Geneses and evolutions of iron-bearing minerals in banded iron formations of >3760 to ca. 2200 million-year-old: Constraints from electron microscopic, X-ray diffraction and Mössbauer spectroscopic investigations

Abstract

Minerals preserved in banded iron formations (BIFs) are diagenetic and/or metamorphic rather than primary. Knowing their geneses and evolutions are essential for understanding the interactions between the atmosphere, ocean and biosphere in the first two billion years of life history on Earth. Here we use high resolution electron microscopy, X-ray diffraction and Mössbauer spectroscopy to investigate the mineralogy of six BIFs aged from >3760Ma to ca. 2200Ma with a variety of genetic conditions. The results reveal that each major iron mineral (magnetite, hematite, carbonate and silicate) has a range of structural and morphological features that represent different geneses and evolutionary histories. Euhedral magnetite crystals with homogeneous internal structures and compositions, 3–5nm hematite particles aggregated in iron-rich bands and submicrometer-sized euhedral hematite in chert matrix of Fe-Si transitional bands are identified to have transformed from precursors directly precipitated from seawaters. These minerals retain some primary environmental information when BIFs are deposited. The other mineral phases display features of alteration due to multi-redox reactions, hydrothermal or supergene processes, such as crosscutting, mineral replacement or overgrowth, deformation, fracture and porous structures that have overprinted the primary signatures. Such minerals include (1) massive magnetite with fragmentation structures, (2) partly to completely oxidized magnetite crystals, (3) acicular/fibrous hematite in fractures or iron and silica layering, (4) microplaty hematite, (5) hematite oxidized from other Fe(II)-bearing minerals (e.g., magnetite and stilpnomelane), (6) carbonates with heterogeneous chemical compositions, (7) carbonates replacing other minerals (e.g., siderite thin film coating the euhedral magnetite crystals), (8) irregular carbonate cementing other minerals (e.g., magnetite), (9) preferentially orientated fibrous/spindle-like silicates cutting iron- and silica-rich bands and, (10) massive silicate pieces. This study provides detailed mineralogical and petrologic characterizations of BIF minerals that can serve as a reference for future in situ geochemical measurements by recent developed techniques.