Sulfide Droplets Research Articles

Timing of S-saturation in the formation of the Oubulage porphyry Cu-Au deposit, Inner Mongolia, Northern China

The Oubulage porphyry Cu-Au deposit, Inner Mongolia, North China, is located in the Langshan Tectonic Belt on the southern margin of the Central Asian Orogenic Belt (CAOB). The ore-forming quartz porphyry intrusion was formed during the Middle Permian (268.4 ± 1.6 Ma). Randomly and directionally distributed sulfide blebs are observed in some porphyry rocks. These blebs are spherical or ellipsoidal in shape. There are two groups of such blebs: polymetallic sulfide blebs and silicate-carbonate-sulfide blebs. Sulfide minerals in these blebs include pyrite, chalcopyrite, and pyrrhotite. Silicate-carbonate-sulfide blebs also contain calcite, epidote, plagioclase, chlorite, fluorite, and bastnaesite. Pyrite of sulfide blebs has δ56Fe ranging from −0.38 ‰ to 1.19 ‰. Calcite in sulfide blebs has C-O-Sr isotopic compositions (δ13CVPDB = −7.35 – −5.35 ‰, δ18OVSMOW = 8.35 – 9.92 ‰, 87Sr/86Sr = 0.7032 – 0.7064) indicative of a magmatic origin. We propose a new possible genetic model in which sulfide saturation occurred in a magma chamber, leading to the formation of sulfide droplets. Assimilation added sulphur (S) into the magma chamber and caused S-saturation. The sulfide melts are transported to the shallow crust attached by fluids.

A discovery of nanoscale sulfide droplets in MORB glasses: implications for the immiscibility of sulfide and silicate melts

Sulfur forms an immiscible liquid upon saturation in magma, and sulfide droplets are commonly found in fresh mid-ocean ridge basalt (MORB). Scanning electron microscopy (SEM) analysis revealed that the fine-grained and weakly phyric MORB samples exhibited hypocrystalline to vitreous textures. Transmission electron microscopy (TEM) of MORB glasses exhibits nanoscale sulfide droplets (10–15 nm) with rounded shapes and smooth edges, showing crystalline and homogeneous composition. Elemental distribution included S, Fe, Cu, and Ni, while Si, Al, and O were lacking. Prior research clarified the immiscibility between sulfide and silicate melts, impacting the size distribution of sulfide droplets. This is the first report on nanoscale sulfide droplets within MORB glasses, and these results suggest that nanoscale sulfide droplets represent the initial phase of sulfide saturation. Such an insight may prove useful in understanding how siderophile and chalcophile elements behaved during sulfide crystallization. In addition, this study determines the immiscibility of sulfides and silicate melts that occur in the early nanometer stage. Therefore, it is speculated that immiscibility phenomena may occur in the nanometer stage during magma evolution.

Caught in the moment: interaction of immiscible carbonate and sulfide liquids in mafic silicate magma—insights from the Rudniy intrusion (NW Mongolia)

The Devonian Rudniy intrusion is a composite magmatic body comprising two gabbroid units. Located in the Tsagaan-Shuvuut ridge in NW Mongolia, it is the only one known to contain disseminated sulfide Ni-Cu-PGE minerals out of numerous gabbroid intrusions surrounding the Tuva depression. The ore occurs as disseminated sulfide globules made of pyrrhotite, pentlandite, chalcopyrite, and cubanite, confined to a narrow troctolitic layer at the margins of a melanogabbro, at the contact with a previously emplaced leucogabbro. Globules generally display mantle-dominated sulfur isotopic signatures but show variable metallogenic and mineralogical characteristics, as well as notably different sizes and morphologies reflecting variable cooling and crystallization regimes in different parts of the intrusion. Sulfides from the chilled margin of the melanogabbro are surrounded and intergrown with volatile-rich (i.e., CO2-, H2O-, F-, and Cl) phases such as calcite, chlorite, mica, amphibole, and apatite. Based on the mineralogical and textural relationships of volatile-rich phases with sulfides, we argue that this assemblage represents the product of the crystallization of volatile-rich carbonate melt immiscible with both silicate and sulfide liquids. We put forward the hypothesis that volatile-rich carbonate melt envelops sulfide droplets facilitating their transport in magmatic conduits and that this process may be more widespread than commonly thought. The smaller sulfide globules, which are interpreted to derive from the breakup of larger globules during transport and emplacement, do not display an association with volatile-rich phases, suggesting that the original carbonate melt could have been detached from them during the evolution of the magmatic system. Variable rates of crystallization may have been responsible for the observed disparities in the mineralogical and metallogenic characteristics of different sulfide globules entrained in the Rudniy intrusion.

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

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

Ultramafic pseudotachylytes in high-pressure metamorphogenic peridotite from Luobusha, Tibet: a record of crustal paleo-earthquakes

In this paper, we report an occurrence of ultramafic pseudotachylytes, providing fault-rock evidence of paleo-earthquakes, from the Luobusha ophiolite complex in the Yarlung Zangbo suture zone. The pseudotachylytes form hairline-thin aphanitic veinlets and vein networks bounded by micro-damage zones cutting through the host harzburgite, forming flow banding in some places. The pseudotachylyte veins are dominated by close-knit ultrafine-grained minerals consisting of olivine, orthopyroxene, serpentine, spinel, and magnetite, cemented by an extremely fine matrix. As the primary component of the pseudotachylyte veins, olivine occurs as microphenocryst showing zoning from core to rim and as irregularly shaped microlite immersed in the interstitial material. Zoned crystals of olivine developed with Mg-rich cores and more Fe-rich rims. Microlite diagnosis of crystallization in a quenched melt includes dendritic, skeletal, and poikilitic olivine crystals, which are typical of ultramafic pseudotachylyte. The olivine microlites contain higher amounts of Ca, Al, and Cr but a lower Ni content compared with the host harzburgite olivine. Irregularly shaped chrome-spinel crystals are chemically zoned as well, indicating an Fe-rich rim overgrowth. Ni sulfide droplets interspersing among the matrix imply melt occurrence. The presence of a micro-fibrous and micro-vesicular interstitial matrix also indicates a melting-related origin. Ultracataclastite veins associated with pseudotachylyte transecting serpentine are observed, which convey that heat was generated during rapid comminution and injection. The characteristic petrography, microtextures, and chemical inhomogeneities meet the criteria of ultramafic pseudotachylyte and reveal a mixed genesis via a combination of crushing and melting. The development of extremely tiny globular prograde serpentine inclusions (∼100 nm) in the olivine microlites is ascribed to the dehydration reaction of serpentine to olivine within the pseudotachylyte. The Luobusha metamorphogenic peridotite was subjected to serpentinization after having emplaced in the crust and subsequently to high-pressure metamorphism. The pseudotachylytes were generated in the crust after the high-pressure metamorphism and did not descend to a greater depth. Flash ultra-comminution associated with frictional heating may release fluids via localized heat-driven prograde reactions in the crust.

Role of volatiles in intrusion emplacement and sulfide deposition in the supergiant Norilsk-Talnakh Ni-Cu-PGE ore deposits

Abstract The Norilsk-Talnakh orebodies in Siberia are some of the largest examples on Earth of magmatic Ni–Cu–platinum group element (PGE) deposits, formed by segregation of immiscible sulfide melts from silicate magmas. They show distinctive features attributable to degassing of a magmatic vapor phase during ore formation, including: vesiculation of the host intrusions, widespread intrusion breccias, and extensive hydrofracturing, skarns, and metasomatic replacement in the country rocks. Much of the magmatic sulfide was generated by assimilation of anhydrite and carbonaceous material, leading to injection of a suspension of fine sulfide droplets attached to gas bubbles into propagating tube-like host sills (“chonoliths”). Catastrophic vapor phase exsolution associated with a drop in magma overpressure at the transition from vertical to horizontal magma flow enabled explosive propagation of chonoliths, rapid “harvesting” and gravity deposition of the characteristic coarse sulfide globules that form much of the ore, and extensive magmatic fluid interaction with country rocks.

Mantle-to-crust metal transfer by nanomelts

The transfer of chalcophile metals across the continental lithosphere has been traditionally modeled based on their chemical equilibrium partitioning in sulfide liquids and silicate magmas. Here, we report a suite of Ni-Fe-Cu sulfide droplets across a trans-lithospheric magmatic network linking the subcontinental lithospheric mantle to the overlying continental crust. Petrographic characteristics and numerical calculations both support that the sulfide droplets were mechanically scavenged from the mantle source during partial melting and transported upwards by alkaline magmas rising through the continental lithosphere. Nanoscale investigation by high-resolution transmission electron microscopy (HR-TEM) documents the presence of galena (PbS) nanoinclusions within the sulfide droplets that are involved in the mantle-to-crust magma route. The galena nanoinclusions show a range of microstructural features that are inconsistent with a derivation of PbS by exsolution from the solid products of the Ni-Fe-Cu sulfide liquid. It is argued that galena nanoinclusions crystallized from a precursor Pb(-Cu)-rich nanomelt, which was originally immiscible within the sulfide liquid even at Pb concentrations largely below those required for attaining galena saturation. We suggest that evidence of immiscibility between metal-rich nanomelts and sulfide liquids during magma transport would disrupt the classical way by which metal flux and ore genesis are interpreted, hinting for mechanical transfer of nanophases as a key mechanism for sourcing the amounts of mantle-derived metals that can be concentrated in the crust.

Experimental Investigation on the Transport of Sulfide Driven by Melt‐Rock Reaction in Partially Molten Peridotite

AbstractExtraction of sulfides from the partially molten mantle is vital to elucidate the cycling of metal and sulfur elements between different geochemical circles but has not been investigated systematically. Using laboratory experiments and theoretical calculations, this study documents systematical variations in lithologies and compositions of silicate minerals and melts, which are approximately consistent with the results of the thermodynamically‐constrained model. During a melt‐peridotite reaction, the dissolution of olivine and precipitation of new orthopyroxene generate an orthopyroxene‐rich layer between the melt source and peridotite. With increasing reaction degree, more melt is infiltrated into and reacts with upper peridotite, which potentially enhances the concomitant upward transport of dense sulfide droplets. Theoretical analyses suggest an energetically focused melt flow with a high velocity (∼170.9 μm/hr) around sulfide droplets through the pore throat. In this energic melt flow, we, for the first time, observed the mechanical coalescence of sulfide droplets, and the associated drag force was likely driving upward entrainment of fine μm‐scale sulfide. For coarse sulfide droplets whose sizes are larger than the pore throat in the peridotite, their entrainment through narrow constrictions in crystal framework seems to be physically possible only when high‐degree melt‐peridotite reaction drives high porosity of peridotite and channelized melt flows with extremely high velocity. Hence, the melt‐rock reaction could drive and enhance upward entrainment of μm‐ to mm‐scale sulfide in the partially molten mantle, potentially contributing to the fertilization of the sub‐continental lithospheric mantle and the endowment of metal‐bearing sulfide for the formation of magmatic sulfide deposits.

The critical role of magma degassing in sulphide melt mobility and metal enrichment

Much of the world’s supply of battery metals and platinum group elements (PGE) comes from sulphide ore bodies formed in ancient sub-volcanic magma plumbing systems. Research on magmatic sulphide ore genesis mainly focuses on sulphide melt-silicate melt equilibria. However, over the past few years, increasing evidence of the role of volatiles in magmatic sulphide ore systems has come to light. High temperature-high pressure experiments presented here reveal how the association between sulphide melt and a fluid phase may facilitate the coalescence of sulphide droplets and upgrade the metal content of the sulphide melt. We propose that the occurrence of a fluid phase in the magma can favour both accumulation and metal enrichment of a sulphide melt segregated from this magma, independent of the process producing the fluid phase. Here we show how sulphide-fluid associations preserved in the world-class Noril’sk-Talnakh ore deposits, in Polar Siberia, record the processes demonstrated experimentally.

Lithologic and Geochemical Constraints on the Genesis of a Newly Discovered Orebody in the Jinchuan Intrusion, NW China

AbstractThe Jinchuan Ni-Cu-platinum group element (PGE) sulfide deposit is one of the world’s major magmatic sulfide deposits. The Jinchuan intrusion originally consists of two independent bodies, named the Western and Eastern intrusions. Recently, an underground exploration program discovered a large economic orebody in the Fine-Grained unit of the Western intrusion, independent of the orebody No. 24 that occurs at the base of the Coarse-Grained unit. The newly discovered orebody dips to the southwest with thicknesses of 20 to 150 m and is composed of olivine-sulfide cumulates with net-textured sulfide overlain by lherzolite with disseminated sulfide. The net-textured olivine-sulfide cumulates have average grades of 2.1 wt % Ni and 1.2 wt % Cu, whereas the disseminated ores in the lherzolite have average grades of 0.6 wt % Ni and 0.4 wt % Cu. This makes this orebody, named orebody No. 3 and hosted in segment III, the fourth largest at Jinchuan. The sharp lithologic and chemical variation between the net-textured olivine-sulfide cumulates and the disseminated sulfide-bearing lherzolite indicates that they were formed by different processes. Evident differences in sulfidefree whole-rock compositions reflect the dunite forming as olivine adcumulate and the lherzolite as olivine orthocumulate with variable trapped liquid abundances. The olivine-sulfide cumulates were formed by percolation of sulfide liquid into the olivine adcumulates, expelling the interstitial silicate melts upward, whereas the disseminated ores in the lherzolite were formed by settling of finely dispersed sulfide droplets in the olivine crystal mush, within which pyroxene crystallized from the interstitial melts. Variations of PGE tenors of the net-textured sulfides demonstrate fractionation of the sulfide liquids. The relatively high PGE tenors of the disseminated sulfides compared with the net-textured sulfides are associated with upgrading of the sulfides due to reaction with fresh magma.

Fluids as primary carriers of sulphur and copper in magmatic assimilation

Magmas readily react with their wall-rocks forming metamorphic contact aureoles. Sulphur and possibly metal mobilization within these contact aureoles is essential in the formation of economic magmatic sulphide deposits. We performed heating and partial melting experiments on a black shale sample from the Paleoproterozoic Virginia Formation, which is the main source of sulphur for the world-class Cu-Ni sulphide deposits of the 1.1 Ga Duluth Complex, Minnesota. These experiments show that an autochthonous devolatilization fluid effectively mobilizes carbon, sulphur, and copper in the black shale within subsolidus conditions (≤ 700 °C). Further mobilization occurs when the black shale melts and droplets of Cu-rich sulphide melt and pyrrhotite form at ∼1000 °C. The sulphide droplets attach to bubbles of devolatilization fluid, which promotes buoyancy-driven transportation in silicate melt. Our study shows that devolatilization fluids can supply large proportions of sulphur and copper in mafic–ultramafic layered intrusion-hosted Cu-Ni sulphide deposits.

Transfer of Gold, Platinum and Non-Ferrous Metals from Matte to Slag by Flotation

One of the stages of extracting gold and platinum from sulfide materials and circulating slags is the melting stage in ore-thermal and electric furnaces, where the melt is separated into matte and slag. Gold, platinum, and non-ferrous metals are concentrated in the matte. However, a significant portion of them ends up in the slag, which reduces recovery and increases environmental pollution. The main reason for their transition to slag is the flotation of sulfide droplets by gas bubbles, a significant proportion of which occurs during the decomposition of sulfides. Gold and platinum are associated with matte droplets during flotation. Evaluation of adhesion showed that it is large and comparable to the cohesion of these metals. One of the options to reduce the loss of valuable components is to add fluxes to the slag. The influence of calcite and fluorite on the distribution of gold and platinum over the melting products of copper–nickel sulfide materials (matte and slag) has been experimentally studied based on the above theoretical concepts of droplet flotation. When calcite was added to sulfide ore, there was a significant decrease in the sulfur content in the slag (more than 3 times). This, in turn, led to a decrease in non-ferrous metals in the slag by 2–3 times, with gold from 0.45 to 0.29 g/t and platinum from 2.15 to 2.06 g/t. The addition of fluorite to the mixture of copper–nickel matte and model slag (CaO/SiO2/Al2O3 = 40/40/20) significantly reduced the sulfur content and non-ferrous metals by 1.5 times, whereas gold was not found in the slag. The decrease in the number of sulfides in the slag is mainly because the listed additives reduce its viscosity. This leads to acceleration of the coagulation of sulfide drops, which are inevitably carried into the slag during flotation, and increases the rate of their settling to the slag–matte boundary, where they merge with the matte mass.

Extremely low-Ni olivine as a tracer of carbonate-rich sediment recycling beneath the southwestern Okinawa Trough

ABSTRACT Carbonate recycling in subduction zones has a strong influence on arc magmatism and carbon reservoirs in the mantle. However, geochemical evidence of carbonate recycling during subduction, especially at sub-arc depths, is rare and the related process is poorly understood. In this study, we document high-Mg olivines (81–91 mol.% forsterite) with extremely low Ni concentrations (mostly <600 ppm), which occur in host dacites recovered from the southwestern Okinawa Trough (SWOT). The temperature–pressure of crystallization had a negligible effect on the Ni contents of the olivines, and reactions between primary basaltic magmas and the lithospheric mantle were also insignificant. Small sulphide droplets are present in the olivines, which suggest that sulphide–silicate melt immiscibility occurred during the early stages of magmatic evolution. Although early segregation of sulphide melts could reduce the Ni contents of the primitive magmas, our forward modelling suggests that sulphide segregation cannot solely explain the low Ni contents. Therefore, the most likely explanation for low Ni concentrations is the interaction between Ca-rich carbonate melts and the mantle wedge, forming an olivine-rich mantle source by consumption of orthopyroxene, as indicated by the extremely low Ni/Mg and high Ca/Fe ratios of magmatic olivines. Given that the SWOT is located just ~100 km above the Wadati–Benioff zone, our results provide rare evidence for carbonate recycling at sub-arc depths in an active subduction zone.

Kinetic controls on the sulfide mineralization of komatiite-associated Ni-Cu-(PGE) deposits

Genesis of magmatic Ni-Cu-(PGE) deposits involves many interconnected and multiple-scale processes in the fields of thermodynamics, kinetics and fluid dynamics. The main focus of quantitative work on magmatic sulfide ore genesis has been on equilibrium thermodynamics since the initial studies over 50 years ago, while the significance of kinetics and dynamics to sulfide mineralization in dynamic magmatic systems has received little attention. Taking the komatiite-hosted Ni-Cu-(PGE) deposits of the Raglan area as an example, we have developed a quantitative model for assimilation of sulfidic substrates during lava emplacement through the consideration of these three fields. We show that the dominant control on the composition of externally derived sulfide xenomelts throughout their residence within the flowing magma is exerted by relative rates of diffusion of sulfur and metals. In rapid channelized flow of komatiites, the kinetically-controlled extraction of chalcophile elements from silicate melt into externally-derived sulfide droplets released from underlying sediments is the quickest way to produce orebodies with economic value, without the necessity of reaching either sulfide saturation throughout the mass of magma or indeed compositional equilibrium in the magmatic system as our long-held understanding would suggest. The kinetic upgrading of initially metal-poor sulfide xenomelts is enhanced for sulfide droplets of smaller size and increasing downstream distance from the source, approaching to equilibrium mass distribution for the fast-diffusing elements such as Cu during prolonged or especially vigorous processes (e.g., chaotic magma mixing). In this kinetically-driven process, differences in diffusivities between Pt and Pd may account for the anomalous excess Pd/Pt ratios (~2) of disseminated ores from many komatiite-related deposits compared with the compositions of the parental magmas. Sedimentation of sulfide on the bottom of the flowing magma terminates this kinetically-controlled process, while the number density of droplets imposes a restriction on sulfide compositions due to mass conservation. Therefore, sulfide mineralization in komatiite-associated deposits is mostly controlled by diffusion kinetics with additional interactions and restrictions from the dynamics of transport and settling of sulfide droplets. By analogy, multiple cycles of dynamic processes predominate in the conduit-type magmatic deposits. A quantitative grasp of the relative importance of these three fundamental mechanisms, i.e., thermodynamic driving forces, kinetics of diffusive equilibration, and fluid dynamics of transport and deposition, is of great value in understanding ore genesis in complex mineral systems and even the origin of igneous rocks in general.

Bronze Age metallurgical slags from the South Urals: types, mineralogy and copper sources

In the paper, an investigation of copper metallurgical slags found in the South Urals and dating to the Bronze Age between 4000 and 1300 BCE is carried out. Four main mineralogical types are distinguished according to their mineral composition: Cr-rich spinel containing olivine, sulphide-containing olivine, sulphide containing glassy and pyroxene type. First type of slags is composed of zoned olivine, magnetite and glass having relicts of Cr-rich spinels and serpentinites that indicate the use of azurite-malachite ores from ultrabasic rocks. These finds were determined in sites of the Early Yamna period in the Cis-Urals and are widespread in Sintashta culture. The second type is composed of skeletal olivine crystals, magnetite and wustite with copper sulphides droplets. It type belongs to Alakul period. The raw material sources are sulphide ores from a secondary enrichment zone of VMS, skarn and Cu-porphyry deposits. The sulphide containing glassy type is mainly composed of glass with copper and sulphide droplets. The slags were recovered in the Srubna sites. The raw material sources were rich sulphide ores of cupriferous sandstones. The pyroxene type is composed of augite, pigeonite and/or wollastonite with a small amount of glass and sulphide inclusions. According to the composition of glass, slags can be distinguished into the Cis-Uralian type, including cupriferous sandstones, and Trans-Uralian type having various volcanogenic-hydrothermal deposits as raw materials. Essential Ca, P, Ba and REE impurities in slag glass demonstrate the use of barite, calcite and bone fluxes. The phase diagrams, crystallisation temperatures and experimental data revealed that slag melt temperatures ranged between 1150–1300 ° C.

The Haidbach deposit in the Central Tauern Window, Eastern Alps, Austria: a metamorphosed orthomagmatic Ni-Cu-Co-PGE mineralization in the Polymetallic Ore District Venediger Nappe System – Hollersbach Complex

Abstract Cu-Ni-Co-PGE mineralization occurs at Haidbachgraben in the Early Palaeozoic, Subpenninic Hollersbach Complex of the Central Tauern Window, Austria. Massive sulfide ore formed from sulfide melt segregated from silicate melt during intrusion of pyroxenite into magmatic rocks formed in an MORB-type environment. Relics of magmatic minerals include chromian spinel and polyphase sulfide droplets composed of pyrrhotite, chalcopyrite and pentlandite preserved in recrystallized pyrite. Both ore and host rocks were multiply deformed and metamorphosed, leading to hornblendite carrying the ore, enveloped by chlorite-epidote schist. Conditions of – likely Variscan – amphibolite facies metamorphism are documented by relict pargasitic cores in hornblende and actinolite-tremolite, and by ternary sulfarsenide compositions in the Co-Ni-Fe solid solution series that are the most common accessory minerals found in the sulfide ore. Pyrrhotite, pentlandite, chalcopyrite and pyrite are the major sulfide minerals. Chalcopyrite is Cd-rich and retains a high-temperature magmatic signature. High Co/Sb and moderate Se/As ratios in pyrite also point to a magmatic environment of mineralization. The accessory mineral assemblage of small grain size (mostly &lt;10 µm) comprises native Au-Ag alloy and petzite as Au-Ag minerals, sperrylite, a variety of Pd tellurides and bismuthotellurides with elevated Sb, irarsite, and Re sulfides such as tarkianite and a Pb-Re sulfide. In addition, minor molybdenite, bournonite, scheelite and selenides have been identified. Two precious metal assemblages are present in individual samples: (1) hessite associated with Pd tellurides, often accompanied by sphalerite and chalcopyrite; (2) tarkianite forming euhedral inclusions in pyrite. Sperrylite and Au-Ag native alloys are present throughout and were also detected in silicate matrix. Most of the precious metal-bearing phases must have formed during recrystallization of base metal sulfides after the magmatic, and probably during later metamorphic events terminating in the Neoalpine Tauern crystallization.

Toward a global conceptual model for metal enrichment in felsic, mafic-ultramafic, and alkaline-carbonatitic magmas

The formation of intrusion-related metal ore deposits commences with a combination of sequential metal and melt interactions in the magma reservoir: (i) metal diffusion within the melt; (ii) melt motion; (iii) metal partitioning between silicate melts and a magmatic immiscible phase (MIP, including immiscible melts and/or immiscible vapor phases, and crystalized solids; (iv) metal advection in the melt; (v) chemical interactions between the MIP and the mush; and (vi) finally metal deposition. Occasionally, these processes may also develop simultaneously, and lead to formation of multiple depositional centers. During those events, the presence of the MIP seems preponderant and a prerequisite to metal segregation and transport with the interactive mush. Based on forward and inverse modeling, novel diffusion-partitioning-advection models are proposed to account for economic metal enrichment (three to five orders of magnitude relative to initial concentrations in parental melts). For felsic to intermediate magmas, metals (Cu, Mo, W, Sn) are enriched in a saline aqueous phase by diffusion and metal partitioning. Progressive stagnation of melt motion enhances metal diffusion according to the tortuosity of a developing mush and destabilization of plagioclases. The metal-rich liquids separate from the magma reservoir by advection and transfer the metals to the country rocks. For mafic/ultramafic magmas, the MIP is a dense, sulfide melt, which preferentially sequesters metals (Ni, Cu, platinum group elements). The sulfide droplets interact with olivine to scavenge more Ni before they sink toward the bottom of the magma chamber due to negative buoyance. The down-going flow may even become turbulent according to the melt viscosity, leading to disperse and chaotic ore. In peralkaline magmas, CO2-rich carbonatitic melts are the MIPs formed by segregation of CO2-poor feldspathoid syenite melts, and attracting rare earth elements. Those are selectively partitioned by fenitization. In all the three different types of magmatic system, metals diffuse from viscous evolved parental melts toward a MIP, which is responsible for metal partitioning, segregation and transports. This global conceptual model for metal enrichment focuses on the competition between metal diffusion within the melt, melt viscosity and metal partitioning between the parental melt and MIP. However, a second stage of metal enrichment occurs during interaction of the already enriched melt and the mush. Such conceptual model is a first order model that could suggests newer ideas to ore formation.

Insights from canonical correlation analysis into sulfide migration during formation of disseminated ores in magmatic Ni–Cu–PGE sulfide deposits

Magmatic Ni–Cu–platinum group element (PGE) sulfide deposits form as the result of segregation of immiscible sulfide liquid from mafic–ultramafic magma. Sulfide ores formed in this process commonly take their places in the lower part of mafic–ultramafic intrusion, and many researchers attribute this phenomenon to gravity settling of dense sulfide droplets within lighter silicate magma. Some laboratory experiments, however, suggest that not all the sulfide droplets can migrate downward through crystal mush, especially those forming disseminated ores. On the basis of geochemical analysis, we employ a multivariate statistical method to investigate whether downward motion of sulfide droplets can happen during formation of disseminated ores. Two typical magmatic Ni–Cu–PGE sulfide deposits, namely the Hongqiling and Kalatongke deposits, were selected, and the relationship between sulfide composition (bulk-rock Cu, Ni, S, and PGE contents) and silicate composition (bulk-rock SiO2, Al2O3, K2O, Na2O, CaO, and MgO contents) was investigated by canonical correlation analysis (CCA). The result shows that there is a strong relationship between the sulfide and silicate compositions in the disseminated ores, which indicates that the sulfide droplets may have formed from the local crystal mushes, and furthermore that they have not migrated away from the crystal mushes. Hence, we conclude that sulfide droplets that form disseminated ores may tend not to migrate downward for long distances after segregating from mafic–ultramafic magma.

Reaction Infiltration Instabilities in Partially Molten Peridotite and Implications for Driving the Transport of Sulfide Liquid

Reaction infiltration instability (RII) can cause the formation of melt channels and potentially facilitate the physical transport of sulfide liquid, which contributes to the geochemical evolution of chalcophile elements in the lithospheric mantle. This study conducted some two-layer reaction experiments to explore the feasibility of reaction-driven sulfide migration along high-velocity silicate-melt channels. With increasing duration, the formation of more silicate-melt channels and the transport of more sulfide droplets into a depleted peridotite were observed due to the increase of the local permeability. However, at a longer duration, the presence of some melt-channel relics implies that melt channels are temporary and ultimately closed when the reaction infiltration of silicate melt reached equilibrium in the depleted peridotite. Furthermore, theoretical calculations indicate that the RII of the system is suppressed, which impedes the formation of melt channels. The homogeneous distribution of silicate melt in a sulfide-free experiment implies that the Zener pinning of sulfide probably enhances the RII, thereby facilitating the formation of temporary melt channels. Therefore, this study demonstrates that sufficient silicate melt disequilibrium with solid phases in a liquid source potentially promotes the mechanical extraction of sulfides during reaction infiltration of silicate melt.

Flotation mechanism of sulphide melt on vapour bubbles in partially molten magmatic systems

The motion of dense Fe-rich immiscible sulphide liquids is generally supposed to be dominated by gravitational settling in crustal magma chambers, however they may become buoyant by attachment to low-density vapour bubbles to form compound drops, possibly contributing to the upward transfer of sulphur and transition metals in degassing magma bodies. Here, using numerical models, we consider constraints on the flotation of compound drops, and find a wide range of morphologies that would be expected to be capable of migrating through magmatic systems, including both melt- and mush-dominated domains, but also some situations where the compound drops will spontaneously separate to allow the vapour to rise while the sulphide phase is sequestered in the mush. The stability of compound drops is empirically related to a function f(Bo)=|σMV−σMS|σMS⋅1/BoS+|σVS−σMS|σMS⋅1/BoV, where M, V, and S denote silicate melt, vapour, and sulphide liquid, σ is surface tension (N⋅m−1), and Bo is the Bond number. Over a range of plausible conditions in magmas, f(Bo) must exceed values of 3 to 4 for capillary forces to overcome buoyancy forces tending to pull them apart. Using published thermodynamic models for vapour and sulphide solubility in silicate melts, we show that in many magmas the second boiling of vapour occurs synchronously with the saturation of sulphide liquid, increasing the potential for coupling of vapour and sulphide droplets. Diffusive coarsening produces mm-sized bubbles within 100-500 years, generating enough buoyancy to trigger the migration of compound drops. During compaction-driven expulsion of interstitial melt, compound drops can pass through constrictions between crystals in mush. The flotation of sulphide-vapour aggregates is likely to occur in crustal magma reservoirs and provides a feasible mechanism for the removal of sulphide liquid from crystal mushes, promoting its ability to participate in the generation of magmatic sulphide and porphyry copper deposits, emissions of the metals during volcanic eruptions, and even the remobilization of chalcophile metals sequestered in deep arc cumulates to generate porphyry Cu-(Au) deposits.