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 chemistr y of an unusual palaeo-fluid, using an integrated approach based on multi-t echnique analyses of fluid inclusions. The investigated samples were retrog rade metamorphic quartz veins from Ouro Fine (Quadrilatero Ferrifero, Minas Gerais, Brazil): (i) hematite-quartz veins which crosscut itabirites and d isplay complex inclusion fluid chemistries, (ii) sulphide-quartz veins foun d in the vicinity of the hematite-quartz veins. Gas and sulphate contents w ere 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 degrees C was m ade from the sulphate/bisulphate concentrations, calibrated using a series of fluid chemistries that bracket the natural fluid compositions. Primary f luid 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-N-2 volatile phase. Quantitative analysis of sulphate and bisulphate c oncentrations by Raman microspectroscopy yields an estimate of the inclusio n fluid pH of around 1 (+/- 1)at room temperature. These fluids are oxidizi ng (So, above the hematite-magnetite buffer at temperature of entrapment) a nd are characterized by very low Na/K(similar to 0.45), and Cl/SO4 (similar to 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 geoth ermal waters. The sulphide-quartz veins, primarily located in the metamorph ic rocks surrounding the itabirites, contain CO2-(CH4,N-2)-rich fluids and varying amounts of water. The dissolved salts are Ca-Na-K chlorides with Na /K similar to 2.3, Na/Ca similar to 0.75 and Cl/SO4 similar to 10. These fl uids are indicative of f(O2) close to the Ni-NiO buffer at temperature of e ntrapment. Despite the significant chemical differences, fluids in both cas es display Br/Cl ratios of similar to 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 chem istry may likely to have been caused by intense fluid-rock interaction. (C) 1999 Elsevier Science B.V. All rights reserved.