I've killed my fair share of these, even while subbing THF for Treasure Hunter, and have only received copper and zinc from them. I would believe that they drop silver, since most of the copper/zinc worms will drop it occasionally, but I'm having a hard time buying that these drop mythril or gold. After all, if they did drop gold ore, the gilfarmers in Gusgen Mines would camp these AND the mining points. -_-
However, if anyone has received mythril or gold from these, please post! ^^