Underground mining is in general reserved for a vein style mineralization.

Pit mining is generally reserved for porphyries.

Underground mining is higher cost, but when it comes down to how many tonnes of non mineralized rock you would have to move to extract the ore it works out cheaper overall. Vein systems are also much smaller than porphyrys.

Porphyrys are much larger systems of mineralization so you really cant do underground mining at all....this is where pit mining comes into play because you have lower overall grades in general, but you're seeing some payback off of every tonnes of rock you move, not just higher payback from the rock you would have moved if you were in a vein style structure.

Think about the dynamics of a pit style mine bing employed to extract whats tantamount to a couple of lines (the veins) of mineralization.....costs are way too high and its extremely inefficient.

Hope this helps,

Rogue