rare earth elements in Wyoming

Extraction of rEE
Aside from generally low concentrations of REE in
their host rocks and minerals, the cost of separation
of the individual elements is quite high
compared to other metals. REE ores are mineralogically
and chemically complex, and are often
radioactive (Long, Van Gosen, Foley, and Cordier,
2010). Because of their similar chemical properties,
individual REE are difficult to isolate by ordinary
metallurgical means. Therefore, separation is a long
and complex process that involves multiple and
repetitive steps in solvent extraction and/or ion
exchange. Other extraction methods may involve
iterative fractional crystallization, fusion, volitization,
or a combination of selective reduction and
vacuum distillation of halides (Hedrick, 1985; Uda
and others, 2000).
The presence of two or more mineral phases in
many deposits requires a different extraction
technology for each mineral, compounding both
complexity and cost (Long, Van Gosen, Foley, and
Cordier, 2010). The cost of building a plant to
separate the individual REE is also high and must
be tailored to the unique mineralogy of a specific
ore. Construction of a new mine, with attached
separation plant, was estimated in 2010 to cost
between $500 million and $1 billion (Hsu, 2010),
with a lead time up to 15 years for full-capacity
completion (Long, Van Gosen, Foley, and Cordier,
2010; Hsu, 2010).
General Geology of rEE
Awareness of the diversity of REE deposit types
is beneficial to exploration efforts. Economically
exploitable concentrations of REE are primarily
derived from crystalline igneous rocks (Long, Van
Gosen, and Johnson, 2010). REE are more common
in alkaline igneous rocks and carbonatites
than in mafic rocks (Long, Van Gosen, Foley, and
Cordier, 2010). REE typically occur as trivalent
cations in rock-forming minerals in carbonates,
oxides, phosphates, and silicates (Hedrick, 2004).
REE are chemically similar to thorium and are
often found in minerals and rocks in association
with thorium (Hedrick and Templeton, 1990).
Economic concentrations of REE host minerals are
known from alkaline igneous rocks, carbonatites,
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and from a wide variety of dikes and veins that
cross-cut alkaline intrusions and surrounding rocks
(Long, Van Gosen, Foley, and Cordier, 2010).
Sedimentary rocks that may preferentially contain
REE include phosphatic rocks, coarse clastic rocks,
and shales (Hedrick, 1985). Phosphate deposits
may be sources for yttrium and lanthanum (Long,
Van Gosen, and Johnson, 2010). Sediments
derived from host rocks that contain even small
amounts of REE-bearing minerals may produce
both placer and paleoplacer concentrations.
Ion-adsorption clays, or laterites, developed as
residuum from chemical weathering, may be enriched
in REE from intensive leaching of igneous
and other rocks. Economic REE concentrations
in ion-adsorption clays, are known from southern
China and Kazakhstan (Long, Van Gosen, Foley,
and Cordier, 2010). Such clays tend to have a
greater enrichment in HREE than other deposit
types (Hedrick, 2004).
rEE Minerals
Numerous minerals are known to contain REE
as essential constituents, and a greater number
contain REE as accessory elements. However,
only a few of these minerals host large enough
concentrations of REE to be considered ore
minerals (Hedrick, 1985). Worldwide, the
principal commercial sources of REE are the
minerals bastnasite, loparite, monazite, and
xenotime, and rare-earth ion-adsorption clays
(Hedrick, 2004). When discussing principal
REE-bearing minerals only in the United States,
Long, Van Gosen, Foley, and Cordier (2010) omit
loparite and ion adsorption clays, but add euxenite
and allanite, although other REE-bearing minerals
are also present. Table 5 lists minerals reported in
Wyoming that contain REE. Of the principal REEbearing
minerals, allanite, bastnasite, euxenite,
monazite, and xenotime are known to occur in
Wyoming (King and Harris, 2002). Mineral
formulas shown in the following discussions are
variable depending on the source cited, and on
subvarieties that may contain differing elemental
substitutions.