The São João Marcos Iron Formation: an example of Neoproterozoic Algoma-type iron formation in Rio de Janeiro State, Brazil

The São João Marcos Iron Formation (SJMIF), located near the municipality of Rio Claro in the state of Rio de Janeiro, was here studied regarding its genetic aspects. The methodology involved geological mapping, petrography, and geochemistry. This iron formation has been dated to the Neoproterozoic in previous works and occurs intimately associated to metamafic and metaultramafic rocks. In the mapping area, the host metamafic and metaultramafic rocks occur over an Archean to Paleoproterozoic basement and, over them, sillimanite quartzite and (sillimanite)-garnet-biotite gneiss occur from the base to the top. Mapping reveals that these lithologies are disposed as an overturned synform. Petrography of the iron formation reveals a modal composition of mainly titanomagnetite and quartz with a few percentages of other minerals (e.g. ferrosilite, muscovite, garnet, zircon, tourmaline, feldspar and amphiboles); the metamafic and metaultramafic rocks are composed basically of plagioclase and hornblende in diverse proportions, and finally, the metaultramafic ones are mainly composed of amphiboles and pyroxenes. Geochemistry of major, trace and rare earth elements points out strong similarities between the SJMIF and classical Neoproterozoic Algoma-type iron formations, besides characteristics of chemical precipitates. For mafic and ultramafic rocks, geochemistry points to a generation in an intracontinental context. These pieces of field, petrography and geochemistry (whole rock and mineral) information suggest an iron formation generated in an extensional environment, possibly accompanying the break-up of the Rodinia supercontinent during the Neoproterozoic. It is the first example of Neoproterozoic Algoma-type iron formation in Rio de Janeiro. (author)