rare earth elements in Wyoming

tertiary-aged Igneous occurrences
Leucite Hills
The Leucite Hills in the Green River Basin make
up one of the largest lamproite fields in the world
(Hausel, 2006). Lamproites are ultrapotassic, mafic
to ultramafic volcanic and subvolcanic rocks that
can be extremely enriched in the LREE (Kuehner,
1980; Ogden, 1979). Notable REE occurrences in
the Leucite Hills are located on Zirkel Mesa, Pilot
Butte, and South Table Mountain (table 13). Detailed
descriptions of the geology and geochemistry
of the Leucite Hills are available in WSGS Report
of Investigations 56 (Hausel, 2006).
Zirkel Mesa, NE¼NW¼ sec. 24, T. 22 N., R.
102 W., Northwestern Sweetwater County
Zirkel Mesa is the largest exposure of lamproite in
the Leucite Hills, and is dominantly composed of
orendite, a diopside-sanidine-phlogopite lamproite
(Hausel, 2006). Wyomingite (diopside-leucitephlogopite
lamproite) from Zirkel Mesa (Sample
ZM1-WY) is enriched in the LREE, europium,
gadolinium, terbium, and dysprosium; lanthanum
and cerium are enriched to greater than five times
crustal abundance.
Pilot Butte, SE¼NE¼ sec. 10, T. 19 N., R. 106
W., Northwestern Sweetwater County
The Pilot Butte lamproite consists of a volcanic
neck, a single lobate lava flow, and two associated
small dikes exposed on the southwest flank of the
butte. The volcanic and subvolcanic rocks on Pilot
Butte are dominantly madupite (diopside-phlogopite-leucite
lamproite) with Green River Formation
39
xenoliths (Hausel, 2006). The Pilot Butte madupite
(Sample PB1-95) is enriched in the LREE, europium,
gadolinium, terbium, and dysprosium; La,
Ce, Pr, Nd, and Sm are enriched to greater than
five times crustal abundance.
South Table Mountain, SE¼SW¼ sec. 6, T. 22
N., R. 102 W., Northwestern Sweetwater County
South Table Mountain is composed of olivine orendite,
and wyomingite. The olivine orendite (Sample
STM-1-2010) is enriched in LREE, europium,
gadolinium, and terbium; only cerium is enriched
to greater than five times crustal abundance.
sediment-hosted occurrences
Important REE occurrences in Wyoming are
hosted in a number of sedimentary environments.
These include fossil beach placers (paleoplacers)
within marginal marine sandstones, uranium- and
phosphate-rich horizons in Eocene lacustrine
deposits in southwestern Wyoming, and phosphorite
beds in western Wyoming (King and Harris,
2002). Minor REE occurrences are also present
within phosphate concretions in the Bighorn Basin
and in alteration zones associated with reductionoxidation
fronts in central Wyoming.
Paleoplacers may host REE-bearing minerals, such
as monazite, zircon, and apatite. The Cambrian
Flathead Sandstone and the Cretaceous Mesaverde
Formation are the most notable REE-bearing
paleoplacers in Wyoming; minor paleoplacers are
also found in the Cretaceous Frontier Formation
(fig. 13). Quaternary alluvial deposits host placers
table 13. Concentration of the REE, niobium, uranium, thorium and zircon in lamproites from the Leucite Hills.
Bold values indicate concentrations greater than five times crustal abundance; scandium was not analyzed for these
samples.
Element La (ppm) Ce (ppm)
Leucite Hills
Pr (ppm) Nd (ppm) Sm (ppm) Eu (ppm) Gd (ppm) Tb (ppm) Dy (ppm) Ho (ppm)
ZM1-WY 185.5 375 41.2 150.5 19.75 4.35 13.85 1.49 4.6 0.66
PB1-95 355 700 75.4 267 33.4 7.02 23.7 2.23 6.18 0.86
STM-1-2010 151 308 34 121.5 15.9 3.48 11.45 1.22 3.87 0.55
Element Er (ppm) Tm (ppm) Yb (ppm) Lu (ppm) Y (ppm)
Cumulative REE
(ppm)
Nb (ppm) U (ppm) Th (ppm) Zr (ppm)
ZM1-WY 2.05 0.17 1.08 0.15 18.1 818.5 57.4 4.45 21.2 1400
PB1-95 2.77 0.21 1.32 0.19 22.5 1497.8 122 8.98 40.1 1160
STM-1-2010 1.67 0.14 0.87 0.13 14.8 668.6 49.4 4.76 16.55 1420