Stochastic modelling of the open pit to underground transition interface for gold mines

Criteria or transition indicators to guide the complex decision problem of open pit to underground (OP-UG) transition are currently not well defined and documented as most mines rely on context-dependent experiences of their respective project feasibility teams. This problem is further compounded by the fact that transition indicators such as net present value (NPV), stripping ratio and commodity price are dynamic over time. Therefore, traditional deterministic approaches to solving the OP-UG transition problem cannot fully address the practicalities that mining companies face due to the variability of the transition indicators. This paper therefore reviews the OP-UG transition decision problem from a stochastic perspective. Transition indicators identified from an extensive literature review were used to benchmark four case study mines against one which had recently made the OP-UG transition, subsequently leading to the development of a generic OP-UG transition model for gold mines. The model indicates that depending on the type of deposit, gold mines can prepare to transition when the gold price to cost per ounce ratio is just greater than 2.0; grade is between 4 and 9 g/t, stripping ratio is between 3 and 15 m3/t and NPV is positive for the underground mining option.