Musselwhite Mine in Northwestern Ontario, Canada began thickened tailings disposal in May 2010. Surface disposal of thickened tailings for the 4,000 tpd gold mine was chosen following a series of studies beginning in 2002 to investigate options to expand the capacity of the tailings management area. This technology was considered most cost effective in maximising storage capacity within the existing site footprint. The elimination of a pond for closure will reduce the risk of dam failure. Further, the non-segregating tailings are expected to be relatively impervious and saturated. These characteristics will help inhibit tailings oxidation and reduce the likelihood of acid generating and metals leaching in the long term. Musselwhite Mine is currently planning to remove the sulphide minerals from the tailings stream using a flotation plant. This paper discusses the design of the thickened tailings disposal system and presents preliminary observations on tailings deposition to date. In preparation for thickened tailings disposal the tailings management area was partitioned by an internal dyke in 2008. This has allowed tailings to be 'stacked' using the upstream method of construction in the west cell and the east cell to serve as a water management pond. The cold climate presents a special challenge to operating the thickened tailings system as there is very little precedent experience. The design has incorporated a number of contingency measures to facilitate tailings thickening and deposition in winter. Field data have suggested that the thickened tailings system is performing very well to date. The thickening plant has consistently produced underflow at about 70% solids with minimal attention. The tailings beaches are steeper than expected. The deposited tailings are non-segregating and relatively saturated. The measured in situ dry density of the tailings is higher than expected. The performance of tailings deposition is being closely monitored.

The ideal outcome for mine closure measures is a walk-away scenario, with all hazards and liabilities resolved and with the land returned to beneficial use. Unfortunately, in the majority of cases, this ideal is not practicably attainable. After a mining company makes large capital expenditures and completes its agreed closure measures, it may still have ongoing obligations, liabilities and risks. These could include: postclosure inspections and reporting, routine care and maintenance, and in some cases, ongoing treatment of effluent. In addition, there may also be residual hazards to the public, safety concerns related to, for example, unstable crown pillars or open pit slopes as well as a risk of failures due to long return period events, such as floods and earthquakes. Uncertainties will also remain regarding the performance of the long-term closure measures. There may also be unresolved liabilities requiring financial reporting and costs associated with ongoing beneficial use of the land. If, alternatively, the mining lease is returned to the government, there may be a requirement for a lump sum payment to cover future costs associated with the unresolved liabilities.It is possible to estimate the aggregate cost of these unresolved post-closure liabilities using risk modelling. The risk cost could then form the basis for the following alternative approaches for managing mining properties after closure:• Retaining the property and establishing internal funding to pay for future costs and risks.• Returning the mining lease to the government, with a payment to cover the future costs and residual risks.• Transferring the ownership to a special purpose company and purchasing insurance against the residual liabilities on the property.• Transferring some of the assets to a local community or government with some arrangement to cover costs of future maintenance and risks.

There has been a growing interest in the surface disposal of high-density tailings in recent years. Such an interest is driven primarily by the need to conduct mining in a sustainable manner by minimising environmental and social impact, conserving water and improving tailings safety. Increasingly, more mines are considering thickened or paste tailings disposal in conjunction with paste tailings backfill to better utilise their infrastructure. Several factors have hindered the wider adoption of high-density surface tailings disposal technology. The ability to predict tailings behaviour in full scale is still relatively limited. There is also little experience in managing a high-density tailings facility under varied climatic conditions. A number of high capacity mines have also had production issues meeting the tailings consistency criterion for deposition leading to major reconfiguration of the disposal system due to poor field performance. Some of these uncertainties are being addressed through laboratory and field research with significant advances being made on our understanding on tailings rheology, transportation, mechanism of deposition and post deposition behaviour change of the tailings. High-density tailings facilities will require a greater degree of management than conventional slurry tailings. Not only are the tailings likely more variable, there are unique challenges associated with tailings deposition and water management. This paper discusses tailings deposition control and water management methods that have been successfully used at a number of high-density tailings facilities with special reference made to central cone deposition in the Kidd Metallurgical Site and perimeter deposition at the Musselwhite Mine. Both of these facilities are located in cold climatic regions in Canada with surplus precipitation. Contingency measures that have been adopted to reduce the risk of system upsets and non-performance, and closure considerations for these facilities are highlighted. Field experience has shown that high-density tailings disposal can offer unique opportunities for progressive rehabilitation and optimisation of the closure design.

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