Petrography and Geochemistry of Organic Matter in Neoarchean Abitibi BIFs Closely Associated with Hydrothermal Vents

Hydrothermal vents and associated banded iron formations in the 2.73 Ga Hunter Mine Group in Quebec have been metamorphosed to sub-greenschist facies and exhibit low degree of deformation. Various Abitibi BIF lithofacies occur in close association with ferruginous and pyritiferous hydrothermal chimneys including siderite-chert-magnetite, chert-jasper, jasper-magnetite and chert-magnetite. Rich and diverse microbial communities are expected to have thrived in such hydrothermal environments and their organic remains may be preserved in these sedimentary rocks. However, hydrothermal systems are also known to be sites where abiogenic hydrocarbon production can occur during Fischer-Tropsch type reactions. Some criteria that can help to distinguish biological and abiological carbonaceous material include modes of occurrence, organic structural characteristics, carbon isotopic compositions, trace levels of H, N, O, S and P, and organic biomarkers. Several micro-analytical methods were used to trace the origin of organic matter in the various Abitibi BIF lithofacies. Petrographic mapping of carbonaceous material in thin section shows preferred distribution in the silicate matrix and the occurrence of finely laminated organic layers in chert-magnetite BIF. Raman microspectroscopy reveals a generally disordered organic structure with similar peak intensities for the D- and G-bands, consistent with exposure to only low metamorphic temperatures. Micron-size crystals of ankerite and apatite inside organic layers were also observed in Raman maps. In both oxide and carbonate BIF lithofacies, SEM analyses revealed microscopic diagenetic apatite grains with inclusions of organic carbon, silicates and magnetite mostly in the chert-magnetite matrix. Chemical maps of trace elements by nanoSIMS and carbon isotopic analyses will help to further constrain the origin of carbonaceous material in these BIFs.