The Precious Ores is an event that takes place in the second level of a cave near the center of 'The Fields of Misery'. It is a random event so you may have to reload the map a few times to find it.

I was only level 12, barely a couple of hours into my first runthrough of Diablo III when it happened. My Witch Doctor was neck-deep in battle on the Fields of Misery, bloody body parts ...