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

Abstract

Stratabound fluorite occurring as thin films in the concomitantly derived dolomitic portions of the calcitic limestone of the Proterozoic Carandaí 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.