Sequential opening and filling of cavities forming vesicles, amygdales and giant amethyst geodes in lavas from the southern Parana volcanic province, Brazil and Uruguay

摘要

The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in ore deposits. This study documents three stages of opening and filling of vesicles and geodes in the basalts and rhyodacites of the southern Parana volcanic province. Each step detailed here is actually part of a sequence of minor hydrothermal events. First, lava degassing at high temperature (1150°C) formed small (<4 cm) vesicles in the crusts of flow units. In sequence, these vesicles were partly to fully filled at low temperature (30-150°C) by hydrothermal minerals, particularly clays and zeolites;;this process also sealed the porosity of the lava. Second, the injection of fluidized sand generated new cavities, which were partly filled with sand;;the newly formed porosity was sealed by the low-temperature fluid. Third, intense alteration of the basalt or rhyodacite core into a claystone favoured the opening of small to giant protogeodes (0.1 mm to 4 m) by dissolution;;cooling of the fluid led to the precipitation of hydrothermal minerals, particularly the spectacular amethyst, calcite, and gypsum-bearing geodes.