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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 water–rock reactions and surface adsorption reactions on a geochemical model of a proposed pit lake at the Martha Au Mine, New Zealand. Our 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 that surface adsorption reactions would lower trace metal concentrations. 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 corresponding to annual turnover. To accommodate for uncertainty in the estimation of surface area, the surface area was increased by × 10, × 100, and × 1000 in three additional models. To model surface adsorption reactions, the mass of ferrihydrite precipitated annually was set equal to the mass of hydrous ferric oxide allowed to adsorb trace metals. Results of the baseline geochemical prediction without water–rock reactions showed the surface water pH dropped from 6.5 to 5.0 over 50 years. The sensitivity analysis on 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 water–rock reactions will not have a significant effect on the pH of the proposed Martha lake owing to the surface area of submerged minerals. Surface adsorption reactions lowered the concentrations of some trace metals (As, Cu, and Pb), whereas other trace metals were unaffected (Cd, Ni, Zn), which emphasized the pH dependence of these reactions. In the absence of model calibration or observed water chemistry, sensitivity analyses are a useful tool to explore uncertainties in pit lake predictions.

Turnover events in pit lakes influence the water quality of pit lakes by homogenizing the chemistry of mixing layers and distributing dissolved oxygen, hence limnologic models that predict the depth of annual turnover are required for predictive studies of post-mining pit lake water quality. This study demonstrates a limnologic prediction of a proposed pit lake at the Martha Au–Ag Mine, New Zealand, created with the hydrodynamic program DYRESM. The model considered 5 years of lake-filling conditions where the pit will receive river water, groundwater, pit wall runoff, and direct rainwater followed by 1 year of steady-state conditions where the lake will receive only groundwater, runoff, and rainwater. Sensitivity analyses showed that input groundwater temperatures ≤ 17 °C produced meromictic conditions, whereas groundwater temperatures ≥ 17 °C produced holomictic conditions. Changes in the chemistry of runoff inputs had no effect on turnover because the quantity of runoff is significantly less than the quantity of other inputs. These results suggest that understanding the density of major lake inputs is critical toward predicting the limnologic evolution of a pit lake as small changes in the water chemistry or temperature between inputs may affect input density and the resulting turnover depth. Lakes filled with water from one principle source, such as groundwater, are more likely to be holomictic due to the relatively homogeneous character of the resulting lake water, whereas lakes filled with water from more than one source, such river water and groundwater, may develop meromictic conditions due to slight density differences between input waters. It may be possible for mine managers to engineer holomictic or meromictic conditions in future pit lakes to improve lake water quality.

Pit lakes that result from open pit mining are potential water resources or potential environmental problems, depending on lake water quality. Wall rock mineralogy can affect lake chemistry if surface water inputs and/or groundwater inputs and/or lake water in contact with submerged wall rocks react with the wall rock minerals. This study presents a mineral quantification method to measure the distribution and concentration of wall rock minerals in open pit mines, and applies the method to the Martha epithermal Au–Ag mine, Waihi, New Zealand. Heterogeneous ore deposits, like Martha, require a large number of wall rock samples to accurately define mineral distributions. X-ray diffraction analyses of 125 wall rock samples identified the most abundant minerals in the wall rocks as quartz, adularia, albite, illite, chlorite, kaolinite, pyrite and calcite. Distribution maps of these minerals defined 8 relatively homogenous areas of wall rock referred to as “mineral associations”: weakly-altered, propylitic, fresh-argillic, weathered-argillic, oxidized, potassic, quartz veins, and post-mineralization deposits. X-ray fluorescence, Leco furnace, and neutron activation analyses of 46 representative samples produced the geochemical dataset used to assign quantities of elements to observed minerals, and to calculate average mineral concentrations in each association. Thin-section petrography and calcite concentrations from Sobek acid-digestions confirm the calculated mineralogy, providing validation for the method. Calcite and pyrite concentrations allowed advanced acid–base accounting for each mineral association, identifying 3 potential acid-producing associations and one potential acid-neutralizing association. The results target areas, where detailed hydrologic and kinetic tests would be valuable in the next stage of pit lake evaluation. Detailed understanding of wall rock mineralogy will help strengthen predictions of pit lake water quality.