Geochemical evidence for the origin of th-ree mineralization in the Lemhi Pass District, Idaho/Montana
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The Lemhi Pass district, recognized as the largest known thorium (Th) occurrence and the fifth largest rare earth element (REE) occurrence in the U.S., is part of a larger trend of Th-REE mineralization extending along the central Idaho-Montana border. The deposits occur in hydrothermal veins, and the nature and origin of the mineralizing fluids are not well characterized. Salinities of the mineralizing fluids were found to vary, with quartz-hosted fluid inclusions ranging from 12 to 31.1 equiv. wt. percent NaCl and fluorite-hosted fluid inclusions from 14 to 25.2 equiv. wt. percent NaCl. Homogenization temperatures (Th) also varied widely. Quartz-hosted fluid inclusion Th ranged from 100 to 392 degrees C. Fluorite-hosted fluid inclusion THanged from 76 to 174 [degrees] C. A combination of fluid inclusion microthermometry and Zr-in-rutile geothermometry from samples of the Last Chance mine suggest mineralizing fluid temperatures likely were between 468 and 538 [degrees] C and pressures between 4.9 and 9.8 kbars. These results indicate a depth of formation between 18 and 33 km. The pressure and temperature data suggest a metamorphic or magmatic origin for the Lemhi Pass fluids, though Lemhi Pass fluid inclusion compositions deviate from the typical compositional ranges of metamorphic and magmatic fluids. Lemhi Passfluid inclusion compositions more closely match those in carbonatites from the Tuva region in Russia and Amba Dongar, India, suggesting a possible carbonatite source for the Lemhi Pass mineralizing fluids. The Lemhi Pass mineralizing fluids were not particularly Th- or REE-rich. Most of the fluid inclusions did not contain detectable concentrations of Th or REEs. Those that did had REE concentrations mostly in the range of tenths to 10s of ppm and Th concentrations from about 1 to 12 ppm. The fluids were rich in base metals, with Zn and Pb concentrations up to 1000s of ppm and Cu concentrations up to 100s of ppm. However, Lemhi Pass contains little base metal sulfide mineralization, possibly due to the low pH of the mineralizing fluids, which may have been between 2.3 and 2.6 at the Lucky Horseshoe deposit. The oxidizing nature of the mineralizing fluids could have further inhibited base metal sulfide mineral precipitation. The elevated levels of fluoride and chloride in the mineralizing fluids are likely to have increased the solubility of Th and REEs, contributing to their enrichment in the Lemhi Pass deposits.
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M.S.