I agree that there probably hasn’t been all that much exploration (and thus, discovery nor enumeration) for REs per se in the US. So, their prevalence is at least partially an unknown. However, where we find most of our non-ferrous goodies in the US = the West, *has* been widely though certainly not exhaustively explored. So the odds are shrunken that we’ll find anything monumental. Sure, there’s always the possibility of finding a mother lode somewhere, but it’s not a great bet.
Still. Never mind the eco considerations, mining exclusively for REs seems pretty far fetched in terms of economic viability within the US. Monazite is generally known as home of Thorium. If we were mining thorium, we’d be getting REs along with the thorium. But we’re not with any great gusto.
I don’t especially agree that they are relatively abundant, even if the WRC says so. The rare earths are 17 elements and ALL OF THEM, taken together, might be a little less rare than “rare” but getting any particular ONE of them, the one you need, remains a needle-in-the-haystack affair. They are usually grouped all together in the type of generalistic reports you linked and IMO that gives the impression they are statistically more common than they are in reality.
Anyone who has ever been in the exploration business knows you have to remain optimistic! I still think there are many areas yet to be thoroughly explored because the US mining industry is essentially dead except for gold. Companies hire and fire geologists based on the fluctuating commodity prices, so since no demand has existed for rare earths or other metals, they have fired many of their geologists. If you look at college programs in geology in the US today, they are all but dead and a lot of them have been converted to “environmental geology” programs.
Companies that explore for gold don’t necessarily pay any attention to rare earths. And when you consider that most of the old time prospectors in the western US had no formal training in geology, I wouldn’t say that the western US has been explored all that well. If there was no gold in a rock, most of the old prospectors just threw it in the waste pile.
As for the whether rare earths are “abundant” or not, I would disagree, but there is no reason to argue over it. Abundance of elements in the earths crust has been studied to death. You might be interested in the graph here:
http://en.wikipedia.org/wiki/Abundance_of_elements_in_Earth%27s_crust
MolyCorp began mining and processing rare earths at the Mountain Pass mine in 1952 with a 50 year permit. In the early 90s they decided to expand and produce for another 35 years. As they were re-permitting they had a major enviro spill so it was 2004 before they got their new permit. But they really haven’t begun operations as yet because of depressed prices. You should go their website, it explains alot of the problems the industry is facing. Another company, US Rare Earths, has extensive deposits of rare earths that have never been mined.