The Garrison Armory is a vital Alliance mining outpost near the Nethergarde Keep in the Blasted Lands. Here Alliance miners toil to gather precious resources and minerals for the Nethergarde Keep. These resources are frequently shipped to Stormwind in exchange for the other vital resources Nethergarde Keep depends upon to survive.
The nodes can only be mined by Horde, according to the official forum post found here: http://forums.wow-europe.com/thread.html?topicId=3778840435&sid=1
"This works as following. If you're Horde and you wander into an Alliance NPC mining establishment, such as the Garrison Armory, you're able to mine the nodes present there, but the NPC's will engage you of course, making the mining a chore (so to speak). Alliance cannot mine there as the task would be trivial and too easy to do. The same works vice versa in Horde NPC mining caves."
This section concerns content exclusive to Cataclysm.
The Horde have taken over the Garrison Armory with their push into the Blasted Lands.