rare earth elements in Wyoming

Black Rock Gap prospect, SE¼SW¼ sec. 32, T.
31 N., R. 91 W., Southeastern Fremont County
The Black Rock Gap prospect is located along
an east-trending shear zone in the north-central
Granite Mountains. Intensive potassic and propylitic
alteration surrounds the prospect with a 4.6 m
(15 ft) long adit in the center of a north-trending
antiformal structure just west of Black Rock Gap.
The adit follows a 95˚ trending, near vertical shear
zone, which cuts mafic and ultramafic rocks and
granite gneiss. Brecciation accompanies shearing
in some areas. Abundant jasper and epidote are
present near the adit, as is some copper staining.
Epidotization is spread across a wide area near the
prospect and is easily seen in aerial imagery. A sample
of sheared ultramafic rock with copper staining
(20121009WS-A) yielded 0.5 ppm (0.015 oz/
ton) gold, 318 ppm cobalt, 516 ppm chromium,
and 1.15 percent copper. A sample of iron-stained
fault gouge from the shear zone (20121009WS-B)
yielded 0.51 ppm (0.015 oz/ton) gold, 318 ppm
cobalt, 515 ppm chromium, and 1.575 percent
copper. Two additional samples from the prospect,
20121009WS-C (brecciated mafic rock with
CaCO 3 cement) and 20121009WS-D (severely
altered and brecciated felsic gneiss) yielded 1,200
ppm and 1,630 ppm chromium, respectively.
UT Creek - Lost Muffler Fault Prospects,
NW¼NW¼ sec. 22, T. 32 N., R. 87 W., Northwestern
Natrona County
The UT Creek Formation is generally east-trending
and inclined steeply south to near vertical, in
the northern part of the Saddle Rock quadrangle
(Sutherland and others, 2012). The Archean
UT Creek Formation is dominated by fine- to
medium-grained metagreywackes and tuffaceous
metagreywackes separated by a relatively thick
metavolcanic member that is locally dominated by
foliated to massive, black, fine-grained to aphanitic
metabasalts (Hausel, 1996).
The Lost Muffler fault parallels the general east
trend of the UT Creek Formation and carries
anomalous gold values west of UT Creek (Hausel,
1996) where it is dotted with small prospects.
Near the north side of the fault, east of UT Creek,
and near a contact between metabasalt and metagreywacke,
a prospect pit is dug into sheared
metabasalt cut by iron-stained quartz veins adjacent
to an outcrop of minor black schist. Sample
20120607WS-B is representative of the sheared
metabasalt and associated quartz veins. Analysis of
this sample showed 1.84 ppm (0.05 oz/ton) gold
and no anomalous REE.
Wagon Bed Formation NW¼SE¼ sec. 30, T. 32
N., R. 88 W., Southwestern Natrona County
The Wagon Bed Formation crops out south of
the North Granite Mountains Fault in the northern
part of the Blackjack Ranch 7.5’ quadrangle.
The Wagon Bed Formation was described by Van
Houten (1964) in the Beaver Rim area, about 55
km (34 mi) west of the Blackjack Ranch quadrangle.
Love (1970) further described the Wagon
Bed in the central Granite Mountains. Regionally,
it varies from pale-olive and greenish-gray, tuffaceous
sandstone, siltstone, and mudstone with
some zeolitic ash layers to a highly variable coarse
conglomerate. Outcrops of the Wagon Bed in this
area are poorly exposed due to weathering and
slope wash. However, ground disturbances such as
excavations or drill holes provide opportunities to
examine the lithology of the formation. At this site,
a very recent (2012) drill hole provided subsurface
material from the Wagon Bed Formation. Three
samples were collected here (20121031WS-E,
20121031WS-F, and 20121031WS-G). Cream to
greenish and rusty-orange, very-fine grained sandstone
and ash (Sample 20121031WS-F) contained
3.15 ppm (0.09 oz/ton) gold.
Wagon Bed Formation Ash Layer, NE¼SE¼ sec.
32, T. 32 N., R. 87 W., Southwestern Natrona
County
The Wagon Bed Formation crops out in this
vicinity, where it is dominated by conglomerates,
sandstones, and ash layers, along with minor silicified
limestone. Conglomerate typical of the Wagon
Bed caps the ridge above and immediately to the
northwest of this location. A tufa deposit from
a paleo-hot spring (Sample 20120613WS-A) is
underlain by 5- to 10-foot thick ash layer (Sample
20120613WS-B). The tufa (Sample 20120613WS-
A) contains 3.68 ppm (0.11 oz/ton) gold, but the
ash layer contains no gold. No anomalous REE
were detected in either sample.
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