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 concomitantlyderived dolomitic portions of the calcitic limestone of the ProterozoicCarandaí Formation in the Macaia-Ijaci Basin was supposedly formedby metasomatic reactions between the carbonate and HF- and Mg2+-rich brines, during metamorphism. Thirteen electron microprobeanalyses from the carbonate in a single petrographic thin section, allowedthe determination of the temperature of the carbonate formation in therange 180o-230oC, using the Mg-calcite solvus geothermometer. Fivekey mineral reactions in the system Mg-Ca-C-H-O-F plotted in diagramsin terms of either, CO2- and HF-fugacities or chemical potentials ofCO2 and HF yielding fluorite + dolomite from calcite + Mg2+, areproposed. Apparently the fluorite formation is related withdolomitization. Additional reaction involving fluorite was calculatedand inserted in the phase diagram, according to the existence of calcite-fluorite veinlets bordered by dolomite selvages, at the Casa de Pedraquarry.

Keywords

• Dolomitization
• HF/CO2 fugacity
• geothermometry