Elevated Platinum-group Elements Research Articles

Further evidence for an impact origin of the Tsenkher structure in the Gobi-Altai, Mongolia: geology of a 3.7 km crater with a well-preserved ejecta blanket

AbstractThe Tsenkher structure in the Gobi-Altai, Mongolia is ac. 3.7 km diameter crater with a well-preserved ejecta blanket. It has been hypothesized to be either of impact or volcanic origin in our previous work. Observations during our 2007 expedition and related sample analyses give further support for an impact origin. The evidence includes the presence of a structurally uplifted near-circular rim surrounded by an ejecta blanket, and abundant breccias, some of which are melt- and millimetre-scale spherule-bearing. Planar deformation features (PDFs) were found in one quartz grain in a breccia sample. Fe-rich grains are found in a vesicular melt sample that is also characterized by elevated platinum group element (PGE) abundances with respect to the sedimentary bedrock of the area (approximately an order of magnitude). Noble gas analysis of one breccia sample yielded an elevated3He/4He value of (5.0±0.2) × 10−6. Although not conclusive alone, these geochemical results are consistent with a contribution of meteoritic components. A volcanic origin, in particular a maar formation, would require explanations for the unusual conditions associated with Tsenkher, including its large size occurring in isolation, the structurally uplifted rim and the lack of a bedded base surge deposit. A pronounced rampart structure observed at the eastern ejecta is also unusual for any volcanic origin.40Ar–39Ar dating of a vesicular melt sample gives an age of the Tsenkher structure of 4.9±0.9 Ma. The rampart structure could provide insights into the formation of similar ejecta morphologies associated with numerous impact craters on Mars.

New insights into the petrogenesis of the Jameson Range layered intrusion and associated Fe-Ti-P-V-PGE-Au mineralisation, West Musgrave Province, Western Australia

The Mesoproterozoic Jameson Range intrusion forms part of the Giles Complex, Musgrave Province, Western Australia. It is predominantly mafic in composition comprising olivine-bearing gabbroic lithologies with variable amounts of magnetite and ilmenite. Lithologies containing more than 50 vol% magnetite and ilmenite are classified as magnetitites. The Jameson Range hosts several of these magnetitites forming laterally extensive layers, which can be traced for at least 19 km as continuous magnetic anomalies. Similar occurrences of magnetitites are known from the upper parts of other layered intrusions, such as the Bushveld Complex. In addition, the intrusion hosts several P-rich zones, one of which is at least 59 m in thickness containing 1.0 wt% P2O5. The P-rich zones are not directly associated with the magnetitites, but they mostly occur slightly above them. The mineral chemistry of the Jameson Range cumulates is relatively evolved with olivine compositions ranging from Fo44 to Fo60 and plagioclase compositions varying between An56 and An59. The Mg# (100 × Mg / (Mg + Fe)) of ortho- and clinopyroxene ranges from 60 to 61 and from 70 to 75, respectively. Magnetite compositions are characterised by low TiO2 concentrations varying from 0.39 to 3.04 wt% representing near end-member magnetite with up to 1.2 wt% Cr and 1.3 wt% V, respectively. The basal magnetite layer reaches up to 68.8 wt% Fe2O3(t) and 24.2 wt% TiO2, and it is also markedly enriched in Cu (up to 0.3 wt% Cu), V (up to 1.05 wt% V2O5) and platinum-group elements (PGE) (up to 2 ppm Pt + Pd). Sulphide minerals comprising bornite, chalcopyrite and minor pentlandite occur finely disseminated in the magnetitite and account for the elevated base metal and PGE concentrations. Modelling indicates that the PGE mineralisation was formed at very high R factors of up to 100,000, which is typical for PGE reefs in layered intrusions. Whole rock geochemical and mineralogical data of the magnetite layers and their host rocks further allow for a refinement of current formation models of layered igneous sequences. Several lines of evidence suggest that the magnetite layers formed in response to primarily density-controlled mineral sorting within crystal slurries, although the grain size also affects the sorting process.

Platinum-group element signatures in the North Atlantic Igneous Province: Implications for mantle controls on metal budgets during continental breakup

The North Atlantic Igneous Province (NAIP) is a large igneous province (LIP) that includes a series of lava suites erupted from the earliest manifestations of the (proto)-Icelandic plume, through continental rifting and ultimate ocean opening. The lavas of one of these sub-provinces, the British Palaeogene Igneous Province (BPIP), were some of the first lavas to be erupted in the NAIP and overlie a thick crustal basement and sedimentary succession with abundant S-rich mudrocks. We present the first platinum-group element (PGE) and Au analyses of BPIP flood basalts from the main lava fields of the Isle of Mull and Morvern and the Isle of Skye, in addition to a suite of shallow crustal dolerite volcanic plugs on Mull, and other minor lavas suites. BPIP lavas display both S-saturated and S-undersaturated trends which, coupled with elevated PGE abundances (>MORB), suggest that the BPIP is one of the most prospective areas of the NAIP to host Ni–Cu–PGE–(Au) mineralisation in conduit systems. Platinum-group element, Au and chalcophile element abundances in lavas from West and East Greenland, and Iceland, are directly comparable to BPIP lavas, but the relative abundances of Pt and Pd vary systematically between lavas suites of different ages. The oldest lavas (BPIP and West Greenland) have a broadly chondritic Pt/Pd ratio (~1.9). Lavas from East Greenland have a lower Pt/Pd ratio (~0.8) and the youngest lavas from Iceland have the lowest Pt/Pd ratio of the NAIP (~0.4). Hence, Pt/Pd ratio of otherwise equivalent flood basalt lavas varies temporally across the NAIP and appears to be coincident with the changing geodynamic environment of the (proto)-Icelandic plume through time. We assess the possible causes for such systematic Pt/Pd variation in light of mantle plume and lithospheric controls, and suggest that this reflects a change in the availability of lithospheric mantle Pt-rich sulphides for entrainment in ascending plume magmas. Hence the precious metal systematics and potential prospectivity of a LIP may be affected by contamination of plume-derived magmas by subcontinental lithospheric mantle at the margins of cratons that have been enriched by Palaeoproterozoic orogenesis.

Petrography and geochemistry of ejecta from the Sudbury impact event

AbstractEjecta from the Connors Creek site in Michigan (500 km from the Sudbury Igneous Complex [SIC]), the Pine River site in western Ontario (650 km from the SIC), and the Coleraine site in Minnesota (980 km from the SIC) were petrographically and geochemically analyzed. Connors Creek was found to have approximately 2 m of ejecta, including shocked quartz, melt droplets, and accretionary lapilli; Pine River has similar deposits about 1 m in thickness, although with smaller lapilli; Coleraine contains only impact spherules in a 20 cm‐thick layer (impact spherules being similar to microkrystites or microtektites). The ejecta transition from chaotic deposits of massively bedded impactoclastic material with locally derived detritus at Connors Creek to a deposit with apparently very little detrital material that is primarily composed of melt droplets at Pine River to a deposit that is almost entirely composed of melt spherules at Coleraine. The major and trace element compositions of the ejecta confirm the previously observed similarity of the ejecta deposits to the Onaping Formation in the SIC. Platinum‐group element (PGE) concentrations from each of the sites were also measured, revealing significantly elevated PGE contents in the spherule samples compared with background values. PGE abundances in samples from the Pine River site can be reproduced by addition of approximately 0.2 wt% CI chondrite to the background composition of the underlying sediments in the core. PGE interelement ratios indicate that the Sudbury impact event was probably caused by a chondritic impactor.

THE FIRST REPORT OF PLATINUM-GROUP MINERALS IN MAGNETITE-BEARING GABBRO, FREETOWN LAYERED COMPLEX, SIERRA LEONE: OCCURRENCES AND GENESIS

Platinum-group minerals (PGM) have been located in magnetite-bearing gabbroic rocks of the Freetown Layered Complex, Sierra Leone for the first time. The PGM occurrences described here are from four stratiform layers characterized by a local, more primitive, composition of the associated silicate minerals and elevated platinum-group elements (PGE) and base metal assays. The uppermost layer (Horizon B) is the best exposed; the three lower layers are currently known only from single outcrops. Platinum-group minerals are irregularly distributed through these layers. Altered PGE arsenides and antimonides, possibly derived from sperrylite (PtAs2) and “mertieite” [Pd8(Sb,As)3 to Pd11(Sb,As)4], occur in the lowest known PGE-enriched layer (Horizon M). Cooperite (PtS) and Pt-Fe alloys were observed in a higher layer (Horizon D). Pd-Cu alloys including nielsenite (PdCu3) are unique to the next stratigraphically higher PGM-bearing layer (Horizon C). The uppermost layer (Horizon B) contains the most PGM. Pt-Fe alloy (usually Pt3Fe) is by far the most abundant PGM and cooperite is a significant component of the assemblage. Bowieite [(Pt,Ir,Rh)2S3], tulameenite (Pt2CuFe), and laurite (RuS2) have also been found in this horizon. Cooperite occurs in a late magmatic, high Ca-amphibole (cannilloite) which has formed interstitially to plagioclase and between plagioclase and olivine at a depth of about 20 km. The cooperite is often attached to rounded magnetite grains within the amphibole. Where cooperite is located at the edge of the amphibole or in interstitial locations it has been altered to Pt-Fe alloy. Platinum-group minerals sometimes occur near the margin of sulfides, mainly chalcopyrite and minor pentlandite. The entire PGM assemblage is found mainly interstitial to olivine, pyroxene, and plagioclase, often at the edge of amphibole or in interstitial sites associated with amphibole, chlorite, or quartz. Many of the PGM grains are less than 1 micron in size, with a few in the 2–4 micron range; the largest example is a grain of Pt-Fe alloy that measures 9 by 6 microns. The PGM described here from the rocks are three orders of magnitude smaller than the eluvial and alluvial PGM of the Freetown complex, which are predominantly Pt-Fe alloys, laurite, erlichmanite (OsS2), and Os-Ir alloys. The large eluvial and alluvial PGM might be derived from a horizon containing coarse-grained mineralization, but no vestige of such a horizon has yet been discovered. Previous studies of this area have suggested that secondary remobilization of the PGE has occurred, leading to the growth of large secondary PGM. The primary PGE-mineralization associated with magnetite and Cu-rich sulfide mineralization maybe due to late sulfur saturation in the magma after Ni has been removed by being incorporated into earlier olivine. The presence of digenite, native copper, and nielsenite indicates a low sulfur fugacity, possibly due to oxidation. The four distinctive PGM-bearing horizons are the result of separate injections of magma. The silicate compositions, the minor element content, the PGM mineral assemblage, and the Cu/Pd ratios of the rocks of the PGM-bearing horizons demonstrate that these four magma pulses are compositionally distinct.

Genesis of chromitites from Korydallos, Pindos Ophiolite Complex, Greece, based on spinel chemistry and PGE-mineralogy

The Pindos Ophiolite Complex, located in northwestern Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. In Korydallos are encountered both types of chromitites. These are podiform chromitites that have small dimensions and occur sub-concordantly to the hosting peridotites. The Cr-rich chromitites contain magnesiochromite with high Cr# [Cr/(Cr + Al): 0.65–0.68] and low platinum-group element (PGE) contents (294.1 ppb), whereas the Al-rich ones host spinel with low Cr# (0.44–0.48) and elevated total PGE grades (28830 ppb). The former display a nearly flat C1-normalized PGE-pattern, whereas the latter show a positively sloped normalized PGE-pattern. The in situ mineralogical investigation of the Cr-rich chromitites revealed a platinum-group mineral (PGM) assemblage dominated by small (≤ 3 μm) sperrylite, laurite and erlichmanite grains (determined by recalculated qualitative analytical data). Textural relations suggest crystallization under conditions of high fS2 and fAs and/or low T. The in concentrates mineralogical study of the Al-rich chromitites showed that the PGM assemblage that they host is dominated by Pd–Cu and Pd–Au–Cu alloys. The vast majority of these alloys is associated with abundant secondary BMS (base-metal sulfides) and BMA (base-metal alloys), thus confirming that a sulfide melt scavenged the PGE + Au of the silicate magma from which chromian spinel had already started to crystallize. Both assemblages were affected by an invasion of an oxidizing aqueous fluid in the investigated chromitites. Combined data indicate that the chromitites from the Korydallos area crystallized from a progressively differentiating MORB/IAT melt, produced in a small backarc basin in a supra-subduction zone setting.

Distribution of platinum group elements in urban soils, comparison of historically different large cities Prague and Ostrava, Czech Republic

The distribution of platinum group elements (PGE) was studied in the soils of municipal parks in two large industrial cities with different histories, Prague and Ostrava in the Czech Republic. Soil samples were taken from depths of 0–10, 10–20 and 20–30cm and the Pd, Pt, Rh and Ir contents were studied in the individual samples following dokimastic separation into NiS using ICP MS. Higher PGE contents were determined in Prague; the highest contents were found in the centres of both cities because of the slower, stop-and-go movement of automobiles during which more PGE's are emitted. The maximum PGE contents found were for Pt in Prague (160μgkg−1), with lower contents of Pd (49μgkg−1), followed by Rh (3.9μgkg−1). Elevated PGE concentrations are observable in the topmost layers of most soils. At some locations, the maximum PGE concentrations were observed in deeper parts of the profile. On the basis of the maximum numbers of registered vehicles (1997) and assumption that new vehicles emit the highest amounts of PGE, the calculated velocity of migration of metals through the profile corresponded to 1.1–2.2cmyear−1. The statistically lower PGE contents in areas under the tree crowns compared to open sites can be explained by the annual collection and removal of fallen biomass containing the captured catalyst particles.

Platinum-group element distribution in base-metal sulfides of the Merensky Reef from the eastern and western Bushveld Complex, South Africa

Base-metal sulfides in magmatic Ni-Cu-PGE deposits are important carriers of platinum-group elements (PGE). The distribution and concentrations of PGE in pentlandite, pyrrhotite, chalcopyrite, and pyrite were determined in samples from the mineralized portion of four Merensky Reef intersections from the eastern and western Bushveld Complex. Electron microprobe analysis was used for major elements, and in situ laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) for trace elements (PGE, Ag, and Au). Whole rock trace element analyses were performed on representative samples to obtain mineralogical balances. In Merensky Reef samples from the western Bushveld, both Pt and Pd are mainly concentrated in the upper chromitite stringer and its immediate vicinity. Samples from the eastern Bushveld reveal more complex distribution patterns. In situ LA-ICP-MS analyses of PGE in sulfides reveal that pentlandite carries distinctly elevated PGE contents, whereas pyrrhotite and chalcopyrite only contain very low PGE concentrations. Pentlandite is the principal host of Pd and Rh in the ores. Palladium and Rh concentrations in pentlandite reach up to 700 and 130 ppm, respectively, in the samples from the eastern Bushveld, and up to 1,750 ppm Pd and up to 1,000 ppm Rh in samples from the western Bushveld. Only traces of Pt are present in the base-metal sulfides (BMS). Pyrrhotite contains significant though generally low amounts of Ru, Os, and Ir, but hardly any Pd or Rh. Chalcopyrite contains most of the Ag but carries only extremely low PGE concentrations. Mass balance calculations performed on the Merensky Reef samples reveal that in general, pentlandite in the feldspathic pyroxenite and the pegmatoidal feldspathic pyroxenite hosts up to 100 % of the Pd and Rh and smaller amounts (10–40 %) of the Os, Ir, and Ru. Chalcopyrite and pyrrhotite usually contain less than 10 % of the whole rock PGE. The remaining PGE concentrations, and especially most of the Pt (up to 100 %), are present in the form of discrete platinum-group minerals such as cooperite/braggite, sperrylite, moncheite, and isoferroplatinum. Distribution patterns of whole rock Cu, Ni, and S versus whole rock Pd and Pt show commonly distinct offsets. The general sequence of “offset patterns” of PGE and BMS maxima, in the order from bottom to top, is Pd in pentlandite → Pd in whole rock → (Cu, Ni, and S). The relationship is not that straightforward in general; some of the reef sequences studied only partially show similar trends or are more complex. In general, however, the highest Pd concentrations in pentlandite appear to be related to the earliest, volumetrically rather small sulfide liquids at the base of the Merensky Reef sequence. A possible explanation for the offset patterns may be Rayleigh fractionation.

Platinum Group Elements in Airborne Particles in Mexico City

Automobile exhaust catalysts using platinum group elements (PGE) have been mandatory on new cars in the Mexico City Metropolitan Area (MCMA) since 1991. Platinum, Pd, Rh, Ir, and Os concentrations and the isotopic composition of Os were determined in PM10 samples from the MCMA. Samples were prepared by isotope dilution NiS fire assay, and analysis was performed by magnetic sector ICP-MS using a single collector instrument for Pt, Pd, Rh, and Ir analysis and a multicollector instrument for Os analysis. Pt, Pd, and Rh concentrations at a downtown location (Merced) increased from < or =1.7 pg of Pt m(-3), 2.7 (4.0) pg of Pd m(-3), and 1.2+/-0.9 pg of Rh m(-3) in 1991 to 9.6 +/- 1.8 pg of Pt m(-3), 10.2+/-1.8 pg of Pd m(-3), and 2.8 +/-0.6 pg of Rh m(-3) in 2003. Concentrations at five sites in MCMA in 2003 averaged 9.3+/-1.9 pg of Pt m(-3), 11+/-4 pg of Pd m(-3), and 3.2+/- 1.0 pg of Rh m(-3). In contrast, Ir and Os concentrations and Os isotopic composition remained relatively constant and were 0.08+/-0.04 pg of Ir m(-3), 0.030 +/-0.007 pg of Os m(-3), and 0.60+/-0.04, respectively, in the MCMA in 2003. Elevated Pt, Pd, and Rh concentrations in the MCMA are attributed to automobile catalysts. A Pt-Pd-Rh concentration peak in 1993 suggests that early catalysts emitted a larger amount of PGE, possibly due to factors inherent in the technology or the use of inappropriate gasoline. Therefore, this study suggests that the current introduction of automobile catalysts in developing countries may result in elevated PGE concentrations if it is not accompanied by infrastructures and policy measures supporting their efficient use.

Geochemistry and mineralogy of the Platreef and “Critical Zone” of the northern lobe of the Bushveld Complex, South Africa: implications for Bushveld stratigraphy and the development of PGE mineralisation

The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration. This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone.

Importance of Automobile Exhaust Catalyst Emissions for the Deposition of Platinum, Palladium, and Rhodium in the Northern Hemisphere

An estimated 500 million vehicles worldwide are equipped with an exhaust catalyst that uses platinum group elements (PGE) as the main active components and thus contribute to global PGE emissions. Although PGE emitted from automobile exhaust catalysts were first believed to remain in the roadside environment, we propose here that fine PGE-containing particles in automobile exhaust have resulted in a widespread distribution of emitted PGE. Regional and long-range transport of PGE from automobile exhaust catalysts is supported by elevated PGE deposition in both a peat bog located 250 m from traffic and in central Greenland, respectively. Russian smelters were also found to contribute to PGE contamination in central Greenland. Deposition rates estimated for the roadside environment, the peat bog, and central Greenland were used to provide a first estimate of PGE deposition in the northern hemisphere. The results show that deposition of regionally or long-range transported PGE accounts for a large fraction of total PGE deposition, and PGE deposition in the roadside environment represents less than 5% of the total deposition. Transport at the regional and global scales represents an important component in the environmental cycle of emitted PGE and needs to be further studied to fully assess the environmental fate of PGE from automobile exhaust catalysts.

Sources of anthropogenic platinum-group elements (PGE): Automotive catalysts versus PGE-processing industries.

Soil samples from the area of Hanau (Hessen, Germany) were analyzed for anthropogenic platinum-group elements (PGE). The results confirm the existence of two different sources for anthropogenic PGE: 1. automotive catalysts, and 2. PGE-processing plants. Both sources emit qualitatively and quantitatively different PGE spectra and PGE interelemental ratios (especially the Pt/Rh ratio). Elevated PGE values which are due to automotive catalysts are restricted to a narrow-range along roadside soil, whereas those due to PGE-processing plants display a large-area dispersion. The emitted PGE-containing particles in the case of automotive catalysts are subject to transport by wind and water, whereas those from PGE-processing plants are preferably transported by wind. This points to a different aerodynamic particle size. Pt, Pd, and Rh concentrations along motorways are dependent on the amount of traffic and the driving characteristics.

Sulfur isotope systematics and platinum group element behavior in REE-enriched metasomatic fluids: A study of mantle xenoliths from Dish Hill, California, USA

The sulfur isotopic compositions, trace and rare earth element (REE), and platinum group element (PGE) contents were determined for a suite of anhydrous spinel lherzolite xenoliths and their metasomatized equivalents from Dish Hill, California, USA, to evaluate the mobility of these elements in metasomatic fluids. Anhydrous xenoliths have REE patterns that vary from being slightly LREE-depleted to flat, as well as variable enrichments of LILE (Ba and Sr) and HFSE (Th and Nb) that indicate they were affected by cryptic metasomatism. The formation of amphibole in xenoliths (modal metasomatism) is accompanied by pronounced enrichments in HFSEs and REES, particularly LREEs. With the exception of a single xenolith with slightly elevated PGE contents, the metasomatic fluids did not produce any appreciable changes in the platinum group element (PGE) contents, suggesting these elements are relatively immobile under these mantle conditions. Typical concentrations of PGEs in xenoliths from Dish Hill are 0.8 ppb Os, 1 ppb Ir, 0.6 ppb Rh, 2.5 ppb Pt, 1.8 ppb Pd, and 1.7 ppb Au. Modal metasomatism has resulted in a variable increase in sulfur content of the xenoliths, from a value of <10 ppm for anhydrous xenoliths, to between 25 and 120 ppm for amphibole-rich xenoliths. The δ 34S values of sulfides in anhydrous xenoliths from Dish Hill are near +7, which are unusually high for the mantle and indicate that the lithospheric mantle under California was most likely affected by subduction of crustal sulfur. Sulfides in amphibole-bearing xenoliths have variable but generally lower δ 34S values of 3 to 8.7, suggesting that some portion of the metasomatic sulfur was derived from an asthenospheric mantle source with a more primordial sulfur isotopic composition.