Wilson Wildner

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.

Post-Collisional Alkaline Magmatism on the Taquarembó Plateau: A Well-Preserved Neoproterozoic–Cambrian Plutono-volcanic Association in Southern Brazil

The Taquarembo Plateau plutono-volcanic association (TPPVA), a magmatic association related to a silica-saturated alkaline series, represents a portion of the last episode of post-Brasiliano/Pan-African collisional magmatism in southern Brazil. It was preceded by a postcollisional high-K calc-alkaline and shoshonitic magmatism, which is more pronounced near the collisional belt. The TPPVA is a sequence of volcanic, volcaniclastic, and volcanogenic sedimentary deposits with hypabyssal associated rocks, lying on Paleoproterozoic granulites. Isotope data suggest that this alkaline postcollisional magmatism occurred over the period from 580 to 537 Ma. Two sequences of magmatic liquids, which evolved by mineral fractionation from low-Ti-P and high-Ti-P basaltic magmas, were identified. The former includes mildly alkaline silica-saturated basalts, metaluminous monzodioritic intrusions, and rhyolitic lavas, whereas the second includes hawaiites, mugearites, syenitic intrusions, and peralkaline to intermediate la...

Epigenetic hydrothermal origin of native copper and supergene enrichment in the Vista Alegre district, Paraná basaltic province, southernmost Brazil

Native copper is widespread in the Lower Cretaceous Paraná basaltic province, southern Brazil, both as films in fractures and as massive balls in amygdules. The focus of this investigation is on the large concentration of occurrences (n  =  85) that forms the Vista Alegre district in the border region of Rio Grande do Sul and Santa Catarina states. The high average of 220 ppm Cu content of the basalts resulted in ore of native copper, Cu oxides, abundant chrysocolla in the top of mineralizations, and minor malachite and azurite. Native copper is associated with dioctahedral and trioctahedral smectites, zeolites (heulandite and clinoptilonite), quartz, and calcite, typical of a low-T (100–150°C) hydrothermal alteration assembly. The PGE distribution shows enrichment in Pd in relation to Pt both in basalts and in native copper, supporting the hypothesis of hydrothermal origin of the mineralization. No evidence was found of direct precipitation of copper from the lava; based on field and petrographic evidence, integrated with BSE images, EPMA analyses, EGP contents of native copper, and bulk rock analyses, this is an epigenetic hydrothermal copper mineralization, followed by supergene enrichment.

ESTÁGIOS EVOLUTIVOS DA BACIA DO CAMAQUÃ (RS)

This chapter presents a revision of concepts and hypotheses previously presented about the evolution of the Camaqua Basin, central portion of the Rio Grande...