Talc-bearing metadolomites of the Itaiacoca Group, Paraná (Southern Brazil): fluid regime and genetic implications

Abstract

Brazil's largest talc deposits occur in the Itaiacoca Group, which is a low-grade metasedimentary belt composed mainly of dolomitic rocks. Fluid inclusions from three geological settings in which talc was formed differently were analysed. Talc schist ore occur along transcurrent shear zones. These talc schists are host to quartz veins, which indicates prolonged circulation of hydrothermal fluids at high water:rock ratios. The fluid inclusions in the quartz veins contain predominantly low-salinity (< 5 wt % NaCl equiv.) and CO2-free aqueous inclusions. The wide range of Thtot values in the primary fluid inclusions in the veins, mainly from 110 to 230ºC, is related to several episodes of fluid percolation during the shear zone evolution. As a contrast, outward of the shear zones, fractured metadolomites with lower concentrations of talc contain quartz veins with CO2-rich fluid inclusions (X CO2 from 0.17 to 0.81) and Thtot ranging mainly from 250 to 300ºC, which indicates restricted fluid circulation during the trapping of these inclusions, shortly after the peak of talcification, in a later brittle stage. Density-salinity relationships for these inclusions suggest supersolvus mixing in variable proportions of aqueous-carbonic fluids. In the contact aureole between the metadolomites and the Cunhaporanga granite complex, where talc was formed in minor amounts by retrometamorphism of high temperature magnesium silicates, fluid inclusions are mainly carbonic to aqueous-carbonic, suggesting restricted fluid circulation. Fluid inclusions in this setting show a wide range of measured and calculated parameters, such as salinity (0 to 20 wt % NaCl equiv.), dCO2 (0.13 to 1.0 g/cm³), dtot (0.2 to 1.0 g/cm³), and Thtot (between 130 and 574ºC). Results obtained in this study did not allow precise determination of the talc-ore forming conditions, considering that the inclusions contain fluids which were trapped after the peak of talcification. Conditions similar to those of talcification are suggested by the Thtot values obtained for the inclusions in quartz-veins from fractured metadolomites, although from a later, brittle stage.