I'm in about the same situation as you are but a bit further along. I've been making swings through the Westfall mines where you see Tin and Silver (a few veins) on a fairly regular basis. Tin and Silver also Northwest of Lakeshire in Redridge. I have also be going Northwest of Darkshire in Duskwood where I have found Tin, Iron and Gold but the Critters there are mostly level 25-29. It SEEMS like every 2 or 3 tin veins result in a level increase. Silver and Iron have done it every time so far. I haven't found any more Gold since I passed 155 but I assume it will level each time too.