rare earth elements in Wyoming

Samples of the Flathead were collected in the area
and analyzed for REE (Samples 20110920WS-
A, 20121004JC-C). No elevated values for REE,
thorium, or uranium were detected for sample
20121004JC-C. Sample 20110920WS-A was only
slightly elevated in thorium, but not in REE.
Placer deposits along Warm Spring Creek, also
known as the Clarks Camp-Union Pass gold placers,
include monazite-bearing gravels. These gravels
are reported to extend 4.8 km (3 mi) up the South
Fork (Wilson, 1960; Bolmer and Biggs, 1965;
Osterwald and others, 1966; Rohrer, 1968; Hausel,
1989). Rohrer (1968) noted that claims for REE
covered the monazite-bearing gravels. The monazite-bearing
gravel deposits along Warm Spring
Creek range from 0.4 to 1 km (0.25 to 0.6 mi)
wide, extend for about 4.8 km (3 mi) in length,
and include high terraces more than 30 m (100 ft)
above the stream bottom (Hausel, 1989).
The area was mapped by Rohrer in 1968, at which
time he sampled and mapped quartzite conglomerate
layers in the Wind River Formation containing
flour gold within a matrix of fine- to coarse-grained
angular quartzite and tuffaceous sandstone. These
conglomerate layers, dominated by percussionmarked
pebble- to small cobble-sized quartzite
round-stones, represent four, 1.6 to 3.2 km (1 to
2 mi) wide, eastward-trending channels. Rohrer
(1968) cited gold values (less than 0.009 to 1.777
g/cy (0.0003 to 0.06268 oz/cy as interpreted from
1966 gold value of $35/oz) from
the conglomerates as sampled by
J.C. Antweiler, but made no mention
of monazite or other REEbearing
minerals within them.
Rohrer (1968) stated that these
auriferous deposits are derived
from similar deposits in the Pinyon
Conglomerate (occurring about 13
km (8 mi) to the west) which, Love
(1973) believed to have originated
in the hypothetical, now vanished,
Laramide (Late Cretaceous to
Paleocene) Targhee uplift west of
Jackson Hole.
47
Modern stream placers at four locations along
Warm Spring Creek and the South Fork of Warm
Spring Creek were sampled by the WSGS in 2010
and 2011. Monazite and xenotime grains identified
in pan samples were generally angular to subrounded,
suggesting a relatively local source, rather
than being reworked from a secondary source in
the quartz cobble-rich conglomerates in the Wind
River Formation or the Pinyon Conglomerate to
the west. Precambrian igneous rocks south of the
Quaternary placers are the probable source of the
monazite and xenotime.
Although analyses of placer concentrates showed
high cumulative REE values, the calculated values
for unconcentrated bank cubic yards (BCY) of
placer material were all less than crustal abundances.
Sample WSC-91510-A showed calculated
total REE at 37.28 g/BCY (1.32 oz/BCY), or about
27 ppm, with gold content calculated at 25.3 mg/
BCY (0.0009 oz/BCY). See figure 16 for typical
monazite and gold flake from Warm Spring Creek.
Sample WSC-91510-B showed calculated total
REE at 3.84 g/BCY (0.136 oz/BCY), or about
3 ppm, with gold content calculated at 1.9 mg/
BCY (0.00007 oz/BCY). Sample WSC-91510-C
showed calculated total REE at 51.92 g/BCY (1.83
oz/BCY), or about 37 ppm, with gold content calculated
at 2.9 mg/BCY (0.0001 oz/BCY). Sample
20110921WS-B showed calculated total REE at
32.5 g/BCY (1.146 oz/BCY), or about 23 ppm,
with no gold.
figure 16. Gold flake and monazite grains from sample WSC-91510-A
concentrate.