Southern Missouri, at the exposed roots of crust that formed the Ozark Mountains, is alone responsible for significant amounts of the world's metals. It used to provide 80% of the world's lead, for instance.

Like almost all mining areas, the people are poor, undereducated, and damaged healthwise (lots of lead there!).

Is it unusually rich for any region that big? I don't actually know, but it'd be interesting to see a comparison to another similarly sized country.