Silver I believe is a "rare" spawn. I don't mean rare as in uncommon, but as far as I understand it spawns in place of Tin spawn points.
Let me try and be a little more clear. For every "regular" spawn point out there, there's a chance that it will spawn as a rarer ore. In this case, every Tin spawn point has a chance to spawn as Silver. The same is true with Gold:Iron and Mithril:Truesilver.
At least that's the way I understand it.