MINING NEWS: Worst is over at former Yukon asbestos mine

Creek stabilization measures, clean-up effort left waste rock and tailings, but fibers pose no significant risk to human health

Sarah Hurst

Mining News Editor

Sawson City, Yukon epitomizes the romance of the Gold Rush, so it was a little surprising that the first field trip arranged as part of this year’s Northern Latitudes Reclamation Workshop was to an extremely unglamorous site: the former asbestos mine at Clinton Creek.

Quite apart from the lack of allure of its toxic ore, the mine probably doesn’t see too many visitors. To reach it from Dawson you have to cross the Yukon on a ferry, drive for almost an hour up the winding Top of the World highway, drive for another hour down a muddy mountain road, cross the Fortymile on a narrow bridge, and finally ford a small creek. It is only accessible by vehicle between May and October.

Clinton Creek mine operated in 1968-78, under the management of Cassiar Mining, which also ran its namesake asbestos mine in British Columbia.

The chrysotile (white) asbestos at Clinton Creek occurs in the serpentine ore body and its fibers can be seen forming a furry carpet on top of the rocks at the site, or sandwiched in the middle of them.

White asbestos is less likely to be inhaled than the other, more dangerous types of asbestos, but nevertheless red and white signs prominently posted around the mine site warn of the dangers of inhaling the fibers, and also of flash floods.

Cassiar extracted 1M tonnes of asbestos

The company went bankrupt in 1992.

In the years when the mine was operating there was no law requiring a reclamation bond.

“In those days the word ‘environment’ was only just entering the vocabulary,” Frank Patch, a project manager with the Yukon government’s Department of Energy, Mines and Resources, told Mining News. “Mining companies are now starting to understand that clean-up is part of the cost of doing business.”

The Yukon government and UMA Engineering intended to show reclamation workshop participants their newly-completed creek stabilization measures, but these were somewhat overshadowed by the 60 million tonnes of waste rock and 10 million tonnes of asbestos tailings that had reshaped the local landscape. Not to mention the 74-hectare Hudgeon Lake, which could be considered a very attractive scenic feature, except that it was created as a result of a slope failure of the waste rock dump in 1974 that blocked natural drainage through the Clinton Creek valley.

Still, the reclamation work that has been done on the creek appears technologically efficient as well as aesthetically pleasing. Previous channel erosion control measures – large boulders and a series of small rock weirs – were unsuccessful and could provide no resistance during a major flood event in the spring of 1997, when they were destroyed altogether.

UMA designed a stable channel over the landslide dam, featuring four gabion drop structures constructed between 2002 and 2004. This should protect the creek from a full breach of the waste rock blockage, which would result in a peak discharge of approximately 500 cubic meters of water per second.

Gabion structures date back to the ancient Egyptians, who used them for bank protection along the Nile about 7,000 years ago. Originally baskets of woven reeds, they are now engineered containers manufactured from wire mesh. They are self-draining units that yield to earth movement but remain structurally sound, as opposed to rigid structures, which can fail if slight changes occur in their foundations.

The channel stabilization measures at Clinton Creek had to be able to accommodate some movements of the waste rock pile and remain functional, as the waste rock is creeping northward at a rate of 5-7 cm per year.

The drop structures here are a set of 0.5m-high steps that provide energy dissipation between each step as the water travels through and over the structure. Dewatering of the channel was required to facilitate construction, and a fish salvage operation was undertaken.

Up to 1,000 fish – mainly Arctic Grayling and Slimy Sculpins – were salvaged and returned to Hudgeon Lake or downstream reaches of Clinton Creek.

Groundwater seepage was controlled during construction by the use of granular blanket drains installed below the drop structures. The granular drains also served as foundation material in some cases.

UMA won a Consulting Engineers of Manitoba award for their Clinton Creek channel stabilization design three years ago.

$3 million at Clinton Creek

In addition to the creek stabilization, old buildings were knocked down and loose asbestos around the mill site was buried.

Much of the work was carried out by First Nation contractor Hän Construction from Dawson City.

UMA considered more radical reclamation plans, according to Gil Robinson, a geotechnical engineer from the company’s Winnipeg office. Removing all the waste rock and restoring the valley would have cost over C$30 million, he said.

The grey mountain of tailings is an eyesore, but not a health hazard. Monitoring has shown that the amount of asbestos in the air does not pose a significant risk.

Robinson joked that the tailings pile would make a great spot for snowboarding. A protective crust has formed on top of it, and if any attempt were made to move the tailings, the crust would be disturbed and dust would be released.

Another surprising benefit of the crust, recently discovered, is that it absorbs carbon dioxide, thanks to the magnesium cations (positively charged ions) in it. In theory the Clinton Creek mine could earn credits for reducing greenhouse gases.

Which goes to show that every cloud has a silver lining, or at least a magnesium one.