Diamond Drilling AND Sampling

Just as one should not judge a book by its cover, surface sampling gives no definitive indication of how tremendous — or how mediocre — a deposit lies underfoot. Thus, after surface sampling indicates a possible concentration of valuable mineral, diamond drilling is undertaken.

The only way to ascertain the quantity (tonnage) and quality (grade) 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. To do this, a special type of drill has been developed, with a rotating core barrel that grinds down through the bedrock. At the end of the core barrel is a cylindrical bit studded with the hardest of natural substances — diamonds.

The size of diamond drill core varies with the size of the machine used, hole depths and material being drilled. However, the most common sizes are:

A — core diameter 27.0 mm, hole diameter 48.0 mm;
B — core diameter 36.5 mm, hole diameter 60.0 mm;
N — core diameter 47.6 mm, hole diameter 75.5 mm; and
H — core diameter 63.5 mm, hole diameter 96.0 mm.

Mechanically, the diamond drill consists of a power unit rotating a tubular steel bit on the face of which are set diamonds. This bit and attached core barrel are rotated under controlled pressure by means of hollow steel rods. Water is pumped through these rods to cool the bit and 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, consisting 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. As it moves through the rock, it pushes the core up into the core barrel.

The rods are withdrawn at intervals of 1.5 or 3 meters (5 or 10 feet) and the core is removed from the core barrel for examination and storage. The core presents a tangible and accurate record of the various rock formations through which the bit has passed.

Sludge samples are also sometimes taken while drilling is under way. These consist of the cuttings made by the drill, and are useful as checks against the drill core samples.