A rock and paleomagnetic characterization of the Duluth complex layered series intrusions associated with the Nokomis Deposit in NE Minnesota.

Abstract

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 mineral foliation. 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.