NSR Cut-off Grade Review When Prices and Costs Both Shift

Copper prices have climbed well above the levels most mine plans were built on, and many companies have updated their long-term price assumptions to reflect that reality. At the same time, operating costs have risen sharply. Mining consumables, labour, fuel and contractor rates all cost more than they did a few years ago. These two forces move the cut-off grade in opposite directions, and when they move together, the net result is easy to misjudge. If your Ore Reserve statement has not been revisited since both factors shifted, it may no longer reflect what your pit should actually be mining.

Why a Polymetallic NSR Cut-off Is Not a Simple Calculation

In a single-metal copper pit, the cut-off grade is already a function of many variables: mining cost, processing cost, general and administrative cost, recovery, smelter payability and the long-term copper price. In a polymetallic Mexican pit producing copper, zinc, lead and silver, the cut-off grade is expressed as a minimum net smelter return value per tonne rather than a single element grade. That NSR value combines all four metals into one revenue estimate per block, accounting for the specific smelter terms and transport charges that apply to each concentrate stream the operation produces.

This matters because the prices of copper, zinc, lead and silver do not move in the same direction or by the same amount at the same time. When copper rises strongly but zinc is flat, the NSR formula weights the revenue gain toward copper-dominant blocks. Blocks that were previously below the cut-off grade may cross into the economic zone, while blocks where zinc or lead provided most of the revenue contribution may not change much at all. The spatial pattern of what becomes ore and what stays waste is not uniform across the pit. A team that looks only at total reserve tonnage and concludes that higher copper prices have enlarged the reserve may be correct on average but wrong in the specific parts of the deposit that drive the first three years of the LOM.

We also look carefully at the smelter terms embedded in the NSR formula. Treatment charges and refining charges are negotiated periodically, and they have moved with market conditions. If the NSR formula in your reserve model uses smelter terms from a contract signed several years ago, the coefficient applied to each metal may overstate the net revenue your operation will actually receive. A cut-off grade that looks conservative on paper may in practice be too low, meaning the pit is treating material as ore that will generate a loss at current smelter terms.

This is one of the first things we check when we conduct a cut-off grade analysis for a polymetallic operation: whether the NSR coefficients are still consistent with the actual commercial agreements the operation is working under today, not the agreements that existed when the reserve was originally declared.

When Costs Rise Faster Than Revenue: The Margin Squeeze in the Block Model

Higher copper prices push the breakeven NSR value down, which normally expands the reserve. Rising operating costs push the breakeven NSR value up, which normally contracts it. When both happen together, the net direction depends on which effect is larger. In many operations we review, the cost increases of recent years have been substantial, particularly in haulage, reagents and labour. If the copper price assumption has risen by a meaningful amount but operating costs have risen by a proportionally larger amount, the cut-off grade may actually be higher than the one currently used in the reserve model, not lower.

The consequence in the pit is direct. If the cut-off grade is too low, material is sent to the mill that generates a loss per tonne processed. That loss does not appear in the daily grade report. It is absorbed into the cost-per-tonne-milled figure and appears later in reconciliation as a margin shortfall rather than a grade difference. When we review metal reconciliation data for operations in this situation, we often find that mill feed grade matches predictions reasonably well but operating margins are below what the LOM plan projected. The reserve model was delivering the right tonnes at the right grade, but that grade was not high enough to cover current costs.

There is also a practical issue with how cost inflation feeds into the reserve model. Mining costs are often estimated in a base year and then escalated or held constant, depending on the convention used by the technical team or the signing professional. If the reserve model holds costs constant at a level from two or three years ago and actual costs have risen since then, the model is systematically underestimating the breakeven NSR. Every tonne near the cut-off grade carries more risk than the reserve statement implies.

The problem is more pronounced in pits with high strip ratios. Haulage is one of the largest single cost items in a truck and shovel operation. When haul distances increase in the later years of a deep pit, the cost per tonne of ore mined rises accordingly. If the cut-off grade analysis used to declare the reserve does not model this cost increase through the LOM, the cut-off grade applied to early years may be the same as the one applied to deep late-year benches, even though the economics of those two situations are different. The result is a LOM plan that appears sound on average but has economic problems in specific periods. Our mine planning and design work routinely addresses this kind of period-specific cost variation when assessing whether a single static cut-off grade is appropriate for the full LOM or whether a variable cut-off grade is more defensible.

What the Signing Professional Needs to Confirm Before the Reserve Is Disclosed

Under any of the three major reporting frameworks, the Competent Person or Qualified Person must be satisfied that the modifying factors used to convert Mineral Resources to Ore Reserves are current and reasonable. That includes the metal prices, the operating costs, the metallurgical recoveries and the smelter terms embedded in the NSR formula. The signing professional is not simply confirming that the calculation was done correctly. They are confirming that the inputs reflect the conditions under which the deposit will actually be mined and processed.

When metal prices and costs have both shifted since the last reserve declaration, the signing professional needs more than a memo stating that management has reviewed the assumptions. They need to see that the NSR formula has been rebuilt with current inputs, that the block model has been rerun with the revised cut-off grade, that the reserve boundary has been checked spatially, and that any blocks changing classification between Measured, Indicated and Inferred have been identified and handled correctly. If Inferred material that was previously excluded from the reserve now falls inside the revised pit shell, it cannot be included in the Ore Reserve regardless of its grade. That constraint requires a careful review of the spatial relationship between the resource classification envelope and the new reserve boundary.

The chain shown above describes this process in sequence. It is not a lengthy process when the block model and cost model are well maintained, but each step must be completed explicitly and documented. A reserve that is disclosed without this review carries regulatory and commercial risk, particularly if the cut-off grade assumption is challenged during a technical audit or a transaction due diligence review.

We assist operations and corporate technical teams with this process through our strategy and reserves work, including independent review of the modifying factors and the NSR methodology before disclosure. The value of that review is not in finding errors. It is in giving the signing professional a documented, independent basis for their judgment.

The Right Time to Review Is Before Disclosure, Not After

When copper prices move and costs move with them, every polymetallic operation should treat that as a reason to review the NSR cut-off, not an optional task for the next annual reserve update cycle. The review does not need to be long, but it must be systematic: rebuild the NSR formula with current inputs, restate operating costs to reflect what the operation is actually spending, rerun the pit optimization, check the reserve boundary, and confirm that the signing professional’s modifying factors are current. If the review shows that the existing cut-off grade is still appropriate, document that conclusion. If it shows that the cut-off grade should change, act on it before the next disclosure.

If you want to discuss how your operation’s NSR cut-off grade holds up under current price and cost conditions, contact the Agmines team. We work with operations across the Americas in English and Spanish, and we can review your NSR methodology, your modifying factors and your reserve documentation before your next reporting period.

By Alejandro Guerra, Agmines Consulting.