The Iron Formations of the South American Platform

Abstract

The Precambrian of South America contains world-class iron formation (IF) occurrences of Archean, Paleo-, and Neoproterozoic age preserved in its different cratonic fragments and surrounding collisional belts developed during their amalgamation along a protracted Neoproterozoic to Early Paleozoic event known as the Brasiliano-Pan-African collage that formed the southwest part of the Gondwana supercontinent. In the domain of South America located west of the Transbrasiliano Lineament, most Archean IFs are spatially and genetically associated with volcanosedimentary sequences and crop out as discontinuous tectonic fragments of relatively small dimensions and seldom with economic concentrations of iron. An exception is the Carajás mineral Province in the Amazon Craton, where voluminous IFs from the Neoarchean Carajás Formation host giant high-grade iron ore deposits associated with the recurrent circulation of fluids of magmatic origin. Giant ore deposits of this metal are also present in the Paleoproterozoic Imataca granulite facies Complex in the Transamazon/Guianas Province of the Amazon Craton, but the information available about their primary geological setting, age, and mineralization processes is very limited. In the domain located east of the Transbrasiliano Lineament, on the other hand, the São Francisco Craton stands out with large Paleoproterozoic platform sequences containing banded IF (BIF) deposits, including some of the most important high-grade iron-ore deposits in the world. These are hosted by the Siderian Cauê Formation of the Minas Supergroup in the well-known Quadrilátero Ferrífero Mining District, and found also in the north–south-trending Orosirian-Statherian Serra da Serpentina IF, all IFs having been deposited along the eastern coast of an Archean protocraton. The Nico Pérez Terrane of southern Brazil and Uruguay, assumed to be a fragment of the São Francisco-Congo Craton, also preserves relics of BIF-bearing Paleoproterozoic platform successions. These BIFs contain significant but sub-economic iron ore resources as a result of a weak secondary iron enrichment and their relatively small size. The Paleoproterozoic Río de la Plata Craton lacks an Archean core and does not show any evident link with the other cratons of the South American Platform. In contrast with the other terranes, it hosts only small, scarce occurrences of BIFs associated with metavolcanosedimentary sequences. Neoproterozoic BIFs associated with Fe-rich clastic sediments are widespread in the Cryogenian sediments of the mobile schist belts, but economic deposits are few. The best-known examples of sequences hosting Neoproterozoic BIFs include the Jacadigo Group of Brazil and Bolivia, and the Yerbal Formation of Uruguay.