The Corsica Mine at McKinley

The Corsica mine represents a typical natural ore mine. The iron containing ore was enriched (oxidized) to the mineral hematite or goethite, many times in small pockets. In the early days of mine operation a small town would form next to each of these mines, many later to become ghost towns. This mine pit now filled with water has become the water supply for the nearby town of McKinley. Just over 14 million long tons of hematite goethite ore were mined from 1901 to 1967.1 We had permission to enter the pit area and collect samples.

Corsica Mine Pit
The pile at the left of the above photo (west) is overburden - the glacial till and other material that is scraped off and piled up to provide access to the ore. In the photo above right members of the workshop are collecting samples of ore containing rock.  One sample (right) shows the metallic gray hematite ore in layers with jasper, a form of micro crystalline quartz. In many places Mary-Ellen Jasper is noticeable. Mary-Ellen Jasper is the fossil of stromatolites, a form of blue-green algae of the late precambrian era. These fossils are 1.8 to 2 billion years old and are believed to be among the earliest forms of life on earth. The stromatolites would produce oxygen in a marine environment that was rich in ferrous (iron in the +2 oxidation state) ions. The combination of the ferrous iron and the oxygen would cause precipitation of the hematite minerals along with the silica (quartz). Mary-Ellen Jasper has a hardness from 6.5 to 7 (the same as glass - quartz) with colors of red, pink, white, yellow and even green. It occurs commonly in the Biwabik Iron Formation forming bands of iron and chert in the Mesabi Iron Range.

  The stromatolites prokaryotes, cellular organisms that lack a nucleus, are in a classification of plant life called cyanobacteria, capable of photosynthesis. Before the Animikean time the earth's atmosphere did not contain free oxygen. The cyanobacteria created the earth's original atmospheric oxygen which directly led to an explosion of life worldwide. Cyanobacteria built the first "reefs" in what was once a sea by grouping together and forming hardened colonial structures called stromatolites. In the early days of Minnesota iron mining the identification of the fossil layers indicated a formation of rich iron ore close by. The quartz rich rock as much as 15 feet thick had to be first removed before the rich iron ore could be mined. The quartz also caused problems with the refinement process of the ore into steel. the quartz would melt and form slag in the blast furnace that then had to be chiseled out by hand.

    About 600 million years ago snails evolved in marine environments. Snails would make fast food of the algae causing a near extinction of stromatolites. A few stromatolite reefs still exist, the most famous in western Australia where the high salt content of Shark Bay prevent snails from invading.  Lake living stromatolites in Minnesota resemble coral like growths of up to a foot in diameter on rocks.  They are usually found in 10 to 20 feet of water, often in sandy locations. The location of the stromatolite containing lakes in Minnesota is kept a secret in large part to protect their fragile formation. If you see living stromatolites the Potamac Museum Group would like a report of the exact location.2

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1. MN DNR Minerals Education Workshop Field Trip 2003
2.  Kramer, Joe The Minnesota Volunteer, July-August 1996