Geochemical discrimination among different types of banded iron formations (BIFs): A comparative review

Abstract

Banded iron formations (BIFs) represent purely sedimentary geological environment, show macrobandings (1 m–100 m), mesobandings (1 mm–10 cm), microbandings (0.2 mm–1 mm) and crypto-nanobandings (26 nm–0.2 mm) of hypogene oxide facies (magnetite, hematite), silicate facies (Fe-Mg-Mn silicates, commonly chert, but not necessarily), carbonate facies (siderite, dolomite, calcite), sulfide facies (pyrite, pyrrhotite, chalcopyrite, sphalerite, galena), contain >15% iron, contribute to more than 60% of the global iron resources, and formed during Archean, Paleoproterozoic and Neoproterozoic time. In terms of their age, volcano-sedimentary associations, size, and possible associated glaciogenic features, BIFs may be globally mapped as Algoma, Superior, Rapitan and Ediacaran ores. This contribution presents new ore structures, major-trace element data in bulk ores and microprobe trace elements in magnetite of different BIFs. Of importance is that the Iranian Ediacaran ores display dropstone-bearing jaspilitic magnetite, diamictite, dropstone-bearing cap carbonates with negative isotopic carbon excursion, reflecting the possible post-Gaskiers glaciation events. Discrimination diagrams of SiO2-(Al2O3 + TiO2)-Fe2O3, SiO2-Al2O3, Zr-Al2O3, Fe2O3-Zr, and Zr-Y/Ho illustrate that all BIFs show an affinity to exhalative hydrothermal-seawater, while the Rapitan and Ediacaran ores to some extents have been subjected to detrital contamination by glaciogenic dropstones. Although in Fe /Ti -Al/(Al + Fe + Mn) biplot all BIFs show affinity to the Red Sea and East Pacific Rise hydrothermal sediments, only the Rapitan and Ediacaran BIFs contribute a minor detrital contamination inputs (<30%), possibly caused by dropstone-bearing jaspilitic magnetite. Y-P2O5 diagram suggests that the source of phosphorus in the Rapitan and Ediacaran BIFs is of hydrothermal as well as deep sea diagenetic-biogenic origin. The chondrite-normalized REE + Y patterns show a distinguished positive Eu anomaly for the Algoma BIFs, a weakly positive Eu anomaly for the Superior ores, and a weak Ce anomaly in both BIFs. By contrast, the Rapitan and Ediacaran ores illustrate a weak negative Eu anomaly and an unusual strong positive Tb anomaly, possibly caused by detrital contamination of dropstones. The depletion of REE patterns in some Carajas iron ores compared to other BIFs may have been caused by dissolution, remobilization and supergene deposition. Magnetite discrimination diagrams display a low-T (<200 °C) distal ore zone for the Algoma and Superior ores, a high-T (300–400 °C) proximal ore facies and a low-T (<200 °C) distal ore zone for the Ediacaran BIFs. The presence of dropstone-bearing jaspilitic magnetite, the similarity of fluid inclusions data in apatite and the similar sulfur isotopic composition of pyrite to that of seawater-exhalative hydrothermal system support an Ediacaran type BIF rather than the magmatic immiscibility El Laco-type Kiruna ores for the Iranian magnetite-apatite ores. The peripheral occurrence of dropstone-bearing jaspilitic magnetite and cap carbonates in distal ore zone may be the possible exploration guides for high-grade proximal magnetite-apatite ores. Further microprobe data in magnetite, hematite, martite, goethite, tourmaline, sulfide minerals and albitite are suggested for a better understanding of the ore genesis and exploration guides.