The secret behind the Copper Country’s native deposits

A unique geological feature made copper mining an integral part of Michigan history in the Keweenaw Peninsula, and this area is also the destination for members attending MNA’s annual Fall Adventure.

The Michigan copper mining boom of the 1800s is often told with photographs and stories. The Keweenaw Peninsula’s history is told with volcanic eruptions and faults.

One billion years old, the peninsula holds a secret—one that Theodore Bornhorst says visitors can only see in a few, specific places. Bornhorst is a professor of economic and engineering geology as well as the A.E. Seaman Mineral Museum director at Michigan Technological University.

Exposed sections of the Keweenaw Fault often look like the rock wall above-Graphic courtesy of Theodore Bornhorst

A clay and broken rock mixture that appears near places like Douglass Houghton Falls and Hungarian Falls is like the spine of a book—it shows what may be there without revealing the full story.

What the mixture isn’t telling is that it is a modern-day remnant of the inactive Keweenaw Fault. If not for this fault, the 6.6 million tons of refined copper produced by mines in the region between 1845 and 1968 would have never been.

The Keweenaw Fault runs from the tip of the Keweenaw Peninsula to the Wisconsin border.

“There has been no movement we know of on the Keweenaw Fault in the past 500 plus million years,” Bornhorst said.

Bornhorst has studied various aspects of the fault for 36 years.

But movement was the key to the fault’s formation.

The Keweenaw Fault was formed by extension of the Mid-Continent Rift. This rift system begins in eastern Kansas, stretching 2,000 kilometers to the western part of Lake Superior before traveling down into Michigan in a southeastern direction.

The rift system was formed by upward moving mantle. Mantle is the solid or partly melted material below the Earth’s surface.

What Bornhorst described as a “mushrooming plume” created “normal faults” when the plume of mantle spread parallel to the crust and on the underside of it, creating a rift.

The mantle created the rift when the crust was stretched and eventually torn apart to form a depression in the middle.

Formation of the rift from which the Keweenaw Fault came began when mantle moved upward, stretching the crust and breaking it- Graphic courtesy of Theodore Bornhorst

“Imagine that you’re taking something that’s rigid and you’re pulling it apart,” Bornhorst said.

A normal fault has one side of rock that moves upward, called the hanging wall. The other side moves downward and is called the footwall.

Lava flows eventually formed eruptions on land, filling the rift and the Keweenaw Fault with volcanic rock. Streams brought additional layers of gravel, silts, sands and mud. But all was to change.

Compression occurred 50 million years after the fault was created when a tectonic plate collided with North America. In a drastic change, the Keweenaw Fault became a reverse fault, in which the footwall slid lower than the hanging wall and volcanic rock from past eruptions and other minerals filling the middle were forced upwards. This created the Keweenaw Peninsula.

But the event helped create something else. Bornhorst wrote in a paper that this event may have created the pathways—fractures, cracks and faults—that allowed copper-carrying fluids to rise.

These fluids began as “hot, mineralizing waters” reaching temperatures of more than 437 degrees Fahrenheit. These waters eventually became ore fluids delivering native copper and other minerals to spaces in the rock filling the rift.

A compression event may have encouraged formation of native copper deposits near the Keweenaw Fault - Graphic courtesy of Theodore Bornhorst

“As they moved upward then they cooled down and had chemical reactions with the other rocks and mixed with other things to make the copper come out of solution,” Bornhorst said.

The compression, which created more pathways, may have helped the fluids to rise by slowly leaving native copper deposits behind.

“This specific situation has happened elsewhere in the world, but the bottom line is never to this extent,” Bornhorst said.

He said a similar example is located in Quebec, where small amounts of native copper are found associated with the Gaspé Peninsula. Despite the similarity, copper deposits formed near the Keweenaw Fault remain the largest.

“We’ve never found any other place that has had this mass of mineable native copper,” Bornhorst said.

That mass of mineable native copper remains the largest on Earth.

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