How to Read a Grade Tonnage Curve

Reference

A grade tonnage curve is a mineral deposit answering the only commercial question it ever gets asked: what do you receive if you insist on better rock? This guide explains how to read one, how to build one, and the traps between the two.

The two curves and the trade

For each cutoff grade, one curve reports the tonnes at or above the cutoff and the other reports their average grade. Tonnage always falls as the cutoff rises; grade always climbs. The shape between those certainties is the deposit personality: a tight porphyry barely bends, a nuggety vein system dives. Build the curve from your own blocks in the Grade Tonnage Curve Calculator and the trade becomes a table you can read.

Where the cutoff comes from

The cutoff is economics, not geology: cost per tonne divided by recovered value per grade unit, computed in the Cutoff Grade Calculator for both the breakeven case that plans the mine and the internal case that governs rock already on the truck. Slide that cutoff along the curve and you are reading different mines out of the same deposit.

Getting the grades right first

A curve built from raw assays lies, because samples of unequal length carry unequal weight. Composite them first with the Assay Compositing Tool, and remember the support effect: curves built from big blocks always look smoother and kinder than the selective mining they promise.

From curve to decision

Contained metal falls slower than tonnage as the cutoff rises, and the pit that chases it must still pay for waste: the Stripping Ratio Calculator closes the loop between the curve and the shovel. For the honest version with uncertainty attached, the Monte Carlo tools in the Risk Suite show how ranges in the inputs become ranges in the answer.

The three reading errors

  • Reading resources at one cutoff as if they survive at another.
  • Comparing deposits whose curves were built on different block sizes.
  • Forgetting that the curve knows nothing about where the tonnes sit, and haulage is paid in distance.

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