Cut-off Grade

A cut-off grade is a threshold used to decide how mineralized material should enter a mine plan or be assigned to a destination. Its meaning depends on the decision: extracting a mining unit, sending already mined material to a plant, or retaining material for later treatment. It combines a stated economic objective with assumptions about recovery, costs and available capacity. It is therefore a planning rule that can change with circumstances, rather than an intrinsic boundary within the deposit.^[1]

Start with the Decision

The first question is what alternatives are actually available. A threshold for developing an underground mining area addresses a different commitment from a threshold for processing material that must already be excavated within a pit. A break-even threshold identifies equality between defined revenues and costs; a policy selected to maximize project value may differ because timing and competing uses of capacity also matter.^[2]

A useful decision statement identifies the material, alternatives, time period and costs that change:

These are analytical boundaries, not interchangeable names for one universal calculation. Before calculating a threshold, make the alternatives explicit enough that another engineer can identify which cash flows belong to each.

Define the Grade and Value Basis

A numerical threshold is incomplete without its grade basis. State the constituent, units and reference material: for example, metal content in undiluted mineralization or in diluted plant feed. Mining dilution changes the relationship between those bases. Metallurgical recovery describes the proportion of contained metal recovered by treatment; payability concerns the proportion credited under sale terms. They should not be silently combined or counted twice. Different mining or metallurgical domains may consequently require different thresholds.^[3]

Cut-off grade is also different from the average grade of accepted material. For a fixed model and selection basis, the grade-tonnage relationship connects a threshold to the tonnage remaining above it and that material's average grade. It does not, by itself, establish a practical extraction sequence.^[4]

Where several metals contribute value, a monetary measure can be more informative than a threshold on one assay. Goldie and Tredger define net smelter return as proceeds after off-site processing and distribution costs; their model requires grades, recoveries, treatment terms and prices. This gives a common value basis for polymetallic material.^[5]

The implication is that a quoted value per tonne needs an explicit boundary. A value after selling charges is not automatically a profit after mining and processing. List deductions once, and compare the resulting value with the costs that remain outside it. An equivalent grade is useful only insofar as its conversion assumptions preserve the decision being evaluated.

A Limited Break-even Relation

For material already committed to mining, compare processing with a discard route yielding no product receipts. The break-even relation is g_be = c / (v × r), adapted from Asad and Topal's mine-limited comparison.^[6]

• g_be: metal mass fraction, tonnes of metal per dry tonne of feed.
• c: treatment-route costs minus discard-route costs, in currency per dry tonne of feed, including handling and residue disposal.
• v: net value per tonne of recovered metal after payability and specified selling deductions.
• r: metallurgical recovery as a fraction; do not include it again in v.

For comparison with assays, convert the resulting mass fraction to the reported grade units. Assume constant positive c and v, constant 0 < r <= 1, and no capacity displacement or other binding constraint. If g_be exceeds attainable feed grades, none breaks even under these assumptions. Other cost signs, revenue-bearing alternatives, deferred treatment or nonlinear responses require direct cash-flow comparisons.

Select Costs That Match the Commitment

Cost inclusion follows the decision boundary. For material that will be mined under either destination choice, a mining cost identical in both alternatives cancels from their difference. Additional haulage, treatment, handling or disposal costs do not cancel merely because they occur within an existing operation. Conversely, a decision to extend mining cannot disregard the development, equipment or continuing services that the extension requires.

This distinction explains why an incremental threshold can be below a life-of-mine threshold. SRK's treatment of incremental material explicitly conditions its use on available capacity, avoiding displacement of planned feed and consistency with the mine plan. Its separate examples distinguish additional extraction from material arising through development.^[3]

Classifying a cost as “fixed” does not settle the issue: fixed over a shift is different from avoidable over several years. Ask what changes if the alternative is rejected, and over what period. Allocating every historical cost to every routing decision can reject worthwhile treatment; excluding necessary future commitments can make an uneconomic expansion appear attractive. These are reasons to reconcile the threshold calculation with the financial model for the actual decision.

Capacity, Timing and Stockpiles

A tonne with positive treatment margin may still be a poor choice for immediate processing when it delays a more valuable tonne. The foregone benefit is an opportunity cost. Minnitt's application of Lane's framework connects cut-off selection with mining, processing and marketing capacities and the present value of future cash flows. The binding stage matters; plant capacity cannot be treated as unlimited merely because the mine can supply more material.^[2]

Consider a plant with spare capacity that later becomes fully utilized. Material previously accepted for its positive treatment margin may now defer more valuable feed. Its mineral content has not changed; the competing use of capacity has. The policy needs reassessment against the revised schedule.

An operational implication is to compare alternative schedules, not simply raise a threshold by an arbitrary margin. Test whether the proposed selection can supply the required feed while respecting access, material availability and the relevant capacity constraints.^[3]

Stockpiling preserves a possible later treatment option, but adds another decision. Asad and Topal's model constrains stockpile capacity and assumes reclaim after deposit exhaustion; its results depend on that timing assumption.^[6]

Accordingly, identify where the material would be stored, how it would remain identifiable, when plant capacity would become available and what expenditure recovery would require. Deferring material is not evidence that it will eventually be profitable. A stockpile policy needs a realizable reclaim schedule as well as a grade boundary.

Selectivity, Uncertainty and Verification

A block above cut-off is an economic screening result under specified assumptions. Its physical extraction still depends on geometry, access and the mining unit that can actually be separated. Birch's investigation of narrow tabular gold deposits links block dimensions to practical mining selectivity and accounts for dilution between stoping and milling. The same study shows how estimated grades can assign material to the wrong side of a threshold.^[4]

Near the decision boundary, review how sampling, estimation and operating selectivity affect classification. Compare like with like during reconciliation: a plant-feed result should not be compared directly with an undiluted model value without accounting for the intervening material flows. Investigate whether a discrepancy reflects sampling, dilution, recovery, routing or an obsolete economic assumption before changing the threshold.

A reproducible policy records the model version, grade basis, destinations, economic date, recoveries, costs and capacity assumptions. Githiria and colleagues' implementation illustrates the dependence of a cut-off policy on grade-tonnage distributions, economic inputs, stage capacities and a resulting production schedule.^[1]

For review, vary the assumptions that could change the decision and examine the whole consequence: selected tonnage, feed composition, timing, capacity use and cash flow. Retain the reasons for choosing the policy and the conditions that would trigger reassessment. This allows a changed threshold to be understood as a revised engineering decision, rather than an unexplained adjustment to reported tonnes.