What is a Mining Block Model?
A block model is how a mine turns scattered drillhole samples into something you can plan against: a three-dimensional array of blocks covering the deposit, each carrying estimated values for grade, density and rock type. It is an estimate everywhere except at the samples themselves, and understanding what that means is the difference between using one well and trusting it too far. This guide explains what a block model is and how it comes to exist.
The short definition
Take the volume that contains a deposit and divide it into a regular grid of rectangular blocks. Give every block a set of attributes — grade of the mineral of interest, bulk density, rock type, confidence classification.
That array is the block model. It is the bridge between geology, which is measured at a few thousand points, and mine planning, which needs a value everywhere.
Where the numbers come from
The input is drillhole data: samples along each hole, each with an assayed grade over a specific interval. These are composited to a consistent length so that samples are comparable.
The block values are then estimated by interpolation. Inverse distance weighting assigns weights by distance. Kriging goes further and uses the variogram — a model of how quickly grade similarity decays with distance — to weight samples in a way that accounts for the deposit's own spatial behaviour.
Whatever the method, the result is an estimate. Only at the sample locations is anything measured; everywhere else is inference, and blocks far from any drillhole are inference over a long distance.
Block size and what it hides
Block dimensions are chosen against drillhole spacing and the mining method — roughly, a block should be something the equipment could realistically mine selectively.
Blocks that are too large smooth the deposit: high and low grades average together and the model shows a uniformity the rock does not have. Blocks that are too small imply a resolution the drilling cannot support — the estimate becomes smooth anyway, but now it looks detailed.
This is why a block model's apparent precision is misleading. A value carried to two decimals is still an interpolation between holes tens of metres apart.
Cut-off grade, ore and waste
A block becomes ore or waste by comparison with a cut-off grade — the grade at which extraction and processing pay for themselves at current prices and costs.
That threshold is economic, not geological. When prices move, blocks change category without a single rock changing, which is why reserve figures move with the market.
Above cut-off is ore, below is waste that has to be moved to reach it. The ratio between them is the stripping ratio, and it is the headline economic measure of an open pit.
What the model is used for
Resource and reserve estimation — how much is there, and how much of it is economically extractable.
Pit optimisation — algorithms work over the block array to find the pit shell that maximises value, subject to slope constraints.
Scheduling and grade control — deciding what gets mined when, and reconciling what was predicted against what the plant actually received.
Resource estimation and mine scheduling are their own disciplines with their own specialist software. STREAM covers the terrain and quantity side of mine engineering — surfaces, volumes, bench and haul road geometry, blast patterns and reporting — not geological block modelling.
Frequently asked questions
What is the difference between a block model and a terrain model?
A terrain model describes a surface — one elevation per position. A block model describes a volume — a three-dimensional array of blocks each carrying estimated properties such as grade and density. One tells you the shape of the ground, the other what is estimated to be inside it.
Are block model grades measured or estimated?
Estimated everywhere except at the drillhole samples. Interpolation methods such as inverse distance or kriging spread those samples across the block array, so a block's value is an inference whose reliability depends on how close the nearest drilling is.
How is block size chosen?
Against drillhole spacing and the mining method — a block should be roughly what the equipment could mine selectively. Too large smooths real grade variation away; too small implies a resolution the drilling cannot support.
Why do reserves change when nothing has been mined?
Because the cut-off grade is economic. When metal prices or operating costs move, blocks cross the threshold between ore and waste, and the reserve figure moves with them even though the rock is unchanged.