The Unusual Bronze Mask is the ninety-eighth treasure that can be found in Uncharted 2: Among Thieves.
The Unusual Bronze Mask is found in Chapter 25 - Broken Paradise. After hoisting yourself upward with the cart, you can grab another treasure before proceeding. This, the fifth treasure of the chapter, is easy enough to acquire if you know precisely where to look. With your back facing the cart, head rightward and carefully jump to the platform over where you just took out the four enemies. Sitting in a dark corner is the treasure; simply stand around and look for the treasure's glimmer if you're having a difficult time spotting its specific location.