English

Abstract

Over the last 15-20 years, there has been a large emphasis on tailings geochemistry and hydrogeology. Waste rocks piles, on the other hand, have received much less attention. Especially in the porphyry copper mining industry, waste rock piles can represent a larger potential for groundwater and surface water contamination than tailings since the waste rocks are commonly unsaturated and coarse. This setting allows for greater oxygen availablility throughout the pile, thus increasing the oxidation potential. In general, there are three flow systems within waste rocks: macro flow (high flow); matrix flow (low flow) and micro flow (within particles) that together with waste rock composition effect seepage water quality. These systems have been investigated at the Tyrone Mine. Continuous seepage water quality monitoring was performed at one of the stockpiles for approximately one year. The selected stockpile consisted primarily of oxide material with low sulfide content, and had never been leached. Seepage was observed throughout the year and seepage monitoring consisted of flow rate, pH and electric conductivity measurements. Waste rock samples were analyzed for acid base accounting and mineralogy, and kinetic tests were performed. The flow rate together with rainfall, hydrogeological data from the stockpile, stockpile infiltration and evaporation rates, and groundwater recharge rates were used to calculate macro/matrix components in the system using the kinematic wave theory. The results of this modeling indicated that only approximately 10 % of the infiltrating rainwater flowed through the macro pores all the way to the bottom. The majority of the water flowed through the matrix. Water quality monitoring showed a pH increase during low flow periods and a rapid drop during heavy rainstorms. Electric conductivity had a slow decrease during the dry periods and a drastic increase shortly after heavy rainstorms. The geochemical data indicated a buildup of easily leachable secondary acid generating minerals during the dry periods that were being leached during heavy rain storms. The matrix also possessed a neutralizing effect from silicate mineral dissolution that was working effectively only in the slow matrix flow zones.