I believe Thulium is supposed to be a resource that comes from a dead planet which gets named Thulium.
In the game, you cannot built a mining starbase for just thulium alone. That is, if there is another resource nearby that is mineable, you can create a mining starbase for that. If the Thulium planet is in its range, it will also grab the thulium. However, if you just try to build a starbase in its range without other resources in range, the starbase does not allow you to build a mining ring.