ABSTRACT: The impetus for this research was the enigma regarding the origin of the extensive silicic volcanic units in the Paraná-Etendeka Igneous Province. Are they rheoignimbrites, lava flows, or a combination of both? The São Joaquim silicic eruptive sequence is comprised of Palmas-Type aphyric dacites and is located in the northern segment of Torres Trough, a dissected mountainous region in southern Brazil. These dacite outcrops form plateaus and remnant hills that are scattered above a basaltic andesitic landscape. The original morphology of the silicic volcanic flows was constrained based on the internal architecture, geometry, and cyclicity of the lithofacies, in conjunction with petrographic and geochemical information. The data suggests the presence of ancient large tabular and lobate silicic lava flows (~100 m thick and ~10-40 km extents). No pyroclastic features were found. Eight interdigitating units were mapped. These large aspect ratios are similar to ones for basaltic flows and those of Snake River-type rhyolites, and they suggest high effusion rates and high temperatures as well as the presence of well-insulated cooled crusts. Lavas would have overflowed from long fissure vents that had created an extensive volcanic silicic surface body composed of conjugated and interdigitated flows above the north segment of the Torres Trough region.

ABSTRACT: The Paraná Continental Flood Basalt (PCFB) province is one of the largest igneous provinces on Earth, but little is known about the architecture and geochemical characteristics of the lava flows. In this context, a set of borehole samples arranged in an NEE-SWW profile - more than 600 km long - located in the northern PCFB was used to define its chemostratigraphy. In this region, the outcrops are rare, and the volcanic piles are covered by hundreds of meters of sedimentary rocks from the Bauru Sub-Basin. This study aimed to explore the chemostratigraphic column from northern PCFB to improve the understanding of its internal architecture and to characterize the flood basalt sequences by using the petrography and elemental geochemistry of surface and borehole samples. The results showed the presence of the main four magma types, which were essentially distinguished by their P 2 O 5 , TiO 2 and Sr concentrations, as well as Zr/Y ratios, with excellent chemostratigraphy relations.

ResumoEsse estudo consiste na aquisição de dados magnetométricos na região de São Sepé (RS), sul do Brasil, com o propósito de avaliar o potencial de mineralização aurífera de um gabro, a partir de indícios obtidos em prospecção geoquímica de sedimentos de corrente prévios. O gabro denominado Santa Catarina está intrudido no Complexo Metamórfico Vacacaí, uma sequência metassedimentar ,onde foram reconhecidas diversas jazidas de ouro. Nesse estudo, foram realizadas nove linhas de magnetometria terrestre, com 1.200 m de comprimento cada, num total de 10.800 m e 720 estações de leitura. Os resultados sugerem a presença de mineralizações filonianas em fraturas, algumas das quais sujeitas à ação intempérica, com liberação e transporte de ouro por ação fluvial e deposição junto a sedimentos de corrente a jusante do gabro. O mapa de sinal analítico descreve as áreas de alta susceptibilidade magnética dentro de seus limites, coincidentes com lineamentos estruturais nas direções NE e E-W, associadas a à última das quatro fases de deformação regional, possivelmente responsável, tanto por fraturamento e abertura de espaços no gabro, quanto pela remobilização de sulfeto e ouro reconhecidamente descritos em diversas jazidas de ouro no âmbito do Complexo Metamórfico Vacacaí.

AbstractThis study involved magnetometric exploration in the region of São Sepé (RS), southern Brazil, to evaluate the potential for gold mineralization in a gabbro using evidence previously obtained from the geochemical exploration of stream sediments. The gabbro named Santa Catarina is intruded in the Metamorphic Complex Vacacaí, a metassedimentary sequence where several gold deposits were recognized. In this study, nine lines of land magnetometry were performed with 1.200m in length each, a total of 10.800m and 720 reading stations. The results suggest the presence of filonian mineralization in fractures, some of which are subject to weathering action, releasing and transporting gold by water action and depositing it along the stream sediments downstream from the gabbro. The analytical signal map outlines areas of high magnetic susceptibility within its boundaries, coinciding with structural lineaments in the NS and EW directions, associated with the last of the four stages of regional deformation, possibly responsible for both fracturing and opening of spaces in the gabbro, as by remobilization of sulfide and gold recognizably described in several gold deposits within the Vacacaí Metamorphic Complex.