English

Abstract

A major difficulty facing many open pit mines where sulfides are present is the pit lake water quality after closure. A reliable prediction of future water quality within an open pit mine, excavated below the water table, requires a large amount of information and predictions for maybe hundreds of years into the future. These long-term predictions can only be useful with thorough understanding of geochemical and hydrological processes of pit walls and surface water and groundwater discharging into the pit during mining and at closure. Tyrone Mine initiated a pit-lake water quality evaluation as part of a closure evaluation. This involved characterization of waste rocks near the mine pit; determination of groundwater discharge and recharge into and from the pit using numerous wells drilled in the vicinity and monitored over many years; analysis of existing pit lake water quality over many years including a period of seven months with no pumping; water quality data from monitoring wells; local climate measurements; simulated rainfall leaching tests; and waste rock discharge water analysis. A modeling scenario was set up dividing the groundwater recharge/discharge into 5 zones based on groundwater flow and geochemistry. The hydrogeological model was calibrated based on the monitoring data where recharge or discharge volumes for each of the zones were calculated. Surface runoff water quality was based on three different mineralogical zones that had a distinctly different leaching potential: an oxide zone with neutral pH and low total dissolved solids; a Chalcanthite zone with weakly acidic and very high TDS (in particular sulfate and copper); and a sulfide zone with low pH and high sulfate. Pit lake modeling was performed to simulate the water quality observed during a seven-month period when there was no pumping taking place in the Main Pit. The model was calibrated with surface runoff and mineral precipitations. This modeling indicated that pit wall runoff was the most important control on water quality, and that the runoff water quality could be well represented by using in- field rainfall simulated leach tests.