​Drilling is the process of penetrating through the ground and extracting rocks from various depths beneath the surface for confirming the geology beneath and/or providing samples for chemical analysis. The tip of the drill is known as a drill-bit, which is attached to the longitudinal drill-rod and penetrates the ground to form boreholes from which rock is collected. Because the rocks produced from drilling can be assigned to specific depths, this method is extremely important for determining the types and structures of the rocks underneath our feet. It is therefore also widely used outside mineral exploration, including investigating the strength of ground prior to the building of infrastructure (geotechnical engineering) and for groundwater studies. Logging is the process whereby the resultant sample is studied and recorded by a geologist to give definitive boundaries between different rock types and develop the picture of the geology below the surface. Assaying is the process of analysing the rock in a laboratory to measure the precise chemical composition of the samples, and is particularly important for investigating the metal contents of potential ore bodies. The dimension of the borehole is usually be 10-30cm in diameter and can range anywhere from 10s of metres to 100s of metres in length. Depending on the purpose and budget of the project, there are several types of drilling methods available, but reverse circulation and diamond are by far the most common for mineral exploration. However, in Ireland, rotary auger drilling in some places can be an additional option, which may also be used for geological mapping purposes.

Drill rig with rotary auger attached.

  • Reverse Circulation (RC)

RC is a quicker and less expensive form of drilling which produces small centimetre-millimetre-sized rock chips. The RC drill bit is known as the down-hole hammer and acts just like one: it is made of a tungsten-steel compound and uses percussive force to create the borehole through breaking-up and crushing the underlying rock. The RC drill rod has a hollow centre and a hole also in the middle of the drill bit, allowing the resulting rock chips to ascend upwards through the rod to the surface where they are collected, logged and – if required – assayed. The force required to penetrate through hard rock is supplied by very high-pressure air, which is generated within an adjoining cyclone and travels through the wall of the rod. When this air reaches the end of the borehole it also acts to flush the rock chips up through the core of the rod. Because the rock is crushed up, it means the resulting rock chips do not preserve the fabric of any structures within the rock.

In addition to being a relatively cheaper type of drilling, in certain ground conditions RC may be the only viable method as it is relatively insensitive to ground conditions (broken rock, permafrost, glacial till etc.) which may prove problematic for other drill rigs. Additionally, due to the use of high-pressure air, recovery of delicate rock types such as clay is still achieved at a satisfactory level, whereas other drilling methods may be prone to lose some of the quantity of such samples. An extremely useful advantage of RC is that it requires no water, and so it is a more environmentally-sensitive option where local water resources are scarce. However, in Irish mineral exploration, diamond is by far the most popular method (see below), and so much of the RC drilling in Ireland is done to create boreholes for the finding and monitoring of groundwater.

  • Diamond

Diamond is the hardest (readily available) naturally occurring substance in the world, and so finds itself an invaluable material in the process of drilling through extremely strong rocks such as granite and basalt. It is the most expensive form of drilling, however unlike RC, is capable of recovering an unaltered cylindrical core of rock, which preserves the structural details of the geology below. The Geological Survey Ireland's core storage facility in Sandyford houses a large proportion of such core that has been extracted from mineral exploration in Ireland over the previous decades.

The drill bit is studded with diamonds and acts a rotary-cutting motion on the rock to produce the borehole, and due to a hollow centre in the drill bit and drill rod, allows a cylindrical core to be preserved. As the drilling continues, a wire/rope is sent down the middle of the rod which then attaches to the resulting core and pulls it up to the surface in segments. These are then stored in trays of approximately 1m length. Core in this form is also extracted during geotechnical studies, which uses these 1m core intervals to measure the density of fractures in the rock, and from this its strength is calculated. However, unlike RC drilling, the rotary action of the drill bit generates enormous friction heat, and so a steady supply of water is required to cool the drill bit, which also acts to flush out any abrasive rock fragments.

Diamond is a very expensive method of drilling, however can be used to determine the nature and angle of the ore body at its margins where it is in contact with the surrounding rock types. Therefore, despite its increased costs, it can provide vital details which RC cannot.

Boxed diamond-drilled core in approximately 1m intervals.

  • Rotary Auger

Rotary auger is on the lower end of drilling in terms of cost and set-up, and is effectively a larger, mechanized version of a soil auger attached to a vehicle rig. During the ice ages in Ireland, extensive movement of glaciers across the country has resulted in the accumulation of significant loose rock and boulders known as till. Therefore when this material needs to be dug through or sampled up to a maximum of about 30m, rotary auger drilling offers a quicker, cheaper form suited to handle this less vigorous task where the rock layers are soft or loose. This is useful in mineral exploration for sampling till to find minerals of interest, and following any occurrences back in the opposite direction of glacial movement to try and locate the metals' source. However, it is also useful for reaching the bedrock layer for geological mapping, as well as sampling for environmental and geotechnical purposes.

  • What Comes After?

Due to the costs of drilling, it is usually a late step in the mineral exploration process. If previous work has yielded sufficiently successful results, drilling may eventually be used to physically confirm any mineralisation, and further develop the geological picture. After drilling a sufficient number of holes to satisfy the geologists that there is/isn't an ore body of a certain size and quality (and in the vast majority of cases it is the latter!), the decision may be made to develop a mine in conjunction with legal environmental requirements. Ireland has a long history of lead and zinc mining, in addition to modern strict environmental regulation, requiring the mining company to invest a significant amount of money to satisfy these requirements. Hence only in locations deemed suitably metal-enriched will mine-setup be considered a profitable venture. However, even if the decision to mine goes ahead, the exploration doesn't stop there, and will continue alongside mining to continue to define a more specific geometry of the ore body. For more information, please see the dedicated FAQs page.