Controls on prolate and oblate geode geometries in the Veia Alta basalt flow, largest world producer of amethyst, Paraná volcanic province, Brazil

Variable intensity of hydrothermal alteration of the Veia Alta basalt flow, Ametista do Sul, Brazil, exerted the fundamental control on the shape and size of the amethyst geodes. The loss on ignition (LOI) of the host basalt is used as a proxy for intensity of alteration and has direct relationship with the height of the geodes (up to 150 cm in the three study mines) and with the prolaticity of geodes. All rocks with LOI > 5 wt.% host prolate geodes and all oblate geodes are hosted in rocks with LOI < 5 wt.%. An additional observation is the extensive mobility of several elements during hydrothermal alteration (variable LOI), including SiO2, K2O and Rb. The hydrothermal origin of the geodic cavities is thus established and their shapes explained by the empirical observation of the results from a previous numerical simulation experiment.