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Thousands of silica gossans are exposed at the top of the Lower Cretaceous basalt hills in the Parana volcanic province, strongly indicating the presence of calcite, amethyst and agate geode deposits along with native copper mineralization. The Embrapa silica gossan in the northwestern portion of the province (Campo Grande region) is an excellent example of such novel geological structure in the continental flood basalts. This silica gossan has a size of 450 × 350 m standing out as a treeless area in the densely wooded savanna and makes part of the stratigraphy of six basalt flows of Paranapanema intermediate-Ti chemical type. The base of the volcanic column is constituted by two Pitanga types and the overlying column is Paranapanema type. Every basalt flow has a silicified sand layer or breccia at the top and these are fed by abundant sand dikes. The Anel Viario Norte (AVN) flow is the most intensely altered by hydrothermal fluids producing voluminous secondary calcite infillings in the amygdales and fractures. In this region the basalts contain higher copper content than the average of the volcanic province. The studied silica gossans display negative anomalies in gamma spectrometry as a response to K, U and Th depletion during alteration. We propose a new exploration methodology by observing GoogleEarth images complemented with field studies and geochemistry to readily locate favorable areas for amethyst and agate geode deposits and native copper mineralization.

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 2 , K 2 O 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.

Blue chalcedony nodules have been mined from the Sundikendagi deposit in the Mayislar–Saricakaya (Eskisehir) region of north-central Turkey since ancient times; however, no modern geological study of this deposit has yet been published. Although ancient and current mining production have both taken place in an area of complex geology, our study and analyses of the deposit suggests a simple model of sedimentary deposition for its origin. The repeated episodes of tectonic activity, accompanied by brittle deformation, metamorphism, and hydrothermal activity, which characterize this part of the Anatolian Peninsula with its complex junction of tectonic plates, appear to have had little influence on the blue chalcedony nodules that make the deposit valuable other than perhaps to influence their trace-element composition. The physical nature of the nodules as revealed by polarized-light microscopy and XRD—they are composed only of fibrous length-fast quartz (chalcedony) and fibrous length-slow quartz (moganite), but contain neither platy opal-CT nor opal-C—is consistent with a sedimentary origin as are their overall shape and strata-bound occurrence in a sandstone (arkose). The relatively high concentrations of some trace elements in the nodules revealed by ICP-AES, suggest involvement of hydrothermal fluids during the direct epigenetic formation of chalcedony concretions during diagenesis of the enclosing sandstone or by alteration of diagenetic concretions of another composition. Sources could include upwardly moving hydrothermal fluids entering the sedimentary basin from underlying older Saricakaya intrusive rocks or sea-floor hydrothermal vents in the vicinity during diagenesis in the Palaeocene and Eocene (65–37.8 Ma) periods. Oxygen isotope analyses (SMOW) (using EA-IRMS) of both the blue chalcedony nodules (δ 18 O = + 28.2‰ to + 30.8‰) and the enclosing sandstone (δ 18 O = + 11.3‰ to + 13.2‰) suggest that the nodules formed during diagenesis at a low temperature of around 55 °C, although they are encased in sandstone whose grains came from rocks that formed at significantly higher temperature, perhaps above 100 °C. The unbanded Sundikendagi chalcedony nodules are similar in occurrence to the banded Fairburn agates of South Dakota, USA and the Dryhead agates of Montana, USA, which formed in Palaeozoic limestones, except that the blue chalcedony is hosted in sandstone. Other sedimentary agates are generally believed to have formed by the alteration of diagenetic concretions from the outside, inward. No other agates or chalcedonies hosted in sandstone are known for comparison with this deposit. Thus, the deposit appears to be unique. It is possible that the Sundikendagi unbanded blue chalcedony formed as epigenetic concretions during diagenesis of the sandstone—a mechanism previously shown for large crystals of other minerals found in sandstones.

The Parana volcanic province is the world's largest producer of amethyst geodes, mostly from the Ametista do Sul mining district (20 by 30 km) 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 3 wt.% TiO 2 ); which occur at the base of the local stratigraphy and the other seven flows belong to Paranapanema magma type (2–3 wt.% TiO 2 ) 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 60 vol.%) 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 200 m 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 250 m 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.