Date of Award
Master of Science (MS)
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.
Coppage, Ethan, "GEOLOGY AND GEOCHRONOLOGY OF THE EAST DOME COMPLEX WITHIN THE HOG HEAVEN VOLCANIC FIELD, FLATHEAD COUNTY, MONTANA" (2022). Graduate Theses & Non-Theses. 278.