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Simplistic example (with rough diagram) comparing two deposits of different grade.

The first is like Estelle, near-surface and Ore Body occupies the majority of the pit shell. The ore body is a large blob with gold in lower-grade sheeted veins disseminated through out the rock. About 50% of the rock moved is waste.

The second is an example of a high grade deposit. Ore body higher grade but in narrower lodes. A very large % of the rock moved is waste.

Every tonne of rock that you remove from the pit costs money to move. Diagrams not to scale and don't reflect ratio in calculations below.



In this example the grade averaged across all tonnes of rock moved is the same.



The key to getting the economics right is to match the right processing method to each ore body type.

Heap leach works for low grade ore. Ore is crushed to between 1/4 inch and several inches in size. So very low power costs for crushing.

For high grade ore the ore needs to be crushed down to a fraction of a mm to liberate the gold. The crushing and grinding is very power intensive.