The Anatomy of a Mine - Mining and the Environment: From Ore to Metal - page 178

thousand and with some base metals commodities, strike rates range from one in i fty to
one in one hundred. However, once the deposit has been located, most of the delineation
drill holes will intersect the deposit.
Drilling can result in signii cant environmental impacts. Although rare, exploration drill-
ing can encounter artesian water which, if not prevented can l ow for many years, wasting
groundwater resources. Also, drill cuttings containing pyrite can oxidize. Where these cut-
tings are left on the surface adjacent to the drill hole, the resulting oxidation can leave a patch
of contaminated ground which resists attempts at re-establishment of vegetation.

Auger Drilling
If disturbed samples are required from relatively shallow depths in unconsolidated sedi-
ments, soils or soft rocks, an auger rig may be used. The auger may be truck or tractor
mounted, similar to a farm post hole drill, or it may be portable. The main limitations of
auger drilling are boulders in the soil, excessive moisture, and inability to penetrate far
into the bedrock.

Rotary-percussion Drilling
This includes a wide range of drilling techniques. In most cases the energy source is com-
pressed air, and the sample obtained is in the form of chips. The chip samples produced at
the bit face are continuously blown to the surface by compressed air and collected at regu-
lar intervals by the crew for examination by the geoscientists. In the reverse circulation
technique the chips are blown to the surface through the centre of the drill rods, reducing
contamination of the sample and danger of hole collapse. If the ground is soft, special bits
may be used with rotary percussion rigs to produce small pieces of rock similar to dia-
mond cores. This is known as air-core drilling. Any form of percussion drilling requires
quite large volumes of compressed air to operate the rig and to lift the samples to surface.
The compressors required are usually mounted on wheeled or tracked carriers. Truck-
mounted rigs generally require a reasonable quality road to access a drill area, although
lightweight rigs are increasingly being used at the reconnaissance stage.

Core or Diamond Drilling
Samples are obtained with this method by rotating a string of drill rods into the ground
under hydraulic or mechanical pressure. The drill bit, which is usually faced with dia-
monds, cuts a cylindrical core of rock that continuously passes up into the drill barrel as
the bit penetrates. Diamond drilling rigs are truck, track, or skid mounted and an access
track is usually, but not always, required. The standard of such tracks is determined by
a number of factors, including the location of the area, terrain, type of drill rig and the
estimated duration of the drilling programme. Lightweight rigs can also be used for this
type of drilling for transport to rough or remote mountainous areas by helicopter. This
approach requires that drill sites be prepared in advance by ground crews. Figure 4.6
shows the distribution of drill sites accessed by helicopter at the Batu Hijau copper deposit
in Sumbawa, Indonesia. Man-portable rigs, which can be transferred from drill site to
drill site without the need for vehicular access, are being used increasingly for drilling in
rugged forest areas. Most of the drilling at the Tampakan site in the Philippines used this
minimally invasive approach.
Diamond drilling usually involves directing the cuttings to a sump where they are
deposited, enabling the water with any drilling additives to be recycled. These sumps
require careful i lling after drilling operations are completed. Otherwise they can pose a
hazard for people, livestock and wildlife.

Man-portable rigs, which can
be transferred from drill site to
drill site without the need for
vehicular access, are being used
increasingly for drilling in rugged
forest areas.