Picture a truck driver at an open-pit mine. She pulls up to a pile of blasted rock, waits while a geologist checks a handheld analyser, and then receives one of two instructions: tip it onto the conveyor to the mill, or drive it to the waste dump. The rock looks identical either way. The difference is a number — the grade — and whether it clears a threshold called the cut-off. That threshold is not carved in stone. It can shift between a Monday and a Tuesday if the metal price moves far enough overnight.
What grade actually means
Grade is simply a concentration: how much of the wanted material is present in a given weight of rock. For most metals it is expressed as a percentage by mass, or for rarer and more valuable metals as grams per tonne. A copper deposit described as grading 0.8% contains roughly eight kilograms of copper in every tonne of rock dug up. The rest — more than 99% — is gangue, the unwanted host rock that the mill must process and then discard as tailings.
Grade varies continuously through an orebody. A single blast might shatter rock ranging from barren granite to a rich vein. Geologists sample drill cores and blast-hole cuttings to build a block model: a three-dimensional grid in which each cell is assigned an estimated grade. The mine plan is then drawn through that grid, deciding which blocks to extract and in what order.
Where cut-off grade comes from
Cut-off grade is the minimum grade at which a tonne of material pays its own way. Below it, processing the rock costs more than the metal it yields. Above it, there is a margin. The calculation behind cut-off grade draws together the metal price, the cost of mining a tonne, the cost of milling and refining it, the recovery rate of the processing plant, and any royalties or transport costs. Change any of those inputs and the cut-off shifts.
The most dramatic changes come from price. When the metal price rises, lower-grade rock that was previously uneconomic crosses the threshold and becomes ore. When the price falls, some of what was ore falls back below the line and reverts to waste. This is not an accounting abstraction — it changes physical decisions about where the shovel goes.
A worked example (illustrative)
Suppose a hypothetical copper mine faces the following simplified economics. The cost of mining and processing one tonne of rock — drilling, blasting, hauling, crushing, flotation, smelting, refining — comes to a combined figure of, say, 80 currency units per tonne of rock. The mill recovers 90% of the copper in each tonne it processes. Copper is selling for 8,000 currency units per tonne of metal.
To find the break-even grade, you ask: how many kilograms of copper must a tonne of rock contain for the revenue to equal the cost? Revenue per tonne of rock equals the grade (as a decimal) multiplied by 1,000 kg, multiplied by the recovery rate, multiplied by the metal price per kilogram. Setting that equal to the cost: grade × 1,000 × 0.9 × 8 = 80. Solving gives a grade of roughly 0.011, or about 1.1% copper. Any block grading above 1.1% goes to the mill; anything below goes to the dump.
Now suppose the copper price rises by a quarter. Re-running the same arithmetic, the break-even grade falls to around 0.9%. That might sound like a small movement, but in a large orebody the volume of rock that now qualifies as ore can increase substantially, because grade distributions in natural deposits tend to produce many more low-grade tonnes than high-grade ones. Conversely, a price fall to three-quarters of the original level pushes the cut-off up toward 1.4%, and a significant portion of the previously economic ore reverts to waste.
Why this matters beyond arithmetic
The cut-off is not just a financial filter; it reshapes the reported size of a deposit. Mineral resource and reserve estimates are always quoted relative to an assumed cut-off grade. When a mining company revises its reserve figure without changing the geology, it is usually because the assumed metal price — and therefore the cut-off — has changed. A reserve that grows without a new drill programme is telling you about economics, not about new discovery.
Cut-off grade also influences the life of a mine in a less obvious way. A mine operator facing a price slump may raise the cut-off to mine only the richest parts of the deposit, keeping cash flow positive in the short term. This practice, sometimes called high-grading, preserves near-term margins but sterilises lower-grade ore that might have been economic later. Rock that goes to the waste dump is rarely retrieved; once buried under years of subsequent dumping, it is effectively gone. Decisions made during a price trough can permanently shrink a deposit's recoverable inventory.
At the other end of the price cycle, a sustained high-price environment encourages operators to lower the cut-off and process leaner rock, extending mine life and recovering metal that would otherwise be left behind. Neither response is irrational; both are straightforward consequences of the same underlying arithmetic applied in different market conditions.
Grade, cut-off and the wider supply picture
Because cut-off grade responds to price, the global supply of a metal is not a fixed physical quantity — it is a number that expands and contracts with market conditions. Ore grades at producing mines have generally trended lower over long periods for many base metals, as richer deposits are depleted and the industry moves to leaner rock. That structural trend sits underneath the cyclical movement driven by price. Understanding both layers is necessary for making sense of production cost data, reserve life estimates, and the economics of new project development.
Readers who want to go further will find that cut-off optimisation is a branch of mine planning in its own right, involving concepts such as the opportunity cost of mill capacity, lane-specific cut-offs for different ore types, and dynamic cut-off strategies that vary the threshold through a mine's life to maximise the net present value of the operation rather than simply covering costs tonne by tonne.