Copper deposits of the western Upper Peninsula of Michigan

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