There are a large number of different drilling techniques. Three basic types that are most commonly used in mineral exploration. In order of increasing cost, these are auger drilling, rotary percussion drilling and diamond drilling.
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| Auger drilling |
Selecting the right technique
Selecting the right technique or combination of techniques is always a trade-off between speed, cost, required sample quality, sample volume; logistics and environmental considerations. Augering and RAB drilling provide relatively low levels of geological knowledge but are quick and cheap and so are useful principally as geochemical reconnaissance tools for collecting samples below areas of shallow overburden.
Large rotary percussion rigs can quickly drill a large-diameter hole (100– 200 mm) with good sample volume and at reasonable cost. They are powerful machines capable of penetrating much deeper and through harder rock than the RAB rig. However, in normal rotary percussion drilling, the long sample return from drill bit to the surface along the outside of the rods may produce contamination from the walls of the hole. This problem can be especially acute when dealing with the low and often erratic concentrations typical of gold mineralization. The sample recovery system used in the RC drill rig is designed to overcome the contamination problem and for this reason RC rigs are nowadays specified in most rotary percussion drilling programmes.
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| Rotary Percussion |
Diamond drilling provides the premier sample for both geology and geochemistry. A diamond impregnated cutting bit produces a solid cylinder of rock. The rock sample can be obtained from any depth that is capable of being mined. Diamond drill core permits sophisticated geological and structural observations to be made, and can also yield a large-volume, uncontaminated sample with high recovery suitable for geochemical assay. Drill core can be oriented permitting structures to be measured. Diamond drilling is also the most expensive technique. As a general rule, for the cost of 1m of diamond drilling, up to 4 m of RC or 20 m of RAB can be drilled.
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| Diamond core bits |
From almost all points of view, the larger the core diameter the better. Large diameter holes provide better core recovery and deviate less. Lithology and structure are much easier to recognize in the larger core sizes and a larger volume sample is better for geochemical assay and ore reserve calculations. However, as the cost of diamond drilling is roughly in proportion to the core size, a compromise on hole size is usually necessary.
The specific requirements of an exploration programme play a large part in the choice of drilling technique. For example, if the area is geologically complex, or the exposure is poor, and there are no clearly defined targets (or perhaps too many targets), it may be imperative to increase the level of geological knowledge by diamond drilling. The geological knowledge gained from the diamond drill core can be used to help prioritize surface geochemical anomalies or develop conceptual targets. On the other hand, if discrete and clearly defined surface geochemical anomalies are to be tested to see if they are the expression of blind but shallow ore bodies, it may be sufficient to simply test them with a large number of RC or even RAB drill holes. In arid terrains, such as the Yilgarn Province of Western Australia, the RC drill has been used in the discovery and development of a large number of gold ore bodies within the weathered rocks of the upper 80 m or so of the surface. It has proved to be an excellent compromise between cost, good sample quality for geochemistry, and some geological return in the form of small rock chips. In spite of this success, the RC rig is principally a geochemical sampling tool and it is dangerous to attempt to define an ore body on assay numbers alone. RC drilling data can seldom give an adequate geological understanding of mineralizing processes and in most cases will need to be supplemented with detailed mapping (where outcrop is available), by trenching and/or a selected smaller number of diamond drill holes.
The logistical requirements of the different drilling types also play a large part in selection of the best technique. RC rigs (and the larger RAB rigs) are generally very large, truck-mounted machines which have difficulty getting into some rugged areas without track preparation, and cannot operate on very steep slopes. Diamond drill rigs are much more mobile; they are truck or skid mounted, have modest power requirements compared to an RC rig, and can be disassembled if necessary and flown to site by helicopter. Some rigs are even designed to be man-portable on dismantling. Diamond rigs, however, require a large, nearby water source. The ability to be flown or carried into a site also makes the diamond rig suitable for operation in environmentally sensitive areas.
The air core machine is a compromise that has some of the features of RC, diamond and RAB drills. In ideal conditions the drill can produce small pieces of core and so provide better material on which to determine lithology and structure than normal RC cuttings. It is often capable of penetrating and producing a sample from sticky clays that might stop a conventional drill rig. As with all RC cuttings, recovery is usually good with minimal sample contamination. However, the sample volume is small compared to that from a large RC rig, and hence less suitable for gold geochemistry. Air core is usually intermediate in cost between normal RC and RAB drilling. Some available rigs are truck mounted and are capable of getting into difficult to access sites.