The Ouray County sheriff's office was called to the Revenue-Virginius mine at about 7:20 a.m., county spokeswoman Marti Whitmore said. The miners were underground and were confirmed dead Sunday afternoon.
"Anything that has been reported is speculative," Whitmore said. "We don't know what the cause is."
Star Mine Operations, LLC, the owner of the mine, couldn't immediately be reached by the Associated Press for comment, but Whitmore said the company has accounted for all of the workers at the site.
She said 20 people were taken to area hospitals, and all but two have been treated and released. The conditions of those two hospitalized workers haven't been released.
Rory Williams, project manager for Star Mine Operations, told the Ouray Watch newspaper (http://bit.ly/1ajQLAs ) the accident was not related to a cave-in or mine collapse but was apparently a "powder-smoke incident," and that the release of chemicals in the smoke injured the miners.
The Mine Safety and Health Administration is at the accident site, which is about 270 miles southwest of Denver.
The last major mining disaster in Colorado occurred on April 15, 1981, when an explosion killed 15 people at the Mid-Continent Dutch Creek No. 1 Mine near Redstone.
In 2000, a 37-year-old man was killed after being struck by a high-pressure hose that snapped off a piece of equipment in the Sanborn Creek Mine.
And in 2011, a New Mexico contract worker died after being hurt at the West Elk Coal Mine in Somerset, in western Colorado. The U.S. Mine and Safety Administration found the 53-year-old slipped and fell from a beam at a tower construction site.
The Watch reported that in its heyday, between 1876 and the late 1940s, the Revenue-Virginius mine produced more than 14.5 million ounces of silver, enough to weather the Silver Panic of 1893.
Star Mine Operations acquired the property in late 2011.