Geochemical processes in sulphidic mine tailings : field and laboratory studies performed in northern Sweden at the Laver, Stekenjokk and Kristineberg mine-sites

This thesis presents geochemical, mineralogical and laboratory studies of unremediated and remediated tailings at two abandoned mine sites; at Laver and Stekenjokk and at one active mine site; the Kristineberg mine. At Laver the Cu released from oxidation is enriched in an enrichment zone just below the oxidation front. Other sulphide bound metals seem to be more evenly spread over a greater volume of tailings and are mainly immobilised as a result of adsorption onto mineral surfaces. Adsorption is also important for Cu, but covellite (CuS) formation is more important, due to transformation of pyrrhotite (Fe1-xS). The groundwater within the impoundment at Laver is affected by the breakdown of sulphides during oxidation. Vertical flow of contaminated water is not the cause of groundwater contamination, since the metals are to a large extent retained in the tailings below the oxidation front. Instead, contamination occurs when the advancing oxidation front pushes the secondary enrichments of metals ahead to meet the groundwater table and the metals are released to the groundwater. The release of metals is caused by desorption due to the low pH in this environment. Areas of the tailings deposit with shallow groundwater table are at present the main sources of metal release. There is a seasonal variation in the composition of groundwater, particularly shallow groundwater, caused by changing levels of the groundwater table. Laboratory studies of the tailings from Laver and Stekenjokk using Humidity Cells tests show that the tailings at Laver are more reactive and have a higher oxidation rate compared with the tailings at Stekenjokk. This is probably due to the large carbonate content at Stekenjokk. The oxidation rate of the Stekenjokk tailings decreases with time, probably due to Fe-oxide-hydroxide coatings on sulphide surfaces. High pH conditions can be maintained and low metal concentrations are measured in the leachate waters. Diffusion of metals from the tailings up into the overlying pond water is occurring at Stekenjokk. Oxygen is available down to depths of 16 to 17 mm in the tailings and minor oxidation seems to occur in the uppermost tailings. Due to the oxygen consumption by decomposing organic material the oxidation rate will decrease in the future. An organic layer and a layer rich in Mn- and Fe-oxide-hydroxides have developed on top of the tailings since the flooding. The layers contain higher element versus Ti ratios compared to the underlying tailings, indicating that these layers, to some extent, control the diffusion of elements from the tailings. The suspended phase in the pond contains high metal concentrations and consists mostly of Fe- and Mn-oxyhydroxides precipitated in the pond and detrital material from the surroundings. Sorption of metals onto Fe- and Mn-oxyhydroxides is occurring. The element concentrations in the pond at Stekenjokk therefore seem to be largely controlled by processes controlling the precipitation and dissolution of Mn- and Fe-oxyhydroxides in both the water column and in the surficial tailings. The flooding works well. Pore water and solid tailings in impoundment 1 at the Kristineberg mine contain high contents of metals. Flush out of elements seems to be occurring at some locations due to inflow of surface- and ground water from the surroundings, since the total molar concentrations are much lower in the upper part of the tailings. The pH is high at most depths in the tailings except in and around the former oxidation zone, where the highest heavy metal concentrations also are generally measured. It seems likely that the aim of remediation has been fulfilled, in that oxidation has ceased. However, desorption and remobilization of metals may occur after remediation due to the raised groundwater table.Godkänd; 2000; 20061117 (haneit