31-8 Late Cretaceous Magmatism in West-Central Montana: Geologic Mapping of the Crow Creek Falls 7.5' Quadrangle and Insights into the Elkhorn Mountain Volcanics

Session: Best Student Geologic Map Competition (Posters)

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Abstract:

The Elkhorn Mountain Volcanics (EMV) represent the onset of Cretaceous magmatism in Montana. Most researchers interpret the EMV as part of a larger magmatic system associated with subduction of the Farallon plate, but their timing, frequency, volumes, and geochemistry remain poorly understood. These gaps limit our ability to investigate EMV magma genesis, storage, evolution, and eruption progression, as well as its genetic relationship to other regional magmatic bodies and mineral deposits. This study begins to address these questions with detailed remapping of the Crow Creek Falls 7.5’ quadrangle located ~26 km southeast of Helena, Montana along the hinge of the Devils Fence Anticline. This area has a history of base and precious metal mining beginning in the early 20^th century, and hosts a complex interplay of Late Cretaceous sedimentation, volcanism, and shallow intrusions. The goals of this study are to: 1) produce a 1:24,000 scale bedrock geologic map of the quadrangle, supported by airborne radiometric (K-Th-U) and magnetic (AMF) data, 2) refine the stratigraphy of the EMV in this area, and 3) use whole-rock geochemistry and petrography to better understand EMV magmatism.

The quadrangle exposes a Mississippian to Cretaceous carbonate and siliciclastic sequence that is overlain by EMV eruptive products, with both intruded by a variety of Cretaceous hypabyssal sills, dikes, and plugs. Eruptive products of the EMV consist primarily of andesitic to rhyolitic ignimbrites, mafic to intermediate lava flows, lahar deposits, and extensive volcaniclastics. While historically the EMV has been divided into lower, middle, and upper members, we identify at least five stratigraphically distinct volcanic units based on componentry differences such as plagioclase abundance, lithic clast abundance and composition, and fiamme character, as well as four intrusive units based on mineralogy and composition. Preliminary whole-rock geochemical results span ~52-69 wt.% SiO₂ and a Rb vs. Y+Nb tectonic discrimination diagram indicates origin in a volcanic arc for all igneous rocks. Petrography reveals that post-emplacement alteration, present as widespread chloritization, is evident in both the volcanics and intrusions. Widespread hydrothermal alteration of the EMV and related intrusions in the region highlights Late Cretaceous magmatism as a mineral exploration target. This work refines the internal stratigraphy of the eastern exposure of the EMV and highlights the importance of detailed geologic mapping in understanding EMV evolution.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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