Dilution and Diluted Grade
Mining never cuts perfectly along the orebody outline: waste rock rides along, and dilution converts the geological grade into what the mill actually receives.
The equation
diluted grade = g / (1 + d)
- g is the in situ grade of the ore zone
- d is dilution as a fraction of extra waste tonnage
- Mining recovery separately accounts for ore left behind
Worked example
A stope grading 1.4 grams per tonne mined with 5 percent dilution feeds the mill at 1.4/1.05 = 1.33 grams per tonne. At a 0.76 cutoff the ore still pays, but a narrow vein taking 30 percent dilution would fall to 1.08 and start flirting with the line.
When to use it
Dilution assumptions are where optimistic feasibility studies go to die. Wider orebodies and bulk methods dilute less as a percentage, narrow veins and open stopes more, and reconciliation against mill data is the only honest check.
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.