Sorry I didn't get back to you sooner, I think you know why, though....

There are a lot of folks out there who could answer your question with more authority and expertise than I can, but I'll tell you how I see it , in a nutshell.

Open pit mining has it's merits, some of which is very little money ( comparatively speaking) has to be sunk into the project before it starts yielding cash flow. It is not without it's detractors, however. From an environmental impact, it creates quite a scar on the land, and subsequent issues arise from that.

I wouldn't say that it's more economical to mine deep underground, it's just that historically in the Greenstone Belt, the really big deposits are found deep in the earth, therefore making it worth the expenditure to get to the deposit.

Just a quick answer for now, gotta grab some lunch. Maybe longbomb could wade in and offer up some insight on this topic...I betcha he knows more than I do and can explain it better.