| Summary: | The open pit to underground transition problem involves the decision of when, how and
at what depth to transition from open pit (OP) to underground (UG). However, the
current criteria guiding the process of the OP – UG transition are not well defined and
documented as most mines rely on their project feasibility teams’ experiences. In
addition, the methodologies used to address this problem have been based on
deterministic approaches. The deterministic approaches cannot address the
practicalities that mining companies face during decision-making, such as uncertainties
in the geological models and optimisation parameters, thus rendering deterministic
solutions inadequate.
In order to address these shortcomings, this research reviewed the OP – UG transition
problem from a stochastic or probabilistic perspective. To address the uncertainties in
the geological models, simulated models were generated and used. In this study,
transition indicators used for the OP - UG transition were Net Present Value (NPV),
ratio of price to cost per ounce of gold, stripping ratio, processed ounces and average
grade at the run of mine pad. These indicators were used to compare four individual
case study mines; with AngloGold Ashanti’s Sunrise Dam Gold Mine in Australia, which
made the OP – UG transition in 2004 and hence develop an OP – UG transition model.
Sunrise Dam Gold Mine is a suitable mine for providing baseline values because it
recently made the OP-UG transition. Only four case study mines were used because it
took nine months to generate transition indicators for each case study mine.
A generic model was developed from the results of the four case studies to help mining
companies make the OP - UG transition decision. The model uses a set of transition
indicators that trigger the decision while recognising the uncertainties in the geological
models, future mineral price as well as cost and processing parameters. From the
generic model, mines can transition when the margin (gold price to cost per ounce
ratio) is greater than 2.0; grade is between 4 g/t and 9 g/t, stripping ratio between 3 and
15 m3/t and positive NPV depending on the type of deposit. With this model mines can
now transition when the critical conditions of the transition indicators (gold price to cost
per ounce, grade and stripping ratio) are achieved. The model also uses the set of
transition indicators to model the probabilistic nature of the OP-UG interface. The
derived generic model will help mining companies in their annual reviews to assess the
OP - UG interface and make decisions early enough with regard to transition timing.
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