The amethyst geodes observed in the tholeiitic basaltic flow from the Triz quarry at Ametista do Sul (Rio Grande do Sul, Brazil) show particular wall-layering infillings with, from the outside inwards, celadonite, chalcedony, fine-grained quartz and large-sized amethyst crystals. The primary fluid inclusions analysed in the amethyst crystals yield a 152 to 238 °C crystallization temperature range. The amethyst geodes are always located in the massive, fracture-free, basaltic part of the lava flows and surrounded by a limited wall-lining alteration halo, the thickness of which depends on the geode radius. The geochemical balances calculated from fresh to altered basalt in the geode environment indicate that the amount of SiO2 released from the alteration halo is always sufficient to produce the siliceous rims in the geodes without requirement for extraneous silica supply. The results point to a volcanic origin for the amethyst geode infillings through basalt ‘autoalteration’ by authigenic high temperature hydrothermal fluids. These fluids can originate from (1) unmixing of volatile coumpounds from melt through pressure release, (2) cooling to supercritical hydrous fluids with ‘autoalteration’ of the surrounding basalt and (3) migration of the residual hot fluids from the basalt to the geodic cavities through the pressure gradient between the geodic cavity and the rock.