Mines
A world's mines will produce a certain amount of metal each turn--one metal per turn produced by each mine.
This requires having sufficient population to work the mines. If there is not enough population to work both the mines and the industry at a given world, the population will work the industry if there is enough metal already stockpiled to build with, and any remaining population will work the mines.
A world still recovering from a recent plunder will not produce anything from its mines.