Fatores controladores da variação de MgO nos mármores da mina Rio Bonito (PR)

Abstract

The Rio Bonito mine of the Portland Itambé Company is a calcitic marble deposit used for the cement industry. The high MgO content of the marble causes volume increase and produces soluble salts, which weaken the cement. There is a dolomitic front in the calcitic marble body, which generate huge amount of barren material. The main goal of the present research is to determine the key factors, responsible for the content variation of CaO and MgO in the marble deposit.The methods applied are: whole rock geochemical analysis by x-ray fluorescence, mineralogical analysis by x-ray diffraction, petrography and structural analysis. The integrated methodological approach of the study shows reasonable results, allowing to delimit the dolomitic front, as well as to identify the key factors, which control the dolomitic front in the calcitic marble. The calcitic marble is composed essentially of calcite, with fine and homogeneous grain size. The dolomitic marble is essentially composed of dolomite, medium to course, heterogeneous grain size. Breccias, with angular clasts in a fine matrix, are common in the dolomitic marble. Both marbles present distinctive colors, according to their porosity. The dolomitic marble has higher porosity because of the secondary dolomitisation. This process occurs by fluid percolation in structural weakness zones, leading to the breccia formation in the dolomitic marble. In spite of the dolomitic front has been generated by dolomitisation process, the structural is the main control. The contact between calcitic and dolomitic front is abrupt, marked by fractures. The fractures are sub- vertical, oblique to the main foliation. Applying Riedel model to the studied area, it is possible to observe that the strike of the contact plane of dolomitic and calcitic front matches up with the open fractures, or extensional faults. The open zones, responsible for the fluid percolation, are genetic related to the most important regional structure, the Cerne strike-slip fault.