RARE EARTH ELEMENTS IN HEAVY-MINERAL SANDSTONES OF THE ROCK SPRINGS FORMATION, SOUTHWEST WYOMING

Cretaceous-age heavy-mineral sandstones in the Rocky Mountain region are potential sources of critical mineral commodities. Although these paleoplacer deposits—which formed along the shores of the Western Interior Seaway in the Sevier foreland—have been studied previously as prospective titanium ores, the rare earth element content of most remain unknown. As a preliminary investigation of rare earth elements in heavy-mineral sandstones in Wyoming, 84 samples from the Rock Springs Formation were collected along the eastern flank of the Rock Springs Uplift, a doubly plunging north–south-oriented anticline in the southwestern part of the state. Geochemical analyses reveal cumulative rare earth element (lanthanum through lutetium plus yttrium) concentrations of 192 ppm to 2.2%. Measured elemental titanium values range from 0.2 to 23%. Titanium enrichment is attributed to abundant ferrian ilmenite and anatase leucoxene. The primary rare earth element-bearing minerals are monazite and xenotime. The heavy-mineral-enriched sandstone bodies are lenticular and relatively small, about one to two meters thick. The largest deposit in the area is about 1,000 meters long and several hundred meters wide. Enrichment trends within the deposits can be explained by depositional models of a wave-dominated shoreface environment, although in some cases fluvial and aeolian processes appear to have played a role in placer formation. The titanium, zirconium, and rare earth element contents of the Rock Springs paleoplacers are considerably greater than that of heavy-mineral beach sands currently mined elsewhere; however, developed deposits of this type are typically Neogene or Quaternary in age and poorly consolidated—therefore readily mined and processed—whereas the Rock Springs Uplift sandstones are well lithified. Furthermore, it is unclear how diagenetic alteration of both titanium and rare earth minerals, which is widespread in the sampled heavy-mineral sandstones, might affect the potential of this type of deposit as a source of critical mineral commodities. Regardless of their potential as critical mineral ores, the heavy-mineral sandstones of the Rock Springs Formation reveal a story of contemporaneous volcanism, depositional trends in the Sevier foreland, and exhumation of the Sevier orogenic belt.