rare earth elements in Wyoming

downstream from older REE mineral concentrations.
flathead sandstone Paleoplacers
The Middle Cambrian Flathead Sandstone, referred
to by some early workers as the Deadwood
Conglomerate, is the oldest sedimentary formation
above the Precambrian in Wyoming and has
a maximum thickness of about 170 m (560 ft)
(Kanizay, 1978). Figure 13 shows the locations
of WSGS samples collected from the Flathead
Sandstone. The Flathead is dominantly quartz-rich,
subangular, medium- to coarse-grained sandstone
with large-scale cross-bedding. Non-quartz grains
include abundant feldspar and crystalline lithic
fragments typical of the underlying granite. Most
of the formation is thin- to thick-bedded and wellcemented
with a predominant reddishbrown
color that grades to purple, rustyorange,
yellow, or gray. The sandstone is
interrupted by thin layers of greenishgray
siltstone and shale, particularly in
the upper part of the formation. The
Flathead represents a fluvial-marine
transition zone along a north-south
oriented shoreline with braided stream
deposits in the conglomeratic base of the
formation (Middleton, 1980).
The basal conglomerate (fig. 14) varies
from 6 to 15 m (20 to 50 ft) in thickness
(McKinney and Horst, 1953).
These braided stream deposits are
quartz-rich, arkosic conglomerates with
subangular to rounded clasts indicative
of both a nearby source and a lack of reworking
(Middleton, 1980). Clast sizes
are up to 5 cm (2 in) in diameter with
cementation varying from moderately
silicic or limonitic to localized areas of
quartzite.
bald Mountain Paleoplacers, big
Horn and sheridan counties
The Bald Mountain area sits along
the upper part of the west flank of the
41
northwest-trending, northern Bighorn Mountains
at elevations between 2,600 and 2,900 m (8,600 to
9,400 ft). This part of the Bighorn Mountains is an
asymmetric, Laramide anticlinal uplift characterized
by west-dipping monoclines. Near-horizontal
to east-dipping exposures of Lower Paleozoic
formations unconformably overlie medium- to
coarse-grained, pink to gray, Archean plutonic
rocks that generally vary from granitic to quartz
dioritic in composition. The granitic rocks locally
contain minor pegmatites and are cut by more than
one generation of mafic dikes (Osterwald, 1959;
Hausel, 1989). Mafic dikes in the Bald Mountain
area were mapped as hornblende-diorite by Cardinal
(1958).
figure 14. Conglomerate of the lower Flathead Sandstone. A, outcrop
of the basal conglomerate from which sample 20130824WS-D was
collected. B, close-up of sample 20110824WS-D, conglomerate
from the lower Flathead Sandstone.