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Michigan’s Massive Copper Boulders

In the early 17th century, fur traders traversing Lake Superior in North America heard tales of a fabulous boulder lying on the banks of the Ontonagon River. The boulder was said to be five tones in weight and as large as a house. And it was made of solid copper.

Stories about such a prize lying unclaimed in the wild set off many prospectors in the hunt, and it wasn’t long before the boulder was located. It really was made of solid copper. Curiously, no effort was made to relocate the treasure until nearly two centuries later. In 1766, when trader Alexander Henry laid eyes on the rock he was so excited that he grossly overestimated the weight of the boulder to be ten tons. Henry reported that the copper was so pure and malleable that he was able to easily remove a large piece.

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A 19-ton piece of native copper displayed at the A. E. Seaman Mineral Museum of Michigan Technological University. Photo credit: Michigan Technological University

Intrigued by these wild rumors, geologists Henry R. Schoolcraft set forth in search of the legendary boulder and when he found it in 1819, he was sorely disappointed by its size. It was far smaller than legends claimed it to be. Schoolcraft estimated the weight of the copper boulder, now famously known as the Ontonagon Boulder, to be only about a thousand kilograms, or one ton. Schoolcraft also erred in his estimate, towards the lower end of the scale. The actual weight of the Ontonagon Boulder was later found to be about 1,680 kilograms.

To make a long story short, a prospector from Detroit called Julius Eldred bought the boulder, first from the natives and then again from the federal government, and after much effort dragged the boulder through the woods, down the river on a raft and then on a ship to Detroit where he proudly displayed his find to the public. After a legal battle against the federal government regarding ownership of the boulder, the government was forced to pay Elder back more than four times he paid when he bought the boulder from the government. The Ontonagon Boulder is now at the National Museum of Natural History, in Washington, although not in display.

Float Copper

Massive copper boulders like the Ontonagon Boulder once lay scattered all over Keweenaw Peninsula in northernmost Michigan. Keweenaw Peninsula that projects into Lake Superior was once covered by glaciers as much as two miles thick until the ice started to melt about 10,000 years ago. As the ice glided across the rocky surface, it sliced pieces of native copper from lodes or veins within the rock and carried them along with the glacial ice. As this happened, the copper slices from veins within the lavas and conglomerates were occasionally freed from their surrounding rock, forming a “floating” piece of nearly pure copper. When the glacier finally melted away these “floating” masses of native copper was left abandoned and dispersed all around upper Michigan. Until recently, a nugget of floating copper, described as “the world’s largest float copper” lay in the woods before it was acquired by an industrialist. That copper boulder (pictured below) weights about 28 tons and is more than 90% copper.

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Much of this copper was laid down 1 billion years ago by ancient lava flows, producing the earth’s only strata where large-scale economically recoverable 97 percent pure native copper is found. Then, around 2.6 million years ago, glaciation started, and this repeated cycle of glaciation and thawing eroded the overlying rocks down to the level of the native copper ore allowing subsequent glaciation to remove the masses of native copper from the bedrock and entrain in the moving glacial ice. This process created not only nuggets of floating copper but also exposed valuable deposits of copper that were to be discovered and mined during the past few hundred years.

Native Americans started exploiting this natural resource as early as seven thousand years ago. By 3000 BC, copper was used for making spear points and axe heads and knife blades, and to decorative ornaments. The native copper nuggets were so pure that copper tools were directly fashioned out of the rock by pounding the copper pieces into shape. It was only much later that the indigenous people learned the tricks of smelting to produce more refined copper.

Industrial mining began in the mid-19th century and for the next hundred years there was a “copper boom” in Northern Michigan. Almost all surface copper nuggets were exploited during this period and were sent to the smelters to produce pure copper metal.

Today a handful of these specimens, ranging from sizes that fit in your hand to as large as a small car, can be found at various locations around the American state.

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A 4,260 kilogram float copper at Calumet in Michigan. Photo credit: www.keweenaw.info

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Another float copper monument at Michigan Technological University. Photo credit: Michigan Technological University

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A piece of float copper sliced and polished as a bookend shows how pure the copper in the ore is. Photo credit: www.turnstone.ca

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