Structural constraints on Paraná basalt volcanism and their implications on agate geode mineralization (Salto do Jacuí, RS, Brazil)

Abstract

The Paraná-Etendeka Continental Flood Basalt province hosts world-class agate and amethyst geode deposits in Rio Grande do Sul (Brazil; Serra Geral Fm.). Salto do Jacuí Mining District (Rio Grande do Sul, Brasil) has different types of agate geode hosted in vesicular basalt. A series of structural features has recently been investigated in the Salto do Jacuí Mining District, and indicates at least two volcanic episodes: i) normal tholeiitic basalt and dacite eruption, and ii) vesicular basalt and dacite intrusions as sills and dikes. These structural features include: basalt and aeolian sandstone xenoliths in vesicular basalts, vesicular basalt apophyses in massive basalts, sandstone and basalt breccias, sandstone dikes cutting across vesicular lavas and connected to mixed sandstone-agate geodes, sandstone assimilation by vesicular lava, and mixed sandstone and agate geodes. These features show that agate geodes were formed by melting of Botucatu sandstone xenoliths. High density contrast between vesicular basalt and Botucatu sandstone melts makes them immiscible during flow. Botucatu sandstone xenoliths melting is favored by degasing of intrusive volatile-rich basalts. The high-silica globs crystallize dynamically in a closed-system environment, giving rise to agate banding and fibrosity.