Page 240 - Introduction to Mineral Exploration

10: EVALUATION TECHNIQUES 223

FIG. 10.11 On the left-hand side are several types of surface-set diamond bits, on the right are bits with
tungsten carbide insets and at the back are impregnated bits. (Reproduced by permission of Diamond Boart
Craelius Ltd.)

necessitates having a smaller diameter core flush away part of the sample and this can
(Table 10.7). sometimes be alleviated with the addition of
3 Circulating medium. Usually water is cir- specialized additives. Water may be used in
culated down the inside of the drill rods, wash- combination with various clays or chemicals,
ing over the cutting surface of the drill bit, and which, in addition to the above functions,
returning to surface through the narrow space have added uses such as sealing the rock face
between the outside of the rods and the wall of the drillhole. The science and use of such
of the drillhole. The purpose of this action is to drilling fluids is a subject in itself and is
lubricate the bit, cool it, and remove crushed much developed in the oil industry which
and ground rock fragments from the bit sur- drills significantly larger diameter and deeper
face. In friable and soft rock this action may holes than are required in mineral exploration