Caracterização petrográfica e geoquímica de brechas hidrotermais da Formação Salitre - Borda Leste da Bacia de Irecê (BA)

Abstract

Breccias are rocks composed of angular rocks or fragments of cemented and filled with fine matrix, observed in the most diverse types of environment and formed by processes of igne-ous, sedimentary or tectonics, or a combination of them. The work aims to characterize geo-graphic and geochemical aspects of breccias located at Mamonas Farm and Melancias Farm, respective in the of Morro do Chapéu and Cafarnaum cities, inserted in the Salitre Formation, Una Group, São Francisco Supergroup - Irecê Basin, BA, dating from the Superior Proterozo-ic. The samples correspond lithostratigraphically to algal laminite (Nova América Unit), a mi-crite matrix and a spatial cement and a characteristic neomorphized block (Gabriel Unit), both characteristic of the tidal plain environment. The methodology is composed by geological and structural bibliographic survey, orthophoto analysis for characterization of veins and fractures, field excursion for sampling, breccia classification by manual sample description, petrographic description through thin sections. In addition, analyzes were performed as Scanning Electron Microscopy (SEM), Dispersive Energy Spectroscopy (EDS), X-Ray Fluorescence Spectrome-ter (FRX), X-Ray Diffractometry (DRX) and Quantitative Evaluation of Minerals by Scan-ning Electron Microscopy (QEMSCAN). Photointerpretations recorded pattern of preferential direction NE-SW venulations and mostly N-S fractures. Moreover, descriptions pointed to lithologies corresponding to hydrothermal breccia formed by the fluid-assisted brecciation process, those with clasts composed of recrystallized, dolomitized and limonitized carbonates, constituted essentially of dolomite, calcite and ankerite, iron oxide matrix and / or magnesium, vugular secondary porosity, presence of styloliths, silicification and fractures filled primarily by quartz and galena, with secondary mineralogy appearances consisting of pyrite, cerussite, sphererite, zincite, chlorite magnesite, barite, apatite and plagioclase. The aforementioned techniques indicate the presence of silica and CO2 enriched in hydrothermal fluids, heated due to the action of granitic body below the basin, percolating through faults generated during the Brazilian. This event is responsible for adding fluid pressure ready to cause breccia-generating hydrofracturing and barium, silver and zinc mineralizations in Mississipi Valey-type deposits associated with late silicification in dolomitized carbonates.