Geochemistry and Petrology of Some Proterozoic Banded Iron-Formations of the Quadrilatero Ferrifero, Minas Gerais, Brazil

Abstract

This study presents detailed geochemical and petrologic data for 16 samples collected from four different iron-formations in the Early Proterozoic Minas Supergroup of the Quadrilatero Ferrifero. These banded ironformation samples represent the best preserved, unweathered, and least altered iron-formation precursors to the major hematite-rich ore deposits of the region. Since the 17th century, the Quadrilatero Ferrifero has been recognized as a major gold- and iron-rich province, which has resulted in it being the best-studied Precambrian region in Brazil. The banded iron-formations, locally known as itabirites, and associated rocks, including dolomites, all are part of the Proterozoic Minas Supergroup with a minimum Pb/Pb age determined on the dolomites of 2.4 Ga. The samples were obtained from fresh outcrops in the field, from fresh exposures in open pit mines or from deep diamond drill cores in four localities: the Santuario da Serra da Piedade, and the open pit iron ore mines of Pico do Itabirito, Aguas Claras, and Mutuca. All these have hematite as the only or principal iron oxide; maghemite was found in only one sample. This is in contrast to the common occurrence of magnetite as a major iron oxide in many Archean and Early Proterozoic iron-formations. Although all four localities have been affected by low-grade metamorphism and complex tectonics, none of the assemblages studied show any metamorphic reaction between the oxides and quartz, or between the oxides and carbonates and/or quartz to form metamorphic iron-rich silicates. All of the samples studied (except one that contains maghemite) consist of quartz-hematite, quartz-hematite-carbonate, or hematite-carbonate. It is unknown whether all or only part of the hematite represents a primary phase. In view of the deep lateritic conditions in much of Brazil it is likely that not all of the hematite reflects a primary Fe 3+ -rich oxide or hydroxide phase, but that some of the hematite is a secondary oxidation product. The overall major oxide bulk chemistry of these four iron-formations is very similar to that of most Archean and Proterozoic iron-formations studied except with respect to their very high Fe2O3 (due to hematite) and correspondingly very low FeO contents. Many of the REE patterns of the iron-formation samples show a considerable depletion in light REE, a small negative Ce anomaly and a positive Eu anomaly. Such positive Eu anomalies are common in most Archean and Proterozoic iron-formations and are considered to reflect the result of deep-ocean hydrothermal input into an otherwise highly seawater-dominated system. Such a hydrothermal origin is corroborated when the sum of the REE is plotted against Co + Ni + Cu with the resulting data points clustering in the hydrothermal deposits for metalliferous ocean bottom sediments. The carbon isotope compositions of the carbonate-containing iron-formations range from δ 13 C of ‐1.055 to ‐5.083, which coincides with a range of values for other Proterozoic iron-formations of South Africa (Kuruman iron-formation sequence).