Date of Award

Summer 2019

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Christopher Gammons

First Advisor

Larry Smith

Second Advisor

Dick Berg

Abstract

The Crystal Mountain Fluorite Mine, known for its vast amounts of massively grown fluorite and rare accessory minerals including fergusonite (YNbO4) and thortveitite (Sc2Si2O7), is located in the Sapphire Mountains 22 km east of the town of Darby, Montana. This site has been mined previously between the years of 1954 through 1973. The Taber collection of drill core, thin sections, mine maps and miscellaneous reports on Crystal Mountain was recently donated to the Montana Bureau of Mines and Geology for future studies. The purpose of the present study has been to use modern methods of mineral identification and fluid inclusions on these samples to better understand the origin of the deposit. Hand samples collected from drill core and in the field were made into polished sections for analysis by SEM-EDS, Raman spectroscopy, and fluid inclusion microthermometry. The fluorite itself varies from colorless to deep purple, and is strongly enriched in yttrium (average of 1.51 wt% Y). The ore bodies were essentially pure fluorite in shallow-dipping masses up to 7 meters thick and > 40m in diameter. Impurities included silicate minerals (albite, phlogopite, quartz), Fe-oxides, titanite, apatite, thortveitite, sulfide minerals (pyrite, pyrrhotite, chalcopyrite), thorite, fergusonite, allanite, xenotime, monazite, and other REE-rich minerals. Thin sections of country rock show weak K-metasomatism in the form of orthoclase replacing plagioclase and phlogopite replacing amphibole. However, this alteration is subtle and is easily missed by visual inspection of outcrops and hand samples. Near contacts with country rock, fluorite is intergrown with silicate minerals in a texture that appears igneous.

Two populations of fluid inclusions were found in the fluorite. Primary inclusions average 10-15mm in size, are distributed randomly throughout the fluorite, and have homogenization temperatures from 350 to > 500ºC. All primary inclusions contain multiple daughter minerals including halite, sylvite, ankerite, siderite, and quartz. Salinities determined by the temperatures of halite dissolution ranged from 35 to 56 wt% NaCl. Secondary fluid inclusions, which occur along healed fractures in the fluorite, are bigger at an average of 30-50mm, with no daughter minerals. However, these inclusions contain a double bubble at room temperature which indicates a high CO2 content. Based on fluid inclusion geobarometry, the fluorite at Crystal Mountain formed at a minimum pressure of about 3 kbar, which corresponds to a depth of > 10 km.

The above observations suggest that Crystal Mountain is a unique example of a magmatic fluorite deposit formed by crystallization of a fluoro-silicate melt that separated from a granitic melt. This model is consistent with the high temperatures, high pressures, and elevated concentrations of incompatible elements (Y, Nb, Ti, Sc, Th, REE) in the deposit, as well as the geometry of the massive fluorite ore bodies. Similar high temperature and high salinity fluid inclusions have been found in other fluorite deposits of western Montana, including the Snowbird, Spar, and Wilson Gulch deposits. More work is needed to determine the ages of these deposits and how they relate to one another.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science in Geosciences: Geology Option

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