Iron Isotopes Applied to BIF-Hosted Iron Deposits

Abstract

AbstractPublished and unpublished iron isotope data from banded iron formations (BIF) and their BIF-hosted hypogene (hydrothermal) iron ores from the Quadrilátero Ferrífero (itabirites), Corumbá, and Carajás iron districts in Brazil, as well as from the Hamersley province in Australia are presented and discussed. BIF constitutes a typically thinly bedded or laminated chemical sedimentary rock, with ≥ 15% Fe and layers of chert, chalcedony, jasper, or quartz, whereas itabirite is considered a laminated, metamorphosed iron formation rich in iron oxides, which may contain carbonate minerals, amphiboles, and abundant quartz. For the Paleoproterozoic Quadrilátero Ferrífero district, the range in δ56Fe values of hypogene iron ores is similar to that of the metamorphosed BIFs, and iron isotope variations are better distinguished in different regional deformational domains. Light isotopic compositions dominate in the low deformation domain (δ56Fe = −0.42 ± 0.12 to 0.29 ± 0.04‰), whereas the eastern, high-strain domain is characterized by heavy values (δ56Fe = −0.09 ± 0.08 to 0.37 ± 0.06‰; Mendes et al., Mineral Deposita 52:159–180, 2017). Iron isotope composition for the Neoproterozoic iron formations of the Corumbá region (hematitic, dolomite-rich: −1.83 and −0.83‰; cherty-hematite: δ56Fe −0.49‰) are controlled by: (1) primary seawater signature, (2) microbial activity, and (3) supergene goethite alteration. Hydrothermal alteration is reflected in the oxygen isotope data, but apparently not in the iron isotope fractionation. Iron and oxygen isotope pairing shows that δ56Fe values increase, while δ18O values decrease. In the Archean jaspilites of Carajás, hypogene ores tend to display lighter δ56Fe values than their host BIF counterparts. Also, there is a correlation between coupled iron and oxygen isotope values that is clearer towards lighter isotopic values, especially for δ18O. In the Paleoproterozoic Hamersley deposits, correlation between δ18O and δ56Fe values suggests a direct correlation of both isotope systems during low-grade, greenschist-facies metamorphism. On the other hand, despite the evident shift to negative δ18O values and apparent preservation of the metamorphic δ56Fe signature, iron ore and hydrothermally modified BIF show a correlation between δ18O and δ56Fe values. In contrast, in supergene-modified samples a negative correlation is apparent. The Carajás (+1.24 to + 0.44; one sample − 0.30‰) and Hamersley (+ 1.02 to − 0.29‰) hypogene ores display δ56Fe in a similar interval, reaching positive values, whereas ores from the Quadrilátero Ferrífero show a tendency towards lower values (to − 0.80‰). This review indicates that the application of iron isotopes in exploration is presently limited mainly due to the restricted dataset available for ore samples. Nevertheless, and despite all local differences, there is a general tendency for hypogene ores to display moderately lighter δ56Fe values for all deposits compared to precursor BIF. In contrast, a strong supergene imprint in ore leads to moderately heavier δ56Fe values. As more data become available, and if these trends are confirmed, the use of this tool may be valuable in the future, for instance to decipher the hypogene or supergene origin of specific ore zones, and as a consequence the probable depth extension or interpretation of concealed, deep orebodies.