CAMBRIAN AND MISSISSIPPIAN MAGMATISM ASSOCIATED WITH NEODYMIUM-ENRICHED RARE EARTH AND THORIUM MINERALIZATION, LEMHI PASS DISTRICT, IDAHO

Recent studies of intrusive rocks and REE-Th mineralization at the Lemhi Pass District, Idaho and Montana, documented an early Cambrian syenite-lamprophyre magmatic suite. The bimodal suite was intruded along a rifted Paleozoic continental margin, later overprinted by Cretaceous magmatism and deformation. Proterozoic metasediments and altered mafic rocks locally host quartz-chalcopyrite-bornite veins. Previous Ar⁴⁰/Ar³⁹ hornblende ages on the mafic porphyries returned complex spectra with ages from pre-400 Ma to 558 Ma. Syenite has been dated at 529.1 ± 4.5 Ma by U-Pb SHRIMP analysis on zircons, and it is cut by specular hematite veins. REE-Th mineralization is present as quartz veins and biotite-rich replacements with abundant specular hematite, thorite and monazite. However, age dating of Nd-enriched (>30 wt. %) monazite and thorite from the Lucky Horseshoe mine by electron microprobe analysis at the University of Massachusetts returned Carboniferous ages (300-350 Ma) for mineralization, much younger than the syenite and not matching known intrusive events. Two new U-Pb age dates by ID-TIMS analysis at Boise State University are reported here. Five zircons from the Cow Creek stock, a pyroxene porphyry lamprophyre, form an equivalent cluster with an age of 534.37 ± 0.22Ma, similar to the age of the syenite. Zircons from the Lucky Horseshoe sill, an unusual ultramafic to mafic intrusion with ferroan magnesite carbonate replacement of primary olivine(?) phenocrysts, are more diverse. Seven single zircons yielded ages from Neoproterozoic to Late Carboniferous. Neoproterozoic and Carboniferous cores appear to be rimmed by younger Carboniferous overgrowths, but four zircon grains yielded concordant results with dates between 317.9 and 315.1 Ma. Two interpretations are possible, both of which support a Carboniferous age for the REE-Th mineralization. The 315.1 Ma zircons could represent an igneous crystallization age for the sill and any related intrusive (carbonatite?) at depth. The second hypothesis is that the younger zircons grew metasomatically as part of the REE-Th mineralizing event at 315 Ma. In either case, the mafic magmas, syenite, and REE-Th mineralization represent recurrent magmatic and hydrothermal systems along a rifted margin of North America.