NEOPROTEROZOIC-CAMBRIAN EVOLUTION OF A CONFINED OROGEN AND ITS RELATION TO ECONOMIC MINERALIZATION: THE ARAÇUAÍ BELT, EASTERN BRAZIL

The Araçuaí Belt and its African counterpart, the West-Congo Belt, make up an orogen that developed in the embayment between the São Francisco and Congo cratons during the assembly of West Gondwana. The unique setting of the Araçuaí-West-Congo Orogen led geologists to conclude that it represented closure of an entirely ensialic basin; this interpretation influenced mineral-prospecting evaluations. However, recent work shows that the Neoproterozoic orogeny involved closure of a narrow ocean and development of a magmatic arc within the Araçuaí Belt. Ages of zircons from bimodal volcanics suggest rifting occurred at about 1000-910 Ma. A glaciomarine diamictite-sandstone-pelite sequence with thick intercalations of sedimentary iron formation records the transition from the rift to passive margin. Manganese deposits and volcanogenic massive sulfide mineralization occur in ophiolite remnants (ca. 800 Ma). The metamorphic gradient within passive-margin sediments outlines a horseshoe shaped pattern, converging toward a granulitic-anatectic core. This pattern of regional metamorphism and deformation controls the distribution of large deposits of graphite in kinzigite and schist-quartzite units, and gold-bearing quartz veins in greenschist-amphibolite facies turbidites. No metallic mineralization seems to be associated with the tonalite-granodiorite roots of the calc-alkalic magmatic arc (ca. 625-575 Ma), but syncollisional intrusions of S-type granites (ca. 591-570 Ma) are the sources of many tourmaline-beryl pegmatites. Late-stage magmatic activity related to the collapse of the orogen generated one of the world's largest gem provinces: a myriad of pegmatites, derived from S-type granites, provide gem-quality tourmaline and morganite, and lithium ores; and pegmatites, derived and hosted by high-K-Fe calc-alkalic granites, are rich in gem-quality aquamarine. Alexandrite and emerald deposits formed in hydrothermal systems related to this last magmatic episode (530-500 Ma).