Lots of things to think about in this linked interview, including those beyond the obvious. Here's a teaser/extract from a "superstar" in the mining and mining royalties businesses:

So do you avoid early-stage exploration stocks?

Mostly, yes – though I do have people looking for the next Hemlo, or the next Voisey’s Bay. Whoever finds one of those, the premium they get to their share price will be beyond belief, because it’s been way too long since we’ve had a really significant new discovery – a Hemlo, at like 20 million ounces of gold, or a GoldStrike, with around 40 million ounces, a Voisey’s Bay or a Diamond Fields. It’s been over 20 or 25 years since we’ve had one of those discoveries. The world is just panting for one and I think it would be helpful to bring back the risk-taking attitude that’s completely gone from that sector.

But those companies I look at are 1 in a 1,000. You can look at 999 dogs before you find one. It’s very difficult.

http://www.mineweb.com/mineweb/content/en/mineweb-fast-news?oid=250851&sn=Detail