Experimental studies on the formation of pyrite and marcasite and the mechanisms of arsenic incorporation

Xizhi Yao

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.

Experimental studies on the formation of pyrite and marcasite and the mechanisms of arsenic incorporation

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