Page 236 - Introduction to Mineral Exploration

10: EVALUATION TECHNIQUES 219

Purpose Quality Type

Reconnaissance and exploration Low Auger, rotary and percussion (chips)
High Reverse circulation (chips)
Resource or reserve evaluation High Reverse circulation coring in soft formations
Diamond or sonic core drilling in hard formations

Fundamentally, drilling is concerned with depths of up to 200 m are possible, but depths
making a small diameter hole (usually less of 100–150 m are more usual. Drill cuttings are
than 1 m and in mineral exploration usually flushed to the surface by compressed air and a
only a few centimeters) in a particular geo- regular and efficient source of this is vital for
logical target, which may be several hundred the successful operation of this technique. As
meters distant, to recover a representative the cuttings come to the surface they can be
sample. Among the available methods rotary, related to the depth of the hole. However, such
percussion, reverse circulation, and diamond direct correlation is not always reliable as
core drilling are the most important. holes may not be cased and material may fall
from higher levels. In wet ground the lifting
effect of compressed air is rapidly dissipated
Rotary drilling
but special foaming agents are available to
Rotary drilling is a noncoring method and alleviate this drawback. Rigs are usually truck
is unequalled for drilling through soft to or track mounted and very mobile. With the

medium hard rocks such as limestone, chalk, latter slopes of up to 30 degrees can be negoti-
or mudstone. A typical rotary bit is the tricone ated and the ability to drill on slopes steeper
or roller rock bit that is tipped with tungsten than 25 degrees partly eliminates the need to
carbide insets. Rock chips are flushed to the work on access roads.
surface by the drilling fluid for examination 2 Top hammer drills. As the name suggests
and advances of up to 100 m per hour are pos- the hammer unit, driven by compressed air,
sible. This type of drilling is typically used in is at the top of the drill stem and the energy
the oil industry with large diameter holes to the noncoring drill bit is imparted through
(>20 cm) to depths of several thousand meters the drill rods. These are usually lighter units
with the extensive use of drilling muds to lift than down-the-hole hammer drills. They are
the rock chips to the surface. The large size of used for holes up to 10 cm diameter and depths
the equipment presents a mobility problem. of up to 100 m, but more usually 20 m. Most
only use light air compressors and this restricts
drilling depths to, at the most, only a few
Percussion drilling
meters below a water table. It becomes impos-
In percussion drilling, a hammer unit driven by sible for the pressurized air to blow the heavy
compressed air imparts a series of short rapid wet rock sludge to the surface. Usually they are
blows to the drill rods or bit and at the same mounted on either light trucks or tractors.
time imparts a rotary motion. The drills vary in
size from small hand-held units (as used in road General remarks
repair work) to large truck-mounted rigs cap- Percussion drilling is a rapid and cheap method
able of drilling large diameter holes to several but suffers from the great disadvantage of not
hundred meters depth (see Fig. 11.3). The units providing the precise location of samples, as is
can be divided broadly into two types: the case in diamond drilling. However, costs
1 Down-the-hole hammer drills. The hammer are one third to one half of those for diamond
unit is lowered into the hole attached to the drilling and this technique has proved par-
lower end of the drill rods to operate a non- ticularly useful in evaluating deposits which
coring, tungsten carbide-tipped, drill bit. Holes present more of a sampling problem than a
with diameters of up to 20 cm and penetration geological one, e.g. a porphyry copper. There is