Recebido em 15/9/05; aceito em 25/4/06; publicado na web em 30/8/06Lateritic iron crust (LIC) samples from Padauari (AM) were analysed by XRD, optical microscopy and SEM-EDS. The equilibrium of iron minerals (IM) was studied using Eh-pH diagram

In the eastern portion of the Brazilian state of Roraima there are two main volcanic terranes of Paleoproterozoic age: the Surumu Group to the north and the Iricoumé Group to the south. The Central Guyana Belt is located between these groups. Geochemical and geochronological data point to a dissociation with the anorogenic granitoids related to Saracura and Mapuera suites, presently grouped into the Uatuma Supergroup (Uatumã Event, 1.90-1.70Ga). Chemical characteristics of volcanic rocks are very similar to those observed in the calcalkaline granitoids such as the Pedra Pintada and Água Branca suites (1.96-1.91Ga) pointing to a post-collisional setting, related to the end of Trans-Amazônian Orogeny. The Surumu Group is usually metamorphosed in the lower greenschist facies, with steep foliation and open folding. The Surumu-Iricoumé groups represent a series of acidic to intermediate volcanics, metaluminous and calc-alkaline affinities, similar to those observed in modern magmatic arcs. The REE pattern shows two characteristic groups of volcanics, suggesting distinctive magmatic evolution. Both volcanic-plutonic associated Surumu-Pedra Pintada and Iricoumé-Água Branca and previous molassic sedimentary covers show a new geological framework and are postulated to the Orocaima Event (1.98-1.88 Ga), Orosirian Period.