Oxygen and sulfur isotope investigations of the oxidation of sulfide mixtures containing pyrite, galena, and sphalerite

Laboratory investigations of sulfide oxidation may help to understand data from Acid Mine Drainage (AMD) field sites. Although numerous oxygen and sulfur isotope studies of sulfide oxidation have been performed with single sulfides, isotopic data from oxidation experiments with sulfide mixtures are lacking. Due to the common occurrence of sulfide parageneses in nature, aerobic abiotic experiments were performed with mixtures of pyrite, galena, and sphalerite at initial pH 2 and 6 for different lengths of time (5 to 100 days). The oxygen and sulfur isotopes as well as surface and geochemical features were studied in order to obtain similarities and/or differences compared to data from single sulfide experiments.The oxidation of sulfide mixtures cannot be simply derived from previously performed single sulfide oxidation experiments. The experiments showed that pyrite was galvanically protected whereas galena and sphalerite were preferentially dissolved during the whole time period of 100 days. Geochemical and isotopic data indicated that the dissolution mechanisms of the acid-soluble sulfides galena and sphalerite are controlled by the concurrent attack of protons and molecular oxygen (or ferric iron, if available) on sulfide surfaces. The non-oxidative dissolution of galena and sphalerite followed by hydrogen sulfide oxidation dominated under acid pH conditions. When the concentration of DO or ferric iron exceeded the proton concentration, galena and sphalerite were dissolved oxidatively by molecular oxygen or ferric iron. The relatively large Δ34SSO4–sulfide values gave evidence that pyrite was not measurably oxidized during sulfide mixture experiments. Sulfide mixture experiments indicated that oxidation rates of waste rock of polymetallic sulfide ore deposits depend on pH conditions, the minerals present and their iron content, the abundance of the minerals, and their grain size (i.e., specific surface area). The δ18OSO4 values indicated that an oxygen isotope exchange between dissolved intermediate sulfur species (e.g., sulfite, thiosulfate) can occur during the oxidation of galena and sphalerite. The observed ε18OSO4–H2O value was 16.1–18.9‰ at a pH of about 5. The results from these experiments indicate that pyrite oxidation may be substantially reduced under AMD conditions as long as galena and/or sphalerite are not completely decomposed. Large δ18OSO4 and Δ34SSO4–sulfide values in combination with only slightly acid pH conditions are indicators of the preferential oxidation of galena and/or sphalerite.

► Oxidation mechanisms of sulfide mixtures differ from that of single sulfides.
► Pyrite is galvanically protected while galena and sphalerite are dissolved.
► The non-oxidative galena and sphalerite dissolution dominates at acid pH.
► Galena and sphalerite are dissolved oxidatively at circumneutral pH.
► An oxygen isotope exchange between intermediate sulfur species and water can occur.