Metamorphic magnesite deposits of Brumado (Bahia, Brazil): Evidence from highly saline H2O-CO2 fluid inclusions

Abstract

Magnesite deposits in Brumado, Bahia, Brazil, occur in the Serra das Éguas, a mountain range, where an Archean greenstone belt is exposed within the Gaviao block in the Sao Francisco Craton in Eastern Brazil. A genetic model is designed using the properties and distribution of fluid and solid inclusions in magnesite. These inclusions are analysed with careful petrography, and several technical methods, such as microthermometry, Raman spectroscopy, cryo-Raman spectroscopy, and ion chromatography. Equations of state of complex water-gas-salt mixtures are used to establish pressure and temperature conditions of ore formation. Two different mineralization types of coarse-grained sparry magnesite are distinguished. Type 1 consists of a coarse-grained reddish coloured magnesite exposed at the Pedra Preta mine and at the Pedra de Ferro prospect, whereas type 2 is composed of more fine-grained, purely white magnesite currently produced at the Pomba mine and explored at the Pirajá prospect. Abundant finely dispersed hematite is found in the reddish coloured magnesite of the type 1 mineralization, and is nearly absent in white magnesite of the type 2 mineralization. Fluid inclusions are exceptionally large (up to 50 μm) and provide excellent objects for the estimation of properties with all analytical techniques. The fluid inclusion assemblages are homogeneous. A sequence of three types of fluid inclusions is observed in both magnesite deposits, that illustrates various gradual modifications of pore fluids properties during formation of the ore: 1. a decreasing content of CO₂, from about 21 mol% to trace amounts; 2. a decrease in volume fraction of the CO₂-rich phases; 3. a decrease in density of the CO₂ phases; 4. nearly constant salinity of the aqueous phase (approximately 14 eq. mole% NaCl). Cryo-Raman spectroscopy reveals that CaCl₂ is the major salt component within fluid inclusions, and the salinity can be expressed in terms of about 7 eq. mole% CaCl₂, which correspond to similar chlorinity values. The ionic ratio of Na/K and Na/Li is sensitive to metamorphic conditions, and can be used as paleo-geothermometer yielding about 320 °C for the white magnesite and 250 °C for the reddish coloured magnesite. However, these values are obtained from bulk analyses and must be regarded as mechanical mixed values of all types of inclusions within a sample. The triple halogen ion content (Cl-Br-I) is not affected by metamorphic processes and reveals the origin of salinity: an Archean bittern, which is the major source of magnesium. This highly saline H₂O-CO₂ fluids with a high density were trapped under various metamorphic conditions in both types of magnesite of the Brumado deposits. Minimum trapping conditions of the fluid, and therefore of magnesite formation is about 670 °C and 371 MPa. The source of CO₂ is of metamorphic origin. The consumption of both CO₂ and magnesium to produce magnesite is recorded in the observed trends of fluid inclusion properties. The late-stage fluid inclusion assemblages are trapped at about 250 °C and 100 MPa.