English

Abstract

Fourteen samples of pyrite-bearing Archean greenstone rock (d < 6.35 mm, 0.08 ≤ FeS2 ≤ 2.25 wt. %) were characterized and subjected to laboratory dissolution testing for periods of 154 or 204 weeks. Rates of pyrite oxidation were determined based on the observed rates of sulfate release between weeks 20 and 60 and the calculated pyrite surface areas exposed. The pyrite surface areas exposed were determined based on the particle size distribution, sulfur content of individual size fractions, and percent pyrite liberation. The pyrite oxidation rates, normalized for exposed surface area, ranged from 4 × 10 −10 to 18 × 10 −10 mol m −2 s −1 and tended to increase as drainage pH decreased from 7.3 to 3.3. For eight rock samples with median pH values above 6.0, rates were roughly 0.6 to 1.3 times those predicted in the literature for the abiotic oxidation of pyrite by oxygen. Median pH values for the remaining six samples ranged from 3.3 to 5.0, and pyrite oxidation rates were roughly 2 to 8 times the published abiotic rates, suggesting the influence of oxidation by ferric iron.