Sediment-Hosted Copper Deposits in the Lake Superior Portion of the Midcontinent Rift System

Science Center Objects

This project is focused on the sediment-hosted copper deposits of the Midcontinent Rift, with three main objectives: 1) describe the ore mineralogy and geochemistry of the White Pine and Copperwood deposits, 2) evaluate whether hydrothermal alteration in footwall rocks can be used as a vector towards ore, and 3) provide a careful scoping study to evaluate whether unconventional platinum-group element (PGE) resources may be associated with sediment-hosted copper systems in the Midcontinent Rift.

geologic map Lake Superior region

Generalized geologic map of the western Lake Superior region, showing igneous and sedimentary rock types, selected geologic structures and mines.

(Public domain.)

Scientific Issue and Relevance

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 the environmental impacts of past and future mineral resource development in the region.

Methodology to Address Issue

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?
carbonate-argillaceous laminite lithofacies photos

Photomicrographs and cathodoluminescence (CL) images of the carbonate-argillaceous laminite lithofacies, Nonesuch Formation, Wisconsin.

Figure 8 from Stewart, E.K., and Mauk, J.L., 2017, https://doi.org/10.1016/j.precamres.2017.03.023

(Public domain.)

Ore mineralogy and genesis of sediment-hosted copper deposits: 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.

Alteration accompanying mobilization and transport of metals to sediment-hosted copper deposits: 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.

Unconventional platinum-group elements (PGE) in sediment-hosted copper deposits: We have compiled and reviewed studies of the Kupferschiefer deposits to document the location of gold (Au) and platinum-group element (PGE) mineralization in those deposits. The resulting information is being used to target lithostratigraphic horizons that are mostly likely to contain these elements in the Midcontinent Rift region. Geochemical analyses of samples from these intervals will be used to test whether Au and PGE mineralization is associated with sediment-hosted copper deposits in the Midcontinent Rift.

 

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