IRON MINING

Iron Ore vs "Iron Formations"
    One of the most important events in the economic development of the Lake Superior region was the (natural) conversion of parts of the iron-formations into naturally formed iron ore bodies. The alteration of the original rock to iron ore involved two processes that probably occurred at essentially the same time: (1) oxidation of the iron minerals caused by waters circulating through the rocks (The soluble ferrous iron in the minerals was converted to insoluble hematite, geothite, or limonite). and (2) Continued circulation of water through the rocks gradually dissolved the silica from the iron silicates and the chert and carried it out of the rock. This leaching of silica amounted to removal of nearly 50% of the original volume of the rock. As a result, the ore was very porous and unable to support the weight of overlying rock layers, which slumped into ore bodies. The removal of silica thus allowed an increase in the iron content from an initial 25 to 35% to about 55 to 60% in the ore bodies.  Mining of these ores was initially carried out in open pit mines, as shown below.  The image below is of the Jackson Mine, near Negaunee.

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Later, when the easily-mined ores (those near the surface, see below) were exhausted, deep shaft mines were required.

    The large areal extent of the ore bodies and their close relationship to the present erosion surface indicates that they probably formed by the circulation of groundwater through the rocks. The largest of the natural ore bodies was mined as a large open pit near Hibbing, Minnesota. It has an irregular outline but is nearly 3 miles long, 2 miles wide, and up to 500 feet deep. Nearly 200 million tons of iron ore were removed from this mine over a period of about 50 years.
    Ore bodies in the more deformed iron-formations of the iron ranges on the south shore of Lake Superior have no obvious connection to the surface, and must have been formed by a different mechanism. On the Menominee range, ore bodies formed in troughs created by folded sedimentary layers. Impervious shales form the bottom of the ore bodies. Many of these ore bodies do not extend to the surface and thus the iron-formations are sufficiently deformed so that it was necessary to utilize underground mining methods rather than the large open-pit operations as on the Mesabi (Minnesota) Range. Since underground mines cannot be mechanized as can surface mines, they have been unable to compete economically, and all have closed. 
The first Michigan iron ores to be mined were soft ones (hematites), and they were mined near the surface.

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These ores, however, were soon exhausted, and mines were developed that were 3,000 to 4,000 ft (914 to 1,219 m) deep.
    Mining was a slow, laborious task.  It was undertaken in dark, cold, wet mine shafts.  Like copper mining, iron mining brought many ethnic peoples to the Upper Peninsula, among them Irish, Welsh, Cornish, Italians, Swedes, Danes, Norwegians, Finns, and French Canadians. So many immigrants flocked to the region that the mining counties had the largest foreign population in the Upper Peninsula in the late 1800s: 12,000 in Houghton County, 10,000 in Marquette County, and 8,000 in Gogebic County.

The shaft below looks dark, cold, and wet, but recall that it has had lights installed on the ceiling for tourists; in the 19th century such lights would have been small candles or lanterns--and the whole shaft would have been much dimmer.

The image below shows how rocks were "hand drilled" by pounding a steel stake into the rock. 

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Cars like these (below) were used to haul the ore from the mine to the surface.

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Some of the images and text on this page were taken from various issues of Michigan History magazine.

This material has been compiled for educational use only, and may not be reproduced without permission.  One copy may be printed for personal use.  Please contact Randall Schaetzl (soils@msu.edu) for more information or permissions.