Yes, I know, it takes millions of years for new metals to work their way up from the mantle. However, if someone's working an active mine the miners could uncover new deposits, making it worth while to check back again after a months or so of in game time or a few levels. I doubt this would happen to veins you find out in the world.. however deposits near water could potentially have new veins exposed by the water's flow in a reasonable amount of time. It Could Happen!

Veins restock every game month according to the strategy guide.

I haven't seen it happen, and personally I just buy the stuff I need. It makes mining useless IMO. It would be cool to know if it does come back, though.

EDIT: Above post answered my question.

I think that veins of ore do refresh after some period of time, but I'm not sure. Honestly, that's a bit unrealistic. I suppose they wanted to have a way to allow you to have a technically infinite amount of ore in the game world, but not allow you to simply mine a metric [censored] ton of it at like level 2 and make a bazillion dollars smithing right off the bat.

But probably technically it would be more realistic if when the veins ran out, they were OUT, but mines had dozens of veins in them.