Petrografia e diagênese dos arenitos da formação Rio Maria, Província Mineral de Carajás, SE do Cráton Amazônico

Abstract

Petrographical analysis of detrital and diagenetic constituents of the sandstones of the Rio Maria Formation allowed characterization of the diagenetic regimes and to infer the provenance of these deposits. These sandstones occur in the southeastern portion of the Amazonian Craton, in the area of the Rio Maria Granite-Greenstone Terrain. The sandstones framework is composed basically of monocrystalline and polycrystalline quartz, with angular- to subangular, and rounded forms. Grains contacts ranging from concave-convex to sutured. The low frequency of feldspar grains suggests prolonged weathering in source area, as well as diagenetic regimes. The feldspar grains are partially- to completely altered to kaolinite and sericite. Although sporadic occurrence, lithic fragments (sedimentary and metamorphic) are important indicators of source rocks, since they indicate the basement lithologies. Diagenétic regime includes: eogenesis (depositional-environment related diagenesis), mesogenesis (burial diagenensis) and telogenesis (uplift and exposure related diagenesis). Eogenetic regime was marked by the mechanically infiltrated of clays, grain coating in quartz and early dissolution of feldspar. Mesogenetic regime prevailed in the development of sutured contacts and concave-convex, intraestratal mineral dissolution, precipitation of silica cement and illitização of kaolinite. According to the diagenect models, we suggest that: the dissolution of feldspar indicates burial from 1.5 to 4.5 m and T ranging from 50-150°C; and the precipitation of silica cement, the reaction of kaolinite with feldspars, and illitization of kaolinite indicates burial temperatures ranging between 70-130°C. By tectonic uplift, the telogenttic regime was characterized by the precipitation of iron hydroxide-oxide and telogenetic porosity. The high mineralogical maturity of the sandstones and low textural maturity indicate intense action of weathering in the source area, a fact which is attested by the Dickinson’s diagram and geological context of the area, indicating sources from craton interior to the sandstones of the Rio Maria Formation.