Fluid and metal provenance of the Kiskamavaara Cu-Co deposit, northern Norrbotten, Sweden

Abstract

Abstract Kiskamavaara is a shear zone-hosted cobaltian pyrite and chalcopyrite deposit in Northern Norrbotten, Sweden. Although previously classified as an Iron Oxide Copper-Gold (IOCG) deposit, the dominance of pyrite is atypical, suggesting either an unusual IOCG variant or a deposit type with overlapping characteristics. Understanding deposits like Kiskamavaara, for which limited documentation exists, is critical for refining genetic models and regional exploration strategies. To determine metal and fluid provenance and grain deformation characteristics in Kiskamavaara, this study combines in-situ Electron Backscatter Diffraction for deformation textures, Laser Ablation Inductively Coupled Plasma Mass Spectrometry for trace element geochemistry and Secondary Ion Mass Spectrometry for δ 34 S. Results indicate early pyrite formed under variable physio-chemical conditions, producing concentrically zoned disseminated grains. A later pyrite phase, hosted in veins and breccias, exhibits crystal-plastic deformation with polygonal grains and 120° triple junctions. High Co and Ni concentrations in pyrite, with Co/Ni ratios between 10 and 1000, suggest a magmatic-hydrothermal metal source, possibly derived from nearby greenstones. Chalcopyrite is restricted to the late mineralisation stage and displays exclusively negative δ 34 S values, indicating precipitation from an evolved Cu-bearing hydrothermal fluid. The wide pyrite δ 34 S range of − 4.94‰ to + 11.16‰ implies sulfur isotope fractionation, likely driven by thermochemical sulfate reduction. Combined Se/S ratios of 3.62 × 10⁻⁶ to 5.35 × 10⁻ 4 , δ 34 S and negligible Au, indicates that an oxidised, saline fluid, plausibly formed through mixing between magmatic-hydrothermal fluids and basinal brines, was reduced by thermochemical sulfate reduction, triggering pyrite precipitation and development of a moderately reduced mineral assemblage.