They seem done once taken/mined.

In the swamps the fire things provide surtling cores, so if you lack burials then you can farm those. Though black forests seem to provide more than enough burial dungeons to get cores from.

Also, always seem to be more than enough copper, tin, silver veins/spots. Iron ore can be a problem if nearby swamps have 0-1 crypts, which can happen. That can be slow going roaming/sailing all over for a swamp with crypts and, of course, stumbling across plains as you sail around or search on foot.