PRECAMBRIAN SUPERGENE ALTERATION OF NATIVE COPPER DEPOSITS IN THE KEWEENAW PENINSULA, MICHIGAN

The Keweenaw Peninsula within the Midcontinent Rift System is host to a world-class native copper mining district from which about 6 billion kg of refined copper was mined from 1845 to 1968. The principal economic minerals are native copper and native silver, though late veins of copper arsenides and sulfides are locally present. Supergene alteration minerals occur irregularly throughout the district, and include cuprite, tenorite, malachite, chrysocolla, and less commonly azurite and paramelaconite. There are also several copper chlorides of uncertain paragenesis. These minerals occur as coatings on or replacing native copper, fracture fillings, or as euhedral crystals in small pockets in the host basalts, conglomerates and calcite veins. Locally, arsenides may be oxidized to arsenates such as olivenite and tyrolite. In some mines, supergene mineralization has been encountered at depths greater than 1000 feet, well below the depth of the current water table.

The rocks of the Midcontinent Rift System were deposited between ~1.1 - 1.0 Ga, and the native copper deposits were hydrothermally emplaced ~1.05 Ga. After deposition, erosion would have enabled supergene oxidation once sufficient cover was removed to allow penetration of oxygenated groundwater. Erosion ended in late Precambrian time, when the rift rocks were unconformably overlain by Paleozoic sediments, which covered the Keweenaw until removed by Pleistocene glaciers, once again making supergene alteration possible. Since the last glacial retreat, the water table probably has not been significantly lower than it is today. Because most of the major supergene alteration is below the current water table, it follows that it did not form since the glaciers removed the Paleozoic cover. Rather it occurred during the protracted period of erosion in late Precambrian time, when the copper deposits were probably relatively near the surface. Pleistocene glaciation probably did not erode much of the rift rocks in the Keweenaw Peninsula proper, else the supergene altered rocks would also have been removed. Thus, the current surface of the Keweenaw Peninsula is likely near the same stratigraphic level that was present when buried by Paleozoic sediments.