Head grade is the concentration of target metal in the ore feed to a processing plant, typically expressed as grams per tonne (g/t) for gold and silver, or as a weight percentage (%) for copper and other base metals. Head grade times recovery times tonnes processed = total contained metal produced. Combined with metal price, head grade defines the economic potential of a deposit.
Typical Gold Head Grades
Heap leach: 0.5–2 g/t is typical for large open-pit operations; some heap leaches operate as low as 0.3 g/t. Mill operations: typically 1–10 g/t; underground high-grade mines may run 10–30+ g/t. The cutoff grade (the lowest grade still economic to mine) depends on metallurgy, costs, and metal price.
Grade vs Cutoff
Cutoff grade is the breakeven grade below which mining is uneconomic. As gold price rises, cutoff drops and more ore becomes "ore"; as price falls, cutoff rises and ore becomes "waste". Mining schedules are typically re-optimized periodically as the gold price changes.
Reconciliation
"Head grade" calculated from sampled mill or plant feed often differs from "block model grade" (the geological estimate). Reconciliation between the two is a routine operations function. KCA's metallurgical bench engages on reconciliation when grade discrepancies indicate a metallurgical issue (e.g. mineralogy variability the model didn't capture).
Why It Matters for Design
Plant design is sized for a specific head grade. A plant designed for 1.5 g/t feed will not be economic on 0.7 g/t feed — recovery losses, fixed operating costs, and capital amortization all rise per ounce produced. Head-grade variability is therefore a key risk factor in any feasibility study.
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