Lemhi Pass mineral district

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area REE-bearing veins. Reported REE minerals in veins. Focus area outline based on USMIN location polygon ID00023. Includes fluorite-Fe-REE-Th veins at Deer Creek in the North Tendoy Mountains in Montana.
Identified resources Identified resources of REE and thorium.
Production No production, early exploration focused on Th resources.
Status Past exploration; trenching and minor underground workings.
Estimated resources Largest concentration of Th resources in the United States. Total reserves of 64,000 metric tons (70,500 tons) of thorium oxide (ThO2) and probable potential resources of an additional 121,000 metric tons (133,000 tons) (Staatz, Sharp and Hetland, 1979). REE resources of the vein deposits of the Lemhi Pass district are approximately equal to its Th resource (Staatz, 1972b; Long and others, 2010). Hedrick (2010) reported, “Based on average percentages of individual REE by recent sampling and previous analyses by Idaho Energy Reserves Co. (a subsidiary of Idaho Power Co.), the Lemhi Pass District had resources, in order of increasing atomic number, 77,345 t of yttrium; 25,780 t of lanthanum; 69,980 t of cerium; 11,050 t of praseodymium; 66,296 t of neodymium; 40,515 t of samarium; 14,735 t of europium; 40,515 t of gadolinium; 1,840 of terbium; 14,730 t of dysprosium; 1,840 t each of holmium and ytterbium; and about 929 t or less each of erbium, thulium, and lutetium.”
Geologic maps Sharp and Cavender (1962), scale 1:24,000; Staatz (1972b, 1973), scale 1:24,000; Staatz, Sharp and Hetland (1979, plate 1), scale 1:31,680; Evans and Green (2003), scale 1:100,000; Burmester and others (2018), scale 1:24,000; Lund (2018), scale 1:50,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Quartz-hematite-thorite veins, which fill fractures, shears, and brecciated zones in Mesoproterozoic quartzite and siltite host rocks at the angular unconformity between the Apple Creek and Gunsight Formations in window beneath thrust fault.
Deposits Last Chance vein (MRDS dep_ids: 10071967, 10093618, 10096395; USMIN Site_ID: MT00018), Lucky Horseshoe veins (MRDS dep_id: 10071945; USMIN Site_ID: ID00024), Silver Queen veins (MRDS dep_id: 10071974; USMIN Site_ID: ID10010).
Evidence from mineral occurrences MRDS; USMIN.
Geochemical evidence Geochemical study (Staatz and others, 1971). Favorable mineralogy includes thorite, allanite, monazite, barite, rutile, and apatite.
Geophysical evidence Unknown.
Evidence from other sources Unknown.
Comments Hybrid deposit type; hydrothermal with some aspects of IOA/ IOCG. Carboniferous age Th-REE mineralization as quartz veins and biotite-rich replacements with abundant specular hematite, thorite, and monazite. Unusual enrichment pattern of the middle rare earths and neodymium on chondrite-normalized plots. REE vein chemistry distinct from typical carbonatite patterns, only partially overlaps igneous peralkaline systems, and may represent a hydrothermal mix of constituents from multiple sources (Gillerman and others, 2021). Since 2008, the veins have been intermittently explored for REE (Gillerman and others, 2021).
Cover thickness and description Variable.
Authors Virginia S. Gillerman, Reed S. Lewis, Christopher A. Tate, Karen Lund, Thomas P. Frost, Allen K. Andersen.
New data needs Geologic characterization of district and mineral deposits in general.
Geologic mapping and modeling needs Geologic mapping adequate. Work needed is reanalysis of archived ore samples and modern revision of deposit descriptions based on available modern geologic mapping and dating.
Geophysical survey and modeling needs Rank 1 geophysical survey complete.
Digital elevation data needs Lidar in progress.