Rally Mode - ON, page-183

Haven't read the report but I would say hey are using the $1,800 pit shell for their planning. This is where the optimisation process will push the pit down to a depth and width where the cost of mining the last tonne equals he value of that tonne when using a $1,800 gold price for that bit of ore. This can result in a slightly larger than optimal pit but the main thing to remember is the $1,800 applies to the last tonne and the average cost per ounce will be well below this.
Usually, the optimisation process produces a whole series of "nested" pit shells, over a range of gold price assumptions. The results from these shells (gold produced, average $/oz, etc.) are graphed and analysed and then a preferred shell chosen based on the trends of the results and risk profile etc.