International Geology Review 2010, iFirst article, 1–14
Sequential opening and filling of cavities forming vesicles, amygdales and giant amethyst geodes in lavas from the southern Paraná volcanic province, Brazil and Uruguay Léo Afraneo Hartmanna*, Lauren da Cunha Duarteb , Hans-Joachim Massonnec , Cassiana Michelin a , Leonardo Manara Rosenstengela , Magda Bergmannd , Thomas Theyec , Juliana Pertillea , Karine Rosa Arena a , Sandro Kucera Duartea , Viter Magalhães Pintoa,e , Eduardo Guimarães Barbozaa , Maria Luiza C.C. Rosa a and Wilson Wildnerd a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Avenida Bento Gonçalves, Porto Alegre, Rio Grande do Sul, Brazil; bDepartamento de Materiais, Escola de Engenharia, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul, Brazil; c Institut für Kristallchemie und Mineralogie, Universität Stuttgart, Stuttgart, Germany;d Geological Survey of Brazil (CPRM), Rua Banco da Província, Porto Alegre, Rio Grande do Sul, Brazil; eDepartamento de Geologia, Universidade Federal da
Roraima, Rio Branco, Roraima, Brazil (Accepted 17 May 2010)
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 Paraná 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 (30–150C) 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. Keywords: vesicles; amygdales; giant geodes; amethyst-quartz; Serra Geral Group
Introduction The opening and filling of cavities in rocks are the major processes related to the generation and sealing of porosity in ore deposits. The Paraná volcanic province of South America (Figure 1) constitutes one of the largest continental flood basalt provinces in the world and has large deposits of amethyst and agate (calcite, gypsum) geodes. Specimen-quality minerals, including several zeolites, are also abundant in the amygdales of the volcanic rocks. The investigation of these different cavities is therefore a major topic in the geology of volcanic rocks. Basalts and rhyodacites (including some andesites) from the Cretaceous (133 Ma) Paraná volcanic province are included in the Serra Geral Group. These units cover 1,300,000 km2 of South America, not only in Brazil but also in Uruguay, Argentina, and Paraguay (Bellieniet al. 1984). Monthly production of geodes from underground mines is nearly 400 tons, mostly from Ametista do Sul (Brazil) and Los Catalanes (Uruguay), with commerce concentrated in Soledade, Brazil.
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[email protected] ISSN: 0020-6814 print/1938-2839 online © 2010 Taylor & Francis DOI: 10.1080/00206814.2010.496253 http://www.informaworld.com
The filling of vesicles with zeolites and geodes with quartz and amethyst is recognized as a low-temperature process (50 C) (see, for example, Juchem (1999), Gilg et al. (2003), Duarte et al. (2009), Morteani et al. (2010), and Commin-Fischer et al. (2010)), and generally it is considered to represent a single hydrothermal event. For instance, Commin-Fischeret al. (2010) observed that the small cavities (amygdales) are rarely filled with SiO2 phases, and attributed this feature to restricted water circulation. The opening of vesicles in the upper and lower crust oflavas has long been regarded as the result oflava C) (see, for examdegassing at high temperature (1150 ple, Aubele et al. (1988) and Sahagianet al. (1989)). The economically significant large cavities (geodes) present in the massive core of the lavas have also been attributed to lava degassing at 1150C (Proust and Fontaine 2007a). However, Duarte et al. (2009) showed that these cavities were formed at low temperature (150C) and therefore have an epigenetic origin. The hydrothermal processes were also evaluated by Hartmann (2008).
