Sulfide Ore Formation Research Articles

Extent and survival of zoned pyroxene within intrusions hosting magmatic sulfides: Implications for zoned pyroxene as a prospectivity indicator

AbstractChromium-zoning patterns in pyroxene from the economically significant Ni-Cu sulfide deposits at Nova-Bollinger (Western Australia) and Kevitsa (Northern Finland) were investigated using XRF mapping, automated mineralogy, and EPMA analyses. At Nova-Bollinger, complex Cr-zoning patterns are found widely throughout the cumulus orthopyroxene and clinopyroxene within the Lower Intrusion, a small chonolith that hosts the vast extent of the sulfide mineralisation. Cumulus pyroxenes with visible sector and abrupt zonation patterns have been found up to 150 m vertically away from the massive sulfide ore. Complex zoning patterns are observed throughout the Kevitsa intrusion, in the form of strong oscillatory zoning in cumulus clinopyroxene and sector zoning in idiomorphic orthopyroxene oikocrysts. Kevitsa pyroxenes show varying degrees of hydration, leading to epitaxial replacement by amphibole. Cr zonation is visible through the early stages of this alteration, with preservation enabled by the presence of Cr-rich epitaxial amphibole; however, the remnant zoning is lost as the amphibole alteration progresses. Results suggest that Cr zonation in pyroxene may be an effective indicator of dynamic recharged conduits and therefore an indicator of favourable conditions for metal enriched magmatic sulfide ore formation. Such indicators have significant vertical extent from the ore body and can survive partial alteration, which makes them a useful tool for prospectivity assessment of drilled intrusions. Furthermore, our data show that there is potential for complexly zoned pyroxene to be used as an ex-situ prospectivity indicator in glacial till.

Porphyry and Epithermal Au-Cu Systems of the Southern Caucasus and Northern Iran

This article presents tangible geological evidence for coexistence of porphyry copper and epithermal gold systems within single polygenic deposits and provides a paleothermophysical model for their origins. Brief metallogenic analysis of the Southern Caucasus and Northern Iran has shown that such deposits are confined to long-living calc-alkaline island arcs and were formed during their orogenesis. Examples of complex Sonajil (Iran), Gharta, and Merisi (Georgia) deposits are considered. Investigation has shown that for combined porphyry and epithermal ore formation some preconditions are suggested to exist: (i) Source of anomalous energy, which exceeds thermodynamics of the enclosing environment; (ii) Existence of temperature gradient, which determines conventional flows of fluids composed of endogenous and meteoric constituents (proven by rhythmical zoning of ore lodes); (iii) Stability of such conditions for a period of sulfide ore formation. However, such a process of sulfide ore formation cannot explain formation of high sulfidation gold deposits. Mass precipitation of free gold requires phreatic collapse in the ore conduit channel already after formation of hydrothermally altered rocks, and this event results in creation of either hydrothermal breccias, often with jigsaw-fit texture or brecciated vuggy silica where host rocks and hydrothermally altered rocks are cemented by a gold-bearing quartz matrix.

Mineralogy and geochemistry of magnetite in the Zhdanov Ni-Cu-(PGE) deposit in the Pechenga ore field, Russia: Implications for the formation of magmatic sulfide ores

The mineralogy and chemical composition of magnetite and chromite series (simplified as magnetite) can provide valuable insights into the formation processes of magmatic Ni-Cu-(PGE) sulfide deposits. Previous studies mainly focus on magnetite in disseminated ores and massive ores, but very little is known about the magnetite in net-textured ores. In this study, magnetite in net-textured ores (type 1 magnetite) and massive ores (type 2 magnetite) from the Zhdanov Ni-Cu-(PGE) deposit in the Pechenga ore field, were recognized and analyzed using scanning electron microscope (SEM), electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Both the type 1 and type 2 magnetite grains show zoned textures with Cr concentration decreasing from the core to the rim. Type 1 magnetite has higher Al and Mg concentrations than type 2 magnetite. These observations indicate that type 1 and type 2 magnetite crystallized from and record the solidification process of silicate melt and sulfide liquid, respectively. Type 1 magnetite within sulfides of net-textured ores has identical chemical composition to that within silicates, which suggests that the chemical composition of magnetite is not significantly affected by sub-solidus re-equilibrium with sulfide minerals. It also suggests that the type 1 magnetite within sulfides was incorporated into sulfide liquid later during the downward percolation of sulfide liquid through the magnetite-bearing olivine crystal mush. On the other hand, type 2 magnetite has comparable concentrations of W, Sc, Ga, Cr, Mo, and Mn to the type 1 magnetite. Such compositional similarity can be explained by the crystallization of type 2 magnetite from the sulfide liquid pool that have been mingled with the downward percolated sulfide liquid. Accordingly, the downward percolation of sulfide liquid plays an important role in the formation of net-textured ores and massive ores in the Zhdanov deposit.

The effect of platinum-group minerals on differentiation of platinum-group elements in magmatic sulfide deposits: Evidence from the Cu-Ni-PGE deposits in the Yangliuping area of the Emeishan large igneous province, SW China

Platinum-group minerals (PGM) are important host phases for platinum-group elements (PGE) in magmatic sulfide deposits. Recent studies have demonstrated that some PGM, such as sperrylite, can crystallize early from sulfide liquids. It is important to determine whether and how these PGM particles affect the enrichment and differentiation of PGE in Cu-Ni-PGE deposits. The magmatic sulfide deposits in the Yangliuping area (mainly consisting of the Yangliuping deposit and Zhengziyanwo deposit) are the largest Cu-Ni-PGE deposits in the Emeishan large igneous province, SW China. Primitive-mantle normalized PGE patterns and fractional crystallization modelling show that massive ores in the Yangliuping deposit are enriched in Pd and Pt. In contrast, massive ores, breccia ores and Cu-rich ores in the Zhengziyanwo deposit are depleted in Pt. Scanning electron microscopy (SEM), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), focused ion beam (FIB) and transmission electron microscopy (TEM) were used to constrain the occurrence of PGE in the massive ores from the two deposits. Mass balance calculation shows that <7% of Ir, Rh and Pt occur in base metal sulfides (BMS, mainly pyrrhotite, pentlandite and chalcopyrite) as solid solution in both the Yangliuping and Zhengziyanwo massive ores. 17% and 91% Pd are present in solid solution within BMS in the Yangliuping and Zhengziyanwo massive ores, respectively. In the Yangliuping massive ores, PtAs2 and Pd-PGM particles were commonly found within BMS crystals, and Ir-Rh-(Pt) sulfarsenides enclosed in cobaltite-gersdorffite solid solution (CGSS) are enriched in Pt. In contrast, in the Zhengziyanwo massive ores, no PtAs2 particles and fewer Pd-PGM particles were found, and Ir-Rh-(Pt)AsS particles enclosed in CGSS are poor in Pt. The distribution, mineralogy and microstructures of Ir-Rh-(Pt)AsS, PtAs2 and Pd-PGM particles in both the Yangliuping and Zhengziyanwo deposits suggest that they mainly crystallized from sulfide liquids or formed from Pd-Pt-semimetal-rich droplets. We suggest that the incorporation of Pd-PGM and PtAs2 particles into MSS cumulates led to the enrichment of Pd and Pt in the Yangliuping massive ores. In the Zhengziyanwo deposit, early fractional crystallization of PtAs2 reduced the Pt content in sulfide liquids and resulted in the formation of Pt-depleted sulfide ores and low-Pt Ir-Rh-(Pt)AsS particles, but no Pt-rich sulfide ores have been found to date. As a result, the role of PGM particles should be considered when using PGE to trace the formation process of Cu-Ni-PGE deposits. This study also provides additional evidence that the behavior of Pt in sulfide liquids is influenced by thermodynamic conditions and As, and suggests a potential for Pt-rich sulfide ores exploration in the Zhengziyanwo deposit.

Formation of Fe-Ti oxide and Ni-Co sulfide ores by concentric tube flow and hydrous metal-rich melt recharge into the cooling crystal mush: Example from the Hongge intrusion in Panxi region, SW China

The Panxi (ultra-)mafic intrusions are relatively small compared with world-class ore-related (ultra-)mafic intrusions, but they host several super-large Fe-Ti oxide deposits and few small Cu-Ni (PGE) sulfide deposits. The mechanisms that concentrate huge amounts of oxide minerals into these small intrusions and form the distinct oxide and sulfide mineralization remain poorly understood. Here, we investigate the magma dynamics that produced extensive Fe-Ti oxide and coexisting Ni-Co sulfide mineralization in the Hongge intrusion of Panxi region, based on detailed field cross-cutting relations and petrographic observations, bulk-ore/rock geochemistry, and in-situ LA-ICP-MS magnetite trace element analyses. The Fe-Ti oxide ores include magnetite-poor (<60 vol%) disseminated and magnetite-rich (>60 vol%) massive types, with the former further divided into the gabbro-hosted (GH) and (olivine-)pyroxenite-hosted (OPH) subtypes. The GH oxide ores have Cr-poor (median 86.9 ppm) magnetite and abundant apatite interstitial to silicate minerals. In contrast, the OPH and massive oxide ores have Cr-rich (median 13,956 ppm) magnetite that envelops resorbed silicate minerals. Principal component analyses of bulk-ore and magnetite elemental contents reveal genetic links between the OPH and massive oxide ores, but remarkable genetic distinction from the GH oxide ores. Considering the overall lopolith-shape orebody geometry of several branch intrusions, the GH oxide ore formation may have involved spontaneous development of concentric tube flow zones within the cooling crystal mush. However, the lower P but much higher Cr-Cu-Co-Ni contents than the GH oxide ores and the sharp intrusion of massive oxide ores into the OPH oxide ores suggests that the OPH and massive oxide ores were probably formed by upward percolation and later intrusion of hydrous Fe-rich melts from a deep-seated magma reservoir into semi-solidified and fossilized mush, respectively. In addition, sulfide ores occur mainly as irregular/lenticular aggregates in massive oxide ores and are depleted in Cu and platinum group elements (PGEs), indicating early, deep-level sulfide segregation that scavenged the Cu and PGEs from the magmas. Hence, we envisage that the assimilation of S-deficient wall-rocks into basaltic magmas may have induced the early sulfide saturation and retain the majority of Fe for later precipitation as principal oxide and minor PGE-Cu-depleted sulfide ores.

Effect of Sulfur Concentration on Olivine Sulfidation under Lithospheric Mantle PT-parameters

Experimental studies aimed at the modeling of interaction processes of sulfur-bearing metasomatic agents with mantle silicates and assessing the effect of sulfur concentration on olivine sulfidation were carried out in the Fe,Ni-olivine—sulfur system using the high-pressure multi-anvil apparatus BARS (1050 and 1450°C, 6.3 GPa, 40–60 h, sulfur concentrations (Xs) 0.1, 2, and 6 mol %). It has been established that as a result of the recrystallization of Fe,Ni-olivine in a sulfur melt, Fe and Ni are extracted from olivine into this melt, and formation of Fe,Ni-sulfides (or sulfide melts) and low-iron, low-nickel silicates takes place. The key indicator characteristics of the olivine sulfidation process are determined depending on the temperature and sulfur concentration, including characteristic phase assemblages, regularities in the evolution of the chemical compositions of mineral and melt phases, and structural features of olivine crystals. It has been experimentally established that reducing sulfur-bearing metasomatic agents, even in minimal concentrations and at relatively low temperatures, are capable of dissolving and transporting mantle silicates and sulfides, and can play an important role in sulfide ore formation in the mantle.

EFFECT OF SULFUR CONCENTRATION ON OLIVINE SULFIDATION UNDER LITHOSPHERIC MANTLE &lt;i&gt;PT&lt;/i&gt;-PARAMETERS

Experimental studies aimed at the modeling of interaction processes of sulfur-bearing metasomatic agents with mantle silicates and assessing the effect of sulfur concentration on olivine sulfidation were carried out in the Fe,Ni-olivine – sulfur system using the high-pressure multi-anvil apparatus BARS (1050 and 1450°C, 6.3 GPa, 40–60 hours, sulfur concentrations (Xs) 0.1, 2 and 6 mol. %.). It has been established that as a result of the recrystallization of Fe,Ni-olivine in a sulfur melt, Fe and Ni are extracted from olivine into this melt, and formation of Fe,Ni-sulfides (or sulfide melts) and low-iron, low-nickel silicates takes place. The key indicator characteristics of the olivine sulfidation process are determined depending on the temperature and sulfur concentration, including characteristic phase assemblages, regularities in the evolution of the chemical compositions of mineral and melt phases, and structural features of olivine crystals. It has been experimentally established that reducing sulfur-bearing metasomatic agents, even in minimal concentrations and at relatively low temperatures, are capable of dissolving and transporting mantle silicates and sulfides, and can play an important role in sulfide ore formation in the mantle.

Multiisotope (δ33S, δ34S, δ36S) composition of sulfur in sulfides of ancient ores: significance for determining sulfur sources, biogeochemical processes and deposit genesis

New data on the isotopic (δ 33 S, δ 34 S, δ 36 S) composition of ancient sulfide ores of the Karelian and Siberian cratons have been obtained. An in situ analysis of the isotope composition in sulfides made it possible to draw conclusions about the sources of sulfur during the formation of Archean deposits, the biological influence on the formation of sulfide ores, and the features of the environmental impact (atmosphere, hydrosphere) on sulfide formation.

Sulfide Ore Formation of the Kalatongke Ni-Cu Deposit as Illustrated by Sulfide Textures

Abstract The Kalatongke magmatic Ni-Cu deposit features high Ni-Cu grades compared with other Ni-Cu deposits in the Central Asian orogenic belt. The sulfides, mainly hosted by olivine norite and gabbronorite, are characterized by high Cu/Ni ratios. There is wide variety of textural relationships in the mineralized rocks, including globular, sulfide matrix, emulsion, disseminated, net-textured, and semimassive to massive textures. Quantitative textural measurements reveal that more than 65 vol % of the total sulfide volume in disseminated ore (defined as containing 4–10 vol % sulfide) and more than 90 vol % of the total sulfide volume in net-textured ore (typically 16 vol % sulfide) are hosted in few interconnected networks with equivalent sphere diameters (ESDs) larger than 6 mm. This illustrates that sulfide coalescence is a critical process. The remaining sulfide blebs define two groups of log-linear particle size distribution (PSD), i.e., a finer group (ESD of 0.080–&amp;lt;0.529 mm) and a coarser group (ESD of 0.529–4.084 mm). The PSD of the finer group differs slightly among different types of mineralization. The origin of this group of sulfides is attributed to sulfide nucleation simultaneously with crystallization of the olivine-orthopyroxene-plagioclase phases from the ore-forming magma in the current magma chamber. The PSD of the coarser group from net-textured ores is parallel to that from the disseminated ore but has lower intercept values. Modeling results show that aggregation of ~40–70% sulfide blebs of different grain size from the disseminated ore into the networks can generate the coarser group sulfide PSD of net-textured ore. In addition, monosulfide solution (MSS, pyrrhotite component)-enriched sulfide globules are commonly located close to sulfide matrix ore breccias and emulsion-textured ores, rimming the net-textured and massive orebodies. These globules are different from the sulfide component that was in equilibrium with the mafic magma, suggesting they were formed by mechanical remobilization of a cumulus MSS-enriched component from a previously segregated and partially crystallized sulfide pool. These observations, combined with the ubiquitous chilled margin xenoliths in the high-grade ores, suggest that the Kalatongke deposit was the result of voluminous magma flow through the current location accompanied by sulfide reworking and percolation. All these textural characteristics could be explained by emplacement within a laterally propagating bladed dike. Moreover, the deformation recorded in the emulsion-textured and massive ores suggests the fault system remains active during magma solidification, driving downward migration of sulfide from the sulfide pool at magmatic temperature to form vein-type massive ore. We suggest that the Kalatongke deposit formed by magma pulses injecting into the current location and that the tectonic movement remains active during the solidification of the intrusion. The syntectonic emplacement model, which may be a common feature in the Central Asian orogenic belt Ni-Cu deposits, indicates that the fault systems beneath the intrusion are of great exploration interest for high-grade ores.

The role of evaporite layers in the ore-forming processes of iron oxide-apatite and skarn Fe deposits: Examples from the middle-lower Yangtze River metallogenic Belt, East China

Iron oxide – apatite (IOA) and skarn Fe deposits are important sources of Fe in China. During the Mesozoic, a large number of large IOA and skarn Fe deposits formed in the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in a short period. These deposits genetically show a close spatial relationship to Triassic evaporite layers. In this study, the S isotope values of IOA deposits and Fe skarn deposits in the MLYRMB were collected and summarized. Most of the sulfides from Fe deposits have higher S isotope values (5–10‰), and the sulfates have S isotope values (17–21‰) that are similar to those of the evaporite layers (28–30‰), indicating the involvement of the evaporite layers. The effects of the evaporite layers on the mechanism of Fe ore formation are discussed. Evaporite layers are the most important oxidation barrier in the formation of Fe ores, as they can oxidize Fe2+ to Fe3+ in magmatic melts and magmatic-hydrothermal fluids, promoting Fe accumulation. Evaporite layers can also provide Na, Ca, K, Cl, and other agents of mineralization that facilitate the formation and transformation of Fe-bearing fluids and cause the deposits to display large-scale Na and Cl alterations. When sulfate from evaporite layers oxidizes Fe2+ to Fe3+, S6+ is reduced to S2−, becoming a major sulfur source for the formation of either sulfide ores or pyrite deposits after Fe mineralization. Sulfate (gypsum) from evaporite layers could be activated and transferred to the upper or side part of the ore system, forming anhydrite/gypsum ores (veins). Therefore, pyrite, anhydrite, and Fe ores often appear in single deposit.

Controls on the disseminated Ni-Cu-PGE sulfide mineralization at the Tubane section, northern Molopo Farms Complex, Botswana: Implications for the formation of conduit style magmatic sulfide ores

The Molopo Farms Complex (MFC) is located near the western margin of the Archean Kaapvaal Craton, in southern Botswana. The complex hosts low grade disseminated Ni-Cu and platinum group elements (PGE) mineralization restricted to the lower ultramafic cumulates, notably in the feldspathic orthopyroxenites. The mineralized feldspathic orthopyroxenites contain up to 0.47 ppm Pt + Pd + Au manifested in the form of platinum group mineral (PGM) assemblages dominated by PGE bismuthotellurides, asernides and minor Au and Ag alloys. The PGM encountered are froodite (PdBi2), moncheite (PtTe2), michenerite (PdBiTe), sperrylite (PtAs2) largely associated with altered silicates and sulfide minerals. The association of PGM with sulfides suggests that they were originally concentrated by sulfide melt, while late stage reaction of sulfides with hydrothermal fluids resulted in the observed spatial proximity of PGM with altered silicates. The Cu/Pd ratios of most of the rocks fall within the primitive mantle range. However, the Cu/Pd ratios of the feldspathic orthopyroxenites are more variable, with few samples falling above and below the primitive mantle range suggesting the prior sulfide extraction from the magmas and the presence of cumulus sulfides. This is supported by the Cu/Zr ratios of these rocks which are also variable above and below unity. Trace element ratios on the (Th/Yb) PM versus (Nb/Th) PM plot indicate strong crustal contamination across all units including those not in proximity to the country rocks. The δ34S of the magmatic rocks are variable between −6.3‰ and −11.4‰. In contrast, the δ34S of dolomitic siltstones of the Transvaal Supergroup which constitute the immediate country rocks vary from −4.1‰ to +0.6‰. These data indicate that crustal contamination largely occurred prior to magma emplacement. It is inferred that the most important contaminant and source of crustal S were the Ramotswa and Ditlhojana Formations that include sulfidic shales at depth. We consider that magnesian basaltic magmas of B1 Bushveld magma composition underwent crustal contamination, assimilation of crustal S and segregation of sulfide melt in the feeder conduit or staging-chamber prior to final magma emplacement. The feeder conduit or staging-chamber was subsequently invaded by further B1 magma. This magma entrained some of the sulfides lodged in the plumbing system and flushed them to the final MFC chamber studied here. After the staging-chamber had already partially solidified, it was invaded by yet another influx of magma, possibly even more primitive than the B1 Bushveld magma. Being a crystal mush, this magma assimilated and entrained some of the previously formed orthopyroxenites manifested by fragments of orthopyroxenite in the harzburgites of the final MFC chamber. The presence of centimeter-sized globular sulfides and fragments of orthopyroxenite within harzburgites suggests that the complex was emplaced as separate pulses of magmas. Therefore, it is considered that sulfide accumulations may remain undiscovered in the feeder zones or the staging-chamber.

Zinc Sulphide Ore Formation – a Cathodoluminescence and X-ray Examination

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Petrography and sulfur isotopic compositions of SEDEX ores in the early Cambrian Nanhua Basin, South China

Widespread Ni-Mo sulfide mineralization in South China during the early Cambrian may have affected the seawater chemistry and biodiversity of this region, but its underlying causes are not well-understood. To better understand the formation of sulfide ores in early Cambrian marine systems, this study evaluated the paragenesis and sulfur isotopic composition (δ34S) of sediment-hosted Ni-Mo sulfide ores in the lower Cambrian Niutitang Formation of the Nanhua Basin of South China. Petrographic analysis revealed three types of sulfide aggregates: (1) ubiquitous framboidal and euhedral pyrite (Py-1), (2) a MoSC phase (i.e., Mo mineralization) that was cogenetic with larger pyrite cubes or nodules (Py-2), and (3) intergrown millerite (NiS), sphalerite (ZnS) and veined pyrite (Py-3) (i.e., Ni-Zn mineralization) characterized by laminated and veined textures. Alternating precipitation of Mo and Ni-Zn sulfides was caused by intermittent emission of Mo-Fe or Ni-Zn-rich hydrothermal fluids into euxinic mid-depth waters of the Nanhua Basin, representing a sedimentary exhalative (SEDEX) process. The different sedimentary ore textures are each associated with a characteristic δ34S distribution. At all study sites, the syngenetic Py-1 phase yields δ34S of −15.7 to + 1.9‰, consistent with a seawater sulfate source that was variably fractionated through microbial sulfate reduction (MSR). In the Nayong and Zunyi areas, δ34S values of −11.8 to +4.4‰ are associated with the MoSC phase, and −20.6 to −6.8‰ with the Py-2 phase. In the Zhangjiajie area, Py-2 yields similar values (−25.1 to −10.8‰) but MoSC higher and less variable values (+8.6 to +18.7‰). These phases are thought to contain a mixture of sulfur sourced from seawater sulfate (via MSR) and hydrothermal fluids in varying proportions. A mode of Ni-Zn sulfides δ34S values between −9.1 and +8.0‰ is likely to represent the isotopic composition of hydrothermal sulfur inputs to the Ni-Zn mineralization. As a consequence, both hydrothermal emissions and biogenic production of H2S had a significant influence on seawater chemistry, with consequences for contemporaneous evolution of early marine animal life (e.g., the small shelly fauna and Chengjiang Biota) in the early Cambrian Nanhua Basin.

MODEL OF MASSIVE SULPHIDE ORE-MAGMATIC SYSTEM OF AKZHARSKY ORE FIELD (SOUTH URALS)

A model of the evolution of a massive sulphide ore-magmatic system was constructed for the Akzharsky ore field, which is based on the analysis of two groups of elements of a typical model that characterize the magmatic and ore stages of its development. According to the proposed model, the ore deposits of the Akzharsky ore field were formed by a combined method. Copper came with deep hydrothermal solutions, separated in the process of magmatic liquation at the final (ore) stage of the development of the ore-magmatic system. Sulfur also had a juvenile source, as evidenced by the closeness of its isotopic composition in pyrites to the meteorite standard, and iron was extracted from the host rocks as a result of their hydrothermal processing. The onset of the ore process is associated with the formation of propylites and the hydrothermal-sedimentary deposition of hydroxide ferrous mineralization, subsequently transformed into magnetite ores under the influence of metamorphism. Massive sulphide mineralization completes ore formation and is a product of partial processing of early hydroxide ferruginous ores by later pyrite-bearing fluids. Ore formation was shortlived and proceeded under conditions of periodically renewed volcanic activity, which contributed to the rapid burial of ores and prevented the full development of the ore cycle from the earliest to the final mineral paragenesis, as well as the creation of large massive sulphide deposits. The process of massive sulphide ore formation was short-term and onestage, which manifested itself in the relative simplicity and uniformity of the texture and structural features of the ores, telescoping of productive mineralization in a narrow vertical interval, and the absence in some cases of distinct mineral and geochemical zoning.

Occurrence of Graphite-Like Carbon in Podiform Chromitites of Greece and Its Genetic Significance

The role of post-magmatic processes in the composition of chromitites hosted in ophiolite complexes, the origin of super-reduced phases, and factors controlling the carbon recycling in a supra-subduction zone environment are still unclear. The present contribution compiles the first scanning electron microscope/energy-dispersive (SEM/EDS) data on graphite-like amorphous carbon, with geochemical and mineral chemistry data, from chromitites of the Skyros, Othrys, Pindos, and Veria ophiolites (Greece). The aim of this study was the delineation of potential relationships between the modified composition of chromite and the role of redox conditions, during the long-term evolution of chromitites in a supra-subduction zone environment. Chromitites are characterized by a strong brittle (cataclastic) texture and the presence of phases indicative of super-reducing phases, such as Fe–Ni–Cr-alloys, awaruite (Ni3Fe), and heazlewoodite (Ni3S2). Carbon-bearing assemblages are better revealed on Au-coated unpolished sections. Graphite occurs in association with hydrous silicates (chlorite, serpentine) and Fe2+-chromite, as inclusions in chromite, filling cracks within chromite, or as nodule-like graphite aggregates. X-ray spectra of graphite–silicate aggregates showed the presence of C, Si, Mg, Al, O in variable proportions, and occasionally K and Ca. The extremely low fO2 during serpentinization facilitated the occurrence of methane in microfractures of chromitites, the precipitation of super-reducing phases (metal alloys, awaruite, heazlewoodite), and graphite. In addition, although the origin of Fe–Cu–Ni-sulfides in ultramafic parts of ophiolite complexes is still unclear, in the case of the Othrys chromitites, potential reduction-induced sulfide and/or carbon saturation may drive formation of sulfide ores and graphite-bearing chromitites. The presented data on chromitites covering a wide range in platinum-group element (PGE) content, from less than 100 ppb in the Othrys to 25 ppm ΣPGE in the Veria ores, showed similarity in the abundance of graphite-like carbon. The lack of any relationship between graphite (and probably methane) and the PGE content may be related to the occurrence of the (Ru–Os–Ir) minerals in chromitites, which occur mostly as oxides/hydroxides, and to lesser amounts of laurite, with pure Ru instead activating the stable CO2 molecule and reducing it to methane (experimental data from literature).

Экспериментальная и расчетная модель образования пирит-сфалерит-баритовой ассоциации при смешении флюидов

Experimental and thermodynamic study of the formation conditions of sulfates and sulfides under the mixing of fluids has been carried. This geochemical process is very important for modern sulfide ore formation at the floor of the oceans. Experimental procedure was based on the scheme H 2 S (H 2 SO 4 ) + МеСl 2 → MeS (MeSO 4 ) + 2HCl. During experiments, sphalerite and pyrite, together with barite, were synthesized by mixing aquatic solutions of chloride of zinc, iron, and barium with sulfide-containing solutions at temperature of 200—275 °C and pressure of 100 bar. Experimental data on the composition of fluid and solid phases were used to validate a computational model. To calculate the composition of non-ideal solid solution of sphalerite (ZnS-FeS), the Margules asymmetric model was applied. Contents of iron in sphalerite calculated through this model and determined from experiments at 243 °С are 1.5 and 1.0 wt % correspondingly. The good similarity of these values indicates the feasibility of the proposed model of deep-sea sulfide ore formation in ocean floor structures.

Минеральный состав палеопротерозойских метаморфизованных колчеданных руд Кольского региона (на примере проявления Брагино, Южная Печенга)

Many occurrences and deposits of massive sulfide ores in the Kola Region are connected with the Paleoproterozoic Pechenga—Imandra—Varzuga Rift Belt (2.5—1.7 Ga). They are placed in volcanogenic-sedimentary complexes of South Pechenga (Bragino occurrence) and western part of Imandra-Varzugskaya structural zones (Pyrrhotite Ravine deposit, Tahtarvumchorr occurrence, etc.). The age of massive sulfide ore formation is expected to be about 1.9 Ga. The ores and their host complexes were metamorphosed under conditions close to the amphibolite facies. High degree of metamorphism of massive sulfide ores of the Kola region determined the features of their mineral composition, which are considered on the example of the Bragino occurrence. The article displays ore types, their mineral assemblages, and data on main minerals of ores: pyrrhotite, pyrite, sphalerite, marcasite, etc.

New constraints on the origin of the Maozu carbonate-hosted epigenetic Zn-Pb deposit in NE Yunnan Province, SW China

Micro-scale isotopic compositions determined by micro-beam in situ analysis techniques are expected to provide new information for the understanding of ore genesis. Here we use the Maozu (∼20 Mt of sulfide ore, grading 7.25 wt% Zn and 4.15 wt% Pb) Zn-Pb deposit in the Upper Yangtze Zn-Pb metallogenic province as a case study, utilizing NanoSIMS in situ S, femtosecond LA-MC-ICPMS in situ Pb and conventional bulk Sr isotopes, as well as field data, aimed to reveal its origin and ore formation process. NanoSIMS in situ δ34S values of sulfide minerals from the Maozu deposit have a wider range (+5.9–+20.7‰) than that of bulk δ34S values (+9.9–+19.9‰). Such S isotope signatures reflect the enrichment of heavy S isotopes in ore-forming fluids, and further indicate the source of S is multiple reservoirs (e.g. marine evaporites, coeval seawater, and metamorphic rocks/fluids). Pb isotopic ratios of galena determined by LA-MC-ICPMS in situ techniques have a narrower range (206Pb/204Pb = 18.20–18.21, 207Pb/204Pb = 15.69–15.71 and 208Pb/204Pb = 38.40–38.43) than that of bulk Pb isotopic ratios (206Pb/204Pb = 18.13–18.38, 207Pb/204Pb = 15.64–15.69 and 208Pb/204Pb = 38.22–38.58). This implies that in situ Pb isotopes can truly reflect the source of Pb, which is due to impurities enclosed by sulfide crystals and pollutants introduced by chemical analysis could significantly affect the quality of the bulk Pb isotopic data. Compared with previously reported Pb isotopic data of potential source rocks (i.e. late Permian Emeishan basalts, Paleozoic ore-bearing sedimentary rocks and Proterozoic basement metamorphic rocks), the in situ Pb isotopic data are significantly different from those of basalts, but are similar to those of both basements and sediments. This means that metal-rich fluids were mainly derived from and/or flowed through basements and sediments. Sphalerite separates have initial Sr isotopic ratios (0.711–0.719) that are distinctly different from those of ore formation age-corrected basalts (87Sr/86Sr200 Ma = 0.704–0.708) and basements (87Sr/86Sr200 Ma = 0.724–0.729), whereas partly overlap with those of age-corrected sediments (87Sr/86Sr200 Ma = 0.707–0.714). Such Sr isotope signatures suggest that hydrothermal fluids were predominantly derived from and/or flowed through basements and sediments with a certain influence from basalts. Hence, the Maozu deposit has complex source rocks that are related to the local geological setting. The mixing of metal-rich fluids (related to basements and sediments, and even basalts) and S-bearing fluids (associated with multiple S reservoirs) plays a key role in the formation of sulfide ores. Therefore, the Maozu deposit, formed during Early Mesozoic (∼196 Ma), is an epigenetic Zn-Pb deposit that hosts in carbonate rocks, occurs along fault-fold structures, and has a mixed source of mineralizing elements and associated fluids.

Caledonian formation of gold-bearing sulfide depositions in Early Proterozoic gabbroids in the northern Ladoga region

We have studied Pb isotopic systems of K-feldspar, pyrite, and pyrrhotine from gabbroids and ore of the Velimyaki Early Proterozoic massif in the northern Ladoga region in the southeastern part of the Fennoscandian Shield. The isochronous Pb–Pb age of sulfides has been determined as ∼450 Ma, which corresponds to intersection of the regression line with the lead accumulation curve with μ = 10.4–10.8; the model Pb age of sulfides is close to isochronous under the condition that the composition of lead evolved from a geochemical reservoir with an age of 1.9 Ga. The isotopic parameters of the lead in sulfides and K-feldspar indicate their formation in upper crust conditions (μ = 238U/204Pb > 10). From the obtained data, it follows that the isotopic composition of lead in K-feldspar corresponds to a Proterozoic age (1890 Ma) of magmatic crystallization of the rocks in the massif, and strongly radiogenic lead sulfides testify, with the greatest probability, to the later (Caledonian) formation of sulfide ores.

Influence of hydrothermal-metasomatic processes on the formation of present-day sulfide ores in carbonate bottom sediments of the mid-Atlantic Ridge (19°–20° N)

Based on materials obtained in Cruises 33 and 34 of the R/V Professor Logachev, the paper addresses formation conditions, morphology, structures, mineral composition of the present-day oceanic sulfide ores, and their relationships with the host (biogenic carbonate) bottom sediments in the 19°–20° N MAR (Zenith-Victoria and Petersburg hydrothermal fields) region. The grain size distribution, mineral composition of the carbonate (background) and ore-hosting sediments, as well as physicochemical parameters of their interstitial waters, are examined. The results suggest a significant role of hydrothermal-metasomatic processes in the formation of ores and ore-bearing sediments. A model is proposed for the formation of sulfide mineralization in oceanic sediments at the geochemical barrier in the zone of their interaction with the acid hydrothermal (diffuse-type) ore-bearing solutions delivered from rocks of the ocean floor.