Platinum-group Minerals Inclusions Research Articles

Inclusions in Placer Pt-Fe Alloy Nuggets, Northwest Ecuador: Evolution of a Ural-Alaskan Type PGE Mineralizing System (Cr-Spinel, IPGE, Sulfarsenides, and Cu-Depleted PGM)

Abstract A Cu- and Rh-enriched magmatic ore system is defined by abundant PGM (platinum group mineral) inclusions in forty-four Pt-Fe alloy nuggets from the Camumbi River gold placer, northwest Ecuador. Isoferroplatinum is depleted in Rh, Os, and Ru compared with native platinum, suggesting most crystallized after Os-(Ir) alloy, laurite, and some Rh-PGM. Two Pt-Fe alloy nuggets have zoned hydrothermal alteration rinds, and an UM (unnamed mineral) is (Rh,Pd)4As3. Our previous work shows that silicate glass inclusions define a fractionated co-magmatic compositional series related to primitive hydrous ferrobasalt, and trace element chemistry matches their Late Cretaceous accreted volcanic arc terrane. Here we report exceptional Cr-spinel (Ural-Alaskan type) inclusions coexisting with primitive ferrobasaltic glass crystallized at highest T. Laurite inclusions also indicate high T and S saturation of early melt. Os-(Ir) inclusions are Ru-depleted while two discrete Ir-enriched osmium crystals have remarkable, extreme Ru enrichment and depletion, confirming crystallization before and after laurite. Laurite and osmium inclusions in one Pt-Fe alloy reflect concomitant crystallization and fluctuating low fS2 melt conditions. In experimental primitive Cu-bearing Pt-Pd-S-(As) melt (cf. exsolved from primitive basalt), first Cu-PGM-sulfide crystallization generates a Cu-depleted, Pt-Pd-As-(S) residual melt. At lower T immiscible melts Pt-As-(S) and later Pd-As-(S) crystallize distinctive PGM. We report analogous natural multiphase PGM inclusion assemblages in separate isoferroplatinum nuggets: (1) zoned sulfarsenides, sperrylite, and genkinite, with rare resorbed cognate xenocrystic cooperite (captured from primary sulfide melt) define a high T, Pt-enriched sub-system [Pt > Rh(Pd,Ir,Ru)As,S ≫ Sb,Bi] and (2) zoned sulfarsenides, arsenopalladinite, sperrylite, törnroosite, and gold define a lower T, fractionated Pd-enriched sub-system [(Pd > Rh ≃ Pt > Ir > Au)As,S > Te ≫ Sb,Bi]. The previously undocumented natural S-rich sperrylite (formerly “platarsite”) solid solution series and later crystallized irarsite series are discriminated in terms of Pt-Ir-Rh. Both trends fractionate toward increasing Rh (hollingworthite). The discrete PGM assemblage, sperrylite-telluropalladinite (with exsolved palladium and electrum) defines an IPGE-depleted Pd > Pt(Au > Ag)As ≥ Te ≥ Sb sub-system and records extreme fractionation. Cu-bearing multiphase PGM inclusions (some coexisting with silicate glass) derived from the fraction of Cu-bearing exsolved Pt-Pd-S-(As) melt will be reported separately.

Behavior of Platinum-Group Elements during Hydrous Metamorphism: Constraints from Awaruite (Ni3Fe) Mineralization

Abstract Natural Fe-Ni alloys are common in meteorites and, presumably, the Earth’s core, where they host significant platinum-group elements (PGE). However, little is known on PGE concentrations in hydrothermal or metamorphic Fe-Ni alloys (i.e., awaruite Ni3Fe) from terrestrial rocks. In this work, we examine the geochemistry of awaruite and related minerals from several placer deposits sourced from the suprasubduction ophiolitic (Kamchatsky Mys, Karaginsky Island, and Mamet) and Ural-Alaskan (Galmoenan) complexes of Kamchatka and the Koryak Highlands (Far East Russia) in order to assess the abundance of PGE in awaruite and constrain their mobility under metamorphic and hydrothermal conditions. Studied awaruite from ophiolitic and Ural-Alaskan type complexes formed via desulfurization of pentlandite during serpentinization. Three groups of platinum-group minerals (PGMs) are associated with awaruite from Kamchatsky Mys: (1) Pt-Fe alloys such as ferronickelplatinum (Pt2FeNi) or unnamed Ni2FePt alloys; (2) Os-Ir-Ru alloys of various composition; (3) Pd-Sb minerals which form together with serpentine during hydrothermal alteration. Despite the abundance of PGM inclusions, no significant PGE concentrations were measured in awaruite from the Kamchatsky Mys, Karaginsky Island, or Mamet ophiolites. In contrast, pentlandite relicts in awaruite from placers related to the Galmoenan Ural-Alaskan type complex contain exceptionally high, previously unreported, Os (up to 540 ppm). Awaruite that forms on behalf of this pentlandite does not show any significant Os enrichment. Rare Galmoenan awaruite analyses yield up to 3 ppm Pd. The new data are not in complete accordance with previous studies that reported relatively high (up to first 10 ppm) PGE content in awaruite. We attribute this to low PGE concentration in precursor sulfides and preferential partitioning of PGE into discrete secondary PGM within awaruite. Nevertheless, abundant inclusions of secondary PGM in awaruite provide evidence of PGE mobility during metamorphic and hydrothermal alteration of ultramafic rocks.

Distributions of Platinum Group Elements and Re-Os Isotope Systematics in Chromite from the Coobina Chromitite Deposit in Western Australia: Implications for Chromite as a Sulfide Mineralization Indicator

ABSTRACTThe abundance of Ru in chromite has been suggested as an indicator of sulfide liquid saturation in komatiites. The komatiite magma-derived Archean Coobina intrusion is known to be barren in terms of sulfide mineralization. Therefore, the Coobina intrusion can serve as a useful case study to test the applicability of Ru abundance in chromite as a potential indicator for sulfide mineralization, as well as for better understanding the PGE-chromite association in general.The Coobina intrusion is a highly deformed layered intrusion interpreted to be a flared dike. It contains multiple massive chromitite seams that have been recently mined for metallurgical-grade chromite. In this study, 18 samples from chromitite seams throughout this intrusion are investigated for their whole-rock platinum group element (PGE) contents, which are compared to their chromite mineral chemistry (including PGE content), the platinum group mineral (PGM) mineralogy, and Re-Os isotope systematics. Each sample has a similar chromite major and minor element chemistry, but a unique trace element signature, even within the same seam. In general, there are higher concentrations of Ru (>300 ppb) within chromite in the southeast (toward the feeder dike) and lower concentrations (<50 ppb Ru) in the northwest. At a sample scale, Ru in the whole rock and Ru in solid solution in the chromite are inversely correlated, while Ir shows a positive correlation between the whole rock and chromite mineral chemistry, indicating differing partitioning behaviors within the iridium-group PGE (IPGE = Os, Ir, Ru). The inverse correlation between Ru in solid solution within chromite and Ru in whole-rock chromitite suggests that, for seams with high Ru in whole rock, Ru is occurring within separate PGM phases. This is supported by the observation that the samples with high whole-rock Ru also have a high number of visible metal alloy and/or PGM inclusions. Although these inclusions are not necessarily Ru-rich phases, their presence suggests that there is a preference for these samples to form nuggets, which may restrict Ru partitioning into the chromite crystal structure. We suggest that the low Ru values in the Coobina chromite are a result of transient sulfide saturation. The Re-Os isotopic composition of the Coobina chromitite is chondritic [γ187Os(3.189 Ga) = −0.63 ± 0.21] and is consistent with derivation of the Coobina parental magma from the convecting upper mantle source, providing evidence for the mantle origin of the Coobina PGE inventory.If using chromite as a detrital indicator mineral for magmatic sulfide exploration, it must be kept in mind that transient sulfide saturation within chromitite seams may give a false positive signature.

Platinum and gold placer from Tugidak Island, Alaska: Platinum-group minerals and their inclusions, gold, and chromite mineralogy

ABSTRACT Black sand beach placers from Kodiak, Sitkinak, and Tugidak Islands, Alaska, have been mined intermittently for gold and minor platinum-group alloys for more than 100 years. High-grade platinum-rich magnetic separate and accompanying black sand from the southern beach placer of Tugidak Island were studied using electron microprobe WDS and scanning electron microscope EDS; mineral classification and identification were based on these techniques. The major platinum mineral is isoferroplatinum, followed by minor tetraferroplatinum and tulameenite, and rare ferronickelplatinum. Two types of alteration were identified in about 3–4% of the alloy grains: rim formation involving Pt loss and increased Fe, Ni, and/or Cu, and fracturing and vein filling by Cu-rich alloy. Ruthenium-Ir-Os-Pt alloys occur as inclusions and veins as well as form part of composite grains. Ten percent of the alloy grains contain a large variety of platinum-group minerals (PGM). Inclusions of cuprorhodsite, malanite, cuproiridsite, laurite, erlichmanite, cooperite, braggite, bowieite, kashinite, miassite, hollingworthite, irarsite, sperrylite, stillwaterite, genkinite, stibiopalladinite, keithconnite, zvyagintsevite, and probable palladodymite and vincentite were identified. Two unidentified inclusion phases also occur. Most of the PGM inclusions are primary and were trapped by a growing crystal from a melt; some inclusions exhibit textures that suggest trapping of an As,Te,S-rich immiscible melt. Secondary inclusions and evidence of deformation were observed in a few alloy grains. Associated with PGM inclusions or as separate inclusions are various base-metal sulfides. Two silicate-melt inclusions in one isoferroplatinum grain have an andesite–shoshonite composition. Minor gold and Ag-rich gold in the high-grade magnetic separate contain magnetite, pyrrhotite, and chromite inclusions. The gold composition suggests that their sources are the numerous quartz veins and apophyses related to granitoids on Kodiak Island. The composition of the placer chromite is similar to chromite from the Border Ranges mélange fault system and suggests that the Uyak Complex ultramafic and mafic rocks are part of a supra-subduction-zone ophiolite and are the source of the platinum-group minerals.

Mineralogical and isotopic peculiarities of high-Cr chromitites: Implications for a mantle convection genesis of the Bulqiza ophiolite

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 (<500 nm) of RuOs bisulfides (laurite; RuS2), OsIr, and Os-Ir-Ru alloys, tolovike, irarsite, IrNi sulfides, and millerite (NiS) are also common in the Bulqiza chromitites. Dispersed spherical, nano-inclusions of IrPt and OsIr may reflect formation of the IrPt and OsIr particles by sulfide liquid immiscibility. The high Cr# of the chromitites, their evolved whole-rock PGE abundances, and IPGE/PPGE ratios (≥ 10) suggest extensive partial melting, likely in a supra-subduction zone (SSZ) environment. This interpretation is supported by the variable 187Re/188Os (0.0103–0.0271) and 187Os/188Os (0.12600–0.13014) ratios and the low γOs isotope values (mean + 0.01) of the chromitites, reflecting derivation from heterogeneously depleted mantle materials. The presence of remarkably zoned zircons with UPb ages (164–3354 Ma) represents the addition of crustal materials.Discrete micro-diamonds, PGM, and sulfide inclusions point to a super-reducing (SuR), ultra-high pressure (UHP) origin. We propose a genetic model that involves incorporation of crustal and UHP phases into the mantle by convection or subduction. The UHP and crustal silicates are consistent with a subduction-related origin, but the presence of super-reduced (SuR) inclusions suggests equilibration under subduction-disparate mantle conditions.

Platinum mineralization and geochemistry of the Matysken zoned Ural-Alaskan type complex and related placer (Far East Russia)

Ural-Alaskan type complexes are the sources for unique placer platinum deposits and contain platinum-group minerals (PGM) within lodes. Such complexes occur along modern and ancient convergent tectonic margins and are comprised of dunite, wehrlite, clinopyroxenite and gabbro, which gradually interchange towards the marginal parts of the complex. In-situ PGM are largely restricted to chromitites, which are comprised of small veinlets and schlieren of chromian spinel in the dunite unit. PGM mineralization in chromitites can be extremely rich; however, their distribution is very sporadic, without any apparent regularity. Moreover, the role of accessory chromian spinel and post-magmatic overprinting of platinum group element (PGE) concentrations is poorly defined. Detailed studies of PGM assemblages, including those in chromitites, serpentine veinlets in dunites, and wehrlites, combined with PGE geochemistry could provide insights into the distribution and processes responsible for PGE accumulation.The Matysken complex in the Koryak Highlands (Far East Russia) is comprised of dunite, wehrlite, clinopyroxenite and gabbro units, and has a classic zoned structure, rendering it an excellent example of an Ural-Alaskan type complex, ideal for this case study. This locality contains mm-scale isoferroplatinum (Pt3Fe) nuggets that cement chromitites and occur as µm-scale euhedral inclusions in chromian spinel. Placer PGM assemblages in the nearby Matysken River consist of exactly the same assemblages of minerals that are found in dunites and chromitites but with larger compositional scatter, reflecting a complex history of alluvium accumulation, which was sourced from different eroded parts of the complex. Detailed textural investigations discovered a diverse array of hydrous silicate–related mineralization, including euhedral isoferroplatinum grains in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys, sulfarsenides and antimonides, which formed in serpentine veinlets together with awaruite (Ni3Fe) and base metal sulfides. This provides further evidence that isoferroplatinum, which is widely accepted to be a magmatic mineral, may form under a wide range of conditions, including during serpentinization.Dunites have fractioned primitive mantle-normalized PGE patterns, typical of Ural-Alaskan type complexes, with strong enrichments in Pt and depletions in Ir, Ru and Pd. PGE, with an exception of Pd, are quite uniformly distributed in dunite containing accessory chromian spinel, with good correlations between PGE and Cr. This correlation, together with data from previous studies, allowed us to calculate the concentration of Pt in accessory chromian spinel (probably hosted as minute PGM inclusions), which is of the same grade as for chromitites (ppm levels). This suggests that the formation of chromitites does not concentrate PGE over Cr, but only assembles them into larger aggregates, causing the observed unevenness in the PGM distribution.

Chalcophile-siderophile element systematics and regional-scale magmatic percolation in the Ronda peridotite massif (Spain)

Eighteen samples from the south-western part of the Ronda lherzolitic massif have been analysed with bulk-rock and in-situ techniques to unravel the fate of Cu, platinum-group element and chalcogens (S, Se) in a middle Proterozoic subcontinental lithospheric mantle segment variably overprinted by kilometre-scale porous flow percolation of asthenosphere-derived silicate melts during the Alpine orogeny. Chalcogen elements (S, Se Cu) and PGE systematics fit well the published data base for orogenic mantle peridotites as a whole. Positive correlations between S, Se, Cu and fertility result from progressive removal of sulfides from the mantle at increasing degrees of melting. A few harzburgites preserved the S-, Se- and Pd-depleted (Pd/Ir < 1)-chondrite normalized PGE patterns of residues after partial melting. The Oligocene thermo-mechanical erosion event triggered secondary partial melting of the sulfide phase liberating a Cu-Ni-rich sulfide melt now identified through Cu-Ni-rich sulfide inclusions (pentlandite + chalcopyrite ± bornite) and a wide range of intergranular Cu-rich sulfides and alloys. Residual monosulfide solid solution (coexisting with refractory platinum group minerals (laurite-erlichmanite) in spinel tectonites) survived throughout all of the tectonometamorphic domains, thus preserving relative and absolute abundances of compatible platinum group elements (Os, Ir, Ru, Rh). Local scale redistribution of the Cu-Ni-rich sulfide melt accounts for the enrichment/depletion trends of Te, Pd, Pt, Au, Ag, Cu identified in granular peridotites. On cooling, the sulfide melt produced pentlandite by reacting with monosulfide solid solution at T < 870 °C while transfering incompatible elements that generated micrometric Pt-Cu-Te-As-bearing platinum-group mineral inclusions (e.g. malanite CuPt2IrS4, moncheite PtTe2, Pt arsenide, secondary Pt–Cu alloys), in addition to superchondritic Pd/Ir and nearly chondritic Se/Te identified in mantle sulfides of metasomatic origin. By contrast, micrometer-sized Au particles identified in Au-enriched spinel harzburgites are clearly primary precipitates from volatile-rich small melt fractions.

Orthopyroxenite hosted chromitite veins anomalously enriched in platinum-group minerals from the Havana-Matanzas Ophiolite, Cuba

The Havana–Matanzas Ophiolite contains one of the few examples of ophiolitic platinum group minerals (PGM)-rich chromitites associated with orthopyroxenites in the mantle section of ophiolitic complexes. The chromitites occur as veins hosted by orthopyroxenite bands within mantle peridotites. The peridotites are mostly harzburgites and their accessory chromite shows high-Al compositions (Cr# [Cr/(Cr+Al), atomic ratio] = 0.39–0.50), which are typical of spinels in abyssal peridotites. Conversely, chromite from the chromitite veins and their host orthopyroxenite are high-Cr (Cr# = 0.72–0.73 and 0.62–0.69, respectively), with lower Mg# [Mg/(Mg+Fe2+), atomic ratio]. This suggests that both the chromitite and the orthopyroxenite formed from melts with boninitic affinity. The abundant PGM inclusions found in the chromitites are mainly Os-rich laurite grains, which is also characteristic of chromitites formed from magmas with boninitic affinity. Therefore, we propose that the chromitite veins and the orthopyroxenite bands probably formed contemporaneously in the fore-arc setting of an intra-oceanic arc during subduction. The chromitite-orthopyroxenite pair of the Havana-Matanzas Ophiolite could form after the reaction of a Si-rich melt with boninitic affinity and mantle harzburgite, with the orthopyroxenite bands preserving fingerprints of the infiltration of boninitic-affinity melts within the mantle. The small volume of forming chromitite could maximize the efficiency for the mechanical collection of the PGM forming in the parental melt of these rocks, resulting in the anomalous enrichment of primary PGM in the chromitites.

Desulphurisation, chromite alteration, and bulk rock PGE redistribution in massive chromitite due to hydrothermal overprint of the Panton Intrusion, east Kimberley, Western Australia

The Central and Western Zones of the Halls Creek Orogen (HCO) of the East Kimberley region of Western Australia host various Paleoproterozoic layered mafic–ultramafic intrusions. The 1856 ± 2 Ma layered mafic–ultramafic Panton Intrusion of the Central Zone contains a reef-type platinum-group element (PGE) mineralisation associated with metamorphosed chromitite horizons. Here we investigate the compositional variation of chromite within the A chromitite layer, or Main Chromitite Layer (MCL), base metal sulphides (BMS), and the associated PGE distribution. Electron-microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data reveal complex chemical zoning of the chromite reflecting post-magmatic sub-solidus re-equilibration and fluid–rock interaction during metamorphism of the host rocks. The sulphur isotopic composition of the BMS (δ34S: −1.67 to 2.89‰) suggests negligible contamination of the parental melt by crustal sulphur and thus favours BMS formation due to fractional crystallisation. Furthermore, pentlandite compositions indicate the highest ƒS2 in the centre of the MCL, which coincides with a considerably heavier sulphur isotope composition compared to pentlandite in the surrounding ultramafic host rock. Alteration of the BMS during amphibolite facies hydration of the Panton Intrusion resulted in the formation of secondary bornite and digenite from the chalcopyrite as well as desulphurisation of pyrrhotite and pentlandite to secondary magnetite. The bulk rock PGE concentrations are controlled by the distribution of platinum-group minerals (PGM) due to the low PGE concentrations in BMS. PGM are present as inclusions in chromite, suggesting that they formed under sulphur-undersaturated conditions. Additional PGM are present within the interstitial serpentine-chlorite matrix suggesting that they precipitated during fluid-rock interaction and hydration of the silicate matrix. Likely sources for the remobilised PGE include PGM inclusions in altered rim zones of magmatic chromite and PGE released due to desulphurisation of BMS despite their low PGE concentrations.

Origin of Platinum Group Minerals (PGM) Inclusions in Chromite Deposits of the Urals

This paper reviews a database of about 1500 published and 1000 unpublished microprobe analyses of platinum-group minerals (PGM) from chromite deposits associated with ophiolites and Alaskan-type complexes of the Urals. Composition, texture, and paragenesis of unaltered PGM enclosed in fresh chromitite of the ophiolites indicate that the PGM formed by a sequence of crystallization events before, during, and probably after primary chromite precipitation. The most important controlling factors are sulfur fugacity and temperature. Laurite and Os–Ir–Ru alloys are pristine liquidus phases crystallized at high temperature and low sulfur fugacity: they were trapped in the chromite as solid particles. Oxygen thermobarometry supports that several chromitites underwent compositional equilibration down to 700 °C involving increase of the Fe3/Fe2 ratio. These chromitites contain a great number of PGM including—besides laurite and alloys—erlichmanite, Ir–Ni–sulfides, and Ir–Ru sulfarsenides formed by increasing sulfur fugacity. Correlation with chromite composition suggests that the latest stage of PGM crystallization might have occurred in the subsolidus. If platinum-group elements (PGE) were still present in solid chromite as dispersed atomic clusters, they could easily convert into discrete PGM inclusions splitting off the chromite during its re-crystallization under slow cooling-rate. The presence of primary PGM inclusions in fresh chromitite of the Alaskan-type complexes is restricted to ore bodies crystallized in equilibrium with the host dunite. The predominance of Pt–Fe alloys over sulfides is a strong indication for low sulfur fugacity, thereby early crystallization of laurite is observed only in one deposit. In most cases, Pt–Fe alloys crystallized and were trapped in chromite between 1300 and 1050 °C. On-cooling equilibration to ~900 °C may produce lamellar unmixing of different Pt–Fe phases and osmium. Precipitation of the Pt–Fe alloys locally is followed by an increase of sulfur fugacity leading to crystallize erlichmanite and Ir–Rh–Ni–Cu sulfides, occurring as epitaxic overgrowth on the alloy. There is evidence that the system moved quickly into the stabilization field of Pt–Fe alloys by an increase of the oxygen fugacity marked by an increase of the magnetite component in the chromite. In summary, the data support that most of the primary PGM inclusions in the chromitites of the Urals formed in situ, as part of the chromite precipitation event. However, in certain ophiolitic chromitites undergoing annealing conditions, there is evidence for subsolidus crystallization of discrete PGM from PGE atomic-clusters occurring in the chromite. This mechanism of formation does not require a true solid solution of PGE in the chromite structure.

A secondary precious and base metal mineralization in chromitites linked to the development of a Paleozoic accretionary complex in Central Chile

Platinum-group element (PGE) and gold inclusions are usually present in peridotites and chromitite deposits associated with ophiolites. Here, we present the first detailed study of the mineralogy of precious metals in ultramafic rocks hosted in the Paleozoic Coastal Accretionary Complex of Central Chile. In these ultramafic rocks the mineralization of precious metals is associated with small meter-size pods and veins of massive chromitite hosted in serpentinite-filled shear zones. Crystallographic orientation maps of single chromite grains, obtained using the Electron-Backscattered Secondary Diffraction technique, allow us to identify two types of chromite in the precious-metal bearing chromitites: (1) Type A chromite, characterized by an average misorientation per grain of ≤2° and chemically homogeneous cores surrounded by a porous rim with abundant inclusions of chlorite, and (2) Type B chromite, which exhibits higher degrees of misorientation (2–8°) and porosity, and abundant silicate inclusions, but a relatively homogeneous chemical composition. In situ analyses using EMPA and LA-ICP-MS for major, minor and trace elements indicate that composition of the magmatic chromite is only preserved in the cores of Type A chromite grains. Core to rim chemical trends in these Type A chromites are characterized by a progressive increase of the Cr# with a decrease of the Mg#, loss of Al and addition of Fe2+ in the porous rim. The observed changes in the microstructure and chemistry of chromite are associated with the infiltration of external fluids through shear zones filled with antigorite (±talc) developed in partly serpentinized peridotites (i.e., olivine–lizardite dunites). Thermodynamic calculations using the phase equilibria relations in the system Cr2O3–MgO–FeO–Al2O3–SiO2–H2O (CrMFASH) indicate that Fe2+-rich porous chromite+chlorite replaced the original assemblage chromite+olivine in the chromitite while prograde antigorite was formed. According to our results this transformation occurred at ~510–560°C when external fluids penetrated the ultramafic/chromitite bodies through shear zones. These temperatures are slightly higher than estimated for the metamorphic peak in the host metapelitic rocks (i.e., ~420°C at 9.3kbar), suggesting that a hotter ultramafic body was captured by the metasediments of the accretionary prism during their exhumation through subduction channel. Chlorite geothermometry yielded a wide range of lower temperature from 430 to 188°C, for chlorite present in the porous chromite rims. These results are in agreement with the retrograde overprint under greenchist-facies metamorphism conditions recorded by metapelitic host rocks and minor volcanogenic massive sulphide deposits in the area (300–400°C, ~3–4kbar). We suggest that although initially decoupled, the chromitite-bearing ultramafic rocks and their metasedimentary host undergone a common metamorphic PT pathway of exhumation during the formation and evolution of the subduction-related accretionary complex.The chromitites contain appreciable amounts of the platinum-group elements (up to 347ppb total) and gold (up to 24ppb), present as inclusions of platinum-group minerals (PGM) and alloys as well as native gold. The PGM identified include native osmium, laurite (RuS2), irarsite (IrAsS), osarsite (OsAsS), omeiite (OsAs2), Pt–Fe alloy (possibly isoferroplatinum) and a suite of inadequately identified phases such as PtSb (possibly stumpflite), PdHg (possibly potarite), RhS, Ir–Ni and Ir–Ni–Ru compounds. Only a few grains of osmium and laurite were identified in unaltered cores of chromite and therefore considered as magmatic in origin formed during the high-T event of chomite crystallisation in the upper mantle. The other PGM were located in the porous chromite associated with chlorite or base-metal minerals (BMM) that often fill the pores of this altered chromite or are intergrowth with antigorite in the host serpentinized ultramafic rock. The assemblage of BMM identified in the studied rocks include sulphides [millerite (NiS), polydymite (Ni3S4), violarite (FeNi2S4), galena (PbS), sphalerite (ZnS), chalcocite (CuS)], arsenides [(orcelite (Ni5−xAs2) and maucherite (Ni11As8)], the sulpharsenide gersdorfitte (NiAsS), and native bismuth. The irregular shape of several PGM grains observed in porous chromite suggest disequilibrium, whereas others exhibit perfectly developed crystal faces with the associated secondary silicate or base-metal mineral suggesting neoformation of PGMs in situ from metamorphic fluids. We suggest that the origin of these PGM inclusions is magmatic, but some grains were reworked in situ when metalloid (i.e., As, Sb, Pb, Zn and Hg)-rich fluids released from metasediments penetrated the ultramafic rocks through active shear zones, once the ultramafic bodies became tectonically mixed with the host metasedimentary host rocks. During this event, gold sourced from the (meta)sediments was also precipitated within chromitites and serpentinites.

Cryptic mineral variation and PGE distribution in the Merensky Reef and its discontinuous mela-gabbronoritic hanging wall (“Brown Sugar Norite” - BSN) at Two Rivers Platinum Mine, Eastern Bushveld Complex, South Africa

Two Rivers Platinum Mine, situated on the southern sector of the eastern limb of the Bushveld Complex, extracts Platinum Group Elements (PGE) from the UG-2 chromitite layer. A trial operation of the Merensky Reef (MR) exposed unusual occurrences of mela-gabbronoritic lenses, ‘brown sugar norite’ (BSN in mine terminology), within the pyroxenite of the hanging wall. The BSN is a fine-grained mela-gabbronorite, which appears to only occur where the upper chromitite stringer of the Merensky Reef unit is present, and forms laterally inconsistent, flat lenses. Textural features, mineral chemistry of the various rock types, Sr analyses of plagioclase separates and whole rock geochemistry indicate multiple magma injections, fractionation and disequilibrium crystallization of the magma within the Merensky interval. Whole rock MgO and enstatite content of orthopyroxene are higher in BSN compared with the surrounding Merensky Reef pyroxenite, thus supporting the notion of the BSN having formed from a more primitive magma. Both the pyroxenite and BSN plagioclase have 87Sr/86Sr ratios representative of Critical Zone magma; that of the BSN has a lower initial 87Sr/86Sr isotope ratio relative to that of the pyroxenite, suggesting a more primitive magma source. It is suggested that the unusual, discontinuous BSN lenses within pyroxenite may have formed by magmatic erosion of a new magma injection, which affected the crystal mush of the earlier deposited BSN layer. This resulted in isolated lenses of relict, but more primitive orthopyroxene of the BSN in a residual plagioclase-rich melt from an earlier pyroxenitic MR magma. In both rock types (MR pyroxenite and BSN), the PGE occur as discrete platinum-group minerals (PGM), and in solid solution in sulphides. At Two Rivers Platinum Mine, the presence of PGE in sulphides has been identified by EMPA and LA-ICP-MS. The PGE occur mineralogically either in solid solution or as PGM inclusions of different size. Pentlandite, chalcopyrite, pyrrhotite and rare pyrite occur disseminated as individual grains with associated PGM throughout the Merensky pyroxenite interval, but more enriched near the bottom and top chromitite stringers. PGM encountered are moncheite, platarsite and irarsite, relatively enriched in Pt, but poor in Pd and Rh. Our results show that most of the Pd and Rh are incorporated in pentlandite, whereas pyrrhotite, chalcopyrite, and pyrite are almost devoid of PGE. Textural features of the encountered PGM grains suggest their redistribution through late stage magmatic (probably hydrothermal) processes. The BSN contains relatively little if any base metal sulphides and PGM, indicating its formation from magma depleted in PGE.

Crystallization of platinum-group minerals from silicate melts: Evidence from Cr-spinel–hosted inclusions in volcanic rocks

The formation of platinum-group minerals (PGM) during magma differentiation has been suggested to be an important process in primitive magma evolution, but decisive textural evidence is difficult to obtain because PGM tend to be very small and very rare. We have investigated Cr-spinel phenocrysts from two oxidized magmas (Siberian meimechite and Vanuatu [Ambae Island] arc picrite) and one reduced magma (Uralian [Russia] ankaramite) for PGM inclusions and their platinum-group element (PGE) contents. We observed Os-Ir and Pt-Fe alloys entrapped as inclusions in Cr-spinel in all three suites of lava. The alloys may occur in association with PGE-bearing sulfides and co-trapped silicate melt. Cr-spinel crystals also contain measurable amounts of Os, Ir, Ru, and Rh, which are at concentrations 2×–100× higher than mantle values. Thermodynamic models indicate that the arc picrite and ankaramite melts were probably both saturated with the observed PGM phases, whereas the Os-Ir alloy grain observed in the meimechite is not in equilibrium with the bulk melt. Our results demonstrate that PGM (alloys and sulfides) occur as liquidus phases in primitive (unfractionated) melts at high temperature and at a variety of redox conditions, and that Cr-spinel is a significant host of PGE, either in the crystal structure or as PGM inclusions.

Mineralogical control of selenium, tellurium and highly siderophile elements in the Earth’s mantle: Evidence from mineral separates of ultra-depleted mantle residues

The budget of Se–Te and other highly siderophile elements (HSEs) in major silicate and oxide mineral separates (olivine, orthopyroxene, clinopyroxene, spinel), and intergranular fine components of an ultra-refractory harzburgite have been determined via isotope dilution hydride generation ICPMS after mineral separation, high-pressure acid digestion and chemical purification. In addition, x-ray computed micro-tomography (micro-CT) has been performed in order to constrain and illustrate the original location, distribution and size of sulfides and/or alloys inside their intact, bulk harzburgite matrix. The results show that the whole-rock Se–Te budget of mantle residues, that are devoid of base metal sulfides (BMS) after high degrees of partial melting (F≈23%), is almost completely controlled by olivine-hosted micro-inclusions and intergranular fractions of platinum group minerals (PGMs). The scarcity and heterogeneous distribution of intergranular PGMs generally sized 3–5μm in the harzburgite are revealed by 3D micro-CT, while PGM inclusions in olivine are smaller than 2.5μm. The heterogeneous distribution of these microphases as well as the range of Se–Te contents in the bulk harzburgite complicate a quantification of individual contributions of separates to the whole-rock Se–Te budget. However, PGM micro-inclusions in olivine with residual-type CI chondrite-normalized HSE patterns and suprachondritic Se/Te ratios between 30 and 218 (Se/TeCI chondrite=9) host close to 100% of the bulk rock Se. Interstitial fine components up to 125μm grain size may contribute only minor amounts of Se, but up to 50% of Te to the bulk harzburgite and show subchondritic Se/Te ratios of ca. 4, resembling metasomatic PGM signatures. In dependence on increasing proportions of interstitial and metasomatic PGMs vs. olivine-hosted residual PGMs, the bulk rock Te contents may increase and the Se/Te ratios decrease. The systematics seen in mineral separates of one harzburgitic sample resembles the entire spectrum of Se–Te signatures of all suites of peridotites and host phases so far published. The results are consistent with a higher compatibility of Se in covalent monosulfides compared to Te that prefers sulfide melts. It is further consistent with the stabilization of residual PGMs after Mss exhaustion as well as with formation of metasomatic PGMs in a S-Cu-volatile-rich sulfide melt. Altogether this reconciles a higher incompatibility of Te over Se during partial mantle melting, subsequent removal of Te over Se with Cu-Ni-rich sulfide fractionation from high-degree partial mantle melts, and explains the similar Te but higher Se abundances in MORBs compared to peridotites that is globally observed.

Platinum-group minerals (PGM) nuggets from alluvial-eluvial placer deposits in the concentrically zoned mafic-ultramafic Uktus complex (Central Urals, Russia)

We report the results of a detailed mineralogical investigation of platinum-group minerals (PGM) and copper-gold nuggets from the Uktus Ural-Alaskan type complex in the Central Urals (Russia). The studied nuggets were sampled in alluvial-eluvial deposits from three small valleys, with temporary water flows, cutting across the Uktus massif. The volume of the washed samples varies from 0.03 to 0.08 m 3 and a few tens of PGM nuggets, ranging in size from about 100 μm to about 2 mm, were collected. According to their chemical composition, the most abundant PGM are native Ir-Os and alloys in the Pt-Fe-Cu-Ni system. The following less abundant PGM were also recognised: sulfarsenides of the irarsite-hollingworthite-platariste series, sulfides such as laurite, cuproiridsite, kashinite and the sulfantimonide tolovkite. One alloy corresponding to the formula Cu 3 Au 2 was found, and proved to be Cu-rich tetraauricupride. The nuggets of Uktus have, in some cases, a polygonal shape. However, most of them have an irregular morphology and are characterised by a porous rim and zoning. The investigated nuggets occur as single-phase crystals or as polyphase grains, composed of different PGM. One nugget displays a very complex texture, being composed of a Pt-Fe alloy associated with osmium and Cu-rich tetraauricupride. These minerals are in contact with quartz that contains minute inclusions of hollingworthite and platarsite. The mineralogical similarity with the PGM inclusions in the Uktus chromitites indicates these rocks as a possible source for the PGM nuggets. The presence of faceted morphology in some nuggets suggests that they were mechanically liberated and transported for a relatively short distance from their lode deposits. The nuggets characterized by a rounded shape and occurring in association with quartz and Cu-rich tetraauricupride have probably been reworked in the placer environment. Therefore, in the Uktus placers deposits, two types of PGM nuggets can coexist: (i) primary with a magmatic origin, i.e ., only mechanically liberated from their source rock, and (ii) secondary, i.e ., reworked and grown in the placers. The mineralogical assemblage of the Uktus PGM nuggets, the fact that the Uktus PGM placers have never been mined and the recent exponential increase in demand for noble metals make the placer deposits associated with the Uktus complex potentially important for the economic recovery of these rare metals, at least on a small scale.

Compositional variations of chromite and solid inclusions in ophiolitic chromitites from the southeastern Turkey: Implications for chromitite genesis

Chromitites, associated with upper mantle peridotites of ophiolites from southeastern Turkey, are lenticular bodies or veinlets made up of massive, banded and disseminated chromite. Chromite shows a wide range of Cr# [Cr/(Cr+Al)] varying between 0.42 and 0.81, and Mg# [Mg/(Mg+Fe2+)] of 0.45 to 0.75. Geochemically, they are divided into two compositional groups, such as high-Cr (Cr#≥0.7) and high-Al (Cr#<0.6). The contents of the platinum-group elements (PGE) in high-Cr chromitites range between 42 and 348ppb (average 158ppb), whereas high-Al ones are represented by a lower total PGE, ranging between 70 and 94ppb (average 84ppb). However, two high-Cr chromitite samples showed significant enrichments especially of Ir-subgroup of PGE (IPGE: Os, Ir, Ru) with total PGE contents reaching up to 2.7ppm. The chromitites mostly contain tiny inclusions (<5μm) of platinum-group minerals (PGM). The most abundant PGM is laurite which is rarely associated with irarsite±Os–Ir alloys; these minerals in particular are more abundant in high-Cr chromitites, especially in PGE enriched chromitites. The primary base-metal sulfide (BMS) inclusions consist of millerite and to a lesser extent pentlandite, and very rarely pyrite. Violarite, polydymite and heazlewoodite form the secondary BMS phases found mostly in altered silicate matrix. Both PGM and BMS are frequently associated with the silicates: amphibole, olivine and clinopyroxene. Textural relationships and the composition of PGM and BMS inclusions suggest that they crystallized at varying temperature (1300°C–1000°C) and sulfur fugacity conditions, before or during the event of crystallization of chromite. The high-Cr chromitites crystallized in equilibrium with boninite melt, probably originated in a supra-subduction zone (SSZ), while the high-Al chromitites crystallized in equilibrium with middle oceanic ridge basalt (MORB) type melts. The presence of amphiboles in high-Al chromitites suggests hydrous parental melts that could be produced, most probably, in back-arc tectonic setting.

Nanometer-sized platinum-group minerals (PGM) in base metal sulfides: New evidence for an orthomagmatic origin of the Merensky Reef PGE ore deposit, Bushveld Complex, South Africa

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum group element-rich (PGE) samples from the Merensky Reef of the Bushveld Complex in South Africa. Chromitites and pegmatitic melanorite from the Northam and Rustenburg platinum mines contain Ru-Rh-Pt arsenide nanocrystals ( Occasionally the PGM inclusions form clusters, develop a flow texture, or are aligned like pearls on a string. These observations are inconsistent with sub-solidus exsolution and suggest that the nano-scale PGM crystallized before solidification of the sulfide melt and the formation of their BMS hosts. There is evidence from recently performed laboratory experiments on silicate melts with additives of Ru and Pd that discrete Fe-rich Fe-Ru-Pd alloys (nanometer-sized) can indeed crystallize early. These experiments raise the possibility that the PGE-bearing nanocrystal inclusions found in the BMS of the Merensky Reef might represent preserved relicts of an early phase of magmatic PGM that precipitated from the silicate melt and afterwards were collected by the sulfide melt. Combined FIB/HRTEM techniques open up a window through which the nano-scale mineral population can be viewed. PGE previously thought to be present in BMS solid solution might actually be an artefact of inadequate spatial resolution.

Mineralogy and distribution of Platinum-Group Minerals (PGM) and other solid inclusions in the Faryab ophiolitic chromitites, Southern Iran

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite were investigated for PGM and other solid inclusions from Faryab ophiolitic complex, southern Iran. Chemical composition of the chromian spinels, Cr#[100*Cr/(Cr+Al) = 77–85], Mg# [100*Mg/(Mg+Fe2+) = 56–73], TiO2≤0.25wt%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Faryab ophiolitic chromitites can be divided into three categories: PGM, base-metal minerals and silicates. Most of the studied PGM occurred as very small (generally less than 20 μm in size) primary single or composite inclusions of IPGE-bearing phases with or without silicates and base metal minerals. The PGM were divided into the three subgroups: sulfides, alloys and sulfarsenides. Spinel-olivine geothermometry gives the temperatures 1,131–1,177 °C for the formation of the studied chromitites. At those temperatures, fS2 values ranged from 10−3 to 10−1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloys. Progressive crystallization of chromian spinel was accompanied by increase of fS2 in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of fS2 and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions respect to their spatial distribution in chromian spinels show that the minerals regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.

Alteration of Platinum-Group and Base-Metal Mineral Assemblages in Ophiolite Chromitites from the Dobromirtsi Massif, Rhodope Mountains (Bulgaria)

The Dobromirtsi Ultramafic Massif, located in the Rhodope Mountains (SE Bulgaria), is a portion of a Paleozoic sub-oceanic mantle affected by polyphase regional metamorphism. This massif contains several small, podiform chromitite bodies which underwent the same metamorphic evolution as their host peridotites. Like other ophiolite chromitites, those found in Dobromirtsi carry abundant platinum-group minerals (PGM) and base-metal minerals (BMM). The PGM consist mainly of Ru-, Os-, and Ir-based PGM (laurite RuS2-erlichmanite OsS2, Os-Ru-Ir alloys, irarsite [IrAsS], Ru-rich pentlandite, and an unknown Ir-sulfide) but minor Rh- and Pd-based PGM (hollingworthite [RhAsS] and a series of unidentified stannides and sulfantimonides) are also present. In contrast, the BMM are dominated by pentlandite (Ni,Fe)9S8, followed by heazlewoodite (Ni3S2), breithauptite (NiSb), maucherite (Ni11As8), godlevskite (Ni7S6), gersdorffite (NiAsS), millerite (NiS), undetermined minerals containing Ni, As and Sb, orcelite (Ni5-XAs2), awaruite (Ni3Fe) and chalcopyrite (CuFeS2). The detailed study of the textural relationships, morphology and composition of the PGM and BMM inclusions indicate the existence of two different PGM-BMM assemblages: (i) a primary or magmatic; and (ii) a secondary related with postmagmatic alteration. The PGM and BMM inclusions in unaltered zones of chromite crystals (mainly laurite-erlichmanite and pentlandite) are considered to be primary magmatic minerals formed under variable temperature (1200–1100°C) and sulfur fugacity (between −2 and −0.5 log fS2). In contrast, PGM and BMM located along altered edges of chromite and serpentinised silicate matrix are considered to be secondary, formed from or re-equilibrated with altering fluids. Secondary PGM and BMM assemblages are considered as result of the combination of reducing and oxidising events related with regional metamorphism. Under low fO2 states, fS2 also drops giving place to the formation of S-poor Ni-rich sulfides and secondary Ru-alloys by desulfurisation of primary S-containing minerals. In contrast, predominance of platinum-group elements and/or base-metal arsenides and sulfarsenides associated with the altered edges of chromite (chromite strongly enriched in Fe2O3) is related with the fixing of remobilised PGE (mainly Ir, Rh and Pd) and base-metals (mainly Ni and Fe) when late oxidising fluids supplied As as well as Sb and Sn.

New data on base metal mineralogy and platinum-group minerals in the Ojen chromitites (Serrania de Ronda, Betic Cordillera, southern Spain)

Three small Cr-rich chromitites occurrences (62 < Cr# < 84) from the Ojen lherzolitic massif (south Spain) were studied for base metal mineralogy, bulk rock PGE (platinum group elements) abundances and platinum-group minerals (PGM). A great variety of base metal sulfides (BMS), Ni arsenides, Ni sulfarsenides and Ni-sulfobismuthides are reported. Bulk-rock PGE contents span more than two orders of magnitude. Two occurrences display the typical PGE pattern of ophiolitic chromitites (high contents of Os, Ir, Ru and Rh and depletion in Pt and Pd; ΣIPGE = 114.1-628.3 ppb: IPGE = Os, Ir, Ru). Another chromitite, the most chromiferous, is very poor in IPGE and Rh, while displaying a positively-slopping Primitive Mantle-normalized pattern from Pt to Pd and Au. PGM inclusions are abundant in the PGE-rich chromitites, consisting in laurite-erlichmanite, Ir-Pt-Rh sulfarsenides, Ru-Os sulfarsenides, Pt arsenides (sperrylite), Pd antimonide (stibiopalladinite) and Pt-Fe and Os-Pt alloys. The igneous crystallization sequence starts with laurite (containing appreciable amount of sulfarsenide in solid solution), erlichmanite, Ru-Os sulfarsenides and Ir-Rh sulfarsenides, followed by Ir-Pt-Rh sulfarsenides, Pt-sulfarsenide and the other Pt-minerals (sperrylite, Pt 3 Fe, Pt-Os alloys). The high As activity during the crystallization of chromite prevented the formation of Os-Ir-Ru alloys. Stibiopalladinite is a late magmatic phase. Pd and Au in the PGE-poor chromitite are accounted for the high modal abundance of intergranular Ni arsenides. PGM-BMS textures indicate late-segregation for the BMS, probably from late-magmatic fluids. The abundance of As minerals and the presence of Bi- and Sb-bearing phases in the Ojen chromitites confirm that volatile-rich, small-melt fractions have migrated through the Ojen Iherzolitic massif just before its tectonic emplacement into the continental crust.