Operum Tuorum Gemmam Castra
A fort south of the Klysma Quarry. You'll kill everyone here during the story mission 'Where the Slaves Die,' so no need to assault it before then.
The camp captain hangs out near the spot where you'll have to kill Tactito in a relatively obvious fashion. The captain can be taken out in the ensuing alarm.
One of the treasures is located in the captain's tent, west of Tacito's speaking platform.
Enter the tent to claim the carbon crystals from the small table there.
The other treasure is above the main camp to the southeast, where the commander hangs out.
You can assassinate the commander if you like, or you can just kill him during the big push of the story mission.
Either way, looting the large red chest in his tent completes the location.
Note that there are tons of rebel captives in the camp- in cages, tied to posts, and in the boat below. Not only do you get XP for freeing them, you also get XP for assisting them in a fight. Tag along after them, backstabbing or sniping foes, to ride a wave of minor XP boons through the camp.
