THERMODYNAMIC EQUILIBRIUM IN THE MN-MINERALIZATIONS OF MORRO DA MINA DEPOSIT IN THE ARCHEAN BARBACENA GREENSTONE BELT, MINAS GERAIS, BRAZIL

The mineralogical studies allowed the identification of the spessartitic garnet, carbonate, mostly rhodochrosite, minor dolomite, amphibole, Mn-cummingtonite and tirodite. Paragenetic relations between the Mn-minerals permitted the use and to perform comparisons between with previous Mn-protore classification of gondite (quartz-spessartite-Mn cummingtonite) and queluzite (rhodochrosite-spessartite-tephroite-rhodonite-Mn cummingtonite), defined by the relative concentrations of SiO₂, Al₂O₃ and fluids H₂O and CO₂. Morro da Mina queluzite or Mn-carbonate and gondite protores constitutes a splendid opportunity to study the equilibrium conditions of the Mn-rich paragenesis under several viewpoints such as chemical potential and temperature-CO₂ molar fraction aspect. Mineralogical composition of the Mn-protore consisting of rhodochrosite, spessartite, Mn cummingtonite, rhodonite and tephroite additionally with quartz and graphite, defined as queluzite propitiated the mineralogical and thermodynamic study of the equilibrium conditions between the phases. High-grade Mn-protore known from Indian deposits as gondite, basically a quartz-spessartite protore, has been recognized at Morro da Mina and several deposits surrounding the Lafaiete District. Skarn and metasomatic contact deposits exist at Morro da Mina, exhibiting beautiful, pink rhodonite crystals, well formed, hyaline quartz, tephroite gray masses, concentrations of yellowish orange, spessatite and graphite as large plaques and lamellae. It could be emphasized the important role of volatile phases, H₂O and CO₂  and the silica control during metamorphism and metasomatism representing in chemical potential and T-XCO₂ diagrams the reactions involving all phases in the queluzite and some in the gondite. Interrelationships between the Mn-minerals, quartz, graphite, pyrophyllite and K-feldspar reflect the accentuated interaction among the phases resulting in a vast number of reactions which represents the complete set.