rare earth elements in Wyoming

Precambrian occurrences
Precambrian rocks across Wyoming are potential
hosts of REE. Known occurrences are present
within granitic igneous rocks and within metamorphic
rocks of various compositions (fig. 4). The
most important Precambrian REE-host rocks in
Wyoming are granitic pegmatites and other pods
and dikes within granitic igneous and metamorphic
rocks. These include granitic pegmatites within the
Mesoproterozoic Sherman Granite near Tie Siding
in southeastern Wyoming, granitic pegmatites that
cut Paleoproterozoic granite gneiss in the Big Creek
district on the west flank of the Medicine Bow
Mountains, and calc-silicate pods and dikes within
Archean granitic gneiss in the southern Bighorn
Mountains. Secondary alteration in some pegmatites
is apparently important in concentrating REE.
Pegmatites
Globally, granitic pegmatites host important deposits
of strategic resources. Strategic resources are
elements that are important for a country’s economy
or defense, but have substantial risk of supply
disruptions (Adams and others, 1980; Linnen and
others, 2012). Strategic resources commonly hosted
in granitic pegmatites include Ta, Nb, Be, Sb, W,
Co, and REE. Two general types of pegmatites
that are important hosts of strategic resources are:
1) Lithium-Cesium-Tantalum (LCT) pegmatites,
which contain high concentrations of Li, Cs, Ta, P,
B, Rb, Be, Nb, and Sn; and 2) Niobium-Yttrium-
Fluorine (NYF) pegmatites, which are enriched in
niobium, yttrium, and fluorine, as well as the REE
(Linnen and others, 2012). The LCT pegmatites
currently have greater economic importance than
the NYF pegmatites (Linnen and others, 2012).
However, the economic potential of the NYF pegmatites
to produce REE resources is demonstrated
by the Strange Lake pegmatite in Canada, which
hosts more than 140 million tonnes (154.3 million
tons) of ore with an average grade of 0.9 percent
TREO (Quest Rare Minerals, 2013).
LCT and NYF pegmatites typically occur within
granitic intrusive complexes (Linnen and others,
2012). However, REE-bearing pegmatites are also
present within metamorphic terrains, as pods and
dikes that can be discordant to foliation, as well
21
as small lenses concordant to foliation (Houston,
1961; Adams and others, 1980).
Across Wyoming, granitic pegmatites are potential
hosts for both high- and low-grade REE-mineralization.
Significant enrichment is present in the
Big Creek and Tie Siding districts, and interesting
REE or other element anomalies occur in the
Weddle claims, the Many Values area, and in the
Allie claims. For granitic pegmatites hosted within
intrusive rocks, post-emplacement processes may
be important factors controlling the concentration
of REE. In the Tie Siding area (fig. 5), post-emplacement
hydrothermal activity was apparently responsible
for further concentration of REE within
alteration zones (fig. 6). In the Weddle claims in
the Laramie Range, the only notable REE-enrichment
is associated with a granite dikelet, reinforcing
the importance of post-emplacement processes
in concentrating REE. However, the secondary
processes appear to be less important for REEenrichment
in pegmatites hosted by metamorphic
rocks. However, in the Allie claims in the southern
Bighorn Mountains, field relations suggest that the
pegmatite bodies are the product of partial melting
of the host-rock gneiss.
tie siding Pegmatites
Pegmatite pods within the Sherman Granite southeast
of Tie Siding (fig. 5) can be highly radioactive
and enriched in some REE (King and Harris,
2002). The Tie Siding pegmatites are typically
zoned, and have an overall alkali feldspar granite
composition. Zonation consists of an outer zone
composed of biotite, microcline, and quartz, a central
zone that is more than 90 percent microcline,
and an inner core of quartz (Griffin and Warner,
1982). Many of these pegmatites exhibit fresh
exposures due to production of potassium feldspar
in the 1940s (fig. 6) (Osterwald and others, 1966;
Griffin and Warner, 1982).
The REE-bearing minerals within some of these
pegmatites are uncertain, but allanite is present in
at least one locality, and zones of elevated radioactivity
(7 – 500 times background) are present at
all of the studied pegmatites (Griffin and Warner,
1982). The highest levels of radioactivity are associated
with biotite. Griffin and Warner (1982)