Gov. Mark Dayton will inspect hard-rock mines in Michigan and South Dakota this week in preparation for what he calls the “most momentous, difficult and controversial decision” of his tenure: Whether to approve a controversial proposal by PolyMet Mining for northern Minnesota.

The two operations, chosen by opposing advocacy groups, will show Dayton the best and worst environmental consequences of modern mining and help him evaluate the risks associated with PolyMet’s plan.

Both mines offer valuable lessons, Dayton said, but PolyMet’s plan to harvest copper and nickel from Minnesota’s northwoods will be judged on its own merits as state regulators enter the final stage of approvals next month. The Dayton administration expects to complete a 10-year environmental review and start making decisions that could move the project forward, including a linchpin permit to mine.

“I’m genuinely undecided,” Dayton said in an interview. PolyMet’s proposal has triggered hopes of a mining comeback on the Iron Range, but also sowed deep fears about the environmental risks posed by a type of mining that would be new to Minnesota. “I can’t help but think this will be very educational.”

The governor’s tour starts Tuesday at a failed gold mine near Deadwood, S.D., that has cost taxpayers more than $105 million for pollution cleanup and continues to cost more than $2 million per year. The Gilt Edge Mine — a large, deep open-pit operation — was abandoned by its owner in a 1999 bankruptcy and became a Superfund site under the U.S. Environmental Protection Agency. Superfund managers will give Dayton a guided tour.

Mining at Gilt Edge dates back to the late 1800s. But Dayton will focus on a modern period in which there were dreadful environmental miscalculations leading up to the 1986 commencement of operations by Brohm Mining Co., said Betsy Daub, policy director at Minneapolis-based Friends of the Boundary Waters Wilderness.

Daub said the company mistakenly assured regulators that local rock was low in sulfide content. Instead, the mining of high-sulfide rock caused uncontained sulfuric acid runoff into the area’s cold-water trout streams. As at many metal mines, the acid forms when sulfide rock is exposed to air and water, creating pollution that can persist for hundreds of years.

South Dakota demanded that Brohm install a water treatment facility to address the problem, but the extra expense, combined with falling gold prices, put the mine out of business, according to officials from the South Dakota Department of Environment and Natural Resources.

“The prediction was wrong,” Daub said.

“There was not a good enough job in geochemistry,” said Mike Cepak, a state mining engineer in Pierre, S.D.

Cepak said the state now employs more rigorous standards for estimating the acid-generating potential at proposed mines.

In another potential lesson for Minnesota, Cepak also said South Dakota now requires mine operators to put up larger “financial assurances” that would pay for environmental cleanup in case something goes wrong. In South Dakota, the only private money available for the Gilt Edge cleanup came from a $6 million bond posted by Brohm. Cepak said the state now prefers doing mining business with large, well-financed corporations.

Dayton, who will be accompanied by Minnesota DNR Commissioner Tom Landwehr and other state officials, will see Gilt Edge’s so-called Ruby waste rock dump, where piles of processed rock have been regraded and covered with a plastic liner, crushed rock subsoil, and topsoil in the cleanup effort to stifle acid production.

The centerpiece of the Superfund site is a sophisticated water treatment plant that simultaneously neutralizes acid and creates metal-laden sludge contained on site. The setup captures and treats all fluids before releasing clean water into Strawberry Creek, South Dakota officials said.

“I want to see the damage it has caused,” Dayton said. “To see any desecration of the environment from a man-made process will be deeply disturbing. I don’t want it repeated in Minnesota.”

On Friday, Dayton will visit Michigan’s Eagle Mine, located near Lake Superior west of Marquette. Jim Kuipers, an independent mining engineer, said the mine is the polar opposite of Gilt Edge and quite different from PolyMet’s proposed mine.

Eagle Mine, which was opened last September by Toronto-based Lundin Mining Corp., has high-grade deposits of nickel and copper, but it differs from the PolyMet proposal in several ways. While PolyMet’s project will be an open-pit mine, the Eagle operation is underground. It will produce less waste rock and fewer tailings than the mine proposed by PolyMet, Kuipers said. And unlike PolyMet, Eagle is not likely to be saddled with long-term water treatment to fight acid mine drainage, he said.

But Bruce Richardson, a PolyMet vice president, said several aspects of the Eagle Mine reflect favorably on PolyMet. Both mines, for instance, would use reverse-osmosis technology to treat wastewater before it is returned to the environment.

So far, observers agree, Eagle’s closely monitored discharges have been clean.

In another parallel to PolyMet’s plan, Eagle uses a converted iron ore processing facility and is dumping its tailings into a water-filled basin. And like PolyMet, Richardson said, Eagle went through at least 10 years of environmental review before receiving a state permit. The Michigan mine has generated 400 new jobs.

“It’s an attractive site for the governor to visit in our minds because it represents modern mining,” Richardson said. “They are doing it safely and within the law.”

Joe Maki, Michigan’s state mining specialist, said he will meet with Dayton’s group to answer questions before and after they tour Eagle with company officials. “We haven’t had any environmental issues with the mine,” Maki said.

But Chris Swartz Jr., president of Michigan’s Keweenaw Bay Indian Community, said he’d like to dissuade the governor from approving PolyMet’s permit. Swartz fought the Eagle Mine because his band fears long-term degradation of the area.

“Sooner or later sulfide mining will be detrimental to water in the area,” Swartz said. “If he’s concerned about clean water, he should take a close look.”

Dayton said he wants to understand Eagle’s early success and what’s different about the mine.

“It’s part of my due diligence,” the governor said. “You learn what you can from each one and decide what’s applicable or not.”