The idea behind the pit is to economically extract as much ore as is possible at a profit.

There are some pretty deep pits out there, Chiquicamata in Chile is around 850m I believe and Bingham Canyon, (same faultline as BXX yeah! Sorry couldnt resist)is over 1km deep.

It really just depends on the size of the deposit.

Underground mining can go even deeper, South Africa has gold mines in excess of 3.5km deep.

Im not sure the specifics but I believe Webgogs had calculated our payback period at around 3.5 years....what this does is cover the cost of the CapEx to build the mine. Once the initial investment has been recovered (and 3.5 years is very quick) it becomes even more economical as all that seperates you from your profit is the operational costs. So even if we have lower grades deeper in the earth our cost to extract the ore is cheaper, however our profit per tonne of rock is also diminished. If ever it gets to the point where the profit does not exceed the operating costs the mine is finished.

Make sense?

Rogue,