Arsenic, which is a naturally occurring element in the severe health issues during exposure. Both the World Health Organisation and The Swedish Environmental Protection Agency (Naturvårdsverket) state that >10 ppm arsenic in bedrock is already concerning. During a construction of a new highway junction in the Arlanda-Rosersberg area soil samples revealed elevated levels of arsenic. Since it poses a risk to public health and further spreading without knowledge about the origin and extent of the contamination, the Swedish Geological Survey undertook an investigation to address this issue which included geological mapping as well as geochemical analysis of the whole rock samples.  This thesis has used the collected geochemical data to visualise the spatial distribution of arsenic over the project area to examine if there is a correlation between elevated concentrations and specific lithologies, or if other processes such as pegmatite formation could be responsible enrichment. Kernel interpolation with ArcGIS Pro was used to create three maps with unique parameters: without geological barriers, with geological barriers including samples taken from nearby pegmatites, and with geological barriers excluding samples taken from nearby pegmatites. The resulting maps indicate that a majority of the project area far exceeds the recommended limit, regardless of the parameters.  Based on the data it is currently impossible to distinctly determine the origin of the arsenic contamination, since the results suggest a role of both lithological correlation and process-related source, and a combination of both cannot be ruled out. Therefore, to improve the resolution and knowledge about the area, more extensive examination and gathering of data would be beneficial, as well as implementing improvements of the method to better simulate the mobilisation of arsenic. / Arsenik är ett naturligt förekommande grundämne i berggrunden som är känt för att orsaka allvarliga problem för människors hälsa vid exponering. Både World Health Organisation och Naturvårdsverket uppger att >10 ppm arsenik i berggrunden är oroväckande. Vid byggandet av en motorvägsavfart i Arlanda-Rosersbergs området visade jordprover höga koncentrationer av arsenik. Eftersom det utgör en risk för människors hälsa, samt fortsatt spridning utan kunskap om ursprung och utsträckning av kontaminationen, utförde Sveriges Geologiska Undersökning (SGU) ett projekt vilket inkluderade geologisk kartläggning samt geokemiska analyser.  Det här självständiga arbetet har använt det geokemiska data för att visualisera den rumsliga spridningen av arsenik över projektområdet för att undersöka om det finns ett samband mellan höga koncentrationer och geologiska enheter, eller om magmatiska processer som bildandet av pegmatit kan vara källan till arsenik. Kernel interpolation med ArcGIS Pro användes för att skapa tre kartor med unika parametrar: utan geologiska barriärer, med geologiska barriärer inklusive prover tagna i närheten av pegmatiter, med geologiska barriärer exklusive prover tagna i närheten av pegmatiter. De resulterande kartorna indikerar att en majoritet av projektområdet överskrider den rekommenderade gränsen, oavsett parametrar.  Baserat på data är det vid nuläget omöjligt att avgöra ursprunget av arsenik, eftersom resultatet indikerar både geologiskt samband och process relaterad källa, därför kan en kombination av dem inte uteslutas. För att förbättra kunskapen om området krävs mer omfattande undersökning och data, samt förbättringar av metoden för att bättre simulera mobiliseringen av arsenik.

Abstract The metamorphosed Palaeoproterozoic Orivesi gold deposit in southern Finland is located within the Tampere Schist Belt, which belongs to the Svecofennian domain. The Orivesi mine, run by Outokumpu Mining Oy, was in production from 1994 to 2003, during which time a total of approximately 1.7 million tons of ore was extracted, with an Au content of 9.31 g/t, implying a total output 13.115 tons of gold in concentrate. The hydrothermal alteration halo can be divided successively into chlorite-dominant, sericite-dominant and quartz-dominant rocks from the outer zone inwards. The host rocks of the ore are quartz rocks with andalusite-rich quartz rocks. Topaz-bearing rocks also occur in the inner part of the alteration halo. In addition to Au, the elements Ag, Te, Bi, Sb, S, As, Se, Cu, Zn, Pb, Sn and Mo are enriched to varying degrees within the alteration halo. The main ore minerals include base metal sulphides, sulphosalts and tellurides. Pyrite is the most common sulphide. The sulphosalts are represented by tetrahedrite, bournonite, boulangerite and meneghinite. The most common gold, gold-silver and silver tellurides are calaverite, montbrayite, petzite, kostovite, sylvanite and hessite. Other known tellurides include tellurobismuthite, altaite, melonite, frohbergite, tsumoite, tetradymite and rucklidgeite. Gold occurs mostly in fine-grained native grains containing an average of 5% Ag. The native gold is usually of very small grain size, generally < 20 µm. Most of the gold grains in the deposit occur as intergrowths with tellurides. The adjacent hypabyssal intrusion is an obvious source of both hydrothermal fluids and metals. A comb quartz layering has been discovered in the transition zone between the intrusion and the alteration halo. The Orivesi deposit is thought to belong to the high-sulphidation epithermal type. Soon after its formation the deposit encountered deformation and metamorphism that amounted to lower amphibolite facies conditions. The subsequent retrograde metamorphism caused the reappearance of some hydrothermal minerals typical of high-sulphidation epithermal deposits.