Permafrost

mechanisms during the compression and tension cycles. In a similar and perhaps even more
explicit form, a thermo-mechanical ratchet process will be suggested to provide a major input
into the formation of ice wedges, ice polygons etc. The paper will attempt to provide an
alternative classification framework for these and other periglacial features in accordance with
the nature of the thermo-mechanical process involved in their formation.
Key words: Contribution, thermo-mechanical ratchetting, periglacial environments
Impact of Degrading Permafrost on Impounded Tailings
at Mount Nansen, Yukon Territory, Canada
John Kwong Y.T.
(CANMET Mining and Mineral Sciences Laboratories, Natural Resources Canada
555 Booth Street, Ottawa, Ontario, Canada K1A 0G1)
Abstract: Mining in the Canadian North has often taken advantage of the presence of
permafrost to manage reactive mine wastes such as potentially acid-generating tailings and
waste rock. However, the wisdom of such practice is recently in doubt due to concerns about
climatic warming, which may result in permafrost degradation. To elucidate the potential
impacts of climatic warming on tailings management in mountainous terrains with
discontinuous permafrost, the stability of impounded tailings at the abandoned Mount Nansen
mine in the Yukon Territory is reviewed.
The Mount Nansen gold mine is located in the Dawson Range at an elevation of about
1300 m. Discontinuous permafrost ranging from 30 to 60 m thick occurs in the area at a depth
varying from 0.4 to 5.0 m. The mine operated for 15 months in 1997-1999, extracting gold
from oxidized ores associated with arsenic mineralization by cyanidation. The operation
generated 250,000 tonnes of As- and CN-bearing tailings impounded in a valley at 1150 m
elevation behind a compacted earth dam keyed into the underlying frozen ground. Improper
initial tailings placement led to partial thawing of permafrost beneath the dam, casting doubt on
its physical stability. Systematic coring in the tailings impoundment in 2001 showed that
whereas permafrost still persisted at the bottom of the impoundment, the depth to permafrost
increased from north to south and from west to east in the direction of the tailings dam.
Permafrost thawing was particularly evident in the vicinity of the seepage pipe located
upstream of the tailings dam near its centre.
The results of detailed physical and chemical characterization of the ponded water in the
impoundment, of the tailings solids and porewater as well as seepage down-gradient of the
containment dam suggest that the prevalent cold temperature and strong sorption with the fine
tailings particles have slowed down the natural degradation of CN-containing species and the
efflux of potentially detrimental trace elements. Column testing and sequential batch leach
testing of sampled tailings indicate that, under thawed conditions, significant releases of As, Sb,
thiocyanate and ammonia may occur with continual flushing. Partial sequential extraction
analyses also suggest that As and Zn are susceptible to remobilization under mildly acidic or
reducing conditions.
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