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By Sarah Hurst
Mining News Editor 

Brewery Creek reclamation worth a toast

Yukon gold mine one of the most northerly cyanide heap leach operations in the world; biological reclamation processes proving effective

 

Last updated 6/19/2005 at Noon



Most people would expect to find Blue, Canadian, Fosters and Moosehead in their local saloon - not up in the hills that surround northern Canada's rugged Dempster Highway. But here at Brewery Creek mine, Yukon, those are some of the names of the eight small open pits that produced more than a few jugs of gold for Vancouver-based Viceroy Resource Corp. between 1996 and 2002. Brewery Creek was one of the northernmost heap leach mines in the world, and it has been the site of numerous successful technological innovations in reclamation.

Brad Thrall, chief operating officer of Alexco Resource Corp. - also in Vancouver - which took over the reclamation from Viceroy (now Quest), led a field trip to Brewery Creek during the Northern Latitudes Reclamation Workshop in May.

Viceroy mined a total of 9.5 million tonnes of ore, extracting 280,000 ounces of gold, according to Thrall.

The mine cost C$70 million to build, plus around C$15 million per year in operating costs and another C$5.5 million for reclamation.

Viceroy did not come out with a profit, Thrall said. The final stage of the closure plan is a 15-year monitoring period which begins this year, and is expected to cost about C$1.5 million.

Site almost immaculate

Apart from the area at the mine entrance where machinery stands next to the warehouse and some small, wooden structures left over from the former 145-man camp, the site is in almost immaculate condition.

It has won the Yukon government's Robert E. Leckie awards for outstanding reclamation in three separate years. The shop and other buildings have been dismantled; most of the roads have been ripped up and reclaimed. The heap itself has been detoxified, recontoured and revegetated, so that a healthy grass cover is now growing on top of it. Waste rock dumps have been resloped to 2.5:1 or flatter slopes, and the areas have been reseeded and fertilized.

Almost all of Brewery Creek's assets were sold last year and shipped to Mexico, where they are being put to use at another mine. Reagent and fuel storage facilities have been removed, and an overall site decontamination survey will take place this summer.

"The primary objective of the revegetation component is stabilization of the soil and erosion control," Thrall said. "We're trying to promote invasion of natural species, native vegetation." Poplar and willow are already growing naturally on the site.

A total of 250 ha of land has been revegetated to date, with 25 kg of seed/ha and about 300 kg of fertilizer/ha applied. The seed mix includes species such as violet wheatgrass, fowl bluegrass and tufted hairgrass.

Most of the seeding takes place in early September, and the seeds sit dormant during the winter, as conditions are often too muddy for seeding in spring.

Reclamation began while operations were still going on. For example, after the North Golden pit was mined out in 1999 it was used as a backfill for waste from the adjacent South Golden pit. Brewery Creek finished up with just one external waste dump, as all the other waste was backfilled into open pits.

"There's a lot of benefits to concurrent reclamation," Thrall said. "It reduces your overall liability during operations and it really provides experience and research into what some of your final reclamation practices are going to be."

To prevent metal leaching from the 10 ha waste dump, a 0.5 m-deep soil cover was built over the top of it, seeded and revegetated. A large-scale lysimeter was then constructed, a device that collects soil water. "We're now monitoring the amount of precipitation that passes through this cover and we can measure that and collect it, sample and analyze for water chemistry, as well as infiltration rates, and validate the design criteria that we've established for that cover," Thrall said.

Detoxifying the heap challenging

Detoxifying the heap was the most technically challenging and also the most rewarding part of the reclamation process, according to Thrall. "This is a cyanide operation ... in that process we not only leach and extract gold and silver, but most other metals as well. We also create compounds such as nitrate and ammonia."

In 2000, a test program was initiated to determine the most effective decommissioning method for the heap. The traditional technology used at most heap leach operations, hydrogen peroxide for cyanide destruction, was compared with a biological process that would achieve the same result. The biological process was chosen.

A carbon source was introduced into the heap, along with phosphoric acid to oxidize the ammonia. Before the process began, the ammonia level in the heap was 25 ppm and the maximum discharge level was 5 ppm.

"Over about a six to eight week period in 2002 we injected about 100,000 kg of molasses, essentially a carbon source to feed bacteria," Thrall said. "Within about a four-month period, the effluent coming off of the heap had met all the direct discharge standards. It was quite successful."

The heap has remained stable for nearly three years, and in another two years' time the government's criterion for demonstrating stability will have been met.

Lined process ponds below the heap will remain in place as a contingency measure until long-term stability has been demonstrated. As long as the solution from the heap meets the criteria in the water license, it can be discharged directly into the environment. If there is an upset in the heap chemistry, the solution can be collected.

The heap is divided into seven cells and if one area of the heap causes problems, the water from it can be segregated and stored in a pond. As an additional precaution, a conventional chemical treatment system has been installed, although it has not been needed yet.

One of the big tasks completed last year was the removal of five culverts along the haul road, which entailed the excavation of about 80,000 cubic meters of material and the construction of five 1 m-deep rip rap channels, designed for one-in-200-year floods.

On a hot spring day it is hard to imagine water ever flowing over these dry rocks, but the reclamation at Brewery Creek is all about contingencies, no matter how remote they may be.

The objectives of the reclamation plan, according to Thrall, are to protect public health and safety, prevent and minimize environmental effects, and facilitate a passive walkaway closure strategy. This is the dream of all mining companies, meaning that an end to the reclamation process can be established and there is no need to do water treatment in perpetuity.

"A major hurdle in this was really overcoming that mindset that it's difficult to do biological processes in the north," Thrall said. "Biological and bacterial type processes are very effective in northern type conditions."

 

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