From drill holes to a resource estimate
How geologists turn scattered drill holes into a 3D model of tonnes and grade, and the choices that move the result.
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A resource estimate is a model, not a measurement. It fills the space between drill holes with estimated tonnes and grades, and its result depends on choices the estimator makes.
What it is
A resource estimate turns a few hundred drill holes into a 3D picture of a whole deposit: how many tonnes, at what grade, and how confidently. Estimators fill the gaps between holes using geology and statistics.
The steps
- Data validation: checking collars, surveys, logs and assays, and the quality-control results.
- Geological interpretation: building 3D shapes of the mineralized zones, called domains, from the logs and assays.
- Compositing and top cuts: combining samples into equal lengths, and capping extreme grades so a few high assays do not inflate the estimate.
- Statistics and variography: measuring how grade varies with distance and direction, which guides how far each sample's influence should reach.
- The : dividing the deposit into blocks, for example 10 × 10 × 5 m.
- Grade estimation: estimating each block's grade from nearby samples, commonly by kriging or inverse-distance weighting.
- Density: assigning a density to each block to turn volume into tonnes.
- Classification: labelling blocks Measured, Indicated or Inferred by drill spacing and confidence.
- Reasonable prospects: showing the resource could plausibly be mined, usually with a pit or underground shape at a stated price.
- Reporting: tonnes and grade above a , signed by a .
The choices that move the result
| Choice | Effect |
|---|---|
| Domain boundaries | Drawing zones too wide adds waste or spreads grade too far |
| Top cut | A high cap lets a few rich assays inflate the grade |
| Search distance | Reaching too far extends grade into undrilled ground |
| Density | A small error changes every tonne |
| Cut-off grade and price | A low cut-off or high price makes the resource larger |
| Classification | Generous rules move material into higher categories |
How estimates are checked
- Independent review by another qualified person.
- Validation: comparing block grades with the drill holes around them.
- Reconciliation: once mining starts, comparing what the model predicted with what was mined.
Why it matters
Every later number, from contained metal to reserves and mine plans, is built on the resource estimate. Two qualified people can produce different estimates from the same drilling, so knowing which choices were made is as important as the headline tonnes and grade.
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.