Date of Award

Spring 5-8-2025

Degree Type

Thesis

Degree Name

Master of Science in Geoscience: Geology Option

Department

Geosciences

Committee Chair

Dr. Christopher Gammons

First Advisor

Dr. Adrian Van Rythoven

Second Advisor

Gary Wyss

Abstract

The Crystal Mountain fluorite mine is located in the Sapphire Mountains of the Bitterroot National Forest, near Darby, Montana. Crystal Mountain was a significant US producer of high-purity fluorspar between 1952 and 1972. As well, the deposit is rich in yttrium (Y) and scandium (Sc), and could possibly be a domestic source of these critical metals. The ore occurred as lodes of massive fluorite up to 10 m thick and 100 m long (now mined out), with few other associated minerals. Recent studies have speculated that Crystal Mountain may be a rare example of a magmatic fluorspar deposit. The main objective of this thesis is to use modern methods of geochemical and mineralogical analysis to better understand the origin of the Crystal Mountain fluorite deposit with special focus on the distribution of scandium and rare earth elements in the deposit. Geologic maps were made showing the distribution of fluorite and other rock types at the main Crystal Mountain open pit, and several outlying areas. Host rocks include late Cretaceous granite, amphibolite, a quartz-clinopyroxene-amphibole (“QCA”) rock, and migmatite. Hand samples were sent to a commercial laboratory for bulk rock geochemistry. Thin sections and polished plugs were examined by optical microscopy, scanning electron microscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The QCA rock is an unusual rock with igneous texture that is rich in clinopyroxene, hornblende, actinolite, biotite, fluorite, apatite, titanite, magnetite, smoky quartz, and the rare scandium mineral, thortveitite (Sc2Si2O7). The migmatite also contains thortveitite. Thortveitite grains locally contain microscopic inclusions of thorite, titanite, parisite, and other REE minerals. Clinopyroxene and amphibole coexisting with thortveitite in the QCA rock contain up to 1-2 wt% Sc2O3, whereas mafic minerals in the amphibolite have low Sc content. Thortveitite is enriched in Y and the heavy rare earth elements (REE). Fluorite is also enriched in Y, although not enough to be of economic interest. All minerals present, including fluorite, show a strong europium (Eu) anomaly in their REE profiles, suggesting a magmatic origin. It is proposed that the Crystal Mountain deposit formed as a result of a parental magma source separating into two immiscible melts: a felsic-silicate melt and a mafic-fluorosilicate melt. The felsic melt formed the granite, while the mafic melt formed the fluorite lodes and the Sc-rich QCA rock. In situ, LA-ICP-MS analyses of titanite and apatite from the QCA rock gave U-Pb ages of 71.2-73.6 Ma and 61.2-64.3 Ma, respectively. An additional date of 61.7 Ma was obtained using 40Ar/39Ar dating of biotite from a fluorite-rich migmatite. The older titanite ages are thought to be most representative of the timing of formation of the Crystal Mountain deposit, and overlap with emplacement of the metaluminous suite of granitoid rocks in the Bitterroot Lobe of the Idaho Batholith. The younger ages for apatite and biotite are explained by isotopic resetting during emplacement of Paleocene, two-mica granites in the Sapphire Mountains. Overall, the results of this study indicate a late Cretaceous age for the fluorite-Sc-Y mineralization at Crystal Mountain.

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