New Mexico Mineral Symposium — Abstracts

The Michigan copper country, also known as the Lake Superior native copper district, is located on the sparsely populated, isolated, and scenic Keweenaw Peninsula that juts out into Lake Superior near the western end of Upper Michigan. The area is inhabited by a race of people called Yoopers that speak a strange dialect indigenous to the area. The summer season can be a mixture of cold-mild-hot weather with a short growing season. From mid-spring to mid-summer the Keweenaw "bug season" is in full swing and is an entomologist's paradise with swarms of mosquitoes, black flies, deerflies, and horseflies filling the skies. The Keweenaw winters can be long and cold with lake-effect snow accumulations of 200-300 inches not uncommon.

The Keweenaw Peninsula rests along the arch-shaped Middle Proterozoic midcontinent failed rift system that stretches for more than 1,200 mi in the Great Lakes region. Over a period of geologic time from 1,109 to 1,060 Ma (million years ago) the rift valley was filled with thick sequences of lava flows and intercalated sedimentary beds. The 9,000-15,000-ft-thick volcanic sequence is made up of more than two hundred individual basaltic and andesitic flows that comprise the Portage Lake Volcanics (PLV) of the Keweenawan series.

The main part of the Lake Superior native copper district lies within the band of PLV that forms the south limb of the Lake Superior syncline. Native copper was deposited in fissure veins, amygdaloidal flow tops, and conglomerate beds during the same general period of mineralization. The age of mineralization in each lode is probably the same, so the lodes themselves are contemporaneous even though the flows and conglomerate beds are of differing ages. The age of copper mineralization is about 1,060 to 1,050 Ma and corresponds to the formation of the Keweenaw fault, which produced additional faults and fractures that channeled the movement of ore fluids. Copper mineralization occurred sporadically along the full length of the PLV but is mainly concentrated in a 28-mi-long zone that extends from Painsdale, Michigan, to just past Mohawk, Keweenaw County.

The basaltic lava flows make up about 95% of the PLV by volume, and the seven major amygdaloidal lodes that were mined produced 5,966,608,021 (56.7%) of the 10,525,922,136 pounds of copper that was recovered from 1844 through 1976. The conglomerate lodes produced 4,387,258,454 pounds of copper (41.7%), whereas the early mined fissure veins produced only 172,055,661 pounds (1.6%) of the total PLV production.

The world-renowned Michigan copper district is host to 135 verified mineral species, plus another 12 that have been reported. Mining started in the district in 1844, supplying specimens to mineral collectors and museums until 1995 when the sediment-hosted chalcocite orebody at the White Pine mine ceased operations. It appears that the first collections were assembled by mine captains who were in charge of the various mining operations and who proudly displayed their specimens in "China cabinets." As the district became better known, more collectors became aware of what was coming out of the ground and sought to acquire specimens for their collections. A brisk business in high-grading specimens by bosses and miners to supply collectors' needs developed. This profitable practice continued until just a few years ago when mining came to an end in the district.

Native copper along with native silver occurred as beautiful single and groups of copper crystals in all three of the different types of lodes mined (fissure, amygdaloidal, and conglomerate). Native silver also occurred as crystallized groups and was much sought after throughout the whole period of mining. The world famous multi-colored porcelaneous datolite nodules that occurred in the fissure veins and blanket-type amygdaloidal lodes are still highly prized by collectors. Other crystallized minerals that occurred in the various lodes include quartz, calcite, epidote, prehnite, pumpellyite, analcime, apophyllite, and barite.

Today, all mining is dormant in the district, but collecting continues on the accessible mine dumps that are open to collectors. The heyday of mineral specimen production is long gone, but every year crystallized copper and silver groups along with datolite nodules are still being recovered by mineral collectors.

If you visit the copper country on a collecting trip, a must stop is the Seaman Mineral Museum, located on the campus of Michigan Technological University. The museum has on display the world's finest specimens of copper, silver, datolite nodules, and accessory minerals from the district, and the admission is FREE. On your menu should be dining on the famous Cornish pasty, which is still a staple meal in the copper country.