Composition Of Pyrrhotite Research Articles

Shock‐induced pervasive remelting of Fe sulfides in the basaltic shergottite Northwest Africa 14672: A benchmark for shock stages S6/S7 on Mars

AbstractNorthwest Africa (NWA) 14672, the most highly shocked Martian meteorite so far, has experienced >50% melting, compatible with peak pressure >~65 Gpa, at a transition stage 6/7. Despite these extreme shock conditions, the meteorite still preserves a population of “large” Fe sulfide blebs from the pre‐shock igneous assemblage. These primary blebs preserve characteristics of basaltic shergottites in term of modal abundance, preferential occurrence in interstitial pores along with late‐crystallized phases (ilmenite, merrillite), and Ni‐free pyrrhotite compositions. Primary sulfides underwent widespread shock‐induced remelting, as indicated by perfect spherical morphologies when embedded in fine‐grained silicate melt zones and a wealth of mineral/glass/vesicle inclusions. Extensive melting of Fe‐sulfides is consistent with the decompression path experienced by NWA 14672 after the peak shock pressure at ~70 GPa. Primary sulfides acted as preferential sites for nucleation of vesicles of all sizes which helped sulfur degassing during decompression, leading to partial resorption of Fe‐sulfide blebs and reequilibration of pyrrhotite metal/sulfur ratios (0.96–0.98) toward the low oxygen fugacity conditions indicated by Fe‐Ti oxides hosted in fine‐grained materials. The extreme shock intensity also provided suitable conditions for widespread in situ redistribution of igneous sulfur as micrometric globules concentrated in glassy portions of fine‐grained lithologies. These globules exsolved early on quenching, allowing dendritic skeletal Fe‐Ti oxide overgrowths to nucleate on sulfides.

Iron sulfides in sediments of glacial lakes of the upper Kolyma region

Iron sulfides are common in the sediments of glacier lakes of the Magadan region -Gryazevoe (61°08’21” N, 152°19’57” E), Nalimnoe (61°07’41” N, 152°20’8” E), Sosednee (62°03’29” N, 149°3122” E), Sapog (63°29’9” N, 147°50’41” E), Vodorazdel’noe (63°44’8” N, 148°13’4” E), Ui (63°49’31” N, 147°53’21” E) and others. Most of the lakes were formed at the end of the Late Pleistocene or at the beginning of the Early Holocene. Sulfides occur in the spheroid forms with a framboidal structure, dusty clusters, and individual crystals. Sometimes sulfides fill up the diatom valves. The size of framboids does not exceed 100 µm. Sulfides have pyrite, greigite and pyrrhotite compositions. They are distributed unevenly in sections. Greigite and pyrrhotite are distinguished by their magnetic characteristics.

Evolution of magmatic sulfide of the giant Jinchuan Ni-Cu deposit, NW China: Insights from chalcophile elements in base metal sulfide minerals

Chalcophile elements differentially distributed among base metal sulfides (BMS) and other discrete minerals are useful for assessing the evolution of magmatic sulfides. The world-class Jinchuan Ni-Cu sulfide deposit is composed of the western and eastern intrusions, but the differences in the distribution of chalcophile elements among BMS between two intrusions have not been well investigated. This study provides new data of in-situ chalcophile elements and platinum-group minerals (PGM) hosted in BMS of two intrusions, and discusses the discrepancy of chalcophile elements distribution and sulfide melt evolution. The variations of all compatible elements (Re, Co, Os, Ir, Ru, and Rh) and most incompatible elements (Pt, Ag, Zn, Cd, As, Sn, Sb, and Bi) in BMS versus whole-rock (Pt + Pd)/(Ir + Rh + Rh) ratios suggest these chalcophile elements were fractionated by progressive crystallization of the sulfide melt. Micro-textures of PGM suggested that (Ir-Rh-Pt)AsS crystallized almost simultaneously with the monosulfide solid solution, and Pd-semimetal minerals crystallized after the loop-textured pentlandite. As for the same type of sulfide mineralization, the contents of chalcophile elements in BMS minerals of the western intrusion are generally higher than those in BMS minerals of the eastern intrusion, which is consistent with the whole-rock variations. However, chalcopyrite of the eastern intrusion hosts significantly higher proportions of Pd and Te than chalcopyrite of the western intrusion with comparable amounts of Pd-semimetal micro- and nano-particles, indicating the diffusion of Pd and Te during the peritectic reactions in the eastern intrusion was inhibited. In terms of the negative correlation between Metal/S ratios of pyrrhotite and elemental ratios of Pd and Te between pentlandite and chalcopyrite, it is likely attributed to the lower sulfur fugacity (fS2) in the eastern intrusion which has been quantitatively calculated by pyrrhotite composition. We propose schematic models of the crystallization history of sulfide melts at Jinchuan, in which fractional crystallization of sulfide melts, crystallization of PGM and other discrete minerals, and formation of loop-textured pentlandite jointly control the distribution of chalcophile elements. The difference of fS2 between western and eastern intrusions resulted in the differential diffusion of Pd and Te from fractionated sulfide melt into loop-textured pentlandite.

Geology and Genesis of the Giant Gorevskoe Pb-Zn-Ag Deposit, Krasnoyarsk Territory, Russia

Abstract The Gorevskoe Pb-Zn-Ag mine is currently the largest producer of Pb and Zn in Russia, exploiting one of the largest sediment-hosted Pb-Zn deposits worldwide. Despite its size and economic importance, the Gorevskoe deposit remains poorly understood. It is located on the western margin of the Siberian craton within the Yenisei Ridge, a Neoproterozoic orogenic belt. Mineralization consists of three tabular orebodies that are in turn composed of multiple stacked stratiform to strata-bound lenses of galena-pyrrhotite-sphalerite-rich massive sulfide ore, hosted in organic-rich marine metalimestones and calcareous slates of Stenian to Tonian age (1,020 ± 70 Ma). Extensive Fe-Mg-Mn-carbonate alteration haloes surround the ore lenses. The Pb isotope signature of the deposit is consistent with derivation of Pb, and probably all associated metals, from an evolved crustal source at the time of formation of the host rocks. The sulfur isotope compositions of pyrrhotite, sphalerite, galena, arsenopyrite, and pyrite (δ34S = 16.0–20.4‰) do not vary considerably across the deposit and are within the range reported for contemporaneous seawater, indicating complete reduction of marine sulfate as the main source of sulfur. The available geologic and geochemical data indicate that the Gorevskoe deposit belongs to the sediment-hosted massive sulfide (SHMS) class of Zn-Pb deposits, with an affinity to Selwyn-type deposits. Hydrothermal mineralization appears to be temporally related to rifting and distal mafic volcanism in a passive margin setting. Geologic relationships suggest that the orebodies formed in a diagenetic environment. Furthermore, the predominance of primary pyrrhotite over pyrite as the major iron sulfide, the presence of abundant siderite, and the relatively homogeneous sulfur isotope signature of the ores indicate highly reducing conditions during ore formation. They also constrain the character of the metal-bearing fluid to be similarly reducing, and of moderate temperature (200°–300°C). Gorevskoe may thus be regarded as one of the world’s largest Selwyn-type SHMS deposits.

Mineralogy and mineral chemistry related to the Au mineralization in the Dunde Fe-Zn deposit, western Tianshan

The Dunde deposit, which is located in the Awulale Fe metallogenic belt, Xinjiang, China, is a uniquely large Fe-Zn deposit and also contains considerable Au resources. We present the results of a comprehensive study of this deposit based on fieldwork, mineralogy, and mineral geochemistry to constrain the Au mineralization. Our study indicates that the mineralization of Fe, Zn, and at least some Au precipitated in the same hydrothermal system. The Au mineralization, which occurs in the sulfide stage, is characterized by invisible Au at high temperatures and by electrum at low temperatures. Unusual sulfide is recognized in the deposit, such as cobaltite and bismuth minerals. Pyrrhotite compositions indicate that the early precipitation of sulfide generated in high temperatures (410–570 ℃) and high sulfur fugacities (−2.47 to −6.68). Arsenopyrite compositions indicate that the late precipitation of sulfide formed in low temperatures (326–393 ℃) and low sulfur fugacities (−7.41 to −12.00). Bismuth minerals exhibit a close relationship with the Au mineralization. In high temperatures and high oxygen fugacities, bismuth dissolved in the hydrothermal fluid as Bi3+ and precipitated as galenobismuthite and cosalite accompanying precipitation of the invisible Au. Accompanying the decrease of oxygen fugacities and temperatures, the Bi melt directly separated from the hydrothermal fluid and scavenged Au. The reduced condition further enriched Au in Bi melts through the decomposition of the invisible Au-bearing Bi-Pb sulfide. The decrease of temperatures caused the decomposition of the Bi-Au melt and the formation of native bismuth, bismuthinite, and electrum. In addition to the mineralization of Fe, Zn and Au, considerable Cu-, Co-, and Mn-bearing sulfide precipitated in the Zn ores. Integrated utilization of the Zn ores may get greater economic values.

A novel application of image analysis to interpret trace element distributions in magmatic sulphides

The Albany–Fraser Orogen in Western Australia hosts significant magmatic sulphide mineralisation but the impacts of its extensive metamorphic history on this mineralisation are poorly understood. Post-formation modification, including fluid-facilitated alteration, can upgrade or destroy mineralisation, with significant consequences for economic ore deposits. We present laser ablation trace element data from samples of variably altered sulphide breccia from the mineralised Octagonal prospect located in the Fraser Zone of the Albany–Fraser Orogen. Mineral compositions are combined with laser ablation mapping and novel co-localisation analysis to characterise the magmatic and post magmatic processes that modified element distributions within magmatic sulphide ore. Higher concentrations of Re, Co, Ni, Os, Ir, and lower concentrations of Cu, Zn and Ag in pyrrhotite and pentlandite relative to chalcopyrite record sulphide crystallisation from a sulphide liquid. Textural evidence and the compositions of pyrrhotite and pyrite are consistent with replacement of pyrrhotite by pyrite and associated magnetite. Element maps reveal intergranular variations in pentlandite Ag content that are systematically linked to the sulphide distribution. Co-localisation analysis shows that the associations among Mn, Pb, Ag and the sample fracture networks are statistically significant; the association is interpreted as a consequence of fluid-assisted element remobilisation. Post-formation alteration processes at Octagonal were accompanied by variable remobilisation and depletion of metal concentrations associated with mineralisation; the co-localisation analysis provides an invaluable tool that enabled quantitative assessments of these relationships.

Composition of Pyrrhotite as an Indicator of Gold Ore Formation Conditions at the Sovetskoe Deposit (Yenisei Ridge, Russia)

Abstract —We present results of an investigation into the composition and parageneses of pyrrhotite at the Sovetskoe gold–quartz deposit (Yenisei Ridge, Russia). The variability of parameters (temperature T and sulfur fugacity fS2) during the stage crystallization of pyrrhotite-containing assemblages has been assessed from the composition of this mineral (Fe0.873±0.02S–Fe0.885±0.02S) and its parageneses. The compositions Fe0.873–0.875S close to Fe7S8 (Apy + Po + Rut + Qz), for which the estimated formation parameters are T = 486–465 °C and log fS2 = –4.71 to –5.28, are typical of early pyrrhotite in the form of microinclusions in arsenopyrite, associated with rutile and quartz. According to the composition of inclusions of pyrrhotite microcrystals (Fe0.873–0.881S) associated with pyrite in native gold (950‰) (Au + Po + Py), the formation parameters are T = 489–410 °C and log fS2 = –4.63 to –6.98. Coarse pyrrhotite grains containing microinclusions of relict arsenopyrite and galena, sometimes, in aggregate with siderite (Po + Apy + Ga + Sid), and pyrrhotite in aggregate with pyrite and siderite (Py + Po + Sid) have composition Fe0.874–0.878S and form at 479–443 °C and log fS2 = –4.9 to –5.9. The xenomorphic pyrrhotite microinclusions present together with galena and native gold (950‰) in pyrite crystals (Py + Po + Ga + Au) are characterized by higher contents of iron (Fe0.878–0.885S) and, correspondingly, lower temperatures of formation, 432–382 °C, and log fS2 = –6.27 to –7.95. The log fS2–T diagrams have been calculated for the systems Fe–S and Ag–Au–S in the temperature range 25–700 °C with regard for the stability fields of iron sulfides (pyrite FeS2, troilite FeS, and pyrrhotite Fe7S8), phases Fe11S12, Fe10S11, and Fe9S10, metallic iron, native sulfur, uytenbogaardtite, petrovskaite, and solid-solution phases Fe1–xS (0 < x < 0.125), Ag1–zAuz (z = 0, 0.25, 0.5, and 1), and Ag2–yAuyS (y = 0, 0.5, 1, and 2). The calculation results have demonstrated that there is a field of petrovskaite and uytenbogaardtite solid solutions and Au–Ag alloys (>670‰, Ag0.5Au0.5–Au) in the stability field of the pyrrhotite–pyrite parageneses of the Sovetskoe deposit. The gold and silver contents in iron sulfides of the Sovetskoe deposit show that the Au/Ag ratios in pyrrhotites (0.002–2.4) and pyrites (0.004–13) are lower than those in high fineness (950–980‰) gold (19–50). The difference in the Au/Ag ratios in these minerals and the results of thermodynamic calculations indicate the possible presence of Au–Ag sulfides and Au–Ag alloys of lower fineness in the pyrrhotite–pyrite ores of the studied deposit. The absence of visible mineral forms of gold sulfides from the ores suggests that these sulfides are present in finely dispersed or invisible microscopic forms. The pyrrhotite compositions in pyrite-containing parageneses as well as Au/Ag in pyrites, pyrrhotites, and visible native gold in sulfide ores of other gold and gold–silver deposits can be used to assess the possible presence of nanosized solid microinclusions of sulfide and other gold and silver forms.

China: Columbian – carbon black

Through the use of the electrum-tarnish method the following equation has been found to interrelate the composition of pyrrhotite, fugacity of sulfur, and temperature: In this equation fs2 is the fugacity of sulfur relative to the ideal diatomic gas at 1 atm, N is the mol fraction of FeS in pyrrhotite (in the system FeS-S2), and T the absolute temperature. The experimental uncertainty in the equation is 0–003 in N. The activity of FeS (aFeS) in pyrrhotite relative to the pure substance at the temperature of consideration follows from the above equation by virtue of the Gibbs-Duhem relation; it is given by:The electrum-tarnish method has permitted us to determine the fs2 vs. T curve for the univariant assemblage pyrrhotite-pyrite-vapor from 743 to 325°C. Our determinations of the composition of pyrrhotite are in excellent agreement with the results of Arnold. The activity of FeS in pyrite-saturated pyrrhotite is very different from unity, a fact that greatly influences the interpretation of some other phase equilibrium studies involving pyrrhotite and their application to sulfide mineral assemblages, but has little effect on the more general calculations of composition of hydrothermal or magmatic fluids.Pressure effects calculated from available volumetric data on the phases are small.

Thermal and thermomagnetic properties of pyrrhotites

A comparative analysis of the thermal and magnetic properties of synthesized samples of troilite (FeS) and hexagonal (Fe0.94S) and monoclinic (Fe0.875S) pyrrhotites was performed by X-ray diffraction, combined thermogravimetric analysis and calorimetry, and thermomagnetic measurements. The pyrrhotite composition affects the temperatures and heats of phase transitions of the samples during heating and cooling in the range of 300–670 K. The temperature and the magnetic field strength determine the magnetization of pyrrhotites. Higher magnetization is inherent in monoclinic pyrrhotite, while troilite has lower magnetization.

Relationship between microstructures and grain-scale trace element distribution in komatiite-hosted magmatic sulphide ores

Komatiite-hosted nickel sulphides from the Yilgarn Craton (Australia) consist of two main sulphide phases: pyrrhotite (Fe7S8) and pentlandite ((Fe,Ni)9S8); two minor sulphide phases: chalcopyrite (CuFeS2) and pyrite (FeS2) and trace arsenides. Samples of massive sulphides from three deposits with diverse deformation and metamorphic histories (the Silver Swan, Perseverance and Flying Fox deposits) have been studied by electron backscatter diffraction and laser ablation inductively coupled plasma mass spectrometry and nano-scale secondary ion mass spectrometry. These ore bodies were selected to investigate the relationship between microstructures and mineral trace element chemistry in three dominant sulphide species in each deposit. In all three samples, pyrrhotite preserves a strong evidence of crystal plasticity relative to both pentlandite and pyrite. The trace element composition of pyrrhotite shows significant variation in specific elements (Pb, Bi and Ag). This variation correlates spatially with intragrain pyrrhotite microstructures, such as low angle and twin boundaries. Minor signatures of crystal plasticity in pyrite and pentlandite occur in the form of rare low angle boundaries (pentlandite) and mild lattice misorientation (pyrite). Trace element compositions of pentlandite and pyrite show no correlation with microstructures. Variations in pyrrhotite are interpreted as a result of intragrain diffusion during the syn- and post-deformation history of the deposit. Intragrain diffusion can occur either due to bulk diffusion, dislocation–impurity pair diffusion, or by “pipe diffusion”, i.e. along fast diffusion pathways at high and low angle, and twin boundaries. This contribution examines three different diffusion models and suggests that dislocation–impurity pair diffusion and pipe diffusion are the most likely processes behind increased trace element concentration along the microstructures in pyrrhotite. The same phenomenon is observed in samples from three different deposits that experienced widely different metamorphic conditions, implying that the final disposition of these elements reflects a post peak-metamorphic stage of the geological history of all three deposits.

Geology, geochemistry, and geochronology of the Dunde iron–zinc ore deposit in western Tianshan, China

The Dunde iron–zinc deposit (185Mt at 35% Fe), located in the Tianshan orogenic belt, Xinjiang, northwest China, is hosted in late Carboniferous volcanic–volcaniclastic rocks characterized by prograde skarn formation and retrograde alteration, but the ore lacks any clear spatial link with intrusive rocks. Four stages of skarn formation and ore development can be recognized: (1) a prograde skarn stage that formed grossularitic garnet (Gr39–80; Ad15–58) and diposidic pyroxene (Di63–97); (2) a retrograde skarn stage dominated by the formation of magnetite with minor epidote, ferropargasite, apatite, hematite, and trace amounts of titanite and spinel; (3) a sulfide stage dominated by the formation of arsenide (loellingite and arsenopyrite) and sulfide minerals (sphalerite, pyrrhotite, pyrite, chalcopyrite, and minor galena), calcite, and traces of quartz; and (4) a chlorite–calcite stage mainly characterized by the formation of chlorite, calcite, and traces of sericite. Exsolution of droplet-like and patchy chalcopyrite is developed within sphalerite of the sulfide stage, indicating an exsolution temperature of 350–400°C. As such, the crystallization temperature of early stage magnetite may have been >400°C. Abundant fluid inclusions occur in calcite, which include daughter-mineral-bearing H2O, H2O, and pure H2O inclusions. The H2O inclusions have a wide range of homogenization temperatures from 147°C to 367°C with salinities of 2.4–23.4wt.% NaCl equivalent. The daughter-mineral-bearing H2O inclusions have homogenization temperatures from 172°C to 347°C with salinities of 31.9–33.0wt.% NaCl equivalent. Using the chlorite geothermometer, the temperature of chlorite formation is constrained to be between 152°C and 222°C (average=194°C). Sulfur isotope compositions of pyrrhotite, sphalerite, pyrite, and loellingite have a narrow range of δ34S values from 3.8‰ to 8.1‰ (average δ34S=6.8‰), suggesting that the sulfur was magmatic-derived. Zircon LA–ICP-MS U–Pb dating of wall rock dacite yields a weighted mean 206Pb/238U age of 316.0±1.7Ma. Combined with previous dating results of magnetite-mineralized diorite stocks, diorite dikes, and garnet skarn, it can be inferred that the Dunde iron–zinc deposit formed in the late Carboniferous after 316Ma and is genetically related to deep dioritic intrusions. During the late Carboniferous, the tectonic setting of this region changed from subduction–collision to extension, accompanied by mantle-derived magma underplating in deep. After the formation of the iron–zinc ore deposit, the Dunde district was intruded by an early Permian K-feldspar granite that yields a zircon LA–ICP-MS U–Pb age of 295.75±0.71Ma.

Fluid-present melting of sulfide-bearing ocean-crust: Experimental constraints on the transport of sulfur from subducting slab to mantle wedge

To constrain the sulfur enrichment of arc magma source-regions and the agent of sulfur transport from subducting slab to mantle wedge, here we report experimental measurements of sulfur content at sulfide saturation (SCSS) of slab-derived hydrous partial melts at 2.0 and 3.0GPa and from 800 to 1050°C, using Ni–NiO (NNO) and Co–CoO (CCO) external oxygen fugacity (fO2) buffers. A synthetic H2O-saturated MORB with 1wt.% S (added as pyrite) was used as starting material. All experiments produced pyrrhotite- and fluid-saturated assemblages of silicate glass, clinopyroxene, garnet, quartz, and rutile (plus amphibole at 2GPa/800°C and phengite at 3GPa/850°C). The silicate partial melt composition evolves from rhyolitic to rhyodacitic compositions with increasing temperature and melting degree in equilibrium with an eclogitic residue, showing substantial decrease in SiO2 and Mg# and increase in FeOT, TiO2 and Na2O. At all temperatures melt sulfur concentrations are very low, with an average of 110±50ppm S, similar to previous measurements at lower pressures. Melt SCSS appears to be mainly controlled by the melt composition, the activity of water, aH2O and the sulfur fugacity, fS2 (calculated from the composition of pyrrhotite). Mass-balance calculations show that the proportion of bulk sulfur dissolved in the silicate melt is negligible (<0.005wt.% of the bulk sulfur). In contrast, diminishing proportion of pyrrhotite with increasing temperature suggests that the fluid phase at equilibrium may contain as much as 10–15wt.% S at ⩾1050°C, and more than 40wt.% of the bulk sulfur initially present in the slab may be transferred to the aqueous fluid. Our data also suggest that fluid/melt sulfur partitioning increases with increasing temperature, from ∼300 at 900°C to ∼1200 at 1050°C, whereas pressure appears to have less of an effect. With respect to fO2, no real difference of fluid/melt S partitioning, within data uncertainties, between NNO and CCO at 2 and 3GPa was observed. This is interpreted as a consequence of comparable S speciation in the hydrous fluid phase in fO2 conditions ranging from NNO−0.5 to CCO−1.0 at 2 and 3GPa and 850 to 1050°C. Under these reducing conditions, the hydrous fluid phase is composed mainly of H2S and H2O, and to a lesser extent of S2 and H2, and S in melt is dissolved almost totally as S2− (i.e., S6+/Stotal∼0). Our study suggests that slab-derived aqueous fluids with reduced sulfur species could be sufficient vectors for the transport of sulfur from the slab to the mantle wedge and only ∼0.3 to 1.5wt.% of slab fluids are required for the mantle wedge enrichment in sulfur over background mantle concentrations. Also, depending on the initial amount of sulfur in the slab, recycling of sulfide to the deep mantle may be favored in cold to intermediate subduction zones, whereas warm subduction zones will cause fluid-mediated slab-mantle wedge transfer of sulfur.

Sulfide petrology of four nakhlites: Northwest Africa 817, Northwest Africa 998, Nakhla, and Governador Valadares

Abstract–The nakhlites contain small proportions of Cu‐Fe‐Ni sulfide minerals; we have studied these sulfides in Northwest Africa (NWA) 998, Nakhla, Governador Valadares, and NWA 817 with optical microscopy, scanning electron microscope, and electron microprobe. Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are uniformly low (0.02 to 0.05 ± 0.03 vol%), i.e., a factor 5 lower than in shergottites. Sulfides occur within the glassy mesostasis, as composite two‐phase Fe‐Ti oxide‐sulfide grains, intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions (e.g., Governador Valadares) in olivine. They exhibit a uniform low‐Ni monoclinic pyrrhotite composition ± chalcopyrite. There is a gradation of sulfide grain sizes and textures across the nakhlites flow(s): droplets in NWA 817; resorbed blebs in Governador Valadares; more massive, true intercumulus blebs in Nakhla and NWA 998. These nakhlites also show evidence for sulfide weathering. Hot desert finds (e.g., NWA 998 and NWA 817) show a few percent fracture‐filling iron (oxy) hydroxides of likely terrestrial origin. Original sulfides are 50% altered in our NWA 998 section, with iron (oxy) hydroxides at grain boundaries and as complete pseudomorphs. The compositions of unaltered pyrrhotites are homogeneous, close to that of the monoclinic endmember Fe7S8, and are too sulfur‐rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite‐magnetite‐quartz buffer. Therefore, the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization, by assimilation of S‐rich regolith and hydrothermal circulation.

Geochemical model of genesis of the Kingash platinoid-copper-nickel deposit

71 Kingash ore field with the deposit of the same name is located within the Archean Kan block of Eastern Sayan and does not go beyond the boundary of the new nickel and platinum bearing province with numerous ore bearing massifs [1]. In plan the Kingash massif forms a lens (Fig. 1) comprising two “megalayers”: lower peridotite–pyroxenite–serpentinite and overly ing gabbroid. In section it presents lopolith with a geo physically reflected gravitational stage approximated with a multichamber incurrent canal [3]. Ultrabasic rocks of the massif contain Pechenga type sulfide ore of chalcopyrite–pentlandite–pyrrhotite composition. Geochemical Model of Genesis of the Kingash Platinoid–Copper–Nickel Deposit O. M. Glazunov and T. A. Radomskaya Presented by Academician A.A. Marakushev April 1, 2009

Thermodynamic studies of pyrrhotite–pyrite equilibria in the Ag–Fe–S system by solid-state galvanic cell technique at 518–723 K and total pressure of 1 atm

The reaction FeS 2(cr) + 2Ag(cr) = ‘FeS’(cr) + Ag 2S(cr) was studied by measuring the temperature dependence of the electromotive force (EMF) of the all-solid-state galvanic cell with common gas space: ( - ) Pt | Ag | AgI | Ag 2 S, ‘ FeS ’ , FeS 2 | Pt ( + ) The measurements were carried out in the flow of argon at atmospheric pressure to prevent oxidation. AgI was used as a solid electrolyte. From the measurements of EMF as a function of temperature, two linear (Δ r C p = 0) trends were obtained, which characterize the equilibrium hexagonal pyrrhotite + pyrite (po + py) and the β–γ first-order phase transition in hexagonal pyrrhotite (Δ trs H m(γ–β) = (4020 ± 200) J mol −1) at (601 ± 2) K. The latter is presumably related to the γ-paramagnetic-β-antiferromagnetic Neel’s transition. For these measurements, the lower temperature limit (518 K) corresponds to the equilibrium sulfides + metallic silver ( E = 0); and the upper temperature limit (723 K) is determined by the upper temperature at which the contribution of electron conductivity to AgI ion conductive properties may be significant. From experimental results of this study and literature data for Ag 2S, the temperature dependence of the gaseous sulfur activity was determined in the following equilibria: pyrite + high-temperature hexagonal pyrrhotite (γ + py), pyrite + low-temperature pyrrhotite (β + py): log a S 2 ( γ + py ) = ( 15.64 ± 0.035 ) - ( 15455 ± 23 ) · T - 1 , ( 601 < T , K < 723 ) log a S 2 ( β + py ) = ( 14.95 ± 0.05 ) - ( 15040 ± 28 ) · T - 1 , ( 518 < T , K < 601 ) The temperature dependence of the FeS activity in hexagonal pyrrhotite ( a FeS po ) calculated for the pyrrhotite–pyrite equilibrium a FeS po ( γ ) = ( 0.528 ± 9.23 × 10 - 3 ) + ( 2.489 × 10 - 7 ± 1.485 × 10 - 5 ) · T , ( 601 < T , K < 723 ) indicates that the value a FeS po = ( 0.528 ± 0.005 ) remains almost constant in the mentioned temperature range. At temperatures below 601 ± 2 K, a FeS po ( β ) and the Fe/S ratio in β-po decreases with decreasing temperature, which also correlates with the appearance of magnetic order. The compositional extremum (on a T– x diagram) where hexagonal pyrrhotite is in equilibrium with pyrite results in a significant and obvious geological consequence. In all the mineral assemblages that contain hexagonal pyrrhotite and pyrite and that have reached equilibrium at a temperature below (601 ± 2) K, the same pyrrhotite composition will correspond to two different temperatures. However, the minimal temperature principle is acceptable for natural parageneses; and in many cases, the pyrrhotite geothermometer can still be very useful.

Sulfide mineralogy and redox conditions in some shergottites

Magmatic sulfide mineralogy has been studied in 2 olivine-phyric shergottites (DaG 476 and SaU 005) and 4 basaltic shergottites (Zagami, Shergotty, Los Angeles, and NWA 480). Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are high (0.16 to 0.53 area percent) and correlate positively with abundances of Fe-Ti oxides. Sulfides are mesostasis minerals, being mostly interstitial grains or locally enclosed in post-cumulus melt inclusions (e.g., SaU 005) in olivine. Sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of Cu sulfides (chalcopyrite and/or cubanite). Hot desert finds (e.g., DaG 476) show abundant fracture-filling iron (oxy)hydroxides of probable terrestrial origin. Unaltered sulfides show metal-rich hexagonal pyrrhotite compositions with metal/sulfur (M/ S) ratio ranging between 0.936 ± 0.005 and 0.962 ± 0.01. This compositional range corresponds to the two-phase structural domain 2C + nC of the Fe-S system; however, the high-temperature disordered hexagonal 1C pyrrhotite structure would be in better agreement with magnetic properties of shergottites. Ni contents in pyrrhotite increase from Los Angeles (<0.05 at%) to Shergotty, Zagami, and NWA 480 (0.2-0.5 at%), and DaG 476 and SaU 005 (up to 3 at%). The higher Ni values of pyrrhotite in olivine-phyric shergottites correlate with the abundance of pentlandite exsolutions, both reflecting more primitive Ni-rich sulfide liquids where abundant olivine crystallized. This result and the strong correlation between sulfide abundances and Fe-Ti oxides argue for a primary magmatic origin of these sulfides. Although they reproduce the trend of magmatic oxygen fugacity conditions determined from Fe-Ti oxide pairs, observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the Fe-S-O system. This suggests post-magmatic oxidation during cooling on Mars, followed by terrestrial weathering for hot desert finds.

EXPERIMENTAL SULFUR ISOTOPE STUDIES OF THE PYRITE TO PYRRHOTITE CONVERSION IN A HYDROGEN ATMOSPHERE

Sulfur isotope effects associated with the conversion of pyrite to pyrrhotite in a dry hydrogen atmosphere were measured at temperatures from 500° to 600°C. H 2 S was the primary fluid species produced, although at temperatures of 500° and 525°C native sulfur in amounts up to 30 percent of the total liberated sulfur condensed in the collection system. At temperatures of 550°, 575°, and 600°C, more than 90 percent of the liberated sulfur was in the form of H 2 S. At all temperatures pyrrhotite was the only residual iron sulfide. Native sulfur was characterized by a δ 34 S value of –0.1 per mil relative to the starting pyrite, whereas H 2 S varied from –0.2 to +0.1 per mil (most values were within analytical error of 0‰). The produced pyrrhotite ranged from 0.3 per mil at 500° and 525°C, to 0.2 per mil at all higher temperatures. Pyrrhotite-H 2 S fractionation factors are near equilibrium values for these temperatures and suggest that potential kinetic isotope effects are suppressed at low f O2 conditions where H 2 S is the principal fluid species. Both experimental and empirical studies suggest that very little sulfur isotope fractionation accompanies the conversion of pyrite to pyrrhotite in carbon-rich metapelites. For this reason the isotopic composition of pyrrhotite in graphitic metapelites may serve as a reliable indicator of the isotopic composition of sulfur that was available for assimilation by magmas or hydrothermal fluids, and that may have been involved in later ore genesis.

Symplectite of pyrite and magnetite in the massive sulphide ore of the Mashan mine, Anhui Province

A symplectite of pyrite and magnetite in the massive sulphide ore of the Mashan mine, Anhui Province, is interpreted to have been formed by their replacing earlier pyrrhotite. The compositions of pyrrhotite, pyrite and magnetite related to this texture are given by electron microprobe analysis. Such a texture is likely to be formed when the ore-forming system reaches the three-phase point of pyrrhotite, pyrite and magnetite from the pyrrhotite stability field. The very small probability for the system to reach this point could be used to account for the rare occurrence of such sympletite in natural ores.

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

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

Sulfide mineralogy at the main magnetite layer, upper zone, eastern Bushveld Complex, and the effect of hydrothermal processes on pentlandite composition

The sulfide assemblages within a few metres above and below the main magnitete layer (MML) in the upper zone of the eastern Bushveld Complex have been studied including detailed microscopic work and elecron microprobe analyses of pentlandite and pyrrhotite. Rocks at this stratigraphic level, especially below the MML, are enriched in sulfides relative to the concentrations normally encountered in the lower part of the upper zone. The main sulfide minerals, are, in decreasing oder of abundance, pyrrhotite, chalcopyrite, cubanite and pentlandite. Pyrrhotite compositions span the whole compositional range from monoclinic pyrrhotite to troilite. Within pairs of coexisting pyrrhotite and pentlandite, the Fe content in puyrrhotite is related to the Fe/Ni ratio of pentlandite. However, pyrrhotite compositions with and without visible contact to pentlandite can not be distinguished. The same applies to pyrrhotite with and without visible contact to chalcopyrite or cubanite, and for the composition of pentlandite with and without pyrrhotite or chalcopyrite. Above the MM, the composition of pentlandite varies considerably within individual samples (by up to 18 at.% Fe, 22 at.% Ni and 28 at.%Co) which is related to hydrothermal processes. These processes led to an increase in the Co content of all pentlandite grains above the MML. While substitution of Co for Ni is dominant, Co also replaces Fe in Co-rich pentlandite. Furthermore, Ni in Co-poor pentlandite is replaced by Fe. In constrast, the composition of pyrhhotite was only found to be affected if in contact with secondary magnetite or Co-rich pentlandite. Although changing the cation proportions in pentlandite, the hydrothermal processes did not influence the metal/sulfur ratio. it is concluded that hydrothermal activity played an important role in changing the sulfide mineralogy and composition in this part of the upper zone. The absence of minerals indicative of hydrothermal overprinting in a sulfide assemblage can not be taken as evidence that this assemblage has not been affected by hydrothermal processes