THE CRYSTAL MOUNTAIN FLUORITE (SC,Y) DEPOSIT, SAPPHIRE MOUNTAINS, MONTANA: IGNEOUS OR HYDROTHERMAL?

The Crystal Mountain Mine is located near the south end of the Sapphire Mountains in Ravalli County, Montana. The mine operated between 1954 and 1973, and in its day was one of the larger fluorspar producers in the U.S. The deposit is also noteworthy as being one of the few documented occurrences of thortveitite (Sc₂Si₂O₇) worldwide (Foord et al., 1993, Can. Mineral. 31, 337-346). The regional geology consists of high-grade metasedimentary rocks and amphibolite (probably Belt aged) intruded by several generations of Cretaceous and younger igneous bodies. Although most of the ore is mined out, records show that fluorite formed shallow-dipping, tabular masses up to 100m long and up to 10m thick. A fine-grained, unaltered granite, the predominant rock type in the open pit, locally intruded through the fluorite masses. A poorly-exposed, mafic intrusion rich in apatite, titanite, and magnetite is spatially and genetically (?) associated with the fluorite. Where infused with fluorite, pyroxene in the mafic rock is altered to actinolite + phlogopite, and numerous thortveitite grains up to several mm long are present. The actinolite itself is scandium rich (> 1 wt% Sc), whereas the phlogopite is not. The fluorite ore bodies have a coarse, granoblastic texture, are enriched in Y and heavy REE, and have strong negative Eu anomalies. Fluorite locally contains inclusions of plagioclase, phlogopite, titanite, rutile, fergusonite, thorite, xenotime, and thortveitite. Early, primary (?) fluid inclusions scattered through the fluorite contain multiple daughter minerals, and show total homogenization at T > 500°C. A comparison of the temperatures of halite dissolution and vapor bubble disappearance indicates pressures of trapping > 3 kbar. A later generation of mixed CO₂-H₂O-NaCl fluid inclusions was trapped along healed fractures. The origin of the Crystal Mountain fluorite deposit is enigmatic. One idea is that it formed by separation of a fluorite-rich melt from a mafic, silicate-rich melt at T > 500°C. Another idea is that the fluorite masses were formed at lower temperature by ordinary hydrothermal processes, but were contact metamorphosed during intrusion of the granite. Research is in progress to further evaluate these hypotheses.