Platinum-group Mineral Assemblages Research Articles

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

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

Inclusions in Placer Pt-Fe Alloy Nuggets, Northwest Ecuador: Evolution of a Ural-Alaskan Type PGE Mineralizing System (Cr-Spinel, IPGE, Sulfarsenides, and Cu-Depleted PGM)

Abstract A Cu- and Rh-enriched magmatic ore system is defined by abundant PGM (platinum group mineral) inclusions in forty-four Pt-Fe alloy nuggets from the Camumbi River gold placer, northwest Ecuador. Isoferroplatinum is depleted in Rh, Os, and Ru compared with native platinum, suggesting most crystallized after Os-(Ir) alloy, laurite, and some Rh-PGM. Two Pt-Fe alloy nuggets have zoned hydrothermal alteration rinds, and an UM (unnamed mineral) is (Rh,Pd)4As3. Our previous work shows that silicate glass inclusions define a fractionated co-magmatic compositional series related to primitive hydrous ferrobasalt, and trace element chemistry matches their Late Cretaceous accreted volcanic arc terrane. Here we report exceptional Cr-spinel (Ural-Alaskan type) inclusions coexisting with primitive ferrobasaltic glass crystallized at highest T. Laurite inclusions also indicate high T and S saturation of early melt. Os-(Ir) inclusions are Ru-depleted while two discrete Ir-enriched osmium crystals have remarkable, extreme Ru enrichment and depletion, confirming crystallization before and after laurite. Laurite and osmium inclusions in one Pt-Fe alloy reflect concomitant crystallization and fluctuating low fS2 melt conditions. In experimental primitive Cu-bearing Pt-Pd-S-(As) melt (cf. exsolved from primitive basalt), first Cu-PGM-sulfide crystallization generates a Cu-depleted, Pt-Pd-As-(S) residual melt. At lower T immiscible melts Pt-As-(S) and later Pd-As-(S) crystallize distinctive PGM. We report analogous natural multiphase PGM inclusion assemblages in separate isoferroplatinum nuggets: (1) zoned sulfarsenides, sperrylite, and genkinite, with rare resorbed cognate xenocrystic cooperite (captured from primary sulfide melt) define a high T, Pt-enriched sub-system [Pt > Rh(Pd,Ir,Ru)As,S ≫ Sb,Bi] and (2) zoned sulfarsenides, arsenopalladinite, sperrylite, törnroosite, and gold define a lower T, fractionated Pd-enriched sub-system [(Pd > Rh ≃ Pt > Ir > Au)As,S > Te ≫ Sb,Bi]. The previously undocumented natural S-rich sperrylite (formerly “platarsite”) solid solution series and later crystallized irarsite series are discriminated in terms of Pt-Ir-Rh. Both trends fractionate toward increasing Rh (hollingworthite). The discrete PGM assemblage, sperrylite-telluropalladinite (with exsolved palladium and electrum) defines an IPGE-depleted Pd > Pt(Au > Ag)As ≥ Te ≥ Sb sub-system and records extreme fractionation. Cu-bearing multiphase PGM inclusions (some coexisting with silicate glass) derived from the fraction of Cu-bearing exsolved Pt-Pd-S-(As) melt will be reported separately.

Sources of Ore Material in the Platinum-Group Element Deposits of Polar Siberia and the Middle Urals Based on the Data from Radiogenic (Re–Os, Pt–Os) and Stable (Cu, S) Isotopes

Abstract —Understanding the main events of platinum-group element (PGE) ore formation is impossible without analysis of the sources and behavior of major ore-forming components, namely, platinum, osmium, sulfur, and copper, which are important indicators of magmatic and hydrothermal processes. In contrast to the Re–Os isotope system, the radiogenic Pt–Os isotope system, as well as stable isotopes of Cu and S in PGE deposits, are still relatively understudied. Our comprehensive research is aimed at filling this gap. The paper presents data for the Guli massif of ultramafic and alkaline rocks and carbonatites in Polar Siberia and on the zonal Nizhny Tagil and Svetly Bor clinopyroxenite–dunite massifs in the Middle Urals, which include: (1) the contents of the highly siderophile elements (HSE) in whole rocks and platinum-group minerals (PGM), (2) the Re–Os and Pt–Os isotope systematics of chromitite, Os–Ir alloys, and Ru–Os sulfides, (3) the sulfur isotope composition in Ru–Os and Ir–Rh sulfides in primary and secondary PGM assemblages, and (4) the copper isotope composition in Pt–Fe minerals from chromitites and placers. The research was performed using scanning electron microscopy, electron probe microanalysis, and high-precision isotope-geochemical analysis. The high-precision Re–Os and Pt–Os isotope data show that the HSE contents in chromitites and PGM of the Guli massif were controlled by the composition of the mantle source that evolved with near-chondritic time-integrated Re/Os and Pt/Os ratios, which are also typical of the sources of most komatiites and abyssal peridotites. The δ65Cu values of the studied samples of ferroan platinum and isoferroplatinum are identical within the analytical uncertainty and are close to 0‰, which is typical of high-temperature Cu-containing minerals. The sulfur isotope compositions of the Ir–Rh sulfides of the kashinite–bowieite series and of the Ru–Os sulfides of the laurite–erlichmanite series in the primary PGM assemblages indicate that the source of sulfur has a chondritic isotope composition, which is in agreement with the osmium isotope composition of the Ru–Os sulfides and Os–Ir alloys. The heavy sulfur isotope composition (δ34S = 5.6 ± 1.5‰) of As-containing erlichmanite is consistent with its secondary origin. The new data on the isotope compositions of osmium, copper, and sulfur can be used as new important parameters that characterize the sources of PGE mineralization.

Copper Isotope Composition of Pt–Fe Minerals from the Nizhny Tagil Massif, Middle Urals: First Results

In order to provide further insights into the origin of Pt–Fe minerals, this study presents the first copper isotope data for Pt–Fe minerals from the Nizhny Tagil massif, an international standard of the zoned Ural-type complexes. The chemical and isotopic composition of Pt–Fe minerals were determined by electron microprobe analysis, chemical sample preparation with selective chromatographic separation of copper from a solution of the studied sample, followed by high-precision determination of the δ65Cu value using multiple-collector inductively coupled plasma mass-spectrometry. The majority of platinum group minerals (PGM) from chromitites of the Alexandrovsk Log and Krutoy Log deposits within the Nizhny Tagil massif are formed by Pt–Fe minerals, among which high-temperature ferroan platinum (Pt2Fe) containing subordinate inclusions of Os–Ir alloys and laurite (RuS2) predominate over other PGM. The concentrations of copper and δ65Cu values in the studied samples of ferroan platinum vary from 0.4 to 1.4 wt % Cu and from –0.37 to 0.26‰, respectively. Secondary low-temperature PGM assemblage is represented by the tetraferroplatinum (PtFe)—tulameenite (PtFe0.5Cu0.5) solid solutions series. The concentrations of copper in these PGM vary in the range of 6.8–11.3 wt %; the values of δ65Cu are characterized by lighter Cu-isotopic compositions ranging from –1.15 to –0.72‰. The lighter Cu-isotopic composition in secondary Cu-bearing PGM compared to that in ferroan platinum (δ65Cu = –1.01 ± 0.17‰, n = 8 and δ65Cu = 0.03 ± 0.23‰, n = 7, respectively) is consistent with a secondary nature of isotopic variations, due to evolved composition of the ore-forming fluid during the low-temperature formation of the tetraferroplatinum—tulameenite solid solution series.

The Mooihoek Platinum Pipe, Eastern Bushveld Complex, South Africa – Geochemistry and Mineralization

Abstract The Mooihoek pipe is one of several platiniferous dunite pipes in the eastern Bushveld Complex that were discovered in 1924 and mined until 1930. It has a diameter of about 180 by 400 m and consists of magnesian dunite that crosscuts a sequence of layered leucocratic wall rocks of the lower part of the upper Critical Zone of the Bushveld Complex. The Mg-dunite pipe at Mooihoek hosts a smaller, carrot-shaped inner pipe, 13–16 m in diameter and cylindrical in shape near surface, consisting of mainly iron-rich dunite/wehrlite pegmatite, comprising the platinum-bearing orebody. Specific characteristics of the mineralization are the near absence of sulfides and Pt being the predominant platinum-group element (PGE). The present work comprises whole rock analyses, ore microscopy, scanning electron microscopy, and electron probe microanalysis. The main findings are as follows: Chondrite-normalized distribution patterns of the PGE in the Mooihoek samples demonstrate that mainly Pt is strongly enriched relative to the other PGE and Au. The shapes of the patterns are subparallel to each other and also to those of the Onverwacht pipe, indicating a similar mode of origin. Olivine of the ore zone is iron-rich and is frequently accompanied by coarse Ti-magnetite and ilmenite, whereas chromite is rare. Two major generations of platinum-group minerals (PGM) are present at Mooihoek, namely: (1) Early inclusions of mainly ferroan platinum in (magmatic) olivine, Ti-magnetite, and ilmenite, and (2) a later, more complex PGM assemblage, commonly associated with thin veinlets of magnetite, mainly found along grain boundaries of silicates and oxides. This assemblage consists of various Pt-Fe alloys and PGE-antimonides, -sulfarsenides, -arsenides, and -sulfides. Overall, and in line with the geochemical data of the mineralization, Pt-dominated PGM predominate (~70%), followed by Rh, Pd, and Ru species. Pt-Fe alloys are the most frequent PGM (∼44% by number of grains). Our hypothesis on the genesis of the Mooihoek pipe and its mineralization largely corresponds to that proposed for the Onverwacht pipe (Oberthür et al. 2021): The magnesian dunite pipe was formed by upward penetration of magmas which replaced the existing cumulates. Fractional crystallization of olivine within a deeper magma chamber and/or during ascent of the melt resulted in the formation of a consanguineous residual, more iron-rich, and PGE-enriched melt. This melt also contained highly mobile, supercritical fluids and was continuously enriched in PGE and other incompatible elements. The PGE-enriched residual melts formed and sealed the inner ore pipe. Apparently, the PGE mineralization is not related to the well-established mechanism of sulfide collection. Instead, it was introduced in the form of nanoparticles and small droplets of PGM, which coagulated to form larger grains during evolution of the mineralizing system. Concomitant supercritical magmatic to hydrothermal fluids were continuously active and caused a certain redistribution of the initial PGE inventory and possibly added further quantities of PGE from lower parts of the mineralizing system. Reactions with ligands like Sb, As, and S led to the formation of composite PGM grains and complex intergrowths in a continuous process of mineralization. The present work underlines that sulfide-poor magmas may constitute fertile potential precursors of PGE deposits and, therefore, deserve more attention in future exploration.

COPPER ISOTOPE COMPOSITION OF Pt-Fe MINERALS FROM THE NIZHNY TAGIL MASSIF, MIDDLE URALS: FIRST RESULTS

In order to identify the source of copper, for the first time in world practice, the features of the Cu-isotopic composition of Pt–Fe minerals are characterized using the example of the Nizhny Tagil massif, the world standard of Ural-type zoned complexes. The chemical and isotopic composition of Pt-Fe minerals were determined by electron probe microanalysis, chemical sample preparation with selective separation of copper from the solution of the studied sample, and subsequent high-precision determination of the δ65Cu value using multiple-collector inductively coupled plasma mass-spectrometry. The majority of platinum group minerals (PGM) from bedrock deposits of the Nizhny Tagil massif, represented by chromitites of the Alexandrovsky and Krutoy logs, are formed by Pt–Fe minerals, among which high-temperature ferroan platinum (Pt2Fe) with inclusions of Os–Ir alloys and laurite (RuS2) dominates. The concentrations of copper in the studied samples of ferroan platinum vary in the range of 0.4–1.4 wt. % Cu. The δ65Cu values for Cu-bearing PGM vary in the range from –0.37 to 0.26‰. Secondary low-temperature PGM assemblage is represented by solid solutions of tetraferroplatinum (PtFe) – tulameenite (PtFe0.5Cu0.5) series. The concentrations of copper in these PGM vary in the range of 6.8–11.3 wt. %; the values of δ65Cu are characterized by lighter isotopic compositions of copper that range from –1.15 to –0.72‰. The lightening of the Cu-isotopic composition in secondary Cu-bearing PGM compared with that in ferroan platinum (δ65Cu = –1.01 ± 0.17, n = 8 and δ65Cu = 0.03 ± 0.23, n = 7, respectively) is consistent with the secondary nature of isotopic variations, due to the evolution of the composition of the ore-forming fluid during the low-temperature formation of the tetraferroplatinum (PtFe) – tulameenite (PtFe0.5Cu0.5) series.

Zaykovite, Rh3Se4, a new mineral from the Kazan placer, South Urals, Russia

AbstractZaykovite, ideally Rh3Se4, is a new mineral, the first natural rhodium selenide. It was discovered in the assemblages of platinum-group minerals from the Kazan gold placer, South Urals, Russia. The mineral occurs as crystals up to 40 μm in size within the grains of Pt3Fe alloy, in association with unnamed Pd–Sb–Te phase and Au–Pd alloy. In reflected light, zaykovite has a grey colour with bluish-greenish tint; it shows weak bireflectance and anisotropy. Reflectance values [Rmax/Rmin (%) for COM approved wavelengths (nm)] are: 30.1/29.3(470), 32.2/31.0(546), 33.4/32.0(589) and 35.1/33.7(650). The chemical composition corresponds to the empirical formula (Rh2.26Pt0.46Ir0.25Ru0.01Pd0.01Fe0.01)Σ3.00(Se2.77S1.21Te0.02)Σ4.00 Zaykovite is monoclinic, space group C2/m, a = 10.877(1), b = 11.192(1), c = 6.4796(6) Å, β = 108.887(2)°, V = 746.3(1) Å3, Z = 6 and Dcalc = 8.32 g cm–1. The crystal structure has been solved and refined to R1 = 0.016 based on 858 unique observed reflections. The strongest lines of the powder X-ray diffraction pattern [d(Å), (I), (hkl)] are: 5.43(37)($\bar{1}$11), 3.275(75)(310), 3.199(100)($\bar{1}$31), 3.061(87)(002), 2.568(62)(400), 2.545(41)(041), 3.413(34)($\bar{2}$41) and 1.697(34)(441). Zaykovite is a Se analogue of kingstonite, Rh3S4. A continuous series of solid solutions between kingstonite and zaykovite was encountered in the samples from the Kazan placer. The possible sources of this unique Rh–Se mineralisation in the South Urals could be serpentinised dunite–harzburgite or gabbro–clinopyroxenite–dunite complexes in the vicinity.

Diversity of Net-Textured Sulfides in Magmatic Sulfide Deposits: Insights from the Eagle’s Nest Ni-Cu-(Platinum Group Element) Deposit, McFaulds Lake Greenstone Belt, Superior Province, Canada

Abstract The Eagle’s Nest Ni-Cu-(platinum group element; PGE) deposit occurs within the 2.73 Ga Esker intrusive complex of the Ring of Fire intrusive suite in the McFaulds Lake greenstone belt of northern Ontario. Mineralization occurs along the northern margin of a formerly ~500-m-high, ~85-m-thick, >1,500-m-long subvertical structurally rotated blade-shaped dike composed of harzburgite, lherzolite, and wehrlite. Three sulfide textural facies are present (percentage as proportion of total mineralization): (1) disseminated (~5%), (2) net texture (~80%), and (3) semimassive to massive (~15%). Five subfacies of net texture have been identified: (1) bimodal olivine-bearing leopard net texture (~50%), (2) inclusion net texture (~5%), (3) orthopyroxene-bearing pinto net texture (<1%), (4) localized zones of disrupted net texture (~30%) containing 3- to 5-cm-thick zones of barren amoeboid crosscutting pyroxenite, and (5) fine-grained patchy net texture (~15%). All textural facies are characterized by typical magmatic pyrrhotite-pentlandite-chalcopyrite-(platinum group mineral) assemblages. Massive sulfides are localized in two embayments along the basal contact separated by a topographic high, grading upward to rare semimassive, laterally more continuous net-textured, and disseminated sulfides, with gradational contacts between all textures except massive. Similar mean ore tenors of different sulfide textural facies (Ni100 ~7.5, Cu100 ~4.8), suggest that the majority of the mineralization formed from similar magma compositions at similar magma/sulfide ratios, but the presence of different inclusion populations (peridotite, gabbro, chromitite) and the presence of disrupted net texture indicates that the olivine, inclusions, and sulfide melts accumulated from multiple pulses in a dynamic system. The smaller, blade-shaped, sulfide-rich, chromite-poor Eagle’s Nest body does not appear to be the feeder to the overlying larger, oblate, sulfide-poor, chromite-rich Double Eagle body. This highlights the need to understand the fluid dynamics of entire plumbing systems when exploring for these deposit types and the significance of smaller, more dynamic magmatic conduits as environments favorable for Ni-Cu-(PGE) mineralization.

Decoupling the effects of alteration on the mineralogy and flotation performance of Great Dyke PGE ores

Ores from a single deposit may exhibit extensive variability in their mineralogy and texture. The ability to quantify this variability and link it to mineral processing performance is one of the primary goals of process mineralogy. This study focuses on the effect of alteration in three platinum group element ore samples from the Great Dyke in Zimbabwe - two of which were more pristine compared to the third, which was locally classified as 'oxidized' ore. These ores are known to be characterized by varying degrees of alteration, resulting in numerous challenges in flotation and affecting both grade and recovery. Alteration, by near-surface oxidation, of the valuable base metal sulphides and platinum group minerals resulted in lower flotation recoveries of Cu, Ni, Pt, and Pd. Evidence of incipient oxidation was more readily observed in the base metal sulphide assemblage than the platinum group mineral assemblage, even though the loss in recovery (because of oxidation) was most significant for Pd. Alteration through hydration resulted in a significant increase in mass pull and dilution of concentrate grade through the inadvertent recovery of naturally floating gangue comprising composite orthopyroxene and talc particles. In this study, the amount of naturally floating gangue was more strongly correlated with the talc grain size distribution than the grade of talc in the flotation feed. The oxidation and hydration alteration reactions are not necessarily mutually exclusive, although one may be more dominant than the other, giving rise to ore variability.

Origin of Ru-Os Sulfides from the Verkh-Neivinsk Ophiolite Massif (Middle Urals, Russia): Compositional and S-Os Isotope Evidence

This study presents new compositional and S-Os isotope data for primary Ru-Os sulfides within a platinum-group mineral (PGM) assemblage from placer deposits associated with the Verkh-Neivinsk massif, which is part of the mantle ophiolite association of Middle Urals (Russia). The primary nature of Ru-Os sulfides represented by laurite (RuS2)–erlichmanite (OsS2) series is supported by occurrence of euhedral inclusions of high-Mg olivine (Fo92–94) that fall within the compositional range of mantle (primitive) olivine (Fo 88–93). The sulfur isotope signatures of Ru-Os sulfides reveal a range of δ34S values from 0.3 to 3.3‰, with a mean of 2.05‰ and a standard deviation of 0.86‰ (n = 18), implying that the sulfur derived from a subchondritic source. A range of sub-chondritic initial 187Os/188Os values defined for Ru-Os sulfides (0.1173–0.1278) are clearly indicative of derivation from a sub-chondritic source. Re-depletion (TRD) ages of the Verkh-Neivinsk Ru-Os sulfides are consistent with prolonged melt-extraction processes and likely multi-stage evolution of highly siderophile elements (HSE) within the upper mantle. A single radiogenic 187Os/188Os value of 0.13459 ± 0.00002 determined in the erlichmanite is indicative of a supra-chondritic source of HSE. This feature can be interpreted as evidence of a radiogenic crustal component associated with a subduction event or as an indication of an enriched mantle source. The mineralogical and Os-isotope data point to a high-temperature origin of the studied PGM and two contrasting sources for HSE in Ru-Os sulfides of the Verkh-Neivinsk massif.

Mineralogical and geochemical constraints on the origin of the Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany/Czech Republic)

The Sohland-Rožany Ni-Cu-(PGE) sulfide mineralization is hosted by a small gabbroic dike that is one of several Ni-Cu sulfide-bearing gabbroic intrusions in the northern Bohemian Massif. The intrusions are associated with a Middle Devonian to Early Carboniferous mafic–ultramafic plumbing system that emplaced within Cadomian granodiorites of the Lausitz Block. The Sohland-Rožany dike comprises varitextured olivine- and hornblende-bearing gabbro and hornblende gabbro which originated from differently fractionated magma batches. Fractionation likely took place in deeper parts of the magmatic systems, e.g. within staging magma chambers. Mineralogical features like Ti-rich primary brown hornblende and diopsidic clinopyroxene as well as overall enriched incompatible element contents suggest moderate alkaline basaltic parental magmas with relatively high volatile contents.The magmatic sulfide mineralization is exclusively hosted by the olivine- and hornblende-bearing gabbro and include disseminated, net-textured and massive sulfides. The ore types are predominated by pyrrhotite-pentlandite-chalcopyrite assemblages accompanied by secondary violarite and pyrite as well as by accessory Pt, Pd, Au and Ag phases. The disseminated and net-textured sulfides have highly variable modal proportions of base-metal sulfides with pyrrhotite-dominated beside chalcopyrite-dominated aggregates on the hand specimen scale. The platinum-group mineral (PGM) assemblage includes Pd-Pt-Ni-bearing bismuthotellurides of the melonite–merenskyite and michenerite–testibiopalladite solid solution series as well as sperrylite. The sulfide ores feature predominantly elevated Ni and highly variable Cu and Pt, Pd and Au tenors (3.5–6.4 wt.% Ni, 0.1–16.6 wt.% Cu, 80–3080 ppb Pt, 50–5320 ppb Pd and 20–3770 ppb Au). The magmatic sulfides are locally characterized by a late-stage hydrothermal overprint where sulfides were replaced by hydrous secondary phases (e.g. actinolite–tremolite and chlorite). A significant redistribution of the PGE due to hydrothermal alteration is unlikely as the majority of PGM show a close spatial association to the base-metal sulfides.Textures and chemical compositions of the sulfides suggest that repeated magma batches eroded already fractionated sulfide pools and entrained early crystalized cumulates of monosulfide solid solution (MSS) together with variable fractionated sulfide melts. Sulfide saturation in the Sohland-Rožany magmas was locally controlled by assimilation of the granodioritic wall-rock in the course of thermomechanical erosion. This process was amplified by high mica contents of the granodiorites. Sulfur isotope signatures (–2.0 to –1.2‰ VCDT) and the S-poor nature of the hosting granodiorites exclude a significant contribution of S from the country rocks.

Platinum mineralization and geochemistry of the Matysken zoned Ural-Alaskan type complex and related placer (Far East Russia)

Ural-Alaskan type complexes are the sources for unique placer platinum deposits and contain platinum-group minerals (PGM) within lodes. Such complexes occur along modern and ancient convergent tectonic margins and are comprised of dunite, wehrlite, clinopyroxenite and gabbro, which gradually interchange towards the marginal parts of the complex. In-situ PGM are largely restricted to chromitites, which are comprised of small veinlets and schlieren of chromian spinel in the dunite unit. PGM mineralization in chromitites can be extremely rich; however, their distribution is very sporadic, without any apparent regularity. Moreover, the role of accessory chromian spinel and post-magmatic overprinting of platinum group element (PGE) concentrations is poorly defined. Detailed studies of PGM assemblages, including those in chromitites, serpentine veinlets in dunites, and wehrlites, combined with PGE geochemistry could provide insights into the distribution and processes responsible for PGE accumulation.The Matysken complex in the Koryak Highlands (Far East Russia) is comprised of dunite, wehrlite, clinopyroxenite and gabbro units, and has a classic zoned structure, rendering it an excellent example of an Ural-Alaskan type complex, ideal for this case study. This locality contains mm-scale isoferroplatinum (Pt3Fe) nuggets that cement chromitites and occur as µm-scale euhedral inclusions in chromian spinel. Placer PGM assemblages in the nearby Matysken River consist of exactly the same assemblages of minerals that are found in dunites and chromitites but with larger compositional scatter, reflecting a complex history of alluvium accumulation, which was sourced from different eroded parts of the complex. Detailed textural investigations discovered a diverse array of hydrous silicate–related mineralization, including euhedral isoferroplatinum grains in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys, sulfarsenides and antimonides, which formed in serpentine veinlets together with awaruite (Ni3Fe) and base metal sulfides. This provides further evidence that isoferroplatinum, which is widely accepted to be a magmatic mineral, may form under a wide range of conditions, including during serpentinization.Dunites have fractioned primitive mantle-normalized PGE patterns, typical of Ural-Alaskan type complexes, with strong enrichments in Pt and depletions in Ir, Ru and Pd. PGE, with an exception of Pd, are quite uniformly distributed in dunite containing accessory chromian spinel, with good correlations between PGE and Cr. This correlation, together with data from previous studies, allowed us to calculate the concentration of Pt in accessory chromian spinel (probably hosted as minute PGM inclusions), which is of the same grade as for chromitites (ppm levels). This suggests that the formation of chromitites does not concentrate PGE over Cr, but only assembles them into larger aggregates, causing the observed unevenness in the PGM distribution.

Genesis of the low sulfide-high-grade PGE mineralization in the W Horizon, Coldwell Complex, Canada: quantitative modeling for PGE reef-style mineralization in syn-magmatic sills

The W Horizon is a thin continuous zone of low sulfide-high-grade platinum group element (PGE) mineralization in the Two Duck Lake gabbro at the Marathon Cu-PGE deposit, Coldwell Complex, Canada, that resembles reef-type PGE deposits found in much larger intrusions. The Marathon deposit was previously shown to have grown by accretion of syn-magmatic, sulfide-bearing sill-like intrusions that formed each of the W Horizon, Main Zone, and Lower Zone mineralization. Each zone can be distinguished by features such as sulfide mineral or platinum group mineral assemblages, or by Cu/Pd, Pd/Se, and Pd/Au values. For example, the W Horizon is defined by the presence of bornite, very high PGE values, low Cu/Pd ( 95% of the original sulfide liquid. In some samples, metal and S abundances indicate that PGE were transported as TABS-PGE phases. This work describes an example of thin PGE-rich sill-like injections that could be of importance to PGE exploration elsewhere.

New Data on Platinum Group Element Bearing Chromitites from the Kurtushiba Ophiolite, Western Sayan

Abstract—Small grains of platinum group minerals (PGMs) have been found in chromitites of the Ergaksky and Kalninsky ultramafic massifs, which are marginal northeastern fragments of the Kurtushiba ophiolite belt in Western Sayan. Along with previously described PGMs, the Cu–Ru–Os solid solution, arsenic-bearing (As up to 3.6 wt %) laurite with minor Pt, Ni, and Co, platinum group element (PGE) sulfides of the MeS and Me3S4 types, sulfoarsenide with the high Rh content (12.7 wt %) in chromitites of the Ergaksky massif, and laurite and sulfoarsenides in chromitites of the Kalninsky massif were found for the first time. The PGM compositions indicate a high formation temperature of Ru–Os sulfide (1200–1250°C) in both massifs, as well as a high partial sulfur pressure during the formation of chromitites and syngenetic PGMs in the Ergaksky massif. The PGM assemblages were formed during several stages. The Os–Ir–Ru alloys and the laurite–erlichmanite series disulfides (predominantly partitioning Ru) are early; the Cu–Ru–Os alloys, sulfides (Ru,Fe)3S4, (Ru,Ni,Fe)S, and PGE sulfoarsenides are late and were crystallized under the influence of reductive fluids as a result of PGE remobilization and redeposition.

Chromite-platinum mineralization of clinopyroxenite-dunite massif Zheltaya Sopka, North Ural

The paper presents the new data on platinum group minerals (PGM) and chromite from the Zheltaya Sopka massif, North Urals. Few chromite bodies along with specifc composition of chromite indicate weak ore-forming processes developed in this massif. Minakawaite (RhSb) and chengdeite (Ir3Fe) are found for the frst time in the Uralian Platinum Belt. The PGM assemblage of the Zheltaya Sopka massif is compared with that of the Iov, Yuda, and Sosnovsky dunite massifs of the northern part of the Uralian Platinum Belt. As a result, similar PGM assemblages with domi¬nant minerals of late assemblages are established in dunites of the Zheltaya Sopka massif. Chromites of the massif show no evident chromite-magnetite trend typical of clinopyroxenite-dunite massifs of the Urals and are relatively enriched in Cr2O3 and Al2O3 in comparison with chromite of other UPB massifs.

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.

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.

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.

Два типа ассоциаций минералов платиновой группы в золотоносных россыпях северо-западной части Кузнецкого Алатау

Два типа ассоциаций минералов платиновой группы в золотоносных россыпях северо-западной части Кузнецкого Алатау