ABSTRACT
High concentrations of arsenic (> 100 ppm) in a 3 km wide swath of bedrock threatens future projects that excavate rock or use rock aggregate from the Arlanda area. The bedrock is a Proterozoic volcanosedimentary sequence of schists/gneisses intruded by multiple intermediate-mafic dykes and pegmatite complexes which were later sheath-folded leading to cm-scale layering of different lithologies. A detailed, layer-by-layer mapping campaign complemented by handheld XRF analyses in each layer and whole rock geochemical analyses of selected samples was developed to create a high-resolution understanding of the distribution of arsenic in this complex bedrock. This methodology is tested as a tool for risk assessment in all types of rock excavation projects, including tunnels.
Mica-rich metasediments, amphibolite sills and dykes, and fractionated pegmatites were found to be the arsenic bearing lithologies. Weighting arsenic content by the thickness of each lithological layer had only a minor effect on results indicating that comprehensive layer-by-layer analysis is not necessary. Arsenic distribution is not consistent within each lithology and their disruption by high strain hinders risk assessment. However, the originally 3 km wide swath of metasupracrustals originally identified as moderate to high risk for As has been substantially reduced to only a 1.3 km portion along the center of the belt.
