http://minnesota.publicradio.org/features/
HIBBING, Minn. — After more than a century in which iron mining has played a central role in the economy and culture of northeastern Minnesota, a new kind of mining is poised to join the taconite industry.
Generally known as copper-nickel mining, for the two main metals companies want to extract, the process is hailed for bringing much-needed jobs to the region. But opponents prefer to call it “sulfide mining,” for the kind of ore the metals are found in — and because unearthing sulfide can cause toxic water pollution.
It’s a matter of mere geologic chance that northeast Minnesota could hold world-class deposits of both iron ore and copper and nickel.
Geologists have determined the Iron Range formed in what had been a tropical sea two billion years ago. The Duluth Complex, where most of the copper-nickel deposits lie, took form nearby a billion years after that, when North America tried to split apart near present day Lake Superior.
Those deposits formed when molten rock deep in the earth called magma encountered rocks containing sulfur, said University of Minnesota – Duluth geologist Jim Miller.