Page 235 - Introduction to Mineral Exploration

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

minimizing the cost of collecting the samples Excavation can be either by hand, mechanical
when compared with re-sampling. digger, or by bulldozer on sloping ground.
Excavated depths of up to 4 m are common.

10.2 PITTING AND TRENCHING
10.3 DRILLING
In areas where soil cover is thin, the location
and testing of bedrock mineralisation is made 10.3.1 Auger drilling
relatively straightforward by the examination Augers are hand-held or truck-mounted drills,
and sampling of outcrops. However in loca- which have rods with spiral flights to bring soft
tions of thick cover such testing may involve material to the surface. They are used particu-
a deep sampling program by pitting, trenching, larly to sample placer deposits. Power augers
or drilling. Pitting to depths of up to 30 m is are particularly useful for deep sampling in
feasible and, with trenching, forms the simplest easily penetrable material where pitting is not
and least expensive method of deep sampling practicable (Barrett 1987). They vary in size
but is much more costly below the water table. from those used to dig fence post holes to large,
For safety purposes, all pits and trenches are truck-mounted rigs capable of reaching depths
filled in when evaluation work is completed. of up to 60 m, but depths of less than 30 m are
Drilling penetrates to greater depth but is more more common. Hole diameters are from 5 to
expensive and requires specialized equipment 15 cm in the larger units, although holes 1 m
and expertise that may be supplied by a con- in diameter were drilled to evaluate the Argyle
tractor. Despite their relatively shallow depth, diamond deposit in Australia. In soft ground
pits and trenches have some distinct advant- augering is rapid and sampling procedures need
ages over drilling in that detailed geological to be well organized to cope with the material
logging can be carried out, and large and, if continuously brought to the surface by the
necessary, undisturbed samples collected. spiralling action of the auger. Considerable care
is required to minimize cross-contamination
10.2.1 Pitting between samples. Augers are light drills and are
incapable of penetrating either hard ground or
In areas where the ground is wet, or labor is boulders. For this purpose, and holes deeper
expensive, pits are best dug with a mechanical than about 60 m, heavier equipment is neces-
excavator. Pits dug to depths of 3–4 m are com- sary and this is described in the next section.
mon and with large equipment excavation to
6 m can be achieved. In wet, soft ground any pit
deeper than 1 m is dangerous and boarding 10.3.2 Other drilling
must be used. Diggers excavate rapidly and pits For anyone interested in understanding the
3–4 m deep can be dug, logged, sampled, and subsurface, drilling is the most frequently used
re-filled within an hour. In tropical regions, technology. The various methods of drilling
thick lateritic soil forms ideal conditions for serve different purposes at various stages of
pitting and, provided the soil is dry, vertical an exploration program (Annels 1991). The
pits to 30 m depth can be safely excavated. Two Australian Drilling Industry Training Com-
laborers are used and with a 1 m square pit, mittee (1997) gives a comprehensive account of
using simple local equipment, advances of up methods, applications, and safety issues. Early
to 2 m per day down to 10 m depth are possible, on when budgets are low, inexpensive drilling is
with half that rate for depths from 10 to 20 m, required. The disadvantage of cheaper methods,
and half again to 30 m depth. such as augering, rotary or percussion drilling,
is that the quality of sampling is poor with con-
siderable mixing of different levels in the hole.
10.2.2 Trenching
Later, more expensive, but quality samples are
Trenching is usually completed at right angles usually collected using reverse circulation or
to the general strike to test and sample over diamond core drilling as shown in the follow-
long lengths, as across a mineralized zone. ing table: