Date of Award
Spring 6-8-2026
Document Type
Thesis (Undergraduate)
Department
Earth Sciences
First Advisor
C. Brenhin Keller
Abstract
The New World Mining District (NWMD), located two miles northeast of Cooke City, Montana, is host to a series of epithermal sills, dikes, and plutons, the intrusion of which is responsible for significant Au-Ag-Cu mineralization. These intrusive bodies were emplaced in the Eocene, potentially contemporaneously with the Heart Mountain Detachment - Earth’s largest subaerial landslide, and have been proposed as a potential trigger for the event. The pre-emplacement crys- tallization conditions for the porphyritic dikes in the region, however, remain unknown and have previously been limited to surface observations. Constraining the magmatic conditions of the in- trusives in the region can help in better understanding the origins of such systems. Here, a laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) derived point-grid mineral identification is presented and used to calculate the modal abundance of seven porphyritic dikes and a pluton from the NWMD. A Markov Chain Monte Carlo (MCMC) inversion of Perple_X major and accessory phase pressure-temperature (P-T) pseudosections is then used to model the equilib- rium crystallization conditions of each sample. The equilibrium phenocryst phase assemblages of all porphyritic samples are found to have crystallized between 620 °C and 680 °C and at depths between 27.5 km and 47 km and near emplacement for the plutonic sample, placing porphyritic magma development in the lower to middle crust. Textural evidence, including fine-sieve plagio- clase, primary resorbed epidote, rounded quartz phenocrysts, and thin or absent reaction rims on hornblende phenocrysts, alongside significant regional volcanic brecciation, support high rates of ascent from each porphyry’s respective magma storage to the emplacement depth of ∼5km, where magmas rapidly and explosively devolatilized and stalled. These findings permit consideration of the emplacement of NWMD porphyries as a possible mechanism for the processes implicated in triggering the Heart Mountain Detachment.
Recommended Citation
Gordon, Jacob Jared, "Constraints on Crystallization Conditions from Pseudosection Thermobarometry of the New World Mining District Porphyries near Cooke City, Montana" (2026). Earth Sciences Undergraduate Senior Theses. 8.
https://digitalcommons.dartmouth.edu/earthsciences_senior_theses/8
