Sulfide Droplets Research Articles

Morphology, Mineralogy, and Composition of Sulfide Droplets in Picrodolerite from a Near-Bottom Apophysis of the Yoko-Dovyren Layered Intrusion

The paper presents data on sulfide globules in picrodolerites from a bottom apophysis of the Yoko-Dovyren massif, northern Transbaikalia. Textural features of the rocks indicate that they crystallized at rapid magma cooling. Their composition is similar to that of chilled olivine gabbronorites from the lower contact of the massif. Many droplet-shaped sulfides, which range from 2 to 15 mm, were identified in one of the samples using X-ray computed tomography. The largest of these globules was drilled out and studied in detail to clarify its inner structure, which consists of the main spheroid and a surrounding halo of disseminated sulfides. The main globule and surrounding halo significantly differ in composition (the halo is richer in Cu), as was calculated using the average compositions of the dominant sulfide phases and their relative proportions. The weighted mean (with regard for halo) composition can be assumed as an approximation of the composition of the initial sulfide liquid that was formed near the chill zone of the gabbronorite apophysis from the main intrusive chamber. The Cu content in the initial sulfide corresponds to the results of thermodynamic modeling (with the COMAGMAT-5 program package) of the geochemistry of primitive sulfides in ultramafite rocks from the basal zone of the massif. The differences in the composition of the halo and the main globule indicate the possibility of local migration (within a few millimeters) of the residual sulfide liquids enriched in Cu when the protosulfide melt differentiated. As a physical mechanism that produced the halo, we assumed that the latest sulfide fraction was pressed off into the pore space of rapidly crystallizing olivine orthocumulates.

Transport of coexisting Ni-Cu sulfide liquid and silicate melt in partially molten peridotite

Transport of coexisting sulfide and silicate melts in partially molten peridotite contributes to the redistribution of chalcophile elements within the upper mantle as well as the genesis of magmatic Ni-Cu sulfide deposits, but has not been investigated systematically. Using laboratory experiments, theoretical calculations, and X-ray synchrotron microtomography, this study documents the topology and considers controls on the extraction of two immiscible liquids during partial melting of mantle peridotite. Under hydrostatic conditions, the measured dihedral angle at silicate melt-mineral-mineral junctions is 13.7–21.3°. Silicate melt is distributed along grain edges forming incompletely interconnected melt channels that disconnected by some dead ends at melt fractions ∼7–9 vol%. Application of theoretically predicted permeability (k∼10−14–10−16 m2) permits estimation of the extraction velocity of silicate melt of 0.7-11.1 μm/day within a single interconnected melt channel. In the absence of silicate melt, isolated sulfide droplets (3.77 vol%) show a sulfide-olivine-olivine dihedral angle of 91.5–101.3°. However, in the presence of silicate melt, sulfide droplets (average size ∼2.53 ± 2.14 μm, 1σ) are partially surrounded by silicate melt and stranded in triple junctions or melt pockets due to the limitation of the smallest dimension (0.3 μm) of melt channels. Thus, the extraction of sulfide liquid is highly restricted by these dead ends and the smallest dimension of melt channels during porous flow of silicate melt. In contrast, during large-strain shear deformation (strain ∼1.6–2.5), initially stranded sulfide droplets were elongated and extracted with silicate melt into liquid-rich sheets with a length of several hundred microns, constantly oriented at 14.3 ± 4.5° to the shear plane and antithetic to the shear direction. The angle is lower than that (18–30°) of those sheets containing sulfide liquid only, indicating that silicate melt dominates liquid-rich sheets. Driven by stress, silicate melt-dominated liquid-rich sheets open the appropriately oriented grain boundaries between silicate minerals, thus providing an efficient pathway for the extraction of sulfide liquid during deformation. When such sheet-like channels remain open, sulfide droplets (> millimeter-scale) can be potentially mobile through high strain domains of the upper mantle, contributing to the addition of chalcophile elements and the fertilization of the lithospheric mantle.

Formation of banded chromitites and associated sulphides in the Neoproterozoic subarc deep-crustal magma inferred from the Alaskan-type complex, Egypt

The concentrically zoned Dahanib Alaskan-type complex in the Southern Eastern Desert (SED) of Egypt contains cumulate dunites associated with banded chromitites in the core, through harzburgites, wehrlites, and gabbroic rocks to the rim. Primary mineral inclusions hosted in chromian spinels of the banded chromitites are usually pargasite, orthopyroxene (Opx), aspidolite, phlogopite, edenite, olivine and PGE-bearing sulphides. The chromian spinel in Dahanib chromitites has a narrow range of Cr# (0.53–0.63) along with high Ti and Fe+3, like that in the host clinopyroxene-rich cumulate dunites. It resembles in chemistry to spinels in the Alaskan-type complex formed in an island arc setting. The early cumulus olivine in disseminated chromitites and host dunites (Fo88-91) shows a typical fractional crystallization trend. The interstitial clinopyroxene (Cpx) in chromitites is similar in chemical properties to Cpx crystallized from tholeiitic basalt melts, and traces a cumulate trend of arc-related Alaskan complexes. The peridotitic and gabbroic rocks contain primary amphiboles, which are pargasite and edenite with high Ti, Na and Al. The sulphide minerals are usually chalcopyrite, pentlandite and horomanite with subordinate Cu-pentlandite, cubanite, samaniite, heazlewoodite and pyrite. Pentlandite, horomanite and cubanite at temperatures between 900 and 1040 °C are rich in some platinum group elements (PGE: Os, Pt, Ir). They are classified into primary and secondary origins, which were crystallized from segregated sulphide droplets and from hydrothermal fluids at temperature < 760 °C during postmagmatic stage, respectively. The Dahanib banded chromitites and their mineral inclusions crystallized from hybrid melts, consisting of hydrous tholeiitic basalt melts mixed with slab-derived melts/fluids. They possibly formed by gravitational settling of semi-consolidated crystal mushes in a sub-arc lower-crustal magma chamber. Chemistry of primary minerals suggests that the Dahanib subarc deep-crustal magma is rich in Mg, Cr, Al, Ti, V, Ca, Sc and Na during the early magmatic stage, followed by increasing Fe, Mn, Ni, Co, Cu, S, Cs, B, Sr, and Pb with a progress of fractional crystallization.

A Preliminary Model for the Migration of Sulfide Droplets in a Magmatic Conduit and the Significance of Volatiles

Abstract A close relationship between Ni–Cu–(PGE) sulfide deposits and magmatic conduit systems has been widely accepted, but our present understanding still rests on empirical inductions that sulfide liquids are entrained during magma ascent and aggregated at hydrodynamic traps such as the opening of a conduit into a larger magma body. In this contribution, a preliminary quantitative model for the dynamics of mm-scale sulfide droplets in a vertical magmatic conduit is developed, examining such limiting parameters as the size, transport velocity and the magmas’ maximum carrying capacity for sulfide droplets. Addition of numerous dense sulfide droplets significantly reduces magma buoyancy and rapidly increases the bulk viscosity, and the resulting pressure gradient in the propagating conduit dyke restricts the maximum volume fraction of droplets that can be carried by ascending magma. For sulfide droplets alone, the maximum carrying capacity is low, but it will be improved dramatically by the addition of volatiles which reduces the density and viscosity of silicate melt. Potential volatile degassing during decompression further facilitates sulfide entrainment by reducing bulk magma density, and the formation of buoyant compound vapour-sulfide liquid bubble drops also greatly enhances the carrying capacity. The breakdown of compound drops by detachment of parts of the vapour bubble or sulfide droplet may occur at low pressure, which liberates sulfide liquids from rising compound drops, potentially to collect in traps in the conduit system. When sulfide-laden magma flows through a widening conduit, many droplets can be captured by the re-circulation flow just downstream of the expanding section, followed by sulfide liquid accumulation and enhanced chemical interaction via diffusive exchange with the recirculating magma, potentially resulting in an economic, high-tonnage ore body. We apply our models to the emplacement of sulfide-rich magmatic suspensions at Noril’sk and show that the disseminated mineralization in intrusions could have formed when magmas carrying re-suspended sulfide liquid entrained from pre-existing sulfide accumulations in the conduit system reached their limiting sulfide carrying capacity as dictated by buoyancy and were deflected into blind sills flanking the principal conduit for flood basalt volcanism.

Up, down, or sideways: emplacement of magmatic Fe–Ni–Cu–PGE sulfide melts in large igneous provinces

The preferential localization of Fe–Ni–Cu–PGE sulfides within the horizontal components of dike–sill–lava flow complexes in large igneous provinces (LIPs) indicates that they were fluid dynamic traps for sulfide melts. Many authors have interpreted them to have collected sulfide droplets transported upwards, often from deeper “staging chambers”. Although fine (&lt;1–2 cm) dilute (&lt;10%–15%) suspensions of dense (∼4–5 g/cm3) sulfide melt can be transported in ascending magmas, there are several problems with upward-transport models for almost all LIP-related deposits: (1) S isotopic data are consistent with nearby crustal sources, (2) xenoliths appear to be derived from nearby rather than deeper crustal sources, (3) lateral sheet flow or sill facies of major deposits contain few if any sulfides, (4) except where there is evidence for a local S source, sulfides or chalcophile element enrichments rarely if ever occur in the volcanic components even where there is mineralization in the subvolcanic plumbing system, and (5) some lavas are mildly to strongly depleted in PGE &gt;&gt;&gt; Cu &gt; Ni &gt; Co, indicating that unerupted sulfides sequestered PGEs at depth. Two potential solutions to this paradox are that (i) natural systems contained surfactants that lowered sulfide–silicate interfacial tensions, permitting sulfide melts to coalesce and settle more easily than predicted from theoretical/experimental studies of artificial/analog systems, and (or) (ii) sulfides existed not as uniformly dispersed droplets, as normally assumed, but as fluid-dynamically coherent pseudoslugs or pseudolayers that were large and dense enough that they could not be transported upwards. Regardless of the ultimate explanation, it seems likely that most high-grade Ni–Cu–PGE sulfide deposits in LIPs formed at or above the same stratigraphic levels as they are found.

Merensky-type platinum deposits and a reappraisal of magma chamber paradigms

Most of the world’s economically-viable platinum deposits occur as ‘reefs’ in layered intrusions – thin layers of silicate rocks that contain sulphides enriched in noble metals. There are two contrasting magmatic hypotheses for their formation. The first suggests accumulation through gravity-induced settling of crystals onto the magma chamber floor. The alternative argues for in situ crystallization, i.e. upward growth from the floor. Here we report on our discovery of the Merensky Reef in the Bushveld Complex that occurs on subvertical to overturned margins of depressions in a temporary chamber floor. Such relationships preclude crystal settling and demonstrate that the reef crystallized in situ. This finding indicates that platinum deposits can grow directly at the chamber floor, with immiscible sulfide droplets sequestering ore-forming noble metals from strongly convecting silicate magmas. Our model also provides evidence for the paradigm that argues for magma chambers being masses of nearly crystal-free melt, which gradually loses heat and crystallizes from the margins inward.

The Lower Banded series of the Stillwater Complex, Montana: whole-rock lithophile, chalcophile, and platinum-group element distributions

The principal rock types of the Lower Banded series of the Stillwater Complex are gabbronorite and norite with minor troctolite and anorthosite. The whole-rock composition is largely controlled by the cumulate minerals plagioclase and pyroxene (±olivine). The rocks are very poor in incompatible elements, at the 0.1 to 2 times mantle levels, indicating < 10% liquid fraction. The Pd and Pt are enriched in a layer known as the J-M Reef which occurs in a zone containing olivine-bearing rocks (OB I). This zone contains the same rock types as the rest of the Lower Banded series, but troctolite, olivine gabbronorite, and anorthosite are more abundant in OB I. The concentrations of lithophile elements in OB I are similar to the rocks above and below OB I. In particular, the reef rocks show no additional signs of continental crust contamination such as additional Th or LREE enrichment, and the reef rocks are not enriched in incompatible elements. The strongly chalcophile elements Cu, Ni, Se, Bi, and Au all correlate with S, indicating that these elements are controlled by sulfides. The PGE show two trends. The rocks below the J-M Reef and the reef rocks have high PGE+Au to S ratios and fall on a single trend, whereas those above the reef have low metal to S ratios. These low ratios for the rocks above the reef could arise because the magma was depleted in PGE+Au as it had already segregated some PGE+Au-rich sulfides. The relatively high PGE, Au, Cu, and Se concentrations of the rocks below the reef imply that the rocks contain ~ 0.1 wt% cumulate sulfides. However, they have very low S contents (< 300 ppm). The low S/Se ratios of these rocks (< 2000) and most of the rocks outside of the reef suggest that they have lost more than half of their S. The Pd/Pt and Pd/Ir ratios of the reef (~ 3.4 and 800, respectively) are much higher than those of the rocks below and above the reef. The high Pd/Pt and Pd/Ir ratios of the reef imply that the sulfides formed from a fractionated magma; however, there is no correlation between the Pd/Ir and Pd/Pt ratios and the host rock type. These ratios are dissimilar to the Bushveld reefs and the Main Sulfide Zone of the Great Dyke. They more closely resemble the reefs in more fractionated rocks such as the AP Reef of the Penikat Intrusion, the Roby Zone of the Lac des Iles Complex, or the disseminated sulfides of the Noril’sk I intrusion. The very high Pd/S and low Cu/Pd ratios of the J-M Reef require that the sulfides collected Pd from a very large volume of magma. None of the current models is entirely satisfactory; however, we favor a model where at depth the OB1 magma partially melted a komatiitic massive sulfide and then transported the sulfide droplets into the magma chamber at the level of the J-M reef.

Evaluation of Oxidation Reaction of Copper Concentrate Mixed with Silica Sand by Hot-Thermocouple Method

The hot-thermocouple method was applied to observe the oxidation reaction of a copper concentrate/silica sand mixture and the separation process of sulfide matte/oxide slag melts. The powder sample (0.3–0.4 mg), attached to a Pt–13%Rh/Pt thermocouple, was heated rapidly up to ≥ 1250 °C in Ar gas with the aid of an exothermic reaction of the chalcopyrite due to the introduction of oxidizing gas flow via a needle located near the sample. The sample was a mixture of Cu concentrate/silica sand with an initial Fe/SiO2 mass ratio of 2.0. The reaction processes were monitored by a charge-coupled device through the microscope and recorded by a personal computer. The hot-thermocouple method revealed chalcopyrite ignition with oxygen flow for optimized conditions (temperature of the sample raised rapidly within 1 s up to ≥ 1300 °C). Then, sulfide matte melt spread along the Pt–13%Rh/Pt thermocouple wire and sulfide droplets were isolated inside the oxide slag melt held between the thermocouple wires. The rapid temperature change of the sample within 2 s was evaluated using a previously obtained relation between the temperature and sample color for fayalite slag melt with a composition of 2FeO·SiO2. The quenched samples were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy to observe microstructures of the compositions of the matte and slag, unreacted mineral particles, and the presence of magnetite precipitates. The conditions to form magnetite under the hot-thermocouple experiments were qualitatively discussed in terms of the Cu–Fe–S–O–SiO2 phase diagram.

Polymetallic droplets within trapped globules in a quartz diorite porphyry from Gangcha–Kemo gold deposit, West Qinling orogen, China: Implications for petrogenesis and prospecting

As immiscibility in magmas appears most significant in magmatic–hydrothermal processes, studies of magmatic fluid immiscibility should focus on the precipitation of metals. A metallogenic belt along the northern margin of the Western Qinling Belt (China) hosts several porphyry-type Cu—Au deposits. Detailed field investigations reveal that abundant ellipsoidal and sub-spheroidal polymetallic globules exist in a quartz diorite porphyry associated with the Gangcha–Kemo gold deposit, which occurs at the eastern contact of the Dewulu intrusion. To reveal the characteristics and composition of these globules, we present mineralogical and whole-rock major- and trace-element data. The globule formation temperature, which was determined using by chlorite and two-feldspar geothermometers, is estimated at 297–429 °C. This temperature is much lower than the host quartz diorite porphyry, which is estimated to be 860–979 °C using an amphibole geothermometer. Based on the geochemistry of the globules, their polymetallic droplets, and the host quartz diorite porphyry, we infer immiscible separation from the volatile-rich quartz diorite porphyry and precipitation of the polymetallic droplets within the globules. Rapid migration and condensation of the fluids with limited escape of a fluid phase are inferred, with the globules representing trapped ore-forming elements. We propose that the fluid globules were exsolved from a rising magma carrying abundant ore-forming metals and that the globules with sulfide droplets can be used as an exploration tool for interpreting the genesis of mineralized rocks and related ore deposits.

Droplets and Bubbles: Solidification of Sulphide-rich Vapour-saturated Orthocumulates in the Norilsk-Talnakh Ni–Cu–PGE Ore-bearing Intrusions

A common feature of the mineralised chonolith intrusions of the Norilsk-Talnakh camp is the presence of globular disseminated Ni–Cu–Fe sulphide ores, particularly well developed within the olivine orthocumulate layers of the intrusions, locally referred to as picritic gabbrodolerites. In many cases these globules partially fill subspherical intercumulus spaces within a framework of crystals of olivine and/or plagioclase. The sulphide-free component of these intercumulus spaces is partially or completely infilled with crystallization products of highly fractionated residual trapped liquid derived from the interstitial liquid of the original crystal mush; these intergrowths typically develop as ‘caps’ above the globules. Similar partially-filled cavities are well known within basaltic and komatiitic lava flows and are referred to as ‘segregation vesicles’; they have also been observed as caps above sulfide globules in quenched dolerite dyke margins in other localities. In the Norilsk-Talnakh intrusions, where the caps are incompletely filled by magmatic material the remaining space is occupied by amygdaloidal linings, which in some cases include anhydrite. These relationships provide evidence that the caps represent original gas bubbles and the close association with the sulphide globules arises from the strong tendency for sulphide droplets to attach to vapour bubbles in magmas. The original outlines of the vapour–bubble pairs are preserved by the rigidity of the surrounding crystal framework, which is interpreted to have developed largely by simultaneous in situ nucleation and growth of olivine and clinopyroxene on the basis of detailed textural observations. Pressure increase during solidification coupled with gas filter pressing forced residual silicate melt into the bubbles, while gas filter pressing and vapour–liquid capillary attraction drove differentiation of the droplets themselves. Whether the globules were deposited already attached to bubbles, or whether the bubbles were generated by volatile saturation within the crystal mush after deposition, cannot be determined definitively, but the geometry of flattened bubble–droplet pairs tends to favour the former option. The distinctive nature of the Norilsk globular ores, along with other unusual aspects of these deposits, is interpreted to be the result of low confining pressures and syn-crystallization degassing, an unusual condition in intrusion-hosted magmatic sulfide ores.

Experimental study of sulfur solubility in silicate–carbonate melts at 5–10.5 GPa

Sulfur solubility in Ca–Mg–Fe silicate–carbonate melt was experimentally determined at 5–10.5 GPa and 1400–1600 °C. Melt was produced by reaction between natural carbonates (calcite and magnesite) and San Carlos olivine (XFo ~ 0.9), which was used as a container, and equilibrated with either Fe-Ni sulfide liquid under reducing conditions (in the presence of graphite) or CaSO4 liquid under oxidizing conditions (Re–ReO2 buffer). To overcome the problem of the presence of sulfide droplets within the silicate–carbonate melt and to reverse the equilibrium, two sample configurations were employed: (1) a carbonate–sulfide mixture placed in the center of the olivine container and (2) sulfide placed in the olivine container and carbonate initially positioned outside the container near the wall of the external Pt capsule. In the former case, silicate–carbonate melt with suspended sulfide melt droplets infiltrated through the olivine capsule, and the two melts were separated owing to their different physical properties. In the latter case, silicate–carbonate melt migrated through thin fractures in the olivine container to come into contact with sulfide. The results of the two experimental series constrained the solubility of sulfide S in silicate–carbonate melt at 0.02–0.10 wt%. The S-solubility increases with increasing FeO content in the melt and is less sensitive to pressure, temperature and other compositional parameters. The solubility of S in the presence of the Re–ReO2 buffer is much higher (up to 2–3 wt%) and dominated by S6+. The obtained results indicate that S is relatively inert during carbonate mantle metasomatism under reduced conditions, but can be transported under oxidizing conditions.

Textural, morphological and compositional varieties of modern arc sulfides: A case study of the Tolbachik volcano, Kamchatka

Magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits. Sulfide inclusions in olivine preserve the original chemical composition of the segregated sulfide melt, whereas the phase compositions of the solidified sulfide droplets and their textures record the final stages of sulfide evolution. Sulfide globules hosted in olivine phenocrysts of magnesian basalts from the Tolbachik volcano (Kamchatka) display a variety of textures ranging from homogeneous to fine-grained to coarse-grained and lamellar. Porous textures and textures associated with the oxidation and dissolution of sulfides were identified as separate categories. The outer surfaces of sulfides vary significantly in shape, which includes normal spherical or elliptical globules, gravitationally layered inclusions, thin sulfide foils and concave deformed sulfides in contact with fluid bubbles. The estimated cooling rate of olivine phenocrysts responsible for observed crystallization textures varies by five orders of magnitude. The Fe:Ni:Cu ratio also exerts an influence on the resulting texture, whereas the size of the globules does not have any significant effect. The morphology and textural patterns of individual grains indicates that fine-grained textures resulted from the rapid breakdown of a homogeneous solid phase, which the sulfide melt solidified into during extremely rapid quenching. The presence of large or abundant small pores in sulfides, along with channels at the globule periphery indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallization. Sulfides in the form of thin foils and planar swarms in healed cracks indicate cyclic pressure changes in the olivine environment, which resulted in phenocrysts rupturing and healing. The sulfide mineralogy is represented by high-temperature sulfides MSS (monosulfide solid solution) and ISS (intermediate solid solution), as well as pyrrhotite-pentlandite-chalcopyrite-cubanite-bornite-magnetite assemblages, which formed at lower temperatures.

Critical Controls on the Formation of Contact-Style PGE-Ni-Cu Mineralization: Evidence from the Paleoproterozoic Monchegorsk Complex, Kola Region, Russia

The Paleoproterozoic Monchegorsk Complex, located in the Russian part of the Fennoscandian Shield, constitutes one of the largest mafic-ultramafic layered intrusions in Europe. The complex hosts extensive contact-style platinum group element-Ni-Cu sulfide mineralization along its margin, irrespective of the host lithology, which ranges from peridotite to pyroxenite and gabbronorite. The mineralized intervals reach up to 3 ppm Pt + Pd and attain a thickness of up to 50 m in the central portions of the intrusion, thinning toward the periphery. Our study shows that the key process controlling the size and grade of a contact-style deposit in the Mon-chegorsk Complex was the efficiency of sulfide collection in distinct zones of the intrusion. Strongly mineralized basal contacts are always associated with intense brecciation and the presence of large amounts of felsic pegmatite, suggesting a multistage emplacement of the mafic-ultramafic succession. Thermal modeling demonstrates that multiple episodes of magma influx are required to allow for significant partial melting of the basement. Moreover, the interaction between magma and basement led to the local addition of water and, potentially, carbon dioxide to the magma, resulting in local small-scale dissolution of cumulus phases and a reduction in viscosity of the interstitial melt. This increased the porosity of the mush in the vicinity of the lower intrusion contact, which promoted preferential sulfide liquid accumulation at the base, while the local decrease in magma viscosity facilitated gravitational settling of sulfide droplets. These factors led to an efficient collection of sulfide liquid, especially in the center of the complex, where permeability was maintained the longest due to slower cooling relative to more peripheral parts.

Three Types of Apatite in Norilsk Sulfide Ores

Apatite is a concentrator of F and Cl, which play a significant role in the formation of minerals of platinum-group elements of pneumatolitic origin. There are three apatite generations in Norilsk magmatic sulfide ores. Apatite I occurs in sulfide bodies and rims of fluid alteration above sulfide droplets in disseminated ores. Its composition evolved from hydroxyl-chlorapatite to chlorapatite. Apatite I associates with Tiрbiotite, titanomagnetite, ilmenite with baddeleyite lamellae, anhydrite, Ti-poor kaersutite, Cl-bearing hastingsite and edenite, djerfisherite, bartonite, and minerals of the Pt and Au groups. Apatite I contains up to 2.3 wt % lanthanides, primarily Ce, La, and Nd. Apatite I is overgrown and replaced by apatite II, the composition of which evolved from hydroxyl-chlor-fluorapatite to fluorapatite. Apatite II often occurs also as individual crystals in massive sulfides and contains up to 0.9 wt % lanthanides. The pneumatolitic chlorapatite and fluorapatite contain ~0.5 wt % SiO2. The composition of apatite indicates discrete evolution of fluids released during the crystallization of Norilsk sulfide melts: from water–chloride to chloride at the first state, and from water–chlorite–fluoride to essentially fluoride at the second stage. The lanthanides released during the replacement of chlorapatite I by fluorapatite II were probably incorporated in pneumatolitic zoned orthite-(Ce). In the areas affected by prehnite–pumpellyite metamorphism, apatite I and apatite II within metamorphosed sulfide ores are partly or completely replaced by apatite III, which varies in composition from hydroxyl-chlorapatite to hydroxylapatite poor in fluorine and lanthanides. The lanthanides released during the replacement of apatite I and II by metamorphic hydroxylapatite III were probably incorporated in metamorphic unzoned orthite-(Ce).

Time scales and length scales in magma flow pathways and the origin of magmatic Ni–Cu–PGE ore deposits

Ore forming processes involve the redistribution of heat, mass and momentum by a wide range of processes operating at different time and length scales. The fastest process at any given length scale tends to be the dominant control. Applying this principle to the array of physical processes that operate within magma flow pathways leads to some key insights into the origins of magmatic Ni–Cu–PGE sulfide ore deposits. A high proportion of mineralised systems, including those in the super-giant Noril'sk-Talnakh camp, are formed in small conduit intrusions where assimilation of country rock has played a major role. Evidence of this process is reflected in the common association of sulfides with vari-textured contaminated host rocks containing xenoliths in varying stages of assimilation. Direct incorporation of S-bearing country rock xenoliths is likely to be the dominant mechanism for generating sulfide liquids in this setting. However, the processes of melting or dissolving these xenoliths is relatively slow compared with magma flow rates and, depending on xenolith lithology and the composition of the carrier magma, slow compared with settling and accumulation rates. Chemical equilibration between sulfide droplets and silicate magma is slower still, as is the process of dissolving sulfide liquid into initially undersaturated silicate magmas. Much of the transport and deposition of sulfide in the carrier magmas may occur while sulfide is still incorporated in the xenoliths, accounting for the common association of magmatic sulfide-matrix ore breccias and contaminated “taxitic” host rocks. Effective upgrading of so-formed sulfide liquids would require repetitive recycling by processes such as re-entrainment, back flow or gravity flow operating over the lifetime of the magma transport system as a whole. In contrast to mafic-hosted systems, komatiite-hosted ores only rarely show an association with externally-derived xenoliths, an observation which is partially due to the predominant formation of ores in lava flows rather than deep-seated intrusions, but also to the much shorter timescales of key component systems in hotter, less viscous magmas. Nonetheless, multiple cycles of deposition and entrainment are necessary to account for the metal contents of komatiite-hosted sulfides. More generally, the time and length scale approach introduced here may be of value in understanding other igneous processes as well as non-magmatic mineral systems.

Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part I. Occurrence and compositions of sulfide melts

Silicate-sulfide liquid immiscibility plays a key role in the formation of magmatic sulfide ore deposits but incipient sulfide melts are rarely preserved in natural rocks. This study presents the distribution and compositions of olivine-hosted sulfide melt globules resulting from silicate-sulfide liquid immiscibility in primitive arc basalts. Abundant sulfide droplets entrapped in olivine from primitive basalts of the 1941 eruption and pre-historic eruptive cone “Mt. 1004” of the Tolbachik volcano, Kurile-Kamchatka arc. Inclusions range from submicron to 250μm in size, coexist with sulfur-rich glass (≤1.1wt% S), and, in some cases, with magmatic anhydrite. Saturation in sulfide occurred early in the evolution of a water- and sulfur-rich magma, moderately oxidized (QFM+1 to +1.5), which crystallized high-Mg olivine (Fo86–92), clinopyroxene and Cr-spinel. The process developed dense “clouds” of sulfide in relatively small volumes of magma, with highly variable abundances of chalcophile metals. The low degree of sulfide supersaturation promoted diffusive equilibration of the growing droplets with the melt in Ni and Cu, resulting in high concentrations (≈38mol%) of CuS and NiS in the earliest sulfide liquids. The Tolbachik samples provide a glimpse into deep arc processes not seen elsewhere, and may show how arc magmas, despite their oxidized nature, saturate in sulfide.

A double Fe-Ti oxide and Fe-sulphide liquid immiscibility in the Itsindro Gabbro Complex, Madagascar

The petrology and mineralogy of the Itsindro complex in south-central Madagascar has been investigated through samples obtained from the 320.7 m-deep Lanjanina borehole. The section consists of a 254 m-thick pyroxenite unit with interbedded gabbro layers that overlies a gabbro unit and is itself overlain by a 19 m-thick granite unit. Most of the structures are sub-horizontal. A weak magmatic layering is locally observed but at the scale of the core, the intrusion does not appear to be a layered complex.Pyroxenite and gabbro show a systematic disseminated mineralization consisting of Fe-Ti-P oxides and Fe-(Cu-Ni) sulphides that takes the form of ilmenite-titanomagnetite ± apatite and pyrrhotite ± chalcopyrite ± pentlandite. In the upper zone, from 90 to 72 m, sub-massive centimetre-to decimetre-sized layers of oxides and sulphides comprise a total of 16 m of sub-massive sulphide (the main mineralized zone). In this mineralized zone the oxide/sulphide ratio is close to 1/1. The sulphide is strongly dominated by pyrrhotite, which may locally contain inclusions of molybdenite crystals with the Re sulphide rheniite (ReS2). Oxides are generally euhedral, included in or attached to the Fe-sulphide, and also locally form sub-massive centimetre-sized bands.Apatite as a cumulus phase is ubiquitous. Locally it may account for 30% of the ore-rich samples and some samples consist of apatite-Fe-Ti oxides-Fe-Cu-Ni sulphides with virtually no silicate. Apatite is the main REE carrier but the total REE content remains low (<90 ppm).Mineral compositions and whole rock geochemistry indicate that the rocks are highly differentiated, and in spite of a relatively limited thickness, the differentiation process is observed. Two zones can be distinguished: from the bottom to 162.8 m we see a decrease in the Mg number of olivine and pyroxene, and a drop in TiO2 and Al2O3 for the latter. A reverse trend is then observed within the pyroxenite unit from the 162.8 m level upwards. The main mineralized zone is located in the upper part of this unit, about 70 m above this discontinuity.The highly differentiated Fe-Ti-P facies of the Lanjanina series in the Itsindro Gabbro Complex have been interpreted as corresponding to the Fe-Ti-P rich, Si-poor member remaining after an immiscible segregation of an evolved mafic magma. The granite dykes and the overlying granite unit represent the second, Si-rich member of the immiscibility process.The presence of large amounts of sulphide is attributed to sulphur contamination of the Fe-Ti-rich liquid. Fe-Ti oxides will tend to crystallize on the sulphide droplets and the accumulation of dense Fe-sulphides (liquid) and associated Fe-Ti oxides (solid) will result in this complex and unusual association taking the form of a net texture.

Roles of xenomelts, xenoliths, xenocrysts, xenovolatiles, residues, and skarns in the genesis, transport, and localization of magmatic Fe-Ni-Cu-PGE sulfides and chromite

Most sulfide-rich magmatic Ni-Cu-(PGE) deposits form in dynamic magmatic systems by partial melting S-bearing wall rocks with variable degrees of assimilation of miscible silicate and volatile components, and generation of barren to weakly-mineralized immiscible Fe sulfide xenomelts into which Ni-Cu-Co-PGE partition from the magma. Some exceptionally-thick magmatic Cr deposits may form by partial melting oxide-bearing wall rocks with variable degrees of assimilation of the miscible silicate and volatile components, and generation of barren Fe±Ti oxide xenocrysts into which Cr-Mg-V±Ti partition from the magma. The products of these processes are variably preserved as skarns, residues, xenoliths, xenocrysts, xenomelts, and xenovolatiles, which play important to critical roles in ore genesis, transport, localization, and/or modification. Incorporation of barren xenoliths/autoliths may induce small amounts of sulfide/chromite to segregate, but incorporation of sulfide xenomelts or oxide xenocrysts with dynamic upgrading of metal tenors (PGE>Cu>Ni>Co and Cr>V>Ti, respectively) is required to make significant ore deposits. Silicate xenomelts are only rarely preserved, but will be variably depleted in chalcophile and ferrous metals. Less dense felsic xenoliths may aid upward sulfide transport by increasing the effective viscosity and decreasing the bulk density of the magma. Denser mafic or metamorphosed xenoliths may also increase the effective viscosity of the magma, but may aid downward sulfide transport by increasing the bulk density of the magma. Sulfide wets olivine, so olivine xenocrysts may act as filter beds to collect advected finely dispersed sulfide droplets, but other silicates and xenoliths may not be wetted by sulfides. Xenovolatiles may retard settling of – or in some cases float – dense sulfide droplets. Reactions of sulfide melts with felsic country rocks may generate Fe-rich skarns that may allow sulfide melts to fractionate to more extreme Cu-Ni-rich compositions. Xenoliths, xenocrysts, xenomelts, and xenovolatiles are more likely to be preserved in cooler basaltic magmas than in hotter komatiitic magmas, and are more likely to be preserved in less dynamic (less turbulent) systems/domain/phases than in more dynamic (more turbulent) systems/domains/phases. Massive to semi-massive Ni-Cu-PGE and Cr mineralization and xenoliths are often localized within footwall embayments, dilations/jogs in dikes, throats of magma conduits, and the horizontal segments of dike-chonolith and dike-sill complexes, which represent fluid dynamic traps for both ascending and descending sulfides/oxides. If skarns, residues, xenoliths, xenocrysts, xenomelts, and/or xenovolatiles are present, they provide important constraints on ore genesis and they are valuable exploration indicators, but they must be included in elemental and isotopic mass balance calculations.

Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

Chromitite from the Harold’s Grave locality in the mantle section of the Shetland ophiolite complex is extremely enriched in Ru, Os and Ir, at µg/g concentrations. High-resolution X-ray computed tomography on micro-cores from these chromitites was used to determine the location, size, distribution and morphology of the platinum-group minerals (PGM). There are five generations of PGM in these chromitites. Small (average 5µm in equivalent sphere diameter, ESD) euhedral laurites, often with Os-Ir alloys, are totally enclosed in the chromite and are likely to have formed first by direct crystallisation from the magma as the chromite crystallised. Also within the chromitite there are clusters of larger (50µm ESD) aligned elongate crystals of Pt-, Rh-, Ir-, Os- and Ru-bearing PGM that have different orientations in different chromite crystals. These may have formed either by exsolution, or by preferential nucleation of PGMs in boundary layers around particular growing chromite grains. Thirdly there is a generation of large (100µm ESD) composite Os-Ir-Ru-rich PGM that are all interstitial to the chromite grains and sometimes form in clusters. It is proposed that Os, Ir and Ru in this generation were concentrated in base metal sulfide droplets that were then re-dissolved into a later sulfide-undersaturated magma, leaving PGM interstitial to the chromite grains. Fourthly there is a group of almost spherical large (80µm ESD) laurites, hosting minor Os-Ir-Ru-rich PGM that form on the edge or enclosed in chromite grains occurring in a sheet crosscutting a chromitite layer. These may be hosted in an annealed late syn- or post magmatic fracture. Finally a few of the PGM have been deformed in localised shear zones through the chromitites.The vast majority of the PGM – including small PGM enclosed within chromite, larger interstitial PGM and elongate aligned PGM – have Os isotope compositions that give Re-depletion model ages approximately equal to the age of the ophiolite at ∼492Ma. A number of other PGM – not confined to a single textural group – fall to more or less radiogenic values, with four PGM giving anomalously unradiogenic Os corresponding to an older age of ∼1050Ma. The 187Os/188Os isotopic ratios for PGM from Cliff and Quoys, from the same ophiolite section, are somewhat more radiogenic than those at Harold’s Grave. This may be due to a distinct mantle source history or possibly the assimilation of radiogenic crustal Os.

Platinum-group element geochemistry of the layered intrusions in the Emeishan large igneous province, SW China: Implications for the principal controls on magmatic sulfide immiscibility

It is widely accepted that the incorporation of external sulfur via crustal contamination is an important trigger for sulfide immiscibility that generates Ni-Cu-(PGE) sulfide mineralization, yet other controlling factors for sulfide immiscibility may also be present. The late Permian Panzhihua, Baima, Hongge, Xinjie and Taihe layered intrusions in the Emeishan Large Igneous Province (ELIP, SW China), are well-endowed with Fe-Ti oxide deposits, whereas their sulfide mineralization is mainly sub-economic. For example, the lower part of the Xinjie intrusion hosts a few thin PGE-rich ore layers, yet other ELIP layered intrusions do not contain any Ni-Cu sulfide mineralization and are PGE-depleted (0.01–1 ppb). Compared with the PGE-undepleted Emeishan high-Ti basalts that are genetically related to the intrusions, the extent of PGE depletion and elevated Cu/Pd ratios (up to 3.2×106) of the Panzhihua, Baima, Taihe and Hongge intrusions suggest PGE-depletion in their parental magmas due to early-stage sulfide removal. Sr-Nd isotopic compositions of the Panzhihua, Baima and Taihe intrusions suggest crustal contamination was insignificant and sulfide saturation produced mainly by crustal sulfur input was unlikely. MELTS modeling shows that extensive fractionation of chromite, olivine and clinopyroxene in deep-seated magma chambers may have induced early-stage sulfide saturation of the primary magmas. The relatively high sulfide contents in the Fe-Ti oxide layers at Panzhihua, Baima, Hongge and Taihe indicate a close relationship between the second-stage sulfide immiscibility and extensive Fe-Ti oxide crystallization. Positive correlations between sulfur and total Fe2O3, V and TiO2 suggest that Fe-Ti oxide (magnetite and ilmenite) crystallization may have triggered the second-stage sulfide saturation via sharply lowering the Fe concentration and oxygen fugacity of the magmas. Moderate degree of crustal contamination for the Xinjie Fe-Ti oxide-barren rocks may have induced sulfide saturation and accumulation at the lower part of the intrusion. Our calculations indicate that the Xinjie PGE-rich rocks have high R-factors (1000–10000), which are ascribed to PGE-upgrading of the sulfides via reaction with new replenishments of PGE-undepleted magmas. A few Panzhihua, Baima and Taihe samples that contain higher PGE concentrations suggest that the early-stage sulfide droplets at depths were entrained in later magma pulses delivered to shallower magma chambers. The very high R-factors determined by mass balance calculation, implies a good potential for discovering more PGE mineralization in the deep-seated intrusions of the magma plumbing system.