I've been fishing for Deviants in the three oasis in the barrens, and often see tin and copper (and occaisionaly silver) pop up on the bottom of the pools. I've also seen more tin in the northern half of the barrens than in the south, but at any rate, it SEEMS more common in the razerback camps and the oasis.
Also, One Thousand Needles has a fair amount of tin and iron, a lot of copper, never seen silver that I recall but I have seen a couple of mithril nodes.
Edited, Mon Oct 24 19:01:33 2005 by Chirrichela