I'm only lvl 100 in Mining, but so far I've found great Copper spots and I think I might've found a good Tin spot.
Copper: Stonecairn Lake.. The island in the lake is full of copper, copper, copper, but don't expect to find anything else. When I say it's full, I mean I gathered 100 copper ore in 20 minutes (and I had competition).
Tin: Redridge Mountains. Any cave around there should have around 2 tin veins. Sometimes more, sometimes less. If you have sneak ability such as the Druid's Cat prowl ability or the Rogue's stealth ability, you can pass right by the guards and sneak around to the various veins. I gathered 20+ tin ore and 20+ copper ore in one cave before getting spottet.
Prepare to die when they spot you. Daze is such a *****.
I haven't really found any Silver, but then again, I'm only lvl 20+ and I can't survive outside familiarized territory :D