Nano- and Micrometer-Sized PGM in Ni-Cu-Fe Sulfides from an Olivine Megacryst in the Udachnaya Pipe, Yakutia, Russia

ABSTRACT This paper focuses on a nanoscale study of nano- and micrometer-size Os-rich mineral particles hosted in a Ni-Fe-Cu sulfide globule found in an olivine megacryst from the Udachnaya pipe (Yakutia, Russia). These platinum-group element mineral particles and their host sulfide matrices were investigated using a combination of techniques, including field emission gun electron probe microanalyzer, field emission scanning electron microscopy, and focused ion beam and high-resolution transmission electron microscopy. The sulfide globule is of mantle origin, as it is hosted in primitive olivine (Fo90–93), very likely derived from the crystallization of Ni-Fe-Cu sulfide melt droplets segregated by liquid immiscibility from a basaltic melt in a volume of depleted subcontinental lithospheric mantle. Microscopic observations by means of field emission scanning electron microscopy and single-spot analysis and mapping by field emission gun electron probe microanalyzer reveal that the sulfide globule comprises a core of pyrrhotite with flame-like exsolutions (usually <10 μm thickness) of pentlandite, which is irregularly surrounded by a rim of granular pentlandite and chalcopyrite. Elemental mapping by energy dispersive spectroscopy (acquired using the high-resolution transmission electron microscopy) of the pyrrhotite (+ pentlandite) core reveals that pentlandite exsolution in pyrrhotite is still observable at the nanoscale as fringes of 100 to 500 nm thicknesses. The sulfide matrices of pyrrhotite, pentlandite, and chalcopyrite contain abundant nano- and micrometer-size platinum group element mineral particles. A careful inspection of eight of these platinum group element particles under focused ion beam and high-resolution transmission electron microscopy showed that they are crystalline erlichmanite (OsS2) with well-developed crystal faces that are distinctively oriented relative to their sulfide host matrices. We propose that the core of the Ni-Fe-Cu sulfide globule studied here was derived from a precursor monosulfide solid solution originally crystallized from a sulfide melt at >1100 °C, which later decomposed into pyrrhotite and the pentlandite flame-like exsolutions upon cooling at <600 °C. Once solidified, the solid monosulfide solid solution reacted with non-equilibrium Cu-and Ni-rich sulfide melt(s), giving rise to the granular pentlandite in equilibrium with chalcopyrite now forming the rim of the sulfide globule. Meanwhile, nano- to micron-sized crystals of erlichmanite crystallized directly from or slightly before monosulfide solid solution from the sulfide melt. Thus, Os, and to a lesser extent Ir and Ru, were physically partitioned by preferential uptake via early formation of nanoparticles at high temperature instead of low-temperature exsolution from solid Ni-Fe-Cu sulfides. The new data provided in this paper highlight the necessity of studying platinum group element mineral particles in Ni-Fe-Cu sulfides using analytical techniques that can image nanoscale textural features in order to better understand the mechanisms of platinum group element fractionation in magmatic systems. These processes may play a crucial role in controlling the background geochemical budgets for siderophile and chalcophile elements in a wide range of mantle-derived magmas.

Genesis of the Jinbaoshan PGE-(Cu)-(Ni) deposit: Distribution of chalcophile elements and platinum-group minerals

ABSTRACT The Jinbaoshan platinum group element-(Cu)-(Ni) deposit in southwest China is a sulfide-poor magmatic platinum-group element deposit that experienced multiple phases of post-magmatic modification. The sulfide assemblages of most magmatic Ni-Cu-platinum-group element deposits in China and elsewhere in the world are dominated by pentlandite-pyrrhotite-chalcopyrite with lesser magnetite and minor platinum-group minerals. However, Jinbaoshan is characterized by (1) hypogene violarite-pyrite 1-millerite-chalcopyrite and (2) supergene violarite-(polydymite)-pyrite 2-chalcopyrite assemblages. The platinum-group minerals are small (0.5–10 μm diameter) and include moncheite Pt(Te,Bi)2, mertieite-I Pd11(Sb,As)4, the atokite Pd3Sn – rustenburgite Pt3Sn solid solution, irarsite IrAsS, and sperrylite PtAs2 hosted mainly by violarite, silicates (primarily serpentine), and millerite. The platinum-group minerals occur in two sulfide assemblages: (1) mertieite-I-dominant (with irarsite, palladium, and Pd-alloy) in the hypogene assemblage and (2) moncheite-dominant (with irarsite, sperrylite, and atokite) in the supergene assemblage. Palladium and intermediate platinum-group elements (Os, Ir, Ru) are concentrated mainly in violarite, polydymite, and pyrite 2. Platinum is seldom hosted by base metal sulfides and occurs mainly as discrete platinum-group minerals, such as moncheite, sperrylite, and merenskyite. Violarite and polydymite in the Jinbaoshan deposit contain more Pb-Ag than pentlandite and pyrrhotite in the Great Dyke and Lac des Iles deposit. The formation of the sulfide assemblages in Jinbaoshan can be interpreted to have occurred in three stages: (1) a magmatic Fe-Ni-Cu sulfide melt crystallized Fe-Ni monosulfide and Cu-rich intermediate solid solutions, which inverted to a primary pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage; (2) an early-secondary hypogene voilarite-millterite-pyrite 1-chalcopyrite assemblage formed by interaction with a lower-temperature magmatic-hydrothermal deuteric fluid; and (3) a late-secondary supergene violarite-polydymite-pyrite 2-chalcopyrite assemblage formed during weathering. Late-magmatic-hydrothermal fluids enriched the mineralization in Pb-Ag-Cd-Zn, which are incompatible in monosulfide solid solution, added Co-Pt into violarite, and expelled Pd to the margins of hypogene violarite and millerite, which caused Pd depletion in the hypogene violarite and the formation of mertieite-I. Supergene violarite inherited Pd and intermediate platinum-group elements from primary pentlandite. Thus, the unusual sulfide assemblages in the Jinbaoshan platinum-group element-(Cu)-(Ni) deposit results from multiple overprinted post-magmatic processes, but they did not significantly change the chalcophile element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (R factors) through interaction of crustally derived sulfide and a hybrid picritic-ferropicritic magma derived from subduction-metasomatized pyroxenitic mantle during impingement of the Emeishan plume on the Paleo-Tethyan oceanic subduction system.

Genesis and mechanisms of metal enrichment in the Baimazhai Ni-Cu-(PGE) deposit, Ailaoshan Orogenic Belt, SW China

ABSTRACT The ∼259 Ma Baimazhai Ni-Cu-(platinum-group element) deposit is located in the Ailaoshan-Red River fault zone on the southwest margin of the Yangtze Plate in the Jinping area of southeastern Yunnan Province. The intrusion is lenticular (∼530 m long × 190 m wide × 24–64 m thick) and concentrically zoned (margin to core) from gabbro through pyroxenite to peridotite. It contains ∼50 kt of Ni-Cu-(platinum-group element) mineralization, concentrically zoned (margin to core) from disseminated through net-textured to massive sulfides with an average grade of 1.03 wt.% Ni, 0.81 wt.% Cu, and 0.02∼0.69 ppm Pd+Pt. The sulfide assemblage comprises pyrrhotite, chalcopyrite, and pentlandite, with lesser magnetite, violarite, galena, and cobaltite. The mineralization is enriched in Ni-Cu-Co relative to the platinum-group elements and the host rocks are enriched in highly incompatible lithophile elements relative to moderately incompatible lithophile elements with high Th/Yb and intermediate Nb/Yb ratios. These host rocks, and those at most other Ni-Cu-platinum-group element deposits in the Emeishan Large Igneous Province, have high γOs and intermediate εNd values, indicating that they crystallized from a magma derived from a subduction-modified pyroxenite mantle source and modified by crustal contamination. The initial concentrations of metals in the primary magma are estimated to have been on the order of 200 ppm Ni and 100 ppm Cu, but only 0.4 ppb Pd, 0.2 ppb Pt, 0.005 ppb Rh, 0.02 ppb Ru, and 0.01 ppb Ir. The δ34S values of ores and separated sulfides range from 5.8‰ to 8.6‰, between the ∼10‰ value of sulfides in the metasedimentary country rocks and the 0 ± 0.5‰ value expected for magmas derived from MORB-type mantle, or the –2.5 ± 0.3‰ value expected for subduction-modified mantle, consistent with equilibration at magma:sulfide mass ratios (R factors) of 100–1000. Variations in Ir100 and Pd100 (metals in 100% sulfide) are consistent with 40–60% fractional crystallization of monosulfide solid solution to form Ni-Co-intermediate platinum-group element (Ru, Os, Ir)-rich massive ores and Cu-palladium/platinum-group elements (Pt, Pd, Rh)-Au-rich residual sulfide liquids. This process is also recorded by magnetite: Type I (early magmatic), type II (late magmatic), and type III (secondary) magnetites exhibit progressively lower Cr-Ti-V concentrations. The platinum-group element contents in base-metal minerals are low, and only pentlandite, violarite, and cobaltite contain detectable concentrations of Pd, Rh, and Ru. There is abundant textural evidence for metamorphic-hydrothermal alteration of sulfides in the Baimazhai intrusion, with secondary violarite, chalcopyrite, and pentlandite being enriched (Ag, Sb, Au, Pb) or depleted (Sn) in more mobile chalcophile elements. The different tectonic and petrogenetic settings of the Baimazhai and other deposits in China highlight the potential of Ni-Cu-platinum-group element deposits to occur in subduction or post-subduction settings and demonstrate that the key controls are magma flux and access to crustal S. Exploration potential remains for the Ailaoshan orogenic belt to host additional magmatic Ni-Cu deposits.

Multi-stage sulfide evolution of the Moran Ni sulfide ore, Kambalda, Western Australia: insights into the dynamics of ore forming processes of komatiite-hosted deposits

The Moran komatiite-hosted Ni sulfide deposit at Kambalda (Australia) is one of the better preserved orebodies at Kambalda. Its geochemical signature is used to investigate the evolution of the sulfide mineralization. The orebody has several parts, including a flanking segment where massive sulfides formed relatively early and a central portion in a 40-m-deep erosional embayment representing a later generation of massive and net-textured sulfides. Basal massive sulfides within the deep embayment vary systematically in their chalcophile element contents (Ni, PGE, Au, Te, As, Bi). Elements compatible in monosulfide solid solution (MSS) exhibit the highest concentration at the edge of the orebody (up to 4.3 ppm Ir + Os + Ru + Rh), whereas incompatible elements are most concentrated in the centre (up to 11.2 ppm Pt + Pd + Au). This difference in element distributions is explained by fractional crystallization of sulfide melt from the edge towards the centre. To explain the vertical movement of the residual fractionated melt, a new model of sulfide crystallization is proposed. A low-viscosity boundary layer containing incompatible elements is formed between MSS and sulfide melt. This melt propagates with the crystallization front towards the centre of the sulfide melt pool. Trace element variations in pentlandite (e.g. Co) and composite Co- and Bi-bearing arsenide-telluride grains suggest that during the final stages of crystallization, an immiscible Co-As-Te-Bi melt is formed.

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.

Metal enrichment as a result of SCLM metasomatism? Insight from ultramafic xenoliths from SW Poland.

<p>Migration of metals such as gold, silver and copper through the subcontinental lithospheric mantle (SCLM) can be tracked by the investigation of sulfides in mantle xenoliths. Therefore, to understand relations between the metal migration and metasomatism of silicate phases in the SCLM beneath SW Poland we studied sulfides in a set of mantle ultramafic xenoliths with variable metasomatic history. The xenoliths occur in the Cenozoic alkaline mafic volcanic rocks from the SW Poland (N Bohemian Massif).  </p><p>The studied sulfides occur in mantle rocks of variable history: 1) strongly depleted (group A0) to weakly metasomatized peridotites (Group A1); 2) strongly melt-metasomatized peridotites (Group B); 3) pyroxenites (Group C; for details of group definition see Matusiak-Małek et al., 2014, JoP). The metasomatism was of mixed silicate/carbonatite nature. The sulfides are either interstitial or enclosed in the silicates and form mostly globular monosulfide solid solution-chalcopyrite (mss-Ccp) assemblages typical of igneous sulfides separated and crystallized from mafic magmas, with mss partially re-equilibrated to exsolutions of pentlandite (Pn) and pyrrhotite (Po) when temperature dropped to <600°C (e.g., Craig and Kullerud, 1969, Econ. Geol. Monogr.).</p><p>The sulfide abundances increase from Group A (≤ 0.008 vol.‰) through Group B (up to 0.060 vol. ‰) to Group C (up to 0.963 vol.‰) xenoliths. The sulfides of Groups C (Po<sub>15–99</sub>Pn<sub>0–20</sub>Ccp<sub>0–70</sub>)<sub></sub>and B (Po<sub>0–85</sub>Pn<sub>14–100</sub>Ccp<sub>0–27</sub>) are generally poorer in Ni compared to Group A (Po<sub>0–74</sub>Pn<sub>24–100</sub>Ccp<sub>0–35</sub>). Consequently, Ni/(Ni+Fe) in the Group C pentlandites (0.41–0.52) is lower than in those in Group A (0.45–0.69). Moreover, the sulfide grains of Group B are enriched in chalcophile elements (e.g., the median content of Zn is 90 ppm) compared to sulfides from Groups C (52 ppm Zn) and A (51 ppm of Zn). The same relations occur in PGE contents, e.g., Pt in Group B is 1.6 ppm, while in Groups C and A it is 0.1 and 1.3 ppm, respectively.  Observed differences in δ<sup>56</sup>Fe between the Groups are probably due to modal composition of bulk sulfide grains between Groups A (Ni-rich), B and C (Fe-Cu-rich). As no difference is observed between the grains of the same composition, any fractionation of Fe isotopes in sulfide melt seems to be possible only upon its differentiation from Ni-rich to Fe-Cu-rich.</p><p>The host peridotites were affected by strong depletion as the degree of partial melting was possibly ~30%. Thus, the observed enhanced sulfide modes in the metasomatized peridotites (Groups A1 and B) are most likely brought by the metasomatic melt. This is also evidenced by their Fe-Cu-rich composition, similar to that of the sulfides from the pyroxenites. In this view, melt metasomatism likely affects the chalcophile and highly-siderophile metal budget of the continental lithosphere.</p><p> </p><p>The measurements of Fe isotopic ratios were financed from funds for years 2020-2024 within program “Diamond Grant” (DI2019 0093 49), the LAICPMS measurements were financed from 2016/23/N/ST10/00288 to J.C., and the EPMA analyses were done within the frame of the Polish-Austrian project WTZ PL/16 and WTZ PL 08/2018.</p>

PGE-Cu-Ni Mineralization of Mafic-Ultramafic Massifs of the Khangai Upland, Western Mongolia

The mafic-ultramafic massifs with the PGE-Cu-Ni mineralization located in North-Central Mongolia: Oortsog, Dulaan, Mankhan, Yamat, and Nomgon were investigated. For the first time we consider these massifs as a single magmatic association and as fragments of Khangai batholith caused by the action of the plume responsible for the formation Permian Khangai LIP. The massifs fractionated from peridotite to gabbro have a similar typomorphic ore mineralogical and geochemical features, which change depending on the degrees of fractionation of magma and evolution of the sulfide melt. The least fractionated Oortsog massif originated from Ni-rich high-Mg basaltic magma. It is characterized by predominance of pyrrhotite mineralization due to exsolution of monosulfide solid solution (MSS). The most fractionated is the Nomgon massif originated from Cu-rich basaltic magma with bornite-chalcopyrite mineralization, formed as an exsolution of intermediate solid solution (ISS). The rest of the massifs have a medium characteristics between these two. The compositions of sulfides in the studied massifs change in accordance with the increase in sulfur fugacity from peridotite to gabbro: enrichment of pentlandite in Ni and pyrrhotite in S. The composition of PGM changes from Pt minerals in Oortsog massif to Pd minerals in Nomgon massif in the same direction. These massifs can be considered as potential for the PGE.

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

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.

Magnetite Chemistry by LA-ICP-MS Records Sulfide Fractional Crystallization in Massive Nickel-Copper-Platinum Group Element Ores from the Norilsk-Talnakh Mining District (Siberia, Russia): Implications for Trace Element Partitioning into Magnetite

Mineralogical and chemical zonations observed in massive sulfide ores from Ni-Cu-platinum group element (PGE) deposits are commonly ascribed to the fractional crystallization of monosulfide solid solution (MSS) and intermediate solid solution (ISS) from sulfide liquid. Recent studies of classic examples of zoned orebodies at Sudbury and Voisey’s Bay (Canada) demonstrated that the chemistry of magnetite crystallized from sulfide liquid was varying in response to sulfide fractional crystallization. Other classic examples of zoned Ni-Cu-PGE sulfide deposits occur in the Norilsk-Talnakh mining district (Russia), yet magnetite in these orebodies has received little attention. In this contribution, we document the chemistry of magnetite in samples from Norilsk-Talnakh, spanning the classic range of sulfide composition, from Cu poor (MSS) to Cu rich (ISS). Based on textural features and mineral associations, four types of magnetite with distinct chemical composition are identified: (1) MSS magnetite, (2) ISS magnetite, (3) reactional magnetite (at the sulfide-silicate interface), and (4) hydrothermal magnetite (resulting from sulfide-fluid interaction). Compositional variability in lithophile and chalcophile elements records sulfide fractional crystallization across MSS and ISS magnetites and sulfide interaction with silicate minerals (reactional magnetite) and fluids (hydrothermal magnetite). Estimated partition coefficients for magnetite in sulfide systems are unlike those in silicate systems. In sulfide systems, all lithophile elements are compatible and chalcophile elements tend to be incompatible with magnetite, but in silicate systems some lithophile elements are incompatible and chalcophile elements are compatible with magnetite. Finally, comparison with magnetite data from other Ni-Cu-PGE sulfide deposits pinpoints that the nature of parental silicate magma, degree of sulfide evolution, cocrystallizing phases, and alteration conditions influence magnetite composition.

The Occurrence and Origin of Pentlandite-Chalcopyrite-Pyrrhotite Loop Textures in Magmatic Ni-Cu Sulfide Ores

Pentlandite is the dominant Ni-hosting ore mineral in most magmatic sulfide deposits and has conventionally been interpreted as being entirely generated by solid-state exsolution from the high-temperature monosulfide solid solution (MSS) (Fe,Ni)1–xS. This process gives rise to the development of loops of pentlandite surrounding pyrrhotite grains. Recently it has been recognized that not all pentlandite forms by exsolution. Some may form as the result of peritectic reaction between early formed MSS and residual Ni-Cu–rich sulfide liquid during differentiation of the sulfide melt, such that at least some loop textures may be genuinely magmatic in origin. Testing this hypothesis involved microbeam X-ray fluorescence mapping to image pentlandite-pyrrhotite-chalcopyrite intergrowths from a range of different deposits. These deposits exemplify slowly cooled magmatic environments (Nova, Western Australia; Sudbury, Canada), globular ores from shallow-level intrusions (Norilsk, Siberia), extrusive komatiite-hosted ores from low and high metamorphic-grade terranes, and a number of other deposits. Our approach was complemented by laser ablation-inductively coupled plasma-mass spectrometry analysis of palladium in varying textural types of pentlandite within these deposits. Pentlandite forming coarse granular aggregates, together with loop-textured pentlandite where chalcopyrite also forms part of the loop framework, consistently has the highest Pd content compared with pentlandite clearly exsolved as lamellae from MSS or pyrrhotite. This is consistent with much of granular and loop pentlandite being formed by peritectic reaction between Pd-rich residual sulfide liquid and early crystallized MSS, rather than forming entirely by subsolidus grain boundary exsolution from MSS, as has hitherto been assumed. The wide range of Pd contents in pentlandite in individual samples reflects a continuum of processes between peritectic reaction and grain boundary exsolution. Textures in metamorphically recrystallized ores are distinctly different from loop-textured ores, implying that loop textures cannot be regenerated (except in special circumstances) by metamorphic recrystallization of original magmatic-textured ores. The presence of loop textures can therefore be taken as evidence of a lack of penetrative deformation and remobilization at submagmatic temperatures, a conclusion of particular significance to the interpretation of the Nova deposit as having formed synchronously with the peak of regional deformation at temperatures within the sulfide melting range.