Platinum Group Element Concentrations Research Articles

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.

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.

Paleoproterozoic East Pana Layered Intrusion (Kola Peninsula, Russia): Geological Structure, Petrography, Geochemistry and Cu-Ni-PGE Mineralization

The East Pana intrusion is a part of the Paleoproterozoic Fedorova–Pana complex (FPC), which belongs to the group of Fennoscandian layered mafic–ultramafic massifs. This article discusses the magmatic stratification of the East Pana intrusion, as well as Cu-Ni and platinum-group elements (PGE) mineralization (PGE zones A, B and C) in its various parts with a total length of more than 20 km, including the East Chuarvy PGE deposit. Based on the whole-rock data on the distribution of major, trace, and ore-forming elements, it is assumed that PGE zone A belongs to the main ore–magmatic system of the FPC, while PGE zones B and C belong to the minor ore–magmatic systems. At the same time, additional magmatic injection played an important role in the formation of economic Cu-Ni-PGE mineralization (PGE zone B), characterized by high PGE concentrations and moderate palladium enrichment. On the normalized distribution spectra of trace elements, the crystallization products of this injection (Gabbronorite Zone 2) have a positive Zr-Hf anomaly, which distinguishes it from host rocks with an anomaly of the opposite sign (Gabbronorite Zone 1, Gabbro Zone). It is assumed that this portion of magma was intruded as a sill of crystal mush, the fractionation of which at depth led to its enrichment with residual liquid.

Platinum Group Element distribution in water and marine biota from two impacted estuarine environments (Douro and Ave estuaries, Portugal)

Platinum Group Elements (PGEs) are contaminants of emerging environmental concern considering their continuous increasing use and subsequent release in the environment. While recent field studies provided PGE levels in seawater, scarce knowledge still exists regarding PGE contamination in marine organisms, especially for rhodium (Rh). Water, macroalgae and mussels were sampled along two representative urbanized estuarine systems and adjacent coastal areas (Douro and Ave estuaries, Portugal). Rhodium and platinum (Pt) concentrations were quantified through both stripping voltammetry and mass spectrometry in collected samples. Spatial mapping of PGE contamination was, to a certain extent, correlated with proxies of urban effluents. The use of Pt/Rh ratios reflected the dominant influence of PGE traffic emissions along the Douro and inputs from various sources (including industries) on the Ave Estuary. Macroalgae and mussels PGE concentrations reflected urban pressure, amplifying environmental signals, and supporting their relevant use as bioindicators of PGE contamination in estuarine/coastal systems.

PGE geochemical and Os-S-Sr-Nd isotopic constraints on the genesis of the Shitoukengde magmatic sulfide deposit in the East Kunlun Orogenic Belt, NW China

Numerous early Paleozoic mafic–ultramafic intrusions that host important Ni-Cu sulfide mineralization occur in the northern part of the Qinghai-Tibet Plateau, NW China. The sulfide-bearing Shitoukengde Ni-Cu deposit, which is hosted in a ∼424 Ma mafic–ultramafic intrusion, is one of the most represent magmatic sulfide deposits in the East Kunlun Orogenic Belt. The intrusion is composed of a large gabbroic body, cut by lherzolite, olivine websterite, websterite, and minor dunite. Disseminated and net-textured sulfide mineralization occurs as lenses in the center and towards the bottom of the ultramafic bodies. The sulfide mineralization has variable sulfur contents of 0.3 to 13.6 wt% with Ni grade of 0.41 wt% on average, and low platinum-group element (PGE) concentrations (<8.2 ppb Ru and <11.9 ppb Pd). Pd and Ru tenors of bulk sulfides in the deposit are characterized by a positive correlation, indicating that PGE tenor variations are mainly controlled by varying R-factors (magma/sulfide mass ratio). The estimated PGE concentrations in the Shitoukengde magma are about three orders of magnitude lower than those in continental tholeiites. The significant PGE depletions in the Shitoukengde deposit may be due to sulfide removal in the magma conduit at depth. Our new sulfur isotope data reported range in δ34S values from −5.0 to 4.9‰ for the sulfide minerals of the Shitoukengde deposit, clearly distinct from those of mantle-derived sulfur (0 ± 2‰) and immediate country rocks (1.1 to 2.0‰). This, together with the modeling for S isotope variation as a function of R-factor, suggest that the current δ34S range is attributed to the addition of external sulfides with anomalous δ34S into the magma at depth, followed by progressive modification to crustal signature by uncontaminated pulses of magma. The ultramafic rocks exhibit low εNd(t) values (−4.71 to 2.12) and high (87Sr/86Sr)i ratios (0.7073 to 0.7130), which are more enriched relative to the isotopic compositions of depleted mantle. Mixing calculations indicate that about 15 to 20 wt% of lower crustal contamination plus variable upper crustal contamination is required to explain the variations of Sr-Nd isotopes in the Shitoukengde intrusion. Significant crustal contamination is also supported by the mineralization at Shitoukengde having high (187Os/188Os)i (0.24 to 4.51) and γOs(t) values (0.24 to 2534) that are comparable to or higher than those of the important Ni-Cu deposits worldwide. Current results provide evidence that assimilation of crustal sulfur at deep crust and siliceous material with variable crustal contamination during magma ascent and emplacement played critical roles in the attainment of sulfide saturation in the parental magma and the formation of the Shitoukengde deposit.

Neoproterozoic Aksu Diabase Dyke, Chinese South Tianshan: Magma Sources and Implications for Regional Gold Metallogeny

Tianshan is one of the world’s largest gold provinces; however, the relationship between gold mineralization and metasomatized subcontinental lithospheric mantle (SCLM) remains poorly understood. To improve our understanding, we present new bulk-rock geochemistry and platinum group element (PGE) concentrations of the SCLM-sourced Aksu Neoproterozoic diabase dykes in Chinese South Tianshan. These data, combined with in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of hydrothermal pyrite grains in the diabase dykes, are used to discuss the SCLM source characteristics in the region and their potential links to formation of gold deposits. The diabase dykes exhibit high Th/Yb (0.47–0.62) and low Nb/U (13.4–16.3) ratios, indicating that magma evolution involves subduction-related fluid metasomatism and limited contamination of the continental crust. This is consistent with little variation in whole-rock Pd/Zr, Cu/Zr, and Ni/MgO ratios, suggesting that no sulfide segregation was caused by crustal contamination and magma mixing. In addition, the diabase dykes show low PGE and Au contents, with high Cu/Pd (&gt;105) and low Cu/Zr (&lt;0.5) ratios, indicating that magmas were derived from low-degree partial melting of the SCLM under S-saturated conditions. Such source characteristics indicate residual sulfides and chalcophile elements (e.g., PGEs, Au, and Cu) were concentrated at the SCLM reservoir in South Tianshan. Hydrothermal pyrite in the studied dykes has similar Au/Ag ratios and trace element distribution patterns to gold-bearing pyrite of lode gold deposits in Chinese South Tianshan, indicating that metasomatized SCLM may have contributed ore metals during the formation of these gold deposits. Adding to the available data, our study highlights that the SCLM may be a potential metal source reservoir, and it may have contributed to formation of the lode gold deposits in Chinese South Tianshan.

Possible source of Au in the Jiaodong area from lower crustal sulfide cumulates: Evidence from oxygen states and chalcophile elements contents of Mesozoic magmatic suites

The genesis of giant Jiaodong gold province remains perplexing although many ore-forming models have been proposed in the past decades. The key issues are what caused the burst of gold around 120 Ma and possible sources of gold. Here we report gold and platinum group elements (PGE) data and magmatic oxidation states of the pre-mineralization Mesozoic plutonic rocks and some mafic dykes to discuss their potential effects on gold mineralization. Gold concentrations of the Late Jurassic Linglong granite are 0.015–0.278 ppb. The Early Cretaceous plutonic rocks (129–127 Ma) have the same levels of Au concentrations (Au = 0.031–0.047 ppb). Both of the Jurassic and Cretaceous magmatic rocks are depleted in Au compared to normal granitic rocks worldwide (∼1 ppb). Palladium and platinum concentrations in the Linglong granite are 0.160–0.385 ppb and 0.046–0.116 ppb, respectively. Meanwhile, Pd and Pt contents of Cretaceous magmatic rocks are 0.069–0.363 ppb and 0.063–0.465 ppb, respectively.Oxygen fugacities of the Late Jurassic Linglong granite calculated by biotite oxybarometer are FMQ + 0.5 to FMQ + 1.2 (where FMQ refers to fayalite-magnetite-quartz buffer). Oxidation states of the Early Cretaceous magmatic rocks estimated via amphibole oxybarometer are from FMQ + 0.3 to FMQ + 1.1 for Guojialing granodiorite. Zircon trace element data yield overall higher fO2 values than those estimated with biotite and amphibole compositions. Oxygen fugacities are from FMQ – 0.6 to FMQ + 1.9 (sample CJ-3) and FMQ – 0.7 to FMQ + 1.1 (sample LL-1) for the two Linglong granite samples. Zircons from the Early Cretaceous Guojialing pluton yield oxidation states between FMQ – 0.8 and FMQ + 1.9 (DCW-1), which are about one log unit higher than those of the Beijie pluton (BJ-1: FMQ – 1.0 to FMQ + 0.7). However, oxidation states from nearly syn-mineralization mafic dyke reach FMQ + 2.0 to + 3.0 when estimated via zircon oxybarometer. Oxidation states given by whole rock Fe3+/Fe2+ ratios also support the results calculated by mineral compositions.There is no fractionation crystallization of redox-sensitive minerals, like magnetite or garnet, as indicated by geochemical data, which implies that the oxidation states of magmas may reflect their source regions. However, the depleted nature of Au and PGE must imply that these elements are missing. We propose that sulfide saturation at the deep crust can be invoked to explain the extremely low gold and PGE contents of these magmatic rocks. The sulfide cumulates act as sinks for sulfide-loving elements (e.g., Au), which are pre-enriched source for later large-scale gold mineralization in Jiaodong area. The underplated water-rich mafic magma from enriched mantle source ponded at the lower crust, continuously crystallized anhydrous minerals (e.g., olivine and clinopyroxene) and reached volatile saturation at the very moment. This volatile, probably carrying gold, resorbed the sulfide cumulate and escaped to shallow levels to form the giant Jiaodong gold deposits.

Distributions of Platinum Group Elements and Re-Os Isotope Systematics in Chromite from the Coobina Chromitite Deposit in Western Australia: Implications for Chromite as a Sulfide Mineralization Indicator

ABSTRACTThe abundance of Ru in chromite has been suggested as an indicator of sulfide liquid saturation in komatiites. The komatiite magma-derived Archean Coobina intrusion is known to be barren in terms of sulfide mineralization. Therefore, the Coobina intrusion can serve as a useful case study to test the applicability of Ru abundance in chromite as a potential indicator for sulfide mineralization, as well as for better understanding the PGE-chromite association in general.The Coobina intrusion is a highly deformed layered intrusion interpreted to be a flared dike. It contains multiple massive chromitite seams that have been recently mined for metallurgical-grade chromite. In this study, 18 samples from chromitite seams throughout this intrusion are investigated for their whole-rock platinum group element (PGE) contents, which are compared to their chromite mineral chemistry (including PGE content), the platinum group mineral (PGM) mineralogy, and Re-Os isotope systematics. Each sample has a similar chromite major and minor element chemistry, but a unique trace element signature, even within the same seam. In general, there are higher concentrations of Ru (&amp;gt;300 ppb) within chromite in the southeast (toward the feeder dike) and lower concentrations (&amp;lt;50 ppb Ru) in the northwest. At a sample scale, Ru in the whole rock and Ru in solid solution in the chromite are inversely correlated, while Ir shows a positive correlation between the whole rock and chromite mineral chemistry, indicating differing partitioning behaviors within the iridium-group PGE (IPGE = Os, Ir, Ru). The inverse correlation between Ru in solid solution within chromite and Ru in whole-rock chromitite suggests that, for seams with high Ru in whole rock, Ru is occurring within separate PGM phases. This is supported by the observation that the samples with high whole-rock Ru also have a high number of visible metal alloy and/or PGM inclusions. Although these inclusions are not necessarily Ru-rich phases, their presence suggests that there is a preference for these samples to form nuggets, which may restrict Ru partitioning into the chromite crystal structure. We suggest that the low Ru values in the Coobina chromite are a result of transient sulfide saturation. The Re-Os isotopic composition of the Coobina chromitite is chondritic [γ187Os(3.189 Ga) = −0.63 ± 0.21] and is consistent with derivation of the Coobina parental magma from the convecting upper mantle source, providing evidence for the mantle origin of the Coobina PGE inventory.If using chromite as a detrital indicator mineral for magmatic sulfide exploration, it must be kept in mind that transient sulfide saturation within chromitite seams may give a false positive signature.