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The geochemistry of Westmin Resources Ltd. Broughton, L. M.


Westmin Resources Ltd. operate a 3000 tpd copper-lead-zinc mine 93 km by highway south-southwest of Campbell River in Strathcona Provincial Park. The mine began operations in 1966. Tailings were initially discharged directly to the bottom of Buttle Lake; a practice which continued until an on-land tailings disposal facility was commissioned in June 1984. Testwork has indicated significant acid production potential in the sulphide tailings. Tailings are now deposited by the "subaerial" method; a procedure designed to produce an unsaturated, high density tailings mass with low horizontal and very low vertical permeability. This is a relatively new technique in Western Canada and the Myra Falls facility provided an ideal opportunity to evaluate the effectiveness of subaerial deposition to limit acid generation and contaminant migration. A number of surveys have been conducted by Environment Canada and Knight and Piesold Ltd. to evaluate the geochemical and geotechnical characteristics of the tailings mass over the deposition history. This report describes a detailed study conducted by Environment Canada and L.M. Broughton in 1989, with reference to the previous study (Broughton and Ferguson, 1989) to evaluate changes over time. The field sampling component of this study included surface solids sampling of fresh and older tailings, deposited over a seven month time period. Surface water samples were collected. Depth profiles through the newer and old tailings layers in the tailings impoundment and an emergency storage pond were established of the geotechnical and geochemical characteristics through solid sampling over depth, pore water sampling, and laboratory testing including paste analyses, shake flask extraction tests. Sulphide oxidation was occurring at the surface and over depth in the tailings. Contaminant fronts over the vertical profile of the tailings mass were identified, and the rate of infiltration estimated. Elevated zinc, acidity, sulphate and conductivity levels were detected at depth in the impoundment. The infiltration of alkaline tailings water from freshly placed tailings neutralized low pH fronts and precipitated metals constrained by pH (Cu and Fe) near the interface of fresh and older tailings. Oxygen and carbon dioxide profiles were developed to determine oxidation rates, oxygen consumption and the ingress of oxygen to the depth in the tailings. Geotechnical characteristics of the tailings mass within the layers sampled were similar to measurements at a conventional saturated tailings impoundment. The placement of a fresh tailings layer did not appear to significantly change the rate of oxidation or contaminant migration through the tailings. Layers of fine silt with high moisture content limited the vertical diffusion of oxygen into the tailings, and the rate of oxidation. Due to operational constraints, there were long periods between tailings deposition which allowed drying and cracking of the subaerial tailings mass, and development of surface oxidation and low pH fronts which have penetrated to depth in the tailings. Vertical cracking of the tailings allowed the ingress of the oxygen to more permeable underlying sand layers promoting oxidation at depth in the tailings. Relatively simple field and laboratory techniques were developed and applied to evaluate the characteristics and performance of the subaerial tailings mass. Such evaluations should be conducted to determine the nature of the tailings during operation, and for design of alternative closure schemes, but must be carefully designed to recover the essential data within a reasonable time frame and scope of laboratory and field work. The results of investigations of the geochemical nature of the Myra Falls tailings impoundment indicate that acid generation, metal leaching, and contaminant migration are established in the subaerial tailings mass. Closure and reclamation strategies must address limiting the rate and extent of acid generation, and address containment of the contaminated drainage water within the impoundment. All acidic drainage from the Westmin site is collected and treated in a lime neutralization circuit and series of settling ponds, and is sampled and analyzed prior to discharge.

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