How do mining companies predict whether a mining site has ore and how much?

Question

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Answer 1

They carefully drill and "map out" the underlying ore bodies' depth, and by extracting samples, they also map out the concentration of the material being mined.  

For example, the company I work for just recently announced that there is another 625 million tons of ore with a grade of 0.45% copper at the mine - thereby extending the expected mine life by another 50 years.

Answer 2

By carrying out test bores with a drilling rig in the area surrounding the mine site.