Platinum-Group Elements in Layered Intrusions Associated with the Midcontinent Rift

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Platinum-group elements (PGE) are critical commodities. They are essential for the U.S. economy; however the U.S. imports about 90 percent of the platinum-group elements that we need to use. The project objective is to assess the Duluth Complex and related mafic-ultramafic intrusions in the 1.1 Ga Midcontinent Rift System in the Lake Superior region. Our current activities focus on a Keweenawan igneous intrusion to determine how much platinum-group elements are present, how much could be found, and how it formed.

core warehouse

Core warehouse in Ely, Minnesota. (Credit: Michael Zientek, U.S. Geological Survey. Public domain.)

Scientific Issue and Relevance

Platinum-group elements (PGE) are critical commodities. They are essential for the U.S. economy; however the U.S. imports about 90 percent of the platinum-group elements that we need to use. There are no available substitutes.

Previous work done in the U.S. shows that the areas with the greatest potential for these commodities are in south-central Montana (Stillwater Complex) and the Great Lakes Region (igneous rocks associated with Keweenawan magmatic event). Igneous rocks associated with the Keweenawan large-igneous province cover more than 160,000 km2 and include flood basalts, mafic intrusions, and minor rhyolite lava flows. Platinum-group elements occur in magmatic contact-type deposits found in two layered intrusions that crop out along the western margin of the Mesoproterozoic Duluth Complex, Minnesota. The Duluth Complex, related to Keweenawan large-igneous province and the Midcontinent Rift, is composed of several discrete intrusions formed from mafic to felsic magmas that were emplaced between 1,108 and 1,098 Ma into older rocks. Contact-type mineralization of interest (few percent of sulfide minerals) is found in the South Kawishiwi and the Partridge River intrusions of the Duluth Complex.

Methodology to Address Issue

The objective of this project is to assess the Duluth Complex and related
mafic-ultramafic intrusions in the 1.1 Ga Midcontinent Rift
System in the Lake Superior region that will (1) highlight the PGE-mineral
resource potential for this region and its importance to the
Nation, and (2) identify areas and topics for future research initiatives
that will serve the national interest. The assessment will put the current
estimates of mineral inventory in a broader context.

Our current activities focus on the Keweenawan igneous intrusion (South Kawsishiwi, MN) to determine how much platinum-group elements are present, how much could be found, and how it formed. Our objective is to study the magmatic processes that lead to the formation of compositionally zoned contact-type deposits in the South Kawishiwi intrusion and to probabilistically estimate the amount of undiscovered resource near the identified mineral occurrences. Our main tasks are listed below.

  1. Lithogeochemical and sulfur isotopic studies of the Maturi and Birch Lake mineral occurrences, South Kawishiwi Intrusion, Minnesota
  2. Gaussian geostatistical simulation of contact-type mineralization in the South Kawishiwi Intrusion, Minnesota

Assessment of the Platinum-Group Element Resources in Mafic-Ultramafic Igneous Intrusions of the Midcontinent Rift System, Lake Superior Region: An earlier phase (2012-2016) of our project focused on conducting and publishing an assessment of the Duluth Complex and related mafic-ultramafic intrusions in the 1.1 Ga Midcontinent Rift System in the Lake Superior region. The research was conducted in coordination with the Minnesota Geological Survey, the Minnesota Department of Natural Resources, the Natural Resources Research Institute, academia, and industry.

Our work focused on:

  • preparing deposit models for reef-type and contact-type platinum-group element (PGE) deposits,
  • preparing the economic filter that will be used with the assessment,
  • acquiring and compiling publicly available data for the Duluth Complex and related mafic and ultramafic intrusions,
  • establishing relationships with partners, and
  • evaluating and selecting software to use for 3-D visualization and geostatistical resource estimation.
scientists examining core

USGS scientists Josh Coyan and Heather Parks examining drill core through the Maturi deposit. (Credit: Michael Zientek, U.S. Geological Survey. Public domain.)

drill core

Textures of rock in drill core. (Credit: Michael Zientek, U.S. Geological Survey. Public domain.)