Copper deposits of the western Upper Peninsula of Michigan

Abstract

The western Upper Peninsula of Michigan is well known for hosting signifi cant 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 signifi cant quantity of silver. Native copper deposits in this district are stratiform and hosted by tops of rift-fi lling subaerial basaltic lava fl ows and interfl ow coarse clastic sedimentary rocks. These deposits are interpreted to be the result of mineralizing hydrothermal fl uids derived from rift-fi lling 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 fl uids during diagenesis and lithifi cation 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 Copperwood suggest a revival of the mining of copper-dominated deposits in the western Upper Peninsula of Michigan. Bornhorst, T.J., and Barron, R.J., 2011, Copper deposits of the western Upper Peninsula of Michigan, in Miller, J.D., Hudak, G.J., Wittkop, C., and McLaughlin, P.I., eds., [[space for volume title space for volume title space for volume title space for volume title]]: Geological Society of America Field Guide 24, p. 83–99, doi:10.1130/2011.0024(05). For permission to copy, contact editing@geosociety.org. ©2011 The Geological Society of America. All rights reserved. 84 Bornhorst and Barron fl d024-05 1st pgs page 84