Caracterização petrográfica de uma ocorrência kimberlítica a noroeste de Picos, estado do Piauí

Abstract

This research deals with a geological mapping conducted at a scale of 1: 25,000, in an area of 40km2, northwest of the city of Picos, in the State of Piauí, eastern portion of the Parnaíba Basin. Its objectives are the preparation of a geological map, macro and microscopic detailing with identification of the lithologies and lithostratigraphy of the region. The identified lithostratigraphic units, from bottom to top, were: Fm. Cabeças and Fm. Sardinha, as well as an important kimberlitic magmatism in the area. The Cabeças Formation, which is a subdivision of the Canindé Group, is composed basically of sandstones distributed in almost 90% of the mapped area, presenting in the form of masses in the light red coloration that occur in walls, slabs and blocks. The Sardinha Formation represents one of the episodes of basaltic volcanism that occurred in the basin and is composed of NW-SE tolematic diabatic dikes that cut the sandstone, making up 2% of the total area of the mapping, emerging as rounded blocks of dark gray to greenish color, with change cover and also revealing a massive structure with medium to fine granulation. The kimberlitic magmatism is posterior to the basaltic magmatism and are rocks that are extremely altered, exhibiting great mineralogical variation and importance because they constitute the matrix rock of the diamond. Considering the high degree of susceptibility to weathering, kimberlites present low preservation of mineralogy, are solid, cohesive and have a greyish coloration with greenish tones. The rock texture is unequigranular, composed of olivine and clinopyroxene phenocrysts enveloped by a faintly colored fiery matrix. In the Parnaíba Basin, Kimberlites were identified in both the southern and eastern portions. The bodies located in the southern portion of the basin were classified in the Kimberly Province of Gilbués, while those located in the eastern portion were grouped as the Kimberly Province of Picos. For a better characterization and identification of the kimberlites, it was necessary to use the MEV-EDS in the aid of the petrography, due to the alteration of the rocks. The mineralochemical analyzes by MEV-EDS were essential because it is an effective method for the mineral certification and definition of the main elements present in the igneous matrix, through the generated images, revealing several tones and shapes as a function of the composition, and semi-quantitative point chemical analysis. Finally, we can conclude the presence of sandstones, diabase and kimberlites in the area, which are Devonian age, Eocretacea and Neocretacea, respectively.