A haul truck carries 250 tonnes of rock. Whether that load is worth processing or belongs on the waste dump often comes down to a fraction of a percent difference in metal content, a difference you cannot see, smell, or feel. Grade control is how mines make that distinction reliably, load after load, shift after shift.
Get it right and the mill receives consistent feed at the target grade. Get it wrong and you’re either sending valuable ore to the waste dump (ore loss) or diluting good ore with worthless rock (dilution). Both cost money. On a 10 Mtpa gold operation, poor grade control can cost $5M to $15M per year in lost metal recovery alone.
The concepts here (grade definitions, material classification, and geological models) underpin every routing decision the FMS makes. The companion article, FMS Grade Control in Practice, covers how the FMS actually executes grade control in the pit: the GPS workflow, destinations, reconciliation, and common failure modes.
What Grade Actually Means
Grade is the concentration of the valuable mineral or metal in a parcel of rock. How it’s expressed depends on the commodity:
| Commodity | Grade Unit | Example |
|---|---|---|
| Gold | Grams per tonne (g/t) | 2.5 g/t Au |
| Copper | Percentage (%) | 0.8% Cu |
| Iron ore | Percentage (%) | 62% Fe |
| Nickel | Percentage (%) | 1.2% Ni |
| Zinc/Lead | Percentage (%) | 5% Zn |
| Platinum group | Grams per tonne (g/t) | 4.0 g/t PGE |
Grade varies spatially. A block of rock at one location might contain 1.5% copper while the block next to it contains 0.3%. This natural variability is what makes grade control necessary. If ore bodies were uniform, you’d just dig everything and process it. They’re not, so you need a system to separate the good stuff from the rest.
Grade Control Looks Different by Commodity
Not all ore bodies are created equal. The geometry of the mineralisation fundamentally shapes how grade control works, and what the FMS needs to manage.
Bulk commodities (iron ore, manganese) tend to occur in large, relatively consistent deposits. Dig blocks can be sizeable because the grade doesn’t change dramatically over short distances. The grade control challenge is more about managing contaminants (silica, alumina, phosphorus, sulphur) that affect product specifications than about finding where the ore is.
Precious metals (gold, platinum group) are a different beast entirely. Gold mineralisation can be thin, irregularly shaped, and high-grade zones can pinch out over a few metres. Dig blocks need to be smaller and more precisely defined because the boundary between payable ore and waste can be razor-sharp. A shovel bucket width in the wrong direction means either ore loss or significant dilution. This is where grade control earns its keep.
Coal operations generally involve chasing the strip, following the coal seam across a face and managing the interface between coal and interburden or overburden. The grade control challenge is less about metal concentration and more about managing ash content, moisture, energy value, and keeping different coal products separate. The geometry is typically more predictable than hard-rock mining, but seam rolls and faults can complicate things quickly.
Lithium (spodumene) mining involves painstakingly separating spodumene-bearing pegmatite from surrounding waste rock. The ore can occur in irregular veins and lenses, and the visual difference between ore and waste isn’t always obvious. Grade control relies heavily on geological logging and assay data, with the FMS tracking material types to keep spodumene ore separate from barren pegmatite and host rock.
Each commodity type demands a different grade control philosophy, but the fundamentals (knowing what’s in the ground, classifying it correctly, and routing it to the right destination) remain the same.
Cut-Off Grade: The Line Between Ore and Waste
Cut-off grade is the minimum grade at which material is worth processing. It’s an economic calculation, not a geological one.
The logic is straightforward: if the revenue from processing a tonne of rock exceeds the cost of processing it, it’s ore. If not, it’s waste. The cut-off grade is the breakeven point. Some sites will create long-term stockpiles for sub-grade or mineralised waste (material that doesn’t pay today but could be processed if commodity prices rise or processing costs fall).
What drives cut-off grade:
- Commodity price: when gold is at $2,500/oz, lower-grade material becomes worth processing. When it drops to $1,800/oz, the cut-off rises and some material that was ore becomes waste.
- Processing cost: cheaper processing means you can economically treat lower grades.
- Mining cost: the cost of digging and hauling material to the processing plant.
- Recovery rate: the percentage of metal your plant actually extracts. Higher recovery makes lower grades viable.
Cut-off grade isn’t static. It changes with market conditions, processing performance, and strategic decisions. A mine might lower its cut-off grade during high commodity prices to maximise throughput, then raise it when prices fall to protect margins. The FMS needs to reflect these changes. When the cut-off shifts, material that was classified as waste might become ore (or vice versa), and truck routing must update accordingly.
Material Classification
Most mines don’t just split material into ore and waste. They use multiple categories to match different processing options:
| Category | Typical Destination | Example |
|---|---|---|
| High-grade ore | Direct mill feed (ROM pad) | Above 1.0% Cu |
| Medium-grade ore | Stockpile for later processing | 0.5–1.0% Cu |
| Low-grade ore | Heap leach pad | 0.3–0.5% Cu |
| Marginal material | Grade-dependent stockpile | Near cut-off |
| Waste | Waste dump | Below cut-off |
The number of categories and where the boundaries sit varies by operation. A gold mine might use three tiers. A polymetallic operation dealing with copper, gold, and molybdenum might use six or more, because different metals have different processing paths.
The FMS enforces these classifications. When a truck is loaded, the system determines which category the material falls into and routes the truck to the correct destination. This is the core grade control function: getting the right material to the right place. Material destination control is covered in a dedicated article.
The Block Model
Grade control starts with knowing what’s in the ground before you dig it. That knowledge comes from the geological block model.
A block model is a 3D grid that covers the entire ore body. Each cell (block) in the grid contains an estimated grade based on exploration drilling, infill drilling, and geostatistical interpolation. Think of it as a three-dimensional map where every block has a predicted grade value attached to it.
How it’s built
- Exploration and infill drilling provides sample points scattered through the ore body
- Geostatistical methods (typically kriging) interpolate between those sample points to estimate grades in unsampled locations
- The result is a grid, commonly 10m × 10m × 5m blocks, though sizes vary, where every block has an estimated grade, a confidence level, and often estimates for multiple elements
That’s the long-range resource model. For day-to-day grade control, most open-pit operations build a higher-resolution grade control model from blasthole sampling. Every production blasthole is sampled and assayed, giving a much denser dataset than exploration drilling ever provides. Some gold operations use close-spaced RC (reverse circulation) drilling ahead of mining instead. Either way, the grade control model is what actually drives dig block definition on the bench, not the long-range resource model.
What it tells you
The block model tells the mine planning team where the ore is, what grade it is, and how much of it exists. Mine plans, pit designs, and scheduling are all built on the block model. It’s the geological foundation that everything else rests on.
What it doesn’t tell you
Block model grades are estimates, not measurements. The actual grade of a particular block might differ from the prediction, sometimes significantly. The further a block is from a drill hole, the less reliable the estimate. This gap between predicted and actual grades is one of the central challenges in grade control. How block models integrate with the FMS is covered in Block Model Integration.
Dig Blocks: Where Geology Meets the Pit
The block model operates at a geological resolution (10m × 10m blocks). Mining equipment doesn’t. A shovel or excavator works across a face that might span dozens of geological blocks. You can’t selectively mine individual 10m blocks because the equipment is too big and the ore body boundaries are too irregular.
Dig blocks bridge this gap. They’re operational mining units, larger polygons that group geological blocks into areas the mining team will actually excavate as a single unit. A dig block might cover a 50m × 50m area and represent the average grade of all the geological blocks within it.
How big a dig block needs to be depends on the commodity and the orebody geometry. An iron ore operation with broad, consistent mineralisation might work with large dig blocks. A gold operation mining narrow, irregular veins needs smaller, more precisely shaped blocks to avoid averaging out the high-grade zones with surrounding waste.
How dig blocks get their grades
The geology team takes the block model, applies the current mine plan and bench schedule, and defines dig block boundaries that balance:
- Selectivity: smaller blocks give better grade separation but are harder to mine cleanly
- Practicality: the blocks must be large enough for equipment to work efficiently
- Grade contrast: dig block boundaries should follow natural grade boundaries where possible
The grade assigned to a dig block is the tonnage-weighted average of all the geological blocks inside it. This averaging smooths out variability, so a dig block never has as extreme a grade as the best (or worst) individual geological block within it. The operational side of dig block management is covered in a dedicated article.
Key Takeaways
- Understand your cut-off grades and review them when commodity prices or processing costs change. Material classification drives every routing decision the FMS makes.
- Different commodities demand different grade control approaches. What works for iron ore won’t work for gold or lithium. Match your dig block sizing and classification system to your orebody geometry.
- Keep dig block definitions current. Stale dig blocks are one of the most common causes of misrouting, and the easiest to fix.
- Treat the block model as an estimate, not ground truth. The grade control model from blasthole sampling is what drives daily decisions.
- Grade control is a team effort across geology, mining, processing, and dispatch. The FMS is the tool that connects them, but it only works if everyone keeps their inputs current.
For how the FMS executes these concepts in the pit, see FMS Grade Control in Practice.