Platinum Group Element Concentrations Research Articles

Age and evolution of the deep continental root beneath the central Rae craton, northern Canada

Canada is host to at least six separate cratons that comprise a significant proportion of its crustal extent. Of these cratons, we possess knowledge of the cratonic lithospheric roots beneath only the Slave craton and, to a lesser extent, the Superior craton, despite the discovery of many new diamond-bearing kimberlites in Canada's North. Here we present the first age, composition and geothermal information for kimberlite-borne peridotite xenoliths from two localities within the central Rae craton: Pelly Bay and Repulse Bay. Our aim is to investigate the nature and evolution of the deep lithosphere in these regions and to examine how events recorded in the mantle may or may not correlate with the complex history of crustal evolution across the craton.Peridotite xenoliths are commonly altered by secondary processes including serpentinization, silicification and carbonation, which have variably affected the major element compositions. These secondary processes, as well as mantle metasomatism recorded in pristine silicate minerals, however, did not significantly modify the relative compositions of platinum-group elements (PGE) and Os isotope ratios in the majority of our samples from Pelly Bay and Repulse Bay, as indicated by the generally high absolute PGE concentrations and mantle-like melt-depleted PGE patterns. The observed PGE signatures are consistent with the low bulk Al2O3 contents (mostly lower than 2.5%) of the peridotites, as well as the compositions of the silicate and oxide minerals. Based on PGE patterns and Os model ages, the peridotites from both localities can be categorized into three age groups: Archean (3.0–2.6Ga overall; 2.8–2.6Ga for Pelly Bay and 3.0–2.7Ga for Repulse Bay), Paleoproterozoic (2.1–1.7Ga), and “Recent” (<1Ga, with model ages similar to the ca. 546Ma kimberlite eruption age). The Archean group provides the first direct evidence of depleted Archean lithospheric mantle forming coevally with the overlying Archean crustal basement, indicating cratonization of the Rae during the Archean. The subtle difference in Os model ages between Pelly Bay and Repulse Bay coincides with the age difference between crustal basement rocks beneath these two areas, supporting the suggestion that the Rae craton was assembled by collision of separate two Archean blocks at 2.7–2.6Ga. The Paleoproterozoic peridotites are interpreted to represent newly formed lithospheric mantle, most likely associated with regional-scale underplating during the 1.77–1.70Ga Kivalliq-Nueltin event via removal of the lower portion of Archean lithospheric mantle followed by replacement with juvenile Paleoproterozoic lithospheric mantle. The existence of multiple age clusters in the lithosphere at each locality is consistent with the observation of present-day seismic lithospheric discontinuities (Snyder et al., 2013, 2015) that indicate two or more layers of fossil lithospheric mantle fabric beneath this region. Our data define a shallow mantle lithosphere layer dominated by Archean depletion ages underlain by a layer of mixed Archean and Paleoproterozoic ages. This lithospheric mantle structure is probably a response to complex tectonic displacement of portions of the lithospheric mantle during Paleoproterozoic orogeny/underplating. The best equilibrated Archean and Paleoproterozoic peridotites at both Pelly Bay and Repulse Bay define a typical cratonic geotherm at the time of kimberlite eruption, with a ∼200km thick lithospheric root extending well into the diamond stability field, in keeping with the diamondiferous nature of the kimberlites. Such thick lithosphere remains in place to the present day as suggested by seismic and magnetotelluric studies (Snyder et al., 2013, 2015; Spratt et al., 2014). The metasomatically disturbed peridotites in the Rae lithospheric mantle, yielding model ages indistinguishable from kimberlite eruption, may represent parts of the Rae craton mantle root that show anomalous magnetotelluric signatures.

Geology, petrography, geochemistry, and genesis of sulfide-rich pods in the Lac des Iles palladium deposits, western Ontario, Canada

The Lac des Iles Pd deposits are known for their Pd-rich sulfide-poor mineralization. However, previously undocumented sulfide-rich pods also occur within the intrusion that hosts the deposits. Given the complex magmatic and hydrothermal history of the mineralization at Lac des Iles, the sulfide-rich pods could have crystallized from magmatic sulfide liquids or precipitated from hydrothermal fluids. Sulfide-rich pods occur throughout the stratigraphy, in all rock types, and along comagmatic shear zones, and contain net-textured to massive sulfides. They can be divided into four main groups based on the variation in mineral assemblages: (1) pyrrhotite–pentlandite ± pyrite–chalcopyrite–magnetite–ilmenite; (2) chalcopyrite ± pyrrhotite–pentlandite–pyrite–magnetite–ilmenite; (3) pyrite ± pentlandite–chalcopyrite–pyrrhotite–magnetite–ilmenite; and (4) magnetite ± ilmenite–pyrrhotite–pentlandite–pyrite–chalcopyrite. Whole rock metal contents and S isotopic compositions do not change with the amount of pyrite present, except for slight enrichments in As and Bi. The presence of an essentially magmatic sulfide mineral assemblage (pyrrhotite–pentlandite ± chalcopyrite) with pentlandite exsolution flames in pyrrhotite in some pods suggests that the pods crystallized from magmatic sulfide liquids. The very low Cu contents of the pods suggests that they are mainly cumulates of monosulfide solid solution (MSS). We propose a model whereby sulfide liquids were concentrated into dilation zones prior to crystallizing cumulus MSS. Intermediate solid solution crystallized from the fractionated liquids at the edges of some pods leaving residual liquids enriched in Pt, Pd, Au, As, Bi, Sb, and Te. These residual liquids are no longer associated with the pods. During subsequent alteration, pyrite replaced MSS/pyrrhotite, but this did not affect the platinum-group element contents of the pods.

Platinum group elements in urban road dust

Platinum group elements in urban road dust

Batch leaching tests of motherboards to assess environmental contamination by bromine, platinum group elements and other selected heavy metals

In this study, a batch leaching test was executed to evaluate the toxicity associated with chemicals contained in motherboards. The leaching solutions used were distilled water, nitric acid, acetic acid and synthetic acid rain solution. A total of 21 elements including Ag, As, Au, Br, Cd, Co, Cr, Cu, Hf, Ir, Mn, Ni, Os, Pb, Pd, Pt, Rd, Rh, Se, U and Zn were analyzed. In this study, the pH values of all the leachates fell within the range of 2.33–4.88. The highest concentrations of metals were obtained from the acid rain solution, whilst the maximum value of bromine was achieved with solution of acetic acid. Appreciable concentrations of platinum group elements were detected with concentrations around 3.45, 1.43, 1.21 and 22.19 µg L−1 for Ir, Pd, Pt and Rh, respectively. The different leaching of the motherboards revealed the predominant presence of the toxic substances in the leached from the e-waste.

Pyasino–Vologochan intrusion: Geological structure and platinum–copper–nickel ores (Norilsk Region)

Rocks enriched in olivine, such as olivine and picritic gabbro–dolerite and troctolite are abundant in the Pyasino–Vologochan intrusion. In contrast to Norilsk type ore-bearing massifs, ultramafic rocks of the intrusion are dominated by troctolite. Picritic gabbro–dolerite and troctolite are located between olivine and olivine-bearing gabbro–dolerite. These rocks are related by gradual transitions. The rocks are enriched in olivine, but depleted in magnesium relative to the Norilsk type ore-bearing massifs. The intrusion is also distinguished by low chromium content in the rocks. Most disseminated Pt–Cu–Ni ores are confined to troctolite and picritic gabbro–dolerite, and to a lesser extent, to contact and olivine gabbro–dolerite. The ore is mainly observed as low-sulfur assemblages of sulfides with troilite, Fe-rich hexagonal pyrrhotite, Fe-rich chalcopyrite, talnakhite, Fe-rich pentlandite, and cubanite. The total content of platinum group elements (PGE) in the ore (ppm) is as follows: from 0.45–3.47 to 7.8; gold, from 0.05–0.24 to 0.49; and silver, from 0.53–4.50 to 8.30. Disseminated ores of the Pyasino–Vologochan intrusion contain the following Pt and Pd minerals: sobolevskite, Te-sobolevskite, Te-insizwaite, maslovite, paolovite, zvyagintsevite, atokite, niggliite, mertieite II, guanglinite, and Pd2(As,Sb), Pd2(Sn,As), Pd3Sb, (Pd,Ni)2As minerals. The similarities and differences between the Pyasino–Vologochan intrusion and Norilsk type ore-bearing massifs are discussed in this paper.

New data on platinum group elements in sulfide deposits of the Southern Urals

New data on the concentrations of gold and platinum group elements (PGE) in sulfide deposits of the Southern Urals show that a substantial share of Au, Pt, and Pd is concentrated during technological ore processing in their dressing tailings. The behavior of Pt, Pd, and, partly, Au is determined by the size of individual mineral particles.

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.

Chalcophile and platinum-group element distribution in pyrites from the sulfide-rich pods of the Lac des Iles Pd deposits, Western Ontario, Canada: Implications for post-cumulus re-equilibration of the ore and the use of pyrite compositions in exploration

The Lac des Iles Pd-deposits are atypical within the scope of platinum-group element (PGE) deposits. This is because the deposits do not resemble a classical PGE deposit in a number of ways: the intrusion is small and concentrically zoned; most of the host rocks to the deposits no longer have a primary mineralogy and equilibrated under greenschist conditions; the textures of the rocks from the ore zones are extremely variable; and the ores have very high Pd/Ir and Pd/Pt ratios. In addition to the disseminated sulfides, there are sulfide-rich pods present throughout the stratigraphy. The sulfide mineral textures and proportions within the pods vary from those which are essentially magmatic to those which consist predominantly of pyrite. The pyrite could have been deposited from hydrothermal fluids or it could have formed by alteration of magmatic sulfides. In order to distinguish between these two origins, the PGE and chalcophile element contents of the pyrite were investigated. It was found that the pyrite contains Os, Ir, Ru and Rh. These elements also concentrate in the magmatic sulfides pyrrhotite and pentlandite. Their presence in the pyrite could be explained by redistribution of Fe from pyrrhotite to silicate minerals that are present within and around the sulfide pods, possibly during cooling. Maps of the distribution of the elements show that there is zoning of the elements. The IPGE–Rh are present towards the cores of pyrite along with As whereas Co and Se are present towards the rims. Mobile elements such as Pb, Bi and Ag are present in thin overgrowths at the edges of pyrite and in a few cases, Pt, Te and Sn are also present in the overgrowths. Comparison of the composition and element distribution with pyrites from other igneous settings (Sudbury and Aguablanca) shows similarities, suggesting a common ore-modifying process. In contrast, pyrites from low-temperature hydrothermal deposits have different compositions. A plot of Co/Se vs Sb/As appears to be effective at separating the igneous pyrites from pyrites found in other settings and could possibly be used in exploration.

Platinum group elements in mantle melts and mantle samples

A large data compilation has been assembled of platinum group element (PGE) analyses in mantle melts and mantle rocks, the latter including an assortment of xenoliths and obducted mantle massifs. The degree of correlation has been investigated among the PGEs and with other major element variables such as Al2O3, TiO2 and Mg number, and the results are considered in the context of the current paradigm for the behaviour of highly siderophile elements in the silicate Earth.Primitive mantle melts have a wide range of PGE contents. Komatiites have the highest abundances of all the PGEs, show the strongest correlations between Pt and Rh, Pt and Pd and between the iridium-group PGEs Ir, Ru and Os (IPGEs). Most basalts of all affinities have lower levels of Pt and Pd and much lower levels of Ir, Ru and Os than komatiites. Within the basalt grouping Rh has stronger affinities with the IPGEs. Picrites and Archaean basalts are intermediate between these two groups. MORBs and a small proportion of continental LIP basalts show strong depletions in all PGEs attributable to retention of sulfide in their mantle source rocks, or sulfide liquid fractionation on ascent. The degree of PGE depletion in other basalts is probably attributable to equilibration with sulfide, but is less than would be expected under conventional models of sulfide extraction, and is instead attributed to mixing of magmas generated at variable depths incorporating both sulfide-saturated and undersaturated components. Basalts with Pt and Pd contents higher than typical komatiites are rare, a notable example being B1-type parent magmas to the Bushveld Complex, which have komatiite-like relative PGE abundances and Pt, Pd and Rh abundances up to a factor of two higher than komatiites for comparable Ti contents.The mantle composition array as a whole is characterized by variable degrees of depletion of Pt, Pd and Rh in Al-poor, melt-depleted harzburgite/dunite lithologies; lack of depletion in these elements in Al-bearing lherzolites; and a lack of systematic variation in IPGEs across this range. Strongest correlations across the entire set are observed between Ir, Ru and Os; and between Pt and Rh. Melt-depleted cratonic mantle samples are notably more deficient in Pd than in Pt, but comparable Pd-enriched components are not represented in the available data from continental environments. The only group of mantle melts that systematically record high Pd/Pt ratios are MORBs; if these are indeed the complement of the depleted cratonic mantle suite then the melt depletion recorded by the cratonic mantle suite occurred at low pressure prior to tectonic underplating of the depleted lithosphere beneath the cratons. A filtered subset of orogenic peridotite compositions that are thought not to have been affected by significant extents of melt extraction or metasomatic refertilization have median concentrations of 3.9ppb Os, 2.9ppb Ir, 6.3ppb Ru, 1.0ppb Rh, 6.2ppb Pt, 5.4ppb Pd, and Cu/Pd ratio of 5500, which we consider to be representative of the modern convecting mantle. The convecting mantle has PGE proportions closely resembling those of lunar impact breccias, diverging considerably from chondritic proportions and attributable to the presence of a late veneer-derived, predominantly sulfide-hosted component. The compositions of mantle peridotites show considerable scatter attributable to the combined effects of measurement error and a strong covariance due to a heterogeneous distribution of sulfide in the small samples typically chosen for pulverization. The intensity of the covariance between all of the PGE due to sampling error gives a false impression of a genetic trend toward highly enriched PGE in some samples which could be mistaken for the effects of metasomatism; however no plausible metasomatic process would be expected to retain the tight interelement correlations shown.

Understanding Re–Os systematics and model ages in metamorphosed Archean ultramafic rocks: A single mineral to whole-rock investigation

187Rhenium–187Os model ages are used to infer the timing of crust-mantle differentiation events and evolution of lithospheric mantle. However, ancient samples often have long and complex histories of metamorphism, metasomatism and deformation that may be problematic if these processes caused disturbance the Re–Os system. Such processes have been shown to disturb the Re–Os systematics of ophiolitic chromitites, but the effect on stratiform chromitites has not previously been investigated. Here we present a detailed petrographic and Re–Os isotopic study of chromitites, chromite-bearing meta-peridotites and single chromite grains from the Ujaragssuit nunât layered ultramafic intrusion, southwest Greenland, which has experienced intense deformation and at least two episodes of metamorphism up to amphibolite facies. We report the first ever Re–Os isotope and platinum-group element concentration data for single chromite grains achieved by single grain dissolution and isotope dilution. Micrometric Os-, Ir-, Ru – sulphide and sulpharsenide inclusions in chromite occur in the massive chromitites; these accessory phases host a significant portion of the Os, Ir and Ru in these samples. The remaining Os-Ir-Ru budget, along with Pt and Pd, appears to be homogeneously distributed within the chromite, occurring either in the lattice itself, as evenly distributed sub-micrometric alloy inclusions in chromite (unresolvable using the techniques applied in this study) or a combination of both. Rhenium is hosted in silicates, as predicted by previous studies. Both single-grain and whole-rock Re–Os isotope systematics yield unfeasibly young Re–Os model ages due to their radiogenic 187Os/188Os compositions. This could result from country rock contamination of the original melt from which the Ujaragssuit nunât intrusion crystallised, and/or from disturbance of the Re–Os isotope systematics of these rocks during regional metamorphic events at ∼2.8 and/or ∼3.75Ga. We conclude that it is vital to establish a complete sample history before interpreting Re–Os isotope data and the validity of model ages, as the Re–Os system is sensitive to disturbance. It is feasible that in some cases single chromite grains or chromite mineral separates may offer a more robust and meaningful isotopic record than silicates, because Re is preferentially hosted in silicates relative to chromite. Alternatively, the 190Pt–186Os decay system, applied to whole-rock samples, may provide a more reliable chronometer than 187Re–187Os model ages in samples that have experienced crustal contamination and/or metamorphic disturbance.

Efficient and Accurate Identification of Platinum-Group Minerals by a Combination of Mineral Liberation and Electron Probe Microanalysis with a New Approach to the Offline Overlap Correction of Platinum-Group Element Concentrations.

Identification and accurate characterization of platinum-group minerals (PGMs) is usually a very cumbersome procedure due to their small grain size (typically below 10 µm) and inconspicuous appearance under reflected light. A novel strategy for finding PGMs and quantifying their composition was developed. It combines a mineral liberation analyzer (MLA), a point logging system, and electron probe microanalysis (EPMA). As a first step, the PGMs are identified using the MLA. Grains identified as PGMs are then marked and coordinates recorded and transferred to the EPMA. Case studies illustrate that the combination of MLA, point logging, and EPMA results in the identification of a significantly higher number of PGM grains than reflected light microscopy. Analysis of PGMs by EPMA requires considerable effort due to the often significant overlaps between the X-ray spectra of almost all platinum-group and associated elements. X-ray lines suitable for quantitative analysis need to be carefully selected. As peak overlaps cannot be avoided completely, an offline overlap correction based on weight proportions has been developed. Results obtained with the procedure proposed in this study attain acceptable totals and atomic proportions, indicating that the applied corrections are appropriate.

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.

Major, trace and platinum group element (PGE) geochemistry of Archean Iron Ore Group and Proterozoic Malangtoli metavolcanic rocks of Singhbhum Craton, Eastern India: Inferences on mantle melting and sulphur saturation history

The geological and metallogenic history of the Singhbhum Craton of eastern India is marked by several episodes of volcanism, plutonism, sedimentation and mineralization spanning from Paleoarchean to Mesoproterozoic in a dynamic tectonic milieu. Distinct signatures of this Archean-Proterozoic geodynamic process are preserved in discrete crustal provinces that constitute the Singhbhum Craton. Here we report new major, trace and PGE geochemical data from the ~3.4Ga Iron Ore Group (IOG) volcanic rocks of the Jamda-Koira basin, a part of the BIF-bearing volcano-sedimentary sequences of the Noamundi-Jamda-Koira iron ore basin in the western part of Singhbhum Granite (SBG), and ~2.25Ga metavolcanic rocks of Malangtoli. The IOG and Malangtoli volcanic rocks are porphyritic basalts and despite belonging to different ages, they exhibit similar mineralogical composition marked by clinopyroxene, plagioclase (present as both phenocryst and groundmass), opaques and volcanic glass (restricted to groundmass). The igneous mineralogy of these rocks has been overprinted by greenschist to lower amphibolite grade of metamorphism. The Malangtoli samples show low and high MgO compositional varieties. Immobile trace element compositions classify the IOG samples as andesite having a calc-alkaline composition, whereas the Malangtoli rocks correspond to basalt and andesite displaying a tholeiitic to calc-alkaline trend. The IOG basalts show low to moderate PGE contents marked by 26.23–68.35ppb of ΣPGE, whereas the Malangtoli basalts display a moderate to high concentration of PGE (ΣPGE=43.01–190.43ppb). The studied samples have relatively enriched ΣPPGE ranging from 24.1–63.3ppb (IOG) and 34–227.3ppb (Malangtoli) against 2.2–4.1ppb and 1.9–8.9ppb ΣIPGE contents respectively. PPGE/IPGE ratios for IOG and Malangtoli samples range from 7.7–17.6 and 4.8–59.9. HFSE, REE and PGE compositions suggest a low degree (<1 to 1%) of partial melting in the garnet lherzolite domain for the generation of IOG volcanic rocks. The parental magma of the Malangtoli basalts were generated by lower to higher degrees (3–<10%) of mantle melting at depths corresponding to spinel to garnet lherzolite regime. Trace element (Zr/Nb, Th/Ta, Th/Nb, Ni/Cu) and PGE (Pd/Ir, Pd/Pt, Cu/Pd, Ni/Pd, Cu/Ir) ratios corroborate a sulphide saturated and PGE depleted character of IOG volcanic rocks that underwent crustal assimilation. In contrast, the high MgO Malangtoli basalts exhibit sulphide undersaturated, PGE undepleted nature devoid of crustal contamination whereas the low MgO Malangtoli basalts are sulphide saturated, PGE depleted and crustally contaminated. The IOG volcanic rocks correspond to intraoceanic arc with polygenetic crustal signatures, and show affinity towards arc-generated calc-alkaline basalts. The low- and high MgO basalts of Malangtoli are affiliated to transitional arc to rift-controlled back arc tectonic setting in a basinal environment that developed proximal to an active convergent margin.

Potential Cretaceous-Paleogene boundary tsunami deposit in the intra-Tethyan Adriatic carbonate platform section of Hvar (Croatia)

An exceptional 47-m-thick succession of Maastrichtian to Paleocene inner-platform carbonates is exposed on the Dalmatian island of Hvar (Adriatic Sea, Croatia) in a seaside locality called Majerovica. The middle part of this succession is an ~5-m-thick intraformational massive deposit, which is underlain by well-bedded peritidal inner-platform limestones containing latest Maastrichtian rudists and shallow-water benthic foraminifera. This deposit includes a polygenic, matrix-supported carbonate breccia characterized by ripped-up platform limestone lithoclasts, up to boulder sized, and polygenic microbreccia in a muddy matrix. The microbreccia contains rare small intraclasts of pelagic mudstone containing terminal Maastrichtian planktonic foraminifera. The deposit is overlain in turn by mudstone containing a planktonic foraminiferal association belonging to the P0 and Pα zones of the basal Paleogene, and by shallow-water muddy limestones containing planktonic foraminifera belonging to the P1 zone. While facies suggest that the deposit was emplaced over the inner platform by a single large tsunami, the biostratigraphic assessment of this section and the presence of enhanced concentrations of platinum group elements, such as iridium, in the topmost part of the massive deposit lend support to the hypothesis that this tsunamite is related to the Chicxulub impact in Yucatan. This is potentially the first case of a tropical carbonate platform sedimentary succession recording the Cretaceous-Paleogene boundary event, which provides a new constraint for modeling both the western Tethyan paleogeography and the catastrophic aftermaths of the Chicxulub impact at the end of the Mesozoic Era.

Platinum group elements in the precipitation of the dry region of Xinjiang and factors affecting their deposition to land: the case of Changii City, China

Platinum group elements and their compounds are a class of incident allergens and some platinum group element compounds also have carcinogenic effects. They accumulate in city environment as a result of emissions from catalysts used for vehicle exhausts. In this study, sixteen precipitation samples were collected on the north campus of Changji University located in the center of Changji. They were analyzed for palladium (Pd), platinum (Pt), and rhodium (Rh) by inductively coupled plasma–mass spectrometry. The average concentrations of Pd, Pt, and Rh were found to be 26.73ng L-1 (range: 3.18–84.25ng L-1), 1.71ng L-1 (range: below the detection limit to 6.38ng L-1), and 1.49ng L-1 (range: below the detection limit to 3.53ng L-1), respectively. Pd deposition was most pronounced for single precipitation events, reaching 35.47ng m-2 (range: 1.27–101.10ng m-2), followed by Rh (max. 4.96ng m-2, range: 0–14.85ng m-2) and Pt (max. 1.38ng m-2, range: 0-7.66ng m-2). Both Pd and Pt were higher in winter than in other seasons in terms of their wet deposition amounts and their concentrations in precipitation, whereas Rh was lower in winter. Moreover, the results indicated that discharge from coal combustion in winter, the amount of precipitation, and the number of dry days before rainfall events all significantly affected the wet deposition amount and precipitation concentration of platinum group elements. Pd deposition flux was highest (reaching 5.47×103 ng m-2) corresponding to 18 and 16 times the Rh and Pt fluxes, respectively. Finally, vehicle exhaust catalyst emissions from motor vehicles were not the only source of atmospheric platinum group metals in the city environment; in fact, combustion of coal in winter was found to be the dominant contributor of Pt and Pd in the atmosphere.

Platinum Group Elements (PGE) geochemistry of komatiites and boninites from Dharwar Craton, India: Implications for mantle melting processes

High MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for platinum group elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. Though their higher PGE abundance is economically significant in mineral exploration studies, their lower concentrations are also valuable geochemical tools to evaluate petrogenetic processes. In this paper an attempt has been made to evaluate the PGE geochemistry of high MgO volcanic rocks from two greenstone belts of western and eastern Dharwar Craton and to discuss different mantle processes operative at diverse geodynamic settings during the Neoarchean time. The Bababudan greenstone belt of western and Gadwal greenstone belt of eastern Dharwar Cratons are dominantly composed of high MgO volcanic rocks which, based on distinct geochemical characteristics, have been identified as komatiites and boninites respectively. The Bababudan komatiites are essentially composed of olivine and clinopyroxene with rare plagioclase tending towards komatiitic basalts. The Gadwal boninites contain clinopyroxene, recrystallized hornblende with minor orthopyroxene, plagioclase and sulphide minerals. The Bababudan komatiites are Al-undepleted type (Al2O3/TiO2=23–59) with distinctly high MgO (27.4–35.8wt.%), Ni (509–1066ppm) and Cr (136–3036ppm) contents. These rocks have low ΣPGE (9–42ppb) contents with 0.2–2.4ppb Iridium (Ir), 0.2–1.4ppb Osmium (Os) and 0.4–4.4ppb Ruthenium (Ru) among Iridium group PGE (IPGE); and 1.4–16.2ppb Platinum (Pt), 2.8–19ppb Palladium (Pd) and 0.2–9.8ppb Rhodium (Rh) among Platinum group PGE (PPGE). The Gadwal boninites are high-Ca boninites with CaO/Al2O3 ratios varying between 0.8 and 1.0, with 12–24wt.% MgO, 821–1168ppm Ni and 2307–2765ppm Cr. They show higher concentration of total PGE (82–207ppb) with Pt concentration ranging from 13 to 19ppb, Pd between 65 and 180ppb and Rh in the range of 1.4–3ppb compared to the Bababudan komatiites. Ir, Os and Ru concentrations range from 0.6 to 2.2ppb, 0.2 to 0.6ppb and 1.4 to 2.6ppb respectively in IPGE. The PGE abundances in Bababudan komatiites were controlled by olivine fractionation whereas that in Gadwal boninites were influenced by fractionation of chromite and sulphides. The Al-undepleted Bababudan komatiites are characterized by low CaO/Al2O3, (Gd/Yb)N, (La/Yb)N, with positive Zr, Hf, Ti anomalies and high Cu/Pd, Pd/Ir ratios at low Pd concentrations suggesting the derivation of parent magma by high degrees (>30%) partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. The komatiites are geochemically analogous to Al-undepleted Munro type komatiites and their PGE compositions are consistent with Alexo and Gorgona komatiites. The S-undersaturated character of Bababudan komatiites is attributed to decompression and assimilation of lower crustal materials during magma ascent and emplacement. In contrast, the higher Al2O3/TiO2, lower (Gd/Yb)N, for Gadwal boninites in combination with negative Nb, Zr, Hf, Ti anomalies and lower Cu/Pd at relatively higher Pd/Ir and Pd concentrations reflect high degree melting of refractory mantle wedge under hydrous conditions in an intraoceanic subduction zone setting. Higher Pd/Ir ratios and S-undersaturation of these boninites conform to influx of fluids derived by dehydration of subducted slab resulting into high fluid pressure and metasomatism of mantle wedge.

A Hydrothermal Ni-As-PGE Geochemical Halo Around the Miitel Komatiite-Hosted Nickel Sulfide Deposit, Yilgarn Craton, Western Australia

The remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes, the recognition of which could potentially enlarge the detectable footprint of this ore type. The Miitel komatiite-hosted nickel sulfide deposit in Western Australia was used as a case study to investigate the nature and 3-D geometry of the geochemical halo created by the remobilization of base metals, gold, and platinum group elements (PGE) into the rocks surrounding the mineralization. At Miitel, anomalous metal enrichment is found in the country rocks surrounding the massive sulfides, up to 250 m away from the ore. This enrichment, detected using portable X-ray fluorescence (pXRF) and backed up by laboratory analyses, occurs in the Mount Edwards footwall basalt within decimeters of the contact with the overlying Widgiemooltha komatiites. It is associated with the presence of nickel arsenides. Gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets, and are interpreted to form during the circulation of arsenic-rich hydrothermal fluids. Results of lead fire assay analyses and in situ laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analyses indicate high PGE concentrations (Pd and Pt) and minor gold associated with the observed nickel and arsenic enrichment. Results from a larger, regional-scale study, combined with this PGE enrichment, suggest that the massive nickel sulfides from the Miitel ore are the source of the remobilized nickel in the country rocks. The presence of Pd- and Pt-enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes.

Noble metals in rocks and ores of Maysko-Lebed ore field (Mountain Shoriya)

First the authors determined platinum and palladium in the ores and rocks of Maysk-Lebed ore deposit via stripping voltammetry. Based on research data the increased platinoid (platinum group elements) content was identified both in the source host rocks and in metasomatically altered ones in ores.

Concentration of Particulate Platinum-Group Minerals during Magma Emplacement; a Case Study from the Merensky Reef, Bushveld Complex

AbstractThe petrology, mineralogy and geochemistry of a section of the Merensky Reef at Bafokeng Rasimone Platinum Mine (BRPM) are described. A model for the formation of platinum-group minerals (PGM), sulphide and chromitite is proposed that explains the stratigraphic relationships observed in the Merensky Reef, both at BRPM and at other locations in the Bushveld Complex. To achieve this it is necessary to understand platinum-group element (PGE) behaviour in naturally occurring mafic systems and for this reason comparisons are drawn from core TN207 through the Platreef at Tweefontein. The common link between the Platreef and Merensky Reef is the presence of unusually high concentrations of As, Sb, Bi and Te that promote the crystallisation of semi-metal bearing PGM from sulphide liquids. Under conditions of increasing semi-metal contamination, Pt is the first PGE to be extracted from a sulphide liquid followed by Rh, Ru, Os and Ir. While some Pd is released to form Pd-PGM much of it remains within the Ni-rich sulphide phase that crystallizes to form pentlandite. A critical aspect is the timing of their introduction into the magmatic system. For the Merensky magmas, contamination occurred predominantly within a staging chamber owing to wall-rock interaction with Transvaal sediments. This led to the formation of sulphide liquids that captured PGE and, ultimately, the crystallization of Pt- and Ru-PGM. The extreme enrichment in PGE and the high Pt/Pd ratios in the Merensky chromitites are attributed to density-driven concentration of PGM transported by magmas displaced from a staging chamber. Emplacement of these magmas into the Bushveld Complex resulted in thermo-mechanical erosion of the floor and deposition of chromites + sulphides + PGM. In places, these assemblages collected in sedimentary-like scour channels. In the Platreef, contamination occurred largely after magma emplacement owing to interaction with the local Transvaal sediments. As a result, mechanical separation of PGM did not occur and most PGM remain spatially associated with their original sulphide hosts.The Merensky Reef is a prime example of highly efficient PGE concentration resulting from mechanical processes, whereas the Platreef is a prime example of highly efficient PGE removal from sulphide liquids in response to extreme contamination by semi-metals.

Determination of Platinum Group Elements in Sulfide-Containing Minerals by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry with Synthetic Calibration Standards

This paper develops sulfide calibration standards for quantitative determination of platinum group elements in natural sulfide-containing minerals by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). A series of sulfide calibration standards containing different concentrations of platinum group elements were prepared using a home-made high temperature furnace. Homogeneity of element distribution expressed as the RSD of signal intensity were less than 10%. The effective concentrations of platinum group elements in synthetic calibration standards were obtained both by pneumatic nebulization ICP-MS and by LA-ICP-MS. Relative percentage differences of these results obtained by two methods were mainly less than 11%. The synthetic calibration standards were then employed as calibrators for quantitative determination of platinum group elements in two natural chalcopyrite samples. The valid results demonstrated that our synthetic standards could be considered for quantitative microanalysis of platinum group elements in natural sulfide-containing minerals.