Changes of sulphide oxidation rates over time in mine tailings , Laver , Northern Sweden

Pyrrhotite-bearing tailings were deposited in an impoundment at the Copper mine at Laver in Northern Sweden 1936-1946. Sulphide oxidation has released acidity and metals to the recipient since deposition. Sampling of surface water was performed in a brook draining the tailings impoundment in 1993, 2001 and 2004-05. The dissolved concentration of sulphide associated elements such as Cu, S and Zn decreased by more than 40 % between 1993 and 2001, and pH increased in 2001 and in 2004-05 compared to 1993. The annual amount of elements released from the tailings was estimated based on the volume of tailings assumed to oxidise each year, which depends on the oxidation front movement with time. The mass transport of S in the brook during 1993 and 2001 corresponded very well with the estimated annual amount of S released from the tailings. Secondary precipitates such as covellite and gypsum which could retain sulphur was observed in the tailings, but in low amounts. The results suggest that the oxidation rate in the tailings decreases with time and might be a consequence of the increased distance that oxygen had to travel to reach unoxidised sulphide grains or cores of grains in the tailings.