Дисертації з теми "Arsenian pyrite"

Iron disulfide (FeS2) has two polymorphs, pyrite and marcasite. Pyrite is the most abundant sulfide in the Earth's crust. Both minerals can host economic amount of gold and environmentally hazardous arsenic and are found to coexist in hydrothermal mineralization. With time, thermodynamically metastable marcasite can transform to pyrite. However, the kinetics of the marcasite to pyrite transformation, and the mechanisms of arsenic incorporation during growth of pyrite are not well-constrained. This thesis presents experimental results and discussions on: (i) the formation of pyrite and marcasite under dry and hydrothermal conditions (Chapter 2 and 3), and (ii) incorporation of arsenic into pyrite during the growth of pyrite on pyrite seeds (Chapter 4). In Chapter 2, the transformation from marcasite to pyrite was studied by in situ synchrotron powder X-ray diffraction (PXRD) at 520 °C and 540 °C, and ex situ anneal/quench experiments at 400 °C, 462 °C, and 520 °C. It was found that the mechanism and kinetics of this transformation depend not only on temperature, but also on particle size, the presence of water vapor, and the presence of pyrite inclusions in marcasite. Under dry conditions, the transformation is limited by surface nucleation and occurs via epitaxial nucleation of pyrite on marcasite, with {100}pyrite//{101}marcasite and {001}pyrite//{010}marcasite. In contrast, in the presence of water vapor, there is little crystallographic orientation relationship between the two phases; the transformation is limited by surface nucleation, but modification of the surface properties by water vapor results in a different nucleation mechanism, and consequently different kinetics. Kinetic analysis estimates a half-life of 1.5 Ma at 300 °C for the transformation under dry conditions with pyrite-free marcasite grains (<38 μm), but this estimation should be used with extreme caution due to the complexity of the process. From synchrotron X-ray fluorescence elemental mapping, trace elements (As and Pb) play an insignificant role in the transformation. However, the presence of a fluid phase changes the behavior of Pb. Under dry conditions randomly oriented particles of galena formed in pyrite, while under water vapor conditions arrays of nano-to-microparticles of galena precipitated in pores. This chapter highlights that although the natural occurrence of marcasite can indicate low temperature environments, precise estimation of temperature should not be made without considering the influences from various reaction parameters. In Chapter 3, combined in-situ synchrotron PXRD and ex situ experiments were conducted under hydrothermal conditions at 190 °C and 210 °C and pH 1, aiming to study the controls on the precipitation of pyrite and marcasite from supersaturated hydrothermal solutions and the kinetics of hydrothermal transformation from marcasite to pyrite. In situ PXRD experiments show the important role of saturation index on the precipitation of pyrite and marcasite; at 190 °C, hydrothermal fluids rich in ΣS(-II) (0.9 mM) favors the precipitation of nanocrystalline pyrite (23 nm) due to high saturation index, while S(-II)-free fluids produce a mixture of marcasite and pyrite nanocrystals (21-46 nm) due to low saturation index. Fluid/rock ratio (70 and 120 g/g at 210 °C) can affect saturation index of the fluids, resulting in complex nucleation and crystal growth dynamics such as the evolution of crystallite size, phase abundance, and pyrite/marcasite ratio. Ex situ experiments show the rapid transformation from marcasite to pyrite at 210 °C; around 83% marcasite is transformed to pyrite in just 3 weeks, compared to 4.3 million years or 6.3 trillion years at 210 °C based on extrapolation using the kinetic models reported in early studies under dry conditions. These results suggest that saturation index influences the dynamics of precipitation under hydrothermal conditions and controls the phase selection between pyrite and marcasite, and that marcasite may not survive over geological time in low temperature environments in the presence of acidic hydrothermal fluids. In Chapter 4, the formation of zoned arsenian pyrite was studied by growing pyrite on pyrite seeds in O2-free, As-enriched fluids at 200 °C and pH 7. The distribution and concentrations of As in pyrite, as well as the morphology of the zoning are influenced by sulfur source; i.e., native sulfur or Na2S2O3·5H2O. For experiments with native sulfur, up to four concentric alternate zones of As-rich (first zone on pyrite seed) and As-free pyrite grow on pyrite seeds. For experiments with Na2S2O3·5H2O, an aggregate of concentrically zoned pyrite microparticles (~1 µm) precipitate on the surface of pyrite seeds. Based on EMPA, the maximum concentration of As is 4.3 wt. %. However, the TEM-EDS analyses reveal ≤5.8 wt. % of As. HRTEM and selected area electron diffraction (SAED) pattern combined with EBSD analyses document epitaxial growth of As-pyrite on pyrite seed in the presence of native sulfur, but aggregation of randomly oriented aggregates of pyrite microparticles in the presence of thiosulfate. High-angle annular dark-field scanning TEM (HAADF-STEM), HRTEM observations, and EDS mapping show a sharp boundary and trails of pores between the pyrite seed and the product and between the growth zones. In the presence of native sulfur, the thickness of the As-pyrite growth zones is ~ 50 nm, while the subsequently formed growth zones of “barren” pyrite are ~5000 nm thick. X-ray absorption near edge structure (XANES) analyses reveal that speciation of As in pyrite depends on the S-source: (i) anionic As(-I) substitutes for S in pyrite as As2 pair when native S is used, and (ii) cationic As(II)/As(III) substitutes for Fe when thiosulfate is used. Our experiments show that the incorporation of As into pyrite and the formation and morphology of pyrite growth zones are controlled by the source of sulfur in hydrothermal fluids. This thesis highlights the factors that control the mechanisms of the formation and transformation of pyrite and marcasite and the dependence of As incorporation into arsenian pyrite structure as a function of S and As source in the presence of pyrite seeds. These outcomes should benefit our understanding of the formation and alteration of Carlin-type, epithermal, volcanic-hosted massive sulfide (VMS), and orogenic Au deposits.

High arsenic concentrations (>100 ppb) have been measured in wells completed in the Ordovician St. Peter sandstone aquifer of eastern Wisconsin. The primary source of arsenic is As-bearing sulfide minerals within the aquifer. There is concern that periodic disinfection of wells by chlorination may facilitate arsenic release to groundwater by increasing the rate of sulfide mineral oxidation. Current guidance from the Wisconsin Department of Natural Resources recommends a â low-doseâ treatment of 20% of the chlorine strength and 10% of the of the contact time of chlorine treatments used in non-arsenic impacted wells for well disinfection and biofilm removal. In order to provide information pertaining to WDNRâ s recommendations, St. Peter sulfide minerals were reacted with a range of chlorine â shock-treatmentsâ similar to those occurring in wells. This study focuses on abiotic processes that mobilize arsenic from the solid phase during controlled exposure to chlorinated solutions.

Thin sections were made from aquifer material collected at Leonardâ s Michael quarry, located in Winnebago County, Wisconsin. Bulk arsenic content of this material was measured as 674 ppm. Quantitative EPMA analysis shows As zoning in pyrite grains with concentrations up to 1 wt. % As. After mineral characterization, the thin sections were exposed to solutions of 60 mg/L â free chlorine,â 1200 mg/L â free chlorine,â and nanopure water (control) at pH 7.0 and pH 8.5 for 24 hours. Thin sections were then analyzed to measure changes in the pyrite surfaces. For solution experiments, aquifer material was crushed to between 250 μm and 355 μm mesh sizes (S.A. ~ 50 cm2/g â 60 cm2/g, Foust et al. 1980) and reacted under the same conditions as the thin sections in a batch reactor. Solution samples were collected periodically during the 24 hour exposure and analyzed for arsenic, iron, and sulfate ion.

Pyrite oxidation is shown to dramatically increase with increasing chlorine concentrations as shown by measurements of released sulfate ion, used here as the reaction progress variable. EPMA maps also reveal complete oxidation of pyrite cements to Fe-oxyhydroxides at 1200 mg/L â free chlorineâ and pH 7.0. This behavior does not occur at lower concentrations or higher pH. Arsenic release to solution does not appear to be directly correlated to increasing chlorine concentrations, but is governed by Fe-oxyhydroxide nucleation, which inhibits the release of dissolved arsenic at higher concentrations of chlorine.
Master of Science

Aquifer storage and recovery (ASR) is a strategy in which water is injected into an aquifer when it is plentiful and pumped from the aquifer when water is scarce. An impediment to ASR in Florida is leaching of naturally-occurring arsenic from limestone of the Upper Floridan Aquifer System (UFAS) into stored water. The concentration of arsenic in surface water, which serves as the recharge water for many ASR systems, and native groundwater is usually much less than 3.0 µ/L. However, data from ASR wells in Florida show that arsenic in recovered water frequently exceeded the 10 µg/L maximum contaminant level (MCL) established by the Environmental Protection Agency and were as high as 130.0 µg/L. The cause of elevated arsenic concentrations is displacement of reduced native groundwater with oxygenated surface water that dissolves arsenic-bearing pyrite in limestone. Although arsenic can be removed from recovered water during final treatment, mobilization of arsenic in the aquifer at levels that exceed the MCL is problematic under federal regulations.

This dissertation investigated a number of aspects of the ASR/arsenic problem to provide additional insights into the mechanisms of arsenic mobilization and measures that could be taken to avoid or reduce the release of arsenic during ASR operations.

Chapter 2, involved development of a geochemical model to simulate an ASR system’s injection of oxygenated surface water into reduced groundwater to determine whether aquifer redox conditions could be altered to the degree of pyrite instability. Increasing amounts of injection water were added to the storage-zone in a series of steps and resulting reaction paths were plotted on pyrite stability diagrams. Unmixed storage-zone water in wells plotted within the pyrite stability field indicating that redox conditions were sufficiently reducing to allow for pyrite stability. Thus arsenic is immobilized in pyrite and its concentration in groundwater should be low. During simulation, as the injection/storage-zone water ratio increased, redox conditions became less reducing and pyrite became unstable. The result would be release of arsenic from limestone into storage-zone water.

Chapter 3 examined the importance of maintaining a substantial volume of stored water around an ASR well to prevent recovery of reduced native groundwater to the vicinity of the well. Depleting the stored water and recovering reduced native groundwater would result in dissolution of arsenic-bearing hydrous ferric oxide (HFO) and release of arsenic into water recovered from the ASR well. Injection/recovery volumes for each cycle for each well were tracked to determine if a substantial volume of stored water was maintained for each cycle or if it was depleted so that reduced native groundwater was brought back to the well. Each well was assigned to either the “storage zone maintained group” where a zone of stored water was established in early cycles and largely maintained through the period of investigation, or the “storage-zone depleted group” where a zone of stored water was either established in later cycles and/or was depleted during the period of investigation. Graphical and statistical analyses verified that maximum arsenic concentrations for storage-zone maintained wells were nearly always lower in each cycle and declined below the MCL after fewer cycles than those of storage-zone depleted wells.

Chapter 4 was a mineralogical investigation of cores located at 20 m (ASR core 1), 152 m (ASR core 2), and 452 m (ASR core 3) from operating ASR wells to determine where mobilized arsenic in limestone is precipitated during ASR. If arsenic is precipitated distally, reduced concentrations of elements in pyrite, (iron, sulfur, arsenic, etc.) would be expected in ASR core 1 relative to more distant cores and there would be noticeable changes in appearance of pyrite crystals due to enhanced oxidation. The results showed that mean concentrations of the elements were lowest in ASR core 2, which did not support distal precipitation. However, scanning electron microscopy identified well-defined pyrite framboids only in core 3 while framboids in ASR cores 1 and 2 were less clear and distinct, indicating pyrite oxidation in cores closest to ASR wells.

Statistical comparison of concentrations of iron, sulfur, and arsenic between the three ASR cores and 19 control cores not subject to ASR, showed that mean concentrations in ASR cores 1 and 2 were statistically similar to concentrations in control cores. This indicated that concentrations in ASR cores 1 and 2 had not been significantly reduced by ASR. The concentrations of elements were higher in ASR core 3 than in ASR cores 1 and 2 and control cores and statistically dissimilar to all but one control core. This indicated natural heterogeneity in core 3 rather than diminution of elements in ASR cores 1 and 2 due to ASR. The statistical analysis supported local precipitation. Once arsenic is mobilized from dissolved pyrite, it is rapidly complexed with precipitated HFO near the well. As long as all of the stored water is not removed during recovery so that reduced native groundwater is brought back to the well, HFO remains stable and complexed with arsenic. The concentration of elements would not have been lowest in ASR core 1 for this reason and because calculations showed that the mass of arsenic removed during recovery events prior to coring was minor compared to the total in limestone surrounding the well. The implications of this are that while large quantities of arsenic are present near the ASR well, only a small percentage may be available for dissolution. Most arsenic occurs with pyrite in limestone, which may insulate it from exposure to oxidized injection water. Water recovered from ASR wells may continue to have low concentrations of arsenic indefinitely because as limestone is dissolved, more pyrite becomes exposed and available for dissolution.

The primary contribution of this dissertation to understanding and overcoming the arsenic problem in ASR systems is the empirical data developed to support or challenge important ASR/arsenic hypotheses. These data were used to 1) establish that background concentrations of arsenic in groundwater of the Suwannee Limestone were less than 1µg/L, 2) demonstrate that redox conditions necessary for pyrite in limestone to become unstable and dissolve occur when oxygenated surface water is injected into the aquifer, 3) demonstrate that the concentration of pyrite in the Suwannee Limestone is spatially variable to a high degree, 4) support the hypothesis that following injection of oxygenated surface water, pyrite in limestone dissolves and releases arsenic into solution and HFO forms and complexes with the arsenic near the ASR well, 5) propose that only a small percentage of pyrite near an ASR well may be available for dissolution during each cycle because most occurs in the limestone matrix and is isolated from injection water, 6) propose that as a result of the previous conclusion, water recovered from ASR systems may continue to have low concentrations of arsenic indefinitely because as limestone that contains pyrite is dissolved with each cycle, additional pyrite is exposed and is available for dissolution, and 7) support the effectiveness of maintaining a zone of stored water in an ASR well as an effective means of minimizing arsenic in recovered water during ASR.

On retrouve des contaminations d’aquifèr à l’arsenic dans touts les deltaï de l'Asie du Sud-Est, y compris dans le delta du Mékong, ce qui affecte la santé de millions de personnes. L’arsenic est très sensible aux fluctuations des conditions redox qui sont générés par les cycles alternés humides/secs pendant la saison de mousson. Une étude sur les caractéristiques géophysiques et chimiques du sol et des eaux souterraines dans le district de An Phu, dans le haut du delta du Mékong au Vietnam, suggère une forté contamination à l’As dans cette région. Les données chimiques et géophysiques indiquent une forte corrélation entre concentrations dans les eaux souterraines anoxiques et conductivité des sols. La liberation de l’arsenic est associée à la dissolution réductrice induih par des microorganisms des colloïdes et (oxyhydr)oxydes de fer dans des conditions d'oxydo-réduction oscillantes. La présence de bactéries sulforéductrices a le potentiel de stabiliser l’arsenic dans la phase solide et de l’atténuer dans la phase aqueuse par adsorption / désorption de l’arsenic sur les (oxyhydr)oxydes, et / ou sulfures de fer via la formation de complexes thiols. En raison de la teneur en pyrite élevée dans les sédiments, l'oxydation de la pyrite peut abaisser le pH et conduire à l'inhibition de la réduction microbienne du sulfate et aime empêcher la séquestration de l’arsenic dissous. Bien que le cycle biogéochimique de l’arsenic dans un système dynamique d’oxydoréduction soit une problématique complexe, il a été possible de renforcer notre compréhension de ce système
Aquifer arsenic (As) contamination is occuring throughout deltaic areas of Southeast Asia, including the Mekong Delta, and affects the health of millions of people. As is highly sensitive to fluctuations of redox conditions which are generated by the alternating wet-dry cycles during the monsoonal seasons. A survey of geophysical and chemical characteristics of soil and groundwater in the An Phu district, located in the vicinity of the Mekong Delta in Vietnam, shows the occurrence high As aqueous concentration in this region. Chemical and geophysical data indicate a strong positive correlation between As concentrations in the anoxic groundwater and conductivity of soils. In addition, mechanisms of As release are shown to be associated with colloidal and iron (oxyhydr)oxides which undergo microbial mediated reductive dissolution under redox oscilatting conditions. The presence of sulfate microbial reduction potentially stabilizes As in the solid phase and diminish As in the aqueous phase through the adsorption/desorption of As onto iron (oxyhydr)oxides and/ or sulfides with formation of thiols complexes in solid phase. Because of the high pyrite content in sediment, pyrite oxidation may drop in pH values, leads to inhibition of sulfate reducing bacteria and reduces sequestration of dissolved As. Although the biogeochemical cycling of redox sensitive species such as As in dynamic systems is challenging, it has been possible to strengthen our collective understanding of such system

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The Ernest Henry deposit is situated within the Eastern Fold Belt of the Mount Isa Inlier, NW Queensland, and is the largest iron oxide copper-gold (IOCG) deposit in the Proterozoic Cloncurry district. The hydrothermal deposit is hosted in brecciated intermediate metavolcanic and metasedimentary rocks with a biotite-calcite-chalcopyrite-gold-magnetite-pyrite-quartz mineral assemblage. This study investigates the mineralogical, textural and geochemical association between gold and pyrite with samples collected from three drill holes (EH768, EH859 and EH864) at ~700m vertical depth within the ore body. Majority of the gold (~98%) at Ernest Henry is in the form of free gold, which is commonly observed in pyrite microfractures, associated with chalcopyrite infill. Free gold has been interpreted to have entered the system with the main economic Cu-Au mineralisation stage. We propose that the semi-conducting potential of pre-existing pyrite surfaces have acted as a catalyst for the precipitation of free gold. Implications from this study may assist in the improvement of gold recovery at Ernest Henry and provide a better understanding of the timing of the ‘C’ and ‘G’ in ‘IOCG’ deposits in the local Cloncurry District.
Thesis (B.Sc.(Hons)) -- University of Adelaide, School of Physical Sciences, 2017

The Goldstrike property, located in northern Nevada within the Carlin Trend, contains one of the largest Carlin-type Au deposits in the world. The vast majority of this mineralization, formed in the Eocene, is in the form of Au-bearing, trace element-rich arsenian pyrite, either as very fine grains, overgrowths on earlier pyrite, or as reported in this study, patchy zones with high As values. Eight samples characteristic of Ore I and Ore II - ore types defined by Almeida et al. (2010) - were selected and analysed using electron microprobe, secondary ion mass spectrometry, and synchrotron !-XRF. !-XRF is a non-destructive technique for the elemental analysis of these samples with additional structural analysis capability. Although Ore I and Ore II yield similar Au values in whole rock analyses, and in the samples selected in this study, Ore II yielded much lower Au and trace element values in pyrite than Ore I. However, free gold was found in an Ore II sample, which explained their similar gold grade in whole rock. Two compositional trends were identified based on the ratio of Au and As in auriferous pyrite from both Ore I and Ore II: 1) those above an Au/As ratio of 0.007, characterized by elevated Ag, Au, As, Cu, Hg, Sb, and Tl that trend positively with respect to Au. The maximum value of Au ranges from 0.12 to 0.15at% (0.56 to 0.68wt%), occurring at an As concentration of 2.5 to 4.3at% (5 to 7.4wt%), and 2) those below an Au/As ratio of 0.007, characterized by As above the optimal range associated with lower Au and lower trace element concentrations. The peak in As corresponds well with the theoretical maximum amount of As that can be incorporated as a solid solution in pyrite (~6wt%) before the structure changes to a two-phase pyrite-arsenopyrite system. The less structurally stable solid solution has a more reactive surface that is more amenable to adsorption of other trace elements, including Au, especially with increased As.
Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2012-08-27 14:03:12.542

Mining for silver, lead, zinc, and copper in Zimapan, Hidalgo State, Mexico has been ongoing since 1576. Unsecured tailings heaps and associated acid mine drainage have presented problems related to soil quality, water quality, and dust emission control in the Zimapan area. Objectives of the study of the mine tailings are (1) to determine mineralogy of the tailings in order to identify acid-producing minerals and heavy metals at risk for release in acidic conditions, and (2) to quantify carbonate minerals and (3) to determine heavy metal content that may be released by the products of sulfide mineral weathering. Representative mine tailings have been sampled from a site located north of Zimapan. Mineralogical characterization has been conducted with X-ray diffraction (XRD), and scanning and transmission electron microscopes (SEM and TEM). Total carbonates have been determined the Chittick procedure. X-Ray Fluorescence (XRF) has been utilized to determine total elemental composition. XRD and SEM analyses have confirmed the presence of pyrite and arsenopyrite indicating a potential for acid mine drainage. Calcite has been confirmed to have a significant presence in the unweathered samples by XRD and the Chittick procedure, with some samples containing an average of 19.4% calcite. NAA and XRF have revealed significant concentrations of toxic elements such as As, Pb and Zn in both the oxidized and unoxidized samples.

Thesis (M.S.)--University of Wisconsin--Madison, 2001.
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