The Copper Lakhe Mask is the thirty-sixth treasure that can be found in Uncharted 2: Among Thieves.
The Copper Lakhe Mask is found in Chapter 9 - Path of Light. As soon as you've shimmied rightward and landed atop the semi-circular pathway to the right of the gigantic statue face, you can acquire the first of three treasures on this chapter. Simply run to the far end of this completely linear corridor, completely devoid of enemies or traps, and you can claim the treasure at the darkened dead end. Easy enough.