Geochemical modelling may not immediately come to mind when scoping a work plan to develop a mine closure strategy. However, geochemical modelling is often essential in forecasting the long-term impacts of mining facilities, results from which can be applied to selecting closure strategies consistent with regulatory requirements. Closure planning for decommissioned tailings, waste rock, pit lakes, and underground mines can all benefit from predictive geochemical modelling calculations, combined with laboratory or field scale testing.
While laboratory and field tests provide crucial data on the leaching characteristics of mining wastes and pit wallrocks, integrating time varying hydrologic fluxes and other scalable parameters work best with an integrated predictive calculation approach. For example, field plots and barrel tests of waste rock are very useful in providing field scale leachate data, but typically offer only an abbreviated time fraction for generating weathering data for long-term environmental impact assessments. The same is true for laboratory testing.
Reliance on quantitative predictions of post-closure pit lake water quality has gained prominence in recent years, not only in the developed world but globally, as operators and regulators appreciate their importance for environmental impact forecasts. They are often required for closure plan approval, to forecast likely changes in future water quality.
This exercise cannot reasonably be expected to produce a valid forecast without using a detailed geochemical model that includes water balance (ground inflow and outflow, precipitation, evaporation, surface water runoff/runon), spatial and temporal geochemical characterisation of solids, secondary mineral precipitation and redissolution, and such limnological processes as mixing and stratification.
SRK recently completed a comprehensive program of geochemical modelling concerning closure alternatives for the pit lake, tailings, and waste rock in a feasibility study for an open pit gold mine in South America. To predict waste rock leachate quality, SRK combined short-term leach test data and accelerated kinetic test data with a detailed waste rock extraction schedule from the mine plan. The spatial variation of the acid generating characteristics of the waste rock combined with the high tropical rainfall posed challenges for managing predicted acidic waste rock runoff. The first material excavated was non-acid generating saprolite overburden, followed by the ore host, an acid generating intrusive rock. The leachate characteristics of this untimely sequence of waste rock was predicted in a geochemical model that provided the impact assessment needed to formulate the closure plan to minimise the handling of acidic runoff.
SRK incorporated detailed geochemical and mineralogical characterisation of pit wall rocks into a pit lake geochemical model that combined a water balance from groundwater flow and Goldsim models to predict post closure pit lake water quality. This forecast provided the information necessary for closure planners to select an appropriate pit closure strategy. Among other options, the pit lake water prediction indicated that available fresh water was adequate to neutralise acid production, so that natural infilling was identified as the optimal approach.
