Copperwood Geology

The western part of the Upper Peninsula (UP) of the State of Michigan was the site of copper mining for millennia. The host rocks for the copper, referred to as Keweenawan rocks, were part of the North American Mid-Continent Rift. Rifting, or the spreading of the earth’s crust, began during the Proterozoic era about 1.1 billion years ago. No fewer than 200 lava flows and interbedded conglomerates comprise the Portage Lake Volcanics sequence. After cessation of magmatic activity, over 8,000 metres of sediments infilled the subsiding rift basin. This sedimentary sequence includes the Copper Harbor Conglomerate (red beds), the Nonesuch Formation (lacustrine), and the Freda Sandstone (fluvial). The Keweenaw sequence was then affected by a compressional event characterized by reverse faulting, as well as thrust faulting and folding. The Nonesuch Formation in the vicinity of the White Pine Mine and Copperwood Project was folded during this phase. The Portage Lake Volcanics host the native copper deposits whereas the Nonesuch Formation hosts the chalcocite-rich deposits.

Copperwood is located on the south limb of the Western Syncline. The mineralized zone lies at the base of the Nonesuch Formation and is stratigraphically equivalent to the mineralized zone at the White Pine Mine. At its shallowest, the mineralized zone at Copperwood subcrops under less than 30 metres of unconsolidated glacial sediments, and lies at about 200 metres depth one kilometre to the north.

  Location Map

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Western Syncline Geology

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Stratigraphic Column

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