Platinum-group Element Geochemistry Research Articles

Platinum-group elements and geochemical characteristics of the Permian continental flood basalts in the Tarim Basin, northwest China: Implications for the evolution of the Tarim Large Igneous Province

Permian continental flood basalts (CFBs), as a main component for Tarim Large Igneous Province (TLIP), are widely distributed in the western and central parts of the Tarim Basin. Here we present a systematic study of platinum-group elements (PGEs) combined with conventional major, trace and Sr–Nd isotopic geochemistry to characterize the Tarim CFBs from the Yingan section in the Keping area, west of the Tarim Basin. The basaltic units with a total thickness of ca. 400m in the section can be divided into two basaltic sequences: the lower Kupukuziman and the upper Kaipaizileike sequences. Both of the sequences display extremely low PGE concentrations, with Os, Ir, Ru, Rh, Pt and Pd contents of 0.014–0.106, 0.007–0.072, 0.035–0.253, 0.011–0.078, 0.043–0.149 and 0.026–0.124ppb, respectively. The Keping basalts exhibit a marked increase in Cu/Pd ratios (>105) albeit with a narrow range of lower Pd/Ir ratios (<50), different from the PGE-undepleted basalts of the Siberian Traps, Emeishan Large Igneous Province (ELIP), East Greenland CFBs and Deccan Traps. Furthermore, the Tarim CFBs in Keping and elsewhere in the basin have very low Cu/Zr ratios (<0.5 commonly), indicating that their parent magmas were significantly depleted in chalcophile elements. The Tarim CFB magmas are estimated to have formed by <5% partial melting, and were probably S-saturated during partial melting, leaving residual sulfide in the mantle source. The differences in Th/Yb and (Th/Nb)N ratios between the Kupukuziman and Kaipaizileike sequences suggest that the Keping basalts incorporated variable degrees of assimilated crustal components from the Precambrian basement in the Tarim Basin. Modeling indicates that the earlier erupted Kupukuziman lavas witnessed relatively larger extent of assimilation than the later Kaipaizileike lavas. Nevertheless, the crustal assimilation did not trigger sulfur saturation and sulfide segregation for the basaltic magma in the crust. Furthermore, a magma mixing process between the relatively primitive and more evolved residual magmas of the same lineage during magma chamber replenishment is suggested according to the PGEs and Sr–Nd isotope variations in the Kaipaizileike sequence. This process might have also induced limited sulfur saturation and sulfide segregation in the late stage of the magmatic evolution. A comparison of the chalcophile element characteristics between the basalts in the Tarim Basin and the mafic–ultramafic rocks in the Eastern Tianshan and Beishan Rift areas indicates that the degree of partial melting plays an important role on the PGE geochemistry and potential PGE–(Ni–Cu) ore deposits in the TLIP.

Platinum‐group Element Geochemistry of Magnetite from Porphyry‐Cu‐Mo Deposits and their Host Rocks (Siberia, Russia)

Abstract:The concentrations of platinum‐group elements (PGE) have been analyzed in primary magmatic magnetite samples from the Zhireken, Shakhtama and Aksug porphyry Cu‐Mo deposits (Siberia, Russia) by laser ablation‐inductively coupled plasma mass spectrometry to determine the range of PGE contents in magnetites and to check whether magnetite from two main rock suites (barren plutonic suite and mineralized porphyry suite) has distinct PGE composition. The results presented here indicate that magnetites are enriched in PGE relative to whole‐rocks. Comparison of ore‐related porphyry and barren plutonic suites shows that magnetite exhibit relatively similar PGE distribution patterns in both suites. Variations in Rh and Ru contents were controlled by the oxygen fugacity during magma crystallization.

Ruthenium in komatiitic chromite

The distinction between Ru in solid solution and Ru-bearing inclusions is essential for the predictive modeling of platinum-group element (PGE) geochemistry in applications such as the lithogeochemical exploration for magmatic sulfide deposits in komatiites. This study investigates the role of chromite in the fractionation of Ru in ultramafic melts by analyzing chromite grains from sulfide-undersaturated komatiites and a komatiitic basalt from the Yilgarn Craton in Western Australia. In situ analysis using laser ablation ICP-MS yields uniform Ru concentrations in chromites both within-grain and on a sample scale, with concentrations between 220 and 540 ppb. All other platinum-group elements are below the detection limit of the laser ablation ICP-MS analysis. Carius tube digestion isotope dilution ICP-MS analysis of chromite concentrates confirms the accuracy of the in-situ method. Time resolved laser ablation ICP-MS analyses have identified the presence of sub-micron Ir-bearing inclusions in some chromite grains from the komatiitic basalt. However, Ru-bearing inclusions have not been recognized in the analyzed chromites and this combined with the in situ data suggests that Ru exists in solid solution in the crystal lattice of chromite. These results show that chromite can control the fractionation and concentration of Ru in ultramafic systems.

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.

High-Cr and high-Al chromitites from the Sagua de Tánamo district, Mayarí-Cristal ophiolitic massif (eastern Cuba): Constraints on their origin from mineralogy and geochemistry of chromian spinel and platinum-group elements

Several chromitite bodies of variable sizes are hosted in dunite-harzburgite of the small (<3km) mining district of Sagua de Tánamo, in the Mayarí-Cristal ophiolitic massif, eastern of Cuba. The chromian spinel in these bodies displays a large range in Cr# (Cr/Cr + Al) atomic ratio from 0.74 to 0.45. In high-Cr chromitites, Cr# and Ti contents increase from harzburgite to dunite, to chromitite. A roughly opposite trend of variation in Cr# is observed in high-Al chromitites, although in this case Ti distributes randomly. The differences in the type of chromian spinel (i.e., high-Cr or high-Al) in the chromitite coincide with a different behavior of the platinum-group elements (PGE). Whereas high-Cr chromitites are rich in PGE and contain abundant grains of platinum-group minerals (PGM), high-Al chromitites are systematically poor in PGE and in PGM. The calculated melts in equilibrium with chromian spinel of high-Cr chromitite are island arc thoeliites (IAT) with boninitic affinity whereas those in equilibrium with chromian spinel of high-Al chromitites are back-arc basin basalts (BABB). The formation of high-Cr chromitites is interpreted as a result of the extensive reaction of harzburgite with migrating island arc tholeiite melts of boninitic affinity. Melt–rock reaction produces boninitic melts with variable composition and porous dunitic channels in which the mixing/mingling of melts promotes crystallization of mononomineralic high-Cr chromian spinel. In contrast, high-Al chromitites formed by the mixing/mingling of BABB melts within conduits not in equilibrium with dunite. Percolation of primitive BABB melts through pre-existing dunite dissolved olivine, producing melt conduits in which BABB melts mixed and formed high-Al chromian spinel. The higher PGE and PGM in high-Cr chromitites is not only a function of the degree of partial melting but is linked to interaction between the migrating melt and mantle harzburgite during the formation of the chromitite. The coexistence of both types of chromitites in one small mining district reflects the temporal and/or spatial variations of separate melt intrusions emplaced in different subarc mantle domains during the opening of a back-arc basin in a supra-subduction zone environment.

Geochemistry of platinum group elements in marine oil shale from the Changshe Mountain area (China): Implications for modes of occurrence and origins

Abstract The Shengli River–Changshe Mountain oil shale zone, located in the North Qiangtang depression, northern Tibet plateau, represents a potentially largest marine oil shale resource in China. Eighteen samples, including oil shale and micritic limestone, were collected from the Changshe Mountain oil shale to determine the concentrations, the modes of occurrence, and origins of platinum group elements (PGEs) in marine oil shale. The oil shale samples from the Changshe Mountain area exhibit low total PGE contents ranging from 3.196 to 8.071 ng/g with a weighted mean value of 5.383 ng/g, while the micritic limestone samples from the Changshe Mountain area exhibit a little lower ranging from 0.471 to 5.669 ng/g. PGEs in oil shale samples are characterized by high contents in Pd (3.23 ng/g), Pt (1.60 ng/g), and Os (0.32 ng/g), as compared to Rh (0.083 ng/g), Ru (0.112 ng/g), and Ir (0.043 ng/g). The individual PGEs of oil shale samples from the Changshe Mountain area exhibit various modes of occurrence. Ru and Ir are mainly controlled by clay minerals and partly present as sulfide. Rh is mainly associated with sulfide and possibly present as clay minerals. Pt is associated with clay minerals and sulfide, and Pd is controlled by organic matter and pyrite, while Os is mainly present in organic matter. Three possible sources of PGEs were identified in oil shale seams in the Changshe Mountain area. Ru and Ir are mainly terrigenous inputs. Rh and Pt are derived from mixed sources (seawater and terrigemnous supply), while Pd and Os are mainly derived from seawater. The terrestrial source of the oil shale seams in the Changshe Mountain oil shale was the Suowa Formation limestone and the Nadi Kangri felsic volcanic rocks.

Geochemistry and mineralogy of platinum-group elements (PGE) in chromites from Centralnoye I, Polar Urals, Russia

The Polar Urals region of northern Russia is well known for large chromium (Cr)-bearing massifs with major chromite orebodies, including the Centralnoye I deposit in the Ray-Iz ultramafic massif of the Ural ophiolite belt. New data on platinum (Pt)-group elements (PGE), geochemistry and mineralogy of the host dunite shows that the deposit has anomalous iridium (Ir) values. These values indicate the predominance of ruthenium–osmium–iridium (Ru–Os–Ir)-bearing phases among the platinum-group mineral (PGM) assemblage that is typical of mantle-hosted chromite ores. Low Pt values in chromites and increased Pt values in host dunites might reflect the presence of cumulus PGM grains. The most abundant PGM found in the chromite is erlichmanite (up to 15 μm). Less common are cuproiridsite (up to 5 μm), irarsite (up to 4–5 μm), and laurite (up to 4 μm). The predominant sulfide is heazlewoodite, in intergrowth with Ni–Fe alloys, sporadically with pentlandite, and rarely with pure nickel. Based on the average PGE values and estimated Cr-ore resources, the Centralnoye I deposit can be considered as an important resource of PGE.

Platinum Group Element Geochemistry of Mineralized and Nonmineralized Komatiites and Basalts

Platinum-group elements (PGE) are strongly chalcophile and are therefore potentially sensitive indicators of processes involving segregation and accumulation of sulfide melts from silicate magmas. Over 500 new high-precision PGE data for komatiites and komatiitic basalts, spanning a wide range of emplacement and crystallization histories, have been combined with literature data on PGE in magmatic systems from other barren and variably mineralized environments, to test the effectiveness of PGE geochemistry as an indicator of processes forming magmatic sulfide ores. Results show that PGE depletion in S-poor komatiites and komatiite basalts spatially and genetically associated with Fe-Ni-Cu sulfide mineralization is not as common or as strong as expected: samples displaying orders of magnitude depletion in PGE represent less than 10 percent of any given data set from any location. The data confirm that most, if not all, komatiites were sulfide undersaturated when they separated from their sources and remained undersaturated on eruption. Some ore-bearing komatiite sequences display no detectable depletion, and the degree of PGE depletion is commonly less than expected based on modeling using experimentally determined partition coefficients. PGE enrichment is more common and spatially widespread than PGE depletion, commonly representing a better approach to lithogeochemical exploration, even where samples containing anomalous Ni or S contents are absent. PGE enrichment and/or depletion associated with sulfide enrichment and/or segregation can be discriminated from secondary hydrothermal and/or metamorphic processes by covariance of all PGE, with the exceptions in some cases of Ir, Ru, and Os whose abundances may be complicated by the presence of saturation in and accumulation of Ir-Os-rich liquidus phases. Variations attributable to other magmatic processes, such as olivine accumulation and fractionation, can be distinguished by variations in PGE/Ti ratios and strong correlations between Pt/Ti, Pd/Ti, and Rh/Ti ratios in mineralized systems. The degree of PGE depletion is consistent with the relatively low R factor estimated for many komatiite-hosted deposits, which fall in the range of 20 to 200 for Thompson, 100 to 500 for Kambalda, and 300 to 1,100 for Raglan, implying that the volume of silicate magma that interacted with sulfide liquid was relatively small. This is also consistent with the relatively small proportion of komatiites displaying PGE depletion within ore-bearing flow sequences, as only magmas in ore-forming channels or conduits will interact with sulfides. False negatives, i.e., mineralized komatiite sequences with no detectable PGE depletion, are associated with systems characterized by high R factors. Basalts and komatiitic basalts show more complex patterns of variation, which can broadly be divided into three categories: (1) systematic PGE depletion over a range of Mg numbers, as in MORB suites, consistent with retention of sulfide in the mantle during partial melting; (2) increasing PGE depletion with decreasing Mg numbers in large igneous province (LIP)-associated basalts, interpreted to reflect attainment of sulfide saturation during fractionation with subsequent cotectic olivine-sulfide segregation; and (3) variable PGE depletion over a range of Mg numbers in komatiitic basalts (e.g., Raglan) interpreted to reflect ore-forming sulfide incorporation and segregation processes. The results of this study confirm that the PGE geochemistry of komatiites and basalts is a powerful indicator of sulfide saturation and ore-forming processes, but that it must be interpreted with the context of physical volcanologic and fluid dynamic processes.

PGE geochemistry of low-Ti high-Mg siliceous mafic rocks within the Archaean Central Indian Bastar Craton: implications for magma fractionation

Boninite-norite (BN) suites emplaced in an intracratonic setting in Archaean Cratons, are reported from many parts of the world. Such high-Mg low-Ti siliceous rocks are emplaced during Neoarchaean-Paleoproterozoic. The Archaean central Indian Bastar Craton also contains such a boninite-norite suite, which occurs in the form of dykes and volcanics. The spatial and temporal correlation of these high-Mg low-Ti siliceous rocks with similar rocks occurring around the northern Bastar and Dharwar Cratons probably represent a Bastar-Dharwar Large Igneous Province during the Neoarchaean-Paleoproterozoic. Platinum group element (PGE) abundances in these rocks provide constraints on their geochemical evolution during the Neoarchaean-Paleoproterozoic. The PGE geochemistry of the boninite-norite suite from the southern part of the central Indian Bastar Craton is presented to understand their behaviour during magma fractionation. In primitive mantle-normalized plots all samples have similar PGE fractionated patterns that are enriched in Pd, Pt and Rh relative to Ru. The Pd/Ru ratios for eight samples range from 2.0 to 7.0 which is higher than primitive mantle (primitive mantle Pd/Ru ≈1.2). The Pd/Pt ratios range between 0.2-2.5 with an average value of 0.7 which is near chondritic (primitive mantle Pd/Pt ≈0.5). PGE variations in these rocks together with those of major and other trace elements are consistent with a model involving olivine fractionation along with chromite as a cotectic phase. The Pt fractionation from Pd and Rh is controlled by both olivine and chromite crystallization at an early stage during high temperature crystal fractionation when the Pt was strongly compatible and Pd and Rh were incompatible. Strong negative correlations of the S content with iron and TiO 2 plus lithophile element contents of the rock suggest a decrease of the S solubility in the parental high-Mg magma and separation of an immiscible sulfide liquid with decreasing temperature. Palladium plus other available chalcophile elements (e.g., Re, Au, Ag) have been fractionated in this immiscible sulfide liquid after considerable olivine fractionation of the magma.

Base metal (BM) and platinum-group elements (PGE) mineralogy and geochemistry of the elmaslar chromite deposit (Denizli, sw turkey): implications for a local BM and PGE enrichment

The Elmaslar Chromite Deposit (ECD) in Denizli (SW Turkey) consists of chromitite enveloped by highly refractory replacive dunite within a depleted harzburgite. The composition of chromian spinel is Cr-rich suggesting derivation from boninitic melt-peridotite interaction beneath a supra-subduction zone. The chondrite normalized PGE patterns show predominance of Ir-group (IPGE) over Pd-group PGE (PPGE) that is consistent with ophiolitic podiform chromitites in general. The primary PGM paragenesis of ECD mainly consists of laurite-erlichmanite series with minor Pd bearing phases. Dominance of Osrich laurite over the Os-poor ones suggests initially high fS2 during crystallization of chromitite. It also indicates a slow cooling rate of the chromitite parent melt. Coexistence of irarsite with erlichmanite suggests increase in As activity during late stages of chromitite crystallization. One peculiar contact zone sample with disseminated-massive chromitite enveloped by dunite shows unusually high PPGE content and positive-sloped chondrite normalized PGE pattern. This enrichment is attributed to a two stage process: (i) primary and local sulfur saturation, (ii) followed by secondary hydrothermal alteration causing smallscale remobilization and precipitation of PGE. No mode of occurrence is suggested for the primary and local sulfur saturation stage, because there is no direct field evidence on the geological setting of the sample. However, it is clear that the effects of this event were limited and not deposit wide. The secondary enrichment of PGE is mainly attributed to introduction of fluids during serpentinization. The amphibole-pyroxene-spinel-silica-carbonate rocks found in the stockpiles of ECD are remarkable. The role of these rocks, in the secondary PGE enrichment in ECD stays to be questioned. They mark further low-temperature CO2-metasomatism in ECD and provide promising indicators for investigating secondary PGE occurrences in chromitites.

Platinum-group elements geochemistry in podiform chromitites and associated peridotites of the Mawat ophiolite, northeastern Iraq

Small lensoidal bodies of massive and disseminated chromitites have been examined in association with ultramafic rocks of the Mawat ophiolite complex (MOC), Iraqi Zagros Thrust Zone, northeastern Iraq. The chromitites are surrounded by dunite envelopes of variable thickness, exhibiting transitional boundaries to harzburgitic host rocks. The primary chromite composition exhibits high Cr varieties; the average Cr# of chromite is 0.73, and have <0.2% TiO 2 content, which may reflect the crystallization of chromite from boninitic magma. Partial melting in the upper mantle and assimilation of wall rocks by primitive melt might have played an essential role in the chromitite formation in the studied area. Unusually high platinum-group elements contents of up to 1094 ppb have been reported for the first time in these chromitites pods. Chondrite normalized patterns of platinum-group elements are typical for ophiolitic chromitite, with enrichment of the IPGE, and depletion in PPGE indicating a high degree of partial melting of the mantle source. The PGE diversity between different chromitite pods in the MOC may be attributed to the differences in chemistry of the magma involving in the chromitite formation. The mineral chemistry data and PGE geochemistry, indicates that the Mawat ophiolite was generated from an arc-related magma above a supra-subduction zone setting.

Geochemistry of Platinum Group and Rare Earth Elements of the Polymetallic Layer in the Lower Cambrian, Weng'an, Guizhou Province

Abstract:The black shales of the Lower Cambrian Niutitang Formation in Weng'an, on the Yangtze platform of south China, contain voluminous polymetallic sulfide deposits. A comprehensive geochemical investigation of trace, rare earth, and platinum group elements (PGE) has been undertaken in order to discuss its ore genesis and correlation with the tectono‐depositional setting. The ore‐bearing layers enrich molybdenum (Mo), nickel (Ni), vanadium (V), lead (Pb), strontium (Sr), barium (Ba), uranium (U), arsenic (As), and rare earth elements (REE) in abundance. High uranium/thorium (U/Th) ratios (U/Th&gt;1) indicated that mineralization was mainly influenced by the hydrothermal process. The δU value was above 1.9, showing a reducing sedimentary condition. The REE patterns showed high enrichment in light rare earth elements (LREE) (heavy rare earth elements (HREE) (LREE/HREE=5–17), slightly negative europium (Eu) and cerium (Ce) anomalies (δEu=0.81–0.93), and positive Ce anomalies (δCe=0.76–1.12). PGE abundance was characterized by the PGE‐type distribution patterns, enriching platinum (Pt), palladium (Pd), ruthenium (Ru) and osmium (Os). The Pt/Pd ratio was 0.8, which is close to the ratios of seawater and ultramafic rocks. All of these geochemical features suggest that the mineralization was triggered by hydrothermal activity in an extensional setting in the context of break‐up of the Rodinian supercontinent.

Platinum-group element geochemistry of the continental flood basalts in the central Emeisihan Large Igneous Province, SW China

In the central zone of the Emeishan Large Igneous Province (ELIP), southwestern China, the Permian Emeishan continental flood basalts (ECFB) can be divided into high- and low-Ti basalts and alkaline basalts. At the southern margins of the central ELIP, the low- and high-Ti basalts comprise the lower and upper parts of the ECFB sequences, respectively; the alkaline basalts occur in the lower part of the ECFB sequence in a few locations. The high-Ti basalts are high in Pd (4-16 ppb) and Pt (7–18 ppb), but have large variations in Ir (0.04–0.88 ppb). The alkaline basalts have low TiO 2 contents (1.8–2.15 wt.%) and are depleted in PGE (< 1 ppb Pt and Pd with exception of one sample). The low-Ti basalts may be platinum-group element (PGE) depleted or undepleted. The PGE-depleted low-Ti basalts and alkaline basalts are marked by < 2 ppb Pd, and < 3 ppb Pt, whereas the PGE-undepleted low-Ti basalts contain 5–18 ppb Pd and have large variations in Pt (3–19 ppb) and Ir (0.02–0.36 ppb). For the PGE-depleted low-Ti basalts and alkaline basalts, Pd and Pt decrease significantly with decreasing of Ir. Very high Cu/Pd ratios (> 30,000) and very low Pt/Y ratios (230 to as low as 6) strongly suggest that the PGE-depletion in the low-Ti basalts and alkaline basalts is related to sulfide removal. The positive relationship between Pt and Pd depletion and increases in Zr/Nb and Th/Nb ratios indicate that crustal contamination played a role in driving the low-Ti basaltic magmas to S-saturation. In contrast, for the PGE-undepleted high- and low-Ti basalts, Pd decreases slightly with decreasing MgO and Ir, and Pd/Cr ratios increase with decreasing Pt/Y, indicating that fractionation occurred under S-undersaturated conditions. Some PGE-undepleted low-Ti basalts have very high Pd/Pt ratios (up to 6, much higher than that of the primitive mantle ~ 0.6) and extremely low Ir concentrations (< 0.03 ppb), suggesting that the magmas experienced fractionation of olivine, chromite, pyroxene and PGE minerals rich in Os, Ir, Ru, and Pt. The remaining high-Ti basalts have moderate Pd/Pt ratios (< 2.3) and very high Ir concentrations, up to 0.88 ppb, indicating that they possibly contain chromite and PGE minerals rich in Os, Ir, and Ru.

PASAVAITE, Pd3Pb2Te2, A NEW PLATINUM-GROUP MINERAL SPECIES FROM THE NORIL'SK-TALNAKH Ni-Cu CAMP, RUSSIA

Pasavaite, Pd 3 Pb 2 Te 2 , is a new palladium-bearing mineral species discovered at the Talnakh deposit, Noril’sk–Talnakh Ni–Cu camp, Taimyr Autonomous District, Russia. It was observed in two polished sections, where it occurs as subhedral grains not exceeding 20 μm in diameter embedded in polarite, and accompanied by unknown Pd–Pb–Bi–Te phases and sperrylite or intergrown with Au–Ag phases. The other associated minerals are: taimyrite, froodite, galena, and the chalcopyrite + pentlandite matrix. Synthetic pasavaite is megascopically grey in color, has a metallic luster and a grey streak. The mineral is brittle, with a weak {001} cleavage. Values of VHN 25 (15 indentations on five grains of synthetic Pd 3 Pb 2 Te 2 ) range between 173 and 281, for a mean value of 233 kg/mm 2 , which corresponds to a Mohs hardness of approximately 2. In plane-polarized light, pasavaite is pale pink with brownish tinge, has strong bireflectance, pleochroic from brownish to light pink, distinctly to strongly anisotropic, and exhibits no internal reflections. Reflectance values of synthetic pasavaite in air (R max , R min ) are: 49.9, 42.4 at 470 nm, 51.8, 44.6 at 546 nm, 52.2, 45.7 at 589 nm, and 52.8, 46.9 at 650 nm. The average result of four electron-microprobe analyses of pasavaite is: Pd 31.51, Pb 41.54, Bi 0.19, and Te 25.75, total 98.99 wt.%, corresponding to an empirical formula (based on 7 atoms) Pd 2.96 (Pb 2.01 Bi 0.01 )Te 2.02 ; synthetic pasavaite (average of seven analyses) gave: Pd 32.17, Pb 41.78, and Te 25.93, total 99.88 wt.%, corresponding to Pd 2.99 Pb 2.00 Te 2.01 . The mineral is orthorhombic, space group Pmmn , with a 8.599(1), b 5.9381(6), c 6.3173(8) A, V 322.6(1) A 3 , and Z = 2. The crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic Pd 3 Pb 2 Te 2 . Pasavaite is structurally related to shandite and parkerite. The strongest lines in the X-ray powder-diffraction pattern of synthetic pasavaite [ d in A(I)( hkl )] are: 6.3152(34)(001), 3.1572(33)(002), 3.0495(100)(211), 2.5456(63) (202), 2.4424(34)(220), 2.2786(42)(221), 2.1637(71)(022), 2.1496(30)(400), 1.8906(42)(203), and 1.5248(31)(422). The mineral is named after Jan Pasava, in recognition of his important contributions to the geochemistry of platinum-group elements in anoxic environments and related ore deposits.

Platinum-Group Element Geochemistry of the Devonian Arc Picrites from the North Junggar Terrane, NW China: Implications for Petrogenesis

Platinum-Group Element Geochemistry of the Devonian Arc Picrites from the North Junggar Terrane, NW China: Implications for Petrogenesis

HRTEM-AEM-HAADF-STEM study of platinum-group elements within a mantle-derived Cr spinel (Lherz; North-Eastern Pyrenees, France)

As platinum group elements (PGE) like Ru, Os, Ir and Rh have appropriate charge and ionic radii to substitute the major cations in the spinel structure, Cr-spinel is one of the potential minerals that can fractionate such elements during the partial melting and fractional crystallization. However, whether PGE occur in solid solution or are physically collected as poly-metallic clusters of atoms on spinel grain boundaries is a central issue for PGE geochemistry. A PGE-bearing Cr-spinel separated from one harzburgite sample from Lherz has been analyzed by Transmission, Analytical and Scanning-Transmission Electron Microscopy (TEM-AEM-STEM). Our analyses did not reveal any exsolution/cluster feature at the nanometric scale, except for Ir, which is concentrated as Ir-bearing lamellar rutile, about 100 nm width. Rutile appears to have enhanced unmixing of Ir from the spinel structure. The D Cr spinel/silicate melt partition coefficients estimated from the Lherz Cr-spinel (Ir = 32 ± 0.7; Os = 28 ± 6; Ru = 12.5 ± 2; Pt = 1.5 ± 0.2; Pd = 0.2 ± 0.05) may be appropriate for modelling the behavior of PGE in sulfur-undersaturated melting processes of the shallow upper mantle, while being lower than those derived from experiments and natural ultramafic melts.

PGE geochemistry and Re–Os dating of massive sulfide ores from the Baimazhai Cu–Ni deposit, Yunnan province, China

The Baimazhai deposit in Yunnan Province is one of the largest Cu–Ni sulfide deposits hosted in mafic–ultramafic intrusions in China. Concentrations of platinum-group elements (PGE) in massive sulfide ores and host rocks from Baimazhai were determined by using inductively coupled plasma mass spectrometry (ICP-MS) following nickel sulfide fire assay pre-concentration. The results show that the total PGE (ΣPGE) are quite low, decreasing gradually from central massive ores (78.2–556 ppb) to olivine pyroxenite (0.472–67.0 ppb), gabbro (0.847 ppb) and, websterite (0.76–0.809). The intruded lamprophyre dykes also show low ΣPGE (2.98–4.07 ppb). The ΣPGE exhibit obvious positive correlations with Au, Ni and Cu contents. Primitive mantle normalized PGE patterns of the massive Cu–Ni ores are of the Pt–Pd type with relatively steep and trough-like patterns, which are similar to those of the host rocks. In addition, the Pt/Pd and Cu/Pd ratios of the massive sulfide ores are similar to those of olivine pyroxenite, gabbro and websterite. These characteristics suggest that sulfides in the massive ores are of magmatic origin, co-genetic with their host rocks. The relatively high Pt/Pd ratios of the Baimazhai massive sulfide ores (averaging 0.83) and their host rocks imply that the Baimazhai sulfides formed in a single sulfide saturation event, but not through multiple sulfide injections. High Ir contents (0.77–5.52 ppb, averaging 2.35 ppb) and dramatically variable Pd/Ir ratios (4.76–296, averaging 138) of the massive sulfide ores suggest that the Baimazhai sulfide ores might have suffered significant late stage hydrothermal alteration. The Baimazhai massive sulfide ores yield a Re–Os isochron age of 259 ± 20 Ma (MSWD = 0.025), which is the same as the major eruption stage of the Emeishan large igneous province and the Baimazhai intrusion, further supporting their magmatic origin. The initial 187Os/ 188Os value of 0.456 ± 0.026 indicates that crustal contamination has played an important role during mineralization. The proportion of crust-derived Os is calculated to be more than 30%.

Platinum-Group Element Geochemistry of the Devonian Arc Picrites from the North Junggar Terrane, NW China: Implications for Petrogenesis

Picritic (MgO>12%) and ankaramitic Devonian arc lavas in the north Junggar terrane provide a rare opportu- nity to evaluate the platinum group elements (PGE) of subduction-generated melts. The picrites are highly porphyritic with olivine, and rare clinopyroxene and chrome spinel (Cr# (Cr/Cr+Al) =0.63-0.86) phenocrysts, embedded in a matrix of groundmass plagioclase, clinopyroxene, and magnetite, while clinopyroxene phenocrysts are dominant in the an- karamite. Minor metal sulfides (pyrrhotite, chalcopyrite, and pentlandite) are present. Despite their highly magnesian na- ture, the lavas contain relatively low concentrations of PGE's (0.2-0.6 ppb Ir, 5.6-7.1 ppb Pt, 3.1-7.0 ppb Pd) compared with other high-MgO lavas such as kimberlites, komatiites and plume-related picrites, but higher than those from other arc picrites, such as the Lesser Antilles arc in the Atlantic. The primitive mantle normalized PGE patterns are characterized by enrichment of the palladium group elements (PPGE) relative to iridium group elements (IPGE), which resembles those from the Lesser Antilles and Izu-Bonin arcs. Thus, it would appear that melts generated in a subduction regime may have common characteristics. Such low PGE concentrations are probably related to relatively low degrees of partial mantle melting. The picritic lavas have high Os/Ir ratios (>1), possibly reflecting the involvement of pelagic sediments. A Pd/Ir versus Ni/Cu diagram implies that both picrites and ankaramites were not primary magmas, but high-MgO basalts with excess olivine or clinopyroxene respectively, whereas a Pd versus Cu diagram suggests S-undersaturated primary magma. The PPGE may have slightly enhanced transport efficiency in slab-derived fluids compared to IPGE.

Mineralogy and geochemistry of platinum-group elements in the Aguablanca Ni-Cu deposit (SW Spain)

The Aguablanca Ni-Cu-(PGE) magmatic sulphide deposit is associated with a magmatic breccia located in the northern part of the Aguablanca gabbro (SW, Iberia). Three types of ores are present: semi-massive, disseminated, and chalcopyrite-rich veined ore. The principal ore minerals are pyrrhotite, pentlandite and chalcopyrite. A relatively abundant platinum-group mineral (PGM) assemblage is present and includes merenskyite, melonite, michenerite, moncheite and sperrylite. Moreover, concentrations of base and precious metals and micro-PIXE analyses were obtained for the three ore-types. The mineralogy and the mantle-normalised chalcophile element profiles strongly suggest that semi-massive ore represents mss crystallisation, whereas the disseminated ore represents an unfractionated sulphide liquid and the chalcopyrite-rich veined ore a Cu-rich sulphide liquid. Palladium-bearing minerals occur commonly enclosed within sulphides, indicating a magmatic origin rather than hydrothermal. The occurrences and the composition of these minerals suggest that Pd was initially dissolved in the sulphides and subsequently exsolved at low temperatures to form bismutotellurides. Negative Pt and Au anomalies in the mantle-normalised chalcophile element profiles, a lack of Cu-S correlation and textural observations (such as sperrylite losing its euhedral shape when in contact with altered minerals) suggest partial remobilisation of Pt, Au and Cu by postmagmatic hydrothermal fluids after the sulphide crystallisation.

PGE mineralization in marginal sulfide ores of the Chineisky layered intrusion, Russia

We have undertaken a detailed study of platinum group element (PGE) mineralogy and geochemistry of disseminated sulfides associated with the marginal zone of the Chineisky layered mafic intrusion. Towards an intrusive contact the marginal zone reveals a gradual progression from gabbro-gabbronorites towards monzodiorite. Sulfides occur in all the rocks of the marginal zone including exocontact sandstone. They occur mainly as pyrrhotite, chalcopyrite, and pentlandite and show progressive enrichment in Cu towards the intrusive contact. In the same direction, PGE mineralogy reveals the following systematic changes: (1) size of PGE mineral grains decreases from 50 µm up to 1 µm; (2) the association of Pd minerals with Ni and Co arsenide and sulfarsenides becomes stronger; and (3) the composition of PGE minerals changes for palladium: Pd-Sn → Pd-As → Pd-Sb → Pd-Te → Pd-Bi; for platinum: Pt-Fe + PtAs2 → PtS + PtAs2 → PtAs2. This zoning pattern is interpreted as the result of fractional crystallization of an immiscible sulfide melt, with the residual liquid, enriched in Cu, PGE, and volatile elements, being expelled towards the periphery of the intrusive body. PGE minerals also decompose in an oxidation zone. The most stable of them are paolovite and sperrylite, which both accumulated in placers derived from the massif. Pd is removed from the decomposed minerals, and then absorbed by brown iron and goethite in the oxidation zone.