Geochemistry formed a critical aspect of closure planning for the Century open cut zinc mine. The mine is located in Queensland and the mining infrastructure includes three out-of-pit waste rock dumps, one in-pit dump and a tailings storage facility.
Closure objectives were developed for the infrastructure at site and the plan for the pit was that it would remain in place with a lake developing within it, post-closure. The water quality that may develop within the lake post-closure was identified as a potential risk to meeting downgradient water quality objectives (surface and groundwater).
To evaluate the risk, SRK developed a pit lake model that integrated the outputs from geochemical characterisation programmes, water balance studies and hydrogeological modelling. The model quantified solute production rates from pit walls and mineralised wastes located within the pit and accounted for potential influences from out-of-pit waste dumps.
Pit lake water quality was calculated over time, allowing the assessment of potential impacts to surface and groundwater, and third party receptors. Using the predictions from the model it was possible to compare potential environmental outcomes for different closure strategies and assumptions, and set priorities for forward works and make informed selection of optimal closure measures.
Modelling indicated that the pit water would be acidic and become more saline with time due to solute accumulation combined with evapo-concentration effects. The effects were greatest where the pit was modelled as a permanent sink, but were still significant for cases where modelling indicated release via groundwater through flow and overtopping. The modelling suggested that pit lake water may not be suitable for discharge.
