rare earth elements in Wyoming

coarse-grained granite (QAP: 30% Q, 40% A, 30%
P) at this location is weakly enriched in some of the
HREE (Sample 20120921BG-7).
big creek district
The Big Creek pegmatite district, on the west side
of the Medicine Bow Mountains, was investigated
and mapped by Houston (1961) and covers an area
several miles wide and 3.2 to 10 km (2 to 6 mi)
south of the Mullen Creek-Nash Fork shear zone
(fig. 8). The Mullen Creek-Nash Fork shear zone
is part of the Cheyenne belt suture, which represents
a continental-arc collision zone (Graff, 1978;
Hills and Houston, 1979) separating the Wyoming
Province to the north from cratonized (1.7 Ga)
Proterozoic basement of the Colorado Province
to the south. The Colorado Province south of the
Cheyenne belt consists of volcanogenic island-arc
basement rocks, intrusive granites, and gabbroic
complexes. The Mullen Creek complex, one of
the larger layered mafic complexes in the western
United States, lies along the northern edge of the
Big Creek pegmatite district. The district is included
within the Preliminary Geologic Map of
the Saratoga 30’ x 60’ quadrangle (Sutherland and
Hausel, 2005).
Two types of granite pegmatite were described
by Houston (1961) that cut Early Proterozoic
amphibole gneiss in close proximity to both a
medium- to coarse-grained, pink to gray foliated
granite intrusion, and a well-foliated amphibolite
of the 1778 ± 2 Ma, Mullen Creek layered mafic
igneous complex. In the vicinity of known REE
occurrences, the amphibole gneiss is dominated by
hornblende gneiss and schist, with lesser, alternating
layers of feldspar-quartz-biotite gneiss.
The most common pegmatite is an unzoned
feldspar-quartz type that is roughly conformable
in direction and attitude to the amphibole gneiss.
These pegmatites tend to be long and narrow, with
widths rarely greater than 3 m (10 ft) and lengths
exceeding 0.8 km (0.5 mi). Their contacts with the
country rock are sharp and exhibit no decrease in
grain size toward the contacts. Mineralogy is simple
and is dominated by feldspar accompanied by minor
quartz and accessory biotite, muscovite, garnet,
magnetite, and ilmenite (Houston, 1961).
29
The second type of pegmatite may have poorly developed
zoning, has cross-cutting relationships with
the amphibole gneiss, and tends to be elliptical in
plan view, with lengths averaging three times their
widths. Similar to the first type, these pegmatites
have sharp contacts and show no decrease in grain
size toward contact with the country rock. However,
the mineralogy is more complicated, and along
with feldspar and quartz, includes garnet, biotite,
muscovite, tourmaline, fluorite, and rare monazite,
allanite, euxenite, and columbite (Houston, 1961).
Economically, these pegmatites can be described
as either copper-bearing or REE-bearing. Houston
(1961) noted five pegmatites in the district that
are known to host one or more of the REE-bearing
minerals, monazite, allanite, and euxenite. However,
only the Platt pegmatite contained economic
quantities of these minerals. Numerous other
pegmatites in the district were not examined and
have the potential to host REE mineralization
(King and Harris, 2002). The nearby Big Creek
mine hosted copper mineralization, but no primary
REE minerals or columbite. Columbite [(Fe,Mn)
(Nb,Ta) 2 O 6 ] may contain REE and uranium as
accessory constituents (King and Harris, 2002;
MinDat.org, 2011).
Platt Mine (Platt Pegmatite, Uranium King
Mine), SW¼SE¼ sec. 3, T. 13 N., R. 81 W.,
Southeastern Carbon County
The Platt mine was excavated to a depth of 23 m
(75 ft) in a zoned, cross-cutting type pegmatite 21
m wide by 49 m long (70 x 160 ft). The outer part
of the pegmatite is feldspar, and the central part
is quartz-mica-feldspar-rare earth minerals. Euxenite
is the most abundant REE-bearing mineral
followed by monazite, and the least abundant,
columbite (Houston, 1961). These occur as large
individual crystals and crystal aggregates both in
the central pegmatite and in gash fractures. Uranium
accounted for most of the production from
the Platt mine in 1956 with 12 tonnes (13 tons) of
ore (King and Harris, 2002). Approximately 4,500
kg (10,000 lbs) of euxenite were reported to be
extracted from the mine between 1956 and 1958
(Houston, 1961; Adams and others, 1980), although
the USBM recorded only 1,413 kg (3,115