Geologia da Formação Ferrífera São João Marcos, município de Rio Claro, Estado do Rio de Janeiro

Abstract

The São João Marcos Iron Formation (SJMIF), named after the homonymous municipality, is hosted by the São João Marcos Archeological Park, located between Rio Claro and Mangaratiba-RJ and has been little studied so far, frequently cited as “magnetite-bearing quartzite” in previous works. Aiming the study of its genetic aspects, the methodology applied in this work included 1:10.000 geological and structural mapping, besides petrography of thin and polished section, total-rock geochemistry and mineral chemistry by microprobe analysis of the Iron Formation and some associated rocks. The geological and structural mapping has revealed that the SJMIF is intimately associated with mafic and ultramafic metavolcanics, impure quartzites, sillimanite quartzites and metapelites, which are disposed over an Archaean to Palaeoproterozoic basement. This set of rocks was submitted to (at least) two deformational events: Dn (main event), which is linked to the development of tight to isoclinal folds with a mainly SW dipping foliation, and NE-SW trending shear zones; and Dn+1, linked to the development of open folds with subvertical axial plane. Petrographic analysis shows that the SJMIF is mainly composed by magnetite and quartz, with minor percentages of other minerals. Exsolution lamelae of hercynite and ilmenite, and incipient martitization are common in magnetite. At least three metamorphic stages related to Dn were identified based in petrography: the first stage marks a metamorphic peak in granulite facies, followed by the second main stage, which is associated to the development of a retrometamorphic assemblage in amphibolite to green-schist facies, with cooling under approximately constant pressure (IBC path). The third stage is probably related to the orogenic collapse, marked by the development of textures that indicate isothermal decompression (ITD path). A fourth stage (post-Dn or sin-Dn+1) is characterized by a higher degree assemblage. The geochemistry and petrography of the SJMIF points to a precipitate origin for the iron with certain contamination by clastics. Regarding the metavolcanics, their geochenistry indicates OIB, N-MORB and E-MORB signatures. Field data, petrography, geochemistry and microprobe data, together with information about genetic models for iron formation and previous works in literature permitted to conclude that the SJMIF can be classified as a Neoproterozoic Algoma-type iron formation developed in a rift-like basin that possibly accompanied the Rodinia break up in Neoproterozoic. The origin for the iron can be associated to leaching of the associated volcanics by low temperature hydrothermal fluids. The Dn metamorphic assemblages were attributed to the development of the southern portion of Brasilia Belt while the post-Dn assemblage might be related to the first metamorphism event of Ribeira Belt.