotope characteristics. The pristine magmatic character of sulfides and PGM in the stratiform reefs at the top of the Platreef strongly resembles the style of Merensky Reef mineralization. However, the basal part of the Platreef pyroxenitic sequence is variably contaminated and mineralized with a significant hydrothermal overprint. Sulfides from underlying Lower Zone peridotite yield δ 34 S values varying from +9‰ to +14.2‰ that are much higher than the values for the overlying Platreef and are a consequence of sulfur assimilation from sedimentary sulfates. The same homogeneous mantle-like S isotope compositions in the high-grade PGE reef of the Platreef and the Merensky Reef can be explained by enhanced S isotope exchange through the orthomagmatic process of sulfide upgrading. The similar values and the limited variations of the initial Os isotopic ratios of all PGMs in both reefs also support their magmatic crystallization and origin from a common source.

We report the first find of Al-rich chromitites in the Nuasahi massif located in the Singhbhum Craton, eastern India. Electron microprobe analyses yielded Al 2 O 3 contents between 19.86 and 25.86 wt%. The Cr 2 O 3 and Fe 2 O 3 concentrations vary (in wt %) from 34.8 to 40.67 and 5.38 to 7.05, respectively. TiO 2 content ranges from 0.04 to 0.71 wt%. The silicates interstitial to chromite consist of enstatite, magnesiohornblende and clinochlore. One sample contains 855 ppb of total platinum-group elements (PGE) and shows a chondrite-normalized PGE pattern with a slightly positive slope, with enrichment in Pt and Pd relative to Os and Ir. Ruthenium and Rh show positive and negative peaks, respectively. In agreement with the PGE distribution, the following platinum-group minerals (PGM) have been found: laurite ((Ru,Os)S 2 ), erlichmanite ((Os,Ru)S 2 ), irarsite (IrAsS), vysotskite ((Pd,Ni)S), braggite or cooperite ((Pt,Pd,Ni)S) and Pd-telluride. The PGM form small grains, less than 10 μm across, and occur associated with base metal sulphides (BMS) and rutile included in magmatic minerals, such as chromite and enstatite. The BMS occur exclusively included in chromite. Mineralogical observations suggest that the (Os, Ir, Ru), Pt- and Pd-bearing phases crystallized in the magmatic stage, before or during the precipitation of the host chromite, under relatively high sulphur fugacity. Part of the Pd, together with Te, was initially enclosed at high temperature as small clusters in the sulphide liquid and then incorporated in the crystallizing BMS. Finally, these elements precipitated as Pd-telluride blebs, as an exsolution product at lower temperatures. Rutile occurs as a minor phase in association with PGM, included in chromite or in the contact chromite–silicate. The mode of occurrence of rutile in the Nuasahi chromitite is similar to that reported from the stratiform chromitites emplaced in stable cratonic settings.