ARD/AMD potential of the waste management area : Goldcorp Inc., Red Lake Mine Division, Balmertown, Ontario : final report.

Abstract

As part of the decommissioning plans of Goldcorp Inc., the potential of acid generation from the tailings was addressed with acid base accounting tests, The occurrence of acid generating minerals along with neutralizing minerals is extremely varied and the limited number of ABA tests lead to uncertain results. Boojum Research Ltd. was retained to assess the acid generation potential further. From a field investigation of the surface water on the tailings it was concluded, that within one year of cessation of tailings discharge, the physical/chemical conditions ofthe ponded water resembles those ofthe surrounding fresh water and supports considerable biological activity. The pH in ponded water on the tailings ranged from 7.0 to 8.6 and in the sediments/tailings pH values as high as 9.1 were measured. The electrical conductivity was low, with values ranging between 200 umhos/cm to 1000 umhoslcm. These surface waters would facilitate ecological approaches to decommissioning. To determine a reliable estimate of the quantity of acid generating minerals in the tailings, data were extracted from mineralogical and milling records in addition to chemical analysis reported for tailings. The data consistently produce an average of 1.5% S, 6.7% Fe and 2.7% Al. Sequential extraction of the tailings for mineral association produced a weight loss of 42%, the remaining 68% of the tailings are totally inert. Of the digestible fraction, 29% consisted of alkalinity generating minerals and 21% of potentially acid generating minerals, thus the neutralizing potential mass is equal to that of the acid generating mass. Through the extraction of weathering products which had formed in the tailings the ongoing oxidation, leading to acid generation and the concurrent neutralization was determined. Tailings material collected from old (32 years) and new ( 10 years) tailings ponds was leached with distilled water. The leach solutions were all above pH 7 and alkalinity between 50 to 210 mg.L-’ CaCO, equivalent. The tailings material clearly has remaining neutralization potential left. In the leachate from visually oxidizing tailings, 36% of S in the solids was mobilized with water, along with 14% Ca afler 10 years of exposure in the tailings pond. In the visually unoxidized sample exposed for the same time only 2.3% of the S and 2.2% of the Ca could be liberated by distilled water. Acid generation and its concurrent neutralization appears to be localized in pockets of the tailings pond. The rate of acid generation and neutralization has not been determined. It can be concluded that effluent problems normally associated with acid generating tailings will not be encountered for the Goldcorp Inc. tailings area. The concentrate stored on the tailings is acid generating. It is at present unclear, if some piezometer water quality is affected by this material.