You can't trade granite or copper, they are only stored in the portal, once you've finished all the tech that need them and the dwarven roads you can then delete your mines.
I sold the mines as soon as I had enough granite/copper stored in the portal and once I'd finished the dwarven roads I sold the portal.
Once you're in the dwarven tech period you will dramatically slow down on the tech tree unless you use diamonds to get you through faster or build a LOT of mines and then you'll just have to wait it out.