Well, the best spot I’ve found for mithril is the Badlands. There’s a section of the Badlands where black whelps spawn. They kind of spawn in two areas with some elite dragons between the two spawn spots. During my sessions of whelp killing, trying to get a pet to drop, it wasn’t unusual for me to port out with 50 plus mithril ore on me. The mithril would spawn in both whelp areas and in the dragon area.
The whelps are about 43, so how effective you’re mining will be will depend upon your level. My rogue was in his 50s during much of this, so the whelps didn’t slow him down any.