Talc-bearing Metadolomites of the Itaiacoca Group, Paraná (Southern Brazil): Fluid Regime and Genetic Implications

• Published in: Geologia USP. Série científica Vol. 11; no. 1; pp. 171 - 187
• Main Authors: Danilo Marques Saunite, Rosa Maria da Silveira Bello, Fábio Ramos Dias de Andrade, Gergely Andrés Julio Szabó
• Format: Journal Article
• Language: English
• Published: Universidade de São Paulo 01-04-2011
• Subjects: Fluid inclusions; Itaiacoca Group; Metadolomite; Southern Brazil; Talc
• ISSN: 1519-874X

Brazil’s largest talc deposits occur in the Itaiacoca Group, which is a low-grade metasedimentary belt composedmainly of dolomitic rocks. Fluid inclusions from three geological settings in which talc was formed differently wereanalysed. Talc schist ore occur along transcurrent shear zones. These talc schists are host to quartz veins, whichindicates prolonged circulation of hydrothermal fluids at high water:rock ratios. The fluid inclusions in the quartzveins contain predominantly low-salinity (< 5 wt % NaCl equiv.) and CO2-free aqueous inclusions. The wide range ofThtot values in the primary fluid inclusions in the veins, mainly from 110 to 230oC, is related to several episodes offluid percolation during the shear zone evolution. As a contrast, outward of the shear zones, fractured metadolomiteswith lower concentrations of talc contain quartz veins with CO2-rich fluid inclusions (XCO2 from 0.17 to 0.81) and Thtotranging 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 suggestsupersolvus mixing in variable proportions of aqueous-carbonic fluids. In the contact aureole between the metadolomitesand 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 % NaClequiv.), 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 didnot allow precise determination of the talc-ore forming conditions, considering that the inclusions contain fluids which weretrapped after the peak of talcification. Conditions similar to those of talcification are suggested by the Thtot values obtained for theinclusions in quartz-veins from fractured metadolomites, although from a later, brittle stage.