Date of Award

Spring 2022

Degree Type

Publishable Paper

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair

Chris Gammons

First Advisor

Kaleb Scarberry

Second Advisor

Gary Wyss

Third Advisor

Jesse Mosolf

Abstract

This study describes the style and timing of volcanic activity and mineralization in the East Dome Complex (EDC) of the 55-km2 Eocene-Oligocene Hog Heaven Volcanic Field (~36-30 Ma; HHVF) located in northwestern Montana. The HHVF consists of three NE-aligned, predominantly high-K rhyodacitic dome complexes (1.8-4.9 wt. % K2O; 63.2-72.9 wt. % SiO2) formed near the crest the Purcell Anticlinorium, a major Cordilleran fold-thrust belt structure deforming metasedimentary rocks of the Mesoproterozoic Belt Supergroup. Dome emplacement was controlled by a NE-striking fault associated with a pull-apart basin formed between two overlapping, NW-trending dextral oblique-slip faults. The objective of this study is to describe the volcanism and mineralization in the EDC with new 1:12,000-scale geologic mapping, bulk geochemical data, U-Pb zircon ages, and a new 40Ar/39Ar date for hydrothermal alunite. The EDC consists of an 18.3 km2 vent complex cut by a 0.64 km2 maar-diatreme. Silicified breccia, tuff rings, and moat deposits mark the contact between the EDC and Mesoproterozoic Belt Supergroup rocks to define the larger vent complex, which is interpreted as a caldera. New U-Pb zircon ages for EDC rhyodacite domes and flows within and adjacent to the caldera vent range in age from about 35 to 32 Ma. The maar diatreme cuts a 31.1 Ma ignimbrite recognized throughout HHVF. Maar-diatreme vent breccia contains sulfide and volcanic clasts of the EDC rhyodacite and quartzite and argillite from the Revett formation of the Belt Supergroup. A vuggy silica alteration halo encloses the diatreme in plan view. HHVF mineral deposits have unusually high Ag:Au compared to most high-sulfidation epithermal systems worldwide. The only mining district in the HHVF is centered on the EDC where zoned, polymetallic base and precious metal (Ag-Pb-Zn-Cu-Au) mineralization occurs in lode veins, stratabound deposits, and disseminated in the diatreme breccia. This study reports a new 40Ar/39Ar age of about 32.0 Ma for alunite in the maar-diatreme. This new age coupled with a previously reported K-Ar age of 30 Ma for alunite in altered rock near the EDC suggests that the mineral deposits formed before, during, and after the caldera-forming ignimbrite eruption at 31.1 Ma. The style and composition of magmatism and mineralization within the HHVF may reflect an advanced stage of crustal extension in northwestern Montana ca. 36-30 Ma.

Comments

A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science in Geo-sciences.

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