Mineralogical and physicochemical constraints on the hydrothermalism of volcanic rocks in the Paraná Igneous Province, northwestern Paraná state, Brazil

Abstract Hydrothermally affected basaltic flows and volcaniclastic rocks in northwestern Parana (Brazil) display textural features associated with secondary minerals, reflecting the temperature intensity and evolution of the hydrothermal process. Determination of the secondary mineralogy and the geochemical processes associated with their precipitation are addressed in this paper. The lithotypes that comprise the bulk of the Parana Igneous Province have millimetric to centimetric vesicles filled by secondary minerals. Field data, petrographic descriptions, mineral characterization and mineral chemistry were used to analyze these secondary mineral phases. Matrix material in volcaniclastic rocks (e.g. shards, glass pisolites) precedes the precipitation of the secondary minerals. Coalescence and necking features are common in vesicles located on top of basaltic flows. Clay minerals identified as vesicle and/or fracture infillings include celadonite, illite/smectite (I/S) interestratifications, as well as montmorillonite and nontronite, with morphologies typical of authigenic volcanic processes such as cauliflower-like and honeycomb. The occurrence of different zeolites species is primarily controlled by CaO and Na2O activity in the hydrothermal fluid. Heulandite is the most pervasive zeolite, followed by stilbite and analcime. Calcite occurs in the center of vesicles, as the last crystallizing phase, or isolated in monomineralic vesicles. Vesicle formation occurred by magma degassing processes at temperatures of up to 1150 °C. However, infilling of these voids occurred later with the percolation of hydrothermal fluids external to the system. Although random from a geographical perspective, gradual decrease in temperature and fluid evolution were responsible for the systematic change in the mineral phases within individual vesicles. The hydrothermal fluid was marked by a Ca–Na signature, high cation/hydrogen ratio and low to intermediate temperatures, ranging between 50 and 200 °C. The controls for precipitation were mainly temperature and the chemical composition of the fluid, with high alkalinity and low H+ activity.