MODELING THE HYDROTHERMAL METAMORPHISM OF THE PORTAGE LAKE VOLCANIC SERIES, NORTHERN MICHIGAN

Hydrothermal metamorphism is responsible for many of the world’s economic mineral deposits as subsurface fluids that physicochemically alter lithic bodies at elevated pressures and temperatures. Since many of these complex fluid systems are now dormant, the most direct way to assess their paleocharacteristics is through the remnant mineralogical compositions of the altered rocks. A petrologic model is developed to assess the timeline of hydrothermal alteration in the Portage Lake Volcanics series during the Proterozoic Eon, where continental flood basalts and interbedded conglomerates were metamorphosed around one billion years ago to include a variety of mineral deposits, including abundant native copper. Mineral identities, optical properties, and microtextures are examined from six samples from the Calumet and Hecla mines in Michigan, and one sample from Minnesota, using petrographic microscopy, and for the first time micro-Raman spectroscopy is used to confirm mineral identities in this series. The results yield a three-stage paragenetic sequence during hydrothermal history, identified by distinct equilibrium mineral assemblages. Grain size distributions depict the kinetic evolution of this system, suggesting that temperatures declined over time. Chemical changes are inferred from the composite minerals to include an increase in X_Fe3+, followed by an equilibrium shift in favor of calcite deposition. The model presents a framework for future experiments and calculations to precisely describe the heat flux, reaction kinetics, and volatile transport in this ancient hydrothermal system.