I came across this last year when looking into how deep a miner can go safely and economically.

http://www.republicofmining.com/2008...

"The Kidd Mine is the deepest base metal mine in the world. This new project will expand the copper-zinc orebody´s mining zone from 9,100 feet below surface to 9,500 feet and extend the mine life to 2017. This zone is estimated to contain 3.4 million tonnes of ore with a grade of 1.48% copper, 6.22% zinc and 80 grams of silver per tonne."