Lauren da Cunha Duarte

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

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 (30–150°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.

Geochemical and scintillometric characterization and correlation of amethyst geode-bearing Paraná lavas from the Quaraí and Los Catalanes districts, Brazil and Uruguay

Geochemical studies of the six lowermost lava flows of the Cretaceous Serra Geral Formation (Parana volcanic province) in Quarai (Brazil) and Artigas (Uruguay) were combined with flow-by-flow field studies of structures and scintillometric profiles to establish a consistent regional stratigraphic framework over at least 100 km. This greatly improves exploration capability for amethyst and agate geodes. A basalt, colada Mata Olho (Alegrete facies, Serra Geral Formation), was the first lava to flow over the ancient Botucatu desert in the region, but an andesite, colada Catalan, overstepped this basalt in many places, perhaps palaeohighs. Four basaltic andesites complete the lava stratigraphy in this formation, adding up to 300 m of lavas. The stratigraphic sequence of contrasting lava compositions is 51.0 wt% SiO 2 in the first lava, followed by 57.5, 52.5, 56.0, 53.0 and finally 54.5 wt% SiO 2 . Overall MgO variation is between 2 and 7 wt%. All lavas in the two districts are low-Ti ( 2 ) of the Gramado type. The characteristic contents of most major and trace elements (124 rock samples analysed) allow the ready identification of each lava. Contrasting rock chemistry also results in strong variation in scintillometric values (270 points measured in the field and nineteen continuous borehole profiles); from bottom to top of the stratigraphy, the cps values are 49±3.2, 123±10.3, 62±4.7, 94±4.6, ~45 and ~85. Colada Catalan has the structure of aa lava, particularly the contorted igneous banding and autobreccias in the upper and lower crusts. In some places, a 2 m thick, silicified sandstone layer lies on top of some coladas, and silicified sandstone forms breccias with volcanic rocks. Geochemistry of the six lavas indicates complex evolution, involving melting of lithospheric mantle, injection into the crust and assimilation of crust followed by fractional crystallization. This study indicates the possibility of world-class deposits of amethyst geodes on the Brazilian side of the border with Uruguay.

Design and territory: laser cutting/engraving applied in the manufacture of products for the promotion of the "Pedra Grande" archeological site, RS, Brazil

The pursuit for knowing the peculiarities of each country or region takes millions of people to embark on journeys to various destinations. The curiosity about other cultures makes the tourism activity a means of learning the history of a place, including the knowledge of its monuments and archaeological sites. Museums have an important role regarding the disclosure of information on the human history legacy in different places and at different times. And their shops, by providing products, extend the visitors relationship with the place. São Pedro do Sul, located in Rio Grande do Sul, Brazil, holds the Pedra Grande Archeological Site, a monument of great historical and ethnographic importance of human occupation in the area. For the promotion of this heritage it becomes interesting to create strategies aimed at the dissemination and development of the museums that make up the cultural center of this city as tourist attractions. As a proposal to support these institutions, this study employs the technique of laser engraving for making objects that can be sold in their stores, diffusing the local culture and generating incomes for these establishments.