Mine managers and regulators use geochemical predictions of pit lake chemistry to assess whether open pit mines will have positive or negative environmental effects after closure. Sensitivity analyses are a useful approach to explore uncertainties in these predictions. This study investigates the significance of subaqueous pyrite oxidation on the pH predicted by a geochemical model of a proposed pit lake at the Martha Au Mine, New Zealand. The hypothesis was that subaqueous pyrite oxidation would lower surface water pH over time because pyrite will be present in over 1/3 of the submerged wall rock area and turnover of the upper lake layers will annually supply dissolved oxygen.

An initial geochemistry prediction was created in PHREEQC based on site hydrology, representative input water chemistry, physical limnology, and the precipitation of ferrihydrite, manganite, amorphous gibbsite, and barite. Modeling water-rock reactions required the surface area of the submerged wall rocks, the concentration (volume %) of the dominant minerals found in the wall rock (i.e. pyrite, adularia, albite, chlorite, illite, and kaolinite), reaction rates for each mineral, and the volumes of circulating lake layers. These variables determined the mass of each mineral that would react with lake water over a one-year period. Water-rock reactions occurred each year after the annual turnover event. The analysis assumed fresh mineral surfaces, constant surface areas over time, and constant reaction rates, making it a “worst case scenario” for pyrite oxidation. To accommodate for uncertainty in the estimation of surface area, the surface area was increased by ×10, ×100, and ×1000 in three additional models.

Results of the initial geochemical prediction (without water-rock reactions) showed the surface water pH dropped from 6.5 to 5.0 over 50 years. Results of the sensitivity analysis (with water-rock reactions) produced nearly identical results to the initial model for surface areas of ×1, ×10, and ×100. Only the surface area ×1000 model lowered pH more than half a pH unit from the initial prediction. These data suggest that subaqueous pyrite oxidation will not have a significant effect on the pH of the proposed Martha lake owing to the surface area of exposed pyrite and the rate of pyrite oxidation.