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.
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Those deposits formed when molten rock deep in the earth called magma encountered rocks containing sulfur, said University of Minnesota - Duluth geologist Jim Miller.
"Sulfur is the great collector of metals in nature," he said. "If it wasn't for sulfur, there would be no economic quantities of copper-nickel to be mined. And so to extract these metals, we're going to have to deal with the sulfur."
Sulfur, specifically a chemical reaction involving sulfur, is at the heart of the controversy over copper-nickel mining -- in Minnesota and elsewhere.
All the miners have to do, he said, is bring sulfide rock up to the surface. Once it is exposed to oxygen in the air, and water, a chemical reaction will occur that creates among other things sulfuric acid -- also called "acid rock drainage."
"It's a very simple reaction," Miller said of the process that can cause severe water pollution. That's why Miller calls sulfur the blessing and the curse in copper-nickel mining. Without it, the metals wouldn't be congregated closely enough together to make mining them economically feasible. But because of that sulfur, mining companies have to manage the acid mine runoff, which he said they historically haven't done well.
"If it wasn't for sulfur, there would be no economic quantities of copper-nickel to be mined. And so to extract these metals, we're going to have to deal with the sulfur."
"You'll see pictures of this orange soupy stuff, in old mine sites, where they haven't accommodated for this reaction, it's run wild," Miller said. "These legacy mines are the bane of the current mining world's existence."
Opponents of copper nickel mining have plastered those pictures on billboards to warn about the risk of this kind of mining.
Kathryn Hoffman, Staff Attorney for the Minnesota Center for Environmental Advocacy, said that acid runoff can release heavy metals that are dangerous to aquatic systems and human health.
"The copper and the nickel themselves are directly toxic to aquatic life," Hoffman said. "But so are many of the other metals that are unearthed, like arsenic, mercury, cadmium, cobalt, all of those have the real potential to destroy a water body."
Minnesota regulators have known about the environmental risks associated with copper-nickel mining for decades.
So for the past 25 years at a lab in Hibbing, state Department of Natural Resources researchers have measured the runoff from different samples of sulfide ore taken from across the region.
Every week, researchers flush rock samples taken from throughout the Duluth Complex with water after exposing them to oxygen, and then test the water for metals, sulfate and acidity.
Doing so allows them to determine how much sulfur is needed from a particular area to create acid, and, how long it takes for that runoff to become acidic.
The results help the agency evaluate mining companies' plans to prevent pollution, said John Engesser, the DNR's assistant director of lands and minerals.
"It gives us an experience of knowing whether the models the companies are going to be using to predict the future is valid based on our experience with this research," he said.
Much of the sulfide ore in northern Minnesota is relatively low in sulfur. That's led some mining supporters to predict little or no risk of acid mine drainage. Environmental groups counter that other low sulfur locations have still produced pollution, even when impact studies have predicted otherwise.
The question in Minnesota is whether the would-be mine operators can convince regulators they have a reliable and permanent solution to a problem that's as fundamental as water, air and rock.
"The ultimate responsibility is the DNR's to make sure that the environmental reviews are done correctly," Engesser said.. "And that the permit to mine is issued with the proper avoidance or mitigation techniques."