Platinum-group Element Geochemistry Research Articles

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.

The Role of Late Sulfide Saturation in the Formation of a Cu- and Au-rich Magma: Insights from the Platinum Group Element Geochemistry of Niuatahi-Motutahi Lavas, Tonga Rear Arc

This research was supported by an Australian Research Council Discovery grant to Ian Campbell and a fund from the Ministry of Oceans and Fisheries of Korea (PM57580). Three-month stipend for Jung-Woo Park was supported by Australian Research Council grant to Oliver Nebel in 2012.

Metal sources for the polymetallic Ni–Mo–PGE mineralization in the black shales of the Lower Cambrian Niutitang Formation, South China

Total organic carbon content (TOC), trace element and platinum-group element (PGE) concentrations were determined in the black shales of the Lower Cambrian Niutitang Formation in the Nayong area, Guizhou Province, South China, in order to study the polymetallic Ni–Mo–PGE mineralization. The results demonstrate that numerous elements are enriched in the polymetallic ores compared to those of the nearby black shale, particularly Ni, Mo, Zn, TOC and total PGE, which can reach up to 7.03wt.%, 8.49wt.%, 11.7wt.%, 11.5wt.% and 943ppb, respectively. The elemental enrichment distribution patterns are similar to those in the Zunyi and Zhangjiajie areas except that the Nayong location is exceptionally enriched in Zn. Whereas positive correlations are observed between the ore elements of the polymetallic ores, no such correlations are observed in the black shale. These positively correlated metallic elements are classified into three groups: Co–Ni–Cu–PGE, Zn–Cd–Pb and Mo–Tl–TOC. The geological and geochemical features of these elements suggest that Proterozoic and Early Palaeozoic mafic and ultramafic rocks, dolomites and/or Pb–Zn deposits of the Neoproterozoic Dengying Formation and seawater could be the principal sources for Co–Ni–Cu–PGE, Zn–Cd–Pb, and Mo–Tl–TOC, respectively. Furthermore, the chondrite-normalized patterns of PGEs with Pd/Pt, Pd/Ir and Pt/Ir indicate that PGE enrichment of the polymetallic ores is most likely related to hydrothermal processes associated with the mafic rocks. In contrast, PGE enrichment in the black shale resembles that of the marine oil shale with terrigenous and seawater contributions. Our investigations of TOC, trace elements and PGE geochemistry suggest that multiple sources along with submarine hydrothermal and biological contributions might be responsible for the formation of the polymetallic Ni–Mo–PGE mineralization in the black shales of the Lower Cambrian Niutitang Formation across southern China.

Major, trace and platinum-group element geochemistry of the Upper Triassic nonmarine hot shales in the Ordos basin, Central China

The Upper Triassic Chang 7 non-marine hot shales have been proven to contribute to Mesozoic oil reservoirs in the Ordos basin, Central China. These shales have been studied extensively in sedimentology, petrology and organic geochemistry. However, the factors controlling the development of the shales are still ambiguously constrained. This paper presents the major, trace, and platinum-group elements (PGEs) data to address these issues. The Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), C-values and clay mineral compositions indicate that the Chang 7 hot shales were developed in a humid paleoclimate. The indicators of redox conditions (Eu/Eu∗, Ce/Ce∗, Ceanom, δU, U/Th, V/(V+Ni) and V/Cr) suggest that the Chang 7 hot shales were developed under anoxic environment. Uranium enrichment (average 46.10ppm) is an important distinguished signature of the Chang 7 hot shales, which was mainly related to synchronous volcanic ash sediments and/or possibly hydrothermal fluids. The PGE concentrations of the Chang 7 hot shales were reported for the first time here, and can be subdivided into two groups. The total abundances of the Group One samples are relatively lower and derived mainly from detrital minerals, while the Group Two samples are relatively higher, and are also likely linked to the synchronous volcanic ash sediments and/or possibly hydrothermal fluids. Combined with the regional geological setting, we tentatively propose that the tectonics is the major control on the development of the Chang 7 hot shales.

The geochemistry of platinum group elements in marine oil shale—A case study from the Bilong Co oil shale, northern Tibet, China

The Bilong Co oil shale zone is located in the South Qiangtang depression. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibet plateau, represents the potentially largest marine oil shale resource in China. Seventeen samples including oil shale and micritic limestone were collected from the Bilong Co oil shale area to determine the concentrations, distribution patterns, occurrences and origins of platinum group elements (PGEs) in marine oil shale. The oil shale samples from the Bilong Co area exhibit very low total PGE contents ranging from 1.04 to 2.96ng/g with a weighted mean value of 1.686ng/g, while the micritic limestone samples from the Bilong Co area exhibit a little lower PGE value ranging from 0.413 to 1.11ng/g. PGEs in oil shale samples are characterized by high contents in Pd (average 0.79ng/g), Os (average 0.123ng/g) and Pt (average 0.644ng/g) compared with Ru (average 0.068ng/g), Rh (average 0.033ng/g) and Ir (average 0.026ng/g). The highest values for individual PGEs are not uniformly distributed in the section. Clearly, the PGEs are generally enriched in the oil shale samples near the boundary between micritic limestone and oil shale.The individual PGEs in oil shale samples from the Bilong Co area exhibit various modes of occurrence. Ruthenium and Pt occur mainly in pyrite, while Pd is associated mainly with organic matter and Mg-minerals. Rhodium and Os are controlled mainly by pyrite and organic matter. Iridium is present mainly in other Fe-bearing minerals, rather than pyrite. The PGEs in the Bilong Co oil shale are mainly of seawater origin and possibly influenced by terrigenous supply.

Origin of the Tudun Cu–Ni sulfide deposit in the eastern Tianshan, NW China: Constraints on the geochemistry of platinum group elements

This study documents the geochemistry of sulfide ores of the Tudun Cu–Ni sulfide deposit, located at the western end of the Huangshan–Jing'erquan ore belt in the northern part of eastern Tianshan. The ores have been classified as (1) disseminated sulfide ores, (2) net-textured sulfide ores, and (3) massive sulfide ores.The geochemistry of barren rocks shows Ni and Cu contents ranged 889–1330ppm and 145–253ppm, respectively. The rocks generally contained ~0.10ppb Rh, Ir, Ru and Os, and ~1ppb Pt and Pd, as well as high Cu/Pd ratios (133–246). The contents of Cu, Ni and PGEs in sulfide ores were calculated on a 100% sulfide basis. Total PGEs on a 100% sulfide basis ranged from 13.2ppb to 443ppb (average=144ppb), which were lower than the values in the Huangshandong deposit (average=186ppb) in the eastern Tianshan ore belt, one order of magnitude lower than the Jinchuan deposit, and two orders of magnitude lower than the Noril'sk–Talnakh deposit. The R factor value ranged from 168 to 1810 in sulfide ores of the Tudun deposit, indicating Cu and Ni enrichment, although PGEs enrichment did not reach recoverable grades.In the Tudun deposit, the Pd/Ir ratio of sulfide ores was less than 100, showing weak hydrothermal fluid alteration. The massive sulfide ores exhibited less primitive mantle-normalized PGEs patterns, which are depleted in PPGEs and different from PPGEs-rich intrusions, as well as disseminated and net-textured sulfide ores. In addition, the massive sulfide ores exhibit a strong negative Pt anomaly. This may be due to fractionation of Pt–Fe alloys in the magma batches prior to its intrusion into the staging chamber. It could also be caused by selective Pt leaching from the sulfides by hydrothermal fluids during remobilization within the Tudun chamber.

Enriched lithospheric mantle keel below the Scottish margin of the North Atlantic Craton: Evidence from the Palaeoproterozoic Scourie Dyke Swarm and mantle xenoliths

The Lewisian Gneiss Complex of NW Scotland represents the eastern margin of the North Atlantic Craton. It comprises mid-late Archaean tonalite-trondhjemite-granodiorite gneisses that were metamorphosed and deformed during the Late-Archaean and Palaeoproterozoic. A major swarm of mafic-ultramafic dykes, the Scourie Dyke Swarm, was intruded at ca. 2.4–2.3Ga during a period of extension that can be correlated across the North Atlantic Craton. The majority of dykes are doleritic, with volumetrically minor picrite and olivine gabbro suites.New major and trace element geochemical data and Re-Os isotopes indicate that the Scourie Dyke Swarm was not solely derived from a ‘typical’ asthenospheric mantle source region. The geochemical signatures of the dykes show significant negative Nb, Ta and Ti anomalies, coupled with enrichment in Th, Light Rare Earth Elements and other large ion lithophile elements. These features cannot be reproduced by simple contamination of asthenospheric sources with Lewisian granulite-facies crust. Instead they are a feature of the mantle source that produced the Scourie Dykes and may have developed during Archaean subduction episodes.Spinel lherzolite mantle xenoliths from the Isle of Lewis offer direct insight into the lithospheric mantle below this region. They display similar geochemical ‘enrichments’ and ‘depletions’ observed in the Scourie Dykes and the magma source is thus considered to reside primarily in the sub-continental lithospheric mantle (SCLM), with some potential contribution from asthenospheric melts. Platinum Group Element geochemistry and trace element modelling indicate that the dolerite dykes were formed by moderate (<15%) partial melting of the source, whilst higher degrees of partial melting led to the formation of picritic and olivine gabbro suites. Magma production was triggered by significant crustal and lithospheric extension, causing both asthenospheric and substantial lithospheric melting.

Platinum group minerals (PGM) in the Falcondo Ni-laterite deposit, Loma Caribe peridotite (Dominican Republic)

Two Ni-laterite profiles from the Loma Caribe peridotite (Dominican Republic) have been investigated for their platinum group element (PGE) geochemistry and mineralogy. One profile (Loma Peguera) i ...

Platinum Group Element Geochemistry of Sulfide-Rich Horizons in the Ural-Alaskan-Type Ultramafic Complex of Duke Island, Southeastern Alaska

The Duke Island Ultramafic Complex in southeastern Alaska is a Ural-Alaskan–type complex that hosts sulfide-rich horizons primarily in the olivine clinopyroxenite unit. Although placer deposits hosting platinum group elements (PGEs) have been found in association with Ural-Alaskan–type complexes in several locations around the world, these ultramafic complexes have traditionally been viewed as sulfide-poor systems and therefore unlikely hosts of economic Ni-Cu sulfide deposits. Sulfide mineralization in the Duke Island Complex consists of massive, net-textured, and disseminated assemblages that formed as a result of the interaction between magma and sulfidic-graphitic country rocks. The sulfide mineralization at Duke Island is characterized by strong depletion in Ir, Os, and Ru (IPGEs), and relative enrichment in Pt, Pd, and Cu. The sulfide-bearing olivine clinopyroxenites formed from a derivative magma produced as a result of the fractional crystallization of olivine and chromite. Although IPGEs may be compatible within olivine and chromite, fractional crystallization of up to 20% cannot explain the observed IPGE depletion. We propose that early crystallization of laurite and Ir alloys accompanied the accumulation of olivine and chromite. R-factor modeling indicates that much of the inter-element variation in the Sulfide mineralization can be attributed to variations in the degree of equilibration between Sulfide and silicate liquids. Mixing between monosulfide solid solution and fractionated Sulfide liquid may also have contributed to the variability. The modeling suggests that the pre-sulfide saturation concentrations of Os, Pt, and Pd in the magmas were approximately 0.03, 2, and 3 ppb, respectively. The concentration of Pd is near that of primitive mantle and the low Pt concentration can be explained by the crystallization of Pt alloy with early-formed dunites and chromitites. This is consistent with models for Pt-rich placers in the Urals that have been derived from chromitites and dunites. The relative enrichments in Pd and Cu are the results of fractional crystallization before Sulfide saturation was achieved. Taken together, the Ni-Cu-PGE elemental characteristics at the Duke Island Complex are similar to those shown by flood basalt-related Sulfide deposits and Cu-rich Sulfide veins studied in different locations around the world.

Re–Os isotope and platinum-group element geochemistry of the Pobei Ni–Cu sulfide-bearing mafic–ultramafic complex in the northeastern part of the Tarim Craton

A number of mafic–ultramafic intrusions that host Ni–Cu sulfide mineralization occur in the northeastern Tarim Craton and the eastern Tianshan Orogenic Belt (NW China). The sulfide-mineralized Pobei mafic–ultramafic complex is located in the northeastern part of the Tarim Craton. The complex is composed of gabbro and olivine gabbro, cut by dunite, wehrlite, and melatroctolite of the Poyi and Poshi intrusions. Disseminated Ni–Cu sulfide mineralization is present towards the base of the ultramafic bodies. The sulfide mineralization is typically low grade ( 4 wt.%. Samples from the Pobei complex have eNd (at 280 Ma) values up to +8.1, consistent with the derivation of the magma from an asthenospheric mantle source. Fo 89.5 mol.% olivine from the ultramafic bodies is consistent with a primitive parental magma. Sulfide-bearing dunite and wehrlite have high Cu/Pd ratios ranging from 24,000 to 218,000, indicating a magma that evolved under conditions of sulfide saturation. The grades of Ni, Cu, and PGE in 100 % sulfide show a strong positive correlation. A model for these variations is proposed where the mantle source of the Pobei magma retained ~0.033 wt.% sulfide during the production of a PGE-depleted parental magma. The parental magma migrated from the mantle to the crust and underwent further S saturation to generate the observed mineralization along with its high Cu/Pd ratio at an R-factor varying from 100 to 1,200. The mineralization at Poshi and Poyi has very high γOs (at 280 Ma) values (+30 to +292) that are negatively correlated with the abundance of Os in 100 % sulfide (5.81–271 ppb) and positively correlated with the Re/Os ratios; this indicates that sulfide saturation was triggered by the assimilation of crustal sulfide with both high γOs and Re/Os ratios. When compared to other Permian mafic–ultramafic intrusions with sulfide mineralization in the East Tianshan, the Poyi and Poshi ultramafic bodies were formed from more primitive magmas, and this helps to explain why the sulfide mineralization has high Ni tenor.

Segregation and Fractionation of Magmatic Ni-Cu-PGE Sulfides in the Western Jinchuan Intrusion, Northwestern China: Insights from Platinum Group Element Geochemistry

The Jinchuan Ni-Cu-(PGE) deposit is one of the world’s largest magmatic sulfide deposits. Recent studies have suggested that the Jinchuan intrusion originally comprised two individual intrusions, the Eastern and the Western intrusions. The Western intrusion consists of an Upper and a Lower unit. The Upper unit is largely free of sulfides with weakly disseminated sulfides only at the base of the unit; these sulfides are characterized by moderate Cu/Pd ratios of 5,500 to 14,900 and relatively high PGE tenors (PGE in 100% sulfides) in the range of 510 to 1,000 ppb Irsul, 420 to 1,200 ppb Rusul, and 2,900 to 8,000 ppb Pdsul. Disseminated, net-textured and minor massive sulfides in the Lower unit comprise the No. 24 orebody, the third largest at Jinchuan. The disseminated ores have remarkably high Cu/Pd ratios (24,200–85,600) and low PGE tenors in the range of 120 to 560 ppb Irsul, 100 to 480 ppb Rusul, and 430 to 3,600 ppb Pdsul. The net-textured and massive ores contain high IPGE contents (240–820 ppb Irsul and 170–630 ppb Rusul) and Ni/Cu ratios (1.7–11.3), low Pd/Ru ratios (0.53–3.1), and exhibit significant Pt depletion (<300 ppb Ptsul). Additionally, small Cu-rich orebodies occur at the base of the No. 24 orebody or in small intrusions within footwall schists, marbles, and granites beneath the Western intrusion. The Cu-rich sulfides have very low IPGE tenors (1.9–16 ppb Irsul and 6.4–21 ppb Rusul), and Ni/Cu ratios (0.3–2.8), high Pd/Ru ratios (37.3–378), and highly variable Ptsul between 43 and 11,000 ppb. The PGE geochemistry and model calculations indicate that the disseminated sulfides of the Upper and Lower units segregated from PGE-undepleted and -depleted parental magmas, respectively. The PGE-depleted magma had undergone a minor amount of earlier sulfide liquid extraction at depth. Furthermore, this PGE-depleted parental magma had slightly higher PGE contents than the magma from which the sulfides of the Nos. 1 and 2 orebodies in the Eastern intrusion segregated. The significant variations in base and precious metal ratios of the net-textured and massive sulfide ores are attributed to fractionation of the sulfide melts that formed these ores. This led to both migration of the residual Cu-rich sulfide melts away from the early-formed monosulfide solid solution (mss) to form Cu-rich ores as well as back intrusion of the Cu-rich ores into the mss. The negative Pt anomalies of the net-textured and massive sulfides resulted from preferential extraction of Pt into the Cu-rich residual melts due to the presence of As, Te, and Bi.

Chromite and PGE geochemistry of the Elekdağ Ophiolite (Kastamonu, Northern Turkey): Implications for deep magmatic processes in a supra-subduction zone setting

The geochemistry of chromium spinel and platinum group elements (PGE) provides significant information about conditions in the mantle during the emplacement and genesis of ophiolitic complexes. The Elekdağ ophiolite is a greenschist to amphibolite facies ophiolite that was emplaced onto the Sakarya Zone, in the central part of the Pontides of northern Turkey. NW–SE trending Elekdağ ophiolite hosts small scale chromitite occurrences in serpentinized harzburgite and dunite, and have maximum resource of several thousands t Cr2O3.The chromitites the Elekdağ ophiolite can be divided in to “high-Cr” and “high-Al” types. The calculated melt composition of the high-Cr chromitites is 9.4–13.2wt.% Al2O3, 0.2–0.4wt.% TiO2 with FeO/MgO ratios of 0.4–1.9, which are indicative of boninitic compositions. However, the high-Al chromitites have 14.7–17.3wt.% Al2O3 and 0.3–0.8wt.% TiO2 with FeO/MgO ratios of between 0.3 and 0.8, which are consistent with MORB-like tholeiitic affinities. The total PGE content of the chromitites range between 82 and 1466ppb, and the average PGE content of the high-Cr and high-Al chromitites are 267ppb and 195ppb, respectively. The chromitites are characterized by low Pd/Ir ratios of 0.02–0.20 that is characteristic of non-fractionated chromitites. The chondrite normalized PGE pattern of the chromitites resemble those of mantle-derived Cr-rich chromitites in ophiolites of the supra-subduction zone (SSZ) elsewhere.Mineral and bulk rock PGE data for the Elekdağ chromitites indicate that high-Cr chromitites formed from Cr-rich and, Al- and Ti-poor boninitic melts, most probably in the fore-arc setting in the deeper part of the mantle. This was succeeded by the mineralization of high-Al chromitites from a differentiated boninitic melt at shallow depths close to the Moho Discontinuity.

Dispersion Pattern of PGE and Ag in Chromiferous Ultramafic Suite of Rocks in Sukinda Valley, India

AbstractPotential chromite ore deposits of India are situated in Sukinda, Odisha, which may also be considered as a potential resource for platinum group elements (PGEs). This paper reports on PGE geochemistry in twenty six samples covering chromite ores, chromitites and associated ultramafic rocks of the Sukinda ultramafic complex. Platinum group element contents range from 213 to 487 ppb in the chromite ore body, from 63 to 538 ppb in rocks that have chromite dendrites or dissemination and from 38 to 389 ppb in associated olivine–peridotite, serpentinite, pyroxenite and brecciated rocks. The PGEs are divided into two sub‐groups: IPGE (Ir, Os, and Ru) and PPGE (Pd, Pt, and Rh) based on their chemical behaviour. The IPGE and PPGE in these three litho‐members show a contrasting relationship e.g. average IPGE content decreases from chromite to chromitite and associated rocks while PPGE increases in the same order. Appreciable Ag in chromitite (270–842 ppb) is recorded. Positive correlation between IPGE with Cr2O3 and with Al2O3 is observed while these are negatively correlated with MgO. Covariant relationships between Au and Mg in rocks devoid of chromite and between Ag and Fe in chromitite sample are observed. Chromite in all seams and some chromitite samples exhibit an IPGE‐enriched chondrite normalized pattern while PPGE are highly fractionated and show a steep negative slope, thereby indicating that PGE in the parental melt fractionates and IPGE‐compatible elements prefer to settle with chromite. The rocks devoid of chromite and rocks containing accessory chromite exhibit a nearly flat pattern in chondrite‐normalized PGE plots and this suggests a limited fractionation of PGE in these rocks. Variation in the distribution pattern of PGE and Ag in three typical litho‐members of the Sukinda Valley may be related to multiple intrusion of ultramafic magma, containing variable volume percentage of chromite.

Platinum-group element geochemistry of the Zhuqing Fe-Ti-V oxide ore-bearing mafic intrusions in western Yangtze Block, SW China: control of platinum-group elements by magnetite

Platinum-group element (PGE) geochemistry combined with elemental geochemistry and magnetite compositions are reported for the Mesoproterozoic Zhuqing Fe–Ti–V oxide ore-bearing mafic intrusions in the western Yangtze Block, SW China. All the Zhuqing gabbros display extremely low concentrations of chalcophile elements and PGEs. The oxide-rich gabbros contain relatively higher contents of Cr, Ni, Ir, Ru, Rh, and lower contents of Pt and Pd than the oxide-poor gabbros. The abundances of whole-rock concentrations of Ni, Ir, Ru, and Rh correlate well with V contents in the Zhuqing gabbros, implying that the distributions of these elements are controlled by magnetite. The fractionation between Ir–Ru–Rh and Pt–Pd in the Zhuqing gabbros is mainly attributed to fractional crystallization of chromite and magnetite, whereas Ru anomalies are mainly due to variable degrees of compatibility of PGE in magnetite. The order of relative incompatibility of PGEs is calculated to be Pd < Pt < Rh < Ir < Ru. The very low PGE contents and Cu/Zr ratios and high Cu/Pd ratios suggest initially S-saturated magma parents that were highly depleted in PGE, which mainly formed due to low degrees of partial melting leaving sulfides concentrating PGEs behind in the mantle. Moreover, the low MgO, Ni, Ir and Ru contents and high Cu/Ni and Pd/Ir ratios for the gabbros suggest a highly evolved parental magma. Fe–Ti oxides fractionally crystallized from the highly evolved magma and subsequently settled in the lower sections of the magma chamber, where they concentrated and formed Fe–Ti–V oxide ore layers at the base of the lower and upper cycles. Multiple episodes of magma replenishment in the magma chamber may have been involved in the formation of the Zhuqing intrusions.

Source of highly potassic basalts in northeast China: Evidence from Re–Os, Sr–Nd–Hf isotopes and PGE geochemistry

The origin of the very young (0.5Ma to 1721AD) Wudalianchi-Erkeshan (WDLC and EKS) highly potassic basalts in northeast (NE) China has been the subject of considerable debate. In this study, we present new major-, trace- and platinum group element (PGE) data together with Re–Os, Lu–Hf, and Sr–Nd isotopic analyses for these potassic basalts in order to further constrain their source. It has been shown that the WDLC and EKS basalts have unradiogenic to only moderately radiogenic osmium isotope ratios with 187Os/188Os ranging from 0.1187 to ~0.17, and only slight PPGE (Pt and Pd) enrichments relative to IPGE (Os, Ir and Ru). The positive correlations between 187Os/188Os and 1/Os suggests that these basalts have probably experienced about 2–8% of lower continental crust addition during magma ascent. On the other hand, the incorporation of sub-continental lithospheric mantle (SCLM)-derived, primary sulphides and/or PGE micro-alloys during magma ascent seems likely in some basalts, which preserve particularly unradiogenic Os compositions. The complex interplay of signatures derived from crustal and SCLM contamination means that the Os isotope systematics of the basalts do not unequivocally fingerprint the source of the WDLC and EKS basalts.The strong enrichments of light rare earth elements (LREE) and large ion lithophile elements (LILE), high-K, EM1-like Sr–Nd–Hf isotopic characteristics, and particularly strong fractionation of the heavy REE ((Sm/Yb)N=9.7±0.6), suggest that the WDLC and EKS basalts mainly originated from phlogopite-bearing garnet-peridotite in the SCLM. Combined with low Ce/Pb ratios (~9.4–13.5), the most likely source of WDLC and EKS basalts is SCLM that has been metasomatized by delaminated old, lower continental crust. This is different from previous hypotheses that implicate a dominantly asthenospheric source with a contribution from delaminated ancient SCLM or subducted oceanic crust with sediments, or a SCLM source that has been metasomatized by melts derived from deep asthenosphere during the Proterozoic.

Platinum-group element (PGE) geochemistry of Deccan orangeites, Bastar craton, central India: Implication for a non-terrestrial origin for iridium enrichment at the K–Pg boundary

We report platinum group element (PGE) concentrations of twelve bulk-rock samples from the Behradih and Kodomali orangeite intrusions in the Mainpur field, Bastar craton, central India, which are emplaced synchronously with the Deccan flood basalts. Their palladium-group PGE (PPGE) (1.8–5.2ppb Pt, 1.2–6.4ppb Pd) contents are distinctly higher compared to their iridium-group PGE (IPGE) concentrations (0.8–2ppb Os, 0.8–1.2ppb Ir, 3.2–4.2ppb Ru, and 0.2–0.8ppb Rh). Their PGE contents as well as Pd/Ir ratios are either similar or even lower than those from the Mesoproterozoic and Cretaceous kimberlites and orangeites from the Kaapvaal craton (southern Africa), Cretaceous kimberlites from the Sao Fransisco craton (Brazil), Ordovician kimberlites from the North China craton and the Mesoproterozoic southern Indian kimberlites from the Eastern Dharwar craton. Anomalously elevated iridium (and other PGE) contents in sediments at the Cretaceous–Paleogene (K–Pg) boundary are commonly attributed either to a large bolide impact triggering the K–Pg mass extinction or to terrestrial causes such as volcanic eruptions (Deccan flood basalts) or even to mantle-plume derived lithospheric gaseous explosions (Verneshots). Lack of unusually high abundances of PGE in the Mainpur orangeties as well as in the co-eval Deccan flood basalts and associated alkaline rocks implies that the anomalous iridium enrichment reported at the K–Pg boundary sections was not sourced from the mantle and likely originated from an extraterrestrial source.

Platinum group elements geochemistry of ultramafic and associated rocks from Pindar in Madawara Igneous Complex, Bundelkhand massif, central India

Ultramafic rocks comprising dunite, harzburgite, lherzolite, olivine webserite and websterite occur as intrusives in the form of small hillocks at Pindar into the granite–gneisses of Bundelkhand Gneissic Complex (BnGC). The peridotites are dominated by olivine cumulates where chromite and precious metal-bearing sulphides crystallized along with pyroxenes, subsequent to crystallization of olivine into the interstitial spaces of cumulates during cooling. Ultramafic rocks of Pindar are characterized by high MgO (up to 46.0 wt%) and FeO (up to 5.8 wt%); low SiO2 (40.8 to 48.0 wt%), TiO2 (0.2 to 0.5 wt%), Al2O3 (~3.2 wt% av.), CaO (~2.7 wt% av.) and Cu (11 to 73 μg/g). Cr and Ni values range from 2297 to 3150 μg/g and 2434 to 2767 μg/g, respectively. Distribution of Ir (up to 20 ng/g), Ru (27 to 90 ng/g), Rh (3 to 14 ng/g), Pt (18 to 72 ng/g), Pd (10 to 27 ng/g) and Au (22 to 57 ng/g) indicate platinum group element (PGE) and associated gold mineralization in these ultramafic rocks. A mineral phase representing sperrylite (PtAs2) was also identified within the sulphides in scanning electron microscopy with energy dispersive spectrometer (SEM–EDS) studies. The primitive mantle-normalized siderophile elements pattern shows platinum group element PGE (PPGE) enrichment (Rh, Pt, Pd). Discrimination diagrams of Pd/Ir vs. Ni/Cu, Pd/Pt vs. Ni/Cu, Cu/Pd vs. Pd, and Cu vs. Pd for the peridotites of Pindar attribute to affinity towards komatiite magma, derived from high degree of partial melting of prolonged depleted mantle, and the sulphur saturation condition incurred during the crystallization of chromite which was favourable for PGE mineralization.

Platinum-alloy and sulfur saturation in an arc-related basalt to rhyolite suite: Evidence from the Pual Ridge lavas, the Eastern Manus Basin

We have measured the platinum group element (PGE) and Re concentrations of arc-type lavas from the Pual Ridge and the surrounding area in the Eastern Manus Basin. These magmas followed an Fe-enrichment trend to produce a wide range of compositions with MgO varying between 8 and 0.1wt.%. We found distinct differences in the PGE geochemistry of the high (>3wt.% MgO) and the low-Mg lavas (<3wt.% MgO). During the early stages of fractionation, the concentrations of Pd, Cu, Au and Re increase whereas Ir, Ru, Rh and Pt decrease. Co-variations of Ir, Ru and Rh with Pt in the high-Mg lavas suggest that the depletion of these elements is due to Pt-rich alloy saturation. This is consistent with the high Pt contents in the high-Mg lavas, which is close to the solubility of Pt in the basaltic melt at similar conditions. In contrast, the concentrations of all PGE and Re drop rapidly in the low-Mg lavas (except for Ru and Ir), with the PGE concentrations falling at a rate that is appreciably faster than Cu, which we attribute to sulfide saturation. As a consequence, there is a marked decline in Pd/Cu in the low-Mg lavas and we suggest that this ratio is the best indicator of sulfide saturation in an evolving magmatic system.A feature of the data is that duplicate analyses of the same sample often do not agree within error. We attribute this scatter to the nugget effect, with nuggets of a Pt-rich alloy in the high-Mg lavas and sulfide blebs in the low-Mg lavas. The PGE concentrations of phenocryst-bearing high-Mg lavas are higher than in the associated glassy lavas, and scatter on MgO variation diagrams is significantly reduced if only glassy lavas are considered, which indicates that the micron scale Pt-rich alloy grains are intimately associated with the phenocrysts. Our results provide strong evidence that Pt-rich alloys can crystallize from a basaltic-andesitic magma, along with the silicate minerals, and fractionate Pt from Pd during magmatic differentiation. As a consequence, Pd/Pt increases during Pt alloy fractionation and this ratio can be used to identify Pt-rich metal saturation. The Pual Ridge alloys are Pt-rich because the primary magmas are Pt-enriched and Ir-depleted, which is typical of arc-related magmas.

Seawater contribution to polymetallic Ni–Mo–PGE–Au mineralization in Early Cambrian black shales of South China: Evidence from Mo isotope, PGE, trace element, and REE geochemistry

The Early Cambrian black shale sequence of the Niutitang Formation in South China hosts a synsedimentary, organic carbon-rich, polymetallic sulfide layer with extreme metal concentrations, locally mined as polymetallic Ni–Mo–PGE–Au ore. In combination with previously reported data, we present Mo isotope, platinum-group element (PGE), and trace and rare-earth element (REE) data for the polymetallic sulfide ores and host black shales from four mine sites (Dazhuliushui and Maluhe in Guizhou Province, and Sancha and Cili in Hunan Province, respectively), several hundred kilometers apart. The polymetallic sulfide ores have consistently heavy δ98/95Mo values of 0.94 to 1.38‰ (avg. 1.13±0.14‰, 1σ, n=11), and the host black shale and phosphorite have slightly more variable δ98/95Mo values of 0.81‰ to 1.70‰ (n=14). This latter variation is due to variable paleoenvironmental conditions from suboxic to euxinic, and partly closed-system fractionation in isolated marine sedimentary basins. Both the polymetallic sulfides and host black shales show PGE distribution patterns similar to that of present-day seawater, but different from those of ancient submarine-hydrothermal deposits and modern submarine hydrothermal fluids. The polymetallic sulfide bed has a generally consistent metal enrichment by a factor of 107 compared to present-day seawater. PAAS-normalized REE+Y patterns of the polymetallic sulfide bed are characterized by a remarkably positive Y anomaly, consistent with an origin of the REE predominantly from seawater. Small positive Eu anomalies in some of the sulfide ores could reflect minor hydrothermal components involved. The Mo isotope, PGE, and trace and rare-earth element geochemical data suggest that metals in the polymetallic Ni–Mo–PGE–Au sulfide ore layer were scavenged mostly from Early Cambrian seawater, by both in-situ precipitation and local re-deposition of sulfide clasts.

Review of platinum-group element distribution and mineralogy in chromitite ores from southern Iran

The platinum-group element (PGE) potential of four chromite mining districts in southern and south-eastern Iran was investigated using geochemical and mineralogical methods. A total of 20 mines and prospects were sampled in the Neyriz ophiolite, the Abdasht and Sikhuran complexes in the Esfandagheh district, and the Faryab district. Chromitite and dunite were analysed for major, trace element and PGE geochemistry, and the platinum-group element mineralogy (PGM). In all mining districts, chromites analysed from mantle tectonite, probable transition zone and cumulate zone chromitites are refractory, low in TiO2 (<0.3wt.%) as well as Cr- and Mg-rich, with Cr# [100∗Cr/(Cr+Al)] ranging from 70 to 81 and Mg# [100∗Mg/(Mg+Fe2+)] ranging from 40 to 83. Associated olivine is highly magnesian (Mg# 89–98) and Ni-rich (0.1–1.0wt.% NiO).PGE concentrations range from <100 to >5000ppb, with median values around 200ppb in all complexes investigated. The 187Os/188Os isotope ratios of chromitite indicate chondritic to slightly suprachondritic initial Os. Mantle-normalised Pt/Ir ratios are generally low (<0.2), but are up to 0.5 in small chromite pods in the harzburgitic mantle section of the Neyriz ophiolite. PGM, mainly laurite–erlichmanite, mostly occur as small (<1 to 30μm) inclusions in chromite. They are associated with hydrous silicates (calcic amphibole), base metal sulphides (Ni- and Cu-sulphides) and other PGM, such as cuproiridsite, Ni–Ir sulphides, and Os–Ir alloys. PGMs interstitial to chromite are represented by irarsite, hollingworthite, Ru–Os–Ir alloys, tolovkite, RhNiAs, Pt–Fe alloy, probable Ru-rich oxides, and others.In the Sikhuran complex, pods of sulphide-bearing chromitite (0.2–1.2% sulphides) occur in a dunite zone interpreted as an ultramafic cumulate zone. Chromites in chromitite are characterised by high Cr# (75–81) and intermediate Mg# numbers (45–65); in associated dunite, chromites (Cr#, 74–80) are more Fe-rich (Mg#, 29–50). Chromitite and dunite carry up to 5.2ppm total PGE and are characterised by high normalised Pt/Ir (up to 5), Pt/Pd<1, and radiogenic Os (moderately suprachondritic initial Os). The interstitial sulphides are dominated by pentlandite that carries on average 0.45% Co, 25ppm Os, 11ppm Ir and 8ppm Rh. Palladium and Pt concentrations in pentlandite are below the detection limit of the Laser Ablation–ICP-MS technique. Pentlandite is associated with rare grains of Pd- and Pt-minerals such as stibiopalladinite and Pt–oxide, and with Cu-rich phases.Large degrees of partial melting in a supra-subduction zone geotectonic setting, probably in a two-stage melting process, produced boninitic melts that crystallised refractory, Cr–Mg rich and Ti-poor chromite. In some cases, sulphide saturation was achieved during and after chromite formation. Saturation during chromite crystallisation is evidenced by Ni–Cu-rich sulphide droplets preserved in some chromitites. Sulphur saturation after chromite crystallisation governed the formation of Ni-rich sulphide liquids that exsolved monosulphide solid solution, which later crystallised to form PGE-bearing pentlandite. Platinum and Pd did not partition into pentlandite but crystallised as Pt- and Pd-rich PGM associated with the pentlandite.