Thermal and hydrothermal effects of Proterozoic events in the Archean terrain of northeastern Minnesota, USA

Abstract

This dissertation investigates the thermal and hydrothermal impacts of Proterozoic and younger geologic events on the Archean crust of northeastern Minnesota, USA. The primary objective is to constrain the timing, nature, and extent of the impact of processes that altered the mineralogical, geochemical, and structural characteristics of the craton. Understanding the modification of Archean crust by later geologic events is critical for unraveling cratonic evolution, including the formation of economically significant ore deposits. The study provides insights into the tectonic, thermal and hydrothermal processes that shaped ancient terrains.The research encompasses two projects: the Soudan Project and the Giants Range Batholith (GRB) Project. The Soudan Project employed hematite U-Pb geochronology and (U-Th)/He geochronology to date iron ore formation and explore possible thermal signatures. Characterization included hand sample analyses as well as petrography. Lithogeochemical analyses, including major oxide and trace element data, were evaluated using the isocon method to quantify mass-balance changes during hydrothermal alteration. Iron stable isotope geochemistry unveiled iron mobility and fluid sources. Additionally, the Soudan Geology Trail Project initiative was produced as a collaborative effort to share the unique geology of northeastern Minnesota with the public, students and teachers. The GRB Project utilized petrography of alteration textures and U-Pb dating of zircon and apatite on samples from the GRB-Duluth Complex (DC) contact to assess thermal effects from DC emplacement and other processes. The Soudan Project identified Paleoproterozoic (1.8–1.6 Ga) hydrothermal events that upgraded Archean BIFs to high-grade hematite ore, as well as a ca. 1.1 Ga thermal overprint linked to the Midcontinent Rift System (MRS). Geochemical data revealed significant silica leaching and iron enrichment, with iron isotope signatures indicating hydrothermal fluid origins and iron sources. The Soudan Geology Trail Project characterized samples that were used to interpret the geologic history of the Lake Vermilion-Soudan Underground Mine State Park, and produced information will be implemented in educational material. The GRB Project documented thermal resetting near the GRB-DC contact, with zircon U-Pb dates primarily reflecting crystallization ages, though one sample at approximately 150 m showed complete resetting due to DC emplacement. Apatite U-Pb results confirmed resetting up to approximately 1500 m from the contact. Proterozoic events, including the Penokean, Yavapai, and Mazatzal orogenies, MRS-related magmatism, and younger events in the Phanerozoic, all significantly altered the Archean crust through thermal and hydrothermal processes. The Soudan Project highlights far-field tectonic and rift-related effects, while the GRB Project underscores localized thermal impacts from DC emplacement. Integrating multiple geochronological and geochemical methods enhances our understanding of hydrothermally driven iron ore genesis and thermal evolution of ancient terrains.