Origin and migration timing of hydrothermal fluids in sedimentary rocks of the Paraná Basin, South America

Clay authigenesis induced by migrating hydrothermal fluids related to the Early Cretaceous Serra Geral magmatism in the Paraná Basin was investigated by mineralogical, geochemical and isotopic means. Stratigraphically equivalent sedimentary samples from the Permian Rio Bonito (in southeastern Brazil) and San Miguel (in eastern Paraguay) formations were collected close to and far from major intrusive bodies. The rocks are generally cemented, in addition to the clay material, by a variety of minerals, such as albite, K-feldspar, calcite, dolomite, zeolite and quartz.The authigenic clay minerals mainly consist of different types of illite-rich mixed layers illite–smectite and chlorite. Corrensite and smectite were also observed. Measured and estimated K–Ar ages indicate that the varied authigenic illite assemblages were most probably generated 140 to 130 Ma ago, during the Serra Geral magmatic event. Hydrothermal fluids depleted in LREE and sometimes enriched in Eu promoted the formation of the well-crystallized illite minerals in rocks characterized by high water / rock ratios. Additional authigenic assemblages with PAAS-type REE patterns probably formed in less porous rocks with low water / rock ratios. Also, some of the authigenic clays yield HREE-depleted patterns, which point to the occurrence of organic matter during crystallization.