CONTRIBUTION OF MOLECULAR OXYGEN TO THE OXYGEN ISOTOPE COMPOSITION OF DISSOLVED SULFATE DERIVED FROM THE OXIDATION OF IRON SULFIDES

Oxidation of iron sulfides to sulfate in mine drainage settings occurs through multiple-step processes in which Fe³⁺, O₂, or both serve as oxidants, and H₂O, O₂, or both can serve as sources of oxygen for the resulting sulfate, leading to distinctive oxygen isotope signatures in the sulfate depending on pathway. The processes can be described by two end member reactions: one in which all of the oxygen in the sulfate is derived from H₂O, and the other in which three-quarters of the oxygen is derived from H₂O, and one-quarter is derived from O₂. In mine-drainage settings, both pathways appear to be important.  The “H₂O-only” pathway dominates at low-pH where it produces little or no oxygen isotope fractionation between sulfate and H₂O. The pathway involving O₂ is more common at higher pH, where intermediate sulfoxyanions are commonly reported in published laboratory oxidation studies, and produces significant fractionation of oxygen isotopes between sulfate and H₂O of over 15 permil. Electrochemical models of pyrite oxidation suggest that only H₂O molecules interact directly with sulfur atoms on the mineral surface at which point sulfate is the first and only sulfur species released to solution. Under these conditions, oxygen isotope exchange between solid sulfur species and H₂O, and associated interactions with O₂ appear to be prohibitively slow. In contrast at high pH (> 6) in laboratory studies, the observation of dissolved intermediate sulfoxyanions, the most important of which from an isotopic perspective may be sulfite, suggests that a significant amount of sulfur oxidation occurs in solution rather than on the mineral surface. Oxygen isotope exchange between dissolved sulfite and H₂O is fast, with a half-life less than one second below pH 7, at which point further oxidation contributes one of the four oxygen atoms to the resulting sulfate from O₂. Thus, a key step in the incorporation of O₂ into the dissolved sulfate is the release of sulfur to solution prior to its oxidation to sulfite. In addition to pH, other factors that may contribute to the early release of sulfur from the mineral surface include bacterial activity, complexation of intermediate sulfoxyanions with transition metals, particularly copper, and non-oxidative dissolution of monosulfides such as pyrrhotite or sphalerite.