SERPENTINE MINERALOGY FROM FE-TI OXIDE-BEARING ULTRAMAFIC INTRUSIONS IN THE DULUTH COMPLEX, MN

The Duluth Complex contains a series of mafic intrusions, formed 1.1 Ga as part of the Midcontinent Rift System. Its western margin hosts ~12 Fe-Ti oxide-bearing ultramafic intrusions (OUIs). Little is known about their genesis, though they contain domestic Ti resources. In this study, we use mineralogy of serpentinized olivine to understand their lower temperature alteration. Two OUIs, Titac and Longnose, were studied using many tools to understand serpentine textures and mineralogy, including petrographic microscopy, scanning electron microscopy (SEM), Raman spectroscopy (Raman), X-ray diffraction (XRD), and electron probe microanalysis (EPMA).

Serpentine textures were found from petrographic microscopy and SEM. In Longnose, serpentine forms in parallel veins that cluster in some parts. Interpenetrative and pseudomorphic textures are seen. The olivine experienced heavy alteration, seen by thicker veins. In Titac, the olivine is not as heavily altered, seen by thinner veins and presence of original olivine. Titac serpentine has mesh and interpenetrative textures. Despite less intense alteration, Titac has a few larger serpentine veins. Both OUIs contain magnetite veinlets within the serpentine, though not all veins have them.

Serpentine mineralogy was determined using Raman and XRD. Serpentinization of olivine formed lizardite, antigorite, and magnetite. We found, with Raman, that Titac contained more lizardite than antigorite. Magnetite veinlets were only found in lizardite. Longnose contained more antigorite than Titac, though lizardite dominated. There is also evidence of mixing of the two phases. XRD revealed similar results. Lizardite indicates alteration at a lower temperature (~400 °C) and antigorite indicates alteration at a higher temperature (~600 °C), indicating multiple alteration events.

EPMA was used to measure serpentine compositions. The serpentine was found to be Fe-rich, containing up to ~24 wt% FeO. The mineralogy matched the results of Raman and XRD, seen by the T/(T+M) cation ratios of minerals. These analyses are part of a larger study of the magmatic formation of the OUIs. They will help us understand the last step of formation, when the remaining silicate melt is rich in H2O and exolves the fluid phase, altering the olivine. This can then be used to study similar intrusions.