The Bald Eagle Intrusion (BEI), Nickel Lake Macrodike (NLMD), and the South
Kawishiwi Intrusion (SKI) are mafic intrusions related to the Layered Series rocks of the
Duluth Complex, which formed approximately 1.1 billion years ago as part of the
Midcontinet Rift. These intrusions represent a complex plumbing system and the
economically important minerals within them are collectively referred to as the Nokomis
Deposit. The model proposed by Peterson, (2001) suggests that magma ascended
through the Virginia Formation via a vertical feeder dike, thereby becoming enriched in
sulfides as the Virginia Formation was consumed. The magma then injected into a riftparallel
fault (the NLMD), where high velocity, constrained unidirectional flow
prevented the sulfides from precipitating out of the melt. The magma then emptied into
the SKI, where the sulfides were eventually deposited along the basal contact. The
adjacent BEI is thought to have formed after the emplacement of the SKI, and represents
the final magmatic pulse through the Nokomis Deposit.
Rock and paleomagnetic methods were used to characterize the emplacement history and the flow directions within these intrusions. Hysteresis measurements, and
petrographic observations indicate that the minerals responsible for the magnetic
properties in the rocks are Pseudo-Single Domain (PSD) magnetite, titano-magnetite, and
to a much lesser degree, pyrrhotite. The anisotropy of magnetic susceptibility (AMS)
indicates that flow within the NLMD was sub-horizontal and unidirectional, parallel to
the strike of the dike, and that flow in the SKI was more turbulent and multidirectional.
AMS scalar parameters also show that the intrusions formed under dynamic conditions, with both fluid mechanical and gravitational processes responsible for the formation of
The characteristic remanent magnetization (ChRM) indicates that NLMD, SKI,
and the BEI all formed at roughly the same time, with a small hiatus between the
formation of the NLMD and the BEI. Q-values indicate that the local magnetic
anomalies are controlled by the natural remanent magnetization, and are not formed due
to induction by the current magnetic field.
University of Minnesota M.S. thesis. June 2012. Major: Earth Sciences. Advisor: Joshua M. Feinberg. 1 computer file (PDF); ix, 63 pages.
Finnes, Evan Michael.
A rock and paleomagnetic characterization of the Duluth complex layered series intrusions associated with the Nokomis Deposit in NE Minnesota..
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