Mine Planning When Cost of Capital Rises

The Problem Is Not Just Cost. It Is Timing.

Interest rates have risen. Debt costs more, and equity investors are far more selective than they were two years ago. A mine plan that looked acceptable back then may no longer work today. A LOM schedule built when capital was cheap now carries hidden risk. The real question is not whether to cut spending. It is where to cut, where to hold firm, and how to keep the Ore Reserve statement honest while you do it.

Start With the Cut-Off Grade, Not the Budget

When capital tightens, the first reaction is to cut the budget. That is understandable. But cutting spend before revisiting the cut-off grade creates a specific problem: you may be protecting the wrong blocks. A cut-off grade is not a fixed number. It depends on metal price, processing cost, mining cost, and the cost of capital. When the discount rate moves from 5% to 8%, the net present value of material mined late in the LOM changes significantly. Blocks that were economic at a lower discount rate may no longer be economic.

A practical recalculation starts with the block model. Take the mining cost per tonne, the processing cost per tonne, the payable metal price, and the recovery factor, then compute the break-even grade. Add a capital charge for the equipment, infrastructure or sustaining spend tied to that block. If the cut-off grade rises, material at the margins of the pit shell moves out of the economic envelope and the Ore Reserve becomes smaller. That is correct. It is not a failure. It is the reserve doing what it is supposed to do.

Under NI 43-101, S-K 1300 and the JORC Code, the Competent Person must disclose the assumptions behind the cut-off grade. If those assumptions have changed and the reserve statement has not been updated, the disclosure is out of date. Regulators and investors both notice. A cut-off grade analysis should come before the budget conversation, not after it.

A simple illustration shows the effect. Take a copper open pit with a mining cost of $2.10/t, processing cost of $8.50/t of ore, a copper price of $3.80/lb and recovery of 88%. Under those assumptions the break-even grade sits near 0.25% Cu. Raising the discount rate increases the capital charge carried by a major pushback, which lifts the break-even grade by roughly 15%, to about 0.29% Cu. Material near that margin moves out of the reserve. It does not disappear. It becomes resource again, properly classified, waiting for economics to improve.

Resequence Capital, Then Validate the Short-Term Plan

Once the cut-off grade is set correctly, the LOM schedule is the next lever. In a high cost-of-capital environment, the principle is straightforward: bring forward cash. Pull high-grade, low-strip-ratio ore into the first years of the plan and defer major capital events, primarily large pushbacks and new fleet purchases, until the project has generated enough cash to fund them internally or until financing conditions improve.

This is not grade-chasing. It is sequencing with financial discipline, and the distinction matters. Grade-chasing ignores the block model and mines opportunistically, often leaving ore stranded and wall angles compromised. Disciplined sequencing follows the block model, moves the mining direction deliberately, and documents the rationale in the LOM. The mine planning and design process must capture this logic explicitly so the plan remains reproducible and auditable.

Consider a major pushback requiring $180 million of pre-strip capital spread over 18 months. Discounting that spend at 5% brings it to roughly $170 million in present value terms. At 8% it falls to about $167 million. The discounted cost of the capital itself changes little over a short drawdown. What changes materially is the value of the ore the pushback unlocks, because that revenue arrives years later and is discounted much harder. If the ore was worth $200 million NPV at 5%, at 8% it may be worth $160 million or less. That is where the margin disappears.

The decision chain shown above illustrates how this logic flows from economic assumptions through to the reserve statement. Each step has to be grounded in actual data: dispatch records, reconciliation outcomes, and the block model. Planning assumptions that have never been validated in the field are not a sound foundation.

Short-term plans are where strategy meets the shovel. A revised LOM that has not been translated into a credible 90-day or 180-day operating plan is not actionable. The short-term plans analysis process checks that the blocks scheduled for mining in the near term match the revised cut-off grade, fit within current fleet capacity, and deliver the grade targets the processing plant needs to meet its cost-per-tonne goals. When they do not align, the disconnect will show up in the reconciliation report before it appears in the reserve statement. That is the earlier warning sign, and it is the one worth acting on.

Fleet efficiency is part of this equation too. Higher diesel costs, higher tire costs, and higher financing costs on replacement equipment all feed into the effective mining cost per tonne. A truck fleet analysis that compares actual payload, cycle times, and fuel burn against plan assumptions will reveal whether the mining cost used in the cut-off grade calculation is still accurate. A cost underestimate in that line translates directly into an overstated Ore Reserve.

Keep the Reserve Statement Honest

The Ore Reserve is a public statement. Under NI 43-101, S-K 1300, and the JORC Code, it carries legal weight. A Qualified Person who signs off on a reserve without updating the underlying economic assumptions to reflect current discount rates, costs, and metal prices is carrying disclosure risk. That risk belongs to the company, and ultimately to the individual who signed the Table 1.

Operational teams sometimes resist reserve updates because a smaller reserve looks like bad news. It is not. A reserve that reflects current economics is a credible reserve. Investors and lenders can plan around it. A reserve inflated by stale assumptions is a liability that will eventually correct in a far more disruptive way: through a resource write-down, a project review, or a failed financing.

The connection between the strategy and reserves function and the operating plan should be continuous, not annual. When economic assumptions change, the reserve needs to be reviewed. When the block model is updated after a significant reconciliation variance, the reserve needs to be reviewed. When a major capital event is deferred, the reserve needs to be reviewed to confirm it is still supported by a current economic pit shell.

For operations reporting under multiple frameworks, say a Canadian company with a project in a JORC Code jurisdiction, the consistency of assumptions across reporting standards is a real practical concern. The cut-off grade, the modifying factors, and the SMU assumptions must be consistent whether you are producing a NI 43-101 technical report or a JORC Code compliant resource and reserve statement. This is not a compliance detail. It is a test of whether the planning team and the technical reporting team are actually working from the same numbers.

The Practical Takeaway

Capital discipline in mining does not start with the budget. It starts with the cut-off grade, moves through the LOM sequence, lands in the short-term plan, and ends with a reserve statement that a Competent Person can genuinely defend. When the cost of capital rises, each of those steps needs to be recalculated with current numbers. Doing that work early protects value. Delaying it compounds risk.

If your team needs support reviewing cut-off grade assumptions, resequencing the LOM, or validating that the reserve statement reflects current economics, the consultants at Agmines are ready to help. We work in English and Spanish across open pit and underground operations throughout the Americas. Get in touch with us here to start the conversation.