Water–rock interaction and acid neutralization in a large schist debris dam, Otago, New Zealand

Routine water chemistry monitoring of a large mine tailings impoundment dam at the Macraes mine provided a site for a 5-year study of progressive water–rock interaction with Otago Schist greenschist facies schist debris. Water of known composition was added at the top, and discharging waters had different compositions due to different pathways. Water which had most interaction with the rock showed progressively increasing sulphate (to 1500 ppm), bicarbonate (to 500 ppm), calcium (to 250 ppm) and magnesium (to 130 ppm). Sodium and potassium were fully adsorbed initially then transmission of alkalis increased, but no decomposition of albite or muscovite was detectable. Oxidation of pyrite produced the dissolved sulphate and hydrogen ions. The hydrogen ions were consumed immediately by decomposition of calcite and chlorite intergrown with pyrite. The rates of combined calcite and chlorite decomposition are governed by the rate of pyrite oxidation, and match that rate, so that pH does not change perceptibly. The acid neutralizing capacity (ANC) of these rocks depends on the combined calcite and chlorite content, and other minerals present have no significant effect on neutralization on the 5-year time scale. Decomposition rate of chlorite in these rocks is relatively high compared to experimental observations and other mine sites because the Macraes mine chlorite is intimately intergrown with acid-producing pyrite at the millimetre scale.