Microstructures, crystallography and geochemistry of magnetite in 2500 to 2200 million-year-old banded iron formations from South Africa, Western Australia and North China

Abstract

As a major mineral in banded iron formation (BIF), magnetite can provide information on depositional environment and mechanism of BIF. It can provide insights into Precambrian biological process. The crystallography and microstructures of magnetite in BIF have been poorly studied due to its simple chemical composition. Here, we report an integrated crystallographic and geochemical study on various magnetites from four BIFs of 2500 to 2200 million-year-old for a better understanding on their geneses and alterations. Euhedral magnetite crystals from the Dales Gorge and Kuruman BIFs all show pure and homogeneous colors in their EBSD phase, Euler and IPF maps, suggesting they are well crystallized and devoid of inclusions or fractures. These crystals are recognized as silician magnetite with appreciable structural Si (ave. 1.22 and 2.05 wt% respectively) but rare Al (ave. 0.27 and <0.05 wt% respectively). They likely precipitated in a reduced and Si-rich environment and were not subjected to later redox alterations. By contrast, irregularly shaped massive magnetite from the 2500 Ma Wutai BIF contains inclusions and/or fractures as revealed by numerous dark areas in its phase map. Such magnetite contains considerable structural Al (ave. 1.51 wt%) but little Si (ave. 0.07 wt%), suggesting a terrestrial contribution. Magnetite from the 2200 Ma Lüliang BIF commonly contains chert inclusions and fractures and has been partially oxidized to hematite/maghemite. Hematite preferentially occurs along the fractures or edges of magnetite crystals, implying a secondary oxidation with possible fluid percolation. It contains Al (ave. 0.19 wt%) and Si (ave. 0.505 wt%) in a range between that for the Wutai and Kuruman/Dales Gorge magnetites. We suggest primary features of magnetite from the Wutai and Lüliang BIFs have been obliterated in environments with increased detrital inputs, owing to oxygenation-enhanced continental weathering during or after the Great Oxidation Event. For all BIFs, the magnetite that coexists with carbonates contains much higher trace elements (Al, Mn, Ba, Ti and V) than that coexists with oxides. The former might have been influenced by its surrounding diagenetic to metamorphic carbonates. The implications of these findings suggest only the well crystallized euhedral magnetite coexists with oxides may retain the signatures of BIF’s depositional environments.