There is no Burnt biome in this version of Darkness Falls, so there might be a misunderstanding. Nitrate can be mined in the Forest and and Snow biomes. In DFalls v5.0.1 it seems very rare (10%?) to find non-Iron boulders on the surface of the Forest biome (Iron, Coal, Nitrate), but keep mining boulders and see what Ore you get alongside Stone. In the Snow biome (Nitrate Coal Lead), it's rare to find non-Nitrate boulders. Just like the base game, there will be a vein of the ore below ground, you just won't find a block of the ore above ground.
Another source of Brass is scrapping Brass Casings (you'll find plenty (for my mostly-melee playstyle) of ammo if you loot POIs) or unwanted brass ammo. To be fair, the main reason to look for Brass is to make Brass Castings. If you have a Crucible, then you could also consider Steel Casings for making bullets.