Design considerations for mine waste management at the Meadowbank Gold Project
Cumberland Resources Ltd.*, is currently evaluating the development of the Meadowbank Gold Project located some 70 km north of Baker Lake/Qamanit’uaq, Nunavut, in the eastern Canadian Arctic region. The project currently consists of a series of gold-bearing deposits within relatively close proximity to one another. It is planned to mine these deposits primarily as a truck-and-shovel open pit operation using conventional gold milling and extraction. The mining plan indicates that approximately 20 Mt of ore will be mined over a mine life of eight to ten years.
The development of the project is faced with climatic challenges similar to those encountered at other mine sites in the North. Extreme temperatures, permafrost, and high winds pose challenges to development. Additional site-specific considerations include the presence of lakes, some of which overly the main deposits, and the absence of suitable construction materials. Along with the geographic and geotechnical constraints surrounding the site, the project will require a strategy of carefully sequencing the stripping and generation of waste products to be stored either by construction disposal methods into dykes and other construction-related facilities, or to be stored in conventional tailings and rock storage facilities.
Operations will generate approximately 7 Mt of overburden soil and organic materials, 160 Mt of mine waste rock, and 20 Mt of tailings. The general absence of suitable construction materials at the site, and the logistics and cost associated with the transportation of materials to the site have necessitated an approach to waste management that is sensitive to the costs of the project and maximizes the use of mining generated waste products as construction materials. The effective use of the mining generated waste products for construction purposes, in turn, relies on an understanding of the geochemical nature of the overburden, waste rock, ore, and tailings. In addition, as large portions of the main deposits at Meadowbank lie beneath the surface of the main lakes in the project area, significant lengths of de-watering dykes are required to be constructed early in the project life. Consequently, the management of the waste products of mining at the project can be divided into two separate methods:
1) construction disposal methods utilizing disposal of mined materials into the de-watering dykes, tailings dyke, and airstrip; and 2) conventional disposal methods relying on surface waste dumps and freezing of the tailings storage facility.
Several kilometres of de-watering dykes are required to allow the development of the open pits. Dyke construction will be scheduled from materials excavated in pre-stripping and pre-mining waste rock. The design of the dykes will optimize the use of waste rock material by selectively segregating material into either upstream or downstream rock fill embankments.
The mine waste rock that is surplus to the construction requirements will be placed in conventional surface waste rock storage areas. Several options for the storage of mine waste rock were evaluated using an approach similar to a multiple accounts analysis, whereby key indicators relating to operational, economic, and environmental considerations were evaluated to assess which option was the most suitable for the specific project location and environment. A similar set of site selection criteria were developed to evaluate possible alternatives for disposal of the tailings, again considering environmental, operational, and economic factors. The preferred alternative for disposal of the tailings is into the natural basin of an arm of Second Portage Lake that will be dewatered so that mining can proceed. The tailings are predicted to freeze over time.
Some of the mine wastes have the potential to generate acid rock drainage (ARD) and may be susceptible to metal leaching (ML) under neutral drainage conditions. Consequently, the proposed mine waste management strategy, to allow the waste dumps and the tailings storage area to freeze over time, is aimed at minimizing the potential for ARD generation and ML. Furthermore, utilizing mine waste, where possible, for the construction of the main infrastructure requirements—the de-watering dikes—will result in an overall reduction of the mine footprint area for the project, as requirements for additional quarry sites for the production of construction materials will be reduced or eliminated entirely.