Early on in our game, my son and I came across copper deposits that were 3 & 4 times larger than the ones on our starting land mass. They were all on very steep inclines, and most were along a Black Forest coast. I still see the gashes in the mountain side, where we cut into the mountain from water level so deep into the mountain 3 longships would be able to moor inside end to end. We have not needed to mine copper since shortly after we killed Bonemass.

I have recently noticed on my solo game seed map, there are always 2 - 3 decent copper mines surrounding Troll Caves. I always tag them now with a special icon, and when I see a Troll spawn in the area, I kite the Troll to the surface formation. I just stand on the copper and block/parry the Troll until he has mined all the surface copper.