Page 237 - Introduction to Mineral Exploration

220 M.K.G. WHATELEY & B. SCOTT

a rapid penetration rate of around a meter a produces a cylinder of rock that is recovered
minute and it is possible to drill 150–200 m in from the inner tube of a core barrel. The bit
an 8-hour shift. With such penetration rates and core barrel are connected to the surface
and several machines, several hundred samples by a continuous length (string) of steel or
can be collected each day. As each 1.5 m of a aluminum alloy rods, which allow the bit plus
10 cm diameter hole is likely to produce about core barrel to be lowered into the hole, and
20–30 kg of rock chips and dust, sample col- pulled to the surface. They also transmit the
lection and examination requires a high degree rotary cutting motion to the diamond bit
of organization. Like all compressed air equip- from the surface diesel power unit, and appro-
ment these drills are noisy in operation. priate pressure to its cutting edge (Fig. 10.10
& Table 10.6).
1 Drill bits. Drill bits are classified as either
Reverse circulation
impregnated or surface-set. The former consist
The reverse circulation (RC) drilling technique of fine-grained synthetic or industrial grade
has been in use in exploration since the mid- diamonds within a metallic cement while the
seventies and can be used in unconsolidated latter have individual diamonds, sized by their
sediments such as alluvial deposits or for drill- number per carat. In general, impregnated bits
ing rock. Both air and water can be used as the are suitable for tough compact rocks such as
drill flushing medium and both cuttings or core chert, while surface-set varieties with large
can be recovered. The technique employs a individual diamonds are used for softer rocks
double-walled string of drill rods (Fig. 10.9), such as limestone (Fig. 10.11).
with either a compressed air driven percussion Diamond bits will penetrate any rock in time
hammer or a rotating tungsten carbide coring but because of their high cost and the need
bit at the cutting end of the string. The medium to maximize core advance and core recovery
is supplied to the cutting bit between the twin- with minimum bit wear, the choice of bit
walled drill rods and returned to the surface up requires considerable experience and judg-
the center of the rods. In the case of percussion ment. Second-hand (i.e. used) surface-set bits
drilling the rock chippings are also transported also have a diamond salvage value and con-
to the surface up the center of the rods and from sequently are not used to destruction. Drill
there, via a flexible pipe, to a cyclone where bit diameters are classified with either a letter
they are deposited in a sample collection con- code (American practice) or in millimeters
tainer (Fig. 10.9). (European practice) (Table 10.7).
The advantages of using this method to col- 2 Core barrels. As the cylinder of rock (the
lect rock chippings, rather than auger, rotary or core) is cut by the circular motion of the drill
percussion drilling, are that the entire sample bit it is forced up into the core barrel by the
is collected, the method is extremely quick (up advancing drill rods. Core barrels are classified
to 40 m per hour can be drilled) and there is by the length of core they contain. They are
very little contamination. The specialized rods, usually 1.5–3.0 m in length but can be as long
the need for a compressor and additional equip- as 6 m. They are normally double-tubed in the
ment makes this a more expensive drilling sense that in order to improve core recovery an
technique than auger or percussion drilling, inner core barrel is independent of the motion
but the additional costs are outweighed by the of the drill rods and does not rotate. Triple-
higher quality of sample collection. The dual tubed barrels can be used in poor ground and
nature of some RC rigs (chips and core) means for collecting undisturbed samples for geo-
that high quality core can be taken through the technical analysis.
zone of interest without the need to mobilize Previously, to recover core the barrel had
a second (core) rig, thus reducing the overall to be removed from the hole by pulling the
drilling costs. entire length of drill rods to the surface, a time-
consuming process. Wire-line drilling (Q series
core) is now standard practice; in this method
Diamond core drilling
the barrel is pulled to the surface inside the
The sample is cut from the target by a connecting drill rods using a thin steel cable.
diamond-armoured or impregnated bit. This This has the advantage of saving time but often