REE-TH DEPOSITS OF THE LEMHI PASS REGION, NORTHERN ROCKY MOUNTAINS - PALEOZOIC MAGMAS AND HYDROTHERMAL ACTIVITY ALONG A CONTINENTAL MARGIN

Idaho’s rare earth placer deposits were mined in the early 1940’s, but post-war exploration discovered rare earth element (REE) and thorite-bearing quartz veins and specular hematite and biotite-rich replacements at Lemhi Pass in the Beaverhead Mountains of Idaho and Montana. The veins (Cu, Fe-Cu-Th-REE, and Fe-REE-Th) are hosted in Proterozoic-age, clastic metasediments intruded by scattered mafic igneous dikes and covered by Tertiary volcanics. The veins are unusually rich in neodymium. Recent work recognized both a syenite (529 Ma by SHRIMP) and distinctive pyroxene lamprophyre dikes, constrained by Ar/Ar work as pre-Cretaceous in age. U-Pb geochronology on zircons, utilizing the ID-TIMS at Boise State University, confirmed the early Cambrian age (534.37 ± 0.22Ma) of the Cow Creek pyroxene lamprophyre. However, the Lucky Horseshoe carbonate-altered, ultramafic sill yielded four zircon grains with concordant U-Pb ages between 317.9 and 315.1 Ma. The 315 Ma date could represent igneous crystallization or zircon growth during REE-Th mineralization. Age dating of Nd-enriched (>30 wt. %) monazite and thorite from the Lucky Horseshoe mine by electron microprobe analysis at University of Massachusetts returned Carboniferous ages (300-350 Ma). The independent techniques strongly confirm a Carboniferous age of mineralization. Epsilon Nd ratios measured on three, hand-picked monazite grains from the Lucky Horseshoe deposit show present-day values of minus 6 and model ages of about 1.3-1.4 Ga, supporting a crustal source. Common lead isotope data on ore and gangue minerals also support an ancient, high-µ crustal source for Fe-Th-REE mineralizing fluids, but a more juvenile, low-µ source for the earlier base metal mineralization. Recycling of magmas, metals and fluids along the rifted margin in response to tectonic activity apparently extended from British Columbia southward into the Idaho portion of the western margin of ancestral North America.