Fig. 1: Petrographic analysis of the studied Francevillian Group samples.
From: Expansion of the aerobic iron biogeochemical cycle during the Paleoproterozoic Lomagundi event
A Photomicrograph of black shale showing authigenic diagenetic Fe-bearing minerals, including pyrite (Py), authigenic clay (Aut. clay), and dolomite (Dol). B Backscattered electron (BSE) image of black shale showing Fe-dolomite (Fe-Dol) and pyrite (Py) as the main diagenetic Fe host minerals. C BSE image of black shale showing Fe mainly hosted in diagenetic clay minerals (Fe-Chl) together with diagenetic Fe-rich and Fe-poor dolomite (Fe-Dol and Dol., respectively). BSE images of the non-fossil related pyrite nodules (D) and pyrite beds (E and F) showing preserved inclusions of sulfate. G–I BSE images of pyritized Francevillian fossilized structure showing pyritization on organic-rich structures (in the core, H and in the edge, I) preserved in the black shale. Pyritization of the non-fossil related nodule only involves organic matter in the host sediment (high S/C ratios and equilibrium Fe isotope fractionation), while pyritization of fossilized structures involves both organic matter in the bodies of the structures and that of the host sediment (low S/C ratios and kinetic Fe isotope fractionation). Qtz Quartz, Fe-Chl iron-rich chlorite, ill illite, H. shale host shale.
