Myths and Facts about Ancient Mining: Susan Martin
Wonderful Power: Susan Martin
Timeline of Michigan Mining History--NPS
Virtual Field Trip--Michigan Cu District
Mineral Land District, by Sean Gohman
Keweenaw Mining
There is a rich lore about ancient copper mining in the Lake Superior region. Most of it is highly speculative and is unsupported, but fervently believed. There is no doubt that copper was mined at Isle Royale thousands of years ago. Most likely this was done by indigenous miners who traded the copper with people who lived far to the south. These early mines found native copper in veins at the surface. They left behind pits and dumps.
Mining by Europeans started in the 1800s on both the Keweenaw and Isle Royale. The Isle Royale mines were all marginal efforts and did not last more than a few years.
The Mid-continent rift is the most important and notable location on Earth for native copper. This is truly a cosmic oddity, because copper in nature is typically found as a sulfide. Indeed Goldschmidt classified copper with a group of elements called “chalcophile”. So why does copper occur in the Midcontinent rift as native copper? This is a major puzzle.
Another explanation of sulfur loss is that loss of sulfur through degassing of magma from magma oceans would be facilitated by the ponding and long solidification times. Awareness of sulfur emissions from eruptions is heightened by recent studies of eruptions and climate. Could extensive degassing during Keweenawan rifting play a role in eventually forming native copper ore deposits? Speculation!
Keweenawan native copper deposits seem to be associated with widespread hydrothermally (hot water) -induced zeolite and prehnite-pumpellyite facies metamorphism (Stoiber & Davidson, 1959; Jolly, 1974) which mineralized the permeable lava flow tops and sediment layers of the Portage Lake Volcanics, apparently about 30 ma after the rift volcanism, during the period of Grenville induced deformation of the rift syncline (Bornhorst & Barron, 2011; Nicholson et al., 1997 ,Cannon, 1994; Bornhorst et al., 1988) when there was faulting of the rift which enhanced fluid flow within the syncline.
Archeological Copper from the Keweenaw
K Schulz, USGS
Sulfur lost to atmosphere from Pinatubo eruption in 1991
AJ Prata
NATIVE COPPER VEIN ON WASHINGTON ISLAND, ISLE ROYALE NATIONAL PARK. THIS VIEW MAY BE LIKE WHAT NATIVE AMERICANS FOUND WHEN THEY FIRST VISITED THE COPPER COUNTRY. SUCH OCCURRENCES ARE NOT COMMON ANYMORE--THEY WERE DUG OUT OF THE WAVE WASHED SHORELINES.
Justin Olson
Copper Mineralization is concentrated in a few areas along the Keweenaw Peninsula. For native copper, the area is mainly the Houghton-Calumet District.
Bornhorst & Barron, 2011
Bornhorst & Barron, 2011
