it is a bit old, but check out war of the walkers, it had copper and such added in biomes, that should give you a good start to go on
here you go, this is old code, but it should help you identify if you're just adding it to the biome wrong, or if its something else
<!-- random prob_min,_max per chunk where 0 equals no decoration and 256 means on every block -->
<!-- Decorations Main: Snow -->
<decorations>
<decoration type="prefab" name="rock_form02" prob="0.00075" rotatemax="7"/>
<decoration type="prefab" name="rock_form01" prob="0.0075" rotatemax="7"/>
<decoration type="prefab" name="resource_nitrate_pile" prob="0.00135" rotatemax="7"/>
<decoration type="prefab" name="resource_coal_pile" prob=".0007" rotatemax="7"/>
<decoration type="prefab" name="resource_copper_zinc_pile" prob="0.00135" rotatemax="7"/>
<decoration type="prefab" name="resource_iron_pile" prob="0.00135" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_Ammo" prob="0.00011" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_Building" prob="0.00012" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_FoodDrink" prob="0.00012" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_General" prob="0.00012" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_HealthArmr" prob="0.00012" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_Weapons3" prob="0.00011" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_Weapons5" prob="0.0001" rotatemax="7"/>
<decoration type="prefab" name="resource_lootCrate3_Weapons7" prob="0.00009" rotatemax="7"/>
<decoration type="block" blockname="lootForestHelper" prob="0.00025"/>
<decoration type="block" blockname="rock05" prob="0.0025" />
<decoration type="block" blockname="rock06" prob="0.0025" />
<decoration type="block" blockname="rock07" prob="0.0025" />
<decoration type="block" blockname="snowRockResource" prob="0.0005" rotatemax="3"/>
<decoration type="block" blockname="snowRockResource02" prob="0.0005" rotatemax="3"/>
<decoration type="block" blockname="cntBirdnest" prob="0.0009" />
<decoration type="block" blockname="treeWinterEverGreen" prob="0.00325" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine01" prob="0.003" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine02" prob="0.003" rotatemax="7"/>
<decoration type="block" blockname="treeWinterPine03" prob="0.003" rotatemax="7"/>
<decoration type="block" blockname="treePlantedWinterPine1m" prob="0.0013" rotatemax="7"/>
<decoration type="block" blockname="cntTreeStump" prob="0.001" rotatemax="7"/>
<decoration type="block" blockname="treeSnowyDeadBush" prob="0.005" rotatemax="7"/>
<decoration type="block" blockname="treeSnowyGrassDiagonal" prob="0.08" />
</decorations>
<terrain class="Mountains" />
</biome>