Fluorite mineralization related to the dolomitization: an equilibrium study of the Proterozoic stratabound carbonate Macaia-Ijaci Basin, Lavras, Minas Gerais, Brazil

Stratabound fluorite occurring as thin films in the concomitantly derived dolomitic portions of the calcitic limestone of the Proterozoic Carandai Formation in the Macaia-Ijaci Basin was supposedly formed by metasomatic reactions between the carbonate and HF- and Mg2+- rich brines, during metamorphism. Thirteen electron microprobe analyses from the carbonate in a single petrographic thin section, allowed the determination of the temperature of the carbonate formation in the range 180o-230oC, using the Mg-calcite solvus geothermometer. Five key mineral reactions in the system Mg-Ca-C-H-O-F plotted in diagrams in terms of either, CO2- and HF-fugacities or chemical potentials of CO2 and HF yielding fluorite + dolomite from calcite + Mg2+, are proposed. Apparently the fluorite formation is related with dolomitization. Additional reaction involving fluorite was calculated and inserted in the phase diagram, according to the existence of calcite-fluorite veinlets bordered by dolomite selvages, at the Casa de Pedra quarry.