Diamond Drilling
Surface sampling does not give any definitive indication of how enormous – or how mediocre – a deposit lies underneath the surface. Due to this, after surface sampling shows a possible concentration of valuable mineral, diamond drilling is undertaken.
The only way to be sure of the tonnage (quantity) and grade (quality) of a deposit is to make a circular cut in the rock and extract the continuous cylindrical core sample from the center of the cut. A special kind of drill has been developed, this drill has a rotating core barrel which grinds down through the bedrock. At the end of the core barrel is a cylindrical bit which is studded with the hardest of natural substances, diamonds.
The hole depths and the material being drilled is what varies the size of the diamond drill core along with the machine which is being used. However, the sizes which are most commonly used are:
- A – core diameter 27.0mm, hole diameter 48.0mm;
- B – core diameter 36.5mm, hole diameter 60.0mm;
- N – core diameter 47.6mm, hole diameter 75.5mm;
- H – core diameter 63.5mm, hole diameter 96.0mm;
The diamond drill mechanically consists of a power unit which rotates a tubular bit of steel on the face of which are set diamonds. This bit and attached core barrel are rotated under pressure which is controlled by means of hollow steel rods. Water is pumped through these rods to cool off the bit and to remove the rock cuttings.
Under ideal drilling conditions and once the drill bit is lowered to the bottom of the hole and the drill started, the bit will cut a core, which consists of a cylindrical piece of rock. Rotating at a high rate of speed, the bit is forced downward by the action of hydraulic cylinders on the drill. This pushes the core up into the core barrel as it moves through the rock.
The rods are removed at intervals of 5 or 10 feet (1.5 or 3 meters) and the core is removed from the core barrel for storage and examination. The core presents accurate and tangible record of the many rock formations through which the bit has passed.
Sludge samples are also taken at times while the drilling is under way. These samples consist of the cuttings which are made by the drill, they are useful as checks against the samples of the drill cores.
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