Weathering of sulphide-mineralized till at Liikavaara, northern Sweden, and consequences for surface waters in the area
Abstract: The Aitik open-pit mine in northern Sweden is the largest Cu-mine in Europe. Another, smaller ore body, Liikavaara Östra Cu-sulphide deposit, located 4 km east of Aitik, will probably be mined in the near future. The till east of the Liikavaara Östra deposit is in some parts rich in Cu and other sulphide-associated metals. Groundwater flowing in this till has higher concentrations of Cu than groundwater in a reference area where the till has low concentrations of Cu and other metals. To explain the metal enrichment in the groundwater, a detailed study of the till where the highest metal concentrations occur was performed. The importance of local diffusion of Cu-bearing airborne dust from the adjacent Aitik mine tailings impoundment was quantified and solid speciation of metals in airborne material was performed by sequential extraction. The results from Liikavaara were compared to results from the reference area. Glacial ice picked up sulphide-rich material from the Liikavaara Östra Cu ore during its flow. This material was deposited in the till east of the ore body when the ice melted. The sulphides have been dissolved in the <2 mm fraction during 8.7 ka of weathering since the glacial ice retreated. After dissolution of the sulphides, Cu was redistributed and secondarily retained in association with Fe-oxyhydroxides and altered biotite. Other sulphide- associated elements such as Co, Ni and Zn show the same pattern in the till profile as Cu, though the concentrations are much lower. The airborne dust is not the source of Cu in the till. The high metal concentrations in the till cause the increased concentrations in the local groundwater. A fluctuating groundwater table washes out Cu and other metals from the till. To determine if weathering of the metal-rich till results in naturally increased metal concentrations in local streamwater, a whole-year cycle geochemical study of streamwater was performed. Sampling was performed at four stations, located in the major river in the area, Myllyjoki, and in a small brook flowing in the area with the highest Cu-concentrations in the till. This brook flows into Myllyjoki. The surface waters at Liikavaara have a higher dissolved concentration of Cu than the reference area. The relative importance of different sources contributing to the streamwater was calculated from geochemical data, clearly showing that shallow groundwater is the most important source most of the year. The most probable explanation for the increased Cu concentrations in Liikavaara surface waters is the local shallow groundwater. However, peaks with increased Cu concentration in the brook during snowmelt indicate that the airborne Cu from Aitik also influences the streamwater concentration. During the snowmelt season, the suspended phase in the streamwater is dominated by detrital silicate particles, whereas during the baseflow season, nondetrital particles are more important. Consequently, the concentrations of Cu and other metals in the suspended phase show considerable seasonal variations. The largest lake in the Aitik-Liikavaara area is Sakajärvi, and Myllyjoki is its most important inflow. To study how important Myllyjoki is as source for the sediments in Lake Sakajärvi, a comparison between the lake sediments and suspended river material was performed. Myllyjoki suspended matter and airborn dust from Aitik are the most important sources to the sediments. Nondetrital phases such as Fe- and Mn-oxyhydroxides carry the major part Cu and other metals in the suspended phase in the river. When this nondetrital material becomes buried by new sediments, dissolution of the oxyhydroxides results in release of trace metals, which diffuse upwards to the surface sediment where Mn-oxyhydroxides precipitate. Most trace metals are, therefore, enriched in the surface sediment where they show strong correlation with Mn, indicating an association to the Mn-oxyhydroxides.
This dissertation MIGHT be available in PDF-format. Check this page to see if it is available for download.