Question 13 of 18: Maximum-NPV Cut-off Grade and Material Categories 1–3

Question 4.3: Maximum-NPV Cut-off Grade and Material Categories 1–3 (12 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(a) The maximum-NPV cut-off study (Category 1). Lane's (1988) opportunity-cost cut-off methodology runs the ENTIRE mine schedule (block model, mining/milling/refining capacity constraints, price deck, discount rate) repeatedly at a series of trial cut-off grades, computing the whole-of-life project NPV for each trial. Because raising the cut-off in an early period accelerates high-grade material forward (increasing early, heavily-discounted cash flow) at the cost of deferring or stranding lower-grade material, there is an interior maximum: too low a cut-off dilutes early cash flow with marginal material, too high a cut-off leaves economic tonnes unmined and shortens mine life. The cut-off grade (which may itself vary by period, driven by whichever of mining, milling or refining capacity is the binding constraint in that period) that maximizes whole-of-project NPV is adopted, and every block whose grade meets or exceeds it is scheduled as ore – this is Category 1.

(b) Category 2 – below the NPV-optimal cut-off but above the corporate hurdle. This material fails the NPV-maximizing cut-off (mining it now would dilute near-term cash flow enough to REDUCE whole-project NPV) but, evaluated on its own, still earns a positive NPV at the corporation's minimum acceptable (hurdle) discount rate – i.e. it is economic in isolation, just not optimal to process THIS period. Rather than being discarded, it should be set aside on a low-grade stockpile rather than sent to the waste dump, preserving the option to process it later (e.g. once higher-grade Category 1 material is exhausted and mill capacity would otherwise sit idle, or if metal prices rise), at negligible incremental re-handling cost relative to fresh mining.

(c) Category 3 – marginally positive at time of mining, outside categories 1–2. This is material that would generate a positive cash flow (revenue exceeds its OWN incremental mining+processing cost) if milled the instant it is mined, but does not meet even the corporate hurdle-rate NPV test of Category 2 (e.g. because it only barely covers direct cost with no return on capital, or because the time value of money erodes its NPV once haulage/rehandle timing is considered). It should still be captured (not sent to permanent waste) and placed on a SEPARATE, lower-priority stockpile – it remains a real economic option since it covers its own direct cost, but is the last tranche to be reclaimed, typically only once no higher-category material remains available to feed the mill.

(d) Selective stockpiling near the mill. Yes – both Category 2 and Category 3 material should be selectively stockpiled (graded separately, not commingled with waste or with each other) as close to the mill as practical, to minimize the FUTURE re-handling/re-haul cost when each stockpile is eventually reclaimed. Co-locating stockpiles by grade/category preserves the option value of each tranche and keeps the incremental cost of reclaiming it low relative to fresh mining, which is exactly what makes their eventual processing economic.

(e) Valuing end-of-life stockpile reclaim in the feasibility study. Stockpiled material reclaimed at the END of mine life is, by definition, the most heavily time-discounted tonnage in the entire project – its revenue arrives many years after the capital that enabled it was spent, so its contribution to NPV is small per tonne even though the ROCK was already paid for (mined) early. It should nonetheless be included explicitly in the feasibility study's schedule and cash-flow model as a distinct late-life "ore" tranche (with its own grade, tonnage and reclaim-cost line, separate from the primary schedule), because: (i) it extends mine/mill life and sweats the already-sunk milling capital for extra years at low incremental capital cost, (ii) although individually low-NPV per tonne, it is high-margin cash flow once fixed costs are already covered by primary production, and (iii) omitting it from the base case would understate total recoverable value and could misstate the true economic cut-off grade decision made in (a)–(c).

(f) Destruction of lower-grade value, and a role for government. The maximum-NPV cut-off process is deliberately time-value-optimal, not RESOURCE-recovery-optimal: it accepts leaving lower-grade (Category 2/3, and below) material permanently unmined at closure whenever reclaiming it later would reduce whole-project NPV, even though that material is geologically real and, evaluated without time discounting, represents genuine national mineral wealth. Once a pit is closed and reclaimed (backfilled, re-vegetated, infrastructure removed), that stranded lower-grade material is effectively lost forever – the sterilization is permanent, not merely deferred. There is a legitimate role for government (through mine-closure/reclamation permitting conditions, royalty design, or "maximum economic recovery" clauses in mineral tenure and closure-plan approval) to require that a company's mine plan and closure design at minimum preserve FUTURE access to identified sub-cut-off material (e.g. by not permanently sterilizing it under waste dumps, tailings or infrastructure, and by stockpiling rather than dumping Category 2/3 material) even where the private NPV-maximizing schedule alone would not choose to mine it, since the private discount-rate-driven optimum and the public interest in maximizing ultimate resource recovery from a non-renewable, publicly-owned mineral endowment are not automatically the same objective.