The Lafaiete Formation of Minas Gerais, SE Brazil, as exposed at the Morro da Mina mine, near to the city of Conselheiro Lafaiete, presents three major lithostratigraphyc units. The lowest stratigraphic sequence exhibits mainly amphibolites and amphibole schists, while the upper section presents amphibole - garnet and garnet schists with one minor interbedded quartz-feldspathic lense, as main rock types. The middle section, very uneven but on the average with about 20 meters thickness, is predominantly a silico-carbonatic protore; its contacts with the upper and lower sections are gradational and characterized by garnet-rich horizons. Rhodochrosite, spessartite and tephroite are the main components of the silico-carbonatic protore. Metamorphosed grafite and sulfide-rich layers are found throught the entire metassedimentary sequence and suggest that the original depositional environment was chemically reducing. On textural grounds several metamorphic crystallization episodes can be observed. The first event, under amphibolite facies conditions, controlled the development of at least two recrystallization phases. The first phase is sinkinematic and contemporaneous to the major deformation affecting the Lafaiete Formation, and characterized by crystallization of the foliation forming minerals (especially micas and amphiboles) and of snowball garnet. The development of non-rotated border zones of snow-ball garnets and thecrystallization of a second generation of porphyroblastic biotites and amphiboles, now growing at an angle to the foliation, ocurred postkinematically, and texturally characterizes the second growth phase. A mild late metamorphic event controls retrogressive reactions such as partial breakdown of plagioclase into albite plus epidote, chloritization of amphiboles, garnets and biotites, and the formation of Mn-Cummingtonite at the expense of primary tephroite. A comparison of the geological characteristics at Morro da Mina (Minas Gerais) and Serra do Navio (Amapá, N Brazil) shows that both deposits present several characteristics in common, thus suggesting originally similar depositional environment. Polymetamorhism is also a common feature in both regions.
