Pyrite Stability Research Articles

Pyrite stability and chalcophile element mobility in a hot Eocene forearc of the Pacific Rim Terrane, Vancouver Island, Canada

Pyrite stability and chalcophile element mobility in a hot Eocene forearc of the Pacific Rim Terrane, Vancouver Island, Canada

Development of one-step process method to separate tin from tin-refining sulfur slag via vacuum sulfuration and volatilization

Sulfur slag is a dangerous solid waste generated during tin refining. To ensure environmental safety, sulfur slag must be treated. As a result, a novel one-step process method was proposed and developed for clean and efficient recovery of tin and copper from sulfur slag. The method was based on the thermal stability of pyrite to simultaneously sulfurize and volatilize tin, which greatly improved the reaction efficiency. The feasibility of the one-step separation of tin and copper was confirmed by theoretical calculations such as Gibbs free energy and saturated vapor pressure. The effects of temperature, pyrite dosage, system pressure, and holding time were experimentally studied. Finally, the volatile was stannous sulfide, and the residue was cuprous and ferrous sulfides. The optimal experimental conditions were as follows: the reaction temperature was 1273 K, the pyrite addition ratio was 40%, the system pressure was 10 Pa, and the holding time was 3 h. The direct recovery rates of tin, copper, and iron were 99.95, 99.77, and 99.57%, respectively. The removal rates of tin, copper, and iron were all above 99.8%. The vacuum sulfuration and volatilization method provides a novel process for the clean and effective cyclic treatment of tin-refining sulfur slag.

Evolution of Gold in the Oxidation Zone of the Kopto Deposit (the Republic of Tuva, Russia)

Abstract —The Kopto deposit (northeastern Tuva) is assigned to gold ore objects with a combination of the Au–Cu–skarn and superposed quartz–gold–sulfide stockwork types of mineralization. From the surface, the ores underwent intense oxidation, which formed a zone of secondary gold enrichment, containing a supergene paragenesis with gold and silver chalcogenides and newly formed gold. The depth of distribution of oxidized ores from the surface is 80–90 m. The Au content varies from fractions of ppm to 150 ppm (on average, 30.8 ppm). Using computer thermodynamic modeling, it is shown how the ore gold–sulfide–quartz association transformed under oxidizing conditions with a decrease in the pH of solutions. Gold becomes more and more high-grade; acanthite appears and disappears; limonite prevails (pH = 1.65; Eh = 0.69 V). The conditions for the stability of pyrite, iron hydroxides, and gold and silver chalcogenides (petrovskaite (AgAuS) and uytenbogaardtite (Ag3AuS2)) have been estimated. It requires weakly acidic solutions with pH = 5–6 and Eh values close to zero, which ensures the stability of thiosulfate and hydrosulfide complexes of noble metals. The main difference between solutions in equilibrium with petrovskaite and uytenbogaardtite is the Ag/Au ratios, which are maximum in the first case and approximately equal in the second. The paper is concerned with a comparative analysis of the morphologic features of gold from primary and oxidized ores of the Kopto gold deposit. The aim of this work is to identify a set of signs of the supergene nature of gold and to assess the extent of its redistribution.

Reaction mechanism of metal and pyrite under high-pressure and high-temperature conditions and improvement of the properties

Pyrite tailings are the main cause of acid mine wastewater. We propose an idea to more effectively use pyrite, and it is modified by exploiting the reducibility of metal represented by Al under high-pressure and high-temperature (HPHT) conditions. Upon increasing the Al addition, the conductivity of pyrite is effectively improved, which is nearly 734 times higher than that of unmodified pyrite at room temperature. First-principles calculations are used to determine the influence of a high pressure on the pyrite lattice. The high pressure increases the thermal stability of pyrite, reduces pyrite to high-conductivity Fe7S8 (pyrrhotite) by Al. Through hardness and density tests the influence of Al addition on the hardness and toughness of samples is explored. Finally we discuss the possibility of using other metal-reducing agents to improve the properties of pyrite.

Olovem bohatá oxidační zóna polymetalické mineralizace na lokalitě Kvasetice u Havlíčkova Brodu: distribuce minerálních asociací s pyromorfitem a arzenáty

Supergene mineral assemblages rich in Pb, As and P occur on the dumps after the historical mining of the base-metal deposit in Kvasetice (Havlíčkův Brod ore district, Bohemian-Moravian Highlands, Czech Republic). The primary hydrothermal assemblage of pyrite > sphalerite > arsenopyrite > galena >> boulangerite hosted in the quartz veins is partly different from other similar localities in this area, which belong to the typical k-pol ores with iron-rich sphalerite, pyrrhotite, pyrite, arsenopyrite and chalcopyrite. Kvasetice locality is characterized by a relatively higher stability of pyrite and sphalerite with a relatively low Fe content (< 0.12 apfu of Fe) compared to arsenopyrite and galena in supergene conditions, as well as a total deficit of Cu, Ca and partly S. In the supergene mineral association rich in Pb-P-Fe-As, two main assemblages can be distinguished. Supergene assemblage I contains common microscopic anglesite replacing galena and two sub-groups of arsenates with sulphide relics in small quartz veins: (a) in close association with the relic arsenopyrite are pharmacosiderite and hydroniumpharmacosiderite, both relatively Pb-rich (≤ 0.18 apfu of Pb), accompanied by a rare scorodite. (b) More abundant assemblage of segnitite >> beudan­tite > mimetite >> carminite and coronadite without a direct association with sulphides. Other phases with a non-stoichiometric Fe/As ratio (HFA) are only locally found. A sporadic greenockite is the only detected decomposition product of sphalerite. The Pb-Fe3+ sulfates (minerals of the jarosite subgroup), gypsum, and Ca-, Zn-, Sb- and Cu-supergene minerals are missing. Supergene assemblage II is mainly represented by pyromorphite (mostly with As below detection limit, but locally with ≤ 0.44 apfu), rarely by mimetite and only locally also by cerussite. They overgrow thin crusts of mainly Fe-oxyhydroxides covering the altered granite matrix or line thin druse quartz veinlets. The supergene mineral assemblages from Kvasetice differ not only from other localities in the ore district studied, but also from other localities of supergene mineralization in the Bohemian-Moravian Highlands. They are the product of weathering in the shallow supergene zone, however, they could also have been arised by subrecent weathering of materials deposited on the dumps during the 13-16th century.

Environmental-sulfur-controlled surface properties of pyrite: a first principles PBE + U study

The structure and stability of pyrite (100), (210), (110), and (111) surfaces in different sulfur conditions were studied in the framework of GGA(PBE) + U implementations of density functional calculations. The Hubbard U correction was found to be a critical parameter in density functional theory (DFT) calculations to describe pyrite surfaces. With the U correction, the surface energy order for stoichiometric pyrite surfaces is (100) < (210) < (110) < (111), which is different with previous knowledge from conventional DFT and classical force field calculations, but consistent with broken bonds number and shape distribution in natural and synthetic systems in different sulfur conditions. Within the considered surfaces, the stoichiometric (100)-S and non-stoichiometric (111)-3S are the most stable under both S-lean and S-rich conditions, respectively. For relative stable surfaces in different environment, (100)-Fe and (100)-2S reconstruct, but (111)-Fe and (111)-3S show no relaxations while (100)-S, (210)-Fe′, and (210)-2S′ show relaxations to certain degree. Electron transfer from surface Fe to S atom on pyrite during relaxation and reconstruction, forming exemplary surface configuration and chemical composition. These findings reveal the nature of pyrite surfaces in various sulfur conditions, proving fundamental not only for surface-related applications but also for geological indicators of pyrite formation conditions.

Highly Efficient Rhodamine B Decolorization by Pyrite/Peroxymonosulfate/Hydroxylamine System

In this study, pyrite/peroxymonosulfate (PMS)/hydroxylamine (HA) system could decolorize rhodamine B (RhB) in a wide pH range of 3.0-10.0. Near-100% decolorization of RhB was realized under optimum conditions of pyrite 0.4 g L−1, HA 0.8 mM, PMS 1.6 mM, and initial pH 4.0. Radical (mainly SO4•−) oxidation was the dominant process for RhB decolorization by pyrite/PMS/HA system, but nonradical (largely PMS) oxidation also played a non-negligible role. HA can act as a metal-free activator for PMS activation, and a reductant to boost Fe(III)/Fe(II) cycle in solution and on pyrite surface, which not only increased the production of radicals but also enhanced the stability of pyrite via inhibiting the S22− consumption. SO4•− that produced from pyrite oxidation was mainly from PMS activated by the Fe(II) in solution. Four other dye pollutants could also be decolorized completely. Our study suggests a promising process of pyrite/PMS/HA for organic dye pollutant treatment.

Controls on cobalt and nickel distribution in hydrothermal sulphide deposits in Bergslagen, Sweden - constraints from solubility modelling

ABSTRACT We address controls on Co and Ni distribution, based on their solubility in hydrothermal fluids as functions of pH, ƒO2 and T, in two end-member types of sulphide deposits in Bergslagen, Sweden. Oxidized hydrothermal fluids, as have been suggested for the formation of the Zinkgruvan deposit, would efficiently transport Co and Ni in solution, even at 150 °C. Formation of Co and Ni sulphides and sulphosalts supersedes or overlaps the precipitation of other sulphides along a reduction or H2S-mixing path. This is consistent with the presence of Co and Ni sulphides and sulphosalts in vent-proximal Cu-Zn mineralization at Zinkgruvan. Reduced, acidic and hot (≥250 °C) hydrothermal fluids that have been invoked for the formation of deposits like Falun and Stollberg could also transport Co and Ni in solution. However, their solubility is strongly dependent on high T and low pH. Cooling and neutralization are here proposed as likely key triggers for the deposition of Co and Ni, yet, unlike in the Zinkgruvan scenario, saturation will occur within the pyrite stability field, whereby these metals may be sequestered as stoichiometric lattice substitutions in pyrite and other sulphides rather than forming minerals of their own. We conclude that at any T or realistic pH, hydrothermal systems involving oxidized brines have a greater ability to traverse and leach large rock volumes of Co and Ni. Consequently, areas hosting deposits that formed from such brines have a significant exploration potential for these metals, even in areas where Co-enriched source rocks are lacking or subordinate.

Study of spontaneous combustion mechanism and heat stability of sulfide minerals powder based on thermal analysis

The spontaneous combustion mechanism of sulfide minerals powder is studied by simultaneous thermal analysis. The chemical reactions of pyrites in the atmospheres of air and nitrogen are deduced and the corresponding reaction products are characterized by the Raman spectroscopy. The characteristic temperatures of the pyrite reaction process are discussed in different conditions. The results show that the process of oxidation spontaneous combustion can be divided into three stages. Fe3FeSiO4(OH)5 in the minerals is decomposed at low temperature in the oxidation process. FeS2, the main component of the mineral, is mainly oxidized in the stage of oxidation spontaneous combustion. However, FeS2 will be decomposed under heating in nitrogen atmosphere. Thermal stability of pyrite in air is lower than that in nitrogen, and its thermal stability will be improved by increasing heating rate and particle size.

Transformation of pyrite to pyrrhotite in the presence of Au-Ag alloys at 500 °C

Dry annealing of Au x Ag 1−x alloys (х = 0.19, 0.35, 0.56 or of fineness 300, 500, 700‰) and pyrite was used to reveal the solubility of Au (Ag) in FeS 2 and study phase equilibria in the FeS 2 -Au x Ag 1−x system at 500 °C. Pyrrhotite, acanthite, and uytenbogaardtite and Au-Ag alloys with increased fineness were established at the contacts of pyrite blocks with Au-Ag plates. The obtained results evidence the absence of solubility between FeS 2 and Au (Ag) at 500 °C. The Ag content in alloys influences the stability of pyrite and contributes to its transformation in pyrrhotite and sulfidation and ennobling of Au-Ag alloys. Au-Ag sulfides and pyrrhotite may be present in the sulfide ores of metamorphogene deposits as annealing products of Au-Ag-pyrite-bearing ores.

Preservation of Re‐Os isotope signatures in pyrite throughout low‐T, high‐P eclogite facies metamorphism

AbstractPyrite in LT–HP eclogites from the western Tianshan orogenic belt yields a Re‐Os age of 378.1 ± 8.9 Ma, which is 30–70 Ma older than ages previously obtained for the same rocks using the Rb–Sr, Sm–Nd, Ar–Ar, U–Pb, and Lu–Hf isotope systems. The Tianshan LT–HP eclogite experienced temperatures of up to ~570 °C combined with pressures of up to 2.1 GPa during metamorphism. These conditions are below the transition of pyrite to pyrrhotite, which defines both pyrite stability and possibly its closure temperature for Re‐Os. Pyrite can preserve Re‐Os signatures through eclogite facies peak metamorphic conditions, and thus allow determination of the formation age of pyrite in the protolith.

The stability of selected sulfide minerals in sulfuric acid and acidic thiocyanate solutions

The stability of pyrite, chalcopyrite, chalcocite, covellite, bornite and enargite at pH 2 in the absence and presence of thiocyanate has been determined by linear sweep voltammetry and chronoamperometry under conditions pertaining to the thiocyanate leaching of gold (pH 2 and 602–702mV vs. SHE). Electrochemical measurements indicated that the decomposition rate of all sulfide minerals increased with an increase in oxidation potential with pyrite and chalcopyrite being the least reactive and chalcocite and bornite being the most reactive of the examined sulfides. The oxidation rate of covellite and enargite was higher than that of pyrite and chalcopyrite but much lower than that of chalcocite and bornite. It was found that the variation of pH from pH 1 to pH 3 has minimal effect on the oxidation rate of pyrite and chalcopyrite. In the presence of 0.05M thiocyanate at pH 2 and in the potential range of interest for gold leaching (602–702mV vs. SHE), the effect of thiocyanate was not significant on the oxidation of selected sulfides.

First-principles electronic structure and relative stability of pyrite and marcasite: Implications for photovoltaic performance

Despite the many advantages (e.g., suitable band gap, exceptional optical absorptivity, earth abundance) of pyrite as a photovoltaic material, its low open-circuit voltage (OCV) has remained the biggest challenge preventing its use in practical devices. Two of the most widely accepted reasons for the cause of the low OCV are (i) Fermi level pinning due to intrinsic surface states that appear as gap states, and (ii) the presence of the metastable polymorph, marcasite. In this paper, we investigate these claims, via density-functional theory, by examining the electronic structure, bulk, surface, and interfacial energies of pyrite and marcasite. Regardless of whether the Hubbard $U$ correction is applied, the intrinsic ${100}$ surface states are found to be of ${d}_{{z}^{2}}$ character, as expected from ligand field theory. However, they are not gap states but rather located at the conduction-band edge. Thus, ligand field splitting at the symmetry-broken surface cannot be the sole cause of the low OCV. We also investigate epitaxial growth of marcasite on pyrite. Based on the surface, interfacial, and strain energies of pyrite and marcasite, we find from our model that only one layer of epitaxial growth of marcasite is thermodynamically favorable. Within all methods used (LDA, GGA-PBE, GGA-PBE$+U$, GGA-AM05, GGA-AM$05+U$, HSE06, and $\ensuremath{\Delta}$-sol), the marcasite band gap is not less than the pyrite band gap, and is even larger than the experimental marcasite gap. Moreover, gap states are not observed at the pyrite-marcasite interface. We conclude that intrinsic surface states or the presence of marcasite are unlikely to undermine the photovoltaic performance of pyrite.

Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

Abstract Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs T hermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which liberates sulfur) in common metamorphic lithologies. The results suggest that most of the continental crust’s metamorphic sulfur is liberated in a relatively narrow temperature–pressure window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by temperature and pressure, and partly by the amount of H2O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H2O is produced primarily through chlorite breakdown in mafic to pelitic bulk compositions. As temperature increases, more sulfur is required from pyrite to maintain equilibrium proportions of H2O, H2S and SO2 in the fluid, and in addition, progressively more sulfur is required at lower pressures. At low temperatures, little sulfur is required by metamorphic fluid released during initial chlorite breakdown, whereas at higher temperatures coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid’s sulfur requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of sulfur-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low pressures and high temperatures where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. However, deformation also plays a key role in pyrite breakdown. Without deformation, small amounts of fluid in chemical communication with individual pyrite grains will quickly acquire equilibrium concentrations of the sulfur species and minimal pyrite breakdown is necessary. Whereas during deformation, there may be a continuous fluid flux past pyrite grains, promoting ongoing sulfur liberation. In this way, periods of deformation may be the major sulfur-liberating episodes during a metamorphic cycle. Since hydrothermal fluids are inherently buoyant and consequently tend to migrate upwards and towards cooler temperatures through the crust, these results imply that orogenic gold deposits are most likely to form at lower-amphibolite to prehnite-pumpellyite facies conditions, and unlikely to form at higher temperatures. The pressure constraint on metamorphic sulfur liberation implies that tectonic settings that allow prograde metamorphism to follow low pressure P-T-t paths in an occasionally compressional or transpressional environment are necessary. Settings that promote extensive injection of felsic magma into a mid-crust that contains a significant proportion of pyritic carbonaceous metasediment are shown to be ideal for orogenic gold deposit genesis. Inverted back-arc basins are interpreted to be the most favourable of these.

Mercury and trace element contents of Donbas coals and associated mine water in the vicinity of Donetsk, Ukraine

Abstract Mercury-rich coals in the Donets Basin (Donbas region) of Ukraine were sampled in active underground mines to assess the levels of potentially harmful elements and the potential for dispersion of metals through use of this coal. For 29 samples representing c 11 to m 3 Carboniferous coals, mercury contents range from 0.02 to 3.5 ppm (whole-coal dry basis). Mercury is well correlated with pyritic sulfur (0.01 to 3.2 wt.%), with an r 2 of 0.614 (one outlier excluded). Sulfides in these samples show enrichment of minor constituents in late-stage pyrite formed as a result of interaction of coal with hydrothermal fluids. Mine water sampled at depth and at surface collection points does not show enrichment of trace metals at harmful levels, indicating pyrite stability at subsurface conditions. Four samples of coal exposed in the defunct open-cast Nikitovka mercury mines in Gorlovka have extreme mercury contents of 12.8 to 25.5 ppm. This coal was formerly produced as a byproduct of extracting sandstone-hosted cinnabar ore. Access to these workings is unrestricted and small amounts of extreme mercury-rich coal are collected for domestic use, posing a limited human health hazard. More widespread hazards are posed by the abandoned Nikitovka mercury processing plant, the extensive mercury mine tailings, and mercury enrichment of soils extending into residential areas of Gorlovka.

Relationship between Pyrite Stability and Arsenic Mobility During Aquifer Storage and Recovery in Southwest Central Florida

Elevated arsenic concentrations are common in water recovered from aquifer storage and recovery (ASR) systems in west-central Florida that store surface water. Investigations of the Suwannee Limestone of the Upper Floridan aquifer, the storage zone for ASR systems, have shown that arsenic is highest in pyrite in zones of high moldic porosity. Geochemical modeling was employed to examine pyrite stability in limestone during simulated injections of surface water into wells open only to the Suwannee Limestone with known mineralogy and water chemistry. The goal was to determine if aquifer redox conditions could be altered to the degree of pyrite instability. Increasing amounts of injection water were added to native storage-zone water, and resulting reaction paths were plotted on pyrite stability diagrams. Native storage-zone water plotted within the pyrite stability field, indicating that conditions were sufficiently reducing to allow for pyrite stability. Thus, arsenic is immobilized in pyrite, and its groundwater concentration should be low. This was corroborated by analysis of water samples, none of which had arsenic concentrations above 0.036 microg/L. During simulation, however, as injection/native storage-zone water ratios increased, conditions became less reducing and pyrite became unstable. The result would be release of arsenic from limestone into storage-zone water.

Use of solid phase characterisation and chemical modelling for assessing the behaviour of arsenic in contaminated soils

A soil, containing waste material from an industrially contaminated site, was found to be heavily contaminated with several heavy metals and As. A risk assessment for As leaching from this material has been carried out in several stages, collation and examination of historical records, solid-phase characterization and chemical modelling. The historical record indicates that the most probable source of As was arsenopyrite. However, the solid phase characterization of the soil, using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive microanalysis (EDAX), did not yield any direct evidence for pyritic phases, although there was clear evidence of known pyrite-weathering products, such as jarosite. The relative stability of pyrite and arsenopyrite have been modelled for the range of acidity and redox potentials likely to be encountered on the site. For adsorption modelling, a surface complexation model was used to predict arsenate desorption as a function of pH. It was assumed that the principal reactive adsorbent for As was hydrous ferric oxide (HFO) and this assumption was supported by the results of direct and indirect measurements and by the mineral stability calculations. This approach was successful at predicting the increased mobility of As at increasingly alkaline conditions. The modelling predictions were supported by results from batch equilibration experiments. Thus, it was possible to link direct observations of mineralogy, mineral stability calculations and adsorption models in order to predict the mobility of As. The success of this approach was dependent on identifying the reactive phase in this particular soil and having the appropriate data required for the adsorption modelling.

A study of the main oxidation products of natural pyrite by voltammetric and photoelectrochemical responses

(Photo)electrochemical experiments on pyrite electrodes in acetic acid–acetate buffer (pH=4.5) are conducted to clarify the main oxidation reactions and the nature of the products. Electrochemical reactions in the −0.40 to 1.25 V (SHE) potential range are studied, and the production of iron (III) polysulfide from anodically formed iron oxides and polysulfides is discussed. Charges experimentally obtained are considered for the estimation of the most likely stoichiometry of the metallic polysulfide. The photoselectivity of the pyrite corrosion process indicates that the oxidation reactions of Fe 2+ and S 2 2− are not consecutive. The changes in stoichiometry and/or annihilation of crystalline structure defects are responsible for the observed photosensitivity of pyrite. A description of light effects on the interfacial behaviour and stability of pyrite is presented in terms of conduction and valence band energies, and thermodynamic potentials.

Framboidal pyrite formation via the oxidation of iron (II) monosulfide by hydrogen sulphide

Pyrite framboids have been synthesised as the product of the oxidation of FeS (mackinawite) by H 2S in aqueous solutions at pH 6, Eh > −250 mV and temperatures above 60°C in the absence of detectable O 2. Under these conditions at Eh < −250 mV single crystals of pyrite are formed. The reaction may proceed by dissolution of solid FeS and subsequent reaction of an aqueous FeS cluster complex with H 2S. The involvement of an aqueous FeS phase facilitates transport and reaction, and allows the development of framboidal textures from dispersed fine-grained FeS reactant. The results suggest that the framboidal texture results from rapid nucleation in environments where pyrite is strongly supersaturated. At the limits of the pyrite stability field, where pyrite supersaturation is low, rapid pyrite nucleation is inhibited and single crystals form. The results demonstrate that pyrite framboids can form in the absence of molecular O 2, magnetic (e.g., greigite) intermediates or biological intervention.

Ion microprobe determination of sulfur isotope variations in iron sulfides from the Post/ Betze sediment-hosted disseminated gold deposit, Nevada, USA

Secondary ion mass spectrometric analyses of ore samples from the Post/Betze sedimenthosted disseminated gold deposit were utilized to constrain elemental distribution of Au and As in iron sulfide phases. Most of the Au was deposited very early in the paragenetic sequence. Although Au and As are covariant in arsenian pyrite, Au apparently was depleted much more rapidly from the hydrothermal solutions than was As. Sensitive high-resolution ion microprobe (SHRIMP) sulfur isotope analyses of iron sulfides from the Post/Betze deposit vary widely from δ 34S = −29 to 23‰ and provide important information on the origin of sulfur and constraints on depositional mechanisms. Ore solutions had high δ 34S values and were most likely derived, at least in part, from thermochemical reduction of lower Paleozoic seawater-derived sulfate, possibly bedded barite. Late-stage ore fluids ( δ 34S sulfide = −12 to −29‰ ) are extremely depleted in 34S relative to main-stage ore fluids ( δ 34S sulfide = 16–23‰ ). Although such low δ 34S values can be generated hypothetically from the original ore fluids by oxidation (possibly boiling), the stability of pyrite is compromised. Introduction of and mixing with Fe-rich fluids is necessary to deposit pyrite having low isotopic values.