The Slow Death of Ore Bodies

Ore bodies are uneven. Early mine schedules tend to take the best zones first because early cash flow pays for equipment, infrastructure, and mistakes. Over time, the mine is left with rock that is lower grade, harder to process, farther away, or more contaminated by unwanted elements.

A simple grade example shows the leverage. If head grade falls from 1.0% copper to 0.6% copper, the mine needs about 1.67 tons of ore to recover the same contained copper once obtained from 1 ton. Nothing dramatic changes underground. The work required per unit of metal rises.

That extra work spreads across every step of the process—drilling, blasting, loading, hauling, crushing, grinding, tailings handling, and water and power use. This is why ore-body aging reshapes economics long before closure is discussed.

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Open pits live under one rule: as you go deeper, the pit must widen to keep slopes safe. That widening forces more waste stripping per unit of ore. Strip ratio is the shorthand for that burden.

Penn State’s mining engineering course illustrates the effect with a block-model example. A small pit (Pit A) contains equal blocks of ore and waste, so the stripping ratio is 1:1. Expand the pit to capture more ore in a steeply pitching ore body, and the overall stripping ratio in the example jumps to 4.75. That is nearly five units of waste for every unit of ore.

Waste has no revenue attached to it, yet it consumes the same fleet, fuel, and labor. Strip ratio is also lumpy: there can be years where stripping surges as the pit pushes back, even if ore output is steady. To outsiders, the mine looks “fine.” Internally, it is paying a larger geometry bill each year.

This is part of what makes depletion feel invisible. The mine is not short of rock; it is short of cheap access to ore.

Even when grade is known, what matters is payable product. Two quiet forces sit between “in-place metal” and “metal sold”: dilution and recovery.

Penn State defines cutoff grade in a practical way: at the boundaries of a deposit, grade trends down until the cost to mine and process a ton exceeds the value of what can be sold. The cutoff grade is the grade at which mining and processing cost equals the selling price of the commodity extracted. In the same lesson, Penn State walks through a copper example showing how dilution and plant recovery reduce the saleable copper yielded from a ton of ore.

This matters for “slow death” because dilution and recovery often get harder with time. As pits deepen and ore contacts become more complex, keeping waste out of ore can become more difficult. As ore changes mineralogically, recovery can drift down unless the plant is upgraded.

The key point is not a single recovery number. It is the direction. If you need more tons mined and milled for each ton of payable copper, the ore body is aging economically even if the reserve statement still looks large.

Some aging shows up as grade and strip. Some shows up as chemistry, and chemistry can change mine economics fast.

Teck’s technical paper on concentrate impurities explains why: smelters have differing limits, and those limits show up as penalties on incoming impurities. The paper notes that for some impurities, such as arsenic, the penalty per incremental unit can escalate with concentration, and certain smelters may reject feeds outright if a particular impurity is too high.

Teck also provides typical penalty thresholds and approximate penalties, including arsenic at 0.2% (with higher penalties when arsenic is above 1%), antimony at 0.05%, and bismuth at 0.02%, with penalty dollars per extra 0.1% of impurity. These are not abstract limits. They are pricing mechanics. A concentrate that crosses them can be worth meaningfully less, even if copper grade is unchanged.

Bismuth is a good example of why tiny numbers matter. Teck notes that bismuth can harm copper wire drawability even at low levels, and that refined copper must keep bismuth below 0.5 parts per million. That is not a “geology” problem in the casual sense. It is a marketability problem that can force blending, pre-treatment, or new buyers.

Academic work supports the idea that this issue is growing, not shrinking. A peer-reviewed review of complex materials in the copper industry reports that arsenic in concentrates processed in Japanese smelters rose from about 400 ppm in 1991 to over 1,000 ppm in 2016, while copper content in concentrates fell from near 33% to 27% Cu. (PMC) Even without forecasting, that is enough to show the direction of travel: more impurity units per unit of copper.

Penalty terms also have a long history in contracts. A McGill University thesis on minor elements in copper smelting describes how custom smelting contracts often set a “No Charge Maximum,” above which penalties are charged per increment, and it reproduces typical ranges for penalty structures for arsenic, antimony, and bismuth. The numbers vary by site and contract, but the framework is consistent: impurities reduce netback.

In plain language, an ore body can “age” when it starts producing a concentrate that fewer smelters want.

One reason depletion feels sudden is that reserve numbers can change quickly even when the rock does not. Penn State’s course summary puts it simply: reserves are often smaller than resources after subtracting material that cannot be economically mined based on cutoff grade or stripping ratio.

As strip ratios rise and penalties increase, cutoff grades move upward. That reclassifies borderline material from ore to waste on paper. Nothing changed in the drill core. The cost structure changed.

This is why “depletion headlines” can mislead. The slow decline is usually visible earlier in operating metrics than in the final closure announcement.

If you want to read an aging asset, look for a few steady trends rather than one-time events.

A persistent rise in stripping intensity is a warning that geometry is turning against the mine. A rising cutoff grade is a warning that economics are tightening around the edges of the deposit. And rising penalty elements are a warning that the mine is not only fighting cost inflation, but also fighting market access.

Bingham Canyon’s history shows why this matters. The transition to open-pit scale and steam-shovel stripping in the early 1900s was not a celebration of new machinery. It was an answer to an ore body that demanded a different economics to stay viable.

Depletion is rarely an event. It is a process of internal aging. Mines tend to lose the easy ore first, then the clean ore, then the cheap ore. Long before the last ton is hauled, the ore body has already been reshaping the business.