BLASTING RESIDUALS IN PORE WATER AND EFFLUENT FROM WASTE ROCK PILES

The oxidation of sulfide minerals can lead to the release of acidic, metal-bearing drainage from mine waste deposits. Mine-water chemistry also may be influenced by residual blasting agents used during mining. Ammonium nitrate mixed with fuel oil (ANFO) is the primary blasting agent used in metal and coal mining, quarrying and civil construction. Ammonium nitrate is highly soluble in water and contributes NH₄ and NO₃ pore water and effluent. These dissolved constituents present the opportunity to examine flow characteristics and geochemical processes within mine wastes. The Rio Tinto Diavik Diamond Mine is located 300 km northeast of Yellowknife, Northwest Territories, Canada, on a 20 km² island in Lac de Gras. Waste rock generated from the open pit mining operation is stockpiled on site and the quality of water derived from this material is being studied. Three large-scale test piles, measuring 60 by 50 m in area and 15 m high, were constructed at Diavik as part of a comprehensive research program. Ongoing monitoring of the water chemistry began in 2007 and blasting residuals were identified as primary components of pore water and effluent. Variations in concentrations and the gradual rates of dissipation of blasting residuals provide an indication of the pile heterogeneity and the relative contribution of different flow paths. As temperatures within the piles increase with ambient temperature increases and higher proportions of the pile contribute to flow, increased concentrations of blasting residuals were observed in waste rock effluent. Mass balance calculations based on the ratios of SO₄:NO₃ can be used to estimate the relative contributions of sulfide oxidation within the piles and sulfate released when sulfur in the host rock is oxidized during blasting. These calculations can also provide an estimate of S mass released during the first flush of the piles. This research will aid in understanding the release of constituents caused by blasting and waste-rock hydrology.