Time-series monitoring of water–rock interactions in mine wastes, Macraes gold mine, New Zealand

The main tailings impoundment at the Macraes mine was active for 23 years until it was decommissioned in 2013. Water compositions in the tailings complex were closely monitored for 20 years, providing a time series of compositional changes during the interaction between mine waters, sulphidic ore (pyrite and arsenopyrite) and greenschist facies schist host rocks. Most waters had circumneutral pH because of abundant (2–10%) calcite in host rocks. Relict Fe²⁺ caused a temporary pH decrease before re-neutralisation. Chlorite, albite and muscovite partially altered to kaolinite, smectites and illite. Dissolved sulphate reached as high as 8000 mg/L, but this was partially attenuated by the precipitation of gypsum in the tailings complex, while the alkalinity (as dissolved ) rose by three orders of magnitude in parallel with the increase in pH. Dissolved As decreased by six orders of magnitude, to < 0.01 mg/L, between the processing plant and the groundwater system below the tailings complex. Dissolved sulphate plumes emanate from tailings and waste rock piles, but are diluted downstream.