Magmatic Sulfide Research Articles

Chalcophile element processing beneath a continental arc stratovolcano

The chalcophile elements are important both in terms of their economic value and as potential tracers of magmatic processes at convergent margins. However, because of analytical difficulties, comprehensive datasets of chalcophile element concentrations for volcanic rocks are rare. Here, we present analyses of a near complete suite of chalcophile elements (S, Cu, Ag, Se, As, Sb, Sn, W, Mo, Pb, Bi, Tl, Zn, Ga, Co) for volcanic rock samples collected from a typical continental arc stratovolcano in southern Chile (Antuco). Enrichment in Pb, Bi, W, Tl, Sb and As relative to Parental-MORB indicates that these elements have been mobilised from the subducting slab into the sub-arc mantle wedge, in contrast to Cu and Ag. Very low Se concentrations suggest that Se, like S, was lost during co-eruptive degassing of the Antuco magmas. Previous studies on oceanic arcs have demonstrated that as higher fO2 subduction-related magmas ascend through the overlying lithosphere, magnetite fractionation may trigger sulfide fractionation during crystallisation. If such a process is extensive and has a sharp onset, this would result in a plummet in the Cu, Se and Ag contents of the residual melt. At Antuco, although a decrease in the Fe2O3(T) and TiO2 concentrations at ∼55 wt.% SiO2 (∼3 wt.% MgO) indicates magnetite fractionation, this is not associated with a corresponding drop in Cu contents. Instead, we observe a general decrease in Cu and a decrease in Cu/Ag with increasing SiO2 and decreasing MgO. Furthermore, Cu/Ag in the most primitive Antuco rocks are lower than the global MORB array, indicating that the melts were sulfide saturated at an early stage in their crustal evolution. Through modelling fractional crystallisation, we show that only a minor volume (0.5–0.6 vol.%) of fractionating sulfide is needed to produce divergent trends in Cu and Ag, as observed in the Antuco samples. Our results show that sulfide fractionation occurred from an early stage during the crustal evolution of Antuco's magmas. We infer that this was promoted by stalling in the lower crust, which for oxidised magmas at depths >20 km is within the sulfide stability field. However, elevated DyN/YbN of the Antuco magmas compared to oceanic island arc magmas provides an additional, or alternate mechanism to inducing sulfide fractionation in the lower crust prior to ascent, through initial garnet fractionation. Fractional crystallisation within this depth range meant that later magnetite fractionation had only a minor impact on the partitioning behaviour of the chalcophile elements. In contrast, arc magmas transiting thinner crust may not experience sulfide saturation until a later stage in their evolution, induced by magnetite fractionation. Our results imply that convergent margin crustal thickness, and therefore the depth range of magmatic differentiation, determines the dominant control on initial magmatic sulfide saturation and therefore the primary distribution of chalcophile elements. This implies that secondary processes are required to explain the transport and concentration of sulfides and chalcophile elements at shallower crustal levels.

Petrochemical characterization of Neoproterozoic Colomine granitoids, SE Cameroon: Implications for gold mineralization

Biotite granites and K-feldspar granites are the dominant intrusive rock types of the Colomine gold district in the Neoproterozoic terrain of SE Cameroon. The silica contents of these granitoids range from 68.93–75.39 wt% and Au contents from 0.6 to 32 ppb. They are weakly peraluminous (ASI = 1.0–1.2) I-type granitoids with high-K calc-alkaline to shoshonitic character, emplaced within a syn-collisional arc setting. They have fractionated trace element patterns (LaN/YbN: ~ 11–331) with enrichment in LREE compared to HREE and weak to strong negative Ba, Nb, Sr, P, Ti, Lu, and Eu (Eu/Eu* = 0.12–0.52) anomalies. The geochemical characteristics suggest that the granitoids formed by partial melting of crustal source rocks with some input from greywackes. Differentiation of the hydrous parent magma was characterized by fractionation of muscovite, biotite, hornblende, plagioclase, and K-feldspar with minor zircon, apatite and Fe-Ti oxide. Mafic garnet-bearing inclusions represent an early crystallization phase that was incorporated in the later felsic melt. The granitoids became emplaced into Proterozoic schist belts. Along the contacts, they became brecciated, sheared, foliated and sericitized. These “contact granites” have higher gold contents (5–32 ppb) than the fresh granitoids (0.6–3 ppb). Remobilisation of gold from early magmatic sulfide minerals by post-magmatic hydrothermal fluids enriched the shear-related granitoids. Hydrothermal alteration results to enrichment in Cu, Ni, Cr, Mo, Ga, V, Tl, and Au and depletion in LREE (e.g., La, Ce, Pr). Micro-shear and fault zones in the vicinity of the granitoids constitute favourable sites for exploration.

Minerals and Allied Natural Resources and Their Sustainable Development: Principles, Perspectives with Emphasis on the Indian Scenario (M. Deb and S.C. Sarkar)

Research Article| June 01, 2019 Minerals and Allied Natural Resources and Their Sustainable Development: Principles, Perspectives with Emphasis on the Indian Scenario (M. Deb and S.C. Sarkar) Simon M. Jowitt Simon M. Jowitt Assistant Professor, Economic Geology Department of Geoscience University of Nevada Las Vegas 4505 S. Maryland Pkwy Las Vegas, NV 89154-4010, USA May 13, 2019 Search for other works by this author on: GSW Google Scholar Economic Geology (2019) 114 (4): 807–808. https://doi.org/10.5382/econgeo.114.4.br01 Article history first online: 24 Jun 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Simon M. Jowitt; Minerals and Allied Natural Resources and Their Sustainable Development: Principles, Perspectives with Emphasis on the Indian Scenario (M. Deb and S.C. Sarkar). Economic Geology 2019;; 114 (4): 807–808. doi: https://doi.org/10.5382/econgeo.114.4.br01 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEconomic Geology Search Advanced Search Minerals and Allied Natural Resources and Their Sustainable Development: Principles, Perspectives with Emphasis on the Indian Scenario. Mihir Deb and Sanjib Chandra Sarkar. Pp. 550. Springer. 2017. Hard or softcover, USD$179.00; e-bookISBN 978-981-10-4564-6) USD$139.00. Springer recently began publishing a spate of textbooks across the sciences, including a total of 47 previously (2012 onward) or soon-to-be-published books that form the Springer Geology family. These textbooks cover a wide range of topics, some of which are very focused (e.g., isotope geology of the Noril’sk-Talnakh magmatic sulfide deposits), others of which cover a broad range of topics (e.g., the fundamentals of physical... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Microchemical and sulfur isotope constraints on the magmatic and hydrothermal evolution of the Black Swan Succession, Western Australia

The Black Swan Succession consists of an Archean bimodal dacite-komatiite association largely dominated by ultramafic cumulates hosting a number of massive and disseminated magmatic sulfide orebodies. Although it was affected by multiple alteration events, the low degree of penetrative deformation allowed the preservation of magmatic textures and stratigraphic relationships. In this context, we apply a combined multiple sulfur isotope and microchemical approach to unravel the potential of sulfides to simultaneously retain information about their magmatic and hydrothermal history. Negative Δ33S signatures in both massive (− 0.56‰) and disseminated (− 0.66‰) sulfide orebodies reflect the assimilation of photochemically derived sulfur into the komatiite magma, which attained sulfide liquid supersaturation through the assimilation of crustal material analogous to the intercalated breccia dacite hosting colloform sulfides (Δ33S = − 0.91‰). The slightly different Δ33S signatures between disseminated and massive orebodies reflect the assimilation of different proportions of crustally derived sulfur, and imply that the respective sulfide liquids segregated from distinct komatiite magma pulses. Subsequent alteration episodes imparted to the Black Swan Succession a concentric zonation consisting of a serpentinite core and talc-carbonate margins. The Black Swan disseminated orebody extends through both alteration styles without major variations in the dominant millerite-pyrite sulfide assemblage. Pyrite in both serpentinite and talc-carbonate samples exhibits a consistent trace element signature suggesting a relative immobile behavior of Ni and Co during the percolation of the talc-carbonate fluids. Conversely, variable δ34S signatures between the two alteration styles reflect mobilization of sulfur during talc-carbonate alteration. Whereas the 2‰ range of δ34S values in serpentinites can be attributed to moderate equilibrium fractionation processes, the 10‰ range of δ34S values in talc-carbonate rocks suggests that highly oxidizing fluids promoted the mobilization of sulfur as SO42−, which decreased the δ34S of the disseminated sulfides. Furthermore, sulfur isotope and microchemical signatures of euhedral pyrite from intercalated dacitic breccia indicate that this pyrite generation was deposited from a late hydrothermal event, which is characterized by variable and least negative Δ33S signatures (between − 0.20‰ and − 0.48‰) relative to all sulfide orebodies. These features suggest that the late fluids developed in a heterogeneous hydrothermal system with a minor component of sulfur recycled from the sulfide orebodies.

Magmatic sulfide formation and oxidative dissolution in the SW Okinawa Trough: A precursor to metal-bearing magmatic fluid

Abstract Magmatic sulfide formation and oxidative dissolution are effective in metal pre-enrichment and remobilization during magmatic processes, and are considered to be a prerequisite for the formation of magmatic–hydrothermal ore deposits in subduction zones. However, this sequential process and sulfide oxidation mechanism are poorly understood. In this study, we report fine-grained sulfides with variable degrees of oxidation distributed within mafic cumulates, brought into felsic magma as mafic magma intrusions in the southwestern Okinawa Trough. Thermodynamic modeling suggests that the oxygen fugacity (fO2) of primitive mafic magma is ∼FMQ + 2.5, reducing to ∼FMQ + 1 in evolved mafic magma and to ∼FMQ + 0.5 in felsic magma. This variation results from the continuous assimilation of reduced country rocks, as reflected by negative δ34SVCTD values (–0.62‰ ± 0.23‰) of magmatic sulfides and highly radiogenic Sr–Nd isotopic compositions of host dacite. Magmatic reduction and addition of crust-derived sulfur result in early formation of crystalline sulfides, dominantly pyrrhotite, enriched in Cu (0.93 ± 0.38 wt.%). A new injection of oxidized mafic magma triggers partial to complete oxidation of the earlier magmatic sulfides, ultimately transforming them into hematite–magnetite (HM) intergrowths. This means that sulfide oxidation is not directly associated with the mafic magma itself, but more likely with high-fO2 magmatic fluid exsolved from the primitive mafic magma. The oxidative dissolution process releases >90% of Cu from the primary sulfides, with this Cu preferentially moving into magmatic fluid. This indicates that metal remobilization and transfer is very efficient under HM buffer conditions, in which trisulfur S3− ions with high affinities for Au and Cu are probably the dominant sulfur species. We suggest that the periodic injection of oxidized mafic magma into continental crust containing reduced lithologies is essential for the sequential process of early sulfide formation and later oxidative dissolution in the Okinawa Trough and in other subduction zone settings with similar tectono-magmatic conditions. This process could be a precursor to the generation of metal-bearing magmatic fluid, contributing to mineralization in subduction zones.

Geology, sulfide mineralogy and petrogenesis of the Angstberg Ni-Cu-(PGE) sulfide mineralization (Lausitz Block, Bohemian Massif, Germany): A potential Ni-Cu exploration target in Central Europe?

Germany’s largest known magmatic Ni-Cu-(PGE) sulfide enrichment is associated with the gabbroic Angstberg intrusion that is one of several small-scaled, Ni-Cu sulfide-bearing intrusions in the northern Bohemian Massif (Central European Variscan Belt). The intrusions are part of a Middle Devonian to Early Carboniferous, mafic-(ultramafic) magmatic system that emplaced within Cadomian granodiorites of the Lausitz Block. The Angstberg intrusion represents an exposed sub-vertical, dike-like body (320 × 60 m in plan view) and comprises olivine-gabbronorite, olivine-gabbro, gabbro, olivine-hornblende-gabbro, and hornblende-pyroxene-gabbro. The hornblende-bearing and hornblende-free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their Ti, LREE, HREE and volatile contents. Fractionation probably took place in deeper-seated sections of the magmatic system, possibly within staging magma chamber(s). Overall enriched incompatible trace-element patterns suggest that the Angstberg lithologies are linked with intra-plate magmatism, likely associated with a transtensional setting in the course of the Variscan orogeny. The previously suggested Devonian age of the intrusion is confirmed by an 40Ar-39Ar biotite age of 373.4 ± 0.9 Ma for the olivine-gabbro. The magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine-richest parts of the olivine-gabbronorite and olivine-hornblende-gabbro lithologies. In addition, some sulfide-matrix breccias (up to 70 modal % sulfides) occur directly on the footwall contact of the intrusion to granodiorite. Both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite, and accessory PGE, Au, and Ag phases. Nickel-rich bismuthotellurides of the melonite–merenskyite solid-solution series and froodite are the most common platinum-group minerals. Disseminated sulfides are characterized by high Ni, moderate Cu, and low Pt and Pd tenors (∼4–9 wt% Ni, ∼2–6 wt% Cu, ∼200–800 ppb Pt, ∼200–1000 ppb Pd). Sulfide-matrix breccias feature comparable Ni, but lower Cu, Pt and Pd tenors (∼4–6 wt% Ni, ∼0.3–1 wt% Cu, ∼10–100 ppb Pt, ∼40–200 ppb Pd). Fractional crystallization and wall-rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the Angstberg intrusion. Sulfur isotope compositions of pyrrhotite-pentlandite-chalcopyrite assemblages (−1.4 to −0.2‰ VCDT) propose a mantle-origin of S, whereas addition of crustal S cannot be completely ruled out as the S isotope values of the sulfides overlap with those of the granodiorite (−7 to +4‰ VCDT). Olivine- and sulfide-textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine-rich crystal mush during magma rise to upper levels. The sulfide-matrix breccias also resulted from entrained sulfide melts, or from gravity-driven downward percolation of sulfide melts from stratigraphically higher, now-eroded levels. In both cases, the formation of sulfide-matrix breccias most likely was amplified by in-situ thermomechanical erosion of the granodioritic wall-rock, caused by flash-boiling of thermally dehydrating biotite. Despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern Bohemian Massif, the Angstberg intrusion, as well as other neighboring magmatic sulfide occurrences, may represent a new exploration target for Ni and Cu in Central Europe.

Geochronology, petrogenesis and tectonic implications of the newly discovered Cu–Ni sulfide-mineralized Yueyawan gabbroic complex, Kalatag district, northwestern Eastern Tianshan, NW China

The early Permian Cu–Ni sulfide-bearing mafic-ultramafic intrusions from North Xinjiang in NW China help to provide constraints on the late Paleozoic crustal growth in the Central Asian Orogenic Belt (CAOB) and improve the understanding of the genesis of magmatic sulfide mineralization in orogenic settings and the identification of future prospecting targets. The Cu–Ni sulfide-mineralized Yueyawan gabbroic complex in Kalatag represents a new breakthrough for Cu–Ni prospecting in covered areas in the northwestern part of the Eastern Tianshan orogenic belt, North Xinjiang. Rocks from this intrusion have similar LA-ICP-MS zircon U–Pb ages (282.4 ± 1.1–281.3 ± 1.2 Ma) to other mafic-ultramafic intrusions containing magmatic sulfide mineralization in North Xinjiang. These early Permian gabbroic intrusions are thus important targets for Cu–Ni mineralization exploration in this region. The rocks of the Yueyawan gabbroic complex are a ca. 1 km2 elongated NW-trending crescent-shaped at the junction of two E-W- and NW-SE-trending faults. The intrusion has the morphology of a conduit tube. The rocks include troctolite, olivine gabbro, hornblende gabbro, and gabbro, and the Cu–Ni sulfides occur as steeply dipping (65–70°) lenses with disseminated textures in the contact zone between the olivine gabbro and hornblende gabbro, indicating that sulfide precipitation was coeval with the crystallization of the magmas giving rise to these rocks. The rocks investigated in this study are gabbroic in composition; they are characterized by moderate concentrations of SiO2 (44.8–48.8 wt%) and low to moderate concentrations of MgO (1.69–13.4 wt%, Mg# values of 39.6–73.3), Ni, and Cr. Furthermore, the rocks have high positive εHf(t) values (+13.1 to +15.9) and flat rare earth element patterns ((La/Yb)N = 2.01–2.89), with marked positive Eu anomalies, high Ti/Zr ratios (70–164), and low Ce/Pb (3.65–14.83) and Nb/Ta values (9–16), indicating very restricted amounts (5–10%) of crustal assimilation. This complex was likely derived from 10 to 20% partial melting (as evidenced by the low Sm/Yb values of 1.48–2.02 and La/Sm values of 1.61–2.23) of weakly slab melt-metasomatized (as evidenced by the low Ba/Nb and Ba/Zr ratios) subcontinental lithospheric mantle (with 2% garnet and 2% spinel lherzolite) at depths greater than 85 km in a post-collisional extensional source that contained sufficient Ni and Cu for the saturation and segregation of immiscible sulfides to concentrate the metals. The magma underwent sulfide saturation and segregation together with olivine fractionation (as evidenced by Ni and TFe2O3 increase and SiO2, CaO, and Zr decrease with increasing MgO content) in the conduit-type intrusion. Therefore, regional fault structures, ore-bearing rock types, and geochemical ratios that reflect high degrees of partial melting are features of the Cu–Ni-mineralized gabbroic rocks in adjacent areas that help support exploration.

Implications of nano- and micrometer-size platinum-group element minerals in base metal sulfides of the Yangliuping Ni-Cu-PGE sulfide deposit, SW China

The concentrations of platinum-group elements (PGE) and semimetal elements (As, Sb, Se, Te and Bi) in the base metal sulfides from the Yangliuping deposit were determined using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). Mass balance calculation reveals that the base metal sulfides only contain <40 wt% Os, <15 wt% Ir, <25 wt% Pd and negligible Pt, Rh and As, as solid solution. Such PGE percentages in the base-metal sulfides as solid solution are evidently lower than in the sulfides from the Jinchuan, Noril'sk and Sudbury NiCu-(PGE) sulfide deposits. Euhedral shape and similar chemical composition of the nanometer-size PGE-arsenides and sulfarsenides in the base-metal sulfides suggest that they crystallized from the sulfide melt before the crystallization of monosulfide solid solution (MSS) and intermediate solid solution (ISS). It is proposed that the nanometer-size PGE-arsenides and sulfarsenides formed from PGE-As molecular or polymolecular clusters in the sulfide liquid at high temperature. The PGE-As molecular or polymolecular clusters prevent As and PGE from partitioning into the base metal sulfide lattice and tend to form discrete nanometer-size PGE-arsenides and sulfarsenides. Thus, semimetal elements, particularly As, play an important role on behaviour of PGE during solidification of magmatic sulfide liquids.

Unravelling the dynamic emplacement of the carbonate-rich Munali Ni–Cu–PGE breccia deposit, Zambia

Magmatic sulphide deposits form in a variety of geodynamic tectonic settings where sulphide ores are typically hosted by mafic-ultramafic silicate magma. Many Ni–Cu-platinum group element (PGE) sul...

Reduced CO2 fluid as an agent of ore-forming processes: a case study of dolomite-replacement skarns at the Yoko-Dovyren massif)

The paper presents newly obtained geochemical data on outer-contact rocks and carbonatereplacement skarns of the Yoko-Dovyren layered ultramafic-mafic intrusion in the northern Baikal area. The rocks initially contained CO2-rich fluid with a high oxygen fugacity (up to NNO + 3–4), which was generated by the partial decomposition of dolomite and by reactions between SiO2 and carbonates. The skarn blue diopside is enriched in Pt (up to 0.2 ppm) and V (300 ppm), and the wollastonite zone of the skarns contains elevated Re concentrations (up to 0.4 ppm). The REE pattern of the contact-zone quartzite is identical to the REE patterns of phlogopite-bearing lherzolites from the lower contact part of the Yoko-Dovyren massif. These geochemical features of the rocks of the intrusion may be explained by the transfer and redeposition of material by reduced H2O-CO2 fluid. According to thermodynamic calculations, a reaction between H2O-CO2 fluid and high-Mg olivine at a subsolidus temperature of T = 950оC and pressure P = 2 kbar should result in a decrease in the oxygen fugacity to QFM – 2 and, hence, generate much CO. According to the calculations, a low oxygen fugacity (close to QFM + 0.7) can also be maintained by pyrrhotite oxidation with H2O and CO2 fluid components under cumulus P-T parameters. As a result of these reactions, the fluid should enrich in Pt extracted from magmatic sulfides, and this Pt can be redeposited in rocks, including those composing the skarn zones.

The mineralogy and mineral associations of platinum-group elements and precious metals in the Aurora Cu-Ni-Au-PGE deposit, Northern Limb, Bushveld Complex

Aurora is a platinum-group element (PGE) prospect hosted in the Northern Limb of the Bushveld Complex, South Africa. It is one of only three deposits discovered in the Northern Limb so far to be hosted in the melanocratic-leucocratic gabbroic cumulates of the Main Zone of the Rustenberg Layered Suite (Aurora, Moorddrift and Waterberg T Zone deposits), rather than in predominantly ultramafic rocks (e.g. Platreef). The host cumulates at Aurora have been divided into three principal units and they intrude the dolomites of the lower Transvaal Supergroup. Base metal sulphide (BMS) mineralisation with PGE is present in the leucogabbronorites and gabbronorites of Unit 2, and in coarse grained gabbronorite veins which intrude the peridotites of Unit 1. These veins contain up to 50% interstitial pyrrhotite-pentlandite-chalcopyrite ± pyrite. Unit 2 contains 1–3% pentlandite-pyrrhotite-chalcopyrite assemblages, and 1–5% chalcopyrite ± pyrite/pyrrhotite associated with hydrothermal alteration. The PGE content of Aurora however is predominantly controlled by the presence of platinum-group minerals (PGM), not BMS. LA-ICP-MS analysis of sulphides shows the BMS in Aurora have lower PGE concentrations than other Bushveld magmatic sulphides, with pentlandite carrying much lower concentrations of Pd (average 23 ppm) than the Platreef or the Merensky Reef. SEM-EDS analysis of 26 sections characterised 995 platinum-group minerals (PGM) and precious metal-bearing minerals (PMM), with a total area of 27850 μm2 and an average size of 28.2 μm2. Of the PGM and PMM identified in Aurora 85% (by area) are Pd-Te-Bi minerals, with 6% Pd-Te minerals, 4% electrum and 3% Ag-Te minerals, along with minor Pd-Bi, Pd-As, Pt-Te-Bi, Pt-As and Pt-S minerals that collectively comprise 2% of total area. Only 25% of the PGM and PMM in Aurora are BMS hosted, with the rest hosted in silicates. Of the total PGM and PMM area 22% are hosted in alteration-silicates (quartz, chlorite or actinolite) in an alteration halo around sulphides. Unusually, 52% of the PGM and PMM are spatially removed from BMS, instead hosted in alteration silicates and within cracks in primary silicates away from any BMS. This indicates a multi-stage ore genesis model, with hydrothermal remobilisation of PGE important for ore formation. The style and host rocks for mineralisation in the Aurora deposit are fundamentally different from other deposits in the Northern Limb of the Bushveld hosted in ultramafic rocks, such as the Platreef, GNPA member deposits and the F zone of the Waterberg deposit, all of which contain a greater diversity of PGM and BMS with higher precious metal contents. The mineralisation most similar to Aurora is the T Zone of the Waterberg deposit, located to the north of Aurora, which been suggested to be an along-strike equivalent of the Aurora Main Zone mineralisation. However, despite strong similarities in PGM mineralogy and S isotope signatures there are significant differences in BMS mineralisation and host lithology meaning it is unlikely they are directly linked stratigraphically. At present it seems more likely that Aurora and the Waterberg T Zone reflect similar fluid-influenced processes operating in different parts of the Main Zone, perhaps at different times and in different structural basins, rather than a continuous mineralised zone along strike.

Spatial distribution of sulfur isotope ratios associated with the Eagle Ni-Cu sulfide deposit in Upper Peninsula, Michigan: Implications on country-rock contamination in a dynamic magma conduit

The Eagle Ni-Cu magmatic sulfide deposit in the Upper Peninsula of Michigan is hosted in mafic-ultramafic rocks associated with the 1.1 Ga Midcontinent Rift event. The deposit consists of massive, semi-massive and disseminated sulfide textural zones located in two closely spaced funnel-shaped intrusive bodies, called the Eagle and Eagle East intrusions. The intrusions are surrounded by metamorphosed supracrustal Paleoproterozoic rocks of the Michigamme Formation and at depth by a Neoarchean migmatitic banded gneiss. δ34S values were obtained from sulfide minerals separated by micro-drilling from different parts of the intrusions and the surrounding rocks. The δ34S values from the intrusions cluster within a small range between 1 and 3‰ relative to V-CDT regardless of the mode of occurrence, textural type, mineral type and location of the sample. However, disseminated sulfide minerals in the metasedimentary country rocks of the Michigamme Formation show a substantially greater range in δ34S values between 4 and 35‰ with a median of 7.7‰ while disseminated sulfides in the basement granite-gneiss show values ranging from −9 to 13‰ with a median of 2.5‰. The narrow range of δ34S values within the intrusions despite wide variabilities in the surrounding country rocks can be explained by the incorporation of crustal sulfur by a mantle-derived magma, followed by a homogeneous mixing of the immiscible sulfide liquid, with a silicate liquid to sulfide liquid ratio (R-factor) of 200 or more. The assimilation of crustal rocks and the mixing of the resultant immiscible sulfide liquid must have been facilitated by the rapid movement of magma along a narrow conduit system.

Reduced CO2 Fluid as an Agent of Ore-Forming Processes: A Case Study of Dolomite-Replacement Skarns at the Yoko-Dovyren Massif

The paper presents newly obtained geochemical data on outer-contact rocks and carbonatereplacement skarns of the Yoko-Dovyren layered ultramafic-mafic intrusion in the northern Baikal area. The rocks initially contained CO 2 -rich fluid with a high oxygen fugacity (up to NNO + 3–4), which was generated by the partial decomposition of dolomite and by reactions between SiO 2 and carbonates. The skarn blue diopside is enriched in Pt (up to 0.2 ppm) and V (300 ppm), and the wollastonite zone of the skarns contains elevated Re concentrations (up to 0.4 ppm). The REE pattern of the contact-zone quartzite is identical to the REE patterns of phlogopite-bearing lherzolites from the lower contact part of the Yoko-Dovyren massif. These geochemical features of the rocks of the intrusion may be explained by the transfer and redeposition of material by reduced H 2 O-CO 2 fluid. According to thermodynamic calculations, a reaction between H 2 O-CO 2 fluid and high-Mg olivine at a subsolidus temperature of T = 950 о C and pressure P = 2 kbar should result in a decrease in the oxygen fugacity to QFM – 2 and, hence, generate much CO. According to the calculations, a low oxygen fugacity (close to QFM + 0.7) can also be maintained by pyrrhotite oxidation with H 2 O and CO 2 fluid components under cumulus P-T parameters. As a result of these reactions, the fluid should enrich in Pt extracted from magmatic sulfides, and this Pt can be redeposited in rocks, including those composing the skarn zones.

Behavior of Noble Metals during Fractional Crystallization of Cu–Fe–Ni–(Pt, Pd, Rh, Ir, Ru, Ag, Au, Te) Sulfide Melts

Abstract —The method of quasi-equilibrium directional crystallization was used for experimental modeling of the behavior of noble metals in the presence of Te during the fractional crystallization of Cu- and Ni-rich sulfide magma. The experimental melt contained (mol.%): Fe = 18.5, Ni = 19.1, Cu = 16.7, S = 44.1, and Pt = Pd = Rh = Ir = Ru = Ag = Au = Te = 0.2, i.e., is similar in composition to the massive pentlandite-bornite ores of platinum-copper-nickel deposits of the Noril’sk group. The crystallized sample consists of six primary zones differing in chemical and phase compositions. The main minerals crystallizing from the melt include the following sulfide phases: bornite solid solution (bnss), quaternary solid solution (tss), described earlier in the literature, and three phases (сfpn, cnpn, npn), which we attributed to pentlandite according to their chemical composition. The primary phases crystallized from the melt decay on cooling with the formation of secondary phases. The сfpn, cnpn, and tss phases decay completely, and the npn and bnss phases, partly. As a result, secondary zoning forms in the sample. Formation of drop-like inclusions of telluride melt was observed in the end zone of the ingot. The obtained data show that pentlandites and tss are the main high-temperature concentrators of PGE, with each of the macrophases showing specific PGE accumulation. Eight types of impurity phases have been detected. They form by different mechanisms: crystallization from sulfide melt of refractory compounds, deposition from telluride melt, and formation through complete or partial decay of primary macro- and microphases. A scheme of the zonal structure of the crystallized sample and the evolution of the phase composition during fractional crystallization has been constructed. It clearly demonstrates the intricate formation of primary and secondary major-component and impurity zonings and can be used to explain the nature of the zoned structure of massive PGE-bearing pentlandite–bornite orebodies.

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

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

The Cu-Ni mineralization potential of the Kaimuqi mafic-ultramafic complex and the indicators for the magmatic Cu-Ni sulfide deposit exploration in the East Kunlun Orogenic Belt, Northern Qinghai-Tibet Plateau, China

The East Kunlun Orogenic Belt experienced two periods of mantle-derived magmatic activity during its post-collisional stages. One occurred during the Late Silurian-Early Devonian, and the other occurred during the Middle-Late Triassic. The Late Silurian-Early Devonian Xiarihamu mafic-ultramafic intrusions were found to be associated with a giant magmatic sulfide Ni-Co deposit discovered in 2012. The U-Pb age of websterite in the Kaimuqi mafic-ultramafic intrusion is 221.1 ± 2.3 Ma. None of the Middle-Late Triassic mafic-ultramafic intrusions in the East Kunlun Orogenic Belt contain Cu-Ni mineralization. Based on simulations by the software MELTS, we found that the crystallization process leads to a drop in the sulfur content at sulfide saturation. The mineralized Late Silurian-Early Devonian mafic-ultramafic intrusions exhibit both a broader range of m/f values and higher m/f values than the non-mineralized intrusions in the East Kunlun Orogenic Belt, which means that the mineralized intrusions experienced a long process of crystallization differentiation. This process is probably responsible for Cu-Ni mineralization in the Early Devonian Xiarihamu mafic-ultramafic intrusion. Compared to the olivine and clinopyroxene in the Xiarihamu giant Cu-Ni deposit, the Late Triassic Kaimuqi olivine contains significantly lower MgO, Fo and SiO2 contents and higher CaO and FeO contents, and the Kaimuqi clinopyroxene contains higher FeO and CaO contents. Therefore, we infer that the Kaimuqi intrusion formed from a melt with higher FeO and CaO contents and a lower SiO2 content. This melt required more sulfur to achieve sulfide saturation. This characteristic most likely hindered mineralization in the Kaimuqi mafic-ultramafic intrusion. All of the mafic-ultramafic intrusions that host Cu-Ni deposits in the East Kunlun Orogenic Belt contain lherzolite, harzburgite, and orthopyroxenite, indicating that the pyroxene in these intrusions is mainly the orthopyroxene. If an intrusion contains more clinopyroxene, it indicates that its magma contains more calcium. The magma with higher calcium content will need a higher sulfur content to achieve sulfide saturation. Although websterite occurred in the Kaimuqi intrusion, the clinopyroxene and orthopyroxene content in the websterite is 75% and 20%, respectively, and the peridotite lithofacies is wehrlite. Therefore, orthopyroxene is rare in the Kaimuqi mafic-ultramafic intrusions. Thus, the Kaimuqi mafic-ultramafic intrusion may not have had the potential to form Cu-Ni ore deposits. Finally, this paper summarizes some geological and geochemical indicators for Cu-Ni mineralization in the mafic-ultramafic intrusions in the East Kunlun Orogenic Belt. These indicators provide some enlightenment to the exploration of Cu-Ni deposits in mafic-ultramafic intrusions, especially for the geological exploration bureaus or companies, which do not have the budget for large numbers of geological dating, isotopes, major elements, and platinum group elements during the exploration evaluation process in the current global mining recession situation.

Degassing-induced fractionation of multiple sulphur isotopes unveils post-Archaean recycled oceanic crust signal in hotspot lava

Mantle source regions feeding hotspot volcanoes likely contain recycled subducted material. Anomalous sulphur (S) isotope signatures in hotspot lavas have tied ancient surface S to this deep geological cycle, but their potential modification by shallow magmatic processes has generally been overlooked. Here we present S isotope measurements in magmatic sulphides, silicate melt inclusions and matrix glasses from the recent eruption of a hotspot volcano at El Hierro, Canary Islands, which show that degassing induces strongly negative δ34S fractionation in both silicate and sulphide melts. Our results reflect the complex interplay among redox conditions, S speciation and degassing. The isotopic fractionation is mass dependent (Δ33S = 0‰), thus lacking evidence for the recycled Archaean crust signal recently identified at other hotspot volcanoes. However, the source has an enriched signature (δ34S ~ + 3‰), which supports the presence of younger 34S-rich recycled oceanic material in the Canary Island mantle plume.

Formation of the Lubei magmatic Ni–Cu deposit in a post-subduction setting in East Tianshan, North West China

The Lubei Ni–Cu sulfide ore deposit with 7.8 Mt sulfide ores at an average grade of 0.88 wt% Ni and 0.5 wt% Cu, is associated with a mafic–ultramafic intrusion in the western part of the East Tianshan in Xinjiang Uygur Autonomous Region, NW China. The primary mineralization is hosted by ultramafic rocks which are part of a 2.1 km2 mafic–ultramafic intrusion located within the Kanggurtag fault zone. In this paper, we intend to probe the ore-formation processes for the Lubei Ni–Cu sulfide ore deposit and discuss key factors that control the ore formation processes in a post-subduction setting. The Lubei primary magmas were derived from 13 to 17 % partial melting of a hydrous mantle containing ∼0.29 wt% H2O estimated using the silicate-liquid model pMELTS. The Lubei sulfide ores are characterized by low platinum group elements (PGE) and high mantle-normalized Pd/Ir ratios, and disseminated sulfide ores have higher Cu, Ni and PGE tenors than massive and vein sulfide ores indicative of crystal fractionation of Fe-rich monosulfide solid solution. The PGE-depleted Lubei sulfides possibly resulted from early sulfide segregation from the Lubei primary magmas as indicated by high Cu/Pd ratios (up to 106), and the proportion of early sulfide losses was 0.02 wt% which has depleted the Lubei parental magmas and later fractionated sulfides in chalcophile elements. This depletion would been compensated at different levels by later sulfide–magma interaction when the parental magmas reached in situ sulfide saturation, with an R factor (mass ratio of silicate to sulfide liquids) of 100–1000. The formation of the observed Lubei disseminated sulfide ores was associated with 5–15 vol% magma crystallization with the main silicate minerals being spinel and olivine, as modeled using the silicate-liquid model MELTS. The δ34S values of the Lubei sulfides vary over a narrow range of −0.3 to +1.8‰, indicating a mantle origin for the sulfur. The key factor that controls the formation of magmatic Ni–Cu deposits in post-subduction settings is a metasomatized mantle accompanied by activation of translithospheric faults during post-collisional extension. This mechanism alternatively interprets the widespread magmatic Ni–Cu deposits in the southern Central Asian Orogenic Belt and implies translithospheric fault as a potential target for Ni–Cu exploration.

Tectonic controls on Ni and Cu contents of primary mantle-derived magmas for the formation of magmatic sulfide deposits

Tectonic controls on Ni and Cu contents of primary mantle-derived magmas for the formation of magmatic sulfide deposits

3D joint inversion of Gradient and Mise-à-la-Masse borehole IP/Resistivity data and its application to magmatic sulfide mineral deposit exploration

Gradient and Mise-a-la-Masse IP/Resistivity surveys were conducted on a group of 19 boreholes in Eagle’s Nest, Eagle One magmatic sulfide deposit in northern Ontario, Canada. The surveys were conducted as a follow-up to the many drilled boreholes, some of which missed the target. The surveys were intended to map the distribution of the ore mineralization, outline the deposit hosted by mafic and ultramafic rocks and then guide the drilling of new boreholes. Joint Gradient and Mise-a-la-Masse data inversion produced 3D chargeability and conductivity models. The inverted 3D models in turn help delineate the outline of the mineralized zone, and determine the shape, size, strength and economic viability of the deposit. The Gradient array determined the direction of the mineralization with respect to the boreholes, and the Mise-a-la-Masse array examined the highly conductive subsurface bodies and their surroundings. The mapped ore zone shows close similarity to the 0.5 Cu% and 1.05 Ni% iso-surfaces that are produced from core assay result confirming the reliability of the results obtained in this study.