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[BCG09-15] Microfossil-like carbonaceous materials in 3.2 Ga banded iron formations of Sheba Gold mine, Moodies Group, Barberton Greenstone Belt, South Africa
Keywords:Berberton Greenstone Belt, Moodies group, Microfossil, Archaean
Rock samples were classified into Sandy-siltstone (6.1–16.3 wt% Fe2O3), Carbonate rich (Carb-) siltstone (21.6–32.8 wt% Fe2O3) and Magnetite rich (Mgt-) siltstone (18.9–50.2 wt% Fe2O3) based on petrographic observations. Sandy-siltstone consists of relatively large detrital quartz (< ~60 µm), whereas Carb-siltstone and Mgt-siltstone show similar size distribution of detrital quartz (< ~30 µm). Modal analysis indicates that sedimentary settings of Carb-silitstone and Mgt-siltstone are almost identical. Contents and stable isotope ratios of organic carbon in these samples indicate 0.03–0.29 wt% and ~−27 ‰. Most organic materials were observed as ~35 µm diameter, transparent thin flakes and scattered in detrital quartz-rich layers, whereas sheet-like and filamentous organic materials were also observed as minor components. Organic materials sticking on surrounding minerals were picked up by microsampling (Axis pro SS, Microsupport, Japan) directly from thin sections and observed by field emission scanning electron microscope (FE-SEM, JEOL JSM-6500F) and transmission electron microscope (TEM, JEOL JEM2100F). Close-packed and deformed ~1 µm hollow projections were observed on surface of the flaky organic materials. The organic materials sometimes show polygonal depressed area, suggesting that they were compressed by adjacent minerals during metamorphism. Nano-sized Fe, Ti and Cr-bearing particles were sometimes aggregated on organic materials. Organic materials were also extracted by acid-treatment and heavy media separation from powdered samples to estimate fraction of each morphology in samples. Morphologies of organic materials are classified into three types: 30–50 µm flake, > 100 µm filament and ~50 µm indeterminate grain. Note that sheet-like organic materials could not be observed in acid-treated and heavy media separation samples, probably due to deformation during centrifugation in the extracted procedure. Raman spectrum of acid-treated samples indicate that organic materials have experienced 340–576 °C metamorphic temperature, which is slightly higher compared to that of metamorphic facies of Moodies group (greenschist facies, 300–500 °C).
Carbon isotope analysis, Raman spectrum and positional relationship of the organic materials indicate that they are indigenous and biogenic origin. Carb-siltstone is rich in filaments compared to Mgt-siltstone, although carb-siltstone and Mgt-siltstone deposited in identical sedimentary settings. These results suggest multiple microbial species and fluctuation of the microbial activities in 3.2 Ga shallow oceans. Distinct iron contents of Carb-siltstone and Mgt-siltstone (Carb-siltstone: 22–33 wt%, Mgt-siltstone: 19–50 wt%) also suggest that fluctuation of multiple microbial activities affected the iron fixation. If filamentous organic materials were originated in oxygenic photosynthetic bacteria, our results suggest that oxygenic photosynthetic bacteria influenced iron-bearing mineral flux depending on depositional setting and nutrient supply in 3.2 Ga shallow ocean. Detail observations of flake organic materials suggest that they could be benthic microbial mats flourished on sands trapping nano-size minerals from detritus, torn apart by waves.