Ocorrência de dolomita em sela de rochas carbonáticas na mina Rio Bonito, Grupo Açungui (Paraná)

The dolomitization process which affected the carbonate rocks in the Acungui Group, exposed in the Rio Bonito quarry, occurred in two distinct episodes, namely of replacement and void-filling types. In the first case, the ancient limestone had its matrix largely substituted by dolomite with finely to averagely-grained crystals, primarily unimodal and secondarily polimodal distribution, in individuals of cloudy aspect. The void-filling type of dolomitization, responsible for the precipitation of micritic material (Dp1) of dolomitic and Fe-rich composition, originated from fluid in chemical disequilibrium with the fluid responsible for the previous dolomitization. The dolomitic cement Dp2 is constituted of saddle dolomite with non-planar individuals, coarsely-grained and with strong undulate extinction. Moreover, this type of dolomite occurs in the process of fracture-filling, in the breccia as well as in the dolomite marble, with generation posterior to that of the previous type. The dedolomitization process occurred in the dolomitic breccia, under superficial conditions, with the precipitation of late calcite as a void-filleing in association with the dolomite from Dp2 generation. Quartz completes the final phase of the void-filling process