Once a mineral deposit is sufficiently well defined, an important decision must be made: should it be mined from the surface or underground?
The depth of the ore naturally influences this choice, but it is not the only consideration. The shape of the deposit, its grade, rock quality, costs and environmental constraints must also be assessed.
The geometry of the deposit
A large, continuous deposit located near the surface is often well suited to open-pit mining.
In contrast, a deep, narrow or steeply dipping deposit may be better suited to underground mining. Underground drifts can follow the mineralization more precisely.
Depth therefore provides an initial indication, but the deposit’s overall geometry remains a determining factor.
The amount of waste rock to remove
In an open-pit mine, the rock located above and around the ore must be removed.
As the pit becomes deeper, the volume of waste rock that must be moved can increase significantly. The ratio between waste rock and ore, known as the stripping ratio, then becomes an important factor in the project’s profitability.
A pit may appear relatively simple to develop at first, but it can lose its economic advantage when stripping requirements become too high.
The costs of an underground mine
Underground mining generally allows for more selective extraction. It can reduce the amount of waste rock removed and follow specific mineralized zones more accurately.
However, it requires complex infrastructure, including ramps, shafts, drifts, ventilation systems, ground support and water management.
Operating costs are therefore often higher. The ore must generate enough value to cover these additional expenses.
Grade and dilution
An open-pit mine can sometimes process lower-grade ore because of its high production volumes and generally lower cost per tonne.
An underground mine can be more selective, but its profitability depends heavily on ore grade and dilution.
Grade should therefore never be assessed on its own. Mining recovery, metallurgical recovery and the amount of waste rock mixed with the ore must also be considered.
Geotechnical conditions
Rock stability directly influences mine design.
In an open-pit operation, weak rock may require flatter pit walls. This makes the pit wider and increases the volume of waste rock that must be removed.
Underground, poor ground conditions may require additional support, backfilling or reinforcement work.
Geotechnical and hydrogeological data are therefore essential before selecting a mining method.
The environmental footprint
An open-pit mine generally occupies a larger area and directly alters the landscape.
An underground mine may reduce the visible footprint, but it still requires roads, buildings, storage areas and processing infrastructure.
The decision must therefore consider the entire project, not only the main excavation.
Sometimes both methods are used
Some projects begin as open-pit operations and transition underground once the pit reaches its economic limit.
This approach can make it possible to extract near-surface zones quickly while maintaining access to deeper extensions of the deposit.
However, the transition must be planned during the early stages of the project.
How to make the right decision
No single mining method is always superior.
An open-pit mine may be advantageous for a large deposit located near the surface. An underground mine may be preferable when the mineralization is deep, narrow or higher grade.
The best option is the one that matches the deposit’s geometry, ground conditions, project costs and acceptable level of risk.
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P.J Lafleur Géo-Conseil supports mining and exploration companies with data interpretation, geological modelling, model validation and mineral resource estimation.