The Bayan Obo deposit in northern China, about 100 km from the Mongolian border, is presently the most productive mine complex for all rare earths. China produces 97% of the world's rare earths. Two North American deposits have recently attracted attention, because Americans are increasingly aware of our use and dependency on China for these elements (averaging 91% of U.S. imports), and because the price for some of the 17 rare earths is approaching levels to make mining them here economic once again.
The mine at Mountain Pass, in California’s Mojave Desert, was the largest rare-earth producer in the world until the 1990s when China took over. Molycorp, owner of the deposit there, has been processing accumulated ores for a couple years, although the mine itself has not reopened. And for final processing, they must ship their product to—you guessed it—China, the site of the only separation plant. Constructing one in the U.S. would be a huge investment, one nobody is presently willing to undertake.
Another promising undeveloped rare-earth deposit lies along and adjacent to the Continental Divide between Montana and Idaho, centered on the Lemhi Pass area. Two sites are held by U.S. Rare Earths, Inc., a private company.
The Idaho deposits were discovered and initially investigated in the late 1940s and 1950s because they held radioactive thorium, important in nuclear weapons development. The geologic setting is complex: igneous (formerly molten) rock bodies that were once thought to be part of the Idaho Batholith (around 100 million years ago, but with a wide range of dates) are now considered to be Mesoproterozoic (something like 1300 million years ago) in age, and to have metamorphosed (changed by heat and pressure) the surrounding rocks of somewhat older age. Sedimentary rocks of the Gunsight and Apple Creek Formations became quartzites and gneisses thanks to that metamorphic cooking. They also became the host rocks for the veins containing rare earths, gold, and other minerals.
The whole area is complicated further by thrusting – faulting, breaking rocks, by pushing older layers up and over younger layers, sometimes on scales of tens of kilometers or more, something that probably happened over tens of millions of years around 60-70 million years ago. Then, about 40 or 50 million years ago, Nature put a pile of volcanic rocks on top of the whole mess.
With all that going on, you can imagine that geologists are still working out the details, and what I write above is just a broad-brush overview. Some of the basic geologic mapping by U.S. Geological Survey, Idaho Geological Survey, and other scientists was published only a few years ago. It is not completely clear (at least not to me) when the valuable minerals came in—some indications say it was associated with the early metamorphic cooking, some suggest later. But the high grade of the ore at Lemhi Pass and near Salmon, Idaho, is clear, making the two sites perhaps the highest potential in the U.S. as undeveloped rare-earth resources.
Exploration and economic evaluation is underway for two other isolated rare-earth deposits, one near Sundance, Wyoming, and one beneath 600 feet of rock in southeastern Nebraska. Another potentially important North American rare-earth deposit is being investigated at Thor Lake, north of Yellowknife, Northwest Territories, Canada. We’ll save these possibilities for another post.