Os Ir Research Articles

The activation energy values estimated by the Arrhenius equation as a controlling factor of platinum-group mineral formation

In ophiolite complexes and Ural/Alaskan-type intrusions the platinum-group element minerals (PGM) occur as laurite (RuS 2), erlichmanite (OsS 2), irarsite (IrAsS) and alloys (Os–Ir–Ru and Pt–Fe). They are commonly found as small inclusions (normally less than 10 μm, occasionally up to 100 μm) in chromite. The origin of coarse-grained PGM, in the form of 0.5–10 mm nuggets, in placer deposits related with mafic/ultramafic complexes remains still unclear. Literature data on grain size ( r) of platinum-group minerals (PGM) and their formation temperature (range of temperatures between 700 and 1100 °C), revealed an Arrhenius temperature dependence. Correlation of the rate of crystal formation that depends on temperature (T) with the size ( r) of the grain results in a linear relationship between ln( r) and 1/T. From the slope of the line n × ln( r) = −const. + E act/ RT the activation energy for the formation of IPGM (Ir–platinum-group minerals) was estimated, for the first time in the present study, to be approximately 450 ± 45 kJ mol −1. Applying the Arrhenius equation, the corresponding formation temperature for extremely large IPGM grains (up to 1.3 mm) in chromite ores related to ophiolites was found to be approximately 740 °C. It seems to be consistent with a lower formation temperature than with the typical formation temperature of small PGM grains associated with ophiolitic chromitites. This suggests that coarse-grained PGM in mafic/ultramafic complexes, along the permeable shear zones, may have been re-crystallized during plastic deformation at relatively lower temperatures (700–800 °C), under appropriate pressure, temperature, redox conditions and an increased H 2O content. Thus, applying the plot of ln( r) versus 1/T on large Os–Ir–Ru-minerals (sulfides or alloys), characterized by an r value falling into the linear part of the graph and having evidence supporting their formation at relatively high temperatures, then the corresponding formation temperature of those IPGM can be found.

Iridium, ruthenium and rhodium in komatiites: Evidence for iridium alloy saturation

A large body of new data (over 400 samples) on platinum-group element (PGE) concentrations in Yilgarn Craton komatiites is combined with all available high-quality literature data, to reveal trends in a spectrum of komatiite lithologies ranging from spinifex-textured rocks to pure olivine and olivine–chromite adcumulates. The data set is restricted to Carius Tube-isotope dilution analyses, and high-precision nickel sulfide fire-assay-ICP-MS analyses, and we demonstrate good agreement between the two methods. Only S-poor samples are considered, and the data set is filtered to eliminate the effects of addition of cumulus chromite, and addition or segregation of magmatic sulfide liquid. The residual trends indicate that Ir, Ru and Rh are decoupled from one another and from Pt and Pd. Rhodium behaves more like Pt and Pd, whereas Ru is intermediate between Ir and Rh. Rhodium and Ru both show indications of olivine control with partition coefficients less than unity. The maximum Ir whole-rock content is a roughly linear positive function of MgO, but the minimum is constant, sub-1 ppb and independent of MgO. Ruthenium shows some evidence for limited control by chromite, but anomalous Ru enrichment is also found in some olivine cumulate samples with no evidence for accumulated chromite or sulfide. Neither Rh nor Ir shows evidence for any detectable control by chromite. The distribution of Ir, and to a lesser degree Ru and Rh, cannot be adequately explained by olivine control, and a major influence of an Ir-rich phase is strongly indicated. The pattern of Ir variance in relation to MgO bears some important similarities to that of Cr. This similarity arises from the fact that both elements are controlled by a temperature-dependent solubility limit: the chromite saturation surface in the case of Cr, and saturation in Ir (or Ir–Os) alloy in the case of Ir. Combining this effect with moderate compatibility in olivine allows us to successfully model the entire range of Ir data. The upper limit of Ir concentration in olivine cumulates, in an exact analogy to that for Cr, is interpreted as the cotectic ratio of olivine to alloy, as determined by the average slope of the tangent to the alloy solubility curve. Sulfide-undersaturated mantle melts of a wide range of compositions are likely to be Ir alloy saturated. Experimental predictions of very low Ir solubilities in magmas are borne out, with the further implication that true thermodynamic sulfide–silicate liquid partition coefficients, at least for Ir and by inference for the other PGEs, are of the order of 10 7 or even higher.

Controls on the PGE distribution of Permian Emeishan alkaline and peralkaline volcanic rocks in Longzhoushan, Sichuan Province, SW China

The ~ 260-Ma Emeishan large igneous province in SW China and northern Vietnam contains low-Ti tholeiitic basalts, high-Ti alkaline basalts and minor peralkaline volcanic rocks. The flood basalts in Longzhoushan, Sichuan Province, SW China, comprise four units, each of which has volcanic breccia at the base. From the base upward, rocks of Units 1 and 2 are high-Ti basalts with relatively high MgO (7.6–11.0 wt.%), TiO 2 (1.7–4.5 wt.%) and Ti/Y ratios (420–790). Unit 3 is composed of tephrites with lower MgO (0.87–6.2 wt.%), TiO 2 (0.96–2.14 wt.%) and Ti/Y ratios (190–530), whereas Unit 4 consists of basaltic andesites with moderate MgO (3.5–4.8 wt.%), TiO 2 (2.2–3.1 wt.%) and Ti/Y ratios (383–606). The high-Ti basalts may have been derived from an OIB-like mantle source and the basaltic andesites were probably generated by crustal contamination and fractionation of high-Ti basaltic magmas. The tephrites show high Nb/Y (1.2 to 2.7) and Th/Y (0.31 to 0.55) ratios, variable Zr/Hf ratios (40 to 49) and relatively low εNd( t) (− 0.7 to + 0.28) values, suggesting an asthenospheric mantle contribution modified by carbonate-rich fluids. The high-Ti basalts contain relatively high and variable PGE and their Cu/Pd ratios are relatively constant (compared to the tephrites) and vary from 8,500 to 86,000. We attribute such relative constancy of the Cu/Pd ratios to the presence of residual sulphide in the source of the high-Ti rocks and subsequent crystallization from S-undersaturated melts. The basaltic andesites have relatively high and constant Cu/Pd ratios (130,000–370,492) and the lowest PGE contents (average 0.12 ppb for Pd, 0.20 ppb for Pt), suggesting an earlier removal of sulfide due to crustal contamination before emplacement. The tephrites have PGE abundances (0.057 to 1.89 ppb for Pt and 0.11 to 2.10 ppb for Pd) higher than those for basaltic andesites, however, the Cu/Pd ratios (33,300 to 429,000) of the rocks are more variable than those for the basaltic andesites, suggesting that the tephrites may have undergone both S-saturated fractionation during emplacement of the magmas and weak removal of sulfides from the source magmas. Early fractionation of chromite together with removal of laurite and/or Os–Ir–Ru alloys from the parental magmas may be responsible for the negative Ru anomalies of the tephrites, high-Ti basalts and basaltic andesites.

Laser ablation ICP-MS study of platinum-group elements in sulphides from the Platreef at Turfspruit, northern limb of the Bushveld Complex, South Africa

The Platreef unit of the northern Bushveld Complex comprises a diverse package of pyroxenites, peridotites and mafic lithologies with associated Ni–Cu–platinum-group element (PGE) mineralisation. Base metal sulphides (BMS) are generally more abundant in the Platreef than in other Bushveld PGE deposits, such as the Merensky Reef and the UG2 chromitite, but the Platreef, though thicker, has lower overall PGE grades. Despite a commonly held belief that PGEs are closely associated with sulphide mineralisation, a detailed study by laser ablation ICP-MS (LA-ICP-MS) on a core through the Platreef at Turfspruit suggests that this is not strictly the case. While a significant proportion of the Pd, Os and Ir were found to be hosted by BMS, Pt, irrespective of its whole-rock concentration, was not. Only at the top of the Platreef is Pt directly associated with sulphide minerals where Pt–Pd–(±Sb)–Te–Bi-bearing inclusions were detected in the chalcopyrite portions of large composite sulphides. In contrast, Pd, Os, and Ir occur in solid solution and as discrete inclusions within the BMS throughout the core. For Os and Ir, this is usually in the form of Os–Ir alloys, whereas Pd forms a range of Pd–Te–Bi–(Sb) phases. Scanning electron microscope observations on samples from the top of the core revealed the presence of ≤0.2-mm-long (PtPd)2(Sb,Te,Bi)2 michenerite–maslovite laths within the chalcopyrite portions of large composite sulphides. Additional Pt-bearing minerals, including sperrylite and geversite, and a number of Pd(–Te–Bi–Sb) minerals were observed in, or close to, the alteration rims of these sulphides. This textural association was observed throughout the core. Similar platinum-group minerals (PGMs) were observed within the felsic assemblages composed of quartz, plagioclase, alkali feldspar and clinopyroxene produced by late-stage felsic melts that permeated the Platreef. Many of these PGMs occur a significant distance away from any sulphide minerals. We believe these features can all be linked to the introduction of As, Sb, Te and Bi into the magmatic system through assimilation of sedimentary footwall rocks and xenoliths. Where the degree of contamination was high, all of the Pt and some of the Pd formed As- and Sb-bearing PGM that were expelled to the edges of the sulphide droplets. Many of these were redistributed where they came into contact with late-stage felsic melts. Where no felsic melt interactions occurred, the expelled Pt- and Pd-arsenides and antimonides remained along the margins of the sulphides. At the top of the Platreef, where the effects of contamination were relatively low, some of the Pt remained within the sulphide liquids. On cooling, this formed the micro-inclusions and blade-like laths of Pt–Pd–(Sb)–Bi–Te in the chalcopyrite.

Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-collector Plasma Ionisation Mass Spectrometry, part II: Laser ablation and its application to single-grain Pt–Os and Re–Os geochronology

The abundance of platinum group alloy (PGA) grains available from some alluvial deposits combined with their great chemical resistance to conventional acid attack generates the need for an alternative method of obtaining routine, rapid yet precise Os isotopic data. Laser ablation multi-collector ICP-MS (LA-MC-ICP-MS) is ideally suited for this purpose and has been applied, in a relatively limited extent, to Os-rich samples by previous workers (e.g., Hirata, T., Michinari Hattori, M., and Tsuyoshi Tanaka, T., 1998. In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem Geol., 144, 269–280.; Malitch, K.N., 2004. Osmium isotope constraints on contrasting sources and prolonged melting in the Proterozoic upper mantle: evidence from ophiolitic Ru–Os sulfides and Ru–Os–Ir alloys. Chem Geol. 208. 157–173.; Walker, R.J., Brandon, A.D., Bird, J.M., Piccolia, P.M., McDonougha, W.F., R.D., Asha, R.D., 2005. 187Os– 186Os systematics of Os–Ir–Ru alloy grains from southwestern Oregon. Earth Planet. Sci. Lett. 230, 211–226.). The wide variety of PGA minerals yield a broad spectrum of Pt, Re and Os contents which translate into a wide spread in parent–daughter ratios that is an attractive proposition for geochronology using the Re–Os and Pt–Os decay schemes. Here we show that single PGA grains, can contain sufficient internal Re, Pt and Os elemental variability and coupled variations in parent/daughter and daughter isotope compositions that they present an opportunity to obtain ‘isochron’ ages from two different isotope decay systems. We present a rapid (40 s acquisition time), precise and accurate LA-MC-ICP-MS methodology suitable for applying Pt–Os and Re–Os geochronology approaches to single PGA grains, for use in dating chromitite deposits and identifying and dating multiple sources in alluvial PGA deposits. Because of isobaric interferences from Pt, PGA analyses by LA-MC-ICP-MS are best corrected for instrumental mass fractionation using the 189Os/ 188Os ratio. We demonstrate that, within our analytical uncertainties, mass bias effects for Os isotope ratios during LA-MC-ICP-MS are similar in nature and magnitude to mass bias variations observed during normal solution-mode analysis. While robust elemental interference corrections can be applied for Re and W on the Os mass spectrum during solution analyses (Nowell, G.M., Luguet, A., Pearson, D.A., Horstwood, M.S.A., 2008-this volume. Precise and accurate 186Os/ 188Os and 187Os/ 188Os measurements by Multi-Collector Plasma Ionisation Mass Spectrometry (MC-ICP-MS) part I: solution analyses. Chem Geol.) we show that the accurate correction for the Re interference on Os during laser ablation is problematic at 187Re/ 188Os ratios > ∼ 0.5 which means that Re-corrected 187Os/ 188Os ratios for some PGAs and high Re/Os sulfides must be regarded as problematic. Our interfering element correction approach can also be used for the correction for Os on 190Pt for obtaining 190Pt/ 188Os ratios, removing the need for an independent Pt monitor isotope. Accurate and precise geochronology by LA-MC-ICP-MS can be further limited by poorly quantified fractionation of the parent/daughter elemental ratio during ablation. Although additional work is required to understand the nature and exact extent of inter-element fractionation of the Pt/Os and Re/Os ratios during laser ablation analysis of Pt- and Re-rich PGA grains, our best estimate is that fractionation of the Pt/Os during laser ablation is around 5% or less. We present multi-grain isochrons and single-grain internal ‘isochrons’ for the Re–Os and Pt–Os isotope systems in PGA grains from Lapland. In all cases, the Pt–Os isotope system gives significantly more precise ‘isochrons’ than the Re–Os system. We interpret this discordance between the two systems as being most likely due to problems inherent with Re interference corrections at elevated Re/Os ratios. We show that a suite of Pt-rich PGA grains from the Central Lapland greenstone belt yields a spectrum of internal Pt–Os isochron ages that coincides well with pulses of formation of chromitite-bearing ophiolitic material within this magmatic terrane and we take this to indicate the record of chromitite formation in different magmatic units. The single-grain Pt–Os and Re–Os geochronology technique shows great promise in the dating of chromitite deposits in ophiolites and in the discrimination of multiple source regions within alluvial PGA deposits.

Platinum-group elemental and Sr–Nd–Os isotopic geochemistry of Permian Emeishan flood basalts in Guizhou Province, SW China

The 260 Ma Emeishan continental flood basalts (ECFB) occur in SW China and northern Vietnam. A 550-m-thick sequence of ECFB in Guizhou Province (SW China) is composed of 12 basaltic lava flows, each ranging from 10 to 140 m in thickness. All samples belong to the high-Ti basalts (TiO 2 = 1.9–4.4 wt.% and Ti/Y = 400–700). Relative to the upper flows (Flows 9–12), those from the lower part of the section (Flows 1–8) are depleted in Nb, Ta, P and Sr and enriched in Zr, La and Th, consistent with higher degree of crustal contamination. Overall, the concentrations of Os, Ir and Ru range from 0.01 to 0.23 ppb and are low relative to those of Rh, Pt and Pd, which range from 0.2 to 1.1, 6 to 17 and 5 to 18 ppb, respectively. Different lava flows have different primitive mantle-normalized PGE patterns, and samples from the same flow show similar patterns. Fractionations between PPGE and IPGE are mainly controlled by olivine and chromite fractionation. The negative correlations between Pd/Ir and Pt/Pd, and Ni/Pd and Pd/Cr are consistent with crystal fractionation. Flows 6–8 and 10–12 show obvious negative Ru anomalies, explained by fractionation of laurite (Ru, Os, Ir)S 2 or Os–Ir–Ru alloys in the primary magma, or by coprecipitation of laurite with chromite or olivine. Flows 9–12 are Os-poor and have higher initial 187Os/ 188Os ratios than Flows 1–8, indicating it is easy to alter the Os isotopic compositions of basaltic magmas with lower Os concentrations.

MIESSIITE, Pd11Te2Se2, A NEW MINERAL SPECIES FROM MIESSIJOKI, FINNISH LAPLAND, FINLAND

Miessiite, a new mineral species of ideal composition Pd11Te2Se2, is cubic, space group Fd 3 m (#227), a 12.448(2) A, V 1929.0 (4) A3, Z = 8. The strongest six lines of the X-ray powder-diffraction pattern [ d in A(I)( hkl )] are: 2.395(80)(511,333), 2.197(100)(440), 1.875(25)(622), 1.555(25)(800), 1.305(25)(931) and 1.271(30)(844). The mineral was discovered in a platinum-group-mineral (PGM) placer sample taken from the Miessijoki river, Lemmenjoki area, Inari Commune, Finnish Lapland, Finland. Associated PGM and other heavy minerals include stillwaterite, isomertieite, mertieite-II, cooperite, braggite, kotulskite, vincentite, tantalite, thorianite, pyrite, magnetite, chromite, and isoferroplatinum with inclusions of laurite and Os–Ir–Ru alloy. Miessiite is black with a metallic luster. The fragments are malleable and silvery grey in color. The average micro-indentation hardness VHN100 is 362 ( n = 4) and corresponds to a Mohs hardness of 2–2½. Under reflected plane-polarized light, the mineral is isotropic with a light grey color in comparison with the associated PGM grains. Measured values of reflectance are tabulated, obtained in air and in oil for a single grain. The reflectance values in air for the standard COM wavelengths are: 48.88 (470 nm), 51.63 (546 nm), 53.91 (589 nm) and 56.82% (650 nm). D (calc) is 10.94 g/cm3 (for the empirical formula and unit-cell parameter refined from powder data). The electron-microprobe analyses yielded (wt.%) Pd 75.17, Se 9.61, Te 17.06, total of 101.84 wt.%, which corresponds to Pd11.02 Te2.09 Se1.90, based 15 atoms. The mineral is named after the locality, the Miessi river (in Saami language, Miessijohka ).

Abundance and distribution of platinum-group elements in orogenic lherzolites; a case study in a Fontete Rouge lherzolite (French Pyrénées)

Orogenic lherzolites exhibit a specific (although reproducible) platinum-group element signature characterized by slight positive deviations of Pd/Ir, Rh/Ir and Ru/Ir ratios from the canonical chondritic model. Such a signature was alternatively considered to be a compositional feature of the primitive upper mantle or resulting from sulfide melt addition by refertilisation reactions in the continental lithosphere. To shed additional light on this conundrum, the distribution of PGE in FON B 93, an unserpentinized, fertile (3.24% Al 2O 3) orogenic lherzolite (French Pyrenees) used as in-house standard by our group, has been studied to different scales, from the bulk rock to trace minerals. In addition to new ICPMS analyses after separation into NiS and Te coprecipitation, its whole-rock PGE concentrations were redetermined by three different laboratories using very precise ID-ICP-MS methods after digestion of the sample in Carius tube at T = 250 °C and 320 °C or in high-pressure asher (HPA-S). The four methods produce reproducible Ru, Rh and Pd contents (7.1 ± 0.18 ppb; 1.43 ± 0.05 ppb; 7.1 ± 0.30 ppb) whereas the non-ID NiS-fire assay method underestimates the Os and Ir concentrations by c.a. 10 and 15% compared to ID-ICP-MS analyses (4.40 ± 0.07 and 4.00 ± 0.17 ppb, respectively). Platinum was the most difficult to analyse. If performed on powder aliquots smaller than 3 g, the ID-ICP-MS analyses generate strong nugget effects while the non-ID NiS-fire assay method yields statistically lower (but highly reproducible) Pt concentrations (6.92 ± 0.26 ppb). These features reflect the strong partitioning of Pt into trace phases that SEM and laser ablation-ICP-MS analyses on thin sections identify as both high-temperature Pt–Ir–Os alloys and Pt–(Pd) tellurides of likely subsolidus origin; ten to fifteen grains ranging in maximum dimension from a few micrometres to a few hundreds of nanometers, were identified by standard polished thin section. LA-ICP-MS data on base metal sulfides (25 grains analysed), coupled with the whole-rock S concentration (277 ± 10 ppm) and modal composition of the BMS (90% pentlandite + accessory pyrite and secondary pyrrhotite + 10% chalcopyrite) allowed the contribution of the BMS phase to the PGE budget of FON B 93 to be estimated. Except Pt that exhibits a 95% deficit in the BMS phase, the PGE concentrations measured by ID-ICP-MS can be balanced by BMS while Al-spinel is a negligible contributor, accounting for less than 0.5% of the Ru budget. The occurrence of Pt in trace phases may bias the whole-rock Pt concentrations because 1) mechanical collection of Pt-rich trace phases remains problematic with the NiS button, and 2) Pt–Ir–Os alloys may be prone to digestion problems in conventional Carius tube procedures. Since they could be stable at mantle depth, such refractory alloys that contain Os and Ir may also have enhanced the heavy PGEs/light PGEs fractionation. Our observations likely pertain to orogenic lherzolites as a whole because BMS assemblages in these mantle rocks record evolution at low sulfur fugacity, which prevents Pt–Ir–Os alloy from entering the Mss in the mantle; moreover, at subsolidus temperature, pentlandite, the major BMS, cannot accommodate 0 valence state Pt.

Investigation of platinum-group minerals (PGM) from Pindos chromitites (Greece) using hydroseparation concentrates

The Pindos ophiolite complex, located in northern Greece, hosts small podiform chromitites characterized by very low platinum group element (PGE) grades. PGE (excluding Os) analyses from four chromitite samples, collected for this study, are below 400 ppb. Consequently, we used the technique of hydroseparation to concentrate PGM from Pindos chromitites. More specifically, we investigated two separate composite samples from Pefki and Milia chromitites. The Pefki concentrate contains 67 PGM that include secondary Ru-bearing minerals, ruarsite (RuAsS), laurite (Ru,Os)S 2, irarsite (Ir,Ru,Rh,Pt)AsS, alloys of Os–Ir–Ru, hollingworthite (Rh,Pt,Pd)AsS, paolovite (Pd 2Sn), braggite (Pd,Pt,Ni)S, sperrylite (PtAs 2) and four unnamed PGM, electrum (Au,Ag) and native silver. The investigation of the Milia concentrate yields fifty one grains of PGM including primary laurite, Os–Ir alloys, erlichmanite, secondary Ru-bearing minerals, irarsite, Ru-alloys, ruarsite and Ru-based metals sulphides. PGM occur as both single and polyphase particles in both concentrates. The bulk of mineralization for grains between 5 and 15 μm is finer-grained in Milia (64.7%) than in Pefki (41.8%). The latter concentrate hosts considerably more altered PGM grains than the former. The hydroseparation process has recovered significantly more, as well as novel, PGM grains than the in situ mineralogical examination of single chromitite samples from the neighboring Korydallos occurrence. Although, most of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. The mineralogical and grain size differences between the two samples may reflect styles of mineralization and indicate significant remobilization of PGE in Pefki. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Pefki and Milia chromitites. In summary, this study provides significant information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.

CHROMIAN SPINEL COMPOSITION AND THE PLATINUM-GROUP MINERALS OF THE PGE-RICH LOMA PEGUERA CHROMITITES, LOMA CARIBE PERIDOTITE, DOMINICAN REPUBLIC

The Loma Peguera chromitites are located in the central part of Loma Caribe peridotite in the Cordillera Central of Dominican Republic. The chromitite bodies are hosted in typical mantle peridotites, have small size (less than a few meters across) and show massive textures. Primary chromite composition is Cr-rich (0.74 < Cr# < 0.78) and exhibits systematically high Ti (average value: 0.84 wt% TiO2) and Fe3+ (average value: 7.82 wt% Fe2O3). The total PGE concentrations vary from 1.82 to 2.04 ppm, and show an enrichment in Os + Ir + Ru relative to Rh + Pd + Pt. Among the latter group, a positive anomaly of Pt (249–269 ppb) is present as a result of the appearance of Pt-bearing alloys. The PGM assemblage comprises Ru, Os, Ir and Pt-rich phases, including laurite, irarsite, unknown Ru3As2, Ru–Os–Ir–Fe oxides, unknown Pt–Ir–Ni–Fe and Ru–Os–Ir–Pt–Fe–Ni alloys, native Ru and Ru-rich Ni–Fe alloy. The Loma Peguera chromitites are rather unusual in chemical composition compared with those commonly occurring in the Moho transition zone or from mantle harzburgites in ophiolites, but bear some similarity to chromitites from Uralian–Alaska-type complex. The Al2O3 and TiO2 contents of the calculated composition of the melt in equilibrium with the Loma Peguera chromitites are similar to those from island-arc magmas and to some oceanic plateau basalts.

Multiple events in the Neo-Tethyan oceanic upper mantle: Evidence from Ru–Os–Ir alloys in the Luobusa and Dongqiao ophiolitic podiform chromitites, Tibet

In situ analysis of major elements, platinum-group elements and Os isotope composition data for 170 grains of Ru–Os–Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites, Tibet, provide new constraints on the sources of the alloy grains, the Os-isotope composition of the Neo-Tethyan oceanic upper mantle, and the timing of melt–rock reactions in the ultramafic peridotites of the host ophiolites. Most alloy grains are osmiridium and iridosmine. Based on elemental and isotopic compositions, two populations can be identified. Group I, found in both ophiolite bodies, has fractionated PGE patterns typical of alloys and homogeneous 187Os/ 188Os 0.12645 ± 4 (2s; n = 145). Group II alloys from the Dongqiao ophiolites show smooth PGE patterns and 187Os/ 188Os ranges from 0.12003 to 0.12194 (yielding Re-depletion ages ≥ 1.1 Ga). Both the PGE patterns and the Os isotopic compositions of Group II resemble those of sulfides residual after melting. Group I alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt. In contrast, Group II alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite, which may represent relics of a sub-continental lithospheric mantle. The coexistence of the two groups of alloys in Dongqiao samples reflects interaction between asthenosphere-derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt–rock reactions. The data on the Group I alloys support the hypothesis that the Re–Os evolution of the convective oceanic mantle resembles that of enstatite chondrites.

New Stable Decagonal Quasicrystal in the System Al—Ir—Os.

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Diversity of platinum-group minerals in podiform chromitites of the late Proterozoic ophiolite, Eastern Desert, Egypt: Genetic implications

Podiform chromitites are frequently distributed as lensoidal pods in the central and southern parts of the Eastern Desert, Egypt. They are, in most cases, hosted by fully serpentinized peridotite which is a part of dismembered ophiolite complexes of the Pan–African belt of Late Precambrian age. Serpentinites are the predominant components in the ophiolitic mélange, either as matrix or as variably sized blocks, and are derived from harzburgite and subordinate dunite. The central Eastern Desert (CED) chromitites have a wide compositional range from high-Cr to high-Al varieties, whereas those of the southern Eastern Desert (SED) have a very restricted compositional range. The Cr# of spinel ranges from 0.5 up to 0.8 in the former, while it is around 0.8 in the latter. Platinum-group element (PGE) mineralization has been recently reported in podiform chromitites from the late Proterozoic Pan–African ophiolite of the Eastern Desert of Egypt. The populations of platinum-group minerals (PGM) in the studied CED and SED chromitites are quite distinguishable; they are mainly sulfides (Os-rich laurite) in the former, and Os–Ir alloy in the latter. Sulfarsenides and arsenides are also found in subordinate amounts from both chromitites. The most abundant base metal sulfides (BMS) in the Eastern Desert chromitites of Egypt are millerite, heazlewoodite, pentlandite, chalcopyrite and pyrite. The sulfur fugacity and temperature conditions are the main controller of PGE mineralogy in the host chromitite at the initial stage within the upper mantle. Os-rich laurite is stable at high sulfur fugacity and low temperature conditions, whereas Os–Ir alloy is stable at lower sulfur fugacity and higher temperature conditions. The diversity of PGE mineralogy combined with the differences in petrological characteristics of chromian spinels from CED chromitites to SED ones suggests different degrees of partial melting of the mantle rocks of this ophiolite which, in turn may be attributed to different tectonic settings. During the post-magmatic processes, i.e., serpentinization, the primary PGM, e.g., Os-rich laurite and Os–Ir alloy, can be modified at low temperatures to secondary PGM.

Composition and textures of chromite and platinum-group minerals in chromitites of the western ophiolitic belt from Pampean Ranges of Córdoba, Argentina

Chromitite bodies hosted in the Neoproterozoic western ophiolitic belt of Pampean Ranges of Córdoba (Argentina) were studied at Los Congos and Los Guanacos ultramafic bodies, with regard to the composition and textures of the chromite and platinum group minerals. Primary chromite composition is only preserved in some massive chromitites from the Los Guanacos ultramafic body, and is similar to Al-rich ophiolitic chromitites, suggesting that they crystallized from melts with back arc basin basalts (BABB) affinity in the suprasubduction mantle. Subsequently, these chromitites underwent a prograde metamorphism. Chromites from chromitites and associated metamorphosed ultramafic rocks show complex replacement and exsolution textures. Mineral chemistry and texture indicate that the chromite composition records two main metamorphic trends. A first trend defined by chromite from massive chromitite, in which there is an enrichment in Fe 3+ and Fe 2+, Cr remain relatively constant, and slightly depleted in Al, Mg. A second trend is defined by chromite from disseminated chromitite and metamorphosed dunite and harzburgite, in which a Fe-rich phase is replacing the Al-rich chromite. This alteration trend is characterized by enrichment in the total iron content (Fe 3+ + Fe 2+) and a strong depletion in Al and Mg. The chemical composition of all analyzed spinels from Los Guanacos and Los Congos, as plotted on the ternary Fe 3+–Cr–Al diagram, correlates well with the Cr-spinels from the upper amphibolite to granulite-facies metamorphism. Platinum group minerals (PGM) identified include native osmium, laurite, erlichmanite, irarsite, platinum and a number of inadequately identified phases such as an oxide or hydroxide of Ru, Pt and Ir–Ru, Pt telluride, Ir–Ru–As–Se and Ir–Ru–Ti compounds. Native osmium was the only PGM which remained unaltered; other PGM underwent mineralogical reworking during metamorphism. Although it is difficult to establish the extent of platinum group element mobilization based on mineralogical observation, our results suggest that the Ru–Os–Ir PGM in the Los Guanacos and Los Congos chromitites were modified in situ, producing re-distribution of these PGE on a small scale. The presence of rare Pt and PGE–As–Se minerals was possibly related to remobilization of Pt, As and Se by fluids during the alteration processes.

High PGE contents and extremely abundant PGE-minerals hosted in chromitites from the Veria ophiolite complex, northern Greece

More than 100 small, isolated chromite occurrences form elongated lenses that follow the NW–SE trend of the host serpentinized dunite within a 60 km 2 area 15 km SE of Veria, northern Greece. They occur only within mylonite shear zones, characterized by a strong overprint of late brittle deformation. Detailed investigation of chromitites by scanning electron microscope with energy-dispersive X-ray analysis shows that chromite is dominantly of high-Cr composition, with the atomic Cr/(Cr + Al) ratio ranging from 0.55 to 0.84, and an average value of 0.7. Interstitial primary silicates in chromitites have been replaced by secondary phases consisting of mostly Cr-andradite, Cr-chlorite and serpentine flakes in lesser amounts. The most salient feature is the high total platinum-group element (PGE) content (up to 25 ppm), coupled with the presence of extremely abundant solitary platinum-group element minerals (PGM) within massive chromite samples located along a shear zone. These PGM fragments are angular, rarely corroded grains or fine-grained aggregates and occur in a matrix of Cr-garnet. They are mostly Os–Ru–Ir–Fe alloys containing small amounts of Cr, Co ± Mn, followed by irarsite. Laurite (RuS 2) with variable Os and Ir contents occurs as remnants surrounding by Ru–Os–Ir oxides of varying composition. Available geological, mineralogical and geochemical data indicate a prolonged multistage evolution (at least four stages) for the PGE mineralization. The relatively high IPGE enrichment in chromitites seems to be related to post-magmatic processes encompassing a long period of deformation episodes, beginning with plastic deformation in the asthenospheric mantle and culminating with brittle deformation in the crust. Metasomatic fluids have, during a process of rodingitization and serpentinization at a low state of sulfidation, substantially modified the composition of primary laurite towards Fe-, Cr-, Co-, and Mn-bearing Fe–Os–Ir–Ru alloys and oxides. The most prospective locations for chromitite–PGM targets seem to exclusively be small chromite occurrences that are found along shear zones within ophiolite complexes. These shear zones post-date the initial magmatic deposition of the host chromitites.

Platinum-group element mineralogy and geochemistry of chromitite of the Kluchevskoy ophiolite complex, central Urals (Russia)

We report the results of investigation of chromitites occurring in the Kluchevskoy ophiolite complex of the Russian Urals. The chromite composition suggests crystallization from a boninitic magma in a supra-subduction zone geodynamic setting. The investigated chromitites are enriched in Os–Ir–Ru over Rh–Pt–Pd, as typical of the mantle hosted ophiolite chromitites. Consistent with the geochemical data, the Platinum Group Mineral (PGM) assemblage is dominated by Ru–Os–Ir phases, whilst specific Rh–Pt–Pd minerals are absent. Two distinct paragenetic assemblages have been recognized: 1) primary magmatic PGM (laurite, erlichmanite, osmium, iridium, unnamed Ir–Ni–S, cuproiridsite, irarsite and ruthenarsenite) and 2) secondary PGM formed by desulfurization of primary sulfides at low temperature (ruthenium). Comparison of the studied chromitites with those hosted in the mantle of the Kempirsai, Ray–Iz and Voikar–Sininsky ophiolites has shown that all these chromite deposits form in the same geodynamic environment. The differences in the temperature calculated on the Fe–Mg exchange between olivine–spinel and observed in the PGM assemblage suggest that the Kluchevskoy chromitite suffered the effects of the metasomatism to a lesser extent compared with Kempirsai and Ray–Iz chromitites.

Gold overprint of PGE alloy: an example from the Fadeevka Au-PGE placer, Russian Far East

We document a rare case of micron-sized gold inclusions in Ir–Os alloy and overgrowth rims on Pt–Fe alloy from an alluvial Au-PGE placer derived from an Uralian/Alaskan type intrusion in Primorye, Russia. The gold inclusions occasionally form complex aggregates with cooperite [PtS] or tolovkite [IrSbS], and replace magmatic inclusions of Ir-rich Pt–Fe alloy which exsolved from the Ir–Os–Pt matrix. Gold has a relatively constant composition (>90 wt.% Au, a few wt.% Ag, and up to 8 wt.% Pt). The gold rims form superfine (3–5 µm) discontinuous films on the Pt–Fe alloy crystals often followed by cooperite overgrowth. Both gold textures suggest an electrochemical control of gold precipitation via selective Pt–Fe leaching during low-temperature overprint and/or weathering of PGE alloy.

Distribution and mineralogy of platinum-group elements in altered chromitites of the Campo Formoso layered intrusion (Bahia State, Brazil): control by magmatic and hydrothermal processes

Polyphase, penetrative hydrothermal metasomatism in chromitites of the Campo Formoso layered intrusion produced spectacular chromite – ferrian chromite zoning and transformed the primary intercumulus silicates into a chlorite – serpentine – carbonate – talc assemblage. Alteration did not substantially modify the composition of chromite cores and the distribution of platinum-group elements (PGE) through the sequence of chromitite layers, which still are consistent with magmatic fractionation processes. Texture and composition of laurite and Os–Ir–Ru alloys included in chromite cores indicate that these PGM were not altered, and are probably magmaticin origin. In contrast, the PGM located in the intergranular chlorite matrix (laurite, Ir–Ru–Rh sulfarsenides and Pt–Pd compounds with Sb, Bi and Te) display evidence of hydrothermal reworking. These PGM are intimately intergrown with low-temperature Ni-sulfides. The paragenesis suggests that the Ni-sulfides-PGM assemblage formed at the expenses of unknown PGM precursors, which must have been originally present in the intercumulus silicate matrix. Mechanism of formation involves a sequence of dissolution-precipitation events controlled by variation of redox conditions during chromite alteration. The presence of a secondary ore mineral assemblage consisting of galena, bismuthinite, native antimony, and various Pb–Sb compounds suggests a possible contribution of fluids derived from the adjacent granite.

Preparation and Characterization of Novel Os−Diolefin Dimers: New Entry to Os−Cyclooctadiene Complexes

The complex [H(EtOH)2][{OsCl(eta4-COD)}2(mu-H)(mu-Cl)2] (1) has been prepared in high yield by treatment of OsCl3.3H2O (54% Os) with 1,5-cyclooctadiene in ethanol under reflux. Under air, it is unstable and undergoes oxidation by action of O2 to afford the neutral derivative {OsCl(eta4-COD)}2(mu-H)(mu-Cl)2 (2). The terminal chlorine ligands of the anion of 1 are activated toward nucleophilic substitution. Thus, reaction of the salt [NBu4][{OsCl(eta4-COD)}2(mu-H)(mu-Cl)2] (1a) with NaCp in toluene gives [NBu4][{Os(mu1-C5H5)(eta4-COD)}(mu-H)(mu-Cl)2{OsCl(eta4-COD)}] (3) as a result of the replacement of one of the terminal chlorine atoms by the cyclopentadienyl ligand. The CH2 group of the latter can be deprotonated by the bridging methoxy ligand of the iridium dimer [Ir(mu-OMe)(eta4-COD)]2. The reaction leads to the trinuclear derivative [NBu4][{(eta4-COD)Ir(mu5-C5H4-mu1)Os(eta4-COD)}(mu-H)(mu-Cl)2{OsCl(eta4-COD)}] (4) containing a bridging C5H4 ligand that is eta1-coordinated to an osmium atom of the dimeric unit and mu5-coordinated to the Ir(eta4-COD) moiety. Salt 1a also reacts with LiC[triple bond]CPh. In this case, the reaction produces the substitution of both terminal chlorine ligands to afford the bis(alkynyl) derivative [NBu4][{Os(C[triple bond]CPh)(eta4-COD)}2(mu-H)(mu-Cl)2] (5). Complexes 1, 2, 3, and 4 have been characterized by X-ray diffraction analysis. Although the separations between the osmium atoms are short, between 2.6696(4) and 2.8633(5) A, theoretical calculations indicate that only in 2 is there direct metal-metal interaction, as the bond order is 0.5.

Platinum-group mineral characterisation in concentrates from low-grade PGE chromitites from the Vourinos ophiolite complex, northern Greece

The Vourinos ophiolite complex, located in northern Greece, hosts various chromite deposits characterised by very low platinum-group element (PGE) grades. Total PGE (excluding Os) concentrations in the fourteen chromitite samples collected for use in this study varied from 200 to 300 ppb.Previous reports on the platinum-group minerals (PGM) from the Vourinos chromitites, obtained data by in-situ investigation on polished sections. Consequently, we used the technique of hydroseparation to study the PGM from the concentrates in the Vourinos chromitites. More specifically, we investigated two separate composite samples from Voidolakkos and Xerolivado chromitites. The Voidolakkos concentrate sample contains 74 PGM that include: laurite (Ru, Os)S2; irarsite (Ir, Ru, Rh, Pt)AsS; erlichmanite (Os, Ru)S2; ruthenium pentlandite; iridium (Ir(Os)); osmium (Os(Ir, Ru, Pt)); secondary phases composed of Ru, Os and Cu; alloys of Ir–Fe; Rh– and Ru–Ni–Fe alloys; and Os–Ir–Fe alloys. The investigation of Xerolivado concentrate sample yield 45 grains of PGM, including laurite, irarsite, erlichmanite, minor other PGE sulphides, Os–Ir–Ru alloys, iridium and secondary phases of Ru–Os alloys. PGM occur as both single and polyphase particles in both samples. The bulk of mineralisation in Voidolakkos is dominated by a finer variety (<10 μm) of PGM than the Xerolivado sample (mainly <20 μm). The former occurrence hosts considerably more altered PGM grains, less laurite and a larger variety of PGM than the latter, whereas Os–Ir alloys are present in almost equal amounts in both samples. The hydroseparation process has recovered significantly more, as well as novel, PGM grains than were known from previous in-situ mineralogical examination of single chromitite samples. Although, most of the PGM occur as free particles and in-situ textural information is lost, single grain textural evidence is observed. The mineralogical and grain size differences between the two samples may reflect styles of mineralization and indicate significant remobilization of PGE. The latter possibility is suggested by the presence of secondary PGM, which may be related to the different alteration processes that affect the Voidolakkos and Xerolivado chromitites. In summary, this study provides significant new information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing of these minerals.