Segregation Of Sulfide Liquid Research Articles

Sulfide Copper‐Iron Isotopic Fractionation During Formation of the Kalatongke Magmatic Cu‐Ni Sulfide Deposit in the Central Asian Orogenic Belt

AbstractCopper and iron isotopic signatures in sulfide and silicate minerals are important genetic indicators in magmatic sulfide deposits. Kalatongke is a large‐scale magmatic Cu‐Ni sulfide deposit in the Central Asian Orogenic Belt, and one that experienced multiple stages of magmatism and contamination. It is an ideal deposit in which to study Cu‐Fe isotopic fractionation during multiple stages of magmatism and sulfide mineralization processes. The Kalatongke sulfide orebodies are hosted by three small mafic intrusions in which pyroxene and sulfides (pyrrhotite, pentlandite, and chalcopyrite) are the most common Fe‐rich minerals, and chalcopyrite is the dominant Cu‐rich mineral. Sulfide liquid and silicate melt ▵56FeSul‐Sil (0.03–0.19‰) and ▵65CuCcp‐Sil (−0.78–0.74‰) values are indicative of non‐equilibrium fractionation. Most of the Cu isotope compositions in the sulfide ores at Kalatongke can be modeled as subduction‐ metasomatized, oxidized mantle source‐derived silicate melt (initial δ57Fe = 0.15‰, δ65Cu = −0.07‰) that underwent lower crustal contamination, and then reacted with silicate melt, having an R factor of 100–1,000. Rapid silicate melt and sulfide liquid Fe isotope exchange and re‐equilibration between chalcopyrite and pyrrhotite in the massive ores is reflected in the similarity of their δ56Fe values. Sulfide in disseminated ores shows a range of Fe isotope ratios, influenced by the proportions of monosulfide solid solution (MSS) and intermediate solid solution (ISS) formed. Copper isotopes can be utilized to characterize crustal contamination and silicate melt‐sulfide liquid interaction, while the Fe isotope ratios of sulfide minerals record sulfide liquid segregation and evolution in magmatic sulfide deposits.

Highly reduced accretion of the Earth by large impactors? Evidence from elemental partitioning between sulfide liquids and silicate melts at highly reduced conditions

The Earth may have formed at very reducing conditions through the accretion of (a) large reduced and differentiated impactor(s). Segregation of Fe-S liquids within these bodies would have left a geochemical mark on the mantles of reduced impactors and on the proto-Earth’s mantle. Here, we study the geochemical consequences of highly reduced accretion of the Earth by large impactors. New insights into the partitioning of trace elements between Fe-S liquid and silicate melt at (highly) reduced conditions (ΔIW = −5 to +1) were obtained by performing 21 high pressure experiments at 1 GPa and 1683–2283 K. The observed Fe-S liquid-silicate melt partitioning behavior is in agreement with thermodynamic models that predict a significant role for O in Fe-S liquid and S in the silicate melt.The experimental results were combined with literature data to obtain new and/or revised thermodynamic parameterizations that quantify the effects of composition and redox state on the elemental distribution between Fe-S liquids and highly reduced silicate melts. The results were used to assess which elements would most likely retain the geochemical signature of accretion of reduced impactors. Under the assumption of instantaneous core merging, impact delivery to the proto-Earth’s mantle was found to be significant (>10% of present-day BSE concentrations) only for S, Zn, Se, Te and Tl, whereas the abundances of the other elements remain largely unaffected.The results also show that present-day BSE S/Se, Se/Te, Tl/S and potentially In/Zn as well as their absolute abundances are inconsistent with their delivery by (a) large, highly reduced chondritic differentiated impactor(s) during terrestrial accretion. Continued core-mantle equilibration in the proto-Earth, volatility-related loss and/or post-accretion sulfide liquid segregation in the terrestrial magma ocean would further increase or not affect these discrepancies. We conclude that a significant contribution of (a) large (>10% of Earth’s mass) reduced and differentiated chondritic impactor(s) during accretion of the Earth is not reflected in the present-day S, Zn, Se, Te and Tl systematics of the terrestrial mantle. This suggests that significant overprinting of the primordial BSE S/Se, Se/Te and S/Tl signature could have occurred and/or (2) that the S/Se and Se/Te ratios were set by accretion of more oxidised CI-like materials.

An experimental assessment of the chalcophile behavior of F, Cl, Br and I: Implications for the fate of halogens during planetary accretion and the formation of magmatic ore deposits

The elemental and isotopic abundances of halogens (F, Cl, Br, I) are used to constrain planetary volatile loss and volatile delivery processes, but their behavior during magmatic differentiation in general, and sulfide liquid segregation in particular, is currently not well constrained. To test whether sulfide liquid segregation could affect halogen behavior during magmatic processes, we performed high-pressure experiments to systematically quantify the sulfide liquid – silicate melt partition coefficients (Dsulliq-silmelt values, defined as the ratio between the wt.% concentration of the halogen in the sulfide liquid and silicate melt, respectively) of F, Cl, Br and I at a pressure of 1 GPa and temperatures of 1683–1883 K.Results show that dry-polishing target surfaces is crucial for obtaining representative halogen concentrations of sulfide liquids. The results also show that no appreciable amounts of F partition into sulfide liquids, whereas Cl, Br and I behave increasingly chalcophile with increasing atomic radius (i.e., DFsulliq-silmelt <DClsulliq-silmelt <DBrsulliq-silmelt < DIsulliq-silmelt), presumably as a result of an increasingly covalent nature of Fe-halogen bonds with increasing radius. This results in I behaving chalcophile (DIsulliq-silmelt > 1) in several experiments. In contrast to previous observations, DCl/Brsulliq-silmelt was found to be <1. The DCl,Br,Isulliq-silmelt predominantly vary with sulfide liquid melt composition, showing an increase with increasing O in the sulfide liquid, which itself is correlated with more oxidizing conditions (i.e., higher fO2) or silicate melt FeO contents. The DCl,Br,Isulliq-silmelt values remain constant and/or potentially decrease again at the highest O concentrations of the sulfide liquids in this study (∼2.5 wt.% O).Results indicate that the magnitude of halogen depletions in the terrestrial, martian and lunar mantle are not strongly affected by segregation of sulfide liquids during their accretion, given the expected low modal abundance of sulfide liquids and/or relatively low DCl,Br,Isulliq-silmelt values. Core formation remains the most important process in establishing iodine depletion in the terrestrial mantle, whereas volatility-related loss seems most likely for F, Cl, Br and I, in case of the martian mantle. However, segregation of sulfide liquids during accretion could have resulted in a relative increase of the offset between the mantle depletions of the lighter and heavier halogens. The experimental results confirm the previously proposed feasibility of sulfide liquids as reservoirs for halogens in magmatic sulfide ore environments. As proposed by Mungall and Brenan (2003), fractional crystallization of these sulfide liquids in the absence of a silicate melt can lead to the formation of halide melts or fluids, consistent with the association between halide minerals and magmatic sulfide ores in some localities.

An experimental assessment of the potential of sulfide saturation of the source regions of eucrites and angrites: Implications for asteroidal models of core formation, late accretion and volatile element depletions

The geochemistry of asteroidal magmas provides fundamental clues to the processes involved in the origin and early evolution of planetary bodies. Although sulfides are important reservoirs for a diverse suite of major and trace elements, it is currently unclear whether the interiors of asteroid Vesta and the Angrite Parent Body were sulfide liquid saturated during petrogenesis of non-cumulate eucrites and volcanic angrites. To assess the potential of sulfide liquid saturation in the interiors of these bodies, high pressure (P) – temperature (T) experiments were used to quantify the sulfur concentrations at sulfide saturation (SCSS) for volcanic angrites and non-cumulate eucrites. The sulfide-silicate partitioning behavior of various trace elements was simultaneously quantified to study their geochemical behavior at sulfide liquid saturation.It was found that the measured SCSS values agree well with the SCSS values predicted from a previous thermodynamic model for high-FeO* melts. To assess the possibility of sulfide liquid saturation of the source regions of non-cumulate eucrites and angrites, their S abundances were compared with the calculated SCSS values for their source regions. Results show that if eucritic and angritic source regions were saturated with FeS liquid, significant degassing (>50–80%) of S must have occurred during or following their magmatic emplacement. Such loss is inconsistent with the S, Cl, Zn and Rb isotopic compositions of non-cumulate eucrites. Sulfide liquid saturation of eucrite and angrite source regions is also excluded from the strongly incompatible behavior of Cu and HSE in non-cumulate eucrites and angrites (Riches et al., 2012).Additional calculations were performed to further explore the timing and extent of S loss during crystallization of the Vestan magma ocean. The assumption of chondritic bulk S abundances of bulk Vesta would correspond with extremely high S contents of the eucrite source region(s), even after consideration of depletion of S due to core formation. In light of the S, Cl, Zn and Rb stable isotopic compositions of eucrites, the S abundances in eucrites are most consistent with the hypothesis that the Vestan mantle was already strongly depleted in S (>70–80%) by the time of Vestan magma ocean crystallization, resulting in more realistic S contents of the eucrite source region(s). The depletion of S could have been established during initial accretion of Vesta or it could simply reflect accretion of volatile depleted components that experienced incomplete condensation (Wu et al., 2018). Modeling of the new experimentally determined sulfide-silicate partition coefficients and previously reported Vestan mantle depletions of the various chalcophile and siderophile elements suggests that sulfide liquid segregation during early Vestan magma ocean crystallization is also unlikely. The lack of sulfide liquid saturation in the source regions of non-cumulate eucrites and angrites, as well as during early Vestan magma ocean solidification, shows that current geochemical models of core formation and late accretion remain valid for these bodies.

Trace element partitioning between sulfide-, metal- and silicate melts at highly reduced conditions: Insights into the distribution of volatile elements during core formation in reduced bodies

Chalcophile and siderophile element abundances are used to provide important constraints on the interior compositions of planetary bodies as well as the pressure (P) - temperature (T) conditions that prevailed during core formation. The oxygen fugacity (fO2) during core formation varied considerably between the various terrestrial planets and asteroidal bodies in our solar system. Mercury, the aubrite parent body (AuPB) and some terrestrial precursor bodies may have differentiated at highly reduced conditions.At present knowledge about how the metal liquid-silicate melt and sulfide liquid-silicate melt partitioning behavior of major and trace elements are affected by high S concentrations in the silicate melt at highly reducing conditions is incomplete. Here, we experimentally study the metal-silicate and sulfide-silicate partitioning behavior of trace elements in reduced silicate melts over a wide range of S contents as a function of redox state at 1 GPa and 1833–1883 K. Silicate melt S contents ranged between ~0.5 and ~20 wt%, with a corresponding silicate FeO range of ~0.4 to ~17.5 wt%, in a fO2 range between 1 and 9 log units below the iron-wüstite buffer. Our results reproduce the decrease of the S concentration at sulfide saturation (SCSS) with decreasing FeO contents down to ~3 wt%, as well as its strong increase at <3 wt% FeO. At S contents exceeding >6–9 wt% S, the FeO contents increase again.Results show that most elements (Mg, Ti, V, Cr, Mn, Cu, Zn, Se, Nb, Cd, Sb, Te, Ta, Tl, Pb and Bi) are more chalcophile than siderophile at reducing conditions, whereas Si, Co, Ni, Ga, Ge, Mo and W preferentially partition into Fe-rich melts instead of sulfide liquids. Silicon, Ti, Se, and Te preferentially partition into Fe-S over (Fe,Mg,Ca)-S liquids, whereas Mn, Zn and Cd are more compatible in the latter. As proposed by Wood and Kiseeva (2015), chalcophile elements such as Cu, Se and Te behave less chalcophile with increasing S concentrations of the silicate melt, whereas the opposite is observed for nominally lithophile elements such as Mg, Ca and Ti.The results can be used to improve interpretations of the observed trace element systematics of aubrites and other reduced achondrites. All of the volatile elements considered here behave chalcophile at the reducing conditions inferred for differentiation of the AuPB. A significant degree of the observed volatile element depletions in aubrites may therefore reflect their preferential partitioning into sulfide liquids, rather than degassing during or after differentiation of the AuPB. These results suggest that, depending of the extent of core merging, precursor body differentiation and the efficiency of sulfide liquid segregation, reduced precursor bodies that were incorporated in the early Earth were likely more rich in volatile elements than currently assumed.

Iron isotope fractionation during sulfide liquid segregation and crystallization at the Lengshuiqing Ni-Cu magmatic sulfide deposit, SW China

Abstract We report the first measurements of Fe isotope fractionation between coexisting pyrrhotite and chalcopyrite from the Lengshuiqing Ni-Cu magmatic sulfide deposit, Southwest China. The δ56Fe values of chalcopyrite (Cp) range from 0.24‰ to 1.25‰, much higher than those in the coexisting pyrrhotite (Po, −1.25‰ to −0.13‰). The δ56Fe values of pyroxene separates from the Lengshuiqing intrusions are in a range from −0.03‰ to 0.18‰. Pyrrhotite in sulfide-mineralized rocks with greater than ∼2 wt.% S has δ56Fe values of −0.3 ± 0.2‰, indicating Δ56FePo-sil (pyrrhotite – silicate melt) in the range of −0.4 ± 0.2‰ with parental silicate magma δ56Fe of 0.1‰ as calculated from the least evolved pyroxene δ56Fe value. This fractionation is similar to that experimentally determined by Schuessler et al. (2007) for pyrrhotite and a rhyolitic melt, suggesting that Δ56FePo-sil of approximately −0.4‰ represents a near-equilibrium value for sulfide-rich mineralization in the Lengshuiqing intrusions. Pyrrhotite-dominated sulfide separates in samples with less than ∼2 wt.% S show δ56Fe values as low as −1.25‰. The low δ56Fe values can be modeled by a non-equilibrium process, where the fractionation between immiscible sulfide liquid and silicate magma decreases from −0.4 to −1.4‰ as the R-factor (mass ratio of silicate melt to coexisting sulfide liquid) increases from 1 to 1000. We suggest that the relative proportion of externally introduced Fe from sulfidic sedimentary country rocks may be higher in low-S samples, controlling the low δ56Fe values of the sulfide assemblage in the mineralized rocks. The trend of Fe isotope enrichment at Lengshuiqing is consistent with Fe-S bond strengths, but the difference between coexisting pyrrhotite and chalcopyrite is not consistent with equilibration at high temperatures. The Δ56FeCp-Po values at Lengshuiqing (1.06–1.77‰) are similar to those measured for samples from the Voisey’s Bay and Eagle Ni-Cu deposits, and suggest that in a cooling sulfide liquid Fe isotope equilibration may continue to temperatures less than 200 °C.

Sulfide-silicate textures in magmatic Ni-Cu-PGE sulfide ore deposits: Disseminated and net-textured ores

A large proportion of in magmatic sulfide deposits consist of mixtures of cumulus silicate minerals, sulfide liquid, and silicate melt, with characteristic textural relationships that provide essential clues to their origin. Within silicate-sulfide cumulates, there is a range of sulfide abundance in magmatic-textured silicate-sulfide between with up to about five modal percent sulfides, called and (or matrix) containing about 30 to 70 modal percent sulfide forming continuous networks enclosing cumulus silicates. Disseminated in cumulates have various textural types relating to the presence or absence of trapped interstitial silicate melt and (rarely) vapor bubbles. Spherical or oblate spherical globules with smooth menisci, as in the Black Swan disseminated ores, are associated with silicate-filled cavities interpreted as amygdales or segregation vesicles. More irregular globules lacking internal differentiation and having partially facetted margins are interpreted as entrainment of previously segregated, partially solidified sulfide. There is a textural continuum between various types of disseminated and net-textured ores, intermediate types commonly taking the form of patchy net-textured ores containing sulfide-rich and sulfide-poor domains at centimeter to decimeter scale. These textures are ascribed primarily to the process of sulfide percolation, itself triggered by the process of competitive wetting whereby the silicate melt preferentially wets silicate crystal surfaces. The process is self-reinforcing as sulfide migration causes sulfide networks to grow by coalescence, with a larger rise height and hence a greater gravitational driving force for percolation and silicate melt displacement. Many of the textural variants catalogued here, including poikilitic or leopard-textured ores, can be explained in these terms. Additional complexity is added by factors such as the presence of oikocrysts and segregation of sulfide liquid during strain-rate dependent thixotropic behavior of partially consolidated cumulates. Integrated textural and geochemical studies are critical to full understanding of ore-forming systems.

Chalcophile element (Ni, Cu, PGE, and Au) variations in the Tamarack magmatic sulfide deposit in the Midcontinent Rift System: implications for dynamic ore-forming processes

The Tamarack magmatic sulfide deposit is hosted by the Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) in the Midcontinent Rift System. The most important sulfide mineralization in the Complex occurs in the northern part, which consists of two separate intrusive units: an early funnel-shaped layered peridotite body containing relatively fine-grained olivine (referred to as the FGO Intrusion) at the top, and a late gabbro-troctolite-peridotite dike-like body containing relatively coarse-grained olivine (referred to as the CGO Intrusion) at the bottom. Disseminated, net-textured, and massive sulfides occur in the base of the FGO Intrusion as well as in the upper part of the CGO Intrusion. The widest part of the CGO Intrusion also hosts a large semi-massive (net-textured) sulfide ore body locally surrounded by disseminated sulfide mineralization. Small massive sulfide veins occur in the footwall of the FGO Intrusion and in the wall rocks of the CGO dike. The sulfide mineralization is predominantly composed of pyrrhotite, pentlandite, and chalcopyrite, plus minor magnetite. Pyrrhotite containing the highest Ni and Co contents occurs in the FGO disseminated sulfides and in the CGO semi-massive sulfide ores, respectively. The most important platinum-group minerals associated with the base metal sulfides are sperrylite (PtAs2), sudburyite (PdSb), and michenerite (PdBiTe). Nickel shows a strong positive correlation with S in all types of sulfide mineralization, and Cu shows a strong positive correlation with S in the disseminated sulfide mineralization. At a given S content, the concentrations of Pt, Pd, and Au in the CGO disseminated sulfides are significantly higher than those in the FGO disseminated sulfides. The semi-massive sulfide ores are characterized by significantly higher IPGE (Ir, Os, Ru, and Rh) concentrations than most of the massive sulfide ores. With few exceptions, all of the various textural types of sulfide mineralization collectively show a good positive correlation between Pt and Pd, and between individual IPGE. At a given Pt or Pd content, however, the semi-massive sulfide ores have higher IPGE contents than the disseminated sulfide samples. Modeling results show that the variations in PGE tenors (metals in recalculated 100 % sulfide) in the Tamarack magmatic sulfide deposit are mainly controlled by variable R factors (magma/sulfide-liquid mass ratios) during sulfide-liquid segregation and subsequent monosulfide solid solution (MSS) fractionation during cooling. The initial contents of Ir, Pt, and Pd in the parental magma, estimated from the metal tenors of the disseminated sulfides, are 0.2, 2, and 1.8 ppb, respectively, which are ∼1/5 of the values for the PGE-undepleted primitive basalts of the Midcontinent Rift System. The variations of PGE tenors in the semi-massive and massive sulfide ores can be explained by MSS fractional crystallization from sulfide liquids. Extreme variations in the PGE contents of the massive sulfides may also in part reflect metal mobility during post-crystallization hydrothermal processes. The higher PGE tenors for the disseminated sulfides in the CGO dike relative to those in the FGO Intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the FGO magma were subsequently upgraded by the subsequent surge of the CGO magma. The relatively low PGE tenors for the semi-massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit.

The roles of magmatic and hydrothermal processes in PGE mineralization, Ferguson Lake deposit, Nunavut, Canada

The Ferguson Lake Ni–Cu–Co–platinum-group element (PGE) deposit in Nunavut, Canada, occurs near the structural hanging wall of a metamorphosed gabbroic sill that is concordant with the enclosing country rock gneisses and amphibolites. Massive to semi-massive sulfide occurs toward the structural hanging wall of the metagabbro, and a low-sulfide, high-PGE style of mineralization (sulfide veins and disseminations) locally occurs ~30–50 m below the main massive sulfide. Water–rock interaction in the Ferguson Lake Ni–Cu–Co–PGE deposit is manifested mostly as widespread, post-metamorphic, epidote–chlorite–calcite veins, and replacement assemblages that contain variable amounts of sulfides and platinum-group minerals (PGM). PGM occur as inclusions in magmatic pyrrhotite and chalcopyrite in both the massive sulfide and high-PGE zones, at the contact between sulfides and hornblende or magnetite inclusions in the massive sulfide, in undeformed sulfide veins and adjacent chlorite and/or epidote halos, in hornblende adjacent to hydrothermal veins, and in plagioclase–chlorite aggregates replacing garnet cemented by sulfide. The PGM are mostly represented by the kotulskite (PdTe)–sobolevskite (PdBi) solid solution but also include michenerite (PdBiTe), froodite (PdBi2), merenskyite (PdTe2), mertieite II (Pd8[Sb,As]3), and sperrylite (PtAs2) and occur in variety of textural settings. Those that occur in massive and interstitial sulfides, interpreted to be of magmatic origin and formed through exsolution from base metal sulfides at temperatures <600°C, are dominantly Bi rich (i.e., Te-bearing sobolevskite), whereas those that occur in late-stage hydrothermal sulfide/silicate veins and their epidote–chlorite alteration halos tend to be more Te rich (i.e., Bi-bearing kotulskite). The chemistry and textural setting of the various PGM supports a genetic model that links the magmatic and hydrothermal end-members of the sulfide–PGM mineralization. The association of PGM with magmatic sulfides in the massive sulfide and high-PGE zones has been interpreted to indicate that PGE mineralization was initially formed through exsolution from base metal sulfides which formed by magmatic sulfide liquid segregation and crystallization. However, the occurrence of PGM in undeformed sulfide-bearing veins and in their chlorite–epidote halos and differences in PGM chemistry indicate that hydrothermal fluids were responsible for post-metamorphic redistribution and dispersion of PGE.

A Review of the Behaviour of Platinum Group Elements within Natural Magmatic Sulfide Ore Systems

The largest and most significant type of geological deposit of platinum group elements (PGEs) is that associated with magmatic base metal sulfide minerals in layered mafic or ultramafic igneous intrusions. The common association of PGEs with sulfide minerals is a result of processes of magmatic and sulfide liquid segregation and fractionation. The mineralogical nature of the ores is dependent on a number of factors during sulfide liquid fractionation. The most significant of these with regard to the mineralogy of the two most important metals, platinum and palladium, is the presence and concentration of semimetals such as bismuth and tellurium within the mineralising sulfide liquid. Whereas rhodium, iridium, osmium and ruthenium are almost always present in solid solution within the resultant base metal sulfide minerals; should sufficient semimetals be present, Pd and especially Pt will form discrete minerals (such as platinum bismuthides) around the margins of, and possibly away from, the sulfides.

Selenium and sulfur concentrations in the Bushveld Complex of South Africa and implications for formation of the platinum-group element deposits

We have determined the S, Se, Cu and La contents through a complete stratigraphic section of the Bushveld Complex. The principle aim was to determine which phases controlled these elements. S, Se and Cu show positive correlations, but these elements do not correlate with La. In most cases, the concentration of S, Se and Cu in rocks containing greater than 800 ppm S can be modeled by segregation of a Fe–Ni–Cu sulfide liquid from a fractionating magma. As the magma evolved, Se and Cu were depleted by the continual segregation of sulfide liquid and the S/Se and S/Cu of the rocks increased. The Se/Cu ratio is higher in the more evolved rocks, which suggests that Se has a slightly lower partition coefficient than Cu into sulfide liquid (1,200 versus 1,700). The Lower and lower Critical Zone of the complex contains on average only 99 ppm S. The low S content of these rocks has led some authors to suggest that these rocks do not contain cumulate sulfides, despite the fact that they are moderately enriched in PGE. These samples fall along the same trend as the S-rich samples on the S-versus-Se plot and the S/La and Se/La ratios are greater than the initial magmas suggesting that despite the low S contents cumulate sulfides are present. Three models may be suggested in order to explain the low S content in the Lower and Critical Zone rocks: (a) the sulfides that were present have migrated away from the cumulate pile into the footwall or center of the intrusion; (b) the magma was saturated in sulfides at depth and during transport some sulfides lagged in embayments; (c) the rocks have lost both S and Se at high temperature. The first two models have important implications for exploration.

Self-Destructive Sulfide Segregation Systems and the Formation ofHigh-Grade Magmatic Ore Deposits

The metal concentrations of sulfide liquids depend upon the mass of silicate magma that they are able to process for metals. Some high-grade magmatic Ni-Cu (±PGE) deposits and many magmatic platinum group element (PGE) (±Ni-Cu) deposits demand very high magma/sulfide ratios that seem improbable in the light of physical and kinetic constraints. Recent models for some high-grade deposits invoke multistage upgrading processes, in which an early-formed sulfide liquid reacts with multiple later batches of silicate magma. Quantitative models of this open-system process demonstrate that it is indeed more efficient than a closed system. However, it is likely that most later magmas in such a system will be sulfur undersaturated and will thus partly redissolve preexisting sulfide liquids, further increasing metal concentrations in the remaining sulfide liquids. This combined process is termed multistage-dissolution upgrading, and quantitative models show that it could reduce the mass of silicate magma that must be processed by as much as two orders of magnitude. Furthermore, if sulfide liquids are extensively dissolved during enrichment, their base metal concentrations will generally stabilize at a limiting value, whereas their PGE concentrations will generally increase without limit. This divergence could account for unusually high PGE concentrations and high PGE/base metal ratios in many PGE-dominated deposits. The multistage-dissolution upgrading model is tested, using reasonable degrees of dissolution, in the context of natural deposits in the Noril’sk area and the Bushveld intrusion. The models reproduce the observed sulfide compositions significantly better than previous models and are also consistent with other aspects of the geology of these deposits. Thus, dynamic sulfide-forming magmatic systems may be intrinsically self destructive, but this apparently undesirable attribute could play an important role in forming high-grade deposits. However, dissolution could lead to the complete destruction of sulfide liquids and the return of all metals to later magma batches. In this case, no sulfide deposit would remain, but metal-depletion signatures indicating sulfide liquid segregation would be preserved in rocks that crystallized from early magmas. Caution is therefore advised in the use of magmatic depletion signatures to infer overall mineral potential or to estimate the possible sizes of undiscovered magmatic sulfide deposits.

Lithogeochemical exploration for komatiite-associated Ni-sulfide deposits: strategies and limitations

Lithogeochemistry of komatiites can be useful in exploration for nickel sulfides in two distinct ways: indirectly, through identification of favourable volcanic environments, and more directly, through identification of signatures of crustal contamination and sulfide liquid segregation. However, the discrimination between mineralised and unmineralised sequences is not straightforward. Komatiite chemistry is influenced by a variety of primary magmatic variables, overprinted to various degrees for various elements by the effects of metasomatic alteration. Once highly metasomatised samples have been filtered, primary magmatic signatures are largely retained for the elements Mg, Fe, Si, Al, Ti and Cr, even in cases of extensive CO2 metasomatism and talc-carbonate alteration. Incompatible trace elements such as Zr and REE are modified to some degree by alteration, particularly in cumulate rocks, but there is evidence for some retention of primary signatures. Favourable volcanic environments, i.e. those containing large-volume lava pathways, can be identified on the basis of their distribution and ratios of whole rock Mg, Fe, Ni, Cr and Ti. Nickel depletion can be recognised in some cases of large mineralised systems where large volumes of sulfide formed at low silicate:sulfide (R) ratios, but is not effective in finding small high-grade ore shoots. PGE in low-S rocks do not appear to be an effective discriminant, owing to the local scale of S-saturation. Crustal contamination is indicated in some mineralised suites on the basis of enriched ratios of strongly to weakly incompatible lithophile elements in fractionated lavas, but the effects of alteration are significant and may be misleading. On the whole, mineralised sequences tend to have more enriched signatures than unmineralised ones, but there may be many false positives and negatives, and if the most CO2-metasomatised suites are discounted the contrast largely disappears. Overall, the presence of Ni-enriched sulfides is by far the best lithogeochemical indicator there is, and it may be the only really effective one.

Late-Stage Sulfide Liquid Mobility in the Main Mass of the Sudbury Igneous Complex: Examples from the Victor Deep, McCreedy East, and Trillabelle Deposits

Massive sulfide in the contact sublayer in three different deposits of the Sudbury Igneous Complex is overlain by substantial thicknesses of norite-hosted disseminated sulfide mineralization with anomalous compositions. In the Trill embayment, the Levack trough (McCreedy East), and the Victor-Nickel Rim embayment (Victor Deep), disseminated sulfides are present at concentrations of 1 to 5 percent over a vertical thickness locally exceeding 100 m within the main mass (i.e., texturally homogeneous, inclusion- free) norites above massive contact ores of the sublayer. Where primary textures are preserved, they show that sulfide liquid remained present as an interstitial liquid during the entire crystallization history of the enclosing silicate cumulate rocks. The disseminated sulfides show remarkably constant compositions over a large stratigraphic interval. They cannot have formed due to fractional segregation of sulfide liquid from the main mass magma accompanying the deposition of the enclosing silicate cumulus phases, because this would lead to extreme depletion in all platimun-group elements within a stratigraphic interval much shorter than the hundreds of meters in which their compositions are observed to remain approximately constant. Geochemical modeling of sulfide liquation and fractional crystallization shows that the massive sulfides are cumulates rich in monosulfide solid solution (mss). The overlying disseminated sulfides represent mss left behind by an upward-migrating fractionated sulfide liquid residual to the formation of the massive cumulates at the base. These results show that Cu-rich sulfide liquids are capable of migrating distances greater than 100 m through almost completely solidified cumulate rocks at volume fractions of sulfide as low as one percent. The presence of a halo of disseminated sulfide mineralization, with a distinctly fractionated geochemical character, up to several hundred meters wide above zones of massive sulfide mineralization could be used to detect deeply buried massive sulfide orebodies at Sudbury and elsewhere.

Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications

Both Ni-Cu and PGE deposits are associated with mafic and ultramafic rocks, but the former often occur in dynamic magmatic systems such as lava channels and magma conduits whereas the later occur in larger layered intrusions. The contrasting modes of occurrence appear to be related to different degrees of preferential partitioning of the metals (PGE versus Cu and Ni) into segregating sulfide liquid and different concentration processes of the sulfide liquid during the formation of the ores. Primary magmas derived from partial melting of the upper mantle are fertile in PGE, Ni and Cu only when the molten sulfides are completely dissolved in the magmas. This requires a high degree of partial melting of the mantle, perhaps higher than 30 percent. The partial melts usually have basaltic to komatiitic compositions, and are unlikely to achieve sulfide saturation during ascent to the crust. Fractional crystallization and crustal contamination are the most important processes known to have induced sulfide saturation in magma upon intrusion. Sulfide liquid segregated in a large chamber may achieve high R-factors required for the formation of reef-type PGE deposits. Crustal assimilation appears to be most effective in dynamic lava channels and magma conduits. A large volume of sulfide liquid segregated from magma in such dynamic environments often has a lower R-factor, and relatively lower metal concentrations. Such metal-poor sulfide liquid needs to be deposited in restricted localities, and/or to be upgraded in metals by reacting with new surges of magma, to become a Ni-Cu sulfide deposit. Because of their extremely high partition coefficient between sulfide liquid and magma, PGE will become significantly depleted in magma even at small amounts of sulfide segregation. Copper, on the other hand, will be much less depleted because of its relatively low partition coefficient. Thus, Pd/Cu ratios in whole rocks can be used to detect PGE mineralization associated with small sulfide segregation in large layered intrusion. The whole rock Cu/Zr ratio is sensitive to large amounts of sulfide liquid segregation from magma and thus is a useful tool for Ni-Cu exploration. Nickel contents of olivine can also be used to identify Ni depletion in magma associated with large amounts of sulfide segregation.

Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications

Research Article| December 01, 2001 Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications Chusi Li; Chusi Li Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. e-mail: cli@indiana.edu Search for other works by this author on: GSW Google Scholar W. D. Maier; W. D. Maier Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: wdmaier@scientia.up.ac.za Search for other works by this author on: GSW Google Scholar S.A. de Waal S.A. de Waal Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: sadw@scientia.up.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Chusi Li Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA. e-mail: cli@indiana.edu W. D. Maier Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: wdmaier@scientia.up.ac.za S.A. de Waal Centre for Research on Magmatic Ore Deposits, Department of Geology, University of Pretoria, Pretoria 0002, South Africa. e-mail: sadw@scientia.up.ac.za Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2001 Geological Society of South Africa South African Journal of Geology (2001) 104 (4): 309–318. https://doi.org/10.2113/gssajg.104.4.309 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Chusi Li, W. D. Maier, S.A. de Waal; Magmatic Ni-Cu versus PGE deposits: Contrasting genetic controls and exploration implications. South African Journal of Geology 2001;; 104 (4): 309–318. doi: https://doi.org/10.2113/gssajg.104.4.309 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Both Ni-Cu and PGE deposits are associated with mafic and ultramafic rocks, but the former often occur in dynamic magmatic systems such as lava channels and magma conduits whereas the later occur in larger layered intrusions. The contrasting modes of occurrence appear to be related to different degrees of preferential partitioning of the metals (PGE versus Cu and Ni) into segregating sulfide liquid and different concentration processes of the sulfide liquid during the formation of the ores.Primary magmas derived from partial melting of the upper mantle are fertile in PGE, Ni and Cu only when the molten sulfides are completely dissolved in the magmas. This requires a high degree of partial melting of the mantle, perhaps higher than 30 percent. The partial melts usually have basaltic to komatiitic compositions, and are unlikely to achieve sulfide saturation during ascent to the crust. Fractional crystallization and crustal contamination are the most important processes known to have induced sulfide saturation in magma upon intrusion. Sulfide liquid segregated in a large chamber may achieve high R-factors required for the formation of reef-type PGE deposits. Crustal assimilation appears to be most effective in dynamic lava channels and magma conduits. A large volume of sulfide liquid segregated from magma in such dynamic environments often has a lower R-factor, and relatively lower metal concentrations. Such metal-poor sulfide liquid needs to be deposited in restricted localities, and/or to be upgraded in metals by reacting with new surges of magma, to become a Ni-Cu sulfide deposit.Because of their extremely high partition coefficient between sulfide liquid and magma, PGE will become significantly depleted in magma even at small amounts of sulfide segregation. Copper, on the other hand, will be much less depleted because of its relatively low partition coefficient. Thus, Pd/Cu ratios in whole rocks can be used to detect PGE mineralization associated with small sulfide segregation in large layered intrusion. The whole rock Cu/Zr ratio is sensitive to large amounts of sulfide liquid segregation from magma and thus is a useful tool for Ni-Cu exploration. Nickel contents of olivine can also be used to identify Ni depletion in magma associated with large amounts of sulfide segregation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The Role of Magma Mixing in the Genesis of PGE Mineralization in the Bushveld Complex: Thermodynamic Calculations and New Interpretations

The elegant magma-mixing model for the formation of the platinum-group elements (PGE) mineralization associated with chromite and sulfide in the Bushveld Complex proposed by Naldrett and von Gruenewaldt (1989) has recently been questioned by Cawthorn (1999). Based on his own calculation, Cawthorn concluded that the concave-upward sulfur solubility curve of a fractionated basaltic magma, which is central to the model, does not exist. Cawthorn (1999) pointed out that both the hanging-wall and footwall sequences of the Merensky reef formed from sulfide-undersaturated magmas instead of sulfide-saturated magmas as predicted by the model. Cawthorn argued that there is a mass-balance problem with the model if these rocks did not contribute PGE to the sulfides in the reef. Our thermodynamic calculations indicate that the concave-upward curve of S solubility, though different in detail from the original, is very much in evidence. Our calculations also indicate that the content of S dissolved in the residual magma may fall below the S solubility curve when the rate of S increase in the magma, due to the removal of silicate cumulates, is less than the rate of increase in its S solubility, due to the change in its temperature and composition during fractional crystallization. Mixing of such fractionated and yet sulfide-undersaturated magma with appropriate amounts of a primitive magma is still capable of achieving sulfide oversaturation in the hybrid. Once the immiscible sulfides settle out of the magma, the content of S dissolved in the hybrid may fall below its solubility curve again during further crystallization, thus no sulfide segregation is expected in the rocks crystallized subsequently. R-factor assessment needs to consider the consequence of sulfide suspension in magma. Small quantities of sulfide droplets formed during the normal course of fractional crystallization may remain suspended in magma until the next replenishment, and in this case they can be redissolved in the hybrid when the proportion of a new input is large enough. Because there is no early sulfide liquid segregation, both the residual and new magmas will not be depleted in PGE and are equally important for contributing PGE to the sulfide liquid segregated during a subsequent magma-mixing event.

Genetic mechanisms of the Jinchuan ultramafic intrusion and associated superlarge sulfide deposit, Northwest China

Superlarge sulfide deposits are hosted in the Jinchuan ultramafic intrusion. The central and west segments of the intrusion may represent magma conduits along which petrological phases are symmetrical zones. The ores are associated with the dunite and lherzolite in the central part of the intrusive body. The east segment of the intrusion is more gently titled, showing vertical zonation of rock phases with ores localized at the base. Cumulate texture is common in various types of rock. Petrological and geochemical studies indicate that flow differentiation may have dominated in the central and west segments while gravity differentiation was important in the east. The original magma was believed to be tholeiitic in composition. The massive ores were the result of filter pressing. Ore metals were supplied for the most part by progressive influx of new magmas. The segregation of sulfide liquid from the silicate melt was controlled to a large extent by volatiles. Necessary conditions for the formation of such superlarge sulfide deposits are discussed.

Constraints on the formation of platinum-group element deposits in igneous rocks

Mechanisms of concentration of the platinmn-group and other chalcophile elements iu several platinum-group element deposits in layered intrusions are evaluated in light of recently published data on element partitioning between immiscible sulfide and silicate melts. The similarity of the partition coefficieut D (super sulfide melt/silicate melt) of Ir to that of Pd indicates that the observed fractionation of these elements in nature resulted from a process other than or in addition to sulfide liquid immiscibility. In experiments designed to test the possibility that this fractionation was due to the presence of Os-Ir alloy, sulfide and silicate melts were equilibrated with Ir metal at 1,450 degrees C, 8 kbars total pressure, and under sulfur and oxygen fugacity conditions appropriate for nature. The Ir content of the sulfide liquid was found to be > or = 3.7 wt percent and that of the coexisting silicate melt inferred to be > or = 1,000 ppb. These concentrations are far in excess of the Ir contents of natural sulfides or rocks and indicate that Os-Ir alloys are not stable in the presence of sulfide or silicate melts in nature.Using our preferred values of D (super sulfide melt/silicate melt) for the chalcophile elements and their estimated concentrations in the mantle, limits are determined for the possible ranges of concentrations and relative abundances of these elements in immiscible sulfide liquid formed in magma derived by partial melting of the mantle. The ratios of highly chalcophile to moderately chalcophile elements and of metal to sulfur of several deposits, including the J-M reef (Stillwater Complex), the UG-2 (Bushveld Complex), and the Lac des Isles Complex, are inconsistent with concentration solely by segregation of sulfide liquid. In contrast, the Merensky reef fits the geochemical criteria for formation by accumulation of magmatic sulfide. Stratigraphic variations in chalcophile element concentrations through the UG-2 and the Main Sulfide zone of the Great Dyke also indicate that metals must have been redistributed on at least the meter scale. Our analysis implies either different relative mobilities of the chalcophile elements relative to one another in a process postdating initial sulfide liquid deposition or concentration by a different mechanism.

Distribution of pd, rh, ru, jr, os, and au between sulfide and silicate metals

Experiments have been conducted on the partitioning of noble metals in the presence of an H-O-C-S fluid in a high pressure gas apparatus at 1200–1300°C and 1–4 kbar fluid pressure. The fugacities of oxygen and sulfur, and the chemical species in the fluid phase, were controlled by the presence of graphite and of a sulfide melt of pyrrhotite composition and by external control of the fugacity of hydrogen. It has been found that PGE and Au partition strongly into a sulfide liquid in equilibrium with a silicate melt. The compositions of the sulfide and silicate melts as well as the redox conditions have only weak effects on the partitioning. The following partition coefficients have been obtained: D Pd = (5.5 ±0.7) × 10 4 ; D Rh = (2.7 ± 0.6) × 10 4 ; D Ru = (2.5 ± 0.7) × 10 3 ; D Ir = (3.1 ±0.8) × 10 5 ; D Os= (2.3 ±0.3) × 10 2 ; D Au = ( 1.6 ± 0.4) × 10 4 . These values are significantly higher than those reported in the recent literature ( Fleet et al., 1991; Crocket et al., 1992); this is attributed to their having been obtained under hydrous conditions. Their magnitudes demonstrate that the segregation of sulfide liquid from silicate magma is an effective mechanism for concentrating PGE and Au, as has been concluded from the study of deposits of believed magmatic origin.