Open cast mining is used where the resource is near the surface and widespread - examples are iron ore, bauxite and any other deposit that is near the earth's surface. A deposit which has a limited areal extent and/or is very deep will be mined by undergroung mining. This is more expensive, so is only justified where the grades are good. The Platinum deposits in the Bushveld in South Africa were originally mined by opencast methods where they outcropped, but they dip downward, and as the depth to the ore zone became deeper, more material had to be removed to get to the ore, and it became necessary to use underground methods. The same is true for the gold reefs in the Witwatersrand. Many kimberlites were mined originally by opecast methods from the surface, and later by underground methods. The Big hole at Kimberley is an example of a mine that was originally worked by opencast methods, then later by underground, shaft methods. In assessing the value of a deposit the mining method is taken into account in deciding whether it will be economical or not.
Quarrying is the term applied to open cast workings for dimension stone, and aggregates. These have relatively low value and would not be economical to work by underground mining methods.
