CONSTRAINTS ON THE STRUCTURAL EMPLACEMENT OF THE NOKOMIS CuNi-PGE DEPOSIT PROVIDED BY ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS)

The Nokomis Deposit is a mineralized zone in northeastern Minnesota enriched in copper, nickel, and platinum group elements. The deposit formed in association with the Layered Series of the Duluth Complex roughly 1.1 billion years ago as a part of the North American Midcontinent Rift System. An integrated geophysical and geologic model for the emplacement of the Nokomis Deposit is essential for the economic development of the site. This study reports rock magnetic data that (1) constrains the emplacement history of the igneous intrusions comprising the Nokomis Deposit and (2) improves the interpretation of aeromagnetic survey data.

Measurements of anisotropy of magnetic susceptibility (AMS) quantify the magnetic mineral fabric in the intrusion, which allows researchers to determine the direction of magmatic flow. AMS results indicate that magma was emplaced from the NE towards the SW along the strike of the Nickel Lake Macrodike (NLMD). Magnetic fabric becomes gradually more scattered after the magma exits the NLMD and enters the South Kawishiwi Intrusion (SKI), consistent with a more turbulent style of flow generated by a decrease in the velocity of magma and a concomitant increase in the Reynolds number.

Aeromagnetic surveys often reveal complex features near mineralized deposits. A rock’s “Q-value” is a ratio of its remanent and induced magnetizations, and is a means for “ground-truthing” aeromagnetic surveys and magnetic anomalies. Here we show how measurements of a unit’s mean Q-value and its standard deviation can be used to better interpret aeromagnetic anomalies associated with the Nokomis deposit.

The geophysical results presented here are consistent with the model of Peterson (2001), who proposed that sulfide-rich fluids traveled through a rift-parallel fault (the NLMD) before emptying into the magma chamber of the South Kawishiwi (SKI) intrusion, where the sulfides precipitated along the basal contact of the Duluth Complex. Subsequent emplacement of the Bald Eagle Intrusion (BEI) represents the final intrusive stage of the Nokomis Deposit.