ND-ENRICHMENTS AND HYDROTHERMAL ALTERATION AT THE LEMHI PASS AND DIAMOND CREEK RARE EARTH (REE) - THORIUM DISTRICTS, LEMHI COUNTY, IDAHO

Three distinct groups of rare earth element (REE)–bearing deposits align in a northwest belt across Lemhi County, Idaho, and adjoining parts of Montana. The Lemhi Pass (LP) district is located at the southeast end of the belt along the Idaho/Montana border. The Diamond Creek (DC) district is northwest of the town of Salmon and between the other two. Further northwest is the Mineral Hill district, known as the Sheep Creek district in Montana. At Mineral Hill, thin horizons of REE-enriched carbonatite are interlayered in metamorphosed mafic and felsic units. To the southeast at Lemhi Pass, Mesoproterozoic metasedimentary rocks host quartz veins and biotite-rich shears and replacements with specular hematite, thorite [ThSiO₄] and rare earth minerals, principally Nd-rich monazite. Geology at DC is similar, but not identical, to LP; most veins in the two districts are highly oxidized, making studies difficult. As noted by several workers, the Lemhi Pass veins are unusually enriched in neodymium (Nd), a valuable element for the green energy economy. Widespread exploration sampling at Lemhi Pass documents the unusual enrichment of the battery metals, Nd, Pr and Sm. Monazite in one deposit contains 35 weight percent (50 mole %) Nd oxide (EMPA), and LP vein samples often show a middle rare earth (MREE) hump on chondrite-normalized diagrams. Wall-rock vein envelopes at LP consist of pinkish-colored potassium feldspar. Monazite mineralization at Lemhi Pass yields an EMPA U/Pb age of ~355 Ma (Gillerman et al., 2008; GSA Abstracts, 2010, 2021), but the monazite rims also record Cretaceous hydrothermal activity. At Diamond Creek, microscope petrography plus limited SEM work and TESCAN TIMA imaging reveal that the planar quartz veins have white alteration envelopes of albite where biotite/chlorite in the siltite has been leached out. Fine-grained monazite is locally present. Primary vein oxides at DC include Nb-rutile or ilmenite plus magnetite, while hematite is dominant at Lemhi Pass. Apatite is common at both, and local columbite is present at DC but not at Lemhi Pass. Geochemical signatures at Diamond Creek appear to be a blend of the Lemhi Pass and Mineral Hill end-member chemistries. Research is ongoing to better characterize the alteration and mineralization and to determine how the three areas relate.