Drilling

Occurs on a mineral deposit at the exploration, development and production stages of a mining operation. ‘Blasthole' drilling, which involves drilling holes in which to place an explosive charge to break the rock, occurs during development and production. This may be to break waste rock or ore either on the surface, in an open-pit or underground.

In exploration, drilling is an important method of delineating a deposit. In the early stages, the term ‘scout' drilling is often used. Drilling not only provides information about the main features of the deposit but also about the surrounding rock for mine design.

Once a deposit has been outlined by drilling on a regular pattern, further detail may be determined by increasing the density of drill holes through ‘infill’ or ‘step-out’ drilling. This allows an increase of confidence in the initial stage of exploration.

Three methods of drilling are commonly used in mineral exploration: diamond, rotary and percussion.

‘Diamond' (or core) drilling uses a hollow diamond studded bit which cuts out a cylindrical core of rock. The column of rock is extracted from inside the drill rod for geological examination and assay. The drilling medium is usually water.

‘Rotary' and ‘percussion' drilling break the rock into chips using rotary or percussion (or combination) methods of penetration. The rock chips are continually flushed up the hole (usually outside the drill pipe) and are collected in sequence for geological examination and assay. Rotary and percussion drills are faster and cheaper than diamond drills.

‘Reverse circulation' drilling is a type of rotary drilling that uses a double-walled drill pipe. Compressed air, water or other drilling medium is forced down the space between the two pipes to the drill bit and the drilled chips are flushed back to the surface through the centre tube of the drill pipe.