SILVERTON - In Little Giant Basin east of here (elevation 11,000 feet), seasons are merging as lingering snow gives way to wildflowers coming into their own.

But the breathtaking setting holds perils for unwary visitors, because the basin is riddled with unprotected vertical shafts and horizontal adits of played-out gold and silver mines.

Twenty such hazards - three shafts and 17 adits - that hard-rock miners abandoned when the vein of ore they were pursuing petered out are being sealed to protect the public.

"The sites were chosen because they're within easy reach of four-wheel-drive vehicles and because they're dangerous," Kirstin Brown with the Colorado Division of Reclamation Mining and Safety told a pair of visitors recently at the portal of the Black Prince Mine, one of the 20 hazards.

Abandoned mines can conceal unstable explosives, rotting wooden supports, hazardous waste, loose rocks and oxygen-depleted air as well as snakes or bats, said Brown, who works out of a Durango office.

"There have been no deaths here, but rocks could fall on your head or you might not recognize - or not even feel - the symptoms of bad air before it's too late," Brown said.

Visitors flock here to photograph wildflowers or to glimpse elusive bighorn sheep. But the old mines from hard-rock mining's heyday and abundant debris - outbuildings, crumbling tram towers that supported ore cars and mounds of scrap lumber from many eras - are a constant attraction.

Brown pointed to the round nails and milled lumber in one pile of scrap lumber. They are telltale signs that the debris doesn't date from the earliest mining days, she said.

The division's reclamation work is free to landowners if the mine closed before the passage of the 1977 Surface Mining Control and Reclamation Act, Brown said. The division receives funding from the Colorado Legislature, the Environmental Protection Agency, the Bureau of Land Management and the U.S. Forest Service.

A crew from Rocky Mountain Consultants of Lakewood is working now in the Little Giant Basin. They had to remove snow that was blocking the 8-foot-high, 6½-foot-wide adit of the Black Prince to begin a 3½-day job of installing a steel grate and a drainage pipe in the portal.

When work is done, visitors will be able to peer into the Black Prince adit but not enter.

Across the ravine, up the mountain and around a hairpin turn, the adit of the King Solomon Mine can be seen. Crew boss Brian Sweet's team sealed that adit earlier this month with rocks and concrete, the other standard method of closing adits.

The Black Prince and King Solomon mines were among the first explored here - in the mid-1870s - said Jim Herron, the division's staff historian in Denver.

"They were primarily silver mines, although they produced some gold," Herron said. "They operated into World War II because the price of the base metals they produced - zinc, lead and copper - went up."

Above the King Solomon Mine, the road is blocked by snow. Sweet was awaiting the repair of a backhoe radiator in order to clear the road to reach other job sites. The radiator was damaged by pieces of a marmot that was chopped up by the fan when it sought warmth in the engine compartment.

Bev Rich, chairwoman of the San Juan County Historical Society, hates to see mining history veiled by closing adits and shafts. But she understands the safety issue.

"You have to balance historical preservation and public safety," Rich said by telephone. "You don't like to see a mine shaft closed, but you'd hate to see a kid fall into one."

Grates are preferable to rocks and concrete, Rich said. At least visitors can peer into the depths.

The division performs mine-safety work from Durango, where Brown works, and from offices in Telluride, Grand Junction and Denver.

"It can be logistically difficult to get approval for a mine-sealing project," Brown said. "An animal or plant on an endangered-species list can hold up a project."

The welfare of bats and ptarmigan has to be considered in some areas, she said. The state Historical Preservation Commission and the U.S. Fish & Wildlife Service also sign off on projects.

Brown, who has a degree in geology from Colorado State University, became familiar with the Division of Reclamation Mining and Safety at age 16 when she participated in a summer program in natural resources sponsored by the division. She went back three more summers before she earned her degree and was hired by the division, where she's now worked for five years.

Brown said that as of last year, 217 abandoned mines in San Juan County have been sealed. But the number is less than half the estimated 500 the county contains. On her fingers, Brown ticked off her upcoming jobs.

"I have an estimated five years of work lined up," Brown said. "I'll be here for a while."

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