Excavating the Rock
To separate ore from waste rock, drill samples are sent to an assay laboratory for analysis. The assay results are used to mark out zones of ore and waste rock, which are mined separately. The definition of ore and waste rock depends on the cut-off grade. If the cut-off grade is one gram per tonne (1.0 g/t) all rock with more than 1.0 g/t of gold will be marked out as ore, while all rock which contains less than 1.0 g/t of gold will be marked out as waste rock. The cutoff grade can be as low as 0.75 g/t in the open pit.
After sampling and mapping, some areas require blasting to loosen the rock prior to excavation. Other areas can be mined using a bulldozer. Ore and waste rock are mined by excavators loading onto trucks. In the open pit the trucks take their loads to the primary crushers or to stockpiles. In the underground mine the ore is taken to stockpiles at the processing plant, while waste rock is stockpiled for reuse as backfill once mined areas are completed.