Abstract

The Snowbird fluorite-rare earth element (REE) deposit, located in western Montana, is unusual in that it is characterized by very coarse grained hydrothermal quartz, calcite, and fluorite. Also, the rare mineral parisite-(Ce) [Ca(Ce,La,Nd,Pr,Y)2(CO3)3F2], which is typically associated with ankerite in the deposit, occurs as exceptionally large (up to 24 cm in length) and abundant crystals. Snowbird is one of a number of ankerite-bearing deposits hosted by the Belt Supergroup metasedimentary rocks that have been linked genetically to mineralization in the Coeur d’Alene district. Fluid inclusions in quartz, fluorite, ankerite, and parisite from the Snowbird deposit were investigated using petrography, microthermometry, bulk leachate, Raman spectroscopy, and gas chromatographic analyses. Based on field and petrographic observations, we conclude that ankerite and parisite are cogenetic and not restricted to a late stage in the development of the deposit, as had been suggested previously. The fluid inclusions most closely associated with parisite and ankerite contain aqueous liquid, one or two carbonic phases, and a halite crystal at room temperature. The fluid from which parisite was deposited was highly saline (33–50 wt % NaCl equiv) and dominated by Na and Ca with Ca/(Na + Ca) = 0.1 to 0.2. The carbonic component was dominated by CO2, with minor CH4 and N2, and the bulk XCO2 was less than or equal to 0.11. The minimum temperature and pressure of parisite deposition is estimated to be 400° to 500°C and 200 to 300 MPa, respectively. Later fluorite was precipitated from similar fluids but with more variable CO2 contents and lower salinities (down to ~29 wt % NaCl equiv). Fluorite deposition was followed by the infiltration of lower salinity aqueous fluids.

Our paragenetic and fluid inclusion observations combined with previously published radiometric age dates, Sr isotope data, and mineral chemistry are most consistent with a model in which the entire Snowbird deposit was formed in the Cretaceous from fluids derived from the Idaho batholith and that, at least partially, equilibrated with the Belt Supergroup metasedimentary rocks. Furthermore, these data suggest that Snowbird is unrelated genetically to the base metal and silver mineralization of the Coeur d’Alene district, which generally have been interpreted to have been formed from dominantly metamorphic fluids.

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