Detailed determination of palaeofluid chemistry: an integrated study of sulphate-volatile rich brines and aquo-carbonic fluids in quartz veins from Ouro Fino (Brazil)

This paper presents the results of a detailed determination of the chemistry of an unusual palaeo-fluid, using an integrated approach based on multi-technique analyses of fluid inclusions. The investigated samples were retrograde metamorphic quartz veins from Ouro Fino (Quadrilatero Ferrifero, Minas Gerais, Brazil): (i) hematite-quartz veins which crosscut itabirites and display complex inclusion fluid chemistries, (ii) sulphide-quartz veins found in the vicinity of the hematite-quartz veins. Gas and sulphate contents were determined from microthermometry and Raman microspectroscopy data, and ion contents by a crush–leach technique and Laser Ablation Optical Emission Spectroscopy (LA-OES). An estimation of the fluid pH at 25°C was made from the sulphate/bisulphate concentrations, calibrated using a series of fluid chemistries that bracket the natural fluid compositions. Primary fluid inclusions in the hematite-quartz veins display uncommon compositions characterized by high sulphate concentrations (0.7 to 2.1 mol/kg H2O) and a CO2–N2 volatile phase. Quantitative analysis of sulphate and bisulphate concentrations by Raman microspectroscopy yields an estimate of the inclusion fluid pH of around 1 (±1) at room temperature. These fluids are oxidizing (fO2 above the hematite-magnetite buffer at temperature of entrapment) and are characterized by very low Na/K (∼0.45), and Cl/SO4 (∼0.1) ratios, Na/Ca ratio around 8, and high B, Li and F contents. Such high K and SO4 values are unusual and are normally only found in acid geothermal waters. The sulphide-quartz veins, primarily located in the metamorphic rocks surrounding the itabirites, contain CO2–(CH4,N2)-rich fluids and varying amounts of water. The dissolved salts are Ca–Na–K chlorides with Na/K ∼2.3, Na/Ca ∼0.75 and Cl/SO4 ∼10. These fluids are indicative of fO2 close to the Ni–NiO buffer at temperature of entrapment. Despite the significant chemical differences, fluids in both cases display Br/Cl ratios of ∼0.012, which could be interpreted as evolved seawater from which halite had precipitated. This suggests that the two fluids may have had the same origin and that other differences in chemistry may likely to have been caused by intense fluid–rock interaction.