Rare Earth Elements Geology and Minerals
Rare Earth Elements Geology and Minerals
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Geology and Minerals
Awareness of the diversity of REE deposit types is beneficial to exploration efforts. Economically, exploitable concentrations of REE are primarily derived from crystalline rocks. REEs are more common in alkaline igneous rocks and carbonatites than in mafic rocks. Rare earths typically occur as trivalent cations in rock-forming minerals in carbonates, oxides, phosphates, and silicates. REEs are chemically similar to thorium and are often found in minerals and rocks in association with this element. Economic concentrations of REE-host minerals are known from alkaline igneous rocks, carbonatites, and from a wide variety of dikes and veins that cross-cut alkaline intrusions and surrounding rocks.
Euxenite from pegmatite at the Platt mine on the west side of the Medicine Bow Mountains. [Credit: Wayne M. Sutherland, WSGS]
REEs occur in a variety of rock units across Wyoming. Reported Precambrian REE occurrences are hosted by pegmatites, veins and dikes, faults and shear zones, metacarbonate rocks, disseminated minerals in generally alkalic igneous rocks, and metasediments. Sedimentary occurrences in paleoplacers are found primarily in the Cambrian Flathead Sandstone and the Cretaceous Mesaverde Formation as well as local, smaller deposits in numerous units. Disseminations of REEs are also reported within phosphate-rich rock of the Permian Phosphoria Formation. Tertiary-aged hosts include alkaline igneous rocks, carbonatite veins, dikes, hydrothermal zones, paleoplacers, and apparent depositions from solution movement within sandstones. Quaternary REE hosts can be found in alluvial placers and paleoplacers (King and Harris, 2002; Sutherland and others, 2013).
Numerous minerals are known to contain REEs as essential constituents, and a greater number contain REEs as accessory elements. Only a few of these minerals host large enough concentrations of REEs to be considered ore minerals. Allanite and monazite occur as relatively common accessory minerals within many types of felsic igneous rocks. Other REE-bearing minerals can be uncommon and are often hosted in generally uncommon rock types. Worldwide, the principal commercial sources of REE are the minerals bastnasite, loparite, monazite, and xenotime, and rare earth ion-adsorption clays. When discussing principal REE-bearing minerals specific to the United States, Long and others (2010) omit loparite and ion-adsorption clays but add euxenite and allanite, although other REE-bearing minerals are also present.
Of the principal REE-bearing minerals, allanite, bastnasite, euxenite, monazite, and xenotime are known to occur in Wyoming. Although not primary REE sources, ewaldite and mckelveyite were actually discovered with association to trona beds in the Green River Formation in Sweetwater County. The table below lists minerals reported in Wyoming that contain REEs. Mineral formulas are variable depending on the source cited and on sub-varieties that may contain differing elemental substitutions.
Contact:
Patty Webber, patty.webber@wyo.gov
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