Strategic & CriticalBase & IndustrialPreciousGuidesNew InvestorsExisting Investors

What is an assay in mining?

An assay measures the concentration of an element in a sample. In mining, it helps turn observations about rock into numerical grades — but the result is only as useful as the sample and analytical method behind it.

Last reviewed:

What does an assay tell you?

A mining assay is a quantitative analysis of a sample, commonly reporting metal concentration as a percentage, grams per tonne (g/t) or parts per million (ppm). One g/t equals one ppm; one per cent equals 10,000 ppm. The reported element and basis matter: copper as Cu is not the same reporting convention as a rare earth total expressed as oxides.

A laboratory result is not a mineral resource estimate, a reserve or proof of economic extraction. It does not by itself show how the metal is distributed through the deposit, which minerals contain it, or how much can be recovered. Those questions require geological interpretation, additional sampling and metallurgical or economic studies.

From rock sample to laboratory result

  1. Define the sampling objective and interval. Drill core, RC chips, channels and grab samples represent different volumes and can have different biases.
  2. Record the sample location, geology and identifier. Use documented handling and security procedures so the result remains traceable.
  3. Prepare the material. Drying, crushing, splitting and pulverising reduce it to an analytical portion; the split must remain representative.
  4. Analyse with a method suited to the target element, minerals and concentration range.
  5. Review QA/QC and investigate failures before using results in interpretation or reporting.

A precise instrument cannot correct a biased field sample. A selected rock chip may confirm that mineralisation is present without representing the average grade of an outcrop. Coarse gold and other unevenly distributed minerals can also make small analytical portions unrepresentative.

Why the analytical method matters

There is no single assay method for every commodity. Fire assay is widely used for gold; acid digestion or fusion followed by instrumental analysis is used for many multi-element suites. The choice depends on the mineral matrix, required precision and concentration range. Some digestions are partial rather than total: a result can understate an element locked in resistant minerals.

Check the laboratory, preparation procedure, digestion or fusion and analytical finish. Accreditation should cover the relevant analytical scope; a laboratory name or an accreditation badge alone does not establish that a method suits every sample.

A less-than result is below a reporting limit, not proof of zero metal. An over-range result may need reanalysis using a method with a suitable upper range. Do not silently treat either as an ordinary measured grade.

How QA/QC helps

  • Certified reference materials test performance against an accepted value and uncertainty for a suitable matrix.
  • Blanks help detect contamination during preparation or analysis.
  • Duplicates test precision at the stage where they are collected; a pulp duplicate cannot test field-sampling variability.
  • Independent check assays can help investigate bias, but do not replace controls throughout the sampling chain.

Controls need documented acceptance criteria, review and follow-up. Saying that standards and blanks were inserted is not the same as demonstrating acceptable performance. CIM’s exploration guidelines emphasise fit-for-purpose sampling, preparation, analysis and quality assurance.

How an assay becomes a reported result

Companies may combine consecutive assays into drill intercepts using stated compositing rules. The average grade of an intercept is therefore a calculated summary, not necessarily a single laboratory measurement. Read drill results and intercepts alongside the underlying sample lengths, cut-off, internal dilution and true-width statement.

For unit conversions, oxide factors and a wider announcement checklist, use our guide to reading assay results. In an operating mine, grade control uses sampling and geological interpretation to help direct material to ore, waste or stockpiles.

Sources

This guide is educational, not investment advice or a site-specific operating procedure. Numerical examples are hypothetical.