Stripping Ratio
Open pit economics ride on how much waste must move for each tonne of ore, summarized in the stripping ratio.
The equation
SR = waste tonnes / ore tonnes
- Instantaneous SR applies to a pushback, life of mine SR to the whole pit
- Breakeven SR is the ratio at which the ore margin exactly pays for stripping
- Units are tonnes per tonne
Worked example
A pushback moving 3.2 million tonnes of waste to release 1.6 million tonnes of ore runs at SR of 2.0. If each ore tonne clears 15 dollars of margin and waste costs 4 dollars a tonne to move, the breakeven SR is 3.75, so the pushback pays.
When to use it
Pits deepen until the incremental stripping ratio hits breakeven, which is what a pit optimizer automates block by block. Rising SR late in mine life is the quiet reason many pits close with ore still in the ground.
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.