Grade and contained metal
Grade is how much metal a tonne of rock holds. Multiplied by tonnes it gives contained metal, and the cut-off grade decides which rock counts.
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Grade is the concentration of metal in rock. It is the single biggest driver of how much metal each tonne of mined rock can deliver.
What it is
measures how much metal sits in each tonne of rock. Higher grade means more metal for the same effort of mining and processing, so each tonne earns more while costing about the same to move.
How it is measured
- Percent (%) for base metals such as copper, zinc, lead and nickel. A 2% zinc grade means 20 kg of zinc in each tonne of rock.
- Grams per tonne (g/t) for precious metals such as gold, silver and platinum. A 1.5 g/t gold grade means 1.5 grams of gold in each tonne. One troy ounce is 31.1035 grams.
Contained metal
Multiplying tonnes by grade gives : all the metal in the deposit, before any is lost.
Contained metal is not what a mine produces or sells. Recovery, payability and costs all take a share before any money is made.
Cut-off grade
Grade fades gradually from the richest part of a deposit into barren rock, so a line has to be drawn. The is the lowest grade that still counts as . Below it, rock is waste.
- A lower cut-off counts more rock: more tonnes, lower average grade.
- A higher cut-off keeps only richer rock: fewer tonnes, higher average grade.
- Contained metal usually falls as the cut-off rises.
- Tonnes of ore
- 14.2 Mt
- Average grade
- 6.1% zinc
- Contained zinc
- 866 kt
14.2 Mt × 6.1% = 866 thousand tonnes of zinc
Tonnes of ore Contained zincCut-off grade along the bottom
The right cut-off is an economic choice: roughly the grade at which a tonne pays for its own mining and processing at a realistic price. The table of tonnes and grades at each cut-off is a .
Mined grade is lower than resource grade
When ore is mined, some waste rock gets mixed in (dilution) and some ore is left behind. The grade delivered to the plant, the head grade, is usually a few percent to over 10% lower than the resource grade.
Typical grades
| Deposit type | Typical grade |
|---|---|
| Open-pit gold | 0.5–2 g/t |
| Underground gold | 4–10 g/t or more |
| Porphyry copper | 0.3–1% copper |
| Zinc-lead (sediment-hosted) | 5–15% zinc and lead combined |
| Nickel sulphide | 1–3% nickel |
Ranges are indicative and vary widely by deposit. 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.