Liberate the mines near Cambria is a quest in Back in Action.
After finding Ira in Drassen and asking about Miguel Cordona, she asks you to prove your intentions with this quest before telling you anymore.
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.