rare earth elements in Wyoming

Oakie Trail, S½NE¼ sec. 15, T. 40 N., R. 88
W., Northwestern Natrona County
The Flathead Sandstone in this area contains up to
134 ppm thorium, but less than 5 ppm uranium,
and is likely a typical Flathead Sandstone paleoplacer
(Malan, 1972). The Flathead here consists
of white to red, quartz pebble conglomerate and
medium- to coarse-grained, poorly sorted, subrounded,
iron-rich subarkose, with coarse weathered
limonite grains (Sample 20121017JC-E).
In places where the Flathead Sandstone is weakly
brecciated (Sample 20121017JC-F), no elevated
REE or thorium values were detected.
Mesaverde formation Paleoplacers
Titaniferous black sandstone horizons are relatively
common in the lower part of the Late Cretaceous
Mesaverde Formation across Wyoming, Colorado,
and New Mexico, and in the Mesaverde Group
in Arizona (Houston and Murphy, 1962; 1970).
The Wyoming deposits (fig. 13) occur at the top
of the formation’s basal sandstone (Houston and
Murphy, 1962). The hosting basal sandstone of the
Mesaverde is typically a tan to buff, fine-grained
sandstone overlain by a succession of interbedded
sandstone, shale, carbonaceous shale, and coal
or, in some areas, by black shale. The Mesaverde
Formation is underlain by black shale of the Cretaceous
Cody Shale. Together, the Cody and the
Mesaverde represent a generally regressive sequence
that transitioned from a deep marine to near-shore
terrestrial environment over the depositional history
of the two units. The black shale that in places
overlies the basal sandstone of the Mesaverde represents
a short-lived transgressive event (Houston and
Murphy, 1962). Titaniferous black sandstones at or
near the top of the basal sandstone are thought to
be fossil beach placers analogous to modern heavy
mineral-rich black sand beaches found in Australia
(Houston and Murphy, 1962; 1970).
The heavy minerals within the titaniferous black
sandstones of the Mesaverde Formation (fig. 17)
were studied in detail by Houston and Murphy
(1962; 1970) and are dominantly volcanic in
origin. The heavy mineral fraction of the black
sandstone consists of 79 to 97 percent opaque min-
erals, mostly iron-titanium oxides, but a radioactive
niobium-bearing mineral may also be present. The
remaining heavy mineral fraction is composed of
varying amounts of zircon, monazite, garnet, rutile,
figure 17. Outcrops of titaniferous black sandstone
within the Cretaceous Mesaverde Formation. A,
outcrop at Mud Creek, black sandstone overlies tan to
buff sandstone of the lower Mesaverde, view is to the
northeast. B, outcrop at Dugout Creek, black sandstone
overlies tan to buff sandstone of the lower Mesaverde,
view is to the northwest. C, outcrop at Grass Creek,
north deposit, black sandstone is at the top of the cliff,
view is to the northeast.
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