Interpretation of isotopic compositions of dissolved sulfates in acid mine drainage

Abstract

Sulfates produced by oxidation of sulfide minerals in mines, mine-and-mill tailings, and waste-rock dumps are characterized by (1) 634S values similar to those of the original sulfide minerals, and (2) low or negative 51ao values similar to those for sulfates in acidic spring waters. Experimental oxidation of pyrite in waters with a wide range of 51ao values has shown that the 51ao value of the product sulfate depends not only on the experimental conditions (aerobic or anaerobic; submerged or intermittently wet and dry; sterile or containing Thiobacillus ferrooxidans), which may determine the actual reaction pathways, but also on the initial 6180 value of the water in which the oxidation takes place. The experimental results can be interpreted as evidence that 29 to 100% of the oxygen in the sulfate was derived from water molecules, whereas dissolved sulfates in field samples incorporated 35 to 90% water oxygen. So far it has not been possible to determine whether the water oxygen is incorporated into the sulfate during the actual oxidation process (implied by proposed stoichiometric equations), or through oxygen-isotope exchange between the water and one or more intermediates (e.g. thiosulfate or sulfite). At room temperature, the exchange of oxygen isotopes between water and sulfate ions is too slow to effect the observed 51ao values. The range in 51ao values for sulfate produced by oxidation of sulfides tends to widen as 618 0 values for rain and snow become more negative at higher latitudes and at greater distances from the oceans. Some uncertainty still exists regarding the actual 5100 value for dissolved oxygen in subsurface waters. A further complication is background sulfate from sources such as marine evaporites, which might be detected using sulfur isotope data. The above components must be characterized before proper interpretation of the oxygen-isotope composition of sulfate produced during sulfide weathering is possible. Only then could oxygen-isotope data be used to monitor the effects of measures designed to reduce (or accelerate) the oxidation process in mines, tailings deposits, and waste-rock dumps.