Stratiform and Globular Organic Matter in the Lower Proterozoic Metasediments at Elliot Lake, Ontario, Canada

Abstract

The lower Huronian strata of the Elliot Lake-Blind River region in Ontario are a good example of the complex evolutionary, diagenetic, chemical, and mobilization processes involved in kerogen paragenesis in ore-bearing metasediments. The present study was undertaken to delineate some of the relationships at this location between uranium-poor kerogen globules and nearby U-rich stratiform kerogen seams. The latter are syngenetic with the host rocks and are probably the progenitors of the younger kerogen globules. Several of the kerogen globules have the morphologies of solidified liquid droplets although some of the other chemically and physically equivalent particles are more angular. Petrological studies suggest that the stratiform kerogens and their host rocks at Elliot Lake underwent diagenetic and mild metamorphic processes, including hydrothermal episode(s) leading to mineral replacement and paragenesis which involved the evolution of the globular kerogens*. This study confirms previous reports which noted that the detrital uraninite grains in the stratiform layers were fractured and invaded by mobilized organic matter. This kerogen at the time of mobilization appears to have been a viscous, liquid substance. Therefore, it is probable that some of the mobilized organic matter also moved away from the stratiform layers under reducing environments to form the globular kerogens, leaving most of the uranium behind. Subsequently, the movement of the globular substance was arrested and it resolidified in situ. The globular kerogens have higher H/C atomic ratios than the stratiform kerogens. The results of vacuum pyrolysis-gas chromato-graphic-mass spectrometric analyses are compatible with an evolutionary scheme of the globular kerogens from the stratiform layers by thermochemical reactions; one possibility is hydrous pyrolysis.