Learn · 4 min read

What is exploration drilling?

Exploration drilling is how a mining company finds out what is under the ground. Every resource, every mine plan and nearly every announcement a junior makes starts with holes drilled into rock and samples sent to a laboratory.

What exploration drilling is for

Surface work only gets a geologist so far. Mapping, soil and rock-chip sampling and geophysics find a target; drilling tests it. A drill hole recovers a continuous sample of rock from the surface down, the samples go to a laboratory for assay, and the results show whether metal is there, how much, and where. Each round of holes guides where the next goes.

Aircore, RC and diamond drilling

Reverse circulation (RC): the workhorse

A hammer bit smashes the rock, and air blown down the outside of a double-walled rod returns the chips up the inner tube to a cyclone, where each metre is bagged and split. It is several times faster than diamond drilling, reaches a few hundred metres, and gives clean, representative samples. Most resource drilling at gold and lithium projects is RC. You see it in releases as holes named like ABC-RC-001 and results per metre.

Why drill holes are angled

Most orebodies are tilted: veins, shear zones, dipping layers. A vertical hole down a steep vein would stay in it for its whole length and give a false width. So holes are drilled at an angle, often 50 to 70 degrees from horizontal, aimed across the ore. The length of ore in the hole (the downhole width) is then close to the real thickness (the true width). Press Play to see why.

Schematic. True width = downhole width × the sine of the angle between hole and vein.

When exploration drilling happens

Target generation: mapping, soil and rock-chip sampling, geophysics, satellite data. No drilling yet.
Scout or first-pass drilling: widely spaced aircore or RC holes to test targets. Most targets fail here.
Extension and step-out drilling: chasing a discovery along strike and down dip.
Infill drilling: closing the spacing, for example to 40 m by 40 m, so a Competent Person can estimate a Mineral Resource with confidence.
Development drilling: diamond holes for metallurgy, geotechnical design and water, ahead of a feasibility study.

What a drill programme costs

Costs are quoted per metre. Aircore is cheapest, RC costs more, and diamond drilling usually costs several hundred dollars a metre all-in once assays, geologists, camp and access are counted. Depth, hard ground, remoteness and water all push costs up. Try a programme:

Reading drilling in a release

A drilling release lists each hole's collar position, azimuth and dip, depth, and the intervals that returned significant assays: for example 12 m at 2.1 g/t Au from 48 m. How to read those intervals, and the traps in them, is in how to read drill results. The open-file record of Western Australia's drilling, millions of holes, is mapped in the WA drilling story, and you can see the open-file holes under any address in Australia, Québec, Finland, Sweden or Saskatchewan in what's beneath your feet. To see how a real discovery unfolded hole by hole, from the first RC hole to a resource, replay Gonneville, hole by hole.

Questions

What is exploration drilling?

Drilling holes into the ground to sample rock that cannot be seen from the surface, so its grade and shape can be measured. It tests targets found by mapping, soil sampling and geophysics, and later outlines a deposit closely enough to estimate a resource.

What is the difference between RC and diamond drilling?

Reverse circulation (RC) drilling breaks rock into chips that are blown up inside the rods to the surface: it is fast and relatively cheap. Diamond drilling cuts a solid core of rock: it is slower and costs more per metre, but shows structure, contacts and textures, and is usually needed for resource estimates and metallurgical testwork.

Why are drill holes angled?

To cross the ore at a high angle. Most orebodies are tilted veins or layers, so a vertical hole would run down their length and overstate their width. Angled holes, often at 50 to 70 degrees, cut across them so the intercept is closer to the true width.

When is mineral exploration drilling conducted?

After surface work has found a target: geological mapping, soil and rock-chip sampling and geophysical surveys come first. Drilling then runs from first scout holes, to infill drilling that tightens the spacing for a resource estimate, to drilling for engineering and metallurgy before a mine is built.

How much does exploration drilling cost per metre?

It varies with method, depth, ground and how remote the site is. Aircore is the cheapest per metre, RC costs more, and diamond drilling usually costs several hundred dollars a metre all-in once assays, geologists and camp are counted. Companies quote a base rate per metre that extra costs for consumables, standby, moving rigs and water often push to about double.

What is a drill programme?

A planned set of holes, given in metres, for example '5,000 m of RC drilling in 40 holes', with the targets and what the company hopes to learn. Results are released as assays return, usually in batches.

Sources

An explainer of how mining companies report their work, written from the public codes and rules linked above. It describes the documents and does not rate any company, project or result. Not investment advice.