Platinum-group Elements Budget Research Articles

Base metal sulphide geochemistry of southern African mantle eclogites (Roberts Victor): Implications for cratonic mafic magmatism and metallogenesis

Platinum-group elements (PGE) display a chalcophile behaviour and are largely hosted by base metal sulphide (BMS) minerals in the mantle. During partial melting of the mantle, BMS release their metal budget into the magma generated. The fertility of magma sources is a key component of the mineralisation potential of large igneous provinces (LIP) and the origin of orthomagmatic sulphide deposits hosted in cratonic mafic magmatic systems. Fertility of mantle-derived magma is therefore predicated on our understanding of the abundance of metals, such as the PGE, in the asthenospheric and lithospheric mantle. Estimations of the abundance of chalcophile elements in the upper mantle are based on observations from mantle xenoliths and BMS inclusions in diamonds. Whilst previous assessments exist for the BMS composition and chalcophile element budget of peridotitic mantle, relatively few analyses have been published for eclogitic mantle. Here, we present sulphide petrography and an extensive in situ dataset of BMS trace element compositions from Roberts Victor eclogite xenoliths (Kaapvaal Craton, South Africa). The BMS are dominated by pyrite-chalcopyrite-pentlandite (± pyrrhotite) assemblages with S/Se ratios ranging 1200 to 36,840 (with 87% of analyses having S/Se <10,000. Total PGE abundance in BMS range from 0.17 to 223 ppm. We recognise four end-member compositions (types i to iv), distinguished by total PGE abundance and Pt/Pd and Au/Pd ratios. The majority of BMS have low PGE abundances (< 10 ppm) but Type iv BMS have the highest concentration of PGE recorded in eclogites so far (> 100 ppm) and are characteristically enriched in Os, Ir, Ru and Rh. Nano- and micron-scale Pd-Pt antimonide, telluride and arsenide platinum-group minerals (PGM) are observed spatially associated with BMS. We suggest that the predominance of pyrite in the xenoliths reflects the process of eclogitisation and that the trace element composition of the eclogite BMS was inherited from oceanic crustal protoliths of the eclogites, introduced into the SCLM via ancient subduction during formation of the Colesberg Magnetic Lineament c. 2.9 Ga and the cratonisation of the Kaapvaal Craton. Crucially, we demonstrate that the PGE budget of eclogitic SCLM may be substantially higher than previously reported, akin to peridotitic compositions, with significant implications for the PGE fertility of cratonic mafic magmatism and metallogenesis. We quantitatively assess these implications by modelling the chalcophile geochemistry of an eclogitic melt component in parental magmas of the mafic Rustenburg Layered Suite of the Bushveld Complex.

Metallogeny of the Chrome Ores of the Xerolivado-Skoumtsa Mine, Vourinos Ophiolite, Greece: Implications on the genesis of IPGE-bearing high-Cr chromitites within a heterogeneously depleted mantle section

Chrome ore deposits comprise less than 1% of the volume of a pervasively serpentinized dunite body (∼3.5km3) that constitutes the Xerolivado-Skoumtsa Mine of the Vourinos Ophiolite in northwestern Greece. Ores have been examined to aid determination of their geological mode of occurrence, their mineralogy and mineral chemistry (Cr-spinel and olivine), and platinum-group element (PGE) geochemistry. The ore bodies are highly deformed and of the schlieren type, where Cr-spinel-rich and serpentine (after olivine)-rich layers (1–20cm thick) alternate. The Cr-spinel displays a limited range in Cr# [100×Cr/(Cr+Al)] atomic ratio (77–85) and TiO2 content (0.05–0.22wt%). The Mg# [100×Mg/(Mg+Fe2+)] of Cr-spinel shows a wider variation (52–74) indicating that subsolidus (MgSp-Fe2+Ol) re-equilibration occurred. Remnant fresh olivine in the serpentine-rich layers is typical forsterite [Fo#: 100×Mg/(Mg+Fe2+)=93–95]. The Cr-rich chromitite ores have total PGE and Au abundances that range between 74.3 and 205ppb: they show a general enrichment in Ir and Ru over PPGE (Rh, Pt and Pd) resulting mostly in a negatively-sloping C1 chondrite-normalized PGE profile with a quite strong Ru peak. Field observations indicate that the schlieren banding of the deformed ore bodies is a tectonometamorphic feature acquired within ductile deformation of the ophiolitic slab. Geochemical calculations demonstrate that the parental melts of the Xerolivado-Skoumtsa high-Cr chromitites were boninitic. Their high Ru content and variation in Cr#Sp values indicate a polygenetic origin from geochemically similar, but spatially distinct melt inputs. These melts originated within a hydrated mantle wedge beneath a forearc basin within an evolving lithospheric slab. We emphasize that the PGE budget of the Xerolivado-Skoumtsa chromitites was not inherited from interactions of the migrating melts with their parental harzburgites, but it is an intrinsic feature of the deep-seated and heterogeneously depleted mantle source of these magmas.

Platinum-group minerals in the LG and MG chromitites of the eastern Bushveld Complex, South Africa

The chromitites of the Bushveld Complex in South Africa contain vast resources of platinum-group elements (PGE); however, except for the economic upper group (UG)-2 chromitite seam, information on the distribution of the PGE in the ores and on the mineralogical nature, assemblages, and proportions of platinum-group minerals (PGM) is essentially missing. In the present geochemical and mineralogical study, PGE concentrates originating from the lower group (LG)-6 and middle group (MG)-1/2 chromitites were investigated with the intention to fill this gap of knowledge. Chondrite-normalized PGE patterns of bulk rock and concentrates are characterized by a positive slope from Os to Rh, a slight drop to Pt, and an increase to Pd again. The pronounced similarities of the PGE patterns indicate similar primary processes of PGE concentration in the chromitites, namely “sulfide control” of the PGE mineralization, i.e., co-precipitation of chromite and sulfide. Further, the primary control of PGE concentration in chromitites appears to be dual in character: (i) base-level concentrations of IPGE (up to ∼500 ppb) hosted within chromite and (ii) co-precipitation of chromite and sulfide, the latter containing virtually the entire remaining PGE budget. Sulfides (chalcopyrite, pentlandite, and pyrite; pyrrhotite is largely missing) are scarce within the chromitites and occur mainly interstitial to chromite grains. Pd and Rh contents in pentlandite are low and erratic. Essentially, the whole PGE inventory of the ores occurs in the form of discrete PGM. The PGM are almost always associated with sulfides. The dominant PGM are various Pt-Pd-Rh sulfides (cooperite/braggite [(Pt,Pd)S] and malanite/cuprorhodsite [CuPt2S4]/[CuRh2S4]), laurite [RuS2], the main carrier of the IPGE (Os, Ir, Ru), sulfarsenides [(Rh,Pt,Ir)AsS], sperrylite [PtAs2], Pt-Fe alloys, and a large variety of mainly Pd-rich PGM. The LG and MG chromitites have many characteristics in common and define a general, “typical” PGM spectrum of Bushveld chromitites. This PGM assemblage is characterized by the predominance of PGE-sulfides including elevated proportions of malanite, variable proportions of (sulf) arsenides, and Pt-Fe alloys in conjunction with a paucity of (bismutho)tellurides. The formation of this specific PGM spectrum is related to the distinct chromitite environment and its depositional and post-depositional history, whereby desulfurization reactions have probably played an important role. The LG-6 samples have higher contents of PGE-sulfides, including extraordinary high proportions of malanite but low PGE-arsenide and PGE-sulfarsenide contents compared to the MG-1/2 samples. This indicates a higher availability of arsenic either in the stratigraphically higher MG-1/2 samples (compared to the LG-6) or regionally in the chromitites south of the Steelpoort lineament.

The Interplay between Melting, Refertilization and Carbonatite Metasomatism in Off-Cratonic Lithospheric Mantle under Zealandia: an Integrated Major, Trace and Platinum Group Element Study

It is widely accepted that stabilization of the continental crust requires the presence of sub-continental lithospheric mantle. However, the degree of melt depletion required to stabilize the lithosphere and whether widespread refertilization is a significant process remain unresolved. Here, major and trace element, including platinum group elements (PGE), characterization of 40 mantle xenoliths from 13 localities is used to constrain the melt depletion, refertilization and metasomatic history of lithospheric mantle underneath the micro-continent Zealandia. Our previously published Re–Os isotopic data for a subset of these xenoliths indicate Phanerozoic to Paleoproterozoic ages and, reinterpreted with the new major and trace element data presented here, demonstrate that a large volume (>2 million km3) of lithospheric mantle with an age of 1·99 ± 0·21 Ga is present below the much younger crust of Zealandia. A peritectic melting model using moderately incompatible trace elements (e.g. Yb) in bulk-rocks demonstrates that these peridotites experienced a significant range of degrees of partial melting, between 3 and 28%. During subsolidus equilibration clinopyroxene gains significant rare earth elements (REE), which then leads to the underestimation of the degree of partial melting by ≤12% in fertile xenoliths. A new approach taking into account the effects of subsolidus re-equilibration on clinopyroxene composition effectively removes discrepancies in the calculated degree of melting and provides consistent estimates of between 4 and 29%. The estimated amount of melting is independent of the Re–Os model ages of the samples. The PGE patterns record simple melt depletion histories and the retention of primary base metal sulfides in the majority of the xenoliths. A rapid decrease in Pt/IrN observed at c. 1·0 wt % Al2O3 is a direct result of the exhaustion of sulfide in the mantle residue at c. 20–25% partial melting and the inability of Pt to form a stable alloy phase. Major elements preserve evidence for refertilization by a basaltic component that resulted in the formation of secondary clinopyroxene and low-forsterite olivine. The majority of xenoliths show the effects of cryptic metasomatic overprinting, ranging from minor to strong light REE enrichments in bulk-rocks (La/YbN = 0·16–15·9). Metasomatism is heterogeneous, with samples varying from those with weak REE enrichment and notable positive Sr and U–Th anomalies and negative Nb–Ta anomalies in clinopyroxene to those that have extremely high concentrations of REE, Th–U and Nb. Chemical compositions are consistent with a carbonatitic component contributing to the metasomatism of the lithosphere under Zealandia. Notably, the intense metasomatism of the samples did not affect the PGE budget of the peridotites as this was controlled by residual sulfides.

Platinum-group elemental chemistry of the Baima and Taihe Fe–Ti oxide bearing gabbroic intrusions of the Emeishan large igneous province, SW China

Nickel-, copper-, and platinum group element (PGE)-enriched sulphide mineralization in large igneous provinces has attracted numerous PGE studies. However, the distribution and behavior of PGEs as well as the history of sulphide saturation are less clear in oxide-dominated mineralization. Platinum group elements of oxide-bearing layered mafic intrusions from the Emeishan large igneous province are examined in this study. Samples collected from the Baima and Taihe oxide-bearing layered gabbroic intrusions reveal contrasting results. The samples from Baima gabbroic rocks have low total PGE abundances (ΣPGE<4ppb) whereas the Taihe gabbroic rocks, on average, have more than double the concentration but are variable ranging from ΣPGE<2ppb to ΣPGE∼300ppb. The Baima gabbro is platinum-subgroup PGE (PPGE=Rh, Pt and Pd) enriched and iridium-subgroup PGE (IPGE=Os, Ir and Ru) depleted, with a distinct positive Ru anomaly on a primitive mantle normalized multi-element plot. The Taihe gabbros are also PPGE enriched but with negative Ru and Pd anomalies on a primitive mantle normalized multi-element plot. The PGE concentrations of Baima rocks are indicative of fractionation of a relatively evolved, mafic, S-undersaturated parental magma that was affected by earlier sulphide segregation. In contrast, the Taihe rocks record evidence of both S-saturated and S-undersaturated conditions and that the parental magma was likely emplaced very close to S-saturation. Comparisons of the platinum group element contents in the Emeishan flood basalts and the Emeishan oxide-bearing intrusions suggest that the PGE budget in a magma is not controlled by magma series (high-Ti vs. low-Ti), but very much by crustal contamination. The unlikelihood of substantial crustal contamination in the Taihe magma allowed the magma to remain S-undersaturated for a longer duration. PGE and sulphide mineralization was not identified in the Taihe intrusion but the presence of one PGE-enriched sample (Pt+Pd=∼300ppb) suggests that the parental magma likely did not experience sulphide segregation and is a potential target for further prospecting.

Effects of melt percolation on platinum group elements and Re–Os systematics of peridotites from the Tan-Lu fault zone, eastern North China Craton

Abstract: Concentrations of platinum group elements (PGE) and Re–Os isotopic compositions were determined for a suite of peridotite xenoliths from Cenozoic Beiyan volcanoes within the Tan-Lu fault zone, eastern North China Craton (NCC). Previous petrographic and geochemical data indicate the existence of considerable modification by melt–peridotite interaction for these xenoliths. Thus, this suite of xenoliths provides an unusual opportunity to examine the effects of melt percolation on PGE abundances and Re–Os isotopes. Our data show that most of the Beiyan peridotites have flat chondrite-normalized PGE patterns whereas a considerable variation in total PGE abundances has been observed in lherzolites to highly fertile cpx-rich lherzolites and wehrlites. Their total PGE contents range between 0.97 and 61.4 ppb and their PGE relative abundances from 0.1 to 0.0001 × CI-chondrites, respectively. In these rocks, the total PGE budget was reduced by more than 90% from lherzolite to cpx-rich lherzolite and to wehrlite, probably because intergranular sulphides were completely removed by silicate melt. However, Beiyan peridotites have low and variable Re (0.0002–0.5118 ppb) and Os (0.194–10.4 ppb) abundances and high 187 Os/ 188 Os (0.12167–0.14978) ratios. The 187 Os/ 188 Os ratios in some wehrlites are much higher than the value for the primitive mantle. The high 187 Os/ 188 Os ratios have also been observed in peridotites found in other localities within the Tan-Lu fault zone such as Shanwang and Nüshan. This could be the result of the addition of radiogenic Os during the melt percolation. Extremely low Os abundances in Beiyan peridotites suggest that Os may behave as an incompatible element during melt percolation and could mobilize with the dissolution of sulphides. The suprachondritic 187 Os/ 188 Os ratios in peridotites within the Tan-Lu fault zone relative to those away from it indicate that the Tan-Lu fault zone played an important role as a melt infiltrating channel in the radiogenic Os enrichment induced by melt percolation. This could be the reason for the Os isotopic heterogeneity observed in the eastern NCC. This study also confirms that Os isotopes and PGE appear to be mobile during massive melt percolation.

Platinum group element (PGE) geochemistry to understand the chemical evolution of the Earth’s mantle

Platinum group elements (PGE: Os, Ir, Ru, Rh, Pt, Pd) are important geochemical and cosmochemical tracers. Depending on physical and chemical behaviour the PGEs are divided into two subgroups: IPGE (Ir, Os, Ru) and PPGE (Pd, Pt, Rh). Platinum group elements show strong siderophile and chalcophile affinity. Base metal sulfides control the PGE budget of the Earth’s mantle. Mantle xenoliths contain two types of sulfide populations: (1) enclosed within silicate minerals, and (2) interstitial to the silicate minerals. In terms of PGE characters the included variety shows IPGE enriched patterns — similar to the melt-depleted mantle harzburgite, whereas the interstitial variety shows PPGE enriched patterns — resembling the fractionated PGE patterns of the basalt. These PGE characters of the mantle sulfides have been interpreted to be representative of multi-stages melting process of the mantle that helped to shape the chemical evolution of the Earth.

Rhenium–osmium isotopes and platinum-group elements in the Rum Layered Suite, Scotland: Implications for Cr-spinel seam formation and the composition of the Iceland mantle anomaly

The Rum Layered Suite is a layered mafic–ultramafic body that was emplaced during Palaeogene North Atlantic margin rifting. It is a classic open-system magma chamber, constructed of 16 repeated coupled peridotite–troctolite units, some of which have laterally extensive ~ 2 mm-thick platinum-group element (PGE) enriched (~ 2 µg g − 1 ) Cr-spinel seams at their bases. In order to investigate Cr-spinel seam petrogenesis and enrichment of the PGE, abundances of these elements and Re–Os isotopes have been determined at three stratigraphic levels of the Rum Layered Suite that represent major magma replenishment events. Individual units preserve a range of initial 187Os/ 188Os ratios, demonstrating heterogeneity in the composition of replenishing magmas. Data for both the Cr-spinel seams and overlying silicates reveal that the processes that formed the Cr-spinel also concentrated the PGE, following magma replenishment. There is no evidence for structurally-bound PGE in Cr-spinel. Instead, the PGE budget of the Rum Layered Suite is linked to base metal sulphides, especially pentlandite, and to PGE alloys contained within the Cr-spinel seams, but which exist as separate phases at Cr-spinel grain boundaries. The range in initial Os isotope compositions ( γ Os = 3.4 to 36) in the Rum Layered Suite can be successfully modelled by 5–8% assimilation of Lewisian gneiss coupled with changing PGE contents in the replenishing magmas associated with sulphide removal. Initial 187Os/ 188Os ratios for Rum rocks range from 0.1305 to 0.1349 and are atypical of the convecting upper mantle, but are within the range for recently erupted picrites and basalts from Iceland and Palaeogene picrites and basalts from Baffin Island, Greenland and Scotland. Thus, the Os isotope data suggest that the North Atlantic Igneous Province magmas were collectively produced from a mantle source with components that remained relatively unchanged in Os isotopic composition over the past 60 Ma, and that likely contain a recycled lithospheric component.

The “Donauplatin”: source rock analysis and origin of a distal fluvial Au-PGE placer in Central Europe

The mineral assemblage and the sedimentological characteristics of the “Donauplatin” (Danubian fluvial placer containing platinum-group elements (PGE) and gold (Au)) are described for the first time in connection with upstream reference placer deposits near the potential source area in tributaries of the River Danube/Donau. Granulometric and morphometric data have been obtained using the CCD-based CAMSIZER technique. The platinum-group minerals (PGM; iridium, osmium, unknown iridium-osmium-sulfide, ruthenium-osmium-iridium alloys, platinum-iron alloys, iridium-bearing platinum, sperrylite) have been derived from ultramafic magmatic rocks, probably belonging to the ophiolitic series in the Tepla Barrandian unit of the Bohemian Massif. The Au-Pd-Cu compounds in the placer originated from dynamo-metamorphogenic processes in a sulfur-deficient environment at the SW edge of the Bavarian Basement. Gold in the “Donauplatin” has been reworked from a “secondary” or intermediate repository of lateritic gold (Boddington-type). Its primary source is supposed to be of orogenic origin. Provenance analyses of the associated non-heavy minerals point to high-pressure metamorphic rocks, igneous rocks (monazite) and high-temperature metamorphic rocks (750° to 850°C, zircon morphology). Garnet compositions indicate that meta(ultra)basic igneous rocks, calc-silicate rocks and skarns prevailed over paragneisses in the provenance area. Extraterrestrial processes creating the well-known Ries impact crater in the environs of Nordlingen during the Miocene have a minor share in the PGE budget by delivering molybdenum-ruthenium-osmium-iridium alloys and iridium solid-solution series (s.s.s.) minerals. Judging by the heavy mineral suites, Saxothuringian source rocks of the NE Bavarian Basement connected with the Donau River via the Naab River drainage system have not contributed to the element budget of the “Donauplatin” under study. Stream sediments which have been derived from this provenance area are characterized by low-temperature (LT) crystalline rocks and a considerable proportion of pegmatitic and metabauxitic material lacking in the Holocene sediments of the “Donauplatin”.

Highly siderophile element behaviour accompanying subduction of oceanic crust: Whole rock and mineral-scale insights from a high-pressure terrain

Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt–Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re–Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust. Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (∼75% loss). In contrast, basaltic eclogites display significant depletion of Pt (⩾60%), Pd (>85%) and Re (50–60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re–Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10–90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide. Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled 186Os– 187Os enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of 187Os– 186Os enrichment.

Low-temperature, platinum-group elements-bearing Ni arsenide assemblage from the Atrevida mine (Catalonian Coastal Ranges, NE Spain)

The Atrevida vein is located at the south-western edge of the Catalonian Coastal Ranges, Spain. This vein extends for more than 3 km and is hosted by Silurian and Carboniferous metasediments, late Hercynian granites and continental conglomerates of Lower Triassic age. The Silurian metasediments contain massive sulphide layers with disseminations of platinum-group element (PGE)-bearing, Fe-Ni arsenides. Vein fi lling largely consists of banded and brecciated baryte rimmed by a complex polymetallic assemblage of Co-Ni-As-Bi-Ag(-U) ores. The Ni arsenide and sulpharsenide assemblage occurs only where the vein cuts the sulphide-bearing Silurian metasediments, and consists of nickeline, maucherite, rammelsbergite, gersdorffi te and Ni-skutterudite. These Ni arsenides and sulpharsenides display rosettes, spherules, colloform and botryoidal textures, as well as subhedral crystals. The chemical compositions of these arsenide-sulpharsenide minerals exhibit limited cation substitution but substantial anion substitution of As by S and Sb, exceeding the expected theoretical values at low temperatures. This suggests that most of them formed under disequilibrium conditions. Ni arsenides and sulpharsenides also contain trace PGE. Palladium (up to 157 ppm) is the most abundant PGE, and occurs in solid-solution in all the phases studied, but shows the highest values in rammelsbergite and Niskutterudite. Pt only occurs in massive nickeline (54–1338 ppm Pt) and in zoned gersdorffi te (29–360 ppm Pt) formed at the end of the depositional sequence of arsenides. The Ni arsenide and sulpharsenide ores at the Atrevida mine formed by reduction of PGE-bearing, oxidizing fl uids at temperatures close to, or slightly above 100 °C. These oxidizing fl uids most probably collected their PGE budget from the nearby mineralized black shales. During the crystallization of the veined, Ni arsenide and sulpharsenide ores, Pd partly concentrated in the early nickeline and maucherite, but mainly fractionated to the late solutions from which rammelsbergite and Ni-skutterudite precipitated. Pt concentrated only in the latest nickeline and gersdorffi te.

Platinum-Group Elements in Sulphide Minerals, Platinum-Group Minerals, and Whole-Rocks of the Merensky Reef (Bushveld Complex, South Africa): Implications for the Formation of the Reef

The concentrations of platinum-group elements (PGE), Co, Re, Au and Ag have been determined in the base-metal sulphide (BMS) of a section of the Merensky Reef. In addition we performed detailed image analysis of the platinum-group minerals (PGM). The aims of the study were to establish: (1) whether the BMS are the principal host of these elements; (2) whether individual elements preferentially partition into a specific BMS; (3) whether the concentration of the elements varies with stratigraphy or lithology; (4) what is the proportion of PGE hosted by PGM; (5) whether the PGM and the PGE found in BMS could account for the complete PGE budget of the whole-rocks. In all lithologies, most of the PGE (∼65 up to ∼85%) are hosted by PGM (essentially Pt–Fe alloy, Pt–Pd sulphide, Pt–Pd bismuthotelluride). Lesser amounts of PGE occur in solid solution within the BMS. In most cases, the PGM occur at the contact between the BMS and silicates or oxides, or are included within the BMS. Pentlandite is the principal BMS host of all of the PGE, except Pt, and contains up to 600 ppm combined PGE. It is preferentially enriched in Pd, Rh and Co. Pyrrhotite contains, Rh, Os, Ir and Ru, but excludes both Pt and Pd. Chalcopyrite contains very little of the PGE, but does concentrate Ag and Cd. Platinum and Au do not partition into any of the BMS. Instead, they occur in the form of PGM and electrum. In the chromitite layers the whole-rock concentrations of all the PGE except Pd are enriched by a factor of five relative to S, Ni, Cu and Au. This enrichment could be attributed to BMS in these layers being richer in PGE than the BMS in the silicate layers. However, the PGE content in the BMS varies only slightly as a function of the stratigraphy. The BMS in the chromitites contain twice as much PGE as the BMS in the silicate rocks, but this is not sufficient to explain the strong enrichment of PGE in the chromitites. In the light of our results, we propose that the collection of the PGE occurred in two steps in the chromitites: some PGM formed before sulphide saturation during chromitite layer formation. The remaining PGE were collected by an immiscible sulphide liquid that percolated downward until it encountered the chromitite layers. In the silicate rocks, PGE were collected by only the sulphide liquid.

Platinum-group element systematics in Mid-Oceanic Ridge basaltic glasses from the Pacific, Atlantic, and Indian Oceans

The concentrations of Ir, Ru, Pt and Pd have been determined in 29 Mid-Oceanic Ridge basaltic (MORB) glasses from the Pacific ( N = 7), the Atlantic ( N = 10) and the Indian ( N = 11) oceanic ridges and the Red Sea ( N = 1) spreading centers. The effect of sulfide segregation during magmatic differentiation has been discussed with sample suites deriving from parental melts produced by high (16%) and low (6%) degrees of partial melting, respectively. Both sample suites define positive and distinct covariation trends in platinum-group elements (PGE) vs. Ni binary plots. The high-degree melting suite displays, for a given Ni content, systematically higher PGE contents relative to the low-degree melting suite. The mass fraction of sulfide segregated during crystallization (X sulf ), the achievement of equilibrium between sulfide melt and silicate melts (R eff ), and the respective proportions between fractional and batch crystallization processes (S b ) are key parameters for modeling the PGE partitioning behavior during S-saturated MORB differentiation. Regardless of the model chosen, similar sulfide melt/silicate melt partition coefficients for Ir, Ru, Pt and Pd are needed to model the sulfide segregation process, in agreement with experimental data. When corrected for the effect of magmatic differentiation, the PGE data display coherent variations with partial melting degrees. Iridium, Ru and Pt are found to be compatible in nonsulfide minerals whereas the Pd behaves as a purely chalcophile element. The calculated partition coefficients between mantle sulfides and silicate melts (assuming a PGE concentration in the oceanic mantle at ∼0.007 × CI-chondritic abundances) increase from Pd (∼10 3) to Ir (∼10 5). This contrasting behavior of PGE during S-saturated magmatic differentiation and mantle melting processes can be accounted for by assuming that Monosufide Solid Solution (Mss) controls the PGE budget in MORB melting residues whereas MORB differentiation processes involve Cu-Ni-rich sulfide melt segregation.

Platinum-group element distribution in the Main Zone and Upper Zone of the Bushveld Complex, South Africa

The platinum-group element (PGE) contents of the upper portions of the Bushveld Complex were investigated with three questions in mind: (a) In natural systems does magnetite concentrate Os, Ir, Ru and Rh (IPGE), as has been observed in experimental systems? (b) Is there a Au–Pd-enriched layer present such as observed in the upper parts of the Skaergaard intrusion? (c) Can changes in metal ratios be used for prospecting for PGE deposits? In the sulfide-poor Main Magnetite Layer of the eastern Bushveld, Ir and Rh are enriched relative to Pt, Pd and Au and this could be because magnetite preferentially concentrated Ir and Rh over Pt, Pd and Au. However, in most other magnetite layers no enrichment was observed. This could be because most magnetite layers contain approximately 1% sulfides and the PGE budget is dominated by the sulfides. These sulfides obscured the effects of magnetite collecting IPGE, because sulfides collect all the PGE and the partition coefficients for the PGE into a sulfide liquid are much greater than the partition coefficient for IPGE into magnetite. The weighted average of the platinum-group elements (PGE) and Au over the 2000 m of sampled stratigraphy is Au 2.1 ppb, Pd 1.7 ppb, Pt 1.7 ppb, Rh 0.18 ppb, Ru<0.5, Ir 0.16 ppb, Os<0.5 ppb. Compared to the marginal rocks (presumed initial liquids) of the Bushveld Complex the PGE and Au are severely depleted. Only one sample (a leuconorite in the first cyclic unit) contained Pt and Pd at economic grade (Pt 2 ppm, Pd 2 ppm). The overall depletion of the PGE in the Upper Zone (despite the presence of 1% to 3% sulfides) could be the result of the PGE having been stripped from the magma by early sulfide liquid which had already settled out of the magma to form the world famous platinum reefs lower in the magma chamber. In addition to the overall depletion of PGE, PGE/S ratios decrease up section indicating that the sulfide fraction is poorer in PGE up section. This is interpreted to be the result of continued depletion of the silicate liquid as sulfides constantly settle out of the silicate liquid. There appears to be little prospect of a Pd-reef type deposit in the Upper Zone. Comparison of the composition of the marginal rocks of the Bushveld with a weighted average for the complete 6 km of the Bushveld cumulates shows that the cumulate pile is much richer in compatible elements (Ir, Rh, Cr) and poorer in incompatible elements (Sm and Hf) than the marginal rocks. Two possible solutions to this are: (a) The magma emplaced into the chamber was a crystal mush and thus more mafic than marginal rocks to the intrusion or (b) fractionated magma has been removed from the cumulate sequence and either irrupted or intruded the country rocks as granites and granophyres.

Platinum-group element abundances in the upper mantle: new constraints from in situ and whole-rock analyses of Massif Central xenoliths (France)

Fourteen peridotite xenoliths collected in the Massif Central neogene volcanic province (France) have been analyzed for platinum-group elements (PGE), Au, Cu, S, and Se. Their total PGE contents range between 3 and 30 ppb and their PGE relative abundances from 0.01 to 0.001 × CI-chondrites, respectively. Positive correlations between total PGE contents and Se suggest that all of the PGE are hosted mainly in base metal sulfides (monosulfide solid solution [Mss], pentlandite, and Cu-rich sulfides [chalcopyrite/isocubanite]). Laser ablation microprobe-inductively coupled plasma mass spectrometry analyses support this conclusion while suggesting that, as observed in experiments on the Cu-Fe-Ni-S system, the Mss preferentially accommodate refractory PGEs (Os, Ir, Ru, and Rh) and Cu-rich sulfides concentrate Pd and Au. Poikiloblastic peridotites pervasively percolated by large silicate melt fractions at high temperature (1200°C) display the lowest Se (<2.3 ppb) and the lowest PGE contents (0.001 × CI-chondrites). In these rocks, the total PGE budget inherited from the primitive mantle was reduced by 80%, probably because intergranular sulfides were completely removed by the silicate melt. In contrast, protogranular peridotites metasomatized by small fractions of volatile-rich melts are enriched in Pt, Pd, and Au and display suprachondritic Pd/Ir ratios (1.9). The palladium-group PGE (PPGE) enrichment is consistent with precipitation of Cu-Ni-rich sulfides from the metasomatic melts. In spite of strong light rare earth element (LREE) enrichments (Ce/YbN < 10), the three harzburgites analyzed still display chondrite-normalized PGE patterns typical of partial melting residues, i.e., depleted in Pd and Pt relative to Ir and Ru. Likewise, coarse-granular lherzolites, a common rock type in Massif Central xenoliths, display Pd/Ir, Ru/Ir, Rh/Ir, and Pt/Ir within the 15% uncertainty range of chondritic meteorites. These rocks do not contradict the late-veneer hypothesis that ascribes the PGE budget of the Earth to a late-accreting chondritic component; however, speculations about this component from the Pd/Ir and Pt/Ir ratios of basalt-borne xenoliths may be premature.

Behaviour of Platinum-group elements in the subcontinental mantle of eastern Australia during variable metasomatism and melt depletion

Increasing recognition of complexities in the Platinum-group element (PGE) and Re concentration patterns in mantle samples are challenging the view of chondritic relative abundances in the upper mantle. To investigate the possible causes of PGE abundance variations, a suite of east Australian, mantle-derived, spinel peridotite xenoliths, ranging from fertile lherzolites to depleted harzburgites, and including apatite ± phlogopite ± amphibole bearing samples, have been analysed for their whole rock PGE and Re abundances. Whole rock abundances for 21 samples, combined with mineral separate analyses of 2 xenoliths, are presented to constrain the distribution of the PGEs and Re, their inherent heterogeneity at difference scales, and their behaviour during both melt extraction and metasomatism. Fertile (>2.9 wt% Al 2O 3) xenoliths have broadly chondritic relative PGE abundances, with the significant exception of positive Rh anomalies and variable negative Os anomalies. The high Rh abundances cannot be attributed to melt extraction or metasomatism. Bulk mineral separate PGE-Re analyses of 2 fertile xenoliths indicate less than 6% of the whole rock PGE budget resides in either silicate or oxide (spinel) phases. The remainder of the PGEs, and at least 80% of the whole rock Re budget, are sited in acid-leachable sulfides and less soluble trace phases such as PGE-sulfides or alloys. Individual PGEs partition into different trace phases resulting in small scale heterogeneity of both PGE ratios and concentrations on the order of 8%–20%. Although these trace phases may be present within the mantle, it is more likely at least some exsolved from monosulfide solid solutions at low temperatures. Ir and Rh abundances are consistent with compatible behaviour during melt extraction, whereas Ru, Pt and Pd abundances are consistent with slightly incompatible behaviour and can be modeled by assuming all reside in sulfides within the mantle, with D sulf Ru ∼ D sulf Pt > D sulf Pd. Comparison of PGE abundances between ‘dry’ xenoliths and modally metasomatised xenoliths, suggests the PGEs are not significantly mobilised during interaction with carbonate melts or during metasomatism leading to hydrous mineral growth. Given the problems of various types of secondary alteration processes, including melt extraction and surficial alteration that commonly affect xenoliths, and as within-locality heterogeneity is on a comparable order to any proposed regional heterogeneity, it may be premature to define significant regional differences, or ‘primary’ non-chondritic PGE patterns in lithospheric peridotites.

Non-chondritic platinum-group element ratios in oceanic mantle lithosphere: petrogenetic signature of melt percolation?

The concentrations of the platinum-group elements (PGE) Ir, Ru, Pt and Pd were determined in 11 abyssal peridotites from ODP Sites 895 and 920, as well in six ultramafic rocks from the Horoman peridotite body, Japan, which is generally thought to represent former asthenospheric mantle. Individual oceanic peridotites from ODP drill cores are characterized by variable absolute and relative PGE abundances, but the average PGE concentrations of both ODP suites are very similar. This indicates that the distribution of the noble metals in the mantle is characterized by small-scale heterogeneity and large-scale homogeneity. The mean Ru/Ir and Pt/Ir ratios of all ODP peridotites are within 15% and 3%, respectively, of CI-chondritic values. These results are consistent with models that advocate that a late veneer of chondritic material provided the present PGE budget of the silicate Earth. The data are not reconcilable with the addition of a significant amount of differentiated outer core material to the upper mantle. Furthermore, the results of petrogenetic model calculations indicate that the addition of sulfides derived from percolating magmas may be responsible for the variable and generally suprachondritic Pd/Ir ratios observed in abyssal peridotites. Ultramafic rocks from the Horoman peridotite have PGE signatures distinct from abyssal peridotites: Pt/Ir and Pd/Ir are correlated with lithophile element concentrations such that the most fertile lherzolites are characterized by non-primitive PGE ratios. This indicates that processes more complex than simple in-situ melt extraction are required to produce the geochemical systematics, if the Horoman peridotite formed from asthenospheric mantle with chondritic relative PGE abundances. In this case, the PGE results can be explained by melt depletion accompanied or followed by mixing of depleted residues with sulfides, with or without the addition of basaltic melt.