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Location of the Lake Superior portion of the Midcontinent Rift, showing significant orebodies and prospects that are the focus of this study. Image courtesy of Dean Peterson, Duluth Metals.
Location of the Lake Superior portion of the Midcontinent Rift, showing significant orebodies and prospects that are the focus of this study. Image courtesy of Dean Peterson, Duluth Metals.

Why is USGS studying sediment-hosted copper deposits in the Midcontinent Rift region of the U.S.?

The North American Midcontinent Rift System is a major metallogenic feature that contains

  • High-grade nickel-copper-platinum group elements mineralization such as the Eagle mine (Michigan) and the Tamarack prospect (Minnesota)
  • Lower-grade, large tonnage disseminated nickel-copper-platinum group elements prospects in gabbroic to troctolitic sheet-like intrusions of the Duluth complex
  • Native copper deposits of the Keweenaw peninsula, which produced 5 Mt Cu between 1845 and 1968, and were the most important source of copper for the United States until metallurgical advances allowed the development of porphyry copper deposits
  • Sediment-hosted copper mineralization, such as the White Pine and Copperwood deposits, which together contain more than 2.5 Mt copper and more than 1,600 t silver.

This study is part of a comprehensive USGS data acquisition and multidisciplinary research effort to image and characterize the Midcontinent rift and related mineral resources, to document mineral resource potential, and to evaluate mineral environmental impact of past and future mineral resource development in the region.

What is this USGS study about?

Researchers will study the sediment-hosted copper deposits of the Midcontinent Rift to address these questions:

  • Where do economically beneficial (e.g., silver) and environmentally deleterious (e.g., mercury) elements partition in ore samples from the Copperwood deposit?
  • Do the sediment hosted copper deposits in the Midcontinent Rift have large hydrothermal alteration haloes that can help to locate additional deposits, particularly in buried portions of the rift?
  • Do sediment-hosted copper deposits in the Midcontinent Rift contain significant concentrations of platinum-group elements (PGEs) or other strategic or critical elements?


Project Activities

Task 1: Ore Mineralogy and Genesis of Sediment-hosted Copper Deposits

Contact: Jeff Mauk,

Our objectives are to describe the ore mineralogy and geochemistry of the Copperwood deposit. We will use modern analytical methods (petrographic analysis, SEM, microprobe, LA-ICP-MS) to document the silver and mercury in the ore minerals and to test for the presence of other critical and strategic elements in the ore minerals. We will collect whole rock geochemical data from the host rock Nonesuch Formation to test for geochemical fingerprints that can reflect widespread hydrothermal alteration.

Task 2: Alteration Accompanying Mobilization and Transport of Metals to Sediment-hosted Copper Deposits

Contact: Timothy Hayes,

We are evaluating whether hydrothermal alteration in footwall rocks can be used as a vector towards ore. We will use whole rock geochemistry and petrography to test for sodium alteration of the aquifer and (or) source rocks related to copper mobilization and transport through the Copper Harbor Conglomerate by ore fluids en route to the White Pine and Copperwood deposits.

Task 3: Unconventional Platinum-grade Elements (PGE) in Sediment-hosted Copper Deposits

Contact: Cliff Taylor,

We will compile and review studies of the Kupferschiefer deposits to document the location of gold (Au) and platinum-group element (PGE) mineralization in the deposits. The resulting information will be used to determine the lithostratigraphic horizons that are mostly likely to contain these elements in the Midcontinent Rift region and the location where those horizons might be mineralized.



Journal Articles

Gallagher, T.M., Sheldon, N.D., Mauk, J.L., Petersen, S.V., Gueneli, N., and Brocks, J.J., 2017, Constraining the thermal history of the North American Midcontinent Rift System using carbonate clumped isotopes and organic thermal maturity indices: Precambrian Research, 294, p. 53-66, doi:10.1016/j.precamres.2017.03.022.

Stewart, E.K., and Mauk, J.L., 2017, Sedimentology, sequence-stratigraphy, and geochemical variations in the Mesoproterozoic Nonesuch Formation, northern Wisconsin, USA: Precambrian Research, 294, p. 111-132, doi:10.1016/j.precamres.2017.03.023.


Stewart, E.K., and Mauk, J., 2016, Sequence stratigraphy and basin evolution of the Mesoproterozoic Nonesuch Formation, Ashland syncline, northern Wisconsin: 62nd Institute on Lake Superior Geology Proceedings v. 62, Part 1-Program and Abstracts, p. 141-142. View ILSG 2016 abstract book. [PDF file, 13.8 MB]


Project Contact

Jeffrey Mauk
Phone: 303-236-5605
Central Mineral and Environmental Resources Science Center

Timothy Hayes
Phone: 520-670-5024
Geology, Minerals, Energy, and Geophysics Science Center

Mineral Resources Program Science Priority

The use of firm, trade, and brand names is for identification purposes only and does not constitute endorsement by the U.S. government.

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