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Nagrand(Outlands) -  Heavy amounts of adamantite scattered throughout the hills and caves.

Hellfire Peninsula - Very heavy amounts of Fel Iron.

 These are the two most heavy areas for those two main ores. (I'll add Khorium deposits appear anywhere in outlands, rare to find though).

Thanks for the info. Do you happen to use the Gatherer addon? It keeps track of all the minerals you find and shows you where they are on the map. Aside from making your life much easier by showing you where items are, it also lets you share your findings with friends. And it makes it really easy to post a screenshot showing a map of all the ore. I posted a thread on it HERE.

It's times like this when I really miss my old Gatherer data. I used to have the whole of outlands mapped out for just about every ore deposit. Because outlands tends to be so active it can be hard to find ore before people swoop down and mine it. I found that in the crater by the Draeni town in hellfire peninsula, fel iron respawns quickly but isn't mined up as fast because of the elite elementals that patrol the area. If you have a way to distract or slow down these already slow enemies, you can ninja the ore and make a clean break. I used to keep my hunter logged out at that spot, log in and mine a bit, then switch back to another character.