Platinum-group Minerals Research Articles

Platinum-group minerals from the Malaya Kamenushka River placer, Middle Urals, Russia

AbstractThis work presents a detailed study of platinum-group mineral (PGM) assemblages from the Malaya Kamenushka River placer, whose formation is associated with the weathering of the Kamenushensky Uralian–Alaskan type massif, Middle Urals, Russian Federation. The deposit is characterised by the dominance of isoferroplatinum, together with significant numbers of inclusions of Os–Ir–Ru alloys and platinum-group element (PGE) sulfides. A study of the Os–Ir–Ru alloys permitted recognition of two types of iridium with different morphology and composition. The similarity of the PGM assemblages from the Malaya Kamenushka River placer and the lode mineralisation of the Kamenushensky massif is demonstrated. A comparison of PGM assemblages from the Malaya Kamenushka River placer with other placers and massifs of the Ural platinum belt demonstrates significant differences in the number of Os–Ir–Ru inclusions. Such differences for minerals of refractory elements cannot be explained by the vertical zoning of the lode mineralisation. Most probably this is associated with the enrichment of the primary substrate with Os, Ir and Ru and/or the degree of melting, depending on the chosen model of formation of the Uralian–Alaskan type massifs.

Implicit Geomodelling of the Merensky and UG2 Reefs of the Bushveld Complex from Open-Source Data: Implications for the Complex’s Structural History

The Bushveld Complex (BC) is the world’s largest source of platinum group metals. Extensive studies on the complex have focused on its geochemistry, magma and platinum group mineral genesis, mineral characterization and intrusion mechanisms. However, relatively little work has been undertaken on the overall 3D geometry of the complex, which detracts from the adequate contextualization of such studies. Furthermore, the absence of a broader 3D model of the complex does not permit the identification of structural trends and mineralization patterns. This contribution details the construction of 3D implicitly-modelled Merensky and UG2 Reefs across the Rustenburg Layered Suite of the BC, using Seequent’s Leapfrog software. Multiple open-source and public-domain data sources and modelling workflows were explored to account for disparities in data resolution, data spacing and clustering, and the resolution of model outputs. Key outcomes are (1) a representative, fully-implicit, dynamic geological model of the Merensky and UG2 Reefs over the main chamber of the BC; (2) identification of modelled features that augment the current understanding of the BC’s kinematic history and cumulative deformation; and (3) identification and analysis of subtle geometrical trends and patterns, such as inter-reef spacing and modelled depths, as well as structural domains that may not have been apparent from numerous, more focused or isolated petrological or geochemical studies. It is anticipated that this baseline 3D model will form the foundation for future, possibly localized, dynamic updates as further information becomes available. The addition of proprietary (viz., non-open-source) data, such as 2D seismic sections and 3D seismic surveys, would enhance the overall resolution and quality of such a model and resulting interpretations.

PLATINUM-BEARING PLACERS: MINERAL ASSOCIATIONS AND THEIR 190Pt-4He AND Re-Os AGES, AND POTENTIAL LINKS WITH LARGE IGNEOUS PROVINCES IN THE SIBERIAN CRATON

AbstractPlacer deposits in the Siberian platform are closely associated with well-known platiniferous intrusions (Kondyor, Inagli, Noril’sk, Chiney, and Guli) but there are also abundant platinum-group mineral (PGM) placers of large areal extent in the river basins of Siberia (Vilyui, Lena, Aldan, Anabar, and Olenek) for which the primary sources are unknown. The 190Pt-4He dating method is introduced, compared with existing Re-Os isotope dating of PGMs, and applied to Pt-bearing placers of Siberia. Correlations between the 190Pt-4He and Re-Os isotope ages of the PGMs and with ages of major magmatic events in the Siberian craton show that the potential sources for some of the placers are likely buried Precambrian mafic-ultramafic intrusions of Large Igneous Provinces (LIP). The approach of this paper can be applied to other cratonic areas with alluvial PGMs of unknown origin.

Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones

Constraining the behaviour of Re and Os during eclogite melting is required to understand the Re and Os budget and 187Os/188Os of recycled slabs produced at warm subduction zones. It is particularly relevant to early Earth history, a period during which slab melting could have prevailed over dehydration due to higher mantle temperatures. There are however currently few constraints on Re and Os mobility during slab melting. Accordingly, we measured Os, Re and 187Os/188Os in primitive submarine lavas (Mg# ˃ 0.6) from the western Aleutian Arc. These include strongly adakitic rocks shown to be derived from eclogite melting (high-Mg# andesite, dacites and rhyodacites), as well as non-adakitic rocks (high-Mg# andesites, basaltic andesites and basalts) with variable sediment and fluid-derived slab contributions for comparison.The 187Os/188Os of the adakitic and non-adakitic volcanic rocks vary significantly but largely overlap. In both groups, the most radiogenic values occur in samples with the lowest Os concentrations, thus implicating crustal assimilation as the main cause of Os isotope variations. Adakitic and non-adakitic rocks least affected by crustal assimilation have overlapping 187Os/188Os of 0.141–0.149. We show that the source of the adakites is very unlikely to comprise significant eclogite-derived Os, which suggests no or minimal mobilization of Os during eclogite melting. Eclogitic Os is inferred to be retained in sulphides or replacement phases formed upon sulphide breakdown for which Os has high affinity, such as a platinum-group minerals (PGMs). The small Os budget of the adakites is most likely derived from limited reaction with the mantle wedge during ascent. Degassing has reduced Re contents in most samples, but not for end-member adakites (SiO2 > 67% and Sr/Y > 200; n = 4) that were erupted at seafloor depths > 2500 m. These undegassed samples have elevated Re concentrations (0.8–1.5 ppb) that are positively correlated with Sr/Y and so are interpreted to be primary magmatic concentrations resulting from the mobilization of Re from the slab. Re could either be derived from the eclogites or from the serpentinite-derived fluids fluxing eclogites during melting. The former scenario would produce recycled residual crusts with lower Re/Os than in unmelted eclogites while the latter would result in Re/Os ranging from similar to higher than prior to melting. In both cases, the Re/Os and therefore the time-integrated 187Os/188Os of residual crust produced at warm subduction zones involving slab melting are likely to be different from that processed at cooler typical modern subduction zones. Therefore, if slab melting was an important process during the early Earth, the use of Re and Os partitioning in modern subduction zones to model the source of magmas comprising old recycled oceanic crust, such as the HIMU (high μ = 238U/204Pb) ocean island basalts (OIBs), might lead to erroneous interpretations.

Emplacement of sharp-walled sulfide veins during the formation and reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario

Broken Hammer is a hybrid Cu–Ni–Platinum Group Element (PGE) footwall deposit located in Archean basement rocks below the impact-induced Sudbury Igneous Complex (SIC), Canada. The deposit consists of massive chalcopyrite veins surrounded by thin epidote, actinolite, and quartz selvedges and low-sulfide, high-PGE mineralization consisting of disseminated chalcopyrite (<5%) and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite, and quartz. The veins are grouped into five steeply-dipping sets, striking northeast-, southwest-, southeast-, south-, and east–west, which were emplaced along impact-related fractures that were reactivated multiple times during stabilization of the crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip shattered the quartz–chlorite veins into fragments that were incorporated in massive sulfide veins that crystallized from fractionated sulfide melts or from high temperature (400–500 °C) hydrothermal fluids, which migrated outward into the basement rocks from a cooling and crystallizing SIC melt sheet. Hydrothermal fluids syn-genetic with the epidote–actinolite–quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGMs from the sulfide veins and redistributed them to form low-sulfide, high-PGE zones in the footwall rocks. During post-impact tectonic events, slip at temperatures below the brittle–ductile transition for chalcopyrite (<200 °C to 250 °C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu–Ni–PGE footwall deposits formed by the reactivation of impact-related fractures that provided conduits for the migration of melts and hydrothermal fluids.

Compositional Variations and Tectonic Settings of Podiform Chromitites Associated with Ultramafic Rocks of the Pan‐African Proterozoic Ophiolites from South Eastern Desert, Egypt

AbstractThe Precambrian podiform chromitites associated with ophiolites are abundant in Pan‐African belt in central Eastern Desert (CED) and south Eastern Desert (SED), Egypt and range from 690 to 890 Ma in age. The studied chromitites associated with Neoproterozoic ophiolites are distributed in southern Eastern Desert, Egypt in Baranis ‐ Shalaten sheet and occur as lenticular bodies with variable dimensions in ultramafic component (serpentinites). We present geochemical and mineralogical data from three areas of ophiolites and associated chromitites namely Gebel Abu Dahr (D), Gebel Arais (A) and Gebel Anbat in the Wadi Hodein area (H) (Fig. 1). The paper studies the compositional variations and tectonic settings of podiform chromitites associated with ultramafic rocks, in addition to the alteration process of chromite during metamorphism.The ophiolite in the present areas is composed of the ultramafic rocks (mainly serpentinites) with minor relics of fresh dunite and harzburgite. All these rocks are affected by metamorphism and subsequent retrograde during subduction and exhumation.Six samples selected from the serpentinites geochemically analyzed for major, trace and some REE elements and the geochemical results reflect that harzburgite and dunite compositions are typical of depleted mantle peridotite. Microprobe analyses and SIMS investigations were carried out for three massive podiform chromitite ore bodies and disseminated chromites in serpentinites (1215 spot probe analyses), and silicate minerals in serpentinite rocks such as serpentine and olivine (102 spots). Serpentine minerals are mainly antigorite with some chrysotile in serpentinite rocks and in chromitites, mainly filling cross‐cutting veins.In this study, we consider that the alteration occurred in two stages: during the first one chromite reacted with olivine and water to form Cr‐ and Fe‐rich, porous chromite and chlorite; during the second event magnetite filled the pores, created in the porous chromite and defused into this chromite to form homogeneous magnetite.According to this, the composition of chromite is a key factor controlling the metamorphic reaction between olivine and chromite because if the primary chromite is very poor in Al, the chlorite‐forming reaction hardly takes place. In this case, during the second event, the addition of magnetite only contributes to create a magnetite corona around the former chromite grains without any diffusion at the chromite‐magnetite boundary as suggested by Gerbilla et al. (2012).Barnes (2000) studied the chromite in komatiites and modification during green schist to mid amphibolite facies metamorphism. He suggested that the chromite cores continually equilibrated with magnetite rims document metamorphic grade conditions. Barnes (2000) suggested that the relative proportions of Cr3+, Al3+ and Fe3+ of chromite are not affected by metamorphism up to lower temperature amphibolite facies implying restricted mobility of these elements occurred under lower amphibolite facies. So, the chromite in lower temperature amphibolite facies preserves its primary igneous chemistry and can be used to estimate the metamorphic grade. Sack and Ghiorso (1991) and Barnes (2000) suggested that all chromite cores are equilibrated at temperature below ∼ 500–550°C corresponding to lowest amphibolite facies metamorphism and reflect magmatic composition not influenced by metamorphism. In this study, there is no alteration but only nearly pure magnetite deposition and development with restricted Cr‐solubility at < 500°C in the chromite rims on crystal boundaries and within fractures as shown in Fig 2a, b. Also magnetite alters later to hematite.The podiform chromitites are common in the Moho transition zone (MTZ) to the mantle section of ophiolites or harzburgite dominant peridotite massifs (e.g., Arai, 1997; Miura et al., 2012). They have been interpreted as a product of peridotite / melt reaction and subsequent melt mixing within the MTZ to the upper mantle; they are basically magmatic cumulates that formed at the upper mantle level (e.g., Arai and Yurimoto, 1994; Zhou et al., 1994). They are thus a good marker of peridotite / melt reaction (e.g., Arai, 1997). The Pan‐African podiform chromitites may have formed in the same way as the Phanerozoic, namely by melt‐ harzburgite reaction and subsequent melt mixing. The podiform chromatites and disseminated chromites are high‐Cr chromites and have range in Cr#(Cr/Cr+Al) atomic ratio from 0.75 to 0.95 and low Ti with boninitic affinity (Fig. 3a), indicating an island arc setting in supra ‐ subduction zone setting. The present massive chromitites and disseminated chromites in serpentinites fall in the field of chromites de Bou Azer, chromites de Cordoba, Argentinia in the Cr# versus Mg# diagram (Fig. 3b, c) (Gervilla et al., 2012)The studied chromatites contain some grains of platinum‐group minerals (PGM) such as sulfides (Os‐rich laurite) and Os–Ir alloy as shown in Fig. 4 and as reported in South Eastern Desert by Ahmed (2007).

Contrasting platinum-group mineral assemblages of the Kondyor massif (Russia): Implications for the sources of HSE in zoned-type ultramafic massifs

This study presents the first highly siderophile element (HSE: Re, Os, Ir, Ru, Rh, Pt, Pd, Au) abundance and Re–Os isotopic data for primary Pt–Fe minerals and Os-rich alloys from the Kondyor zoned-type ultramafic massif located in the southeastern part of the Siberian Craton, Russia. The Kondyor massif is composed of three zones: (A) the oldest dunite core with associated chromitites, with an age of emplacement of ~250 Ma, (B) a younger rim represented by metadunite, wehrlite, clinopyroxenite, and melanocratic gabbro that has transitional contacts with the dunite core, and (C) the youngest apatite-phlogopite-magnetite-rich clinopyroxenite stockwork exposed in the southwestern part of the massif. It has intrusive contacts with the dunite core and formed at ~125 Ma. Chromitites from zone (A) and clinopyroxenites from zone (C) analyzed in this study are characterized by distinct platinum-group mineral (PGM) assemblages dominated by Pt–Fe alloys in the former and cooperite with sperrylite and Pt–Fe alloys in the latter, with subordinate amounts of Os–Ir alloys present in both assemblages. The Re–Os isotope results identify a narrow range of initial 187Os/188Os values for Os–Ir alloys from the zone A chromitites (0.1249–0.1254) and the chromitites themselves (0.12466 ± 0.00005). Such chondritic initial 187Os/188Os values (γ187Os(T =250 Ma) ranging between −0.5 and + 0.1) indicate that the HSE budget of the chromitites and PGM from zone (A) was largely controlled by that of the mantle domain that evolved with long-term near-chondritic Re/Os, a notion that is also supported by the recent data of Luguet et al. (2019). In contrast, Os alloys from the zone C clinopyroxenites have initial 187Os/188Os values ranging between 0.1302 and 0.1308 (initial γ187Os(T =125 Ma) = +3.2 to +3.7), indicative of a suprachondritic time-integrated Re/Os in the source of these rocks. The mineralogical and isotope-geochemical data point to a high-temperature origin of the studied PGM and at least two distinct sources of HSE in the ultramafic rocks of the Kondyor massif.

Low-Sulfide Platinum Group Element Ores of the Norilsk-Talnakh Camp

Abstract Low-sulfide platinum group element (PGE) mineralization of the Norilsk-type intrusions is located within the Upper Gabbroic Series, which comprises rocks heterogeneous in texture and composition. The highest grade of 10 to 50 g/t PGEs is confined primarily to chromitiferous taxitic gabbrodolerite, which forms irregular lens- and vein-like bodies that interfinger with contact gabbrodolerite, intrusion breccia, leucogabbro, and gabbrodolerite variably enriched in olivine, from olivine free up to picritic compositions. The abundant amygdules and pegmatoidal textures in Upper Gabbroic Series taxitic rocks, as well as the high enrichment of halogen in minerals (e.g., ≤4.6 wt % Cl in apatite), indicate a higher volatile content of the local magma compared to the magma that precipitated the Main Series. The observed diversity in spinel compositions, which evolve from chromite to Cr magnetite as well as toward hercynite, titanomagnetite, and ulvöspinel, is also indicative of crystallization from a fluid-saturated mush that subsequently reacted, to varying degrees, with contaminated trapped melt and immiscible fluid. The high PGE/S ratio is a primary feature of this mineralization style, albeit the ratio partly increased during sulfide replacement and resorption. The PGE tenor of bulk sulfides calculated as ΣPGE (g/t) in 100% sulfides exceeds 160 and may reach up to 1,400 to 2,500 in low-S ores (0.2–3 wt % S), whereas the value does not exceed 42 in the Talnakh disseminated ore and ranges from 35 to 120 in the Norilsk disseminated ore (1–10 wt % S). Several PGE peaks in the vertical sections correlate well with Cu, Ni, S, and Cr peaks, as well as with observed elevated proportion of amygdules. Low-sulfide ores are composed of two primary sulfide assemblages of pyrrhotite + pentlandite + chalcopyrite and pentlandite + pyrrhotite. The primary sulfides are depleted in the heavier 34S isotope relative to sulfides of the corresponded main orebodies (e.g., mean δ34S = 8.9‰ versus δ34S = 12.3‰, respectively, in the Kharaelakh intrusion). A secondary pyrite + millerite + chalcopyrite assemblage has isotope composition enriched in 34S by 2 to 6‰ δ34S with respect to primary sulfides. The directly measured PGE content in sulfides (e.g., 11–2,274 g/t Pd in pentlandite and 0.10–33.3 g/t Rh in pyrrhotite) is within the range of the typical Norilsk-type magmatic sulfide compositions. The textural setting and diversity of platinum group minerals (PGMs) favor the hypothesis of fluid-controlled crystallization. However, the distinct PGM assemblages in Norilsk 1 and Talnakh-Kharaelakh low-sulfide ores are comparable with those of the corresponding presumably magmatic disseminated and massive orebodies. The most remarkable characteristic is the widespread Pt-Fe alloys in Norilsk 1 and their absence in Talnakh-Kharaelakh, which is interpreted to reflect better preservation of the high-temperature PGMs in Norilsk 1 in contrast to their substantial replacement in more oxidized fluid-enriched environments in Talnakh-Kharaelakh.

Platinum Group Element Enrichment of Natural Quenched Sulfide Solid Solutions, the Norilsk 1 Deposit, Russia

Abstract The deepest terminations of the Mount Rudnaya subvertical massive sulfide offshoots of the Norilsk 1 orebody are composed of exceptionally fine grained sulfides that are believed to be natural quenched sulfide solid solutions. Copper-rich intermediate solid solution (ISS) and Fe-rich monosulfide solid solution (MSS) form an equigranular and lamellar matrix hosting MSS- and ISS-dominant globules. The nonstoichiometric chemical compositions of the solid solutions plot within their high-temperature fields known from experiments. MSS contains 19 to 35 wt % Ni, 0.09 to 0.45 wt % Co, and up to 0.6 wt % Cu and is heterogeneously enriched in Rh (up to 32 ppm), Ir (up to 0.6 ppm), Pt (up to 65 ppm), and Pd (up to 168 ppm). ISS occurs as the lamellar intergrowths of the chalcopyrite (Ccpss) and cubanite (Cubss) solid solutions, which bear up to 4.74 wt % Ni and 0.2 wt % Co and are heterogeneously enriched in Zn, Ag, and In. The assemblage of platinum group minerals (PGMs) is hosted mostly in the ISS and is dominated by Pt-Fe alloys and minerals of the rustenburgite-atokite series, like the set of PGMs at the Norilsk 1 deposit. Similar Pt-Pd-Sn compounds in the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spectra of profiles through MSS and ISS are interpreted to be trapped microinclusions. The pentlandite contains up to 0.13 wt % Pt, up to 4.62 wt % Pd, <0.53 wt % Co, and <0.4 wt % Cu according to electron microprobe analysis. LA-ICP-MS data and mapping show that Pd content in the pentlandite increases toward contacts with ISS and decreases toward contacts with MSS, supporting a reaction origin of pentlandite. The wide variations of the concentrations of major and trace elements in the solid solutions, as well as the coexistence of Pd-poor (a few ppm Pd) and Pd-rich (over 4.62 wt % Pd) pentlandite within a single sample, seem to characterize the different generations of the MSS to MSS-ISS globules, antecrysts, and phenocrysts with the distinct histories of enrichment due to exchange with fractionated Cu-platinum group element-rich residue. The directional distribution of Pd of high-temperature primary magmatic origin is preserved due to rapid quenching of the sulfides from ~650°C.

Controls on the disseminated Ni-Cu-PGE sulfide mineralization at the Tubane section, northern Molopo Farms Complex, Botswana: Implications for the formation of conduit style magmatic sulfide ores

The Molopo Farms Complex (MFC) is located near the western margin of the Archean Kaapvaal Craton, in southern Botswana. The complex hosts low grade disseminated Ni-Cu and platinum group elements (PGE) mineralization restricted to the lower ultramafic cumulates, notably in the feldspathic orthopyroxenites. The mineralized feldspathic orthopyroxenites contain up to 0.47 ppm Pt + Pd + Au manifested in the form of platinum group mineral (PGM) assemblages dominated by PGE bismuthotellurides, asernides and minor Au and Ag alloys. The PGM encountered are froodite (PdBi2), moncheite (PtTe2), michenerite (PdBiTe), sperrylite (PtAs2) largely associated with altered silicates and sulfide minerals. The association of PGM with sulfides suggests that they were originally concentrated by sulfide melt, while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of PGM with altered silicates. The Cu/Pd ratios of most of the rocks fall within the primitive mantle range. However, the Cu/Pd ratios of the feldspathic orthopyroxenites are more variable, with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides. This is supported by the Cu/Zr ratios of these rocks which are also variable above and below unity. Trace element ratios on the (Th/Yb) PM versus (Nb/Th) PM plot indicate strong crustal contamination across all units including those not in proximity to the country rocks. The δ34S of the magmatic rocks are variable between −6.3‰ and −11.4‰. In contrast, the δ34S of dolomitic siltstones of the Transvaal Supergroup which constitute the immediate country rocks vary from −4.1‰ to +0.6‰. These data indicate that crustal contamination largely occurred prior to magma emplacement. It is inferred that the most important contaminant and source of crustal S were the Ramotswa and Ditlhojana Formations that include sulfidic shales at depth. We consider that magnesian basaltic magmas of B1 Bushveld magma composition underwent crustal contamination, assimilation of crustal S and segregation of sulfide melt in the feeder conduit or staging-chamber prior to final magma emplacement. The feeder conduit or staging-chamber was subsequently invaded by further B1 magma. This magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final MFC chamber studied here. After the staging-chamber had already partially solidified, it was invaded by yet another influx of magma, possibly even more primitive than the B1 Bushveld magma. Being a crystal mush, this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final MFC chamber. The presence of centimeter-sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas. Therefore, it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging-chamber.

Platinum mineralization of the Epilchik Ural-Alaskan type zoned complex (Far East Russia)

The Epilchik Ural-Alaskan type mafic-ultramafic zoned complex, located in the northernmost part of the Koryak-Kamchatka Platinum Belt (Russian Far East), contains lode platinum mineralization. The outcrops of the complex are crossed by the Snegovaya River, which hosts placer platinum-group minerals (PGM) occurrences. The primary PGM assemblage of the Epilchik lode chromitites is typical for Ural-Alaskan type complexes, dominated by Ir-rich isoferroplatinum which occurs as small euhedral inclusions within chromite. Secondary alloys include tetraferroplatinum (PtFe), tulameenite (Pt2FeCu), an unnamed Fe3Pt compound, and Pt-bearing native iron, which form alteration rims around isoferroplatinum or occupy fractures in chromite. Other secondary minerals from lode chromitites are hollingworthite (RhAsS) and Os-Fe alloy (“hexaferrum”). The PGM assemblages of the Snegovaya River placer occurrences are comparable with those of the lode chromitites with a predominance of Ir-rich isoferroplatinum and secondary alloys, including tetraferroplatinum, tulameenite, and an unnamed Fe3Pt compound. However, placer occurrences are much richer in regard to mineral diversity, comprising native osmium (Os,Ir), native iridium (Ir,Os), laurite (RuS2), erlichmanite (OsS2), cuprorhodsite (CuRh2S4), cuproiridsite (CuIr2S4), Co-malanite (Cu(Pt,Co)2S4), unnamed sulfides Rh3S4, (Rh,Pt)S, sperrylite (PtAs2), platarsite (PtAsS), irarsite (IrAsS), hollingworthite (RhAsS), osarsite (OsAsS), cooperite (PtS), native gold, unnamed telluride (Pt2Te), and an unnamed Fe-Ir-Rh alloy. Similarities between lode and placer mineralization indicate that the Epilchik complex was the dominant source for the placers. The larger diversity of PGM in placers may be attributed to the heterogeneity of samples which could be derived from unsampled or unexposed outcrops of the complex. Alteration of isoferroplatinum to secondary intermetallic compounds occurred via progressive development of reduced conditions as a result of serpentinization. Subsequent pervasive As-mineralization redistributed platinum-group elements, including Os and Ir. A final mineralizing pulse deposited sporadic native gold and Pt telluride minerals. A comparison of key mineralogical and physical features between the Epilchik complex and other Ural-Alaskan type complexes is used to determine the economic potential for placers along the Snegovaya River. Key criteria include (1) the size and degree of erosion of the outcrop area of the complex; (2) the predominance of isoferroplatinum over native platinum; (3) high Ir content in Pt-Fe alloys; and (4) the predominance of secondary intermetallic compounds over sulfides. Based on these criteria, the occurrences associated with the Epilchik complex are considered prospective for high volume and economic grade accumulations of PGE. This method is useful for assessing the economic potential of placer occurrences during indirect early-stage prospecting with a limited sample suite.

Tilkerodeite, Pd2HgSe3, a New Platinum-Group Mineral from Tilkerode, Harz Mountains, Germany

Tilkerodeite, ideally Pd2HgSe3, is a new platinum-group selenide from the Eskaborner Stollen (Adit Eskaborn) at Tilkerode, Harz Mountains, Germany. Tilkerodeite crystals occur as euhedral inclusions in tiemannite or as extremely fine-grained lamellar aggregates (grain-size up to 3 μm) in a dolomite–ankerite matrix, together with clausthalite, tiemannite, jacutingaite, stibiopalladinite, and native gold. Neighbouring Se-bearing minerals include tischendorfite and chrisstanleyite. Tilkerodeite is opaque with a metallic luster, and is flexible in blade-like crystals, with perfect basal cleavage {001}. In plane-polarized light, tilkerodeite is brownish-grey. It is weakly bireflectant, and weakly pleochroic in shades of light-brown and grey. The anisotropy is weak, with rotation tints in weak shades of greenish-brown and grey-brown. The range of reflectance is estimated in comparison to clausthalite with 45–50%. Electron-microprobe analyses yield the mean composition (wt. %) Se 32.68, Hg 26.33, Pt 20.62, Pd 15.89, Pb 2.72, Cu 0.66, S 0.27, total 99.17 wt. %. The empirical formula (based on six atoms pfu) is (Pd1.08Pt0.76Pb0.09Cu0.07)Σ2.00Hg0.95(Se2.98S0.07)Σ3.05. The ideal formula is Pd2HgSe3. Tilkerodeite is trigonal, with Pt4Tl2Te6-type structure, space group P3–m1, a = 7.325(9) Å, c = 5.288(6) Å, V = 245.7(9) Å3, and Z = 2. It is the Pd-analogue of jacutingaite. Tilkerodeite formed hydrothermally, possibly involving the alteration of tiemannite by low-temperature oxidizing fluids. The new species has been approved by the IMA-CNMNC (2019-111) and is named after the locality. Tilkerode is the most important selenide-bearing occurrence in Germany and type locality of naumannite, eskebornite, and tischendorfite.

Mineralogy of the HSE in the subcontinental lithospheric mantle —An interpretive review

The highly siderophile elements (HSE: Os, Ir, Ru, Rh, Pt, Pd, Re, Au) exist in solid solution in accessory base-metal sulfides (BMS) as well as nano-to-micron scale minerals in rocks of the subcontinental lithospheric mantle (SCLM). The latter include platinum-group minerals (PGM) and gold minerals, which may vary widely in morphology, composition and distribution. The PGM form isolated grains often associated with larger BMS hosted in residual olivine, located at interstices in between peridotite-forming minerals or more commonly in association with metasomatic minerals (pyroxenes, carbonates, phosphates) and silicate glasses in some peridotite xenoliths. The PGM found inside residual olivine are mainly Os-, Ir- and Ru-rich sulfides and alloys. In contrast, those associated with metasomatic minerals or silicate glasses of peridotite xenoliths consist of Pt, Pd, and Rh bonded with semimetals like As, Te, Bi, and Sn. Nanoscale observations on natural samples along with the results of recent experiments indicate that nucleation of PGM is mainly related with the uptake of HSE by nanoparticles, nanominerals or nanomelts at high temperature (> 900 °C) in both silicate and/or sulfide melts, regardless of the residual or metasomatic origin of their host minerals. A similar interpretation can be assumed for gold minerals. Our observations highlight that nanoscale processes play an important role on the ore-forming potential of primitive mantle-derived magmas parental to magmatic-hydrothermal deposits enriched in noble metals. The metal inventory in these magmas could be related with the physical incorporation of HSE-bearing nanoparticles or nanomelts during processes of partial melting of mantle peridotite and melt migration from the mantle to overlying continental crust.

Bioprocessing of oxidized platinum group element (PGE) ores as pre-treatment for efficient chemical extraction of PGE

Platinum-group elements (PGE) are relevant for many high technology applications, prompting the search for potential alternative resources and environmentally friendly processing technologies. The largest deposit of PGE worldwide is the Bushveld Complex in South Africa. The pristine sulfide-bearing ore is mined using conventional metallurgical methods, while the near-surface oxidized PGE ore is currently stockpiled. Economic extraction of PGE from these stockpiles using conventional technologies is not feasible to date. Hence, biohydrometallurgical approaches to recover valuable transition metals and liberate platinum-group minerals for subsequent chemical PGE extraction have been considered. The four stockpile samples taken from a heap of oxidized PGE ores from the Platreef formation showed a similar mineralogical composition with >90 wt% silicates and minor amounts of Fe-(oxy)/hydroxides, both associated with Ni, Mn and Cu. Platinum-group minerals were present as sperrylite (PtAs2), cooperite (PtS low Pd), native platinum and Pd‑bismuthotellurides and mostly associated with silicates. Stirred tank bioleaching under various conditions was proven to be most feasible for an oxidative process at moderately-high temperatures using a consortium of iron- and sulfur-oxidizing acidophiles dominated by Acidithiobacillus caldus and Sulfobacillus thermosulfidooxidans. Bioleaching achieved up to 86% total metal recovery (including Co, Cu, Mn and Ni) and led to a liberation of platinum-group minerals. The bioleach residues were subsequently chemically leached by a HNO3/NaCl and a cyanide leach, allowing for the enhanced extraction of up to 89% Pt and 96% Pd after bioleaching. Chemical and mineralogical analysis validated the potential of oxidative bioleaching as a promising processing option for a pre-treatment of oxidized PGE ore, allowing separate and efficient recovery of valuable base metals and enhancing the subsequent chemical PGE extraction.

Magmatic PGE Sulphide Mineralization in Clinopyroxenite from the Platreef, Bushveld Complex, South Africa

The Platreef, at the base of the northern limb of the Bushveld Complex in South Africa, hosts platinum-group element (PGE) mineralization in association with base-metal sulphides (BMS) and platinum-group minerals (PGM). However, whilst a magmatic origin of the stratiform mineralization of the upper Platreef has been widely confirmed, the processes responsible for the PGE and BMS mineralization and metasomatism of the host rocks in the Platreef are still under discussion. In order to contribute to the present discussion, we present an integrated petrographical, mineral-chemical, whole-rock trace- and major-element, sulphur- and neodymium-isotope, study of Platreef footwall clinopyroxenite drill core samples from Overysel, which is located in the northern sector of the northern Bushveld limb. A metasomatic transformation of magmatic pyroxenite units to non-magmatic clinopyroxenite is in accordance with the petrography and whole-rock chemical analysis. The whole-rock data display lower SiO2, FeO, Na2O and Cr (<1700 ppm), and higher CaO, concentrations in the here-studied footwall Platreef clinopyroxenite samples than primary magmatic Platreef pyroxenite and norite. The presence of capped globular sulphides in some samples, which display differentiation into pyrrhotite and pentlandite in the lower, and chalcopyrite in the upper part, is attributed to the fractional crystallization of a sulphide liquid, and a downward transport of the blebs. In situ sulphur (V-CDT) isotope BMS data show isotopic signatures (δ34S = 0.9 to 3.1 ‰; Δ33S = 0.09 to 0.32‰) close to or within the pristine magmatic range. Elevated (non-zero) Δ33S values are common for Bushveld magmas, indicating contamination by older, presumably crustal sulphur in an early stage chamber, whereas magmatic δ34S values suggest the absence of local crustal contamination during emplacement. This is in accordance with the εNd (2.06 Ga) (chondritic uniform reservoir (CHUR)) values, of −6.16 to −6.94, which are similar to those of the magmatic pyroxenite and norite of the Main Zone and the Platreef in the northern sector of the northern Bushveld limb. Base-metal sulphide textures and S–Se-ratios give evidence for a secondary S-loss during late- to post-magmatic hydrothermal alteration. The textural evidence, as well as the bulk S/Se ratios and sulphide S isotopes studies, suggest that the mineralization in both the less and the pervasively hydrothermally altered clinopyroxenite samples of Overysel are of magmatic origin. This is further supported by the PPGE (Rh, Pt, Pd) concentrations in the BMS and mass-balance calculations, in both of which large proportions of the whole-rock Pd and Rh are hosted by pentlandite, whereas Pt and the IPGE (Os, Ir, Ru) were interpreted to mainly occur in discrete PGM. However, the presence of pentlandite with variable PGE concentrations on the thin section scale may be related to variations in the S content, already at S-saturation during magmatic formation, and/or post-solidification mobilization and redistribution.

Nanoscale constraints on the in situ transformation of Ru–Os–Ir sulfides to alloys at low temperature

We report new results of a combined focused ion beam high-resolution transmission electron microscopy (FIB/HRTEM) investigation of two grains of platinum-group minerals (PGM) corresponding to the laurite (RuS2)-erlichmanite (OsS2) solid solution series. The grains are included within unaltered chromite and the serpentine-dominant matrix of the mantle-hosted chromitite body of Monte Bueno in eastern Cuba. At the micron-scale, the grain hosted in unaltered chromite preserves magmatic features including a complex oscillatory zoning defined by a core of erlichmanite surrounded by alternating bands of Os-rich/Os-poor laurite, revealed by conventional field emission scanning electron microscopy (FE-SEM) and X-ray mapping by electron microprobe analyzer (EPMA). Combining high-magnification (HMEM), high angle-annular dark field (HAADF) images and the corresponding Energy Dispersive Spectra (EDS), the elemental mappings acquired using HRTEM showed that the oscillatory zoning occurs at both the micron and nano scales. A specific feature highlighted by HRTEM observations is the rather complex structure of the erlichmanite core, consisting of an aggregation of much smaller cores (<1 µm) characterized by an oscillatory zoning linked to bands thinner than 10 nm that are not detectable when the whole grain is imaged using conventional FE–SEM and X–ray EPMA elemental mapping. The nanotextural features identified in this study suggest: (1) formation of the PGM grains characterized by the coalescence of laurite/erlichmanite nuclei that originally formed independently and coalesced in the silicate magma, (2) growth and crystal zoning of compositionally different bands as a result of sudden changes in T, fS2, fO2 and composition of the melt. These thermodynamic and chemical changes also allowed the segregation of nano-sized Fe–Ni–S solid particles/droplets from the silicate magma, which are now found within the PGM grain as nanoparticles of pentlandite of <100 nm in size and had potential implications for laurite nucleation. On the other hand, backscattered FESEM images and X-ray EPMA mapping of the altered PGM grain associated with the silicate matrix of the chromitite revealed a magmatic sector zoned core, which is surrounded by a rim of S-deficient laurite (i.e., desulfurized laurite) that hosts Ru-Os-Ir alloys. The FIB/HRTEM observations on the rim of desulfurized laurite ± Os–Ir–Ru alloys revealed an intergrowth of nano-sized (<5 nm) laurite with ad Ru-Os-Ir-(S-As) and Ru-Os-Ir alloys (50–100 nm) within a Fe-Mn oxide/hydroxide matrix. This microstructure and the coexistence of these nanoparticles within the Fe-Mn oxide/hydroxide matrix illustrates an alteration sequence whereby magmatic laurite was desulfurized and decomposed into smaller laurite nanoparticles, which in turn were progressively transformed to Ru-Os-Ir-(S-As) particles and, finally, to Ru-Os-Ir nanoalloys. Self-reorganization (coarsening) of the desulfurizing laurite within a matrix transforming to Fe-Mn oxide/hydroxide matrix resulted in the formation of nanoscale intergrowths of Ru-Os-Ir-(S-As) particles and Ru-Os-Ir alloys with the Fe-Mn oxide/hydroxide . This took place concomitant to incorporation/oxidation of Fe and Mn by relatively more oxidizing fluids. These alteration fluids were generated during to serpentinization/ocean-floor metamorphism that affected the chromitite body. The observations provided here are relevant to the current debate about the platinum-group element (PGE) mobilization and the possible existence of PGE oxides and hydroxides minerals

In-Situ Crystallization and Continuous Modification of Chromian Spinel in the “Sulfide-Poor Platinum-Group Metal Ores” of the Norilsk-1 Intrusion (Northern Siberia, Russia)

Layers rich in chromian spinel (Cr-spinel) occur in numerous differentiated and layered intrusions. These layers are often characterized by elevated and even economic concentrations of platinum-group-elements (PGEs), but only scarce sulfide mineralization. One particular type of such lithology occurs in the roof parts of the Norilsk-type differentiated intrusions (Russia) and is referred to as the “sulfide-poor PGE ores”. We investigated rocks containing variable enrichments in Cr-spinel, sulfides, and platinum-group minerals (PGMs) from two sections of the upper zone of the Norilsk-1 intrusion, with a focus on Cr-spinel. The rocks are dominated by two lithological types: (1) leucogabbro/troctolitic and (2) olivine-gabbro. Fine-grained (5–100 μm) disperse disseminations with varying modal abundances of Cr-spinel are characteristic for the rocks studied. Those abundances range from scarce mineralization through to very dense (up to 30 vol. % Cr-spinel) cloud-like accumulations. However, compact-grained accumulations and cumulate-like textures, which are typical for chromitites of layered intrusions, are not characteristic for the studied rocks. Instead, the disseminations exhibit chain- and trail-like alignments of Cr-spinel grains, which cross the boundaries between enclosing silicates, and sub-circular arrangements. The study revealed millimeter-scaled patchy distribution of Cr-spinel compositions within a given dissemination with Cr-spinel chemistry being strongly correlated with a kind of the enclosing silicate. (1) In unaltered rocks, plagioclase hosts more magnesian Cr-spinel (Mg# 30–60), while Cr-spinel in mafic minerals is less magnesian (Mg# 18–35). (2) In altered rocks, more magnesian Cr-spinel is hosted by less altered silicates, while strongly altered silicates mainly host less magnesian Cr-spinel. Systematics of trivalent cations exhibits divergent trends, even on a scale of a thin section, and depends on a kind of hosting lithology. Leucogabbro/troctolite lithologies contain Cr-spinel with anomalously low Fe3+ and extremely high Ti contents, whereas Cr-spinel from olivine-gabbro lithologies have moderate Fe3+ and moderately-high Ti contents. It is envisaged that crystallization of Cr-spinel and their host rocks occurred from viscous mingled magmas, which had different compositions and redox state. Subsequent processes involved (1) high-temperature re-equilibration of Cr-spinel with enclosing silicates and (2) post-magmatic alteration and partial recrystallization of Cr-spinel. During these processes, Cr-spinel was losing Mg and Al and gaining Fe and Ti. These chemical trends are generally coincident with those established for other intrusions worldwide, but the upper zone of the Norilsk-1 intrusion seems to possess an exceptional variety of Cr-spinel compositions, not recorded elsewhere.

Alaskan-type nature and PGE mineralization of the Wuxing mafic–ultramafic complex in eastern part of the Central Asian Orogenic belt

The Wuxing mafic–ultramafic complex is located in the eastern part of the Central Asian Orogenic Belt (CAOB). It consists of hornblende-olivine clinopyroxenite, hornblende clinopyroxenite, clinopyroxenite, gabbro and diorite. Hornblende occurs in all the rock types of the complex and is pargasite in composition. Clinopyroxene is diopsidic in composition with high CaO and low TiO2 contents. The flat REE and sub-parallel trace element patterns of both clinopyroxene and hornblende suggest that these rocks were formed by fractional crystallization from common magma with no significant crustal contamination. Enrichment in large ion lithophile elements relative to high field strength elements of the silicate minerals, together with the variable sulfur isotope compositions (δ34S = -2.55 ~ 11.01‰) of the sulfides, indicate the involvement of subducted oceanic materials into the mantle source. These petrological, mineralogical and geochemical features strongly suggest the Alaskan-type nature of the Wuxing complex. The hydrous parental magma of the Wuxing complex was derived from partial melting of the mantle wedge, which had been metasomatized by subducted melts/fluids.The Wuxing complex is a unique Pt-Pd-rich Ni-Cu sulfide deposit in the CAOB. The ore minerals, which are dominated by pyrite, pyrrhotite, pentlandite, chalcopyrite and platinum-group mineral (PGM), mainly occur in clinopyroxenite and hornblende clinopyroxenite. The sulfide ores are predominantly present as net-textured and disseminated textures. Pentlandite and chalcopyrite associated with pyrrhotite commonly occur in the marginal zone of the pyrrhotite grains. The PGMs occur as interstitial grains at the contact between base-metal sulfides and silicate minerals or along the crack of pyrrhotite or clinopyroxene, or as inclusions in pyrrhotite. The occurrences of sulfides and PGMs in the Wuxing complex, together with sulfur isotope compositions and PGE patterns, reveal that the PGE enrichment of the Alaskan-type complexes can be attributed to the high-degree partial melting of the mantle source, sulfur involvement in the source, and PGE transportation of hydrous melts.

Bowlesite, PtSnS, a new platinum group mineral (PGM) from the Merensky Reef of the Bushveld Complex, South Africa

AbstractBowlesite is a new mineral discovered in the Merensky Reef of the Rustenburg Platinum Mine, Bushveld complex, South Africa. Bowlesite forms tiny grains (maximum dimension 20 μm). It is associated with sulfides including chalcopyrite, pyrrhotite and pentlandite, in contact with silicates including plagioclase, pyroxene- and minor serpentine-subgroup and amphibole-supergroup minerals. Bowlesite is brittle and has a metallic lustre. In plane-polarised light, bowlesite has a light bluish grey colour. It shows weak bireflectance, no pleochroism and has weak anisotropism. Internal reflections were not observed. Reflectance values of bowlesite in air (R1, R2 in %) are: 50.3–51.4 at 470 nm, 48.5–48.9 at 546 nm, 47.9–48.6 at 589 nm and 47.8–48.7 at 650 nm. Ten spot analyses of bowlesite give the average composition: Pt 56.85, Pd 0.02, Sn 34.03 and S 9.15, total 100.05 wt.%, corresponding to the empirical formula (Pt1.001Pd0.001)Σ1.002Sn0.997S1.001, based on 3 atoms per formula unit. The simplified formula is PtSnS. Due to the small size of bowlesite, the crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic PtSnS. The calculated density is 10.06 g⋅cm–3. The mineral is orthorhombic, space group: Pca21 (#29) with a = 6.11511(10), b = 6.12383(10), c = 6.09667(11) Å, V = 228.31(1) Å3 and Z = 4. Bowlesite is isotypic with cobaltite, CoAsS. The origin of bowlesite is probably related to low-T exsolution of Pt–Sn phases from high-T sulfides crystallised from the sulfide melt. The mineral honours Dr. John Bowles (Manchester University, UK) for his contributions to ore mineralogy and mineral deposits related to mafic–ultramafic rocks.

Differential platinum group elements (PGE) re-mobilization at low fS2 in Abdasht and Soghan mafic-ultramafic complexes (Southern Iran)

The Abdasht and Soghan ultramafic complexes, Southern Iran, host major actively exploited chromitite bodies, variably enriched in PGE. The Platinum Group Minerals (PGM) – Base Metal Minerals (BMM) assemblage was studied in order to assess PGE remobilization during post-magmatic processes. Studied chromitites have variable textures: massive, banded, nodular and disseminated. Primary silicates are almost completely replaced by serpentine. Ferrian chromitization is present but not widespread. BMM are mostly found in the silicate matrix, while PGM are more common as inclusions within chromites.Primary BMM (pentlandite and bornite) within the silicate matrix are poorly preserved, while primary PGM (laurite) are more abundant. Secondary BMM (heazlewoodite, millerite and NiFe alloys) and PGM (PGE-alloys) are the dominant mineralogical species. During serpentinization the pervasive percolation of low fS2 fluids induced the desulfurization of the metallic assemblage. Primary sulfides were replaced by S-poor (heazlewoodite) or S-free (NiFe alloys, PGE alloys, PGE-BM alloys) phases.Low fS2 differentially mobilized PGE. The release of IPGE (Ir, Os, Ru) into the fluids follows the order: Ru> > Os>Ir. PPGE (Pt, Pd, Rh) mobility is more difficult to assess, due to their small concentration, but the evidence suggests that Rh is more mobile than Ru, and that Pd and Pt were partially added to the system by fluids. A mass balance calculation on a polyphasic grain, showing differential stages of desulfurization, allowed a semi-quantitative analysis on the desulfurization degree of Abdasht and Soghan complexes, estimated, in the more serpentinized portions, between 75 and 100%.