Central Range Mining
Central Iron Range Geology
Over
1 1/2 billion years ago when a vast, shallow sea covered much of the area which
we know today as Northeastern Minnesota. The
great Mesabi Range began in this sea, which
contained concentrations of iron and silica. The iron and silica settled to the
bottom of the sea and formed thick layers of iron-bearing sediments. As time
went on, the sea disappeared leaving these iron-bearing sediments buried under
thousands of feet of sand, clay and mud. As a result of this deep burial, the
iron-bearing sediments were subjected to heat and pressure which transformed
them into a hard, flinty rock which we call taconite. Tacointe consists
primarily of chert, a form of silica, and of magnetite, a black, magnetic iron
mineral.
The Mesabi
Range, largest of three iron ranges in Minnesota,
consists of a thick layer of taconite extending from Babbitt on the east, 110
miles southwest to Grand Rapids.
The taconite varies from one to three miles in width and up to 500 feet in
thickness.
During the
long period since its formation, Mesabi taconite has been subjected to a
variety of geologic processes which have altered its character. Certain areas
of taconite were affected by solutions which dissolved out portions of the
silica, and the black magnetite was converted to red hematite or what we call
"natural" ore. These natural ore mines, scattered throughout taconite
of the Mesabi Range like raisins in a cake, played an important part in Minnesota's history. In
some area, particularly on the western end of the Mesabi
Range, magnetic taconite has been changed to red hematite, leaving
the silica essentially unchanged, forming non-magnetic taconite and what we
call "semi-taconite". However, the great bulk of the Mesabi Range
iron formation remains as hard, magnetic taconite, enough to last hundreds of
years using conventional mining methods.
