What I did to make a copper vein explode, is starting to mine around it until at least 2 meters is gone. After that I mined all the way to the bottom until I wasn't able to go any deeper.
Next step was to start removing copper from below; more or less creating a floating dome.
Eventually I went to the top and started to remove the copper from the sides, again working around in a circle. At some point I would hit the spot where it was "attached" to the rest and the entire vein would explode. After that it was just a matter of picking up the stones and ores, transport all back to my base and smelt it.

Thorin :)