FE-TI OXIDE-BEARING ULTRAMAFIC INTRUSIONS OF THE DULUTH COMPLEX, MN: UNRAVELING THE GENESIS OF A DOMESTIC TI RESOURCE

The Duluth Complex hosts numerous types of mineral deposits, including Cu-Ni, PGE, Mn, Ti, and others, yet remains largely undeveloped. Significant Fe-Ti±V mineralization has remained particularly unstudied over the past 20 years and represents one of the United States’ most promising domestic resources of these energy- and infrastructure-critical metals. Fe-Ti±V mineralization is hosted in ~12 Fe-Ti oxide-bearing ultramafic intrusions (known in the literature as OUIs) along the Western Margin. There exists incredibly scarce geochemical data on the OUIs, and the main objective of this project is to unravel the genesis of Fe-Ti oxide-bearing ultramafic intrusions.

Focusing on the Longnose and Titac OUIs, we are using detailed geochemical methods on multiple minerals to gain insight into their origin. Both intrusions show similar complex Fe-Ti oxide textures despite differing proportions of ilmenite and titanomagnetite, pointing to a protracted cooling history from >900 C and deformation that redistributed Ti, Mg, and other trace elements. Olivine is heavily serpentinized in both intrusions, offering a glimpse into hydrothermal alteration that affected the systems. Antigorite is more abundant in Longnose, which indicates a more pervasive higher temperature fluid event (~600 C) compared to Titac. The high abundance of lizardite in both OUIs points to lower temperatures (~400 C) of fluid alteration occurring later. Vanadium appears to be concentrated in magnetite within the serpentinized veins and not in the more abundant titanomagnetite and ilmenite, suggesting that V enrichment occurred as a secondary, remobilization process.

The results thus far point to a similar magmatic origin for Longnose and Titac despite differing intrusion geometries and proximity to the Biwabik Iron Formation, which was hypothesized as a potential source of Fe. Secondary fluid alteration was pervasive in both OUIs, and the fluids were potentially sourced via devolatilization of the Virginia Formation, a metamorphosed slate that forms the footwall to the Duluth Complex nearby. Future modeling work will help us fully unravel the genesis of these important domestic Ti resources.