229-18 The Geochemistry, Petrography, and Implications of the Subaerial Rhyolites of Mt. Houghton and Mt. Gratiot, Keweenaw Peninsula, MI

Session: Crustal Petrology (Posters)

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In the Keweenaw Peninsula of Michigan, a group of nine subaerial rhyolite domes make up the majority of the regional topography. Little research has been completed on these rocks due to their small scale and low economic value. Two of these domes, Mt. Houghton (MH) and Mt. Gratiot (MG), are geochemically and mineralogically distinct, yet geographically juxtaposed with each other. MH rhyolites display long, elegant hydrothermal flow bands that are filled with quartz, whereas MG rhyolites display randomized vesicles filled with epidote. Furthermore, the matrix of the MG rhyolites has a greater heterogeneity than those of the MH rhyolites. While quartz and epidote are both secondary minerals in these rocks, they are useful, in combination with whole rock geochemistry, as geochronological markers of hydrothermal alteration events. Results from this project indicate that the two domes experienced different stages of the hydrothermal metamorphic event that deposited the prolific native copper deposits of the Keweenaw. The appearance of sulfur defines the transition from main to late-stage mineral deposition, making it possible to differentiate between the stages. This project demonstrates the potential for using whole-rock geochemistry in conjunction with mineralogy to assist in the determination of structural environments that controlled hydrothermal mineral deposition in different areas of the Keweenaw Peninsula.

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

doi: 10.1130/abs/2025AM-6584

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