COPPER DEPOSITS OF THE MIDCONTINENT RIFT IN MICHIGAN

Since 1840, Midcontinent rift igneous and sedimentary rocks (~1.1 to 1.0 Ga) in Michigan have been subjected to a wide variety of geologic studies because of potential for mineral resources. The rift rocks in Michigan are notable for having already yielded about 8 billion kg of refined copper. The earliest concentration of copper in Michigan Keweenawan rift rocks is represented by a recently discovered (not yet studied in detail) small high-grade magmatic nickel copper sulfide deposit in a Keweenawan dike complex in Marquette County. At the giant White Pine mine, the deposition of chalcocite occurred during diagenesis of post-magmatic rift-filling sedimentary rocks. Subsequently, regional metamorphic/hydrothermal ore fluids generated from buried basalts were released late in the history of the rift (~1.05 Ga) during a major compressional event. This event provided pathways for upward migration and focusing of ore fluids to the sites of copper deposition at shallow depths. These 200-225^oC CaCl-rich and extremely low sulfur fluids resulted in the precipitation of copper and a wide variety of associated non-metallic minerals that was likely controlled by a combination of cooling, fluid-rock reactions, and fluid mixing. This resulted in the unique world-class copper deposits of the Keweenaw Peninsula composed of > 99 % native copper (plus native silver). These same fluids also deposited significant amounts of native copper at White Pine that is younger than the diagenetic chalcocite and small amounts of native copper elsewhere in the rift. There are small amounts of late chalcocite that cross-cuts the native copper deposits of the Keweenaw Peninsula and multiple small, yet to be mined, chalcocite deposits. After deposition of copper ores and rift rocks (~ 1.0 Ga), there was sufficient erosion during Precambrian time to expose the copper deposits at the surface nearly in the same position as they are today. Supergene alteration during the Precambrian prior to deposition of Paleozoic sediments resulted in irregular deposition of the copper minerals cuprite, tenorite, malachite, and chrysocolla. Much later Pleistocene glaciers removed the Paleozoic cover from the western Upper Peninsula of Michigan, again exposing the rift rocks that allowed humans access to impressive quantities of copper.