Grade-Tonnage Curves: How to Read Them and When They Lie

The grade-tonnage curve is one of the most misunderstood plots in resource estimation. It looks simple — a curve showing how tonnage and average grade change as you vary the cut-off grade. But the simplicity is deceptive. That single curve contains a huge amount of information, and if you don’t know what to look for, it can lead you badly astray.

I’ve seen investors make multi-million dollar decisions based on grade-tonnage curves they didn’t understand. I’ve seen geologists use them to justify inflated resource estimates. And I’ve seen mining engineers plan operations based on curves that assumed things the deposit couldn’t deliver.

This post covers how to read a grade-tonnage curve correctly, what assumptions it embeds, and the three situations where the curve lies.

What a grade-tonnage curve shows

The grade-tonnage curve plots:

• X-axis: Cut-off grade (COG), from low to high
• Y-axis (left): Tonnage above the COG
• Y-axis (right): Average grade of the tonnage above the COG

As you increase the COG, tonnage decreases (fewer blocks qualify) and average grade increases (only higher-grade blocks remain). This is the fundamental trade-off in mining: mine more tonnes at lower grade, or fewer tonnes at higher grade.

A typical curve for a gold deposit looks like this:

COG (g/t)   Tonnage (Mt)   Avg Grade (g/t)   Contained Au (oz)
0.1         85.0           0.45              1,228,000
0.2         72.0           0.52              1,203,000
0.3         58.0           0.61              1,137,000
0.4         45.0           0.72              1,041,000
0.5         34.0           0.85              928,000
0.6         25.0           1.02              820,000
0.8         14.0           1.35              607,000
1.0         8.0            1.75              450,000
1.5         2.5            2.60              209,000

Reading this table: at a 0.5 g/t COG, you have 34 Mt at 0.85 g/t containing 928,000 oz. At 1.0 g/t, you have 8 Mt at 1.75 g/t containing 450,000 oz.

How to read the curve correctly

1. Look at the contained metal line

The most informative line on the plot is contained metal (tonnage × grade). This tells you where the value sits:

• If contained metal peaks at a low COG (0.2-0.3 g/t), the deposit is a large-tonnage, low-grade system. The value is in volume.
• If contained metal peaks at a high COG (1.0+ g/t), the deposit is a high-grade vein system. The value is in grade.
• If contained metal is flat across a wide COG range, the deposit has a uniform grade distribution. This is typical of disseminated mineralization.

For the example above, contained metal peaks at 0.1 g/t (1.23M oz) but stays above 1M oz until 0.4 g/t. This tells you the deposit has significant low-grade material that contributes to the total metal inventory.

2. Check the slope

The slope of the tonnage curve tells you about grade distribution:

• Steep slope — grade changes rapidly. A small COG increase removes a lot of tonnage. This means most of the material is near the average grade — uniform mineralization.
• Gentle slope — grade changes slowly. A COG increase removes little tonnage. This means there’s a wide spread of grades — a mix of high-grade and low-grade material.

Indonesian epithermal deposits typically have gentle slopes in the low-grade range (0.2-0.5 g/t) and steep slopes in the high-grade range (1.0+ g/t). This reflects the vein geometry — a large low-grade halo around a narrow high-grade core.

3. Find the “knee”

The “knee” of the curve is the COG where tonnage drops off dramatically. Below the knee, you’re adding lots of tonnes at low grade. Above the knee, you’re losing tonnes without gaining much grade.

The knee is often a good starting point for your operating COG — it represents the point of diminishing returns. For the example above, the knee is around 0.4-0.5 g/t: below that, you add 13-30 Mt but only 90,000-285,000 oz. Above that, you lose tonnage rapidly.

When the curve lies

Lie #1: The curve doesn’t include dilution

The grade-tonnage curve is calculated from the block model — it represents in-situ grade. But mining doesn’t recover in-situ grade. Mining mixes ore with waste (dilution), and not all ore is recovered (recovery).

A curve that shows 0.85 g/t at 0.5 g/t COG might deliver 0.72 g/t to the plant after 15% dilution at zero grade. That’s a 15% reduction in feed grade — which could be the difference between profit and loss.

Always ask: “Is this curve diluted or in-situ?” If in-situ, apply dilution before using it for economic analysis. A good report will show both curves.

Lie #2: The curve assumes perfect selectivity

The curve assumes you can perfectly separate ore from waste at the chosen COG. In reality, mining has a minimum selectivity — the smallest block you can selectively mine. For open pit, this is typically 5m × 5m × 5m (a blast pattern). For underground, it might be 2m × 2m × 2m (a drift round).

If your block model has 10m × 10m × 10m blocks, the grade-tonnage curve assumes you can mine at that resolution. But if the mining method can only achieve 15m × 15m × 15m selectivity, the actual mined grade will be different — usually lower, because the larger mining unit mixes ore and waste.

Always ask: “What is the block size, and what is the mining selectivity?” If the block size is smaller than the mining selectivity, the curve overstates the grade that can actually be delivered.

Lie #3: The curve doesn’t account for metal recovery

A curve showing 1M oz of contained gold doesn’t mean 1M oz will be recovered. If metallurgical recovery is 85%, the recoverable metal is 850,000 oz. And recovery often varies by grade — low-grade material may have lower recovery than high-grade material.

For Indonesian deposits with oxide-sulfide transitions, recovery can vary from 88% (oxide) to 45% (sulfide). A single grade-tonnage curve for the entire deposit masks this variation. You need separate curves for each metallurgical domain.

Always ask: “What recovery assumptions are embedded in this curve?” If the answer is “none” — the curve shows contained metal, not recovered metal. Apply recovery before using it for economic decisions.

The three curves you should see

A proper resource report should include three grade-tonnage curves:

1. In-situ curve — what’s in the ground (from the block model)
2. Mining curve — what can be mined (after dilution and mining recovery)
3. Process curve — what can be recovered (after metallurgical recovery)

The gap between curve 1 and curve 3 is the difference between geological potential and economic reality. Understanding that gap is the difference between a geologist who reports resources and a geologist who creates value.

Practical tips for Indonesian deposits

1. Generate separate curves for oxide and sulfide. The grade-tonnage relationship is different for each weathering state. A combined curve hides the metallurgical challenge.

2. Run sensitivity on metal price. The COG changes with price. A curve at $1,750/oz Au is different from one at $1,500/oz. Show both.

3. Compare curves between estimation runs. If your curve shifts significantly between estimation iterations, something in your model changed. Investigate before reporting.

4. Don’t use the curve to pick your COG. The curve informs the decision, but the COG should be based on economic analysis, not curve shape. The “knee” is a starting point, not a final answer.

The bottom line

The grade-tonnage curve is a powerful tool — if you know what it’s telling you and what it’s hiding. Read it for the grade distribution, the contained metal trend, and the knee. Then apply dilution, mining selectivity, and recovery to get from geological potential to economic reality.

The geologists who do this well produce resource estimates that connect to mine plans and cash flow models. The ones who don’t produce numbers that look good in a presentation and fail in production.