NEW GEOLOGIC MAPPING IN THE MINERAL HILL REE DISTRICT, LEMHI COUNTY, IDAHO

Recent geologic mapping of the Ulysses and Shoup 7.5’ quadrangles (1:24k) in Lemhi County, Idaho is providing new context and rock and mineral analyses of rare-earth element (REE)-bearing carbonatite occurrences of the Mineral Hill district. Mapping is aided by recent USGS aeromagnetic, gamma-radiometric, and LiDAR surveys. Handheld magnetic susceptibility and gamma-ray spectrometer readings are being used to further characterize lithology. The area is dominantly comprised of a Mesoproterozoic interlayered alkaline granitic suite and mafic complex, with the carbonatite occurrences appearing to be hosted within gabbro and amphibolite. These units are in contact with metasedimentary rocks of the Belt Supergroup in the NE corner of the map and are intruded by Cretaceous granodiorite to the SW, while the carbonatite occurrences continue NW to the Sheep Creek district of Ravalli County, Montana. Smaller bodies of paragneiss, migmatite, trondhjemite, tonalite, and rhyolite are also present within the map. The granitic and mafic rocks are both variably foliated, with a dominant strike direction of NW-SE and dipping both to the NE and SW. The majority of lineations recorded exhibit a NW or SE trend. The Mineral Hill district has been investigated and prospected for carbonate-bearing rocks from the early 1950s to present, due to their high REE concentrations. The principle REE-bearing minerals are monazite and allanite, which are dominantly comprised of Ce and La, but contain additional light REEs, including Nd, Sm, and Pr. The rise in demand for critical minerals has placed new importance on the Mineral Hill occurrences as a prospective source of REEs, and we have mapped and sampled several known prospects to provide a new view and characterization of these occurrences. The major phases include calcite, dolomite, actinolite, and magnetite, with variable minor phases of ilmenite, fluorapatite, allanite, barite, rutile, and monazite.