Sulfide Saturation Research Articles

Sulfide Dissolution on the Nickel Isotopic Composition of Basaltic Rocks

AbstractGlobal basaltic rocks show large variations in nickel (Ni) isotopic compositions relative to peridotites. The origin of this difference remains unknown. We analyzed Ni isotopic compositions of mineral separates from seven Beiyan peridotite xenoliths and 16 nephelinites from eastern China. Spinel is isotopically heavier, and clinopyroxene is systemically lighter than coexisting olivine and orthopyroxene. An ionic model predicts that olivine, orthopyroxene, and spinel are in Ni isotopic equilibrium, whereas clinopyroxene of metasomatic origin is out of equilibrium. The nephelinites have higher Fe3+/ΣFe but lower δ60/58Ni values than the peridotites. Mantle silicate melting likely leads to enrichment of heavy Ni isotopes in melts, and, importantly, it cannot explain the negative correlation between δ60/58Ni and Fe3+/ΣFe of the nephelinites. Therefore, the light Ni isotopic signature requires the involvement of a low‐δ60/58Ni component. Sulfide is a minor Ni‐rich component in the mantle and is isotopically lighter than the silicates. The relative proportion of Ni from sulfides vs. that from silicates varies in mantle‐derived magmas, depending on the sulfur content at sulfide saturation and melting degree. Thus, low‐degree melts with high abundance of dissolved sulfides can be variably enriched in light Ni isotopes. We propose that enhanced sulfide dissolution at high oxygen fugacity is a key reason for light Ni isotopic compositions in the nephelinites. This sulfide dissolution model may also be responsible for producing the Ni isotopic variation in global mafic lithologies. When the highly oxidized, low‐degree melts react with the peridotitic region, they can impart light Ni isotopic signatures to mantle rocks.

Contrasting oxidation states of low-Ti and high-Ti magmas control Ni-Cu sulfide and Fe-Ti oxide mineralization in Emeishan Large Igneous Province

Magmatic Ni-Cu-(PGE) sulfide and Fe-Ti oxide deposits in plume-related large igneous provinces (LIPs) are commonly related to low-Ti and high-Ti series magmas, respectively, but the major factors that control such a relationship of metallogenic types and magma compositions are unclear. Magma fO2 controls sulfur status and relative timing of Fe-Ti oxide saturation in mafic magmas, which may help clarify this issue. Taking the Emeishan LIP as a case, we calculated the magma fO2 of the high-Ti and low-Ti picrites based on the olivine-spinel oxygen barometer, and the partitioning of V in olivine. The obtained fO2 of the high-Ti series magma (FMQ + 1.1 to FMQ + 2.6) is higher than that of the low-Ti series magma (FMQ − 0.5 to FMQ + 0.5). The magma fO2 of the high-Ti and low-Ti picrites containing Fo > 90 olivine reveals that the mantle source of the high-Ti series is likely more oxidized than that of the low-Ti series. The results using the ‘lambda REE’ approach show that the high-Ti series may have been derived from relatively oxidized mantle with garnet pyroxenite component. The S contents at sulfide saturation (SCSS) of the two series magmas were calculated based on liquid compositions obtained from the alphaMelts modeling, and the results show that the low-Ti series magma could easily attain the sulfide saturation as it has low fO2 with S being dominantly as S2−. In contrast, the oxidized high-Ti series magma is difficult to attain the sulfide saturation, but could crystallize Fe-Ti oxides at magma MgO content of ∼7.0 wt.%. Thus, contrasting magma fO2 of low-Ti and high-Ti series in plume-related LIPs may play an important role in producing two different styles of metallogeny.

Genesis of Sublayer, Footwall Breccia, and Associated Ni-Cu-Platinum Group Element Mineralization in the Sudbury Igneous Complex

Abstract The Ni-Cu-platinum group element (PGE) sulfide mineralization in the 1.85 Ga Sudbury impact structure occurs in two discrete environments: (1) mineralization along or near the basal contact, including disseminated to semimassive mineralization in an inclusion-rich sublayer, semimassive to fragmental mineralization in the underlying anatectic footwall breccia, and veins and disseminations in the underlying pseudotachylitic Sudbury breccia, and (2) disseminated to semimassive mineralization in the inclusion-bearing quartz diorite phase of radial and concentric offset dikes that extend up to 20 km from the basal contact. The sublayer, inclusion quartz diorite, and footwall breccia all contain abundant mafic-ultramafic inclusions—local xenoliths derived from nearby country rocks, exotic xenoliths derived from unexposed upper-middle crustal target rocks and, locally, anteliths derived from an olivine melanoritic early border phase of the Sudbury Igneous Complex—but the quartz diorite margins of offset dikes contain very few local inclusions and no exotic ultramafic inclusions. The similar inclusion populations indicate a genetic relationship between the sublayer and inclusion quartz diorite and interaction between the sublayer and footwall breccia. But the sublayer is characterized by a cumulate noritic matrix, the inclusion quartz diorite by a noncumulate quartz dioritic matrix, and the footwall breccia by an anatectic felsic-intermediate matrix. The overlying main mass norite is very homogeneous in terms of Hf isotopes, indicating that the impact melt sheet was well mixed, but ores, sublayer, inclusion quartz diorite, and to a lesser degree overlying main mass norites vary widely in their Pb-S-(Os) isotope compositions. The majority of mafic-ultramafic inclusions, except for anteliths, contain no sulfide and exhibit no signature of Ni-Cu-PGE depletion caused by prior sulfide saturation, which indicates that the association between mafic-ultramafic inclusions and Ni-Cu-PGE mineralization is attributable primarily to the refractory nature of the inclusions and to a lesser degree their similar hydrodynamic behavior as the sulfide melt and, secondarily, to derivation of sulfides from the same sources as the inclusions. The very large Sudbury impact event volatilized much of the Pb-S-Zn-Cd-Se-Bi and significant amounts of Sb-Ag-Cu-Au-As from the target rocks. It generated large amounts of superheated impact melt and only minor fragmental debris during the compression and excavation stage, which is when the inclusion- and sulfide-poor marginal phase of the offset dikes was emplaced. Large amounts of debris, including sulfide-bearing Huronian basalts, Nipissing and East Bull Lake intrusions, and Archean mafic gneisses, were generated during isostatic rebound, formation of a central uplift, and collapse of the central uplift and crater walls, which is when the inclusion- and sulfide-bearing internal phase of the offset dikes was emplaced. Convective and gravity flow aided horizontal transport of residual exotic inclusions, local inclusions, and sulfide xenomelts into embayments and funnels to form the protosublayer. Olivine-saturated melts, generated by thermomechanical erosion of local olivine-bearing country rocks, locally crystallized an olivine melanoritic early border phase of the main mass, which was disrupted and preserved in the funnels of some North Range offset dikes. Continued thermomechanical erosion of country rocks enlarged dike funnels and exploited other fractures to generate footwall embayments and significant geochemical and Pb-S-(Os) isotope heterogeneities. As the rate of thermomechanical erosion decreased and the rate of heat conducted into the footwall rocks increased, contact ores and some offset ores fractionally crystallized to form residual melts that generated footwall vein systems.

The Paleo-Mesoarchaean Gondpipri Mafic-Ultramafic Intrusions, Western Bastar Archaean Craton, Central India: Insights from Bulk-Rock Geochemistry and Sm-Nd and S Isotope Studies on the Formation of Ni-Cu-PGE Mineralization

Abstract Magmatic Ni-Cu-platinum group element (PGE)-Te mineralization in the Gondpipri mafic-ultramafic layered intrusion of ca. 3323 ± 74 Ma age, western Bastar craton, central India, is one of the most prospective exploration targets for magmatic sulfides in India. The Gondpipri layered intrusion is divided into two distinct groups of rocks based on their mineralization potential, which include (1) mineralized layered gabbro and pyroxenite and (2) a barren olivine gabbro intrusion. The host rocks show Cu + Ni concentrations up to 5,000 ppm with a Cu/Ni ratio <1 and all PGE values between 0.1 and 1.1 ppm. Mineralization occurs in two modes: type I mineralization occurring as blebs, specks, and dissemination and type II mineralization occurring as stringers and minor veins. The geochemical data suggest that the parental magma of the host rock was generated at depths between spinel and garnet peridotite mantle source regions and subsequently modified by assimilation fractional crystallization (AFC) of the continental crust. High large ion lithophile elements, Th/Yb ratios of the studied rocks, and Sm-Nd isotope studies are consistent with a depleted mantle source. The geochemical proxies such as Th versus Ba/Th and (Ta/La)PM versus (Hf/Sm)PM and higher Sr/Nd (2.21–82.58) ratios indicate involvement of fluid-related subduction metasomatism and enrichment processes in an island-arc tectonic setting. Mineral assemblages and textural relationships between platinum group minerals (PGMs) and base metal sulfides suggest that sulfide-silicate liquid immiscibility was brought about by the precipitation of magnetite/Cr magnetite resulting in sulfide saturation in the melt by decreasing S solubility. Sulfur isotope compositions (δ34S: 1.61–3.30‰) and Sm-Nd geochemistry suggest that the sulfur was added in the tholeiitic magma by magmatic process. Crustal contamination played a significant role in sulfide saturation and in bringing about PGE and Te, As, Bi, Sb, Se (TABS) mineralization. PGM-NiTeBi developed at relatively low temperatures, whereas moncheite (PtPd)Te2 and merenskyite (PdTe) were formed at 650°C. The identification of Ni-Cu-PGM-Te in the margin of the western Bastar craton boosts deeper subsurface exploration.

Role of magma differentiation depth in controlling the Au grade of giant porphyry deposits

Porphyry deposits are the world's most important source of Cu and a major source of Au. It has been recognized that low Au grades generally characterize porphyry deposits in thick continental arcs, like the Andes, where the magmas are likely to differentiate at depth. In contrast, Au-rich porphyries are mainly found in thin island arcs, for example, those of the southwest Pacific, or associated with short extensional periods in continental arcs, where the magmas are likely to experience relatively shallow differentiation. However, the key factors that control this difference remain debated. This study shows that the Au grade of giant porphyry deposits and La/Yb ratios of the ore-associated suites are negatively correlated (r=∼0.7, p=10−7). We attribute the negative correlation to be mainly due to varying sulfide saturation histories, modulated by the depth of magma differentiation.Magmas differentiating in deep crustal reservoirs are likely to reach sulfide saturation early due to high pressure and early depletion of FeO (calc-alkaline trend). Early sulfide saturation causes most Au to be held in cumulus sulfides, making it unavailable to enter ore-forming fluid released in the upper crust, resulting in Au-poor porphyry systems. In contrast, magmas evolving in shallow reservoirs are likely to experience late sulfide saturation because of the high sulfur solubility induced by low pressure and the high FeO content of the melts (tholeiitic trend). Late sulfide saturation enhances the potential of a magmatic system to form Au-rich porphyry deposits due to the high Au content of the magma at voluminous fluid saturation and efficient Au transfer from melt to the ore-forming fluids via sulfide-fluid interaction. The link between average magma differentiation depth and Au content is supported by platinum-group element geochemistry of the porphyry ore-forming suites, which shows that the magmas differentiating at shallower depths (lower La/Yb) reach sulfide saturation later and therefore have higher Au concentrations than those that differentiate at deeper levels (higher La/Yb). Numerical models for ore-associated magmas with different sulfide saturation histories indicate that variations in the Au grades of giant porphyry deposits can be explained by the variations in the timing of sulfide saturation. Based on the results, we propose that Au concentration in the magma, modulated by average magma differentiation depth and sulfide saturation history, is one of the critical factors controlling the Au grade of giant porphyry Cu deposits.

Genesis of the Xiarihamu Magmatic Ni-Co Sulfide Deposit in the East Kunlun Orogen, Northern Tibetan Plateau: In Situ Oxygen Isotope and Geochemical Perspectives

Abstract Convergent margins are commonly deemed to have poor potential to form giant magmatic Ni-Cu-(platinum group element) sulfide deposits, due to the possibly low lithospheric geothermal gradient and the small basaltic magma fluxes. This view, however, is challenged by the recent discovery of the Xiarihamu Ni-Co deposit in the East Kunlun orogen (northern Tibetan Plateau), which contains ~157 million metric tons (Mt) of sulfide ores at 0.65% Ni, 0.013% Co, and 0.14% Cu, making it the world’s 20th largest magmatic sulfide deposit. To investigate the mantle source nature, magmatic processes, and their genetic linkage to Ni-Co mineralization in orogenic belts, we presented new in situ orthopyroxene oxygen isotope and pyroxene trace element data from the mafic-ultramafic cumulate rocks at Xiarihamu. Our results show that orthopyroxene crystals have much higher δ18O (6.40–8.58‰) than those in unmodified mantle peridotite (5.72 ± 0.14‰). Orthopyroxene grains in the primitive harzburgite cumulates have the lowest δ18O values (6.40–7.71‰) and the highest Mg# contents (88.7–83.9). From the harzburgite to the evolved lithologies (e.g., websterite and gabbronorite), the δ18O values of orthopyroxene increase, whereas the Mg# values, Ni contents, and heavy/light rare earth element (HREE/LREE) ratios of orthopyroxene decrease, implying crustal contamination during magma fractionation. The δ18O variations and sharp increases in V/Sc and LREE/HREE ratios from the primitive orthopyroxene crystals (with the highest Ni and Mg#) to the evolved ones (with low Ni and Mg#) indicate that crustal contamination played a critical role in sulfide saturation. The absences of any trend in orthopyroxene δ18O values versus Mg# contents, Ni concentrations, and La/Yb ratios toward unmodified mantle values suggest an enriched mantle source. On the mid-ocean ridge basalt (pyroxene)-normalized multielement plots, both orthopyroxene, clinopyroxene, and calculated parental magma show LREE and large ion lithophile element (LILE) enrichments and depletions in Nb-(Ta), Sr, and Zr-(Hf). The predicted primitive magma trace element compositions and the high-δ18O characteristics of the high-Mg# orthopyroxene grains suggest that the Xiarihamu primitive magma was partly originated from the pyroxenite mantle metasomatized by subducted slab-derived melts and fluids. Given the lower solidus temperature of clinopyroxene relative to olivine, pyroxenites in the mantle wedge may generate voluminous high-Ni magmas during partial melting. The high-Ni magmas are likely conducive to magmatic Ni-Cu sulfide mineralization in orogenic belts.

Sulfide and sulfate saturation of dacitic melts as a function of oxygen fugacity

Sulfur is a key element in terrestrial magmatic processes yet its geochemical behavior remains one of the most difficult to model due to its heterovalent chemistry. The maximum amount of sulfur a silicate melt can dissolve before saturating with sulfide (e.g., pyrrhotite) or sulfate (e.g., anhydrite) changes with the redox state of the system and has important implications for the sulfur budget of a magmatic system. Several empirical models have been developed to predict the sulfur content of a silicate melt at either sulfide (under reducing conditions) or sulfate (under oxidizing conditions) saturation, but only one model existed that systematically assessed how the sulfur content of a basaltic melt changes as a function of oxygen fugacity (fO2) across the transition from sulfide- to sulfate-dominated conditions. The applicability of that model to intermediate and felsic melts rests on the assumption that changes in melt composition do not affect how sulfide or sulfate dissolves in the melt. Here, we report new experimental data that constrain the sulfur concentration at sulfide saturation (SCSS) and the sulfur concentration at anhydrite saturation (SCAS) in a dacitic melt as a function of fO2. The experiments were conducted using a H2O-saturated natural dacitic melt at 1000 °C, 300 MPa, and at log fO2 varying over four orders of magnitude encompassing the sulfide-sulfate transition (log fO2 = ΔFMQ−0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.48, ΔFMQ+1.54, ΔFMQ+1.75, ΔFMQ+2.08 and ΔFMQ+3.3). New SCSS and SCAS data and modeling for dacitic melts reveals that the sulfide-sulfate transition occurs at ΔFMQ+1.81 ± 0.56, defined by the following equations to predict the sulfur content of intermediate to evolved silicate melts as a function of fO2:SCSSdacitic = [S2−] (1 + 10(2.00ΔFMQ – 3.05))SCASdacitic = [S6+] (1 + e(1.26 – 2.00ΔFMQ))The results presented here demonstrate that the basaltic-derived SCSS-SCAS model is not appropriate for dacitic melts and that the sulfide-sulfate transition is shifted to higher fO2 in more evolved silicate melts. Implications include the stability of sulfides to higher fO2 in more evolved silicate melts and the potential for a narrower transition from a sulfide- to a sulfate-dominated melt than that predicted by thermodynamics.

Refertilization of depleted mantle peridotite in the Nagaland–Manipur ophiolite, north‐east India: Constraints from PGE, mineral, and whole‐rock geochemistry

This paper discusses whole‐rock geochemistry, mineral chemistry, and platinum group element (PGE) systematics of depleted mantle rocks (harzburgite and dunite) from the northern part of Nagaland–Manipur Ophiolite (NMO), north‐east India, to comprehend their source features, fractionation behaviour of PGE during magmatic evolution, and its tectonic origin. The studied ultramafic rocks are characterized by a low concentration of CaO (0.57–0.71 wt%), Al2O3 (0.18–0.92 wt%) with ∑REE of 1.135–2.702 ppm and high concentrations of MgO (38.70–44.21 wt%), Cr (1,843–4,572 ppm), and Ni (894–4,138 ppm). They show U‐shaped REE patterns [LREE and HREE enrichment (La/Sm)N = 1.85–4.11, (Dy/Yb)N = 0.51–0.85]. Olivine ranges Fo 88.18 to Fo92.23, whereas Cpx and Opx range En44.84 to En47.89 and En86.37 to En93.37 respectively. The chrome spinel Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)] are 0.47–0.83 and 0.31–0.60, respectively, which indicates recrystallization from a boninitic magma in a Supra‐Subduction Zone setting. Conventional thermometry indicates the equilibration temperatures of the dunite sample yielded high temperatures of ~850°C, suggesting their formation due to later interaction with high‐temperature percolating melts. The PGE contents in harzburgite are low (125.6–142.8 ppb) as compared to the dunite (248–360 ppb). They have high PPGE/IPGE and negative Pt* (Pt/Pt* = 0.73) anomaly, which is characteristic of re‐entry of PPGE into the system via reaction with percolating basaltic melt in the mantle wedge. Significantly higher concentration of PPGEs than IPGEs in the samples, indicating recrystallization of PPGEs with early sulphide fractionation. The presence of significant Rh and Pd enhancements relative to Pt in all samples suggests that Pt was removed during PGE fractionation. This could be one of the reasons for both harzburgite and dunite's sulphide undersaturation. PGE distribution in NMO ultramafic rocks was therefore validated as being governed by sulphide saturation in parental magma and altered not only by partial melting but also by fractionation during their production in the Supra‐Subduction Zone environment.

Platinum-Group Element Geochemistry and Magma Evolution of the Mount Hagen (Papua New Guinea) Magmatic System

Abstract Mount Hagen is a mid-Pleistocene stratovolcano located in the Papua New Guinea Highlands. It was selected for this study because of (1) its close location to several giant Cu–Au porphyry deposits in the region; (2) its high-K calc-alkaline character (absarokite–shoshonite) and (3) its wide compositional range (2 to 12 wt % MgO). Whole-rock geochemistry, petrography and QEMSCAN observations are consistent with fractional crystallisation of the evolving magma being controlled by the phases now represented by the megacrysts in the lavas: olivine and clinopyroxene at high MgO and hornblende and plagioclase at low MgO. However, the role of plagioclase is enigmatic. Although it is an abundant megacryst phase in the low-MgO samples, plagioclase has little or no influence on fractional crystallisation, suggesting that it may be an antecryst. The appearance of plagioclase megacrysts does not slow the rate of increase in Al2O3 or Sr with decreasing sample MgO and the low-MgO samples do not have Eu anomalies. At ~5.5 wt % MgO, there are significant changes in the system that are inconsistent with simple fractional crystallisation. These changes include hornblende and plagioclase replacing olivine and clinopyroxene as the principal megacryst phases, a sharp rise in platinum-group elements (PGE) concentrations, the brief reappearance of high Fo olivine megacrysts and reversed zoning in the plagioclase megacrysts. These changes are interpreted to have resulted from a new pulse of magma entering the system, with higher water, MgO, SiO2 and PGE concentrations than the original parent magma. Scatter in the PGE data is attributed to the presence of micronuggets in all samples, including the most mafic samples. We conclude that the magma system became sulphide saturated during an early stage in its fractionation history, probably before leaving the crust to deep crustal magma chamber, and then became undersaturated as a consequence of decreasing pressure as magma ascended into a mid-crustal magma chamber. The early saturation episode had a pronounced influence on PGE concentration but had little affect Cu and Au, due to their lower partition coefficient into sulphides. The magma became sulphide saturated again in the crustal magma chamber at ~8.5 wt % MgO, this time affecting all chalcophile elements. Given the early episodes of sulphide saturation, depletion in Au in the Mount Hagen magma system and the absence of a known porphyry system, it is unlikely that Mount Hagen produced economic porphyry mineralisation.

A thermodynamic model for sulfur content at sulfide saturation (SCSS) in hydrous silicate melts: With implications for arc magma genesis and sulfur recycling

Modeling the effect of H2O on the “sulfur content at sulfide saturation” (SCSS) in silicate melt is essential for the estimation of SCSS in both arc magmas and slab melts. Here we present a thermodynamic approach, in which, SCSS in hydrous silicate melt has been modeled as a combination of S dissolving as S2− and HS−/H2S, based on the sulfide capacity (CS2−) and the hydro-sulfide capacity (CHS), respectively. By adopting the thermodynamic framework of O’Neill and Mavrogenes (2002), S dissolution as HS−/H2S can be modeled in an analogous fashion to that for modeling S2− in anhydrous melt with the relation: lnHSSCSS=-ΔG°Fes-FeO/RT+lnCHS-lnaFeOmelt+lnaFeSsulfide. To obtain an expression for CHS, we compiled published experimental data on SCSS in hydrous silicate melts covering a PT range of 0.15–3 GPa and 785–1600 °C, and melt H2O contents of ∼1–13 wt%. While the contribution of S dissolving as S2− in basaltic and andesitic melts can be calculated based on the updated SCSS model for anhydrous basic melt from O’Neill (2021), S2− is considered negligible in rhyolitic and dacitic melts, i.e., when melt FeO content <∼5 wt%. We propose an expression for CHS as a function of temperature, mole fractions of cations for the normalized anhydrous melt composition, ln(XOH + XH2O), and a Na + K-Al term for alkaline-enriched rhyolitic melts. The coefficients for cation mole fractions are adopted from the expression for CS2− from O’Neill (2021); the coefficients for the 1/T and Na + K-Al terms are obtained from regression. The reproducibility of compiled SCSS values is ±50% for hydrous rhyolitic and dacitic melts, and ±10–30% for hydrous basaltic and andesitic melts, representing a significant improvement in comparison to previous models.Our model produces SCSS values for the primitive arc magmas compiled by Ruscitto et al. (2012), that are in most cases higher than the measured S contents, implying sulfide under-saturated conditions during mantle wedge melting. The contribution of H2S dissolution to the calculated SCSS values varies in a range of 70–1234 ppm, which increases with the increase of H2O content (0.3–6.2 wt%). H2S dissolution therefore contributes to the higher S content in arc basalt compared to MORB, an observation which is further corroborated by the positive correlation between H2O and S contents in primitive arc magmas. Applying our current model to experimentally produced sediment melts spanning a PT range of 690–1050 °C and 2.5–4.5 GPa, demonstrates that sediment melts, especially those of intermediate supercritical character with >25 wt% H2O and peralkaline in composition, can have high SCSS values as a result of H2S dissolution, and act as the transfer medium for S recycling between the slab and mantle wedge under reduced conditions.

Complex Effects of Assimilation on Sulfide Saturation Revealed by Modeling with the Magma Chamber Simulator: A Case Study on the Duluth Complex, Minnesota, USA

Abstract Wall-rock assimilation can cause effective sulfide saturation in magmas and lead to the formation of base and precious metal sulfide deposits. Detailed assessments of how assimilation affects the sulfur content at sulfide saturation (SCSS) in magmas have been scarce because of the lack of suitable thermodynamic modeling tools. The Magma Chamber Simulator (MCS) is the first geochemical modeling software that accounts for thermodynamic wall-rock phase equilibrium in open magmatic systems experiencing recharge-assimilation-fractional crystallization. We used the MCS to model SCSS in a magmatic system corresponding to the parental melt of the Partridge River intrusion of the Duluth Complex, Minnesota. This intrusion hosts several Cu-Ni deposits in troctolitic and noritic rocks, which both show evidence of assimilation of the adjacent Virginia Formation black shale. Our simulations show that the dominantly troctolitic rocks can form via fractional crystallization if the parental melt is hydrous (≥ 1 wt % H2O), while gabbroic rocks dominate when the parental melt is H2O poor (≤ 0.14 wt % H2O). Formation of norite from the hydrous parental melt requires ~20–30% of selective assimilation of black shale partial melts or bulk assimilation of stoped blocks. In the fractional crystallization simulations, increasing the H2O content of the parental melt lowers SCSS. In the hydrous fractional crystallization scenarios, SCSS is lowered further by the depletion of FeO from the residual melt, owing to enhanced olivine stability. In the assimilation simulations, the residual melt in the magma subsystem becomes enriched in SiO2, Al2O3, K2O, and H2O with simultaneous depletion in FeO, MgO, CaO, and Na2O. These compositional changes promote sulfide saturation—an effect that is more pronounced in selective rather than in bulk assimilation scenarios. Trace element models, used as a proxy for the efficiency of sulfur assimilation, show that sulfur should behave as an incompatible element (DWR (S) ≤ 1) to wall rock in the selective assimilation simulations, i.e., enriched in early-assimilated wall-rock fluids and/or partial melts, in order to fulfill the natural sulfur isotope criteria of the Duluth Complex. Bulk assimilation may also be efficient enough to modify the sulfur isotope composition, but it requires a large amount of crystallization in the magma and is, hence, considered less likely to be the main process for sulfur assimilation. If wall-rock sulfur is effectively transported to the magma, in situ precipitation of sulfides without notable subsequent upgrading by dynamic processes could produce the sulfide grade of an average Cu-Ni deposit in the Partridge River intrusion.

Deep to Shallow Sulfide Saturation at Nisyros Active Volcano

AbstractWe report new petrographic and geochemical observations on magmatic sulfides occurring in different enclave types, including hornblende and/or clinopyroxene‐rich cumulates, and in the host lavas, in Nisyros (South Aegean volcanic arc). We discuss our findings in the context of pre‐existing petrological and thermobarometry studies. Our results indicate that sulfides become less abundant and Cu‐richer (Cu median values) with magma differentiation at progressively lower pressure/P‐temperature/T‐depth conditions. Starting with high T‐P sulfide‐free pillow lavas and wehrilitic cumulates still representing a sulfide undersaturated system, passing to high T and lower P, deep‐forming sulfide‐rich and hornblende‐rich gabbroic enclaves (SiO2 = 53–55 wt.%, 4–5 × 10−5 area %, Cu = 260 μg/g) at the base of the crust (∼25–30 km), then to clinopyroxene‐rich gabbroic micro‐cumulates (2–3 × 10−5 area %, Cu = 570 μg/g) forming in shallower crustal levels (∼10 km), then to more evolved sulfide‐poor hybrid enclaves (SiO2 = 56–70 wt.%, 1 × 10−5 area %, Cu = 602 μg/g), and finally to even more sulfide‐poor rhyodacitic host lavas (SiO2 = 66–76 wt.%, &lt;0.5 × 10−5 area %, Cu = 6.4 wt.%) differentiating at even shallower crustal levels (∼7 km). Sulfide‐free quenched basaltic andesitic enclaves differentiating near surface levels, carry no textural evidence of pre‐existing magmatic sulfides suggesting that the system returned to a sulfide‐undersaturated state. Finally, we point out two important processes for sulfide evolution, a reaction replacement of clinopyroxene by amphibole observed in the deep‐forming hornblende‐rich gabbroic enclaves triggering the onset of sulfide saturation, and an increased mafic input followed by magma mingling and enclave disaggregation leading to sulfide dissolution and Cu‐enrichment of the magmas.

Distribution of Te, As, Bi, Sb, and Se in Mid-Ocean Ridge Basalt and Komatiites and in Picrites and Basalts from Large Igneous Provinces: Implications for the Formation of Magmatic Ni-Cu-Platinum Group Element Deposits

Abstract In magmatic nickel-copper-platinum-group element (PGE) deposits, the PGEs are found both in solid solution in base metal sulfides and as platinum group minerals (PGMs). Apart from S, the most common elements that the PGEs combine with to form PGMs are Te, As, Bi, Sb, and Sn (TABS). Whether the TABS play a role in collecting the PGEs or simply partition into the sulfide liquid along with the PGEs and later combine with PGEs when the sulfide phase becomes saturated with PGMs is not currently clear. This is in part because the concentrations of TABS in the magmas (picrites and basalts from large igneous province and komatiites) that form these types of deposits are not well established, and hence it is not evident whether the magmas contain sufficient TABS to control PGEs. In order to establish the concentrations of Te, As, Bi, Sb, and Se (TABS+) in these rock types and to document the processes affecting these concentrations, we have determined TABS+ concentrations in komatiites, in mid-ocean ridge basalt, and in picrites and basalts from large igneous provinces. Using TABS+ mantle-normalized diagrams, the affects of different mantle sources, crystal fractionation, crustal assimilation, degassing, and alteration are considered. We estimate the concentrations of TABS+ in komatiites to be approximately twice primitive mantle values. In picrites the concentrations vary: approximately 10 times primitive mantle values for As and Sb and decreasing through Bi to Te from 7 to 2 times primitive mantle. Assimilation of S-bearing sedimentary rocks is thought to be important in triggering sulfide saturation leading to the formation of Ni-Cu-PGE deposits. Assimilation of such sediments would enrich the magma in Th over Nb and in As, Sb, and Bi. Evidence of assimilation in the form of TABS and Th enrichment is clear in the PGE reef deposits of the Bushveld and Stillwater Complexes, but the deposits do not contain sufficient TABS to control the PGEs. This is also true in the Norilsk-Talnakh Ni-Cu-PGE deposits. However, at Norilsk degassing of the magma has resulted in the loss of TABS, which results in negative As, Bi, Se, and Te anomalies on primitive mantle-normalized plots.

Mechanism of organic matter assimilation and its role in sulfide saturation of oxidized magmatic ore-forming system: insights from C-S-Sr-Nd isotopes of the Tulaergen deposit in NW China

Mechanism of organic matter assimilation and its role in sulfide saturation of oxidized magmatic ore-forming system: insights from C-S-Sr-Nd isotopes of the Tulaergen deposit in NW China

Sulfide enrichment along igneous layer boundaries in the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge

Reactive porous or focused melt flows are common in crystal mushes of mid-ocean ridge magma reservoirs. Although they exert significant control on mid-ocean ridge magmatic differentiation, their role in metal transport between the mantle and the ocean floor remains poorly constrained. Here we aim to improve such knowledge for oceanic crust formed at slow-spreading centers (approximately half of present-day oceanic crust), by focusing on specific igneous features where sulfides are concentrated. International Ocean Discovery Program (IODP) Expedition 360 drilled Hole U1473A 789 m into the lower crust of the Atlantis Bank oceanic core complex, located at the Southwest Indian Ridge. Coarse-grained (5–30 mm) olivine gabbro prevailed throughout the hole, ranging locally from fine- (<1 mm), to very coarse-grained (>30 mm). We studied three distinct intervals of igneous grain size layering at 109.5–110.8, 158.0–158.3, and 593.0–594.4 meters below seafloor to understand the distribution of sulfides. We found that the layer boundaries between the fine- and coarse-grained gabbro were enriched in sulfides and chalcophile elements. On average, sulfide grains throughout the layering were composed of pyrrhotite (81 vol.%; Fe1-xS), chalcopyrite (16 vol.%; CuFeS2), and pentlandite (3 vol.%; [Ni,Fe,Co]9S8), which reflect paragenesis of magmatic origin. The sulfides were most commonly associated with Fe-Ti oxides (titanomagnetites and ilmenites), amphiboles, and apatites located at the interstitial positions between clinopyroxene, plagioclase, and olivine. Pentlandite exsolution textures in pyrrhotite indicate that the sulfides formed from high-temperature sulfide liquid separated from mafic magma that exsolved upon cooling. The relatively homogenous phase proportion within sulfides along with their chemical and isotopic compositions throughout the studied intervals further support the magmatic origin of sulfide enrichment at the layer boundaries. The studied magmatic layers were likely formed as a result of intrusion of more primitive magma (fine-grained gabbro) into the former crystal mush (coarse-grained gabbro). Sulfides from the coarse-grained gabbros are Ir-Platinum Group Element-rich (PGE; i.e., Ir, Os, Ru) but those from the fine-grained gabbros are Pd-PGE-rich (i.e., Pd, Pt, Rh). Notably, the sulfides from the layer boundaries are also enriched in Pd-PGEs, and therefore elevated sulfide contents at the boundaries were likely related to the new intruding melt. Because S concentration at sulfide saturation level is dependent on the Fe content of the melt, sulfide crystallization may have been caused by FeO loss, both via crystallization of late-precipitating oxides at the boundaries, and by exchange of Fe and Mg between melt and Fe-bearing silicates (olivine and clinopyroxene). The increased precipitation of sulfide grains at the layer boundaries might be widespread in the lower oceanic crust, as also observed in the Semail ophiolite and along the Mid-Atlantic Ridge. Therefore, this process might affect the metal budget of the global lower oceanic crust. We estimate that up to ∼20% of the Cu, ∼8% of the S, and ∼84% of the Pb of the oceanic crust inventory is accumulated at the layer boundaries only from the interaction between crystal mush and new magma.

Petrogenesis of the Paleoproterozoic Näränkävaara layered intrusion, northern Finland, Part I: The northern peridotites and their relationships with the layered series and recharge events

The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion includes a 1.5–2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic layered series, the latter with two peridotitic reversals related to magma recharge. In addition, a series of poorly known elongate poikilitic harzburgitic intrusions (the northern peridotites) are found along the NE contact between the intrusion and the granite-gneiss basement complex. We investigate new mineral and whole-rock geochemical data from the northern peridotites, with the aim of clarifying their petrogenetic relationship to the main layered body of the intrusion. The northern peridotites form a 200–400 m thick cumulate series grading from olivine orthocumulates (OC) at the northern basement complex contact to olivine-orthopyroxene heteradcumulates (HAC) towards the main intrusion body in the south. The OC show whole-rock and mineral chemical trends consistent with origin as rapidly cooled olivine-melt mixtures. The HAC have crystallized in situ from a relatively Cr- and SiO2-rich magma. Based on lithological and stratigraphical correlations, the northern peridotites are linked to the emplacement of the magma that caused the first reversal in the layered series: marginal orthocumulates were formed at the initial emplacement of a new pulse of LREE-enriched siliceous high-MgO basaltic (SHMB) magma into the Näränkävaara chamber, followed by heteradcumulate formation from a fractionating magma with added external SiO2 and fluid. Ubiquitous granite-gneiss xenoliths and felsic veins in drill core suggest assimilation may have been a local process. The northern peridotite parental magma shows undepleted metal ratios suggesting no sulfide saturation occurred prior to emplacement.

A Melt Inclusion Approach to Reconstructing Sulfur Contents and Sulfide Saturation of Primitive Basaltic Melts

A Melt Inclusion Approach to Reconstructing Sulfur Contents and Sulfide Saturation of Primitive Basaltic Melts

Petrogenesis of the Permian Luotuoshan sulfide-bearing mafic-ultramafic intrusion, Beishan Orogenic Belt, NW China: evidence from whole-rock Sr–Nd–Pb and zircon Hf isotopic geochemistry

The Permian Luotuoshan mafic-ultramafic intrusion is one of the ~280 Ma mafic-ultramafic complexes located in the Beishan Orogenic Belt at the southern margin of the Central Asian Orogenic Belt, NW China. The intrusion is predominantly composed of wehrlite, olivine clinopyroxenite, troctolite, olivine gabbro, and gabbro. Disseminated sulfides occur in wehrlite and olivine clinopyroxenite at the center of the intrusion. SHRIMP zircon U–Pb dating of a gabbro sample yielded a concordia age of 282.6 ± 2.6 Ma. The Luotuoshan mafic-ultramafic rocks are characterized by elevated εNd (282.6Ma) values from +0.42 to +6.10 and εHf (282.6Ma) values from +7.9 to +14.1, relatively low initial 87Sr/86Sr (0.703919–0.705272) and Pb isotope composition, and low ratios of TiO2/Yb, Nb/Yb, etc., suggesting the Luotuoshan magma was derived from a depleted asthenosphere mantle source. Some arc-like geochemical characteristics such as enrichment of light rare earth elements and large ion lithophile elements (Ba, Sr, Rb, etc.), pronounced negative Nb–Ta–Zr–Hf anomalies, and La/Nb, Ce/Pb, and Th/Yb ratios indicate that the mantle source was previously modified by subduction-related metasomatism. Petrographic features and mixing calculations of Sr–Nd–Hf isotopes suggest that the parental magma of the Luotuoshan intrusion underwent a moderate degree of crustal contamination (5–15%) followed by fractional crystallization process. Petrological modeling reveals that both fractional crystallization and crustal contamination may have played the important role in triggering the sulfide saturation in the Luotuoshan magma. The comparable formation age, petrographic and mineralogical characteristics, geochemistry of the parental magma and evolution process compared with intrusions in the Pobei area, indicate that the Luotuoshan intrusion may also have similar mineralization potential. This also suggests that the mineralization prospection of Permian mafic-ultramafic intrusions in the Gansu Beishan area should not be neglected, and it is worth considering that whether further exploration is worthy of being implemented in this region.

Petrogenesis and sulfide saturation in the Yueyawan Cu-Ni sulfide deposit in Eastern Tianshan, NW China

The Yueyawan mafic intrusion hosted Cu-Ni mineralization is located at the Dananhu island arc in Eastern Tianshan Orogenic belt, NW China, and is unique in its paucity of ultramafic rocks. It formed by two intrusive phases: the phase I is composed of gabbro, troctolite and olivine gabbro, and the phase II is composed of fine-grained olivine gabbro and fine-grained troctolite. The phase I troctolite yielded SMIS U-Pb age of 283.4 ± 2.0 Ma, broadly coeval with the regional Cu-Ni sulfide mineralization. The mineral components and geochemistry characteristics suggest that the phase I rocks mainly occurred plagioclase fractionation whereas the phase II rocks primarily occurred olivine fractionation. Some plagioclase grains show reverse zoning with 63.4 to 52.1 from rim to core indicating the intrusion underwent magma mixing. The olivine is FeO-rich (16.61–32.58%), and its forsterite (61.8–80.7 mol%) and Ni (358–1218 ppm) contents are relatively low. The olivine and clinopyroxene thermobarometry show that the later pulse magma has higher temperature and higher pressure than the early pulse magma. The estimated equilibrated magma was likely much highly-evolved with 8.49 wt% MgO and 11.02 wt% FeO. The sulfide δ 34S values (–0.61 to 2.87‰) are consistent with those of the Cu polymetallic deposits in Kalatag and the Se/S ration of most samples range from 125 to 180×10−6 suggest that crustal assimilation caused prominent addition of external sulfur into the magma. The olivine crystallization, crustal sulfur addition and pressure rising were likely the triggers of sulfide saturation at Yueyawan deposit.

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) &amp;gt; 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.