Platinum Group Element Concentrations Research Articles

Serpentinised Dunites of the Neoarchean Shimoga Greenstone Belt, Western Dharwar Craton, India: Insights on Ni‐PGE Mineralisation and Genesis

ABSTRACTUltramafic rocks of the Archean greenstone belts worldwide are potential hosts for Cu‐Ni, precious metal deposits like platinum group elements (PGEs) and gold. This study highlights the geochemical evidence and genesis of Ni‐PGE mineralisation in the ultramafic rocks of Shimoga greenstone belt of the Western Dharwar Craton, India. Petrographically, the studied rocks are identified to be serpentinised dunites, while their geochemical signatures indicate komatiitic affinity. Presence of disseminated sulphides and pronounced serpentinisation in these rocks suggest a combination of Type II (disseminated sulphides) and Type IV (post‐magmatic alterations) komatiite‐related Ni–Cu‐PGE deposits. Chondritic Al2O3/TiO2 (9.65–16.38) ratios, superchondritic Gd/YbN (1.3–1.6), depleted high‐field‐strength elements (HFSEs) (Zr, Hf), enrichment of LREE over HREE and negative Nb–Ta anomalies reflect the generation of parental melts from plume‐sourced Al‐depleted komatiites with significant crustal contamination during their emplacement. The major, trace element and PGE relationships (FeO vs. MgO, Cu vs. Pd, Ni/Cu vs. Pd/Ir) infer the origin of sulphur undersaturated primary melts through moderate to higher degrees of partial melting followed by crustal assimilation that led to PGE enrichment. These observations suggest their formation from melts derived from greater depths of the upper mantle (> 400 km) at high pressure (> 10 GPa), wherein, the mantle residue retained majorite garnet. The high Ni (avg. Ni = 6511 ppm) substantiated by high Kambalda ratio ([Ni:Cr] × [Cu:Zn] = 5.6), Ni/Cr ratios (> 1) with high concentration of PGEs (avg. ∑PGE = 3078 ppb) confirm the fertile/mineralised nature of the komatiitic source and potential Ni‐PGE mineralization in the ultramafic rocks of the Shimoga greenstone belt.

Platinum-Group Element and Trace Element Contents of Chromite Grains from the Alexo Komatiite Flows Compared with Chromites from Other Volcanic Rocks: Implications for the Use of Chromite Compositions in Petrogenesis

Platinum-Group Element and Trace Element Contents of Chromite Grains from the Alexo Komatiite Flows Compared with Chromites from Other Volcanic Rocks: Implications for the Use of Chromite Compositions in Petrogenesis

Deccan volcanism's influence on Chicxulub impact signatures at the continental Cretaceous-Paleogene boundary in the Sichuan Basin, China

The Cretaceous-Paleogene (K-Pg) extinction is linked to two primary causes: the Chicxulub asteroid impact in Mexico and the Deccan Traps eruptions in India. While substantial geological evidence supporting both events has been found worldwide, to understand their potential interactions, it is crucial to study sites that preserve records of both events simultaneously. The terrestrial K-Pg boundary located in the Sichuan Basin, China (∼15,500 km from the Chicxulub Crater and ∼5,600 km from the Deccan Traps correspond to 66 Ma), preserves evidence of both the Chicxulub impact and the Deccan Traps eruptions, along with the effects of subsequent events on the earlier ones. It offers a new perspective on the K-Pg boundary. Detailed petrological examination and geochemical analyses, including platinum-group element (PGE) and black carbon concentrations, were conducted across a 2 m K-Pg boundary section. The K-Pg boundary layer (∼2.5 cm thick) is composed of very fine-grained, brick-red sandstone and exhibits significant enrichments in Ir (9.050 ppb) and Os (10.72 ppb), along with elevated black carbon (1.10 ‰), strongly indicating contributions from the impact. Intriguingly, another PGE anomaly was detected approximately 50 cm below the K-Pg boundary. The strata surrounding the K-Pg boundary display evidence of alteration by acidic and HSˉ-rich fluids. These fluids likely caused the observed downward migration of PGE within the boundary, resulting in two distinct PGE anomalies - one at the K-Pg boundary itself and another approximately 50 cm below. These fluids resulted in the overlying sandstones exhibiting minimal matrix content and bleached the underlying brick-red sandstones. Volcanic gas emissions from the Deccan Traps after the impact are a potential source for these fluids. Our findings support the hypothesis that the most voluminous Deccan eruptions had occurred close to, but not necessarily immediately after, the Chicxulub impact, potentially lasting 100–131 ka. The hiatus after the impact weakens the possibility of a direct causal link between the Deccan eruptions and the impact.

Ruthenium isotopes show the Chicxulub impactor was a carbonaceous-type asteroid.

An impact at Chicxulub, Mexico, occurred 66 million years ago, producing a global stratigraphic layer that marks the boundary between the Cretaceous and Paleogene eras. That layer contains elevated concentrations of platinum-group elements, including ruthenium. We measured ruthenium isotopes in samples taken from three Cretaceous-Paleogene boundary sites, five other impacts that occurred between 36 million to 470 million years ago, and ancient 3.5-billion- to 3.2-billion-year-old impact spherule layers. Our data indicate that the Chicxulub impactor was a carbonaceous-type asteroid, which had formed beyond the orbit of Jupiter. The five other impact structures have isotopic signatures that are more consistent with siliceous-type asteroids, which formed closer to the Sun. The ancient spherule layer samples are consistent with impacts of carbonaceous-type asteroids during Earth's final stages of accretion.

Re−Os Isotope and PGE Abundance Systematics of Coast Range Ophiolite Peridotites and Chromitite, California: Insights into Fore-Arc Magmatic Processes

Abstract We report platinum-group element (PGE) and Re concentrations, and Re−Os isotopic data for peridotites and podiform chromitite from the mid-Jurassic Coast Range ophiolite (CRO), California. Our aim is to provide insights into the formation and evolution of the CRO in a fore-arc tectonic setting. The CRO peridotites are divided into two groups: abyssal and supra-subduction zone (SSZ). They have Ir-group PGE concentrations similar to estimates for the primitive mantle and nearly chondritic relative abundances [(Os/Ir)N ≈ 1.1]. Abyssal-type peridotites have slightly subchondritic Pd-group PGE (PPGE)−Re abundances and flat chondrite-normalized patterns, whereas the SSZ-type ones are depleted overall with highly fractionated PPGE−Re patterns. The CRO peridotites have 187Os/188Os values of 0.1188 to 0.1315 (γOs = −8.3 to 1.4) and 187Re/188Os ranging from 0.022 to 0.413. The oxygen fugacity based on the V/Yb ratios of the CRO peridotites is equivalent to the fayalite−magnetite−quartz buffer. The abyssal-type peridotites are residues after ≤5% melting of the primitive upper mantle and represent a remnant of oceanic lithosphere trapped in an SSZ setting but before it was re-melted or modified by subduction processes. The abyssal-type peridotites yield an aluminachron model age of ~1.5 Ga, implying that the CRO mantle had experienced episode(s) of melt extraction before the CRO crust was formed. The SSZ-type peridotites are refractory residues after ~5% to 15% melting. Extraction of fore-arc basalts generated mainly by decompression melting resulted in the SSZ-type peridotites. The chromitite has 187Os/188Os value of 0.1250 (γOs = −3.5) and PGE−Re patterns complementary to that of boninite, indicating a genetic link to fore-arc magmatism.

Early fluid exsolution suppressed sulfide saturation in Triassic arc volcanic rocks from the Gangdese belt: Implications for Cu depletion in arc magmas and mineralization potential

The formation of porphyry–epithermal CuAu deposits depends critically on Cu and Au fertility of arc magmas. Arc magmas commonly exhibit decreasing Cu and Au contents during magmatic differentiation, which is believed to result from sulfide saturation. However, the Cu and Au contents of the arc magmas would also be impacted by fluid exsolution, particularly when magma differentiation occurs in thin arcs. Here we report the variations of chalcophile element contents, including Cu and platinum-group elements (PGEs), with magmatic differentiation in the Qushui–Changguo (QS–CG) arc volcanic rocks from the Gangdese magmatic belt, Tibet. Zircon ages for QS–CG arc volcanic rocks range from 235 to 232 Ma and represent the earliest magmatism correlated to the subduction of the Neo-Tethys Ocean. Magmatic zircons yield an oxygen fugacity of FMQ +1.0 ± 0.6, which is within the range of typical arc magmas worldwide. The crystallization pressures (2.5 kbar) and magmatic H2O contents (5 wt%) estimated from amphibole and clinopyroxene compositions suggest the arc magmas were hydrous and differentiated at low pressures. The primitive rocks have Cu/Pd ratios within the mantle range and relatively high PGE contents, indicating that most chalcophile metals were transferred to the melt during partial melting to form the chalcophile fertile arc magma. The Cu contents decreased from >100 ppm in the primitive rocks to <30 ppm in the evolved rocks (e.g., ∼2 wt% MgO), which resembles the trend observed in global arc magmas. However, the Pt contents exhibit a slight reduction as the MgO decreases, which is in accordance with the crystallization of Pt-rich alloys. No decrease in Pd contents is observed in the samples, and Pd contents increase to ∼10 ppb in the most evolved samples. The absence of a dramatic decrease in PGE contents reveals that the magma did not reach sulfide saturation during differentiation, even in the highly evolved magma after magnetite crystallization. In contrast, the decoupling of Cu and PGE contents in the QS–CG arc magmas and decreasing Cu/Pd ratios are indicative of early aqueous volatile exsolution during magmatic differentiation. We suggest that early volatile exsolution in the arc magmas suppressed sulfide saturation during magmatic differentiation. As a result, early fluid exsolution may also be a key factor in reducing Cu contents in arc magmas in thin arcs, especially for magmas with high volatile contents that undergo differentiation at low pressures. Cu and Au would have been enriched in exsolved fluid prior to sulfide saturation. The Triassic metallogenic potential of the Gangdese belt should be re-evaluated based on recent discoveries. Our results indicate the Triassic magmatism could also be fertile, with the ability to form subduction-related porphyry CuAu deposits in the Gangdese metallogenetic belt.

The chromitites of the Herbeira massif (Cabo Ortegal Complex, Spain) revisited

The ultramafic rocks of the Herbeira Massif in the Cabo Ortegal Complex (NW Iberia) host chromitite bodies. The textural and compositional study of the host rocks and the chromitites classified them into: (1) Type-I chromitites, forming massive pods of intermediate-Cr chromite (Cr# = 0.60–0.66) within dunites; and (2) Type-II chromitites forming semi-massive horizons of high-Cr chromite (Cr# = 0.75–0.82) interlayered with dunites and pyroxenites. Minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V and Sc) contents in the unaltered chromite cores from both types show patterns very similar to fore-arc chromitites, mimicked by the host dunites and pyroxenites. Calculated parental melt compositions suggest that Type-I chromitites crystallized from a melt akin to fore-arc basalt (FAB), while Type-II chromitites originated from a boninite-like parental melt. Both melts are characteristic of a fore-arc setting affected by extension during rollback subduction and have been related to the development of a Cambrian-Ordovician arc. These chromitites are extremely enriched in platinum-group elements (PGE), with bulk-rock PGE contents between 2,460 and 3,600 ppb. Also, the host dunites and pyroxenites exhibit high PGE contents (167 and 324 ppb, respectively), which are higher than those from the primitive mantle and global ophiolitic mantle peridotites. The PGE enrichment is expressed in positively-sloped chondrite-normalized PGE patterns, characterized by an enrichment in Pd-group PGE (PPGE: Rh, Pt and Pd) over the Ir-group PGE (IPGE: Os, Ir and Ru) and abundant platinum-group minerals (PGM) dominated by Rh-Pt-Pd phases (i.e. Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides). The PGM assemblage is associated with base-metal sulfides (mostly pentlandite and chalcopyrite) and occurs at the edges of chromite or embedded within the interstitial (serpentinized) silicate groundmass. Their origin has been linked to direct crystallization from a S-As-rich melt(s), segregated by immiscibility from evolved volatile-rich small volume melts during subduction. At c. 380 Ma, retrograde amphibolite-facies metamorphism occurred during the exhumation of the HP-HT rocks of the Capelada Unit, which affected chromitites and their host rocks but preserved the primary composition of chromite cores of the chromitites. This event contributed to local remobilization of PGE as suggested by the negative slope between Pt and Pd and high Pt/Pd ratios in the studied chromitites, and host dunites and pyroxenites. In addition, it promoted the alteration of primary PGM assemblage and the formation of secondary PGM. Nanoscale observations made by focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) analysis of a composite grain of Rh-bearing arsenide with PGE-base-metal bearing alloys suggest the mobilization and accumulation of small nanoparticles of PGE and base-metals that precipitated from metamorphic fluids forming PGE-alloys. Finally, we offer a comparison of the Cabo Ortegal chromitites with other ophiolitic chromitites involved in the Variscan orogeny, from the Iberian Peninsula to the Polish Sudetes. The studied Cabo Ortegal chromitites are similar to the Variscan chromitites documented in the Bragança (northern Portugal) and Kraubath (Styria, Austria) ophiolitic massifs.

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

Exploring the tectono-magmatic evolution of intraoceanic fore-arc setting during subduction initiation: perspectives from trace and platinum group element systematics of the Jijal ultramafic arc system, NE Pakistan

ABSTRACT The Jijal Complex is considered one of the largest Neo-Tethyan remnants in the intra-oceanic Cretaceous–Palaeogene Kohistan Arc of Pakistan. This complex largely consists of gabbros, peridotites, and chromitites. Previous studies documented the genesis of gabbros and chromitites in detail; however, detailed geochemistry and magma genesis were lacking. In this investigation, we present whole-rock geochemical and platinum group elements (PGEs) data of Jijal peridotites (dunites and harzburgites). The objectives of this study are to elucidate magmatic processes and tectonic regimes involved in the formation of peridotites, factors influencing the concentration and dispersion of PGEs in them, and their tectonic evolution. The investigated peridotite samples are depleted in magmaphile major elements (Al2O3: 0.23–0.57, TiO2: 0.01–0.04, and CaO: 0.08–0.69 wt%), relative to the primitive mantle values, reflecting melt-rock interaction during their magmatic evolution. In addition, rare-earth element (REE) contents in harzburgites are relatively higher compared to the dunites; however, both rock types exhibit depletion with respect to the chondrite-normalized values. The obvious negative anomalies of Nb, Zr, and enriched LREE and large ion lithophiles collectively substantiate their association with depleted arc-type mantle components, metasomatism, and melt/fluid-rock reactions after partial melting episodes. Bulk-rock and geochemical modelling of PGEs data of the peridotites suggest their shallow depth spinel-bearing depleted heterogeneous mantle source and generation from refractory mantle residuum along with boninitic signatures in a supra-subduction zone (SSZ). The PGE’s geochemistry additionally reveals that partial melting, fractionation, metasomatism, and sulphur under-saturation were the key factors that controlled the concentration and distribution of PGEs in the studied rocks. The boninitic features of the Jijal peridotites are equated with intraoceanic fore-arc followed by subduction initiation, and melt-rock reactions in a hydrated forearc mantle, indicating robust evidence of mantle depletion and pervasive refertilisation in an embryonic Neo-Tethyan arc system. Tectonically, the investigated rocks encapsulate vestiges of the Cretaceous Tethyan Ocean and record multifaceted history from boninitic to slab-proximal island arc affinity in compliance with an intraoceanic SSZ fore-arc regime coherent with the subduction inception.

Incorporation of palladium into pyrite: Insights from X-ray absorption spectroscopy analysis and modelling

Pyrite (FeS2) often accommodates elevated concentrations of platinum-group elements in ores of magmatic and hydrothermal origin. In order to elucidate the role of pyrite in concentrating Pd, Pd-doped synthetic crystals were studied via X-ray absorption spectroscopy (XAS). Crystals were obtained by salt-flux method in the system saturated with respect to Pd at the temperature of 580 °C and sulphur fugacity of log f (S2) = −0.4. Scanning electron microscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry studies demonstrated a uniform distribution of Pd within the pyrite crystals. The median and average values of Pd content of ∼0.7 ± 0.1 wt% were measured. Comparison of the Pd K-edge X-ray absorption near edge structure (XANES) spectra with the spectra of standards revealed that the formal oxidation state of Pd was close to +2. Fitting of the extended X-ray absorption fine structure (EXAFS) and Finite Difference Method for Near-Edge Structure (FDMNES) theoretical simulations of XANES spectra showed that Pd substituted for Fe in the crystal structure of pyrite. The isomorphous Pd in pyrite was octahedrally coordinated by S atoms at ∼2.385 Å. The PdS interatomic distance was 5.6 % larger than that of FeS due to the difference in their covalent radii of ∼5.3 %. The expansion caused by the incorporation of Pd into the pyrite structure disappeared at the distance of R > 3 Å. The information on the state of Pd in pyrite, including the local atomic environment and formal oxidation state, is essential for scientific and industrial purposes, e.g. physical-chemical modelling and improvement of leaching and extraction processing respectively.

Fine-grained interplanetary dust input during the Turonian (Late Cretaceous): evidence from osmium isotope and platinum group elements

The Turonian age (~ 90–94 Ma) was the hottest geological interval in the Cretaceous and also marked by the K3 event, a pronounced enrichment of 3He in pelagic sediments (i.e., massive input of extraterrestrial materials). Here, we present Os isotopic (187Os/188Os) and platinum group element (PGE) data from Turonian sedimentary records. After a sharp unradiogenic shift during the end-Cenomanian oceanic anoxic event 2, the 187Os/188Os ratios declined continuously throughout the Turonian, which could be ascribed to the formations of several large igneous provinces (LIPs). Because the interval with the most unradiogenic 187Os/188Os ratios (i.e., enhanced LIP volcanism) does not correspond to the warmest interval during the mid-Cretaceous, additional sources of CO2, such as subduction zone volcanism or the kimberlite formation, may explain the Cretaceous Thermal Maximum. As Os isotope ratios do not show any sharp unradiogenic shifts and PGE concentrations do not exhibit a pronounced enrichment, an influx of fine-grained cosmic dust to the Earth’s surface, possibly from the long-period comet showers, can be inferred at the time of the 3He enrichment during the mid-Turonian K3 event. Our findings highlight the different behaviors of 3He and PGE information in the sedimentary rocks during the input of fined-grained extraterrestrial materials.

Field and petrological characteristics of PGE-rich podiform chromitites: Implications for efficient exploration

Two distinct types of podiform chromitites have been extensively described within the mantle sections of Neoproterozoic ophiolites in Saudi Arabia and Phanerozoic ophiolites in Oman. The first type is concordant with the foliation plane of the harzburgite host, generally lensoidal in shape and is characterized by its low platinum-group elements (PGE) contents. The second type occurs as discordant or dike-like bodies and predominantly rich in PGE. In addition to their mode of occurrence, several lines of evidence can be employed to differentiate these two types of chromitites, both in the field and during laboratory investigations. Concordant PGE-poor chromitites are typically homogeneous in appearance (with regards to chromite grains distribution) and massive, featuring chromian spinel (Cr-spinel) with olivine as the dominant silicate inclusions. In contrast, discordant PGE-rich chromitites are often heterogeneous in appearance, exhibiting a semi-massive to disseminated, and are rich in both hydrous (amphiboles) and anhydrous (clinopyroxene) silicate inclusions. Discordant dunite veins, sharing similar petrological characteristics, are consistently associated with PGE-rich chromitites, with Cr-spinel containing a notable abundance of clinopyroxene inclusions. Compared to podiform chromitites found worldwide, concordant chromitites have lower to intermediate PGE contents (typically < 500 ppb), display a gently sloping negative trend in PGE patterns, contain very few platinum-group mineral (PGM) grains, and exhibit a wide compositional range of Cr-spinel (Cr# = Cr/(Cr + Al) atomic ratio, varying from 0.5 to 0.9). In contrast, discordant chromitites have higher PGE contents (commonly > 500 ppb), show a strong decoupling between the IPGE (Os, Ir, Ru) and PPGE (Rh, Pt, Pd) subgroups of the PGE distribution patterns, contain numerous PGM grains, and consist of high-Cr varieties (Cr# mostly > 0.6). In-situ analysis of clinopyroxene inclusions within Cr-spinel of discordant chromitites and associated dunite veins revealed characteristic U-shaped rare-earth elements (REE) patterns. This suggests the involvement of a hydrous SiO2-rich melt in the formation of PGE-rich chromitites and their associated discordant dunites. The wide range of Os-isotope composition of the Os-rich PGM, spanning from sub-chondritic to supra-chondritic 187Os/188Os values in the PGE-rich chromitites serves as further evidence for the heterogeneous hydrous melting process that led to the formation of these chromitites. These features, encompassing field and petrological characteristics, can be attributed to variations in the tectonic setting during the formation of podiform chromitites and can serve as valuable guidelines for PGE exploration.

Platinum-group minerals and the genesis of the sulfide PGE-Cu-Ni deposit “Ore Horizon 330” of the Monchegorsk Pluton, Kola Region, Russia

The sulfide PGE-Cu-Ni deposit “Ore horizon 330” (OH330) of the reef type is located within the orthopyroxenite of the Sopcha intrusion, which is part of the Paleoproterozoic layered Monchegorsk pluton (Monchepluton) in the Kola Region. OH330 is a sill-like body with a thickness of 4–6 m, traced along the entire perimeter of the Sopcha intrusion at a distance of 3.3 km and a width of 1.2 km. The OH330 deposit was studied in two well-exposed sections of its western flank. Here, OH330 is a thinly layered horison that lies among the host olivine orthopyroxenite. It consists of interlayers of dunite, harzburgite, and orthopyroxenite containing fine sulfide dissemination with increased contents of Ni (1290–5220 ppm), Cu (up to 2320 ppm), and platinum group elements (PGE) (0.43–1.40 ppm of the PGEtot). It is assumed that the OH330 parental magma was significantly contaminated with crustal material, saturated with sulfur and chalcophile elements. The separation of immiscible sulfide droplets during cooling led to the development of ore sulfide concentrations. The association of platinum group minerals (PGM) in harzburgite is represented by the predominant PtFe alloys and subordinate PGE bismuth-tellurides, arsenides, and tellurides. Compared to harzburgite, PGE bismuth-tellurides and tellurides are widely distributed in orthopyroxenite, whereas arsenides, PtFe alloys, and sulfides are minor. The PGE-Cu-Ni ore formed in a relatively wide temperature range from 1000 to 900 °C (base metal sulfides) to 900–600 °C (PGE sulfides and arsenides, and PtFe alloys) and ended at a temperature of 600–400 °C with a crystallization bismuth-tellurides and tellurides of Pt and Pd.

Behavior of Platinum-Group Elements during Hydrous Metamorphism: Constraints from Awaruite (Ni3Fe) Mineralization

Abstract Natural Fe-Ni alloys are common in meteorites and, presumably, the Earth’s core, where they host significant platinum-group elements (PGE). However, little is known on PGE concentrations in hydrothermal or metamorphic Fe-Ni alloys (i.e., awaruite Ni3Fe) from terrestrial rocks. In this work, we examine the geochemistry of awaruite and related minerals from several placer deposits sourced from the suprasubduction ophiolitic (Kamchatsky Mys, Karaginsky Island, and Mamet) and Ural-Alaskan (Galmoenan) complexes of Kamchatka and the Koryak Highlands (Far East Russia) in order to assess the abundance of PGE in awaruite and constrain their mobility under metamorphic and hydrothermal conditions. Studied awaruite from ophiolitic and Ural-Alaskan type complexes formed via desulfurization of pentlandite during serpentinization. Three groups of platinum-group minerals (PGMs) are associated with awaruite from Kamchatsky Mys: (1) Pt-Fe alloys such as ferronickelplatinum (Pt2FeNi) or unnamed Ni2FePt alloys; (2) Os-Ir-Ru alloys of various composition; (3) Pd-Sb minerals which form together with serpentine during hydrothermal alteration. Despite the abundance of PGM inclusions, no significant PGE concentrations were measured in awaruite from the Kamchatsky Mys, Karaginsky Island, or Mamet ophiolites. In contrast, pentlandite relicts in awaruite from placers related to the Galmoenan Ural-Alaskan type complex contain exceptionally high, previously unreported, Os (up to 540 ppm). Awaruite that forms on behalf of this pentlandite does not show any significant Os enrichment. Rare Galmoenan awaruite analyses yield up to 3 ppm Pd. The new data are not in complete accordance with previous studies that reported relatively high (up to first 10 ppm) PGE content in awaruite. We attribute this to low PGE concentration in precursor sulfides and preferential partitioning of PGE into discrete secondary PGM within awaruite. Nevertheless, abundant inclusions of secondary PGM in awaruite provide evidence of PGE mobility during metamorphic and hydrothermal alteration of ultramafic rocks.

Accessory mineralisations in lherzolites of Northern Kraka massif (South Urals)

The findings of platinum group metal mineralization (PGM) and the distribution of platinum group elements (PGE) in lherzolites of the Northern Kraka massif are described. The total contents of PGE are approximately two orders of magnitude lower than those in chondrite and are close to pyrolite, relative to which the studied lherzolites are enriched in Pd and depleted in Ru. In segregations of PGMs, the presence of all PGEs (except rhodium) in various proportions was established. All found grains are divided into three contrasting types: the refractory triad Os–Ir–Ru, essentially platinum with the participation of Pd, and Cu–Pd. Almost all found PGM grains are localized either in the peripheral parts of grains of altered sulfides (heazlewoodite, pentlandite) or in the silicate matrix in the immediate vicinity of sulfide segregations. Based on the mineral associations and PGE distribution, a probable genesis of segregations has been suggested. Associations of Cu–Pd and Pd–Pt(+Cu) composition most likely formed during the crystallization of sulfides from the extracted partial melts. This is indicated by their close association with clinopyroxenes and the presence of relatively fusible platinoids and copper. The associations of Pt–Ir and Os–Ir–Ru(+Pt) composition are most likely restitic, formed in place of primary mantle sulfides as a result of extraction of more fusible elements and further desulphurization. The isolation of platinoids as their own mineral phases is associated with the influence of superimposed low-temperature processes – subsolidus redistribution during cooling and subsequent serpentinization.

Noble and base metal geochemistry of late- to post-orogenic mafic dykes from central Spain

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

Platinum-group elements (PGE) in the New Afton alkalic Cu-Au porphyry deposit, Canadian Cordillera, II: PGE distribution and models for the hydrothermal coprecipitation of Co-Ni-Pd-Pt in pyrite

The first part of this article can be found here: 10.3389/feart.2023.819129 (DOI). Occurrences of significant concentrations of platinum-group elements (PGE) in porphyry deposits have seen limited attention and the geochemical controls on their deposition are poorly understood. New Afton is a late Triassic alkalic Cu-Au porphyry deposit situated 10 km west of Kamloops, British Columbia with Pt and Pd enrichments locally exceeding 30 ppm. Primary PGE deposition accompanied hypogene Cu-Au mineralization linked to potassic alteration, with Pt and Pd deportment dominated by their inclusion in early Ni- and Co-rich pyrite with secondary amounts of platinum-group minerals (PGM) mertieite-II, isomertieite &amp;gt; temagamite, and kotulskite. Secondary hydrothermal alteration of hypogene PGE-rich pyrite promoted remobilization of PGE, creating vein-controlled PGE enrichment where essentially all Pt and Pd are hosted by PGM dominated by temagamite &amp;gt; mertieite-II, and sperrylite, leaving behind complementary PGE-depleted domains. Thermodynamic modelling shows that primary co-precipitation of Pt with Co and Pd with Ni can be explained solely through the cooling of a hydrothermal system, regardless of whether the mineralizing fluid was a brine or a low-density vapour. Pd and Pt were transported from an underlying magma reservoir into the porphyry system by a magmatic-hydrothermal fluid as Cl− complexes. Pt-rich cobaltian pyrite formed at the highest temperatures, whereas Pd-rich nickeliferous pyrite formed at somewhat lower temperatures. Zonation of metals in pyrite suggests that PGE were incorporated in pyrite under conditions of fluctuating fluid temperatures during transport and cooling of orthomagmatic fluids during the main Cu-Au mineralization stage. There exists a hitherto overlooked potential for significant Pt and Pd mineralization in early hypogene pyrite that may be spatially separated from visible Cu mineralization, especially in potassic or ultrapotassic systems.

Geological and geochemical characteristics of shallow-buried ferromanganese crusts from Weijia Guyot and their resource potential

Numerous buried ferromanganese (Fe-Mn) crusts covered by surface sediments, such as foraminifera ooze, were found on the platforms of certain guyots in the Western Pacific Ocean. To examine the origin of these shallow-buried (<100 cm below the seafloor) Fe-Mn crusts and evaluate their resource potential, we conducted mineralogical and geochemical studies on seven drill cores from the Weijia Guyot(Ita Mai Tai Guyot), which is in the Co-rich Fe-Mn crust contract area of the China Ocean Mineral Resources Research and Development Association (COMRA).The buried Fe-Mn crusts exhibit multilayer structures and are composed of disordered vernadite, similar to the exposed Fe-Mn crusts. Moreover, they show comparable contents of major metallogenic elements (e.g., Mn, Cu, Co, and Ni) and platinum group elements when compared to exposed Fe-Mn crusts. However, they have much higher contents of rare earth elements (REEs) than those of exposed Fe-Mn crusts, indicating a comparable resource potential. The high P contents and collophanite observed in the petrographic microscope indicate that an evident phosphatization event occurred. Based on the tectonic evolution and sedimentation rate, we infer that the phosphatization event mainly took place before the crusts were buried. The phosphatization of shallow-buried Fe-Mn crusts results in an enrichment of REEs and their fractions. This process can also modify the sedimentary environment for Ce precipitation, which may, in turn, alter the diagenetic microenvironment of the shallow-buried Fe-Mn crusts. The mineralogical and geochemical results reveal that burial doesn't cause a significant difference between shallow-buried and exposed Fe-Mn crusts from Weijia Guyot, and they are still of hydrogenetic origin. The findings and research on shallow-buried Fe-Mn crusts indicate that they have comparable resource potentials with exposed Fe-Mn crusts. These results will influence future decisions related to the exploration, exploitation, and utilization of Co-rich Fe-Mn crusts in the contract area of COMRA. Furthermore, it could provide valuable insights for other contractors investigating the platform.

Mantle-hosted ophiolitic chromitites from Colombia: implications for petrogenesis and geodynamic evolution

ABSTRACT Chromitite bodies can be found within ophiolitic mantle sections in three locations in Colombia: Medellín, Planeta Rica, and La Guajira. These chromitites are typically massive pods that are often surrounded by lateritic soil in Medellín and Planeta Rica, or small discoidal bodies in La Guajira. Inclusions in chromite are preserved to varying degrees in the three localities: olivine and pargasite are partially preserved in Medellín and Planeta Rica, whereas inclusions in La Guajira chromitites are completely altered to chlorite. Compositionally, chromite from the chromitites is high-Al in Medellín (Cr#[Cr/(Cr+Al) atomic ratio] = 0.41–0.51) and Planeta Rica (Cr# = 0.49–0.57), and high-Cr in La Guajira (Cr# = 0.75–0.76), whereas TiO2 contents are low in the three localities (0.12–0.43 wt%). Regarding trace elements composition, the chromitites are similar to others formed in supra-subduction zone settings; the chromite from the high-Al chromitites is similar to that in MORB/BABB, whereas chromite from the high-Cr chromitites has a strong boninitic affinity. Chromitites from Planeta Rica are richer in Ti, possibly due to the presence of nearby gabbroic bodies and impregnated peridotites. The alteration in chromite rims and silicate matrix is related to metamorphic processes. Bulk-rock platinum-group element (PGE) contents range from 30 to 487 ppb, and chondrite-normalized PGE patterns show the typical enrichment in IPGE relative to PPGE observed in most ophiolitic chromitites. The estimated melts in equilibrium with the different chromitites correspond to MORB-like melts for the Medellín and Planeta Rica chromitites and to boninites for La Guajira chromitites. However, the estimated TiO2 content in the melt is too low for a typical MORB, indicating back-arc basin basalts and a back-arc setting for the formation of the Medellín and Planeta Rica chromitites during the Late Triassic and the Late Jurassic, respectively. La Guajira chromitites are interpreted to have formed in a fore-arc basin related to subduction-initiation during Cretaceous times.

Two-stage emplacement mechanism and symmetrical differentiation process of mafic-ultramafic sills in an arc setting: A case study of the Middle Tonian rock assemblage from the Fanjingshan area

The emplacement and differentiation mechanisms are significant parts of understanding the evolution of a magmatic system. There is debate regarding the emplacement mechanism of long-distance lateral transport of magma. And the differentiation mechanisms mainly target the rock assemblage in mantle plume-related or rift-related settings. As yet, few studies had been followed in the arc setting. In this contribution, the mafic-ultramafic sills formed by long-distance lateral transport of magma in the Fanjingshan area in South China have been selected to decode the magmatic emplacement and differentiation mechanism in an arc system. Voluminous basaltic lava and mafic-ultramafic sills are emplaced, concordant to bedding, within the lower part of the Middle Tonian Fanjingshan Group and laterally extend for tens of kilometers. Some mafic-ultramafic sills display symmetrical lithology and geochemical distribution in vertical sections. We provide a detailed stratigraphy and geochemical analysis of the mafic-ultramafic sills of the Fanjingshan Group. The mafic sills within the underlying strata (the Taojinhe Formation) of the lower part of the Fanjingshan Group were intruded at 832 Ma, which is >8 Ma younger than the basaltic eruptions in the overlying strata (the Huixiangping Formation). Moreover, these mafic sills within the Taojinhe Formation have a positive ɛNd(t) value (+0.9 − +1.6), the lowest Platinum Group Elements (PGE) content and highest Rare Earth Elements (REE) content. Compared with other mafic sills and basalts within different formations (the Huixiangping, Xiaojiahe, and Yujiagou formations), mafic sills within the Taojinhe Formation display a separate field on the principal component analysis diagram, suggesting two stages of mafic magma emplacement with a short interval for the Fanjingshan Middle Tonian volcanic plumbing system. The magma channel of the first stage magma is subsequently used by the second stage magma as suggested by ɛNd(t) values and Nb-Ti anomalies. Although island arc magmas differ greatly from mantle plume magmas in volume, temperature and composition, the calculated dimensionless temperature and dimensionless flux of magma imply that the solidification can affect the formation of mafic-ultramafic sills in the island arc systems. At the same time, the difference in rigidity of the upper and lower strata is not the main reason to control the formation of the mafic-ultramafic sills as suggested by the statistical analysis of the mafic-ultramafic sills occurrence between different wall-rock lithology associations. Through the comparative study of the sills formed by the long-distance lateral transport of magma, it is proposed that the clastic wall-rock provides the first-order control for the sills formed by the long-distance lateral transport of magma. According to the detailed geochemical study of an M-shaped geochemical profile within the Huixiangping Formation, two steps of in situ crystallization model have been suggested. In the first step, minerals crystallizing from the inflowing magma on the floor and roof of mafic-ultramafic sills will become more primitive inwards. In the second step, the magma evolves both downward and upward by uninterrupted in situ crystallization in a closed system that results in the whole-rock becoming more evolved in composition inward.