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Testing the Viability of Floater Dredging in Frozen Ground
This report is an account of our work testing the viability of floater dredging for placer gold in permafrost. Floater dredging is a method of mining which uses a hydraulic excavator to feed placer gravel to a floating gravel processing plant. We wanted to see if the digging face of a dredge pond dug in permafrost would thaw fast enough to supply gravel for a production-scale mining operation. We stripped the vegetation and muck from an area on a permafrost terrace above the Fortymile River. We dug a pit in the stripped area and pumped it full of water to form a pond in which to operate our floater dredge. Water from the pond seeped into the surrounding gravel, thawing as it penetrated. We confirmed that when frozen gravel is exposed to water, it thaws faster than when exposed to air. We experimented with various methods of accelerating the rate of thaw at the dredging cut face. We found that sloping the cut face at 30° to the vertical exposed more surface area to the thawing action of the water, without sacrificing digging efficiency of the excavator. We found that equipping the excavator with spade-type abrasion teeth gave the best combination of penetration, digging efficiency, and wear. Towards the end of the experiment, we found that water had penetrated at least 12 feet back from the cut face. We believe that the artificial water-table, created by seepage from the pond, may have started thawing large blocks of the permafrost. Further work is required to test this theory. We found that the cut face thawed back at a rate of at least 2 feet in 24 hours. With a long enough face, this would provide enough thawed gravel to support a production-scale operation. A 600-foot cut face is required for a 100 yd/hour operation over a 10-hour sluicing day. The gravel did not thaw at an even rate along the face, and we dug more gravel in the thawed areas, causing the face to become uneven. This made mining difficult because the cut became disorganized. The solution to this problem was to even the cut face by using a dozer to push thawed material from the stripped area in front of the face into the sections of the cut where the deepest advances had been made. Using this technique, we were able to dredge the frozen ground successfully on a sustainable basis. Floater mining offers an option to miners working permafrost deposits. However, because there are factors limiting its applicability, a careful analysis should be made before committing resources to this type of operation.
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Placer Gold Dredging Using an Excavator and Floating Processing Plant
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This report provides information on a method of placer gold dredging. This dredging method differs from conventional bucket line gold dredging in that it uses a combination of land-based and floating dredging equipment. An excavator is used for digging the gravel, and a floating processing plant is used to process the gravel and stack tailings. Using this combination of equipment offers many advantages. The cost of mining placer gravel is extremely economical; we have calculated mining costs at 90 cents per loose yard of gravel processed. This mining method also has environmental advantages. Total reclamation of the mined area is achievable and inexpensive. All water used in processing the gravel is recycled, so that there is no discharge of effluent to the watercourse. The method has limitations. Only certain types of placer reserves can be mined. For example, the ground must be thawed to bedrock depth, and have a higher water table or the ability to hold a pond. Design details and specifications for the equipment used in the mining system are discussed. General design information is given for each of the components used in the dredging system. We have provided detailed specifications for our own equipment as an example. Operating procedures are outlined; topics covered include equipment assembly and start-up, mining through a deposit, shut down procedures, and reclamation of the mined area. Because of the current low gold price and rising operating costs, placer mine operators must explore new mining methods to remain profitable. As well, mining methods which minimize environmental impact must be adopted. This dredging method has the advantage of both lower production costs and minimal environmental disturbance. As well, reclamation is easily achieved and cost-effective.
Research of Low Permeability Cover Performance at the Arctic Gold and Silver Mine Site, Carcross, Yukon
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The Arctic Gold and Silver Mine Site located on Montana Mountain near Carcross, Yukon (the Site) operated during the late 1960's. After mining operation was ceased in 1969, there remained potential environmental issues with regards to the tailings that had accumulated at the Site during mine operation. A potential environmental issue associated with these tailings is the possibility that Acid Rock Drainage (ARD) could occur. To ensure that ARD reactions do not occur in the future at the site, the federal government (with assistance from consultants) developed a reclamation plan for the tailings at this site. A silty clay material was used to cover the tailings to prevent oxygen and water from entering and reacting with the iron-sulphides to produce ARD. EBA Engineering Consultants Ltd., the Carcross Tagish First Nation and partners, on behalf of Mining Environment Research Group and Indian and Northern Affairs Canada, conducted a study to evaluate the performance of the silty clay cover over a tailings impoundment at the Site between September 2000 and January 2001. Data-gathering equipment was installed at the Site. Three rounds of testing and monitoring at the site were completed to gather data regarding the thickness, oxygen concentrations, temperatures and moisture levels within the tailings and the cover. This report presents the conclusions and recommendations of the study:: - Observations show that the cover system was installed properly and according to the design - Preliminary data show that the cover is working to reduce the amount of moisture and oxygen entering the tailings - It would be valuable to collect more data at the Site to observe trends and see how well the cover systems will work over time in a northern climate.
Geophysical and borehole investigations of permafrost conditions associated with compromised infrastructure in Dawson and Ross River, Yukon.
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The effects of permafrost degradation in Yukon have serious negative implications for the structural integrity of vertical infrastructure. This is especially pertinent for critical buildings such as hospitals, schools, etc., in small communities that are situated on top of warm, ice-rich permafrost. Projections of mean annual air temperature over the next few decades, based on regional climatic models, indicate that air temperature will rise, hastening the thaw of permafrost. The combination of rising of air temperatures and buildings situated on warm permafrost has prompted this investigation into the vulnerability of Yukon Government vertical infrastructure. The application of DC resistivity and ground penetrating radar in conjunction with borehole drilling indicates that in Dawson there is warm ice-rich permafrost beneath the Palace Grand Theatre; the Old Territorial Administration building is underlain by primarily unfrozen sediment; and permafrost under the St. Andrew’s Church is characterized by high variability. A deep active layer was observed at Ross River School and geophysical surveys indicate that warm water drainage from the roof is contributing to the thaw of the underlying permafrost.
Williams Creek Project, Thermal Leach Project Test Heap Leach Project at Carmacks, Yukon Territory
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An extensive heap leach field test was conducted during the 1993-1994 winter season at Carmacks, Yukon Territory. The test itself utilized an approximately 5-m-diameter crib loaded with composite ore from the northern (higher grade) end of the Williams Creek deposit. The ore was stacked to a total height of 7 m, including one meter of ore atop the emitter system. This matched the commercial heap height planned for Williams Creek at the time the test was started. By insulating the side walls of the crib, lateral heat flux was minimized. Thus, the crib replicated an interior segment of the commercial heap. Leaching was done at a flow rate that matched those commonly used in industry. Leachate temperature was about 21°C, a level achieved with no external heat input other than normal process heat transferred from electrowinning to the leachate via solvent extraction. An analysis of the test results showed that the winter conditions at Carmacks were quite typical of those expected at the mine site. Conditions included ambient temperatures below -40°C and an average temperature of -13°C. In terms of leachate flow to and from the crib, the test ran continuously from late September through mid-February. In spite of some flow system problems, leaching continued unabated. There was no freezing in the interior of the crib or in the solution reservoir at the bottom. Freezing was limited to the insulating ore over layer and a few isolated points high in the crib near the outer walls. The test clearly demonstrates that year-round heap leaching of Williams Creek ore is practical. The heap appears to be adequately insulated by a 1 m ore layer on top of the emitter system. Normal process heat should be sufficient to maintain a leachate return temperature of approximately 20 degrees Celcius at the heap. However, provisions for some supplemental heating in the commercial operation may be desirable. This would permit recycle of the heated solution if electrowinning goes off-line or if there is a long run of exposed leachate pipeline back to the heaps.
Experimental Trials for Restoring Disturbed Sites in Permafrost Areas Using Bioengineering Techniques
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Placer mining has over a one hundred year history in the Yukon Territory. The majority of placer mining has taken place in the zone of discontinuous permafrost. Within the discontinuous permafrost zone, vegetated valley flats and north-facing slopes generally are underlain with permafrost. The permafrost thickness is highly variable and may reach depths of 60 meters. The active layer covering the permafrost also varies greatly in thickness. To access gold bearing gravels, the removal of extensive quantities of overburden is usually required. Once this insulative cover is removed, the thermal equilibrium of the permafrost is disrupted and thawing occurs. This affects the stability of soils and vegetation and slope failure may occur. The extent that exposed permafrost melts depends partly on the amount and form of ice content. As melting progresses, the potential for mass movement of soil increases. Melting permafrost usually results in a wetter environment than was initially present, which further affects the natural revegetation process. Bioengineering is the use of living plant materials to perform engineering functions such as erosion control and stabilization of steep slopes. Although bioengineering methods are now commonly used in the stabilization of steep problem slopes in more moderate climates (Polster, 1997), experimentation with these techniques in areas underlain with permafrost is still required. Advantages of using bioengineering systems are that they can be installed without machine access (which in permafrost areas would disturb the site even more), they strengthen with age, and they are less expensive than traditional hard engineering practices.
Monitoring of Low Permeability Cover Performance, Arctic Gold and Silver Mine Site, Carcross, Yukon
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The production of acid rock drainage (ARD) from mine tailings is a significant environmental concern at various abandoned mine sites in Yukon. Leachate with low pH and high dissolved metals concentrations deriving from mine tailings impoundment areas can negatively impact various groundwater and surface water resources. The presence of both oxygen and water is required for ARD to develop. Therefore, the reduction of the oxygen source and the water source from the mine tailings through the use of a low permeability cover (which acts as an oxygen/infiltration barrier) will limit ARD production. This is the basis of the design and recent reclamation (1998-99) of the Arctic Gold and Silver Tailings Site in Carcross, Yukon that EBA has been involved with in association with PWGSC.
Preliminary data from the establishment of longterm ground temperature reference sites in five Yukon communities
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A drilling program was conducted in fall 2023 to install deep (up to 35 m) ground temperature monitoring boreholes in communities at risk from permafrost thaw. These boreholes will serve as long-term reference sites in both undisturbed and developed locations within or adjacent to communities. These boreholes also filled knowledge gaps that included the thickness of permafrost, and temperature data at the bottom of permafrost. Prior to this program, only 3 of the 34 permafrost-monitoring boreholes managed by the Yukon Geological Survey recorded temperatures through to the bottom of permafrost; furthermore, none of these boreholes are in the extensive discontinuous or continuous permafrost zones. Fifteen boreholes were drilled in five different communities across the Yukon: Dawson City, Mayo, Ross River, Beaver Creek and Haines Junction. Permafrost was encountered in 10 of the 15 boreholes, and the bottom of permafrost, or bedrock, was reached in at least one borehole in each community.
Initial results from the first year of the Permafrost Outreach Program, Yukon, Canada
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In 2007, a permafrost outreach program was initiated in Yukon, Canada by installing long-term permafrost monitoring stations near public schools in Whitehorse, Faro, Ross River, Dawson, Old Crow and Beaver Creek. Shallow boreholes were drilled near participating schools, and data loggers were installed to measure hourly air and ground temperatures at a variety of depths. Frost tubes were also installed in fall 2008 to start monitoring seasonal freezing and thawing trends in the active layer. School students are actively engaged with field data collection and interpretation of results posted on a central website. The program also provides baseline data that can be used to characterize local permafrost conditions and detect long-term changes. A snapshot of current permafrost conditions is provided for each monitoring station, based on the first year of data collection.
Reconnaissance Survey of Erosion Site at Gold Run Creek
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Disturbance of frozen ground, through placer mining activities, may lead to slope failure. A stockpile of frozen overburden has recently begun eroding from the formation of a runoff channel. A large cut, approximately 300 metres long and up to 50 metres deep has been created adjacent to Gold Run Creek, a heavily mined area southeast of Dawson City. This stockpiled overburden has been in place for several years and has revegetated with several species of mature willows (age dated to 15 years). Either due to heavy rainfall or climate change, melting has commenced in the stockpile resulting in the formation of a runoff channel. This has increased in size and slope failure continues to enlarge the cut. Laberge Environmental Services conducted a reconnaissance survey of the site in July 2003, to assess the site with the purpose of exploring ways of halting the slope failure and stabilizing the disturbed section of overburden. Several bioengineering techniques have been suggested to assist in controlling the erosion.
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