120-1 Evaluating magma flow controls on Ni-Cu-PGE enrichment in the South Kawishiwi Intrusion, Duluth Complex, MN, using anisotropy of magnetic susceptibility

Abstract:

Magmatic Ni-Cu-PGE deposits form through complex interactions between magma transport dynamics and sulfide segregation, yet the physical flow regimes governing ore genesis in layered mafic intrusions remain difficult to constrain directly. The South Kawishiwi Intrusion (SKI), part of the Duluth Complex, is an ideal location to advance our understanding of this problem, as it hosts Ni-Cu-PGE mineralization concentrated along its basal mineralized zone (BMZ). While prior work in the region has proposed that mineralization style reflects differences in magma transport dynamics, the internal flow architecture of the intrusion has not been systematically evaluated. This study applies anisotropy of magnetic susceptibility (AMS) as a petrofabric proxy to test whether magnetic fabrics preserve primary magmatic flow signatures associated with sulfide enrichment, offering a broader methodological framework applicable to layered intrusions elsewhere.

Low-field AMS measurements were obtained from 32 oriented samples along two structural transects spanning previously characterized mineralized and non-mineralized zones. Bulk susceptibility (Km), corrected anisotropy degree (Pj), shape parameter (T), and principal susceptibility orientations (K1, K2, K3) were compared across non-mineralized, open-style, and confined-style mineralized samples. Results show elevated bulk susceptibility in mineralized zones, with confined-style samples exhibiting the highest mean Km, stronger anisotropy, and a shift toward triaxial-to-prolate fabrics, in contrast to the weaker, oblate fabrics characteristic of open-style and non-mineralized samples. Principal susceptibility orientations differ systematically between domains: non-mineralized samples show steep, near-vertical K3 axes consistent with vertical compaction, whereas mineralized samples exhibit shallower K1 inclinations, indicating a shift toward laterally organized flow within mineralized zones, with open- and confined-style domains further distinguished by contrasting K1 declinations. These patterns indicate that magnetic fabrics record primary magmatic transport rather than post-emplacement deformation.

These findings support a genetic model in which confined-style mineralization develops under higher-velocity, spatially restricted flow conditions that promote sulfide-silicate interaction and metal upgrading, whereas open-style mineralization reflects lower-energy emplacement. Despite constraints on sample density relative to intrusion scale, this dataset spans a substantial spatial extent and establishes AMS as a robust, quantitative tool for linking magma flow dynamics to sulfide distribution in layered mafic systems more broadly. These results underscore the utility of magnetic fabric analysis as a diagnostic tool for reconstructing ore-forming processes across magmatic Ni-Cu-PGE systems worldwide.

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

doi: 10.1130/abs/2026AM-21229

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