Figure 3
Oxygen isotope compositions of aqueous fluids in equilibrium with magnetite and carbonates from Bastnäs-type deposits at temperatures of 200 to 600 °C. (a) Equilibrium fluid compositions calculated based on the average, minimum and maximum δ18O values measured in magnetite, respectively. The red stippled lines indicate fluid compositions for specific temperatures, within the range given by magnetite-quartz oxygen isotope pairs and published fluid inclusion data for bastnäsite-(Ce)31. Reference fields include primary magmatic waters35,37,38,39,40,41, local carbonate rocks34 and the SMOW line. (b) Similarly modelled oxygen isotope compositions of equilibrium fluids for carbonates. The combined magnetite-carbonate data imply an initially high-temperature, magmatic-dominated hydrothermal system that evolved through decreasing temperature and an increasing input from non-magmatic, low-δ18O fluid sources. See text for detailed explanation. Abbreviations: Bas – bastnäsite; Cb – carbonate; FI – fluid inclusions; Mgt – magnetite; Qtz – Quartz; SMOW – Standard Mean Ocean Water.
