Geochemical stratigraphy of lavas and fault-block structures in the Ametista do Sul geode mining district, Paraná volcanic province, southern Brazil

Abstract

The Paraná volcanic province is the world's largest producer of amethyst geodes, mostly from the Ametista do Sul mining district (20 by 30km) in southern Brazil. The description of the lava flows in the Ametista do Sul mining district, and adjacent areas, combined with flow-by-flow geochemistry and scintillometry, defines the stratigraphy of the flows. Nine flows were identified in the Ametista do Sul region and four additional flows were identified in the Frederico Westphalen region, totalizing 13 flows in the stratigraphy of the Ametista do Sul mining district. Six of the flows belong to Pitanga magma type (more than 3wt.% TiO2); which occur at the base of the local stratigraphy and the other seven flows belong to Paranapanema magma type (2–3wt.% TiO2) at the top of the stratigraphy. The amethyst-bearing geodes occur in the uppermost three Pitanga flows, known as Veia Alta (main producer), Veia do Meio and Veia Baixa. These flows were highly altered (more than 60vol.%) to clay minerals by a low temperature alteration process. The massive presence of clay minerals is an important component that controls the rheology of the rock, transforming the basalt into a very low grade smectite and celadonite metabasalt. Two Paranapanema flows, COOGAMAI and Linha Alta, from Ametista do Sul were identified in a lower position, at the base of the section in Frederico Westphalen which were correlated using scintillometry and flow-by-flow geochemistry. In addition, Veia Alta and Veia do Meio flows were also identified in lower positions, compared to Ametista do Sul. Thus, the identification of these flows in different places with different elevations is an evidence of the presence of downthrown blocks in a fault-block structure, lowering the stratigraphy more than 200m to the west in four steps. The tectonic patterns that generate this structure are three NW-strike lineaments. The identification of the COOGAMAI flow at the base of the Frederico Westphalen region, suggests that the Veia Alta geode-hosting flow is below the ground or below the elevation of 250m in this region. These evidences show that the fault-block structure is controlling vertically the deposit of amethyst-bearing geodes and is of major significance in the exploration for new amethyst geode deposits in the region.