BRAZILIAN IRON FORMATIONS AND THEIR GEOLOGICAL SETTING

Abstract

In the Quadrilatero Ferrifero banded iron formations (BIFs) are metamorphic jaspilites named itabirites that occur in the Paleoproterozoic platformal sequence of the Minas Supergroup (2.6 - 2.12 Ga). Several types of itabirites can be defined according to primary (sedimentary) and secondary features, acquired under the influence of metamorphic, deformational and weathering processes. A complex interaction of primary and secondary processes was responsible for the anomalous concentration of iron in high-grade deposits. In the Carajas Range in the central north of Para, BIF-related iron ores are weathered anchimetamorphic jaspilites with preserved primary textures and structures, making up approx. 18 billion tons of ore with 66 % Fe. The jaspilites are weakly deformed and extend over 2500 km 2 in the eastern portion of the region in the Archean Grao-Para Group (2.7-2.6 Ga) between metavolcanic rocks. The Urucum District in Mato Grosso do Sul, at the Brazilian - Bolivian border, displays the youngest Brazilian banded iron formation as flat- lying anchimetamorphic jaspilites of Proterozoic age (ca. 0.6 Ga). These are associated with fine glacial marine sediments and manganese layers that outcrop as inselbergs locally called Morrarias. Urucum jaspilites belong to the group of Rapitan-Type iron formations and can be distinguished from the Carajas jaspilites by the presence of glacial-clastic components and nodules of uncertain origin. Tropical weathering as well as primary concentration are the principal processes responsible for the high grade iron ores with more than 63% Fe and reserves of approximately only 100 million tons, that are very appreciated for direct reduction plants.