A pit deepens and its best ore goes first. A mine modelled at one strip ratio and one grade across its life is not being conservative. It is describing a mine that does not exist.
Life-of-mine average strip ratio is a real number that appears in real technical reports, and it is a perfectly good summary statistic. It is a terrible model input.
An open pit gets deeper. Waste per tonne of ore climbs as it does, usually steeply in the later benches. Meanwhile the mine plan sends the highest-grade, lowest-cost material first, because every mine plan ever written does that. So unit cost rises through the life and unit revenue falls, and both effects land in the same years.
Apply a single average to both and the model understates cost and overstates revenue in exactly the back half of the life, while overstating cost and understating revenue at the front. Those errors do not cancel, because discounting weights the front end more heavily. The model reports a mine that is more profitable, later, than the one the engineers designed.
It also quietly breaks the cutoff decision. Whether marginal material is worth hauling depends on the strip ratio and grade at the time it would be mined, not on a life average that no single period ever experiences.
Look for a schedule. Does the model carry strip ratio and grade per period, or one value each? If it is one value, the mine plan's own sequencing has been discarded and the answer cannot be right.
Check the last five years. Take the modelled unit cost in the final years of the life and compare it to what the pit design implies at those benches. A flat unit cost across a deepening pit is not a modelling simplification, it is a wrong answer.
Ask where the stockpile went. Most schedules stockpile marginal material and reclaim it late. If the model has no stockpile, tonnes and grades are arriving in the wrong years.
Take the profile from the mine schedule. The engineers already produced strip ratio and grade per period. The number you want exists and someone has already defended it.
Carry stockpiles explicitly. Model what is mined, what is processed, and what is sitting on a pad in between. The gap between those three is where a lot of working capital and a lot of grade actually lives.
Recompute the cutoff each period. With a real strip and grade profile, cutoff grade becomes an output of the economics rather than an input somebody chose once.
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