Document Type

Honors Thesis

Semester of Graduation

Spring 2026

Abstract

The Boulder Batholith of southwest Montana is a geologically significant locality due to its high concentrations of critical metals like copper, lead, zinc, silver, gold, and molybdenum. The Crazy Sphinx Mine, a locality discovered and named by Bart Cannon, is well known for producing high quality mineral specimens of blue spinel, colorless grossular, and amber vesuvianite. The Crazy Sphinx Mine is interpreted to be a skarn deposit of the Boulder Batholith which is hosted in interbedded limestone and clay mudstone. To date there have been no publications on Crazy Sphinx Mine characterizing the overall mineral assemblage of the skarn deposit. This study aims to analyze a series of representative samples from the Crazy Sphinx Mine in order to establish the overall mineralogy and general geologic history of the deposit. A combination of petrographic analysis, cathodoluminescent imaging, electron probe microanalysis, and powder X-ray diffraction, including clay smear analysis, was used to characterize a general mineral paragenesis defined by a main rock-forming stage, a dominant primary and localized secondary hydrous overprint, and late-stage supergene mineralization. An assemblage of apatite+phlogopite+grossular+diopside+meionite+vesuvianite+Mg-spinel+corundum represents the main rock-forming stage of Crazy Sphinx, while an assemblage of clinozoisite+prehnite+clinochlore+margarite+muscovite+calcite±gahnite-enriched spinel forms a dominant primary overprint, and an assemblage of Ca-rich dravite+Na-rich meionite+Fe-rich clinozoisite+muscovite+calcite a localized secondary overprint, with a late-hydrothermal to supergene assemblage of zeolites+clays+descloizite/mottramite. The distinct cobalt blue color of Crazy Sphinx spinel is attributed to a combination of trace cobalt content and gahnite-enrichment along fractures and rims that is paired with an increase in ferrous and manganese content. Extensive margarite alteration potentially explains the opaque, grey-blue spinel color variety in weathered crystals, but a systematic study is needed to properly distinguish the cause for spinel color variation. The Crazy Sphinx Mine deposit is interpreted to be an aluminous, magnesian-calcic, small, roof pendant, reaction skarn primarily produced through contact metamorphism and hydrothermal alteration by the Unionville granodiorite pluton of the Boulder Batholith.

Awardee Name

Samuel Juneau

Academic Major

Geology

Project Mentor

Matthew Loocke

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