Aspectos geológicos e petrográficos de ocorrências de ferro magmático/hidrotermal na porção nordeste da Bacia do Parnaíba

Abstract

The knowledge about iron deposits in sedimentary basins of the Fanerozoic, such as the Parnaíba Basin and similar ones, for example Amazonas and Paraná basins, was restricted to Ironstone sedimentary deposits that generally exhibit a mineralogy dominated by hematite in oolitic and pisolithic textures and low iron content. However, the present research, involving geological mapping, with aerogeophysical data support, on a semi-detail scale, identified important iron occurrences with magmatic / hydrothermal origin, associated to the basic magmatism known in the literature as Sardinha Formation (cretaceous). This magmatism cuts off sandstones from the Devonian age, on the northeastern edge of the Parnaíba Basin, in the municipality of Piripiri and adjacent areas. The occurrences takes place in two forms: a magnetitic plateau exhibiting fragmentary aspect or autoclastic breccia, composed of centimetric fragments of massive magnetite, fine granulation, homogeneous, involved by a limonite matrix, with flow structures, sustained by Cabeças Formation; and the other, disseminated, in altered apical portions of the basic rocks, which are in the form of rolled blocks, centimetric/ decimetric, but with strong magnetism, supporting small hills. The set of petrographic data combined with field information and MEV analysis allowed to relate these mineralizations to the basic Sardinha magmatism, suggesting processes of imscibility and magmatic differentiation. Additionally, vulcanoclastic sandstones intercalated to the common sandstones of the Cabeças Formation were identified, most of which are composed of volcanic quartz. This represents an important finding since there is no record of acid volcanism in the Parnaíba Basin. Petrography of this lithological type together with field data suggest that precursor acid volcanism developed in a subaquatic environment concomitant with the deposition of the Cabeças Formation. Columnar disjunctions were also identified in the diabase of Sardinha Formation and the sandstones of Cabeças Formation. The presence of such structures in sedimentary rocks represent unusual features since they are more common in igneous rocks. The formation of these structures in the sandstones is associated with the thermal effect caused by the diabase sills.