Price of gold dropped substantially last night, and today the share price of at least 2 gold miners - Newcrest and Lihir - is also down substantially. But the gold that's mined today won't be sold at today's gold price - will it? Either it was sold months ago or it will be sold at some future price. So what's the justification for an instant drop in the price of the mining companies?
Same kind of violent share price reaction to general announcements happens in other businesses too, but mining seems the most obvious case.
(If necessity is the mother of invention, curiosity is the mother of discovery)