gold bars are use for crafting certain recipes (like a corpse harvester) and for upgrading weapons at the upgrading altar (I know it's in the secret passage under the statue in town in act 1 and on the ship thereafter).

I don't know what tenebrium bars are used for the Blackrock weapons and armor (see the Weapons tab on the spreadsheet).

I haven't found any use in the mod for silver bars. I guess you could sell them.