| Summary: | Abstract. The Raja Au-Co mineralization in the Paleoproterozoic (2.5–1.8 Ga) Peräpohja belt in northern Finland is a part of the wider Rompas-Rajapalot area with several known Au occurrences. The area is characterized by a distinct occurrence of tourmaline, locally spatially associated with the known mineralizations. In addition, recently, an unusual rock unit with bright purple anhydrite and white gypsum veins has been intersected by diamond drilling. While the sulfate-rich unit is non-mineralized, the atypical mineralogy and close spatial association to the Raja mineralization is interesting. In this master’s thesis, the occurrence of tourmaline and sulfates are studied, and their relationship to the Raja mineralization is examined. In-situ geochemical, major and trace elements, and boron isotopic data of tourmaline were obtained by EPMA and LA-ICP-MS from thin section samples prepared from Raja drill core. The analytical data was used to complement the petrographic observations from thin sections, and to quantitatively evaluate tourmaline chemical composition. Additionally, orientations of quartz-tourmaline veins from drill core samples were measured and analyzed with stereographic projections. In-situ sulfur isotope data was obtained for the sulfate phases by LA-ICP-MS. Three texturally different types of tourmaline are identified: 1. Coarse grained euhedral tourmaline as abundant dissemination in metasomatic bands, 2. Tourmaline in quartz-tourmaline veins crosscutting the host rock, and 3. Rare footwall tourmaline in healed microfractures, and associated to ambiguous pegmatitic quarts-felspar-mica veins. All assayed tourmaline belongs to alkali-group and are classified as dravite. They have variable chemical composition that suggests crystallization in reduced and low- to moderate saline conditions. Tourmaline geochemistry together with boron isotope data (δ11B from -11.5 to -0.9 ‰) suggests that tourmaline could have formed from mixed fluid source: from devolatilization of clastic metasedimentary rocks or ...
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