[CPT Clay]

I think this is pretty cool. Rare Earths are supposed to be found in pegmatites. I suspect that much will be discovered in the Rockies.

[Smokin' Joe]

Iirc, they can be eroded out of pegmatites, and possibly concentrated with host minerals.

I know of some very pure niobium/Tantalum Rutile deposits--but they'll likely never be mined. Too close to a major urban water supply, but they are in a carbonate, too.