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The Bastns-type magnetite-skarn-hosted rare earth element (REE) mineralisations are located in the Palaeoproterozoic Bergslagen ore province, central Sweden, and occur along a ca 100 km long belt of 1.90-1.87 Ga metavolcanosedimentary rocks. Ten different Bastns-type deposits have been sampled in order to assess the origin of ore-forming fluids in these systems._x000D_
Magnetite and quartz have 18O values ranging from -2.3 to +1.1 and +7.2 to +8.3 respectively. Mineralised carbonates have 18O values between +5.8 and +7.2 and 13C values between -5.4 and -3.3. Temperature calculations based on the O-isotope composition of mineral pairs, combined with available fluid inclusion data, suggest primary mineralisation at ca 650-400C. For these temperatures, calculated equilibrium fluids have 18O values that range from those typical of magmatic fluids (+6 to +8) to lower values (down to +3)._x000D_
Variation in carbonate O and C isotope data are consistent with interaction between local Palaeoproterozoic carbonate rocks (18O = +12.5 to +19.6 and 13C = -1.8 to +1.7) and fluids of a magmatic origin. It is suggested that the Bastns-type REE-magnetite mineralisations formed as a result of reactions between local carbonate units and high-temperature, magmatic-dominated hydrothermal fluids during 1.90-1.87 Ga (Svecofennian) syn-volcanic activity.CITATION:Sahlstrm, F, Jonsson, E, Hgdahl, K, Harris, C, Troll, V R and Jolis, E M, 2015. O and C isotope study of Bastns-type rare earth element mineralisation, Bergslagen, Sweden, in Proceedings PACRIM 2015 Congress, pp 439-444 (The Australasian Institute of Mining and Metallurgy: Melbourne).