I have noticed that often Tin veins and silver veins are the SAME vein. They will typically just alternate which spawns. I don't know if it is to alternate EACH time between or on a more rare schedule, but I do know that at least TWO veins alternate in Duskwood between Tin and silver. And there are others in Lockmoden which do the same. Also, the caves with the Ogres in Lochmoden spawn both tin and silver with at least two alternating between tin and silver.
I decided to drop skinning and take up engineering and researched and was sure that this is what I wanted to do. I was playing one evening, and towards the end I made my way to IF. I decided to do the switch... stupidly when already very sleepy. I clicked on unlearn... and boom unlearned one of my skills...
UNFORTUNATELY, given my less than stellar mental condition and awareness at the time, I unlearned MINING!!! Lost over 175 points in mining with the click of the mouse. So now, I have had to work on releveling that all over again. I did download a mod to help, 'Gatherer' from soulforge, however, it doesn't seem to be working. I got the 2.04 beta version, but I think I should have taken the 2.03 regular version, because NOTHING shows up on my worldmap.
Anyway, there is tin and silver in both Duskwood and Lochmoden and at least SOME of these alternate between what spawns at a given node.