Grade, Tonnage & Scale
How Much Metal, in How Much Rock?
Why a little metal over a lot of rock can still add up to a real mine.
- Grade measures concentration. Tonnage measures the amount of material. You need both to understand a deposit.
- A large, lower grade deposit can contain more total metal than a small, higher grade one.
- Contained metal is not the same as recoverable metal, revenue or profit.
- Mining costs, processing recovery and the deposit's shape decide whether scale is actually useful.
- Compare deposits using consistent units, cut-off assumptions and resource categories.
Concentration and Quantity
Grade tells you how much metal sits in a given amount of rock. Tonnage tells you how much rock has been estimated overall. Together they describe a deposit's total metal content.
Picture two bowls of soup. One is small and packed with vegetables. The other is much larger but less concentrated. Concentration alone can't tell you which bowl actually has more vegetables in it. Mining has the same tension between grade and scale.
A Simple Gold Example
Take a hypothetical Deposit A with 10 million tonnes averaging 1 gram of gold per tonne. Multiplying tonnage by grade gives 10 million grams of contained gold. Divide by about 31.1 grams per troy ounce and you get roughly 321,500 ounces.
Now take Deposit B, with 1 million tonnes at 5 g/t. It contains about 160,750 ounces. Its grade is five times higher, but it holds only about half as much total gold.
These numbers describe metal content only. They say nothing yet about whether either deposit could become a profitable mine.
The Metal You Can Actually Recover
Some material never makes it into the mine plan. Some metal is lost during mining and processing. Metallurgical recovery is the share of metal actually recovered by the proposed processing method, and it depends on the mineralogy and the test work behind it.
In a simplified example, processing material containing 100 ounces at 90 percent recovery would recover 90 ounces. That still leaves operating costs, capital spending and other charges before any of it reaches shareholders as profit.
Beginner Benchmark
What "cut-off grade" means and roughly where it sits. A cut-off grade is a threshold used to determine which material qualifies for a stated resource estimate or processing plan under specified assumptions. Its purpose and calculation can differ between resource reporting and operating decisions. Check the metal prices, recoveries, costs, mining method and effective date; do not apply one generic gold-grade cutoff to all projects. Lowering a cutoff can increase reported tonnes while reducing average grade.Why Low Grade Can Sometimes Work
A broad deposit near the surface may suit large equipment and high processing volumes. A narrow deposit deep underground may need more selective, expensive mining. That's why the same grade can look attractive in one setting and marginal in another.
Scale can spread certain fixed costs across more tonnes, but it can also demand a bigger plant, more power, more water and a larger construction budget. Bigger only helps when the plan can actually turn that size into a worthwhile margin.
Questions to Ask Before Comparing
A lower cut-off can bring more material into an estimate while pulling the average grade down. So a bigger reported tonnage needs an explanation. Look for a genuinely improved economic picture, not just a larger headline number.Want more mining news explained this way?
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Mineral Resources vs. Reserves
The difference between "it's there" and "we can actually mine it."
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