The Grade Tonnage Relationship
Raise the cutoff and a deposit shrinks but sweetens: the grade tonnage curve is that tradeoff drawn for every possible cutoff at once.
The equation
metal = tonnes × grade
- Tonnes above cutoff fall as cutoff rises
- Average grade above cutoff rises with cutoff
- Contained metal always falls as cutoff climbs
Worked example
A deposit holding 5 million tonnes at an average 1.4 grams per tonne contains 7 tonnes of gold. Push the cutoff up and you might report 3 million tonnes at 1.9, richer ore but only 5.7 tonnes of metal: the rest just moved to the waste dump.
When to use it
The curve is the negotiating table between geology and the mill. Selectivity assumptions hide inside it: a curve built from big blocks smooths away high grade that small scale mining could capture, and vice versa.
Related Calculators and Reference
- Cutoff Grade Calculator: breakeven grades from your own costs.
- Grade Tonnage Curve Calculator: see the whole deposit response to cutoff.
- CIM: source of reporting standards for mineral resources.