Oxidation Of Pyrrhotite Research Articles

ChemInform Abstract: The Biogeochemistry and Microbiology of Sulfidic Mine Waste and Bioleaching Dumps and Heaps, and Novel Fe(II)‐Oxidizing Bacteria

AbstractReview: 122 refs.

The effect of polytetrafluoroethylene on pressure oxidation of nickeliferous pyrrhotite

Nickeliferous pyrrhotite tailings, from Vale's Clarabelle Mill, were oxidized at 140 °C and 690 kPa (using compressed O 2) in an electrolyte with an initial composition of 50 g/L H 2SO 4 and 2.25 g/L of Fe 2(SO 4) 3. These conditions are similar to those employed in the Sherritt Zinc Pressure Leach Process that promotes the formation of elemental sulfur. The kinetics of oxidation was tracked by measuring the extraction of nickel to the electrolyte as a function of time. As expected, the pyrrhotite oxidation kinetics under these conditions was inhibited by the occlusion of the pyrrhotite particles by molten elemental sulfur formed during the leaching process. Addition of a commercially available granular form of PTFE increased the degree of leaching of nickel from the pyrrhotite, increasing the ultimate extraction of nickel at low and high impeller speeds from ca. 65% to over 90% after 90 min. This effect was attributed to the PTFE particles acting as a solid sorbent for the molten sulfur, which mitigated the effect of pyrrhotite occlusion and allowed oxidation to continue. The results of the use of PTFE on pyrrhotite oxidation kinetics are similar to those with addition of lignin sulfonate.

High-temperature bioleaching of nickel sulfides: thermodynamic and kinetic implications

The effect of temperature on nickel sulfide bioleaching was studied in the presence of mesophile ( Acidithiobacillus ferrooxidans) and moderate thermophile ( Sulfobacillus thermosulfidooxidans) strains and the results were discussed in terms of sulfide dissolution thermodynamics (Eh–pH diagrams) and kinetics (cyclic voltammetry). It was observed that in the pH range 1.8–2.0 the highest nickel dissolution was achieved which reached 50% for mesophiles and over 80% for moderate thermophiles. External ferrous iron addition had no effect on the metal dissolution at 34 °C, but adversely affected nickel leaching at higher temperatures. The best outcomes were accomplished with low FeSO 4 additions (2.5 g/L) at 50 °C. Pyrrhotite dissolution avoided the need for external iron addition, providing Fe 2+ concentrations as high as 7 g/L during bioleaching, which supports bacterial growth. Eh–pH diagrams for pentlandite and pyrrhotite show a negligible effect of temperature on the stability field of each sulfide whilst cyclic voltammetry indicated that temperature has the strongest influence on pyrrhotite oxidation. The latter along with a rapid increase in solution potential (Eh) explains the higher and faster extraction observed with S. thermosulfidooxidans.

The biogeochemistry and microbiology of sulfidic mine waste and bioleaching dumps and heaps, and novel Fe(II)-oxidizing bacteria

More than 70 microbiological studies of sulfidic mine dumps and heaps have been published. The microbiology and the relevant biogeochemical processes in the dumps and heaps are reviewed and have to be understood for improving copper heap leaching operations and to develop and control countermeasures for the formation of acid mine drainage (AMD). Pyrite or pyrrhotite oxidation is the most relevant biogeochemical process in sulfidic mine waste dumps and heaps and different techniques have been applied to measure reaction rates: column experiments, humidity cells, heat flow measurements, or oxygen consumption measurements. Acidophilic Fe(II)- and sulfur-oxidizing microorganisms are most relevant for metal sulfide oxidation. Anaerobic biogeochemical processes in sulfidic mine dumps and heaps are Fe(III)- and sulfate reduction, but little is known about the reaction rates. Fe(III)-reducing microorganisms dissolve Fe(III)(hydr)oxides and may thereby release adsorbed or precipitated metals. Sulfate-reducing microorganisms precipitate and immobilize many metals. Mainly culturing approaches were used to study the microbial communities in sulfidic mine dumps and heaps. More recently, molecular biological techniques have been applied to investigate the microbial diversity and to quantify and monitor particular microorganisms. Since acidophilic Fe(II)-oxidizing bacteria and archaea are most relevant for metal sulfide oxidation, several novel cultures were enriched from sulfidic mine dumps from 10 different countries and maintained in the geomicrobiology laboratory of the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR). Of 68 analyzed acidophilic Fe(II)-oxidizing cultures, 40 almost full (> 1300 bp) 16S rRNA gene sequences were obtained and subjected to phylogenetic analysis. Most of the cultivated Fe(II)-oxidizers belonged to four genera. The Gram-positive strains related to Acidimicrobium or Ferrimicrobium were phylogenetically more diverse than the strains of the Gram-negative genera Acidithiobacillus and Leptospirillum and partly represent so far undescribed bacteria. Several cultures also contained novel representatives of the heterotrophic genus Acidiphilium, often previously found in co-culture with Acidithiobacillus ferrooxidans.

Jarosite formation by an AMD sulphide-oxidizing environmental enrichment: Implications for biomarkers on Mars

The formation of jarosite in acid mine drainage (AMD) accompanies microbial oxidation of reduced sulphidic minerals such as pyrite (FeS2) and pyrrhotite (Fe1−xS) found in mining waste materials. The discovery of jarosite at Meridiani Planum, Mars has been used to interpret past hydrologic conditions as episodically wet, acidic and evaporative based on evidence from jarosite formed abiotically or through microbial iron oxidation. To date, no study has specifically assessed jarosite formation via microbial sulphide oxidation, the most likely pathway in AMD, or the mineral stability of any jarosite formed via this pathway. Here, using both aerobic and anaerobic conditions to control iron- and sulphide oxidation metabolisms, we demonstrate that oxidation of pyrrhotite, a putative Mars mineral, by a sulphur oxidizing environmental AMD enrichment produces crystalline jarosite at a higher pH than previously described. We further show that jarosite produced via microbial sulphide oxidation is stable under the drying, heating and wet-aging conditions shown to impact abiotic- and microbial iron oxidation-derived jarosite. Our results indicate that preservation of microbially produced jarosite via pyrrhotite-associated sulphide oxidation does not require the evaporitic conditions that have been proposed to date. If water is established to have persisted on Mars, then this new evidence signals that the occurrence of stable crystalline jarosite at Meridiani Planum may itself be a clear biomarker indicating that not only water occurred at some time(s) on Mars, but also life.

Geomicrobiology of Sulfidic Mine Dumps: A Short Review

The geomicrobiology of sulfidic mine dumps is reviewed. More than 30 microbiological studies of sulfidic mine dumps have been published. Mainly culturing approaches such as most probable number (MPN) or agar plates were used to study the microbial communities. More recently, molecular biological techniques such as FISH, CARD-FISH, Q-PCR, T-RFLP, DGGE, or cloning have been applied to quantify microorganisms and to investigate the microbial diversity. Aerobic Fe(II)- and sulfur compound oxidizing microorganisms oxidize pyrite, pyrrhotite and other metal sulfides and play an important role in the formation of acid mine drainage (AMD). Anaerobic microorganisms such as Fe(III)-reducing microorganisms dissolve Fe(III)(hydr)oxides and may thereby release adsorbed or precipitated metals. Sulfate-reducing microorganisms precipitate and immobilize metals. In addition to the microbial communities several biogeochemical processes have been analyzed in mine dumps. Pyrite or pyrrhotite oxidation rates have been measured by different techniques: Column experiments, humidity cells, microcalorimetry, or oxygen consumption measurements. Analyses of stable isotopes of iron, oxygen and sulfur have yielded valuable information on biogeochemical reactions. The microbiology and the biogeochemical processes in sulfidic mine dumps have to be understood for control and prevention of AMD generation and to provide different possibilities for remediation concepts. Today, remediation measures, e.g. under water storage of the waste or covering of the dumps, focus on the inhibition of pyrite oxidation to keep the toxic compounds inside the mine waste dumps. As an alternative to the inhibition of pyrite oxidation, metals which also have economic value could be extracted from mine dumps by the application of different metal extraction technologies including bioleaching.

Leaching of a low-grade, copper–nickel sulfide ore. 1. Key parameters impacting on Cu recovery during column bioleaching

This study was prompted by the disparate recoveries of nickel (> 70%) and copper (< 20%) from a test heap of copper–nickel sulfide ore after about 200 days of leaching. Variables tested in bioleaching columns charged with a pyrrhotite-rich, chalcopyrite and pentlandite ore were acid pre-conditioning, inoculation and aeration. The results indicated that the rapid reaction of pyrrhotite with acid created conditions that impacted directly and/or indirectly on copper recovery. Important reactions were hydrogen sulfide formation, high soluble iron concentrations and the formation of large amounts of elemental sulfur. It was hypothesized that copper loss, evidenced by copper re-distribution during passage through the ore, was mainly the result of reaction with hydrogen sulfide to form covellite, although this could not be confirmed by XRD analysis of leached residues. A layer of iron-oxy-hydroxy-sulfate “scale” on particle surfaces encapsulated sulfide grains as well as elemental sulfur formed by the oxidation of pyrrhotite and was of sufficient depth and integrity to have hindered but not prevented leaching and bioleaching. Lack of aeration (oxygen, carbon dioxide) impacted on ferrous ion biooxidation and probably sulfur biooxidation. More extensive sulfur biooxidation to form acid might have lowered the solution pH and reduced the amount of scale formation, resulting in higher ferric ion concentrations and better chalcopyrite oxidation.

Age and depositional environment of the Draa Sfar massive sulfide deposit, Morocco

The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints.

Quantitative Microbial Community Analysis of Three Different Sulfidic Mine Tailing Dumps Generating Acid Mine Drainage

The microbial communities of three different sulfidic and acidic mine waste tailing dumps located in Botswana, Germany, and Sweden were quantitatively analyzed using quantitative real-time PCR (Q-PCR), fluorescence in situ hybridization (FISH), catalyzed reporter deposition-FISH (CARD-FISH), Sybr green II direct counting, and the most probable number (MPN) cultivation technique. Depth profiles of cell numbers showed that the compositions of the microbial communities are greatly different at the three sites and also strongly varied between zones of oxidized and unoxidized tailings. Maximum cell numbers of up to 10(9) cells g(-1) dry weight were determined in the pyrite or pyrrhotite oxidation zones, whereas cell numbers in unoxidized tailings were significantly lower. Bacteria dominated over Archaea and Eukarya at all tailing sites. The acidophilic Fe(II)- and/or sulfur-oxidizing Acidithiobacillus spp. dominated over the acidophilic Fe(II)-oxidizing Leptospirillum spp. among the Bacteria at two sites. The two genera were equally abundant at the third site. The acidophilic Fe(II)- and sulfur-oxidizing Sulfobacillus spp. were generally less abundant. The acidophilic Fe(III)-reducing Acidiphilium spp. could be found at only one site. The neutrophilic Fe(III)-reducing Geobacteraceae as well as the dsrA gene of sulfate reducers were quantifiable at all three sites. FISH analysis provided reliable data only for tailing zones with high microbial activity, whereas CARD-FISH, Q-PCR, Sybr green II staining, and MPN were suitable methods for a quantitative microbial community analysis of tailings in general.

Model Development and Parameter Estimation for the Oxidation of Pyrrhotite-Containing Rock Surfaces

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Lithogeochemical constraints on the host rock, hydrothermal alteration and weathering of the Groundrush gold deposit

Lithogeochemical research at the Groundrush gold deposit, Tanami region, Northern Territory, has identified new lithologies and provides a new understanding of the hydrothermal alteration, metamorphism and weathering processes that have affected the suite of metamorphosed igneous host rocks. Metamorphosed hornblende–chlorite–plagioclase-bearing mafic–intermediate intrusive rocks were classified, using conserved constituents, as dolerite and previously unrecognized basaltic andesite and diorite, the last of which hosts Au mineralization. The diorite forms a steeply dipping dyke, whereas the dolerites are sills intruding clastic sedimentary rocks. Major element geochemical variation in dolerite is consistent with the fractionation of plagioclase and pyroxene, whereas diorite is more chemically homogenous and does not exhibit evidence of fractional crystallization. Hydrothermal alteration, principally developed within the diorite, was accompanied by addition of S and carbonate, and the loss of Ca and Na, giving rise to a propylitic hydrothermal alteration assemblage of chlorite, albite, carbonate and pyrite. A regolith profile of c. 40 m thickness overlies these rocks. If originally present, Fe-rich upper units have been removed. The major weathering processes observed in the regolith profile are: (1) sulphur oxidation and (in diorite), calcite dissolution at 40 m depth; (2) ferrolysis (Fe oxidation) and the production of smectite, goethite and quartz from chlorite and hornblende at 20 m depth; (3) breakdown of albite to kaolinite, breakdown of apatite, and deposition of secondary carbonate in the top 5 to 15 m. No evidence for significant near-surface gold depletion is evident, suggesting that regolith processes remobilizing gold were generally not operating to any significant extent. However, gold was leached from a metre-scale quartz vein, possibly as thiosulphate complexes formed from pyrrhotite oxidation in a relatively low-oxygen environment.

Characterization of stockpile oxidation of pentlandite and pyrrhotite through kinetic analysis of their flotation

A kinetic analysis of the flotation of various nickel–copper sulphide samples is presented. Two approaches of first order kinetics have been used to estimate the flotation rate constants of pentlandite and pyrrhotite for stockpile samples and fresh sample as a reference. Stockpile oxidation of samples was characterized by variations of the rate constants for fast ( k f) and slow floating fractions ( k s) as well as the slow floating fraction ( Φ) according to the Kelsall approach. An overall rate constant along with parameters, ultimate recovery (RI) and time correction factor ( θ) was determined according to the Agar approach. Selectivity index for pentlandite–pyrrhotite separation was also determined for all cases investigated. Both the collectorless flotation and collector induced flotation were investigated for a more complete view on the behaviour of these minerals. Fundamental differences in behaviour of these two minerals have been discussed with respect to development of floatability based on metal ion activation and elemental sulphur/polysulphide induced hydrophobicity. While both of these primary effects promote fast flotation kinetics, the former mechanism appears to be more important in practice. Over-oxidation results in flotation losses by transforming the fast floating particles into slow floating particles and those in turn into non-floatable particles. The values of Φ estimated from the kinetic analysis have been shown to correlate with nickel losses.

Geomicrobiological and geochemical investigation of a pyrrhotite-containing mine waste tailings dam near Selebi-Phikwe in Botswana

An acid mine drainage (AMD) generating tailings dam consisting of waste from about 32 years of Ni-, Cu-, Zn- and Co-sulfidic ore processing near Selebi-Phikwe, Botswana, was investigated to quantify the AMD generation potential for developing a remediation strategy within the mine closure plan. The climate in the region is semiarid with an average annual temperature of 21 °C. The approximately 40 m high dam covers an area of ca. 1 km 2. The surface of the central part of the dam is water covered, but the periphery is dry. Three holes were drilled through the water unsaturated periphery down to the saturated zone at about 25 m depth. Altogether 65 solid samples were taken in 1 m intervals and geomicrobiologically and geochemically analyzed. Brown precipitates of iron(hydro)oxides were discernible throughout the entire unsaturated zone with a paste pH in the range of 3–4. The tailings had sulfur contents in the range of 2–8 wt.% (average 4%). The total sulfur consisted mainly of reduced inorganic sulfur. Pyrrhotite was the main metal sulfide, pyrite, Ni- and Co-sulfides occurred in minor amounts. Metal sulfide oxidizing Acidithiobacillus ferrooxidans-like bacteria were present in high numbers (mean value of most-probable-number method: 3 × 10 6 cells g − 1 dw) throughout the whole unsaturated zone. Mean numbers of living bacteria (CARD-FISH) and total microorganisms (SybrGreen direct counts) were 1 × 10 7 cells g − 1 dw and 8 × 10 7 cells g − 1 dw, respectively. The average potential pyrrhotite oxidation rate measured by microcalorimetry was 1.9 × 10 − 9 mol pyrrhotite m − 2 s − 1 . The ratio of biological to chemical pyrrhotite oxidation was ca. 50:50. For the assumed 0.1 m thickness of the oxidation zone within the 25 m unsaturated zone the potential average pyrrhotite oxidation rate was 3.4 × 10 − 5 mol m − 2 s − 1 (94 kg FeS m − 2 year − 1 ). At this rate, all pyrrhotite in the tailings dam would be oxidized within 80 years.

Geomicrobiological investigation of two different mine waste tailings generating acid mine drainage

The impact of microbiological metal sulfide oxidation on acid mine drainage generation was studied for two different mine tailings. Microorganisms were quantified using different methods: (1) SYBR Green II direct counting, (2) TaqMan quantitative, real-time PCR (Q-PCR), (3) catalyzed reporter deposition–fluorescence in situ hybridization (CARD-FISH) and (4) most probable number (MPN) cultivation of acidophilic Fe(II) oxidizers. Potential pyrite or pyrrhotite oxidation rates were measured by microcalorimetry. In the uncovered, pyrrhotite-containing tailings near Selebi-Phikwe, Botswana, acidophilic Fe(II)-oxidizing microorganisms were present in high numbers (MPN) of up to 10 7 cells g − 1 dw (mean value 3 × 10 6 cells g − 1 dw) throughout the entire water unsaturated, oxidized zone of about 25 m (at the tailings dam periphery) with a paste pH in the range of 3–4. Mean numbers of living Bacteria (CARD-FISH) and total microorganisms (SYBR Green II) were 1 × 10 7 cells g − 1 dw and 8 × 10 7 cells g − 1 dw, respectively. Cell numbers obtained by Q-PCR analysis were in the same range. The average potential pyrrhotite oxidation rate measured by microcalorimetry was 3.4 × 10 −4 mol pyrrhotite m − 3 tailings s − 1 at 25 °C. About half of the pyrrhotite oxidation activity was biologically catalyzed. By contrast, in the covered pyrite-containing tailings in Impoundment 1 in Kristineberg, northern Sweden, acidophilic Fe(II)-oxidizing microorganisms (mean value 5 × 10 5 cells g − 1 dw) were only detected in a distinct zone of oxidized tailings between the cover and the unoxidized tailings where low pH values down to 3 prevailed. Bacterial numbers obtained by Q-PCR analysis were much higher (mean value 3 × 10 8 cells g − 1 dw). The proportion of biological pyrite oxidation was up to 100% for the oxidized zone. The average potential pyrite oxidation rate was 1.6 × 10 −5 mol pyrite m − 3 tailings s − 1 at 10 °C, an order of magnitude lower than that for the pyrrhotite-containing tailings.

Hydrochemical models of the sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana)

The sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana) are located in a similar semiarid environment but have a contrasting mineralogical composition. The Matchless tailings are pyrite-rich, whereas the Selebi-Phikwe tailings are dominated by pyrrhotite. Hydrochemical models are established with computer codes for water-balance, sulphide oxidation rate and hydrochemical equilibrium calculations. The data input is based on detailed mineralogical, chemical and kinetic investigations carried out on the core of boreholes drilled in 2000 and 2003. The oxidation of pyrrhotite proceeds at a much faster rate than the oxidation of pyrite. The PYROX code, which is used for kinetic calculations, can take these differences into account by applying different oxide-coating diffusion coefficients (D2) for pyrrhotite and pyrite. Humidity-cell testing is widely used to predict the post-mining composition of drainage water in humid climates. However, the semiarid conditions at Matchless and Selebi-Phikwe only allow a minimal water flux within the dump. Under such conditions, humidity-cell testing is likely to overestimate the seepage-water pH. This is suggested by the hydrochemical equilibrium calculations for the post-mining period at Selebi-Phikwe, which predict a seepage-water pH about one unit lower than the pH at the end of the 26-weeks humidity-cell testing period. The acidity of the seepage water can be reduced by about half a pH unit, if an oxygen barrier below the evaporation zone is installed. A clay layer 0.5 m thick covered by >1.5 m tailings represents the optimal design for a wet barrier. All three computer codes used for water-balance calculations (HELP3, UNSAT-H and HYDRUS-1D), predict >85% average water saturation for such a layer, which diminishes the diffusion of oxygen into the pile and production of SO4−2 and H+. The alternative design for a dry barrier consists of a vegetated silt layer 1 m thick on top of the tailings. This barrier does not significantly influence the diffusion of oxygen although it reduces the net infiltration to ≤11 mm/year.

Gold Enrichment and the Bi-Au Association in Pyrrhotite-Rich Massive Sulfide Deposits, Escanaba Trough, Southern Gorda Ridge

High gold contents (to 10.1 ppm, avg 1.4 ppm, n = 34) occur in pyrrhotite-rich massive sulfide samples from the sediment-covered floor of the Escanaba trough, the slow-spreading, southernmost segment of Gorda Ridge. These concentrations reflect the presence of primary gold, formed during high-temperature hydrothermal activity in mounds and chimneys, and secondary gold deposited during sea-floor weathering of massive sulfide. Primary gold occurs as fine-grained ( 2 μm) secondary gold grains have a porous, flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by Fe oxyhydroxides, Fe sulfate, and sulfur. Mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite, chalcopyrite, and Fe-rich sphalerite were formed by high-temperature (estimated range 325°–275°C), reduced, low-sulfur vent fluids. The mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1.1 percent organic carbon. As the deposition of pyrrhotite, Cu-Fe sulfides, and sphalerite waned, a volumetrically minor suite of sulfarsenide, arsenide, Bi, and Au minerals was deposited from highly reduced, late main-stage fluids diffusing through mounds and chimneys. The low solubility of Au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution. Instead, gold precipitation is thought to be linked to elevated concentrations of Bi in the late main-stage fluids. The textural relationships of Au and Bi minerals in pyrrhotite-rich samples, low melting point of native bismuth (271.4°C), and recent experimental results on Au and Bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the Escanaba trough vent fluids by coexisting droplets of liquid bismuth. Additional phase relationships of alloys in the Au-Bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241°C. Bismuth droplets trapped in void space between main-stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late-forming pyrrhotite. The limited solid solution of Au in Bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth. The electrum, native bismuth (with gold inclusions), and galena represent the last traces of gold mineralization from late main-stage fluids. During sea-floor weathering and the oxidation of pyrrhotite in the mounds and chimneys, secondary gold formed as aggregates of colloidal particles along pH gradients between acidic pore waters and ambient seawater. Gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes. The reduction of Au(III) by residual Fe2+ in partly altered pyrrhotite and adsorption of colloids by Fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front. Solubility differences between gold and silver chloride complexes at low temperature account for the low Ag content of secondary gold grains. The high concentrations of Bi, and thus the association of Au and Bi minerals in pyrrhotite-rich massive sulfide, can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the Escanaba trough. In contrast, the absence of the Au-Bi association in massive sulfides at other ridges, including other sediment-covered sites at Middle Valley and the Guaymas basin, reflects the predominantly basaltic source rocks.

Oxidation/alteration of pentlandite and pyrrhotite surfaces at pH 9.3: Part 1. Assignment of XPS spectra and chemical trends

Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite [(Fe,Ni) 9 S] and pyrrhotite (Fe 1– x S) from Voiseys Bay has been studied in aqueous solution at pH 9.3. The Fe 2 p , Ni 2 p , S 2 p, and O1 s spectra were obtained for each mineral from polished surfaces, and from surfaces reacted with the solutions for up to 180 minutes. Both minerals oxidize rapidly to give mostly a relatively thin layer (tens of angstroms) of Fe(III) oxyhydroxide (probably FeOOH). In addition, violarite (FeNi 2 S 4 ), Ni(OH) 2 , and NiSO 4 are found in the pentlandite oxidized overlayer, and Fe(III)-S and polysulfide species are found in the overlayers of both minerals. A protocol was developed to fit quantitatively all spectra in a consistent fashion. Fe 2 p 3/2 spectra were fitted with two Fe(II)-S peaks depending upon Fe coordination (four and six) in pentlandite, pyrrhotite, and violarite. An Fe(III)-S multiplet, an Fe(III) oxyhydroxide multiplet, as well as Ni Auger peaks, were also required to fit the Fe 2 p spectra. The Ni 2 p 3/2 spectra for pentlandite were fit with two Ni(II)-S peaks from the original pentlandite or violarite (four and six coordination), as well as Ni(OH) 2 and NiSO 4 peaks. The S 2 p spectra for both minerals were fit with four and five coordinate S peaks from the original minerals and violarite, as well as polysulfides and sulfate. Finally, the O 1 s spectra of both minerals were fit with peaks from oxide, hydroxide, and adsorbed water. The pyrrhotite surface oxidizes more rapidly than the pentlandite surface. For example, after five minutes of reaction the extent of oxidation of pyrrhotite to give an FeOOH overlayer is similar to that of pentlandite after thirty minutes of reaction. In both minerals, iron diffuses from the bulk to the surface and is oxidized. Pentlandite shows small amounts of Ni(OH) 2 and NiSO 4 in the Fe(lII) oxyhydroxide-dominated overlayer even after 180 minutes of reaction. These observations of the oxidation and alteration of the pentlandite surface are entirely consistent with thermodynamic calculations of the relative stabilities of pentlandite, violarite, Ni(OH) 2 , NiSO 4 , and FeOOH. The first stage of reaction is characterized by formation of violarite and FeOOH on pentlandite. With continued oxidation, this assemblage yields NiSO 4 plus FeOOH plus violarite, which is eventually converted to Ni(OH) 2 plus violarite plus FeOOH. Finally, after the violarite is consumed, the NiSO 4 plus FeOOH combination occurs as the thermodynamically stable assemblage.

Effects of a reactive barrier and aquifer geology on metal distribution and mobility in a mine drainage impacted aquifer

The Nickel Rim aquifer has been impacted for five decades by a metal-rich plume generated from the Nickel Rim mine tailings impoundment. Metals released by the oxidation of pyrrhotite in the unsaturated zone of the tailings migrate into the downgradient aquifer, affecting both the groundwater and the aquifer solids. A reactive barrier has been installed in the aquifer to remove sulfate and metals from the groundwater. The effect of the reactive barrier on metal concentrations in the aquifer solids has not previously been studied. In this study, a series of selective extraction procedures was applied to cores of aquifer sediment, to ascertain the distribution of metals among various solid phases present in the aquifer. Extraction results were combined with groundwater chemistry, geochemical modelling and solid-phase microanalyses, to assess the potential mobility of metals under changing geochemical conditions. Reactions within the reactive barrier caused an increase in the solid-phase carbonate content downgradient from the barrier. The concentrations of poorly crystalline, oxidized phases of Mn and Fe, as well as concentrations of Cr(III) associated with oxidized Fe, and poorly crystalline Zn, are lower downgradient from the barrier, whereas total solid-phase metal concentrations remain constant. Iron and Mn accumulate as oxidized, easily extractable forms in a peat layer overlying the aquifer. Although these oxides may buffer reducing plumes, they also have the potential to release metals to the groundwater, should a reduced condition be imposed on the aquifer by remedial actions.

Microbially driven acidity generation in a tailings lake

ABSTRACT In situ characterization and geochemical modelling of acid generation in a mine tailings lake (Moose Lake, ON, Canada) over a 2‐year period (2001–2002; surficial lake pyrrhotite slurry disposal initiated in 2002) show that bacteria significantly impact acidity behaviour through particle‐associated S oxidation and that they do so under conditions that differ from those controlling abiotic pathways. Seasonal epilimnetic pH decreases occurred in both years, decreasing from approximately 3.5 in May to 2.8 by September (2001) or July (2002). Epilimnetic acid generation rates were depth‐dependent, with maximal rates observed not at the surface of lake where O2 concentrations were highest, but rather within a geochemically reactive zone (approximately 1 m thick) of steep, decreasing O2 gradients and dynamic Fe and S geochemistry in the lower epilimnetic region of the lake. Acid generation occurred dominantly through particle rather than aqueous pathways, but model predictions of acid generation via abiotic pyrrhotite oxidation involving either O2 or ferric iron (Fe3+) predicted neither the observed rates nor the depths at which maximal rates occurred. In contrast, model predictions based on microbial pathways involving both O2 and ferric iron (Fe3+) agreed extremely well with both the observed depth profile of H+ generation and the observed rates at any given depth. Imaging showed extensive microbial colonization of epilimnetic‐associated pyrrhotite particles commonly with significant biofilm formation. FISH (fluorescence in situ hybridization) probing of the community in both pelagic and particle compartments indicated mixed communities occurred in both, and that Acidithiobacillus spp. accounted for 2–46% of the total community in either compartment. Initiation of pyrrhotite slurry discharge at the lake surface in 2002 was accompanied by a relative increase in the number of particle‐associated microbes, as well as a relative proportional decrease of Acidithiobacillus spp. in the total microbial community. Given the widespread occurrence of bacteria across mining environments, the implications of our results extend beyond this specific site and provide new insight into bacterially driven processes contributing to bulk system characteristics which are not currently well constrained.

A review on pyrrhotite oxidation

The non-stoichiometric compounds of iron sulphide named pyrrhotite (Fe 1− x S) are often associated with pyrite (FeS 2) in sulphidic ores and their waste products. The factors affecting pyrite and pyrrhotite oxidation are similar but the latter has received much less attention. As it is the case for pyrite, an increase in temperature has an increasing effect on the oxidation of pyrrhotite and the process follows the Arrhenius behaviour. Both ferric iron and bacteria act as catalysts in the oxidation reactions and play a significant role in the oxidation kinetics. Several spectroscopic techniques are used to study the oxidation mechanisms and products of pyrrhotite oxidation. This paper presents and discusses the main factors controlling pyrrhotite oxidation and the proposed mechanisms of oxidation under normal conditions of temperature and pressure.