Post-crystallization fluid behavior in acidic volcanic context: genesis and isotopic signature of fracture-filling calcite in the Paraná Basin, Brazil

Abstract

Basaltic lava flows exhibit a complex internal structure dominated by faults, fractures, disjunctions and vesicles. The percolation of meteoric or hydrothermal fluids is observed in these structures, sometimes resulting in the precipitation of minerals such as quartz, zeolites, and calcites. Analyzing these mineral fillings enables the recognition of different geological parameters, especially when calcite is present. Calcite within fractures can record rock-fluid interaction processes, serving as a marker of secondary precipitation events. The Paraná Basin, in the central-southern portion of the South American Platform, records the Serra Geral Formation, a volcanic event linked to Mesozoic fissural volcanism, responsible for the extrusion of large volumes of basaltic flows. The Serra Geral Formation has been intensely affected by a complex system of fractures and faults resulting from various geological processes over time. Calcite plays a fundamental role in filling void spaces in fractures within this occurrence, with remarkable mineralogical and chemical properties associated with fluid circulation. The infill of these fractures with calcite reflects fluid movement episodes that occurred after the initial formation of the host rocks. The Pedreira Caxiense Fagundes, an outcrop of the Serra Geral Formation, displays three verticalized fractures selected for sampling, followed by petrographic and geochemical analyses, X-ray fluorescence, X-ray diffraction, cathodoluminescence, stable isotopic ratio measurements, and fluid inclusion analysis. Chemical and isotopic evidence suggest that the percolating fluid is homogeneous and of surface origin (modern "fresh" meteoric/groundwater) with inorganic carbon contributions and alteration of acidic volcanic rock. The minerals formed in a highly stable environment, with fluid percolation occurring slowly through fault spaces, and the fractures remained stable. The carbonates are inferred to be younger than 11,700 years, a date associated with the region's last neotectonic movements.