Geochemistry and genesis of the banded iron formations of the Cauê Formation, Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract

The Cauê Formation of the Paleoproterozoic Minas Supergroup hosts banded iron formations (BIFs), locally called itabirites, deposited in shallow marine passive margin settings. Two major compositional types of itabirite, dolomitic and quartz itabirites, are found in the northwestern part of QF. The former consists of alternating dolomite-rich and hematite-rich bands, whereas the latter is formed with alternating quartz-rich and hematite-rich bands. Accessory minerals are chlorite, sericite, and apatite in both types. Dolomitic and quartz itabirites have a very simple chemical composition. In the dolomitic itabirite, Fe 2O 3 plus CaO, MgO, and LOI range from 95.8 to 97.8%, while in the quartz itabirite, Fe 2O 3 plus SiO 2 range from 94.4 to 99.6%. Both itabirites are highly oxidized and present Fe 3+/(Fe 2+ + Fe 3+) ratios higher than 0.98, by far superior than the average ratios of Paleoproterozoic BIFs. Trace element concentrations in itabirites are very low, ranging from <10 to 55 ppm. Dolomite shows negative δ 13C values varying from −2.5 to −0.8‰ versus PDB, while the oxygen isotope data display δ 18O values varying from −12.4 to −8.5‰ versus PDB. The δ 13C values of the dolomitic itabirite are in the same range of those of the overlying stromatolitic dolomites of the Gandarela Formation. C and O isotopes, REE signatures, and Y/Ho ratios suggest a marine origin for the sediments of the Cauê Formation. The HREE enrichment pattern exhibited by the itabirites shows a modern seawater REE signature overprinted by a hydrothermal pattern marked by positive Eu anomalies. Very low contents of Al 2O 3 and TiO 2 and a strong positive correlation between them indicate a minor terrigenous component in the chemically-precipitated marine sediments of the Cauê Formation. Differences in the HREE signatures of itabirites suggest that dolomitic itabirite precipitated in shallower waters receiving sediments from the continent, while quartz itabirite precipitated in deeper waters. Sea-level fluctuations caused by marine transgression–regressions possibly contributed to changes in the composition and varied input of the terrigenous sediments. These changes are expressed by the co-existence of dolomitic, quartz, and amphibolitic itabirites in the Cauê Formation, which represent lateral and vertical facies transitions of dolomitic, cherty, and shaly BIFs, respectively.