Poor Metals
If you've ever used Railcraft, you're probably familiar with this mod's concept. For those unaware, it's quite simple - this addon introduces impoverished versions of ores from the standard set, base metals, and Nether materials.
Each such ore yields three nuggets when smelted. An interesting feature: impoverished ores of base metals and Nether materials won't load if the corresponding mods are missing, but there's a configuration option to disable this restriction.
Currently, the ores don't generate automatically in the world. The developer left this question open - perhaps it's better to entrust generation to specialized mods.
Important clarification: this is NOT an official MMD project, although it's related to it.
The developer is temporarily focused on another project, so pull requests on GitHub are welcome for further mod development.