O SUPERGRUPO RIO DAS VELHAS DA FAIXA MATEUS LEME-PITANGUI - PARTE MERIDIONAL DO CRÁTON DO SÃO FRANCISCO, MG – E SEU SISTEMA DE ALTERAÇÃO HIDROTERMAL

The studied area is located at the southern São Francisco Craton, between the northwest comer of the Quadrilátero Ferrífero and the Upper Proterozoic Bambui basin. The region comprises Archaean granite-gneisses and two supracrustal sequences: a) The Archaean Rio das Velhas Supergroup, which consists of a thick sequence of a greenstones associated with metamorphic felsic volcanites, volcano-clastic and sedimentary rocks; and b) the overlying Early Proterozoic Minas Supergroup with its widespread banded iron formations.Two major deformational events are present. The oldest Dn event can be detected only in the Rio das Velhas rocks and produced large folds with mainly E-W trending axis. The latter Dn+1 of Transamazonian age (±2,0 Ga) is a progressive deformational event, that has also affected the Minas Supergroup. It produces NW-SE to NE-SW folds associated to conspicuous planar and linear structures. This event envolved to a NW-vergent thurst system with the development of a strong stretch lineation. This event also produced strike-slip faults interpreted as tear-faults parallel to the direction of the thrusts.This paper deals with the large hydrothermal alteration which has affected the lower volcanoclastics rocks of the Rio das Velhas Supergroup. This alteration has been produced by a solfataric hydrothermal system in mesothermal P-T conditions. It produced a high aluminium mineralogical assemblage of corundum, kyanite (minor andaluzite) and muscovite. This mineralogy is latter retrograde altered to diaspore, pyrophyllite, kaolinite. Minor amounts of tourmaline, rutile, chloritoid are also present. A hydrothermal zoning with three major subdivisions is proposed as follows: high aluminium zone, sericitic (muscovitic) zone and propilitic zone. Some cristallochemical aspects of the mineralogical phases are discussed, and also the origin  of the protolite and the relationship between alteration and deformatiom. The hydrothermal system has evolved under pre to syn tectonic conditions during the Archean Dn tectonism.For this event, a model of collisional tectonics is also proposed for the area, as the Rio das Velhas Supergroup represents a former continental back-arc basin.