Sampling, assaying and quality control
How drill core becomes a grade, and the checks that prove the numbers can be trusted.
2 min read
On this page
Every grade in a resource comes from a small sample sent to a laboratory. Quality control is how a project proves those numbers are right.
What it is
Sampling turns drill core or chips into small portions that a laboratory can analyse. Assaying measures the metal in them. Quality control runs alongside both, to prove that the results are accurate and that nothing went wrong between the drill rig and the laboratory.
The process
From core to sample
- Logging: geologists record rock type, minerals, structures and how much core was recovered from each run.
- Photography: every box of core is photographed.
- Cutting: core is sawn in half along its length. One half is sampled; the other is kept as a permanent record.
- Sample intervals: usually about 1 m, adjusted to geological boundaries.
- Chain of custody: samples are bagged, numbered and tracked from site to laboratory so they cannot be swapped or tampered with.
RC drilling produces chips rather than core. They are split at the rig and sampled by the metre.
At the laboratory
- Preparation: the sample is dried, crushed and pulverised, and a small portion is taken for analysis.
- Gold: usually fire assay, which melts the sample to extract the gold. Coarse gold may need a screen fire assay on a larger portion.
- Base metals and other elements: usually dissolved in acid and analysed by instruments that measure many elements at once.
Reputable laboratories are independently accredited for their methods.
Quality control
samples are inserted among the drill samples without the laboratory knowing which they are.
| Control | What it is | What it catches |
|---|---|---|
| Certified standards | Material with a known grade | Inaccurate results |
| Blanks | Material with no metal | Contamination between samples |
| Duplicates | A second split of the same sample | Poor precision, or sampling problems |
| Check assays | Samples re-assayed at a second laboratory | Bias at the main laboratory |
A common practice is to insert each control at around 1 in 20 samples, and to send about 5% of samples for check assays.
Two other things that affect the numbers
Density
Each rock type's density is measured on core. It converts volume into tonnes, so a wrong density means wrong tonnes, however good the grades.
The nugget effect
In some deposits, especially gold, metal occurs as scattered coarse particles. Two samples a few centimetres apart can assay very differently. This makes grades erratic, so estimators cap the highest assays (a top cut) and need closer drilling.
Why it matters
Without quality control, there is no way to know whether a grade is real. Codes require it for resource estimates, and missing or failed quality control is one of the most common reasons estimates are rejected.
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.