Immiscible Sulfide Research Articles

The Role of External Sulfur in Triggering Sulfide Immiscibility at Depth: Evidence from the Huangshan-Jingerquan Ni-Cu Metallogenic Belt, NW China

Abstract The Huangshan-Jingerquan belt in Northwest China is one of the most important orogen-hosted magmatic Ni-Cu sulfide metallogenic belts worldwide. The Huangshandong, Huangshan, and Tulaergen deposits are the three largest Ni-Cu deposits along the belt. The orebodies are situated inside mafic-ultramafic complexes. Sulfur isotope data and trace element composition of the sulfide ores and country rocks are used to evaluate the importance of crustal S addition for sulfide mineralization and speculate as to the source of the sulfur that triggered sulfide segregation. The S/Se ratios of >95% of the sulfide ores range from 2,398 to 85,222—higher than that of the mantle (2,850–4,350) but lower than the country rocks (S/Se = 3,889–160,769). The sulfide ores from the Huangshandong, Huangshan, and Tulaergen deposits have restricted δ34S values ranging from –0.86 to 1.33, 0.26 to 0.75, and –0.2 to 1.4‰, respectively. However, the country rocks of these mafic-ultramafic complexes have highly heterogeneous δ34S, ranging from –22.3 to 18.8, –22.3 to 2.12, and –1.4 to 5.3‰, respectively. Arsenic/bismuth and Sb/Bi ratios of the sulfide ores range from 0.22 to 7.59 and 0.02 to 2.88, respectively, which are lower than those of the country rocks (3.17–243 and 2.8–33) and mid-ocean ridge basalt (MORB) (5.09–127 and 0.51–9.25). The values of δ34S and S/Se as well as ratios of As/Bi and Sb/Bi of the sulfide ores indicate that the sulfide segregation and consequently the formation of the deposits were closely associated with the addition of crustal sulfur, whereas the sulfides in the country rocks have not been incorporated into the mineralization. Thus, it is proposed that assimilation of external crustal sulfur at depth might play a critical role in triggering sulfide immiscibility and the formation of the magmatic Ni-Cu deposits in the Huangshan-Jingerquan belt.

The effects of bulk composition on planetesimal core sulfur content and size

This study explores the compositions and sizes of metallic cores that result from planetesimals forming from a range of chondritic bulk compositions. Our models examine the influence of starting bulk composition on core size and composition, how oxygen fugacity (fO2), temperature, pressure, and bulk composition affect sulfur partitioning between the core and silicate mantle of planetesimals, and the formation and fate of immiscible sulfur-rich liquid during core solidification. We apply experimentally-derived equations for the sulfur distribution coefficient to the bulk compositions of ordinary chondrites (H,L,LL) and carbonaceous chondrites (CM, CI, CO, CK, CV) under conditions appropriate for melting planetesimals.The sulfur content of all modeled cores is above 6 wt% S, which is greater than the amount of sulfur needed to form an immiscible sulfide liquid in the presence of other light elements (e.g., C, Si, and/or P). We concluded that early planetesimal cores likely formed either an immiscible sulfide liquid, a eutectic sulfide liquid, or most surprisingly, were composed of mostly monosulfide solid solution, [(Fe, Ni)1-xS].

Sublithosphere Mantle Crystallization and Immiscible Sulfide Melt Segregation in Continental Basalt Magmatism: Evidence from Clinopyroxene Megacrysts in the Cenozoic Basalts of Eastern China

AbstractThis study explores the effects of high-pressure crystallization and immiscible sulfide melt segregation under mantle conditions on the compositional variation of basaltic magmas, using clinopyroxene megacrysts in the Cenozoic basalts of eastern China. These clinopyroxene megacrysts are large (up to >10 cm in size) and homogeneous at the grain scale. They were crystallized from variably evolved parental magmas and then captured by their host basalts. The large and systematic variations of [Sm/Yb]N, Lu/Hf, Fe/Mn, Sc/La, Ni and Cu with Mg# in the clinopyroxene megacrysts suggest their co-precipitation with garnet and with immiscibility between sulfide and silicate melts. This is consistent with the appearance of garnet megacrysts in the host basalts and abundant sulfide globules in the clinopyroxene megacrysts. The covariation between Ni contents of sulfide globules and Mg# of the clinopyroxene megacrysts suggests a genetic relationship between sulfide globules and clinopyroxene megacrysts. High-pressure crystallization of clinopyroxene and garnet results in decrease of Mg# and concentrations of CaO, MnO and heavy rare earth elements (e.g. Yb) and increase of Fe/Mn and [Sm/Yb]N in the residual melts. Therefore, geochemical characteristics of low Mg#, low CaO and MnO contents and high Fe/Mn and [Sm/Yb]N in basalts do not necessarily indicate a pyroxenite mantle source. In addition, caution is needed when applying the olivine addition method to infer the primary compositions of alkali basalts without considering the effects of high-pressure crystallization of clinopyroxene and garnet. The calculated pressure (P) and temperature (T) conditions of the clinopyroxene megacrysts are close to those of the lithosphere–asthenosphere boundary (LAB) beneath eastern China, and the low primitive [Sm/Yb]N (~4.0) of melts parental to the clinopyroxene megacrysts suggests final equilibration at relatively low pressures most likely beneath the LAB. Hence, a melt-rich layer is expected close beneath the LAB. Melt pools in this melt-rich layer provide a stable and closed environment for the growth of compositionally homogeneous clinopyroxene megacrysts. As a result, melts in these melt pools are compositionally evolved with low and variable Mg#. Subsequent pulses of melt aggregation/supply from depths with primitive compositions and high Mg# will disturb these melt pools, cause magma mixing and trigger the eruption of magmas carrying clinopyroxene and garnet megacrysts.

Origin of Fe–Ni–Cu (Co) sulfide and Fe–Ti oxide minerals in the c. 1.77 Ga dolerite dyke, Singhbhum Craton (eastern India)

Abstract Dolerite dyke swarms are widespread within the Singhbhum Craton (eastern India) that emplaced from the Neoarchean to Paleoproterozoic era just after the stabilization of crust before c. 3 Ga. These dyke swarms are oriented in NE–SW to NNE–SSW, NW–SE to WNW–ESE, east–west and north–south directions. The WNW–ESE-trending c. 1.77 Ga Pipilia dyke swarm is sampled from the Satkosia area of Orissa state. The dyke shows a noticeable disparity in terms of the modal proportion and grain size of pyroxenes, plagioclase, Fe–Ti oxide minerals and texture across the trend. In places the primary silicates are altered to secondary hydrated mineral assemblages of amphibole, chlorite and sericite. The primary silicates are clinopyroxene (augite – Mg# = 65.7–82.6; En 37–48 Fs 11–17 Wo 36–41 ), orthopyroxene (clinoenstatite – Mg# = 68.5–78; En 63–70 Fs 20–29 Wo 4–5 ) and plagioclase (An 11–39 Ab 44–82 Or 1–7 ) and the Fe–Ti oxides are titanomagnetite (FeO, 34.38–39.50 wt%; Fe 2 O 3 , 48.26–56.21 wt%; TiO 2 , 5.05–9.60 wt%) and ilmenite (FeO, 40.75–43.79 wt%; Fe 2 O 3 , 3.54–10.03 wt%; TiO 2 , 47.82–50.87 wt%). Application of two-pyroxene thermometry yields an equilibration temperature range of 1065–978°C, and coexisting titanomagnetite–ilmenite pairs reveal 731.39–573.37°C under the oxygen fugacity ( f O 2 ) condition NNO + 0.3 to FMQ − 1.03. The dyke contains disseminated sulfides at the interstices of Fe–Ti oxides and silicates. Major sulfide minerals are pyrite, chalcopyrite and vaesite; pyrite–vaesite assemblages occur in association with secondary silicate minerals. Pyrite grains contain variable concentrations of Co (0.01–5.70 wt%) and Ni (0.02–1.95 wt%). Coexisting vaesite contains Co (2.42–10.44 wt%), Ni (26.40–47.88 wt%) and Fe (7.32–26.55 wt%). Texture, sulfide–silicate assemblage and the presence of low metal/S sulfides such as the pyrite–vaesite assemblage indicate that primary Fe–Ni sulfides (pyrrhotite–pentlandite) that segregated from immiscible sulfide liquid at high temperature are modified by late magmatic/hydrothermal fluid activities. Numerous sulfide-bearing deposits hosted in ultramafic–mafic intrusions of Paleoproterozoic age have been recorded globally and the occurrence of Fe–Ni sulfides in the c. 1.77 Ga Pipilia dyke swarm in the Singhbhum Craton enhances the exploration potential of this craton in eastern India.

Gypsum Precipitating From Volcanic Effluent as an Archive of Volcanic Activity

Records of volcanic activity are a key resource in volcano monitoring and hazard mitigation. The time period for which such records are available and the level of detail vary widely among volcanic centers and there is, therefore, a need for supplementary sources of this information. Here, we use growth-zoned gypsum as a mineral archive of the activity of Kawah Ijen volcano in East-Java, Indonesia. Gypsum precipitates where water seeps from the crater lake and hydrothermal system, and it has formed a 100 m long cascading plateau. A 19 cm plateau cross-section was analysed for minor and trace elements using laser-ablation ICP-MS. Absolute ages were assigned to this transect based on 210Pb dating. This 210Pb age model was corrected for variations in the 210Pb0 resulting from fluctuations in the volcanic radon flux by using 84Kr/36Ar and 132Xe/36Ar. The age model indicates that the transect covers a period from 1919 ± 12 to 2008 ± 0.2. Gypsum-fluid partition coefficients (D) permit the gypsum compositions to be converted to the concentrations in the fluid from which each growth zone grew. The D-values also show the compatibility of the elements in the gypsum structure, and identify the LREE, Sr, Pb, Tl, Ni, Co, Cu, Zn, Cd, Sb, Th, and Mo as least susceptible to contamination from rock fragment and mineral inclusions, and therefore as most reliable elements of the gypsum record. Compositional variability in the timeseries correlates with known element behavior in the Kawah Ijen system and shows three element groups: the LREE, Sr, and Pb that represent rock-leaching; Cu, Zn, and Cd, which have previously been linked to immiscible sulfide destabilization in a deep-seated basalt; and Sb, Tl, and As which point to a contribution from the shallow system and evolved magma. Moreover, the gypsum record shows that episodes of unrest and quiescence have a distinct compositional signature in Kawah Ijen seepage fluids, and can be distinguished. Thus, we show that gypsum is a sensitive recorder of volcanic activity and can provide detailed information on the state of the magmatic-hydrothermal system in the past.

Redox-controlled chalcophile element geochemistry of the Polaris Alaskan-type mafic-ultramafic complex, British Columbia, Canada

ABSTRACT The Early Jurassic Polaris Alaskan-type intrusion in the Quesnel accreted arc terrane of the North American Cordillera is a zoned, mafic-ultramafic intrusive body that contains two main styles of magmatic mineralization of petrologic and potential economic significance: (1) chromitite-associated platinum group element (PGE) mineralization hosted by dunite (±wehrlite); and (2) sulfide-associated Cu-PGE-Au mineralization hosted by olivine (±magnetite) clinopyroxenite, hornblendite, and gabbro-diorite. Dunite-hosted PGE mineralization is spatially associated with thin discontinuous layers and schlieren of chromitite and chromitiferous dunite and is characterized by marked enrichments in iridium-subgroup PGE (IPGE) relative to palladium-subgroup PGE (PPGE). Discrete grains of platinum group minerals (PGM) are exceedingly rare, and the bulk of the PGE are inferred to reside in solid solution within chromite±olivine. The absence of Pt-Fe alloys in dunite of the Polaris intrusion is atypical, as Pt-enrichment of dunite-hosted chromitite is widely regarded as a characteristic feature of Alaskan-type intrusions. This discrepancy appears to be consistent with the strong positive dependence of Pt solubility on the oxidation state of sulfide-undersaturated magmas. Through comparison with experimentally determined PGE solubilities, we infer that the earliest (highest temperature) olivine-chromite cumulates of the Polaris intrusion crystallized from a strongly oxidized ultramafic parental magma with an estimated log f(O2) > FMQ+2. Parental magmas with oxygen fugacities more typical of volcanic arc settings [log f(O2) ∼ FMQ to ∼ FMQ+2] are, in turn, considered more favorable for co-precipitation of Pt-Fe alloys with olivine and chromite. More evolved clinopyroxene- and hornblende-rich cumulates of the Polaris intrusion contain low abundances of disseminated magmatic sulfides, consisting of pyrrhotite and chalcopyrite with minor pentlandite, pyrite, and rare bornite (≤12 wt.% total sulfides), which occur interstitially or as polyphase inclusions in silicates and oxides. The sulfide-bearing rocks are characterized by strong primitive mantle-normalized depletions in IPGE and enrichments in Cu-PPGE-Au, patterns that resemble those of other Alaskan-type intrusions and primitive arc lavas. The absolute abundances and sulfur-normalized whole-rock concentrations (Ci/S, serving as proxy for sulfide metal tenor) of chalcophile elements, including Cu/S, in sulfide-bearing rocks are highest in olivine clinopyroxenite. Sulfide saturation in the relatively evolved magmas of the Polaris intrusion, and Alaskan-type intrusions in general, appears to be intimately tied to the appearance of magnetite. Fractional crystallization of magnetite during the formation of olivine clinopyroxenite at Polaris resulted in reduction of the residual magma to log f(O2) ≤ FMQ+2, leading to segregation of an immiscible sulfide melt with high Cu/Fe and Cu/S, and high PGE and Au tenors. Continued fractionation resulted in sulfide melts that were progressively more depleted in precious and base chalcophile metals. The two styles of PGE mineralization in the Polaris Alaskan-type intrusion are interpreted to reflect the evolution of strongly oxidized, hydrous ultramafic parental magma(s) through intrinsic magmatic fractionation processes that potentially promote sulfide saturation in the absence of wallrock assimilation.

Platinum-group element geochemistry of the Panjal Traps: constraints on mantle melting and implications for mineral exploration

Abstract Forty-two volcanic rocks of the Panjal Traps were analysed for platinum-group elements (PGEs) to investigate the magma genesis, high-temperature behaviour and exploration potential of these elements. The PGE data exhibit substantial variability and show no systematic relation to their low- or high-Ti affinity. Instead, the basalts can be subdivided into a PGE-undepleted group (group 1) that has ΣPGE >10 ppb and Cu/Pd <30 000, and a PGE-depleted group, which consists of a subgroup showing limited (group 2A) or substantial depletion in Ir-series PGEs relative to Ni (group 2B). The group 1 samples indicate an S-undersaturated history, whereas the group 2 samples might have different origins in terms of S-saturation. Fractionation of a tiny amount of sulfide melts (0.075–0.1%) from a representative group 1 sample accounts for the chalcophile element patterns observed in the group 2B samples. The relatively high Cu/Pd, unfractionated Ni/Ir and low PGE abundances observed in the group 2A samples cannot be explained by equilibration of an immiscible sulfide melt alone, and probably require decomposition of residual sulfides into sulfide melt and a monosulfide solid solution in the mantle restite. Our results question the notion that the coexistence of PGE-undepleted and -depleted magmas is prospective in the exploration of magmatic Ni–Cu–(PGE) sulfide mineralization.

Geology, geochronology, mineral chemistry and geochemistry of the Hongnieshan mafic–ultramafic complex in the Beishan area, southern Central Asian orogenic Belt, NW China: Implications for petrogenesis and regional Ni mineralization

In recent decades, new breakthroughs in Ni prospecting have been made in the Beishan area, southern Central Asian Orogenic Belt, NW China—more vein-type sulfide mineralization with high Ni tenor has been discovered. Here, novel discussions on the petrogenesis and sulfide immiscibility of the Permian mafic–ultramafic rock are presented based on petrological, geochronological, mineralogical, and Hf–S isotopic studies of the newly discovered Hongnieshan Ni occurrence. The complex is predominantly composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro and norite. The gabbro has a zircon age of 282.9 Ma and positive εHf(t) values (+5.5–+7.8). The complex may have originated from partial melting of the depleted mantle, which was metasomatized by subduction fluid in a post-orogenic extensional setting. The parental magma is characterized by high temperature (1396 ℃) and high Mg (picritic affinity, 14.4 wt% MgO) and Ti (2.3 wt%) contents, and an oxidized nature (FMQ + 0.59). The disseminated and vein-type sulfide mineralization are the results of multistage segregation. The first segregation was induced by the assimilation of Archean strata, as indicated by the presence of Δ33S of sulfide minerals (1.4‰ to 3.6‰), which is within the range of Archean crust (-2.5‰–+11‰) and is clearly beyond the range of post-Archean rocks (0 ± 0.1‰). The sulfide was removed at depth, leading to that the later segregated sulfide was poor in platinum group element. Vein-type high Ni tenor mineralization was ascribed to the second segregation before olivine crystallization and later injection of sulfide ore pulp. The third sulfide segregation was accompanied by fractional crystallization and gave rise to disseminated mineralization with a low Ni tenor. The δ34S in the Beishan Ni-Cu deposits ranges from-2.01 to 2.07‰, exhibiting a mantle-like variation and arguing against the incorporation of crustal S. Crustal assimilation, especially the addition of Si-rich components from country rocks is the main trigger factor. The Beishan area is still a favorable exploration target in future prospecting.

Geochemical evolution of PGE-sulfide mineralization of the Khudolaz differentiated complex in the South urals: The role of R-factor and hydrothermal alteration

The article highlights the results of mineralogical and geochemical studies of disseminated Cu-Ni-PGE sulfide mineralization from three intrusions of the Khudolaz Differentiated Complex (KDC) in the South Urals. Sulfide mineral assemblages are shown to have formed across a wide temperature range (about 1100–150 °C) during the cooling of an immiscible sulfide melt and subsequent hydrothermal dissolution and redeposition of sulfides. During the solidification process, a crystallization of sulfide solid solutions (mss, iss, etc.) occurred, from which pyrrhotite, pentlandite, and chalcopyrite were subsequently exsoluted. The oxidation, substitution and dissolution of primary sulfides at the hydrothermal stage led to the formation of mineral aggregates of complex morphology, containing complete or partial pseudomorphs of secondary sulfides, silicates and oxides, as well as redeposited sulfide aggregates. According to LA ICP MS data for sulfides and the bulk trace element composition of ores, the depletion of sulfide minerals by some chalcophile and platinum-group elements (PGE) took place during their hydrothermal alteration (partial dissolution and replacement). Secondary sulfides such as pyrite and violarite largely inherited the geochemical features of the primary minerals during pseudomorphic replacement of pyrrhotite and pentlandite. The redeposited sulfides (pyrite and chalcopyrite) are primarily enriched by chalcophile trace elements. Leaching of PGEs from pyrrhotite and pentlandite took place during hydrothermal alteration. Here, the most significant loss was determined for Te, resulting in the depletion of sulfide minerals and ores in Te by about 5 times. The identified decrease of Te/Sb values in altered sulfides by a factor of 3–20 indicates an enrichment in Sb and a high Dhydrothermal fluid/sulfide partition coefficient (≫ 1). It is shown that the hydrothermal fluid was enriched in Sb. On the other hand, the S/Se value increases by a factor of ~ 1.5, demonstrating an Se mobility during metasomatic processes comparable to other chalcogenides.Based on the study of assemblages and traced evolution of the chemical composition of PGE minerals, three stages of mineralization are proposed: 1) crystallization of immiscible chalcogenide and highly fractionated sulfide melts; 2) segregation of isomorphic impurities of chalcogenides and PGE during the sulfides exsolution from sulfide solid solutions; 3) interaction of Pd leached from primary sulfide and PGE minerals with Sb-rich hydrothermal fluid.The calculated value of the R-factor (150–502) indicates a moderate productivity of the KDC intrusions for Cu-Ni sulfide mineralization, but a rather low potential for PGE. Although the degree of primary sulfide melt PGE-enrichment is not inferior to that of many Cu-Ni-PGE deposits in the world, the economic potential of the Khudolaz Complex intrusions is significantly reduced by their small sizes. At the same time, the strong depletion of primary sulfides in Ni, Cu, Pd, and Pt during hydrothermal alteration led to the enrichment of host rocks in the external contact.

Genesis of the Neoarchean subduction‐related Taoke Ni–Cu‐(PGE) sulphide deposit in the North China Craton: Constraints from Os–S isotopes and PGE geochemistry

Archean magmatic Ni‐Cu‐[platinum‐group element (PGE)] deposits are very important and mainly associated with komatiitic rocks in intracratonic rift zones throughout the world. However, the Taoke Ni‐Cu‐(PGE) deposit in the North China Craton, which is the oldest (Neoarchean, 2,537 Ma) magmatic sulphide deposit in China, and is genetically related to the subduction zone. The intrusion is predominantly composed of a small ultramafic body (olivine websterite) and a much larger gabbroic body (gabbronorite). Sulphide mineralization is mainly associated with the ultramafic body with average grades of 0.66 wt% Ni, 0.62 wt% Cu and ~1 g/t PGE. Sulphides of the Taoke deposit are characterized by a chalcopyrite + pentlandite + pyrrhotite + millerite assemblage, which hosts variable amounts of platinum‐group minerals. Good correlations are found for Pd versus individual Ir subgroup of platinum‐group elements (IPGEs; i.e., Pd vs. Os, Ir, Ru, and Rh) and for Ir and Pd subgroup of platinum‐group elements (PPGEs; i.e., Ir vs. Pt and Pd), indicating that the PGE contents of immiscible sulphide liquids in the Taoke deposit are mainly controlled by the initial PGE concentrations in the parental magma and R factor (magma/sulphide mass ratio). Based on our calculations, the initial contents of Ir and Pd in the parental magma are estimated to be 0.13 and 5.6 ppb, respectively, suggesting that the parental magma of the Taoke deposit is PGE undepleted. The multiple sulphur isotopes can provide direct indications of the source of sulphur for the Archean intrusion. The γOs(t) values and Archean crustal sulphur (three samples have anomalous Δ33S values <−0.1‰) suggest that the addition of crustal sulphur occurred in the magma. Fractional crystallization and the addition of crustal sulphur are the most likely key factors for sulphide saturation in the Taoke mafic magma.

Trace elemental behavior in the solar nebula: Synchrotron X-ray fluorescence analyses of CM and CR chondritic iron sulfides and associated metal

We have performed a coordinated focused ion beam (FIB)-scanning and transmission electron microscopy (SEM, TEM)-electron probe microanalysis (EMPA)-synchrotron X-ray fluorescence (SXRF) microprobe study to determine phase-specific microstructural characteristics and high-resolution in situ trace element concentrations of primary pyrrhotite, pentlandite, and associated metal grains from chondrules in CM2 and CR2 carbonaceous chondrites. This work is the first of its kind to link trace element chemical and microstructural observations in chondritic sulfides in an attempt to determine formation mechanisms and conditions of primary sulfides in these meteorite groups. SXRF and TEM analyses were performed on a small number of FIB sections which act as representative samples of primary sulfides and associated metal present in pyrrhotite-pentlandite intergrowth (PPI) grains and sulfide-rimmed metal (SRM) grains.SXRF microprobe analyses allowed the concentrations of the minor and trace elements, Co, Cu, Ge, Zn, and Se to be quantified, in addition to Fe and Ni, at a spatial resolution of 2 µm. The similarity between the CM and CR PPI sulfide trace element patterns provides evidence for a common formation mechanism for this type of sulfide grain in both meteorite groups. In addition, the SRM sulfide and metal have comparable trace element patterns that indicate a genetic relationship between the two, such as sulfidization of metal. Enrichments in Ni, Co, Cu, and Se are consistent with the chalcophile/siderophile behavior of these elements. The observed depletions in Ge suggest that it may have been lost by evaporation or else was never incorporated into the metal or sulfide precursor materials. The depletion in Zn may also be attributable to evaporation, but, being partially lithophile, may also have been preferentially incorporated into silicates during chondrule formation. Trace element concentrations support crystallization from an immiscible sulfide melt in chondrules for formation of the PPI grains and sulfidization of metal for the origin of the SRM grains.

Magma mixing, degassing and late sulfide saturation: Insights into the 1976–2000 eruptive sequence at White Island, New Zealand

This paper reports new petrological and chemical data of ejecta from the 1976–2000 eruptive sequence of White Island, New Zealand. We provide evidence of mixing between shallow stored (0.7–1 km) andesitic to dacitic melts and deeper primitive mafic magma carrying high-Fo olivine, repeatedly injected throughout the sequence. Melt inclusions hosted in plagioclase, pyroxene and olivine were analyzed for major, trace and volatile elements. The low H2O contents in inclusions suggest it exsolves at depth while SO2 degassing is recorded during the rise and crystallization of the mafic magma. The higher solubility of Cl (as HCl and NaCl) delays its exsolution until shallow depth. Convection degassing is suggested to explain the large imbalance between elevated gas emissions and low amount of lava erupted. Frequent inputs of gas-rich, deeply derived magma are essential drivers for the counterflow of viscous degassed melts. Metal contents (Cu, Zn) of the residual mafic magma increases during olivine crystallization. This behavior changes during shallow differentiation of the andesitic magma, with metals leaving the silicate melt. While some metals may be lost to degassing, Cu, Ni, Ag and Au strongly partition into an immiscible sulfide melt. Saturation of such a sulfide melt phase occurs in the shallow environment, likely related to changes in redox state associated with magnetite crystallization and/or SO2 degassing.

Iron Isotope Compositions of Coexisting Sulfide and Silicate Minerals in Sudbury-Type Ores from the Jinchuan Ni-Cu Sulfide Deposit: A Perspective on Possible Core-Mantle Iron Isotope Fractionation

Many studies have shown that the average iron (Fe) isotope compositions of mantle-derived rocks, mantle peridotite and model mantle are close to those of chondrites. Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope composition. However, this is a brave assumption because nearly 90% of Fe of the Earth is in the core, where its Fe isotope composition is unknown, but it is required to construct bulk Earth Fe isotope composition. We approach the problem by assuming that the Earth’s core separation can be approximated in terms of the Sudbury-type Ni-Cu sulfide mineralization, where sulfide-saturated mafic magmas segregate into immiscible sulfide liquid and silicate liquid. Their density/buoyancy controlled stratification and solidification produced net-textured ores above massive ores and below disseminated ores. The coexisting sulfide minerals (pyrrhotite (Po) > pentlandite (Pn) > chalcopyrite (Cp)) and silicate minerals (olivine (Ol) > orthopyroxene (Opx) > clinopyroxene (Cpx)) are expected to hold messages on Fe isotope fractionation between the two liquids before their solidification. We studied the net-textured ores of the Sudbury-type Jinchuan Ni-Cu sulfide deposit. The sulfide minerals show varying δ56Fe values (−1.37–−0.74‰ (Po) < 0.09–0.56‰ (Cp) < 0.53–1.05‰ (Pn)), but silicate minerals (Ol, Opx, and Cpx) have δ56Fe values close to chondrites (δ56Fe = −0.01 ± 0.01‰). The heavy δ56Fe value (0.52–0.60‰) of serpentines may reflect Fe isotopes exchange with the coexisting pyrrhotite with light δ56Fe. We obtained an equilibrium fractionation factor of Δ56Fesilicate-sulfide ≈ 0.51‰ between reconstructed silicate liquid (δ56Fe ≈ 0.21‰) and sulfide liquid (δ56Fe ≈ −0.30‰), or Δ56Fesilicate-sulfide ≈ 0.36‰ between the weighted mean bulk-silicate minerals (δ56Fe[0.70ol,0.25opx,0.05cpx] = 0.06‰) with weighted mean bulk-sulfide minerals (δ56Fe ≈ −0.30‰). Our study indicates that significant Fe isotope fractionation does take place between silicate and sulfide liquids during the Sudbury-type sulfide mineralization. We hypothesize that significant iron isotope fractionation must have taken place during core–mantle segregation, and the bulk Earth may have lighter Fe isotope composition than chondrites although Fe isotope analysis on experimental sulfide-silicate liquids produced under the varying mantle depth conditions is needed to test our results. We advocate the importance of further research on the subject. Given the close Fe-Ni association in the magmatic mineralization and the majority of the Earth’s Ni is also in the core, we infer that Ni isotope fractionation must also have taken place during the core separation that needs attention.

Igneous Rock Associations 27. Chalcophile and Platinum Group Elements in the Columbia River Basalt Group: A Model for Flood Basalt Lavas

The Columbia River Basalt Group is the youngest and best preserved continental Large Igneous Province on Earth. The 210,000 km3 of basaltic lavas were erupted between 16.6 and 5 Ma in the Pacific Northwest, USA. The peak of the eruptions occurred over a 700,000-year period when nearly 99% of the basalts consisting of the Steens, Imnaha, Picture Gorge, Grande Ronde and Wanapum Basalts were emplaced. In this study we examined the Platinum Group Elements (PGEs) Pt and Pd, and the chalcophile elements Cu and Zn in the Columbia River Basalt Group. The presence of Pt, Pd and Cu in the compositionally primitive Lower Steens, Imnaha and Picture Gorge Basalts suggests that the Columbia River Basalt Group magma was a fertile source for these elements. The PGEs are contained mainly in sulphides in the earliest formations based on their correlation with immiscible sulphides, sulphide minerals and chalcophile elements. Grande Ronde, Wanapum and Saddle Mountains Basalts are depleted in PGEs and chalcophile elements compared to earlier formations. Sulphur was saturated in many flows and much of it probably came from assimilation of cratonic rock from a thinned lithosphere. We propose a model where the presence or absence of PGEs and chalcophile elements results primarily from the interaction between an advancing plume head and the crust/lithosphere that it encountered. The early lavas erupted from a plume that had little interaction with the crust/lithosphere and were fertile. However, as the plume head advanced northward, it assimilated crustal/lithospheric material and PGE and chalcophile elements were depleted from the magma. What little PGE and chalcophile elements remained in the compositionally evolved and depleted Grande Ronde Basalt flows mainly were controlled by substitution in basalt minerals and not available for inclusion in sulphides.

Petrology and geochemistry of the Deccan basalts from the KBH-7 borehole, Koyna Seismic Zone (Western Ghats, India): Implications for nature of crustal contamination and sulfide saturation of magma

The Deccan basalt erupted at the K-T boundary and comprises multiple flow units with spatial variations in physiography and geochemistry. The Koyna borehole-7 (KBH-7 of MoES-BGRL) of the Koyna Seismic Zone is situated in the western part of the Deccan continental flood basalt province, where the lava pile is 1251 m thick. Thirty-one samples of four flow units (named FA, FB, FC, FD; 1248.20–1110.78 m) from the lower part and another four flow units (named FV, FW, FX, FY; 202.79–42.85 m) from the upper part of the KBH-7 borehole show significant petrographic, mineral chemical and bulk-rock geochemical variations. Individual flows are characterized by higher abundances of plagioclase and clinopyroxene phenocrysts in their lower than upper parts; conversely, their upper parts are relatively enriched in magnetite and ilmenite. In addition, the flows become amygduloidal near their tops. Seven samples from the lower part of the FB flow unit have relatively higher MgO (≈ 8.94–12.9 wt%) and lower TiO2 (≈ 1.43–1.67 wt%) contents compared to samples from other flow units and are identified as low-Ti high-Mg basalt. Clinopyroxene phenocrysts from low-Ti high-Mg basalts are more magnesian (Mg-number ≈ 62.4 to 82.3) and chromium-rich, and not in equilibrium with plagioclase phenocrysts in the rocks. The magnesian-rich clinopyroxene phenocrysts were probably concentrated in the flow by flow differentiation. The trace element geochemistry indicates that the flows in the KBH-7 borehole are crustally contaminated. Extremely low Nb/Ta (<10) values of a few basaltic samples indicate contamination by the Gondwana sediments (e.g., Gondwana greywacke Nb/Ta ≈ 3.8–12.8) along with the tonalitic crust (Nb/Ta ≈ 5.59–10.2). The major and trace element geochemical character of the KBH-7 basalts can be explained by assimilation and fractional crystallization (AFC) modeling using Archean tonalitic crust and Gondwana sediments as major contaminants. AFC modeling reveals that 6.5–14.5% partial melting of the primitive mantle followed by 64–85% AFC in the crustal magma chamber accounts for the variable geochemical character of the KBH-7 basalts. Despite assimilation by crustal components, the basalts do not show any signs of sulfide supersaturation or segregation of an immiscible sulfide liquid, unlike the Nadezhdinsky formation of the Siberian trap flood basalt that produced the Noril'sk Ni-sulfide deposits. The variable concentration of chalcophile elements (Ni ≈ 11–858 ppm, Cu ≈ 32–342 ppm) or ratios like Cu/Zr (> 1) and Ni/MgO (≈ 1.9–163) of the KBH-7 basalts also indicate the absence of an earlier sulfide liquation of the magma. This may be related to the absence of crustal sulfur from the tonalitic basement rocks or the Gondwana sediments (greywacke) in conjunction with high iron content (FeO(T) ≈ 11.9–15.9 wt%) of the basaltic magma.

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

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

PGE distribution in the chromitite layers at Eastern Chrome Mine, Eastern Bushveld Complex, South Africa: A descriptive study with comparison of EPMA and LA-ICP-MS methods for detection of trace PGE in base metal sulphides

Abstract A descriptive petrographic study was carried out on the chromitites (LG-6 to UG-3a) of a single borehole from the Winterveld Chrome Mine, Eastern BC, along with analysis by both EPMA and LA-ICP-MS of the PGE contents of base metal sulphides (BMS) found within these layers. EPMA measurements were optimized to measure trace amounts of PGE by increased probe current and counting time, yielding very low detection limits for this method (12 to 38 ppm). However, analytical error proved to be very high (over ~30%) in samples that showed PGE values below 100 ppm. Also, the possibility of interference on PGE spectra by other elements, especially on Rh, is high. Therefore, these measurements can only be considered semi-quantitative indicators of elemental PGE concentration. Our findings confirmed that pentlandite of the BC chromitites contains Pd and Rh, probably in solid solution. Pt was found in discrete minerals (braggite, cooperite) throughout the sequence, and Ru, Ir and Os were found as early-stage minerals such as laurite, associated with chromite rather than sulphide. The PPGE increase upwards in the chromitite layers relative to the IPGE. An immiscible sulphide phase contributed to collection of the PGE in the chromitites of the MG-4 and above. The chromitites of the LG-6 to MG-1 show less evidence of hosting an immiscible sulphide phase, and pentlandite of these layers does not appear to host Rh, although Pd was found. Although this borehole is located only 1.5 km away from the centre of the Steelpoort Fault Zone, a possible feeder zone for the BC, large-scale disruption of the chromitite layers was not observed. An intrusive pegmatoidal pyroxenite was found to occur beneath the LG-6, and contained PGE-bearing sulphides, although the origin of this mineralization is most likely from the LG-6 chromitite itself. A sulphide vein occurring in silicate was also analysed and was found to be PGE-deficient compared to the sulphides of the chromitite layers.

Mineral Chemistry, S-Pb-O Isotopes, and S/Se Ratios of the Niubiziliang Ni-(Cu) Sulfide Deposit in North Qaidam Orogenic Belt, NW China: Constraints on the Parental Magma Composition, Evolution, and Sulfur Saturation Mechanism

The Niubiziliang Ni-(Cu) deposit is the first magmatic Ni-Cu sulfide deposit in the North Qaidam Orogenic Belt (NQOB), NW China, and plays a significant role in geological evolution, Ni-Cu mineralization, and exploration in the NQOB. Here, we report on the mineral chemistry, S-Pb-O isotopes, and S/Se ratios of the mafic-ultramafic complex, which provide insights on the parental magma, evolution, and sulfur saturation mechanism. The Niubiziliang mafic-ultramafic intrusion contains four ore blocks and about ten Ni-(Cu) ore/mineralization bodies. Olivines in Niubiziliang belong to the species of chrysolite with Fo values of 88~89, and the pyroxenes are mainly orthopyroxene (En = 79~82) and clinopyroxene (En = 44~40). The olivines and some pyroxenes likely crystallized in a magma chamber at a depth of 35.45~36.55 km at a high temperature (1289~1369 °C) and pressure (9.38~9.67 kbar), whereas the Niubiziliang complex formed at a moderate depth (8.13~8.70 km) with a temperature and pressure of 1159~1253 °C and 2.15~2.30 kbar, respectively. The parental magma was considered to be high-Mg picritic basalt with MgO and NiO contents of 14.95~16.58% and 0.053~0.068%, respectively, which indicated high-degree partial melting of the depleted mantle. The mantle-derived primary magma underwent significant fractional crystallization and crustal assimilation and contamination, which was strongly supported by S-Pb-O isotope data and S/Se ratios, resulting in sulfur saturation and sulfide immiscibility in the magma. Crustal assimilation and contamination contributed more to sulfur saturation than fractional crystallization.

Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9–90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation).

Platinum-group element geochemistry of the volcanic rocks associated with the Jaguar and Bentley Cu–Zn volcanogenic massive sulfide (VMS) deposits, Western Australia: implications for the role of chalcophile element fertility on VMS mineralization

We report whole-rock major, trace, and platinum-group element (PGE) geochemistry of volcanic rocks from the Teutonic Bore complex that hosts the Jaguar and Bentley Cu–Zn volcanogenic massive sulfide (VMS) deposits. This study aims to understand their sulfide saturation history and chalcophile element evolution during differentiation of the Jaguar and Bentley magmas, and investigate the role of chalcophile element fertility on the formation of VMS deposits. The fractionated primitive mantle–normalized trace element patterns, with negative Nb and Ti anomalies of basalts, andesites, dacites, and rhyolites from Jaguar and Bentley, are similar to each other. The trace elements and PGE show continuous variations when plotted against fractionation indices such as Yb, which can be explained by a two-stage fractional crystallization model: stage 1 Rayleigh fractionation of plagioclase + clinopyroxene + Cr-spinel, and stage 2 the fractional of plagioclase + clinopyroxene + magnetite + 0.1 wt% sulfide liquid. Dolerites, which postdate the mineralization, differ from the other rock types and require a different magma source. Andesite and basalt are the most PGE-enriched lithologies in Jaguar and Bentley. The PGE behave incompatibly in the early stage of magma differentiation at < 4 ppm Yb, whereas they abruptly decrease at > 4 ppm Yb, indicating sulfide saturation at this point. When Pd/MgO and Pd/Pt are used as chalcophile element fertility indicators, the andesite before sulfide saturation (< 4 ppm Yb) is as fertile as the magmas associated with porphyry Cu-only deposits. In contrast, the andesite after sulfide saturation and other lithologies are characterized by markedly depleted fertility similar to those of barren suites. This suggests that sulfide-undersaturated andesite, and probably basalt, may have been a significant source for Cu in the Jaguar and Bentley Cu–Zn VMS deposits. However, the Au fertility of the Jaguar and Bentley andesite must have been low and not enough to form Au-rich VMS deposits because their Pd/MgO and Pd/Pt values are 5–10 times lower than those of andesite and dacite from the modern Au-rich seafloor massive sulfide deposits. This can be explained if ore formation occurred shortly after sulfide saturation. If the amount of sulfide melt to precipitate was small, Au, with its high partition coefficient into immiscible sulfide melts, would have been largely stripped from the silicate melt, whereas Cu, with its lower partition coefficient, would be little affected. Our study shows that chalcophile element fertility may play an important role in the formation of VMS deposits, especially in controlling the Au contents of the ore, if the magmatic-hydrothermal component is the dominant source for metals in VMS systems.