120-2 Petrophysical Groundwork for Earth MRI Aeromagnetic Interpretation: Rock Magnetic Stratigraphy of a Drill Core through the South Kawishiwi Intrusion, Duluth Complex, Minnesota

Abstract:

Airborne magnetic data acquired across northeastern Minnesota through the USGS Earth Mapping Resources Initiative (Earth MRI) will image the geologic framework of the Midcontinent Rift at unprecedented resolution. Whether bedrock is exposed or beneath patchy glacial cover, magnetic anomalies constrain the subsurface geometry of intrusive complexes and volcanic successions. These include intrusions hosting magmatic Cu-Ni-Co-PGE sulfide and Fe-Ti oxide mineralization. Realizing this potential requires rock property data that remain sparse across the rift's diverse lithologies. In the Duluth Complex, remanent magnetization typically dominates induced magnetization (Koenigsberger ratio Q > 1), so processing and modeling approaches that assume magnetization parallel to the present field will not be successful without constraints on remanence.

As part of hands-on training on state-of-the-art instrumentation at the Institute for Rock Magnetism Summer School, participants characterized a single drill core that intersects the South Kawishiwi intrusion, whose basal zone hosts magmatic Cu-Ni-PGE sulfide mineralization above footwall granitoids of the Archean Giants Range Batholith. Their measurements quantify natural remanent magnetization (NRM), susceptibility, and Q, and constrain magnetic mineralogy, domain state, coercivity, and anisotropy of magnetic susceptibility (AMS).

The main augite troctolite composing most of the intersected intrusion is magnetically consistent, with remanence carried by low-Ti titanomagnetite and Q > 1 throughout. The mineralized basal zone is markedly heterogeneous. Ultramafic horizons—marker units associated with oxide concentrations and elevated PGE values—have the highest NRM intensities in the core and are most strongly remanence-dominated. Hematite becomes the principal carrier in granophyre and oxidized troctolite, while footwall granitoids pair relatively high susceptibility with low Q. Remanence directions and AMS fabrics are consistent with modest post-emplacement tilting.

These data anchor interpretation of the intrusion's aeromagnetic expression: remanence must be incorporated into anomaly models of this mineralized district, and the contrast between the consistent troctolite interior and heterogeneous basal zone is a petrophysical fingerprint for tracing mineralized stratigraphy in the subsurface—a step towards the much larger foundation needed to fully leverage Earth MRI aeromagnetic data across the rift.

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-18636

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