U-PB AND RE-OS GEOCHRONOLOGY SUGGEST A MULTISTAGE PRECAMBRIAN-MESOZOIC HISTORY FOR THORIUM AND COPPER MINERALIZATION, LEMHI PASS, IDAHO

Thorium deposits of the Lemhi Pass District of Idaho and Montana were discovered in 1949, and subsequent exploration defined a significant Th-REE resource. The thorium deposits and a few associated copper veins are hosted by deformed, Proterozoic (?) Apple Creek Formation siltites and argillites over a wide area in the Beaverhead Mountains southeast of Salmon, Idaho. Previous workers concluded the thorium was related to Eocene volcanic rocks which unconformably overlie Precambrian metasedimentary rocks. Based on field and petrographic relationships, Gillerman et al. (2000) suggested that the ores were Proterozoic and belonged to the Fe-Cu-U/Th-REE association. Two independent methods were used to test this hypothesis. Molybdenite from the dump of the Copper Queen mine (qtz-cp-bn-mo) was collected for Re-Os geochronology. The molybdenite, which contained a remarkably low Re concentration (0.2716 ppm), provided an age of 1055 +/- 4 Ma. Monazite and thorite from two fresh samples of specularite-biotite-monazite-(feldspar)-thorite ore from the Lucky Horseshoe mine were dated using the electron microprobe (Th-U-total Pb). Imaging and chemical mapping of monazite revealed cores (800-1100 Ma) and distinctly younger rims (200-400 Ma). Thorite (ThSiO4) fills spaces and fractures within the monazite. Core domains of nearly stoichiometric thorite yield Th-U-Pb ages of 330-370 Ma, similar to monazite rims. Two altered populations of thorite give ages of 200-280 Ma and 100-180 Ma, but lack any Tertiary signature. The results, which are consistent with field relations, provide a minimum age for host rocks, confirm a Precambrian age for regional mineralization, suggest a Paleozoic hydrothermal event, and document secondary alteration, possibly related to Mesozoic thrusting, magmatism, uplift and erosion.