Date of Award

Fall 12-12-2025

Degree Type

Thesis

Degree Name

MS Geosciences/Geochemistry

Department

Geosciences

Committee Chair

Christopher H. Gammons

First Advisor

Jarred Zimmerman

Second Advisor

John Kirtley

Third Advisor

John Childs

Abstract

The Ruby Graphite project (RGP, also known as the Crystal Graphite deposit) is located in the southern Ruby Mountains, Beaverhead County. The deposit contains coarse-grained and highly crystalline graphite, which is a sought-after critical commodity in today’s global economy. The graphite is hosted by high-grade meta-sedimentary and meta-igneous rocks that reached peak metamorphism during the ~ 1.7 Ma Big Sky Orogeny. In this study, samples were collected from outcrops, mine dumps, and new (2023) drill core from the Ground Hog and Bird’s Nest locations. Rock types include calcitic marble, biotite-garnet schist, pegmatite, diopside-rich skarn, altered ultramafic intrusions, and a possible banded iron formation. The samples were examined by thin section and reflected light microscopy, SEM-EDS, and Raman spectroscopy. In addition, stable isotope analyses were collected on graphite (δ13C), calcite marble (δ13C and δ18O), and pyrite (δ34S) that is locally intergrown with graphite. This study shows that there are multiple generations of graphite mineralization at the RGP. Much of the graphite is early, and has experienced peak metamorphic temperatures in the range of 790 ± 20ºC (based on Raman geothermometry), consistent with the presence of sillimanite + biotite + garnet (+ graphite) in metasediment and meta-pegmatite. A second generation of graphite formed during retrograde metamorphism at an unknown date. This later graphite is intergrown with lower temperature minerals, such as pyrite, calcite, prehnite, and pumpellyite, and therefore must have formed at temperatures < 300ºC, at shallower depths. The C-isotope compositions of both flake and vein graphite at RGP fall in a fairly narrow range of -5.8 to -9.9 ‰. Several mechanisms are proposed for deposition of the graphite: 1) mixing of CO2-rich and CH4-rich metamorphic fluids; 2) a decrease in the activity of water due to retrograde, rehydration reactions; 3) reduction of CO2 by H2(g) generated during metamorphism of nearby ultramafic intrusions; and 4) reduction of CO2 by conversion of pyrrhotite to pyrite. The possibility that some graphite could have formed by metamorphism of primary organic carbon in the host metasediments cannot be discounted, although there is no direct evidence for this in this study.

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