At what was billed as a pro-mining rally at the Duluth Entertainment Convention Center on Monday evening, leaders of the conservative Center of the American Experiment were preaching to the choir - with one dissenting voice.

Personnel from the think tank were on hand to tout a study they produced in August with the thesis that expanded mining on the Iron Range would be a boon to the entire state's economy without harming the environment.

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"How can we have mining and still have a clean and healthy environment?" asked geologist Debra Struhsacker, one of the authors of the "Unearthing Prosperity" study. "The fact of the matter is that the two go hand-in-hand."

There was no sign of any actual miners among the 40-or-so people in the room. The Center of the American Experiment doesn't have a cozy relationship with unions; it celebrated the U.S. Supreme Court's Janus v. AFSCME ruling earlier this year that allowed non-union employees to be exempt from union dues.

But "We Support Mining" stickers were in evidence, along with "Team Stauber" sweatshirts and one pro-Trump cap.

The organization's study contends that mining undeveloped deposits of copper, nickel and platinum would add about $3.7 billion to Minnesota's economy and generate nearly $200 million in tax dollars for state and local governments.

But Sheila Coughlin of Kettle River quickly rose to her feet to question the report. She became involved in a testy exchange with John Hinderaker, president of the organization. Coughlin, who at one point walked to the front of the room and took over the podium, cited a study by Harvard economist James Stock. His paper argued that economic benefits of expanded mining would be temporary and would do more harm than good in the long term.

Hinderaker, who threatened to have security remove Coughlin from the room if she didn't confine her question to a single sentence, called Stock's report "unbelievably bad." Hinderaker said he had contacted Stock suggesting they debate the issue and offered at least 60 possible dates. Stock responded that he wouldn't be available to come to Minnesota any time in 2018, Hinderaker said.

After Coughlin stood again to raise a question as co-author Isaac Orr begin his presentation, a DECC security officer spoke to her quietly, and a uniformed police officer stood at the doorway behind her. She held her tongue until the formal question-and-answer period, then questioned the Minnesota Department of Natural Resources' environmental impact study on PolyMet's proposed NorthMet mining project. She said it used 20-year-old precipitation data.

"How as a downstream community can we feel comfortable with 10 percent of the world's freshwater right in our backyard with this not being included in the design?" she asked.

Struhsacker didn't answer her question directly but defended the DNR, saying it was one of the most transparent among such agencies in the country.

The center's staff plans a second pro-mining rally in Hibbing today.