Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Investigations were conducted into the mechanisms of pyrrhotite oxidation and dissolution. Analysis of pyrrhotite (Fe{dollar}\rm\sb7S\sb8{dollar}) near surfaces (50A) identifies 32% Fe(III) and 68% Fe(II), both bonded to sulphide. These results show pyrrhotite has the stoichiometry Fe(III){dollar}\sb2{dollar}Fe(II){dollar}\rm\sb5S\sb8{dollar}.;Air oxidized pyrrhotite surfaces are compositionally zoned. The outermost surface layer is Fe(III)-oxyhydroxide, below which exists an Fe-deficient, S-rich layer that displays a continuous, gradual decrease in S/Fe until that of the unaltered pyrrhotite. Compositional zones develop by electron and iron migration along a sequence of metal vacancies. At the surface, Fe(III)-oxyhydroxides form through reaction with adsorbed oxygen and hydroxyl species.;Pyrrhotite reacted in {dollar}\rm H\sb2SO\sb4{dollar} (pH 3.0) develops Fe(III)-oxyhydroxide coated surfaces. Desiccation results in Fe(III)-oxyhydroxide spalling and exposure of underlayers containing enhanced S/Fe ratios. Pyrrhotite surfaces reacted in HCl-{dollar}\rm H\sb2SO\sb4{dollar} acid mixtures (pH 3.0) are S-rich and Fe-deficient, with minimal amounts of Fe(III)-oxyhydroxide. Suppression of Fe(III)-oxyhydroxides is attributed to interactions from chloride ions.;Iron concentrations measured in HCl-{dollar}\rm H\sb2SO\sb4{dollar} mixtures (pH 3.0) give linear trends when plotted as a function of the square-root of time, indicating diffusion limited dissolution. Sulphate concentrations remain constant at 1.0 mg/L and iron is 95% ferrous, demonstrating dissolution proceeds incongruently with respect to both iron and sulphur species. A 2:1 removal-release relationship between H{dollar}\sp+{dollar} and Fe{dollar}\sp{lcub}2+{rcub}{dollar} provides mass balance evidence that dissolution proceeds via ion exchange. Calculated activation energies are approximately 25 kJ/mol, indicating that pyrrhotite dissolution is limited by Fe diffusion into bulk solution.;Pyrrhotite and magnetite XRD patterns from the Mattabi Mine tailings yield anomalous peaks. Intense pressures and stresses associated with milling are found to increase the potential for mineral oxidation through development of magnetite (100) cleavages and conversion of monoclinic pyrrhotite to hexagonal pyrrhotite. Surface analysis of coatings on pyrrhotite collected from the Mattabi tailings unsaturated and saturated zones shows Fe(OH){dollar}\sb3{dollar} as the dominant unsaturated zone secondary phase and that FeOOH is the prevailing saturated zone phase. Fe(III) sulphate salts form in the capillary fringe immediately above the water table.



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