The Snowbird deposit is a lenticular, rare earth- and fluorite-rich quartz-carbonate body with a pegmatitic texture, which intrudes Belt Supergroup metasediments at the Idaho-Montana state line west of Missoula, Montana. The deposit, which narrows with depth, consists of a peripheral quartz zone surrounding a core containing ferroan calcite, ankerite, fluorite, and quartz with minor parisite, xenotime, pyrite, and gersdorffite. Two stages of mineralization are recognized, each preceded and/or accompanied by brecciation.Isotope geochemistry of the deposit is consistent with a hydrothermal origin: the delta 18 O ranges from 5.6 to 16.5 per mil; the range of carbonate delta 13 C from -4.5 to -7.6 per mil is permissive evidence of a deep source; the 87 Sr/ 86 Sr of fluorite and carbonates range from 0.8125 to 0.8970 (Rb/Sr approximately 0) with total Sr between 5 and 42 ppm, suggestive of a highly radiogenic, low total Sr source region; the U-Th-Pb parisite ages (71.1 + or - 1.0 m.y.) indicate emplacement during the Late Cretaceous, probably associated with the intrusion of the nearby Idaho batholith; and the temperatures of formation based on oxygen isotope geothermometry, fluid inclusion studies, and other considerations are in the range of 400 degrees to 500 degrees C.The Snowbird deposit is rich in Th and rare earth elements but is impoverished in U. Hydrothermal fluids probably responsible for the deposit initially contained mainly volatile constituents; the cation content was acquired largely by leaching of either Belt metasediments or pre-Belt schists and gneisses. Snowbird mineralization bears definite resemblances to the Th deposits of the Lemhi Pass area and to late, hydrothermal "carbonatite" dikes associated with the Iron Hill carbonatite.