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Grade control in mining explained

Grade control supports decisions about what to mine and where to send it. It combines close-spaced geological information, representative samples and operating constraints rather than relying on a single assay threshold.

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What is grade control?

Grade control is the short-term process used to interpret mineralisation and support material classification, mining boundaries and destination decisions. A team may combine geological mapping, drill or channel sampling, assays and a local model to decide which material goes to the plant, a stockpile or waste.

A long-term resource model describes a deposit at a broader scale. Grade control adds information relevant to current mining and the practical size of material that equipment can selectively extract. It does not replace resource and reserve estimation or remove geological uncertainty.

A typical grade-control workflow

  1. Collect and map geological information at a spacing suited to the deposit and mining method.
  2. Sample with procedures designed to represent the material being classified, maintaining traceable identifiers and locations.
  3. Prepare and analyse samples with appropriate QA/QC and turnaround times.
  4. Interpret mineralised boundaries and estimate grade or material characteristics at an operational scale.
  5. Issue dig, development or stope instructions and track where material actually moves.
  6. Compare model predictions with mined and processed outcomes, then investigate discrepancies.

Open pits may use dedicated grade-control drilling or blast-hole samples. Underground operations may use face mapping, channels and drilling. No sampling method is automatically best: access, geology, particle size, contamination and the volume represented all affect suitability.

Why representative sampling matters

An assay measures the material submitted to the laboratory. If the field sample misses coarse mineral particles, mixes adjacent intervals or preferentially collects fines, an accurate assay can still support the wrong mining decision. Laboratory precision is only one part of the total uncertainty.

Dominy and colleagues’ grade-control review emphasises integrating the Theory of Sampling into the whole sampling chain. Sample mass, collection, splitting and preparation must suit the material and the decision. Repeated measurements of a biased sample can reproduce the bias rather than fix it.

Use suitable reference materials, blanks and duplicates, with acceptance criteria and corrective action. A duplicate taken after pulverising checks a different source of variation from an independent field duplicate. Record where controls enter the process.

Ore, waste and cut-off decisions

A cut-off grade is one input to classification, not a universal boundary that every mine shares. Relevant assumptions include metal prices, recovery, processing and other applicable costs, plant capacity and blending constraints. Material characteristics such as hardness, deleterious elements and mineralogy can matter alongside grade.

The ability to mine selectively also matters. A very narrow high-grade band may not be separable from surrounding rock with the available equipment. Destination decisions should match the mine’s operating strategy and practical mining units rather than imply that every assay interval can be extracted individually.

Dilution and ore loss

  • Dilution is the inclusion of unwanted or lower-grade material with ore, commonly reducing the grade delivered to the plant.
  • Ore loss occurs when material intended to be recovered as ore is left behind or sent to an inappropriate destination.
  • Uncertain boundaries, blast movement, equipment selectivity and routing errors can contribute to either.

Trying to minimise dilution without considering ore loss can exclude valuable material. Conversely, taking a wider mining envelope may recover more ore while adding lower-grade rock. Grade control supports that trade-off; it cannot eliminate it.

What reconciliation checks

Reconciliation compares model predictions with subsequent observations, such as surveyed mined volumes, material movements, stockpile changes and processing measurements. Comparisons need consistent periods, boundaries, density and moisture assumptions, plus allowance for stockpile and plant residence times.

A discrepancy is a prompt to investigate, not proof that the laboratory is wrong. Causes can include geological estimation, sampling bias, survey or density errors, dilution, ore loss, misrouting and measurement timing. Review the whole chain before changing the model or sampling programme.

For public announcements, read drill results and intercepts in their exploration context and use our guide to reading assay results for units and QA/QC. An exploration headline is not a substitute for operational reconciliation.

Sources

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