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Designing A Drill Programme
The design of a drilling programme depends on whether it is designed to discover a new deposit or for
delineating an existing resource.
Resource definition requires drilling at close spacing to provide a high level of certainty and confidence
in the results. If an ore deposit is variable in grade then a large number of drill-holes will be required to
define the resource. For deposits with a consistent grade, much less drilling is needed. Gold deposits are
usually variable in grade over short distances, so a large number of drill-holes are needed to define this
type of resource. Resource geologists and modellers “cut” the data to remove the highest gold values.
This provides a closer approximation of the total resource and its average grade.

Planning a Drilling Programme
In the example shown in the image below a number of angled holes at 60° are designed to intersect at
depth with the mineralization that is causing the surface geochemical anomaly. The mineralization is
assumed to plunge almost vertically due to a known steeply dipping rock structure in the area.
Hydrothermal alteration along this structure has resulted in a strong magnetic low, which is coincident
with the soil geochemical anomaly.
Dispersion of elements from the ore body into the neighboring country rock has resulted in the formation
of an alteration halo. Dispersion of elements at the surface has also occurred causing a soil geochemical
anomaly. The coincident geochemical and geophysical anomalies provide very good evidence for the
drilling proposal.
Scattered highly mineralised rock chips found at the surface in the area indicate the potential
underground ore grades. The spacing between drill-holes is designed to maximize the chance of
intersecting the ore body while also gaining some understanding of its extent along the line of strike.

Hypothetical plan for drilling a narrow sub-
vertical ore body.

Drilling Methods
There are various drilling methods used in
exploration. Cost is often the biggest decider of
what technique is chosen.
Diamond Drilling
The best sample achievable through drilling is the
diamond drilling method which produces core. A
great deal of geological, mineralogical and
structural information can be obtained from
diamond drill core. Cross-contamination from one
adjacent interval to the next is virtually absent.
Diamond drilling is used for deep holes of
approximately 300m or deeper. Diamond drilling is also the most expensive and slowest form of drilling.
Reverse Circulation
Reverse circulation or RC for short, is the most commonly used drilling method in the minerals
exploration industry, mainly because it is cost effective and fast. A drawback to RC drilling is that less
information can be obtained when using this method. This is because the samples produced consist of a
mixture of small rock chips over each meter interval, so structural information is lost. Some
contamination between adjacent intervals can occur and this should be kept in mind when interpreting
the results. RC is useful for drilling holes to a depth of 300m. With the addition of extra high powered
compressors and boosters, depths of up to 500m are possible.
Percussion Rotary Air-Blast
Percussion rotary air blast or RAB is one of the cheapest forms of drilling. It is also the fastest. It is a very
good technique for quickly drilling a large number of shallow (0-30m) holes across a deposit.
Unfortunately, the technique produces samples of fine dust and chips that are often cross contaminated

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