MICHIGAN’S COPPER DEPOSITS AND MINING

The first mineral boom in the United States occurred in 1843 when people from all over the continent began to chase the copper, silver and iron of Michigan’s upper peninsula. These mines were in commercial operation for more than a hundred years. 
   Michigan has produced over 14 billion pounds of copper since mining began in 1844. Silver is often extracted along with the copper deposits. The copper mined in Michigan help fuel the industrial revolution that occurred in the US, because copper products were needed by the growing nation.  All mining of native metallic copper ceased in the UP in 1969. Currently, Arizona leads the nation in copper production.

   Michigan’s copper deposits were remarkable for their quality and purity. Bands of native copper were contained in outcrops 2 to 8 miles wide and of varying depth. The surface deposits first attracted the notice of Native Americans who dug out the easily accessible chunks and fashioned copper tools and adornments from them. The British and French saw the indigenous pieces but did little to exploit the resource.
    Michigan’s unique contribution to archeology is found in the ancient copper mines of the Lake Superior region. The presence of copper in the rocks of the Keweenaw Peninsula has been known for centuries. Copper artifacts show that long before Europeans arrived, native Americans throughout the upper midwest used the metal for making a variety of ornaments, tools, utensils, and weapons. Much of the copper used by the Native American was "drift copper" — removed from the rocks by the glaciers and dropped randomly in the glacial debris.  The Indians were made aware of the existence of the metal by masses of float copper carried south by the glaciers and left lying on the surface. Sometime in the remote past, an unknown tribe began to mine the native copper in the Upper Peninsula. They dug pits in the ground and separated the copper from the stone by hammering, by the use of wedges, and, possibly, by the use of heat. Thousands of hammers have been found in and about the old pits. Copper from these mines was widely distributed throughout the country, and it is probable that numerous tribes made summer pilgrimages to the Upper Peninsula to get supplies of the precious metal.
    Copper jewelry and amulets worn by the Indians excited the interest of the early white explorers. They learned the Indians had not mined the copper but had found it scattered on the surface west of Pictured Rocks. Explorers from the time of the Jesuits on have searched without success for the "mother lode."   Although the French explorers were told of the copper deposits by the Chippewa Indians in the early 17th century, no mining was attempted until late in the 18th century, and none of the French mining ventures were successful.
    In the early 19th century, settlers moving westward from the eastern United States bypassed the UP because of its reputation as an inhospitable wilderness.   However, favorable attention was drawn to the region by Douglass Houghton’s (images below) 1841 geological report describing the presence of copper on the Keweenaw Peninsula.  Douglass Houghton was Michigan's first state geologist.
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Houghton's untimely death

On 14 May 1846, the Wisconsin steamed into Detroit’s harbor. Its flags hung at half-mast. The remains of Douglass Houghton – Michigan’s first state geologist, a mayor of Detroit, a physician and a pioneer in the development of the Upper Peninsula – were being brought home. Houghton had drowned near Eagle Harbor on the night of 13 October 1845 while making a survey of mineral lands in the Upper Peninsula. He was 36 years old. On 15 May 1846 a throng of mourners, including the entire Michigan House of Representatives, attended Houghton’s funeral. Later, Bela Hubbard, assistant state geologist from 1837 to 1841, would remember, "The peculiar characteristics of one not easily forgotten - his diminutive stature - his keen blue eye - his quick active motions - the strong sense and energy of his words when dealing with matters of science and his indomitable perseverance in carrying out his designs."

 

The first copper mine opened in 1771, and its owner, Alexander Henry, sent the massive Ontonagon Boulder--of pure native copper--to Detroit as evidence of the region’s richness. But speculative activity did not begin in earnest until 1841, hastened by the favorable reports of the first state geologist, Douglas Houghton. These reports, combined with publicity given the Ontonagon Boulder, sparked the first mineral rush in the nation, predating by several years the more famous California gold rush. Between 1843 and 1846, thousands of prospectors arrived from the East with the hope of "striking it rich."
    These events brought a rush of copper-seeking prospectors into the Upper Peninsula in the early 1840s.  Copper mining has been almost continuous in the Keweenaw Peninsula area for the 150 years since then.  During that time over 12 billion pounds of native copper have been mined.   The image below shows pure copper ingots, awaiting shipment.
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Iron and copper are the two most economically important minerals in Michigan.  Michigan produced 5,385,849 tons of copper between the beginning of commercial operations in 1845 and 1969 when several companies ended operations because of labor troubles.
   Both these deposits have formed in Precambrian rocks (older than 600 Ma [million years]), but by two very different mechanisms.  In order to understand the formation mechanisms, and why these minerals occur where they do, we must first know the general geologic "story" of the western UP.
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   The state’s copper deposits extend in a narrow band from the Michigan-Wisconsin border to the tip of the Keweenaw Peninsula and out into Lake Superior. The copper-bearing rocks are late Precambrian and mostly volcanic (basaltic lava flows) and sedimentary in origin. Into the lava flows of Keweenaw, Houghton, and Ontonagon counties percolating hot waters rising from great depths brought copper and silver in solution. As it cooled, the waters filled the fissures and the gas cavities (amygdules) of the lavas (trap rocks) with pure copper and silver, making the greatest deposit of native copper in the world. Copper was also deposited in the cracks in the Copper Harbor conglomerate. Many beautiful minerals, as yet of no economic importance, are also found associated with the copper.

The sequence of diagrams above illustrate the events that occurred in the Lake Superior region, during Keweenawan time. They resulted in the formation of the Lake Superior syncline.


A. The crust cracks, and molten rock wells up from great depth, through these cracks (joints). Lava flows cover the surface.
B. Large parts of the crust subside and form a graben. This subsidence is thought to have occurred due to crustal extension, or thinning, as it pulled apart. Some of the subsidence may have occurred due to the tremendous weight of the lava (up to 12 miles in thickness!) that was now at the surface, and within the graben.
C. Crustal extension and volcanic activity end. Sedimentation begins within the now-sinking graben.
D. Compressional forces, pushing together from the east and west, squeeze and fold the graben, and the lavas and sediments within, into a syncline. Reverse faults occur on each side of the syncline as the rocks slide upward, past each other. Sedimentation of sand and gravel (later these will become sandstones and conglomerates) continues within the Lake Superior syncline.

 

The location of the axis of the Lake Superior syncline is shown below.  The syncline formed as the tremendous weight of the lavas caused the crust of sink and fold in on itself, further facilitating even more lava deposition within the syncline, or rift.

The map above shows the major geologic structures and physiographic regions of the western UP. 

 



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The generalized geologic column of the western UP provides us with many clues about what was happening in this region during Keweenawan time.  The basement rocks, including the quartzites, are the eroded remains of the Penokee Range. 
    On top of them were deposited many thousands of feet of lavas, which poured out of fissures in the crust as it was rifting, or spreading apart.  These rocks are called the Portage Lake volcanics.  The image below, from the Copper Country, shows the polygonal cracking that is so common on hardened lava flows.

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    Above them are sediments laid down after the volcanism stopped.   These sediments, now turned into rocks, were laid down in the rift, by rivers, as fans of sediment coming off the uplands on either side.  Today, the Copper Harbor Conglomerate is known for its range in sediment sizes--from silt to sand and lots of gravel too.  It is one of the main sources of copper ore.  The image below shows the Copper Harbor Conglomerate in a mine shaft.  Note the old wooden pillar in the foreground, the veins of greenish, oxidized copper on the rock itself, and the large rocks and stones that make up the conglomerate.
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    Still higher in the column, the Nonesuch Shale is represented as a silty and clayey deposit laid down in a lake in the still-low-lying rift.  The rocks even retain mud cracks--sure evidence that the sediments were laid down in an intermittent water body.  The Freda and Jacobsville Sandstones have similar origins: they too were laid down by rivers in the rift zone.  However, for some reason the rivers that carried the sediment were now carrying mostly sand, rather than silt (as in the Nonesuch) and gravel/sand mixtures (as in the Cooper Harbor).

In summary, the ore deposits of the Michigan Copper District are of two basic types that differ in mineralogy, host rock type, and possibly in origin. Furthermore, they are separated from one another both geographically and stratigraphically.
    1. Deposits of native copper occur in the tops of basaltic lava flows and in conglomerates interbedded with the basalts in the Portage Lake Lava series of rocks. More than 96% of the native-copper production has come from an area 45 km long, extending from southwest of Painesdale to east of Mohawk. Over 11 billion pounds of metallic copper have been recovered from these native-copper deposits.  When this type of copper occurs in large "chunks" or masses, it is referred to as "Mass Copper" (see image below).
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    2. Deposits of copper sulfides (mainly chalcocite) with smaller amounts of native copper occur in the lower 6 meters of the Nonesuch Shale, which lies just above the Copper Harbor Conglomerate. Minor amounts of copper minerals are found along this contact over a distance of 200 km, but important deposits occur only in the Iron River syncline at White Pine (the White Pine mine is still operating today) and in the Presque Isle syncline on the west side of the Porcupine Mountains. Over a billion pounds of copper have been recovered from the White Pine copper deposit.

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Native Copper Deposits. Michigan’s native-copper deposits are found almost exclusively in the Portage Lake Lavas. The lava "series" consists of over 200 lava flows with 20 interbedded conglomerates, sandstone and shale. The rocks strike parallel to the length of the Keweenaw Peninsula and dip to the northwest beneath Lake Superior. Some lava flows have been traced for 65 km along the surface.

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    The main ore deposits are widespread, thin, blanket-like bodies (as are the lavas) found both in the amygdaloidal "bubbles" in the tops of the lava flows, and in some of the intervening conglomerates.  Note the green, copper-rock vesicles in the ore shown below--there vesicles were originally bubbles in the lava that have been filled with copper.
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Commercial ore deposits are present at only about a dozen levels in the Portage Lake Lavas.
    The copper is believed to have been deposited from hot water solutions rising from great depth along permeable layers in the rock. Because the layers are (and presumably were at the time of ore deposition) tilted down beneath Lake Superior, the solutions evidently rose along the porous tops of lava flows and up through the interbedded conglomerates. The copper carried in solution in the water was deposited in cavities and pore spaces in the rocks due to changes in chemistry and/or cooling of the fluids. Assuming that the solutions were widely dispersed in the rocks, barriers to circulation caused more copper-bearing fluids to pass through certain areas, which probably resulted in greater accumulation of copper at these sites.

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For a continuation of this page (part II of the "copper story"), click here.

Some of the images on this page were taken from an issue of Michigan History magazine, and some were donated by Nate Verhanovitz, a mechanical engineering grad. student at MSU.