44 Rare Earth Elements and Other Critical Metals in Idaho Because of recent international trade issues, as well as rare earth elements (REE) are not any rarer than many other their incorporation into a wide variety of critical modern tech- elements, but they are so regarded owing to the difficulty in nologies, the rare earths have made it out of the geological separating and isolating the pure element, usually derived and technical journals and into the popular media (internet from the rare earth oxide (REO). Because REEs share simi- and printed newspapers such as the New York Times) and lar chemical properties, REE-bearing minerals commonly halls of Congress. Prices for rare earth metals have risen contain multiple REEs bound together in the crystal lattice, dramatically. Mining companies and the federal govern- which makes the processing and separation of individual ment are renewing the search for domestic sources of rare REOs expensive. Yet, whatever the cost, individual REEs earth elements necessary in the manufacture of important are critical components in numerous U.S. military applica- consumer and national defense-related products. In reality, tions as well as such high-tech yet common items like smart Hall Mountain 95 Sandpoint 90 Coeur d’Alene 90 95 95 0 50 Miles Moscow 0 50 Kilometers North Lewiston Vein/Lode REE Deposit 12 Placer REE Deposit Mineral Hill District Districts with 93 associated REE 95 Diamond Creek Salmon Lemhi Pass McCall Blackbird Co District 95 Challis Yellows to ne N. P Weiser 55 20 . 15 84 33 BOISE Caldwell Hailey Arco Idaho Falls 95 26 93 15 75 84 Pocatello Phosphate 86 District 15 30 Twin Falls 93 84 Figure 1: Map of rare earth deposits in Idaho. MOSCOW OFFICE BOISE SATELLITE POCATELLO SATELLITE MORRILL HALL, THIRD FLOOR IDAHO WATER CENTER, SUITE 242B PHYSICAL SCIENCE, ROOM 201B UNIVERSITY OF IDAHO 322 E. FRONT STREET IDAHO STATE UNIVERSITY PO BOX 443014 BOISE, ID 83702-7359 MS 8071 MOSCOW, ID 83844-3014 208-332-4420 Fax 208-332-4400 POCATELLO, ID 83209-8071 I DAHO 208-885-7991 Fax 208-885-5826 208-282-4254 Fax 208-282-4414 GEOLOGICAL SURVEY www.idahogeology.org email: [email protected] phones, fluorescent lighting, NiMH rechargeable batteries carbonate. The rare earths contained in carbonate minerals in hybrid vehicles, powerful Nd-bearing permanent mag- are typically the easiest to process. Rare earths are also en- nets used in wind turbines and electric and hybrid vehicles, riched in pegmatites, where they were first mined, but these Eu-rich red phosphors in television sets, and computer hard were eclipsed in commercial importance by mining from drives (Hedrick, 2010; Cordier and Hedrick, 2010; U.S. placer deposits of the mineral monazite, a REE-bearing DOE, 2010). phosphate (Ce, La, Nd, Th) PO4, in the second half of the 20th Century. Beach placers were mined in the southeastern Few people realize how many minerals the United States United States, Australia, and elsewhere, and river placers imports to supply companies and manufacturing plants. In were mined in Idaho (Castor and Hedrick, 2006). Other 1996, the U.S. relied 100% on imports for nine non-fuel types of REE deposits include hydrothermal veins, lateritic mineral materials; by 2006, the number had grown to 17; clays, and alkaline intrusive complexes. Rare earths are also and in 2009, the country was 100% dependent on imports enriched – though not to commercial amounts – in phospho- for 19 mineral materials (USGS Mineral Commodity Sum- rites, which are mined in southeastern Idaho. maries 2010, p. 6). Of those 19 commodities, Idaho hosts significant resources or is a past producer of at least eight Idaho is an obvious target for new exploration because (arsenic, fluorspar, mica, niobium, rare earths, tantalum, it has several known rare earth deposits, and its geologic thorium, and vanadium). Mining of rare earths in the U.S. make-up holds the promise of additional discoveries (An- ended in 1998 when the Mountain Pass mine in California derson and Savage, 1964; Figure 1). Rare earths and thorium closed. Since the mid-1980s, China has increased produc- (Th), a common associate, are typically found in geologic tion from its large REE reserves, and today dominates rare terrains underlain by old continental crust, which includes earth mining and processing. In 2010, China supplied over much of Idaho. Black sand placer deposits of central Idaho 95% of the global production of rare earth elements. How- were mined in the late 1940s and 1950s for thorium, ura- ever, recent announcements of higher export tariffs and low- nium, and rare earths, principally contained in the mineral er export quotas for Chinese rare earth exports have worried monazite (Savage, 1961; Breckenridge, and others, 1980). consumers, governments, and trade organizations. Federal Like uranium, thorium is naturally radioactive. The mon- agencies have been tasked by Congress with reporting on azite is a primary accessory mineral of the granitic rocks REE supply for needs in the defense and energy industry. and pegmatites in the Idaho batholith and other intrusions. In the private sector, the prospect of possible future supply Weathering of the granites contributed monazite and other limits and rising commodity prices have prompted resur- minerals to river sands which were deposited and preserved gent exploration worldwide for new rare earth deposits. in Idaho’s central drainage basins. This is similar to process- What are these esoteric substances that have garnered es that formed the gold-bearing placer deposits. Dredges op- headlines in the business news? Rare earths, also called erated in the Bear Valley region, Cascade-Long Valley, and the lanthanides, are little known elements that chemists did other areas (Mackin and Schmidt, 1957; Staley, 1948). th not fully isolate until the 19 Century (Castor and Hedrick, In 1949, prospectors looking for uranium discovered 2006). On the periodic table of the elements, they start thorium in the Lemhi Pass area of Idaho and Montana. with lanthanum (atomic number 57) and end with lutetium Subsequent study and exploration revealed numerous tho- (no. 71). Yttrium (no. 39) and scandium (no. 21) are com- rium- and rare earth-bearing veins (Anderson, 1958; Staatz, monly included as well, for a total of seventeen elements. 1979a; Gillerman and others, 2003, 2008; Gillerman, 2008) The lower atomic weight elements, lanthanum (La), cerium in the Beaverhead Range of Lemhi County, Idaho, and ad- (Ce), praseodymium (Pr), neodymium (Nd), promethium joining areas in Montana. Early studies by the U.S. Geo- (Pm), samarium (Sm), and europium (Eu) are classified as logical Survey concluded that this district had the largest light rare earths (LREE); the others—gadolinium (Gd), ter- thorium resource in the nation and also hosted an equal bium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), amount of rare earths (Staatz, 1979a,b). These enigmatic thulium (Tm), ytterbium (Yb), and lutetium (Lu)—are the deposits, hosted in Precambrian quartzites and siltites, have heavy rare earths (HREE). A third, middle rare earth group been explored at the surface by government and industry. A (MREE) is sometimes delineated as well. A new Department modest reserve of Th plus REE was delineated from under- of Energy report particularly noted five rare earths (Dy, Nd, ground work by IERCO on the near-surface portions of the Tb, Eu, and Y), plus indium, as being the most critical in Last Chance vein in Montana in the early 1980s, before the the short term for the nation’s clean energy economy (U.S. companies lost interest owing to low thorium prices. Since DOE, 2010). then, no underground exploration or drilling has been done, Rare earth mineral deposits are found in several geo- and there is little subsurface information for the district. On logic settings. Perhaps the most famous type of REE de- average, the veins contain about as much thorium, bound up posit, and one of considerable economic importance, is a in thorite (ThSiO4), as total rare earths. District-wide grades carbonatite, an unusual igneous rock which is composed of of the veins are approximately 0.4% to 0.5% total rare earth carbonate minerals. At both the Mountain Pass mine in Cal- oxides, but individual veins vary significantly (Staatz, 1972, ifornia and the renowned Bayan Obo deposit in China, rare 1979a; Gillerman, 2008; Reed and Gillerman, 2008). The earth minerals are found in carbonatites, though the exact major REE host mineral is monazite. The Lemhi Pass ores genesis of the Bayan Obo is complex and still controversial. are unusual in that they consistently show a strong enrich- The major economic mineral, bastnasite, is a REE-bearing ment in the middle rare earths, especially neodymium. Elec- tron microprobe analyses of monazite reveal as much as 35 better determine distribution of the rare earth concentrations weight percent Nd oxide (Gillerman, 2008; Gillerman and in the Idaho Cobalt Project deposit and the potential for rare others, 2009), which is possibly the highest content known earth byproduct production from the future mine. Occurrenc- in the world. Most of the deposits are quartz veins with as- es of Te (used in solar panels), Nb and Ta (mined in Brazil), sociated hematite, apatite, feldspar, thorite, and sparse mon- and of course Sb (used in flame retardants and batteries), are azite. A few deposits in more argillaceous and sheared rocks noted in several Idaho deposits, and Re is a potential byprod- have biotite, abundant specular hematite, thorite, apatite, uct of molybdenum deposits in Idaho, such as the giant Cumo and Nd-monazite. The geology is complex, and the origin of deposit being explored today in Boise County. the deposits is a topic of ongoing IGS research. Age dating of the monazite and other evidence suggests that the veins Discovery of a “mineral resource” requires significant formed during the late Paleozoic times, subsequent to Early exploration work, but it is only the first step.