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Copper deposits of the western Upper Peninsula of Michigan

By
Theodore J. Bornhorst
Theodore J. Bornhorst
A.E. Seaman Mineral Museum, Michigan Technological University, 1404 E. Sharon Avenue, Houghton, Michigan 49931, USA
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;
Robert J. Barron
Robert J. Barron
Department of Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA
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Published:
January 01, 2011

Abstract

The western Upper Peninsula of Michigan is well known for hosting significant concentrations of copper in copper-dominated deposits. Most of the copper is hosted by rocks of the Mesoproterozoic Midcontinent Rift. Copper deposits in the western Upper Peninsula can be subdivided into two overlapping world-class copper mining districts. The Keweenaw Peninsula native copper district produced 11 billion lbs of copper and a lesser unknown but significant quantity of silver. Native copper deposits in this district are stratiform and hosted by tops of rift-filling subaerial basaltic lava flows and interflow coarse clastic sedimentary rocks. These deposits are interpreted to be the result of mineralizing hydrothermal fluids derived from rift-filling basaltic volcanic rocks that migrated upwards, driven by late Grenvillian compression of the rift some 40–50 million years following cessation of active rifting. The Porcupine Mountains sediment-hosted copper district produced or potentially will produce 5.5 billion lbs of copper and 54 million ounces of silver. These stratiform/stratabound deposits are hosted in rift-related black to gray shale and siltstone and dominated by chalcocite rather than native copper. Chalcocite is interpreted to be the result of introduction of copper-bearing fluids during diagenesis and lithification of host sediments. At the now-closed White Pine Mine, the chalcocite mineralizing event was followed by a second stage of native copper deposition that demonstrates a spatial and temporal overlap of these two world-class mining districts. While these two districts have been dormant since 1996, favorable results from recent exploration at Copper-wood suggest a revival of the mining of copper-dominated deposits in the western Upper Peninsula of Michigan.

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GSA Field Guide

Archean to Anthropocene: Field Guides to the Geology of the Mid-Continent of North America

James D. Miller
James D. Miller
Department of Geological Sciences and Precambrian Research Center University of Minnesota-Duluth Duluth, Minnesota 55812 USA
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George J. Hudak
George J. Hudak
Precambrian Research Center Natural Resources Research Institute 5013 Miller Trunk Highway Duluth, Minnesota 55811 USA
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Chad Wittkop
Chad Wittkop
Department of Chemistry and Geology Minnesota State University Mankato, Minnesota 56001 USA
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Patrick I. McLaughlin
Patrick I. McLaughlin
Wisconsin Geological and Natural History Survey Madison, Wisconsin 53705 USA
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Geological Society of America
Volume
24
ISBN electronic:
9780813756240
Publication date:
January 01, 2011

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