Liberate the mines near Cambria is a quest in Back in Action.
These mines are north-east of Cambria. There, you'll have to claim the area by killing every enemy unit and then talk to Oswald, the head miner, to complete this quest and receive a Flawless Diamond as payment for your efforts.
Note: You're hinted to report back to Ira after completing this quest, she has more work for you.
Strategy: Pretty simple, can easily be done with Blood knifing all opponents.