ND-ENRICHED LEMHI PASS REE-TH DISTRICT, IDAHO-MONTANA: A LINK BETWEEN CRUSTAL HYDROTHERMAL CIRCULATION AND DEVONIAN-CARBONIFEROUS MARINE ENVIRONMENTS?

Rare earth element (REE) deposits at the Lemhi Pass district along the Idaho/Montana border include quartz veins and biotite-rich shears with specular hematite, thorite [ThSiO₄] and rare earth minerals, principally monazite, hosted in Mid-Proterozoic clastic metasedimentary rocks. As noted by early workers (Staatz, 1972), the veins are unusually enriched in neodymium (Nd), a valuable REE for the green economy. Monazite in one deposit contains up to 35 wt. percent Nd oxide (EMPA analysis), and in general, REE patterns of district vein samples show a middle rare earth (MREE) hump on chondrite-normalized REE diagrams. Early Cambrian (ca. 530 Ma) syenite and mafic lamprophyre intrusive rocks are present in the district, but the age of the REE-Th mineralization is constrained as Latest Devonian (ca. 355 Ma with a broad range) by EMPA chemical ages on monazite and thorite. Lead isotopes of minerals in the Fe-Th-REE vein deposits are highly radiogenic, and single crystal determinations (ID-TIMS) on Nd-monazite from the Lucky Horseshoe mine returned values of Epsilon Nd from -6 to -7 (model ages of ca. 1.3 – 1.4 Ga), suggesting derivation of rare earths from ancient crustal sources. Still, the unusual chemistry of the deposits has remained unexplained. Studies of REEs in sedimentary phosphorites and biogenic phosphate over geologic time suggest a new hypothesis (Piper, 2001; Lecuyer et al., 2004; Emsbo, et al. 2015). Shale-normalized REE patterns of marine phosphorites of different ages and locations world-wide typically show flat to hump-shaped patterns, partly due to redox and iron scavenging, not unlike those of some of the Lemhi Pass rocks. Emsbo (2017) proposed a model of brine-expulsion during formation of sedimentary exhalative (SEDEX) base metal deposits, including large deposits in Mississippian and Devonian times in central and western North America. Lemhi Pass is situated near a Precambrian to Mississippian unconformity, and late Paleozoic-age anoxic strata and SEDEX deposits are present in central Idaho. Did hydrothermal circulation through anoxic, REE-enriched Devonian strata and underlying, oxidized continental basement mix fluids of continental and marine origin, resulting in the unique rare earth concentrations at Lemhi Pass?