Abstract
The 60-million tonne, pyritic waste rock stockpile at the Red Dog Mine produces acid rock drainage that impacts the mine’s water treatment and closure planning. This study evaluated the acid generation potential of the Main Waste Stockpile through evaluation of drill cuttings collected from the top to base of the stockpile at a single location near its centerline. Evaluation of waste rock chemical properties indicated major constituents of quartz or barite, along with the primary sulfides of sphalerite, pyrite, marcasite, and galena. Five-day batch reactor tests were used to identify groups of acid generating and acid neutralizing intervals that responded similarly to the continuously saturated conditions. Results of the batch reactor tests indicated initial acidic conditions in 15 of 21 sample intervals due to stored acidity, with acidic conditions in nine intervals for the duration of the test and continued decreases in pH observed in four intervals, attributed to the dissolution of secondary jarosite and the initiation of primary pyrite oxidation. The zone with the highest acid-generating potential is located at the base of the pile. Temporal trends of increasing pH from carbonate and phosphate mineral dissolution were observed at intermediate depths, and probable acid neutralization by clay minerals in the near surface zone. Estimates of iron sulfide content and batch reactor results were used to predict a potential acid generation window of 70 to 2,490 years under saturated conditions. Results of the study indicate that relatively short batch reactor tests can yield valuable reaction data for understanding acid generating potential. These estimates provide context for the mine site operator to evaluate stockpile management and closure activities.