Drilling
Drilling is the only way to see what is below the surface. Different drill types, programmes and hole spacings serve different purposes.
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Drilling is the only way to test what lies underground. How a project is drilled, and how closely, decides how much anyone can know about it.
What it is
A drill rig bores a narrow hole into the ground and brings up rock from depth, either as solid core or as chips. Each metre is logged and sampled, and the samples are assayed. Drilling is how a target becomes a discovery, and a discovery becomes a resource.
Drill types
| Type | What comes up | Typical use | All-in cost per metre |
|---|---|---|---|
| Rotary air blast (RAB) and air-core | Rock chips, shallow | Cheap early testing under soil cover | Lowest |
| Rock chips | Fast drilling for exploration, resources and grade control, usually to a few hundred metres | About $100–250 | |
| A solid cylinder of rock | Structure, density, metallurgy, geotechnical data and deep holes | About $200–500 | |
| Sonic | Undisturbed soft material | Gravels, tailings, placers | Varies |
All-in cost includes the drill contractor, assays, geologists, camp and access. Remote or helicopter-supported drilling can cost twice as much or more.
How the drill type is chosen
The choice is a trade-off between what the hole needs to show and what it costs. Geologists weigh these factors:
| Factor | Points towards cheaper chip drilling (RAB, air-core, RC) | Points towards diamond core |
|---|---|---|
| What the hole must answer | "Is there metal here, and roughly how much?" | "How is the metal arranged?": structure, vein orientation, rock types |
| Depth | Shallow to moderate: air-core to about 100 m, RC usually to 300–400 m | Deep: diamond holes can go beyond 1,000 m |
| Ground water | Above the water table | Below it, where RC samples get wet and can be lost or contaminated |
| Ground conditions | Soft weathered rock (air-core) or competent hard rock (RC) | Broken or faulted ground, where precise intervals matter |
| Budget and speed | Tight budget or many holes needed: RC is often 2–3 times faster and cheaper per metre | Fewer, higher-value holes |
| Data for studies | Not needed yet | Density, metallurgical samples, rock strength for pit and tunnel design |
| Access | Roads for larger RC rigs and compressors | Remote or steep ground: small diamond rigs can be moved by helicopter |
Coarse gold is a special case. RC brings up all the rock from each metre, a larger sample than thin core. In deposits with a strong nugget effect, that can give more reliable grades than small-diameter core, unless larger core is used.
What a typical project uses at each stage:
- Grassroots: RAB or air-core to test geochemical targets under soil cover cheaply.
- Discovery drilling: RC for most holes, with some diamond holes to understand the geology and structure.
- Resource drilling: mostly RC, with diamond for deep holes, holes below the water table, and density and quality checks.
- Studies: diamond for metallurgical, geotechnical and hydrogeology holes.
- Mining: RC for grade control.
A common combination is the RC pre-collar with a diamond tail: RC drills quickly and cheaply through barren rock above the target, then the rig switches to diamond core through the mineralized zone, where detail matters.
Core sizes run from smaller (BQ, NQ) to larger (HQ, PQ). Larger core costs more but gives bigger samples, which matter for metallurgical tests and coarse gold.
Drill programmes
- Scout drilling: a few holes testing a target for the first time.
- Step-out drilling: holes further from known mineralization, to find how far it extends.
- Infill drilling: holes between existing holes, to raise confidence.
- Twin holes: new holes next to old ones, to check historical results.
- Technical holes: for geotechnical (rock strength), hydrogeology (water), metallurgical samples, and condemnation (proving there is no ore under planned buildings or waste dumps).
Drill spacing
The closer the holes, the more confidently grade and continuity can be estimated. Spacing is set by the qualified person, based on how continuous the deposit is, but typical ranges are:
| Purpose | Typical hole spacing |
|---|---|
| Inferred resource | About 80–200 m |
| Indicated resource | About 40–100 m |
| Measured resource | About 15–50 m |
| Grade control during mining | About 5–15 m |
Erratic deposits, such as high-grade gold veins, need closer spacing than continuous ones, such as porphyry copper.
Holes, angles and surveys
Each hole starts at its . Holes are usually drilled at an angle, aimed to cross the mineralized zone as close to a right angle as possible, so the length is close to the . Downhole surveys record where each hole actually went, because holes bend.
Why it matters
Drilling is the largest exploration cost and the main source of information. The amount, type and spacing of drilling decide which resource category a deposit can reach. See resources and reserves.
Ranges are indicative and vary by deposit and region. This is educational material, not investment advice.
Key terms
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Educational material only. Nothing here is investment advice or an offer to buy or sell any security or mineral interest.