REE Geology & 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. REE 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. REE 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.
Madison Limestone flatirons in the Ferris Mountains.
WSGS photo by Wayne M. Sutherland.
Sedimentary rocks that may preferentially contain REE include phosphatic rocks, coarse clastic rocks, and shales. Phosphate deposits may be sources for yttrium and lanthanum. Sediments derived from host rocks that contain even small amounts of HREE-bearing minerals may produce both placer and paleoplacer concentrations.
Ion-adsorption clays, or laterites, developed as residuum from chemical weathering, may be enriched in REE from intensive leaching of igneous and other rocks. Economic REE concentrations in ion-adsorption clays are known from southern China and Kazakhstan. Such clays tend to have a greater enrichment in High Rare Earth Elements than other deposit types.
Precambrian REE occurrences in Wyoming are hosted by pegmatites, veins and dikes, faults and shear zones, metacarbonate rocks of uncertain origin, disseminated minerals in generally alkalic igneous rocks, and in metasediments, including paleoplacers. Paleozoic and Mesozoic occurrences include paleoplacers in both the Cambrian Flathead Sandstone and the Cretaceous Mesaverde Formation, and disseminations within phosphate rock of the Permian Phosphoria Formation. Tertiary REE concentration hosts include alkali igneous rocks, related carbonatite veins, dikes, hydrothermal zones, and some paleoplacers. Quaternary REE hosts are alluvial placers, paleoplacers, and some hot spring deposits.
Numerous minerals are known to contain REE as essential constituents, and a greater number contain REE as accessory elements. However, only a few of these minerals host large enough concentrations of rare earths to be considered ore minerals for commercial mining. Worldwide, the principal commercial sources of REE are the minerals bastnasite, loparite, monazite, and xenotime, and rare-earth ion-adsorption clays. REE-bearing minerals in the United States do not include loparite and ion adsorption clays, but they do include euxenite and allanite. Other REE-bearing minerals in the nation are also present. The table below lists minerals reported in Wyoming that contain REE. Of the principal REE-bearing minerals, allanite, bastnasite, euxenite, monazite, and xenotime are known to occur in Wyoming.