5:15 PM - 7:15 PM
[BCG06-P10] Study on the Causal Relationship between Banded Iron Ore Formations and Black Shales at the Sherman Mine, Ontario, Canada
Keywords:Archean, Banded Iron Formation, Pyrite, Black shale
The role of submarine hydrothermal activities is still unknown for the genesis of Archean banded iron formations (BIFs). On the other hand, many BIFs are known to developed on sulfidic black shales. Such sulfidic black shale may contain the information of the submarine hydrothermal activities. Therefore, in this study, geological and geochemical studies were performed on sulfidic black shales at Sherman Mine, Ontario, Canada, to investigate the hydrothermal activity and associated environmental changes in their depositional settings.
Changes of lithologies and mineralogizes in shale and chert were constructed by geological surveys. In particular, black shales exhibit various sulfur contents and different types of pyrite through the stratigraphy. This indicates that the intensity and nature of hydrothermal activity changed over time. In the initial stages, relatively mild hydrothermal activity contributed to the deposition of sulfidic black shales. As hydrothermal activity intensified, sulfur supply increased, leading to the formation of colloform pyrite and massive sulfides. Furthermore, subsequent changes in the composition of hydrothermal fluids likely resulted in the deposition of chart and oxide BIF. Sulfur isotope analyses indicate that the activities of sulfate-reducing bacteria in organic rich sediments. Such organic rich sediments (shale) represent the earliest sediments. Analyses of organic carbon isotope composition (δ13Corg) suggested the presence of methanogenic and methanotrophic microorganisms in sulfidic shales. Those indicate large production of methane during the hydrothermal stage. It is found that carbon isotope compositions of organic matter changed with stratigraphy most likely corresponded to ecological change with evolution of hydrothermal system. On the other hand, sulfur isotope composition (δ34S) of massive sulfides indicated that the sulfur originated from submarine hydrothermal activity. These findings demonstrate that hydrothermal activity was already active in the depositional environment before the formation of BIF and played a significant role in influencing the characteristics of the sediments and the surrounding environment.
Changes of lithologies and mineralogizes in shale and chert were constructed by geological surveys. In particular, black shales exhibit various sulfur contents and different types of pyrite through the stratigraphy. This indicates that the intensity and nature of hydrothermal activity changed over time. In the initial stages, relatively mild hydrothermal activity contributed to the deposition of sulfidic black shales. As hydrothermal activity intensified, sulfur supply increased, leading to the formation of colloform pyrite and massive sulfides. Furthermore, subsequent changes in the composition of hydrothermal fluids likely resulted in the deposition of chart and oxide BIF. Sulfur isotope analyses indicate that the activities of sulfate-reducing bacteria in organic rich sediments. Such organic rich sediments (shale) represent the earliest sediments. Analyses of organic carbon isotope composition (δ13Corg) suggested the presence of methanogenic and methanotrophic microorganisms in sulfidic shales. Those indicate large production of methane during the hydrothermal stage. It is found that carbon isotope compositions of organic matter changed with stratigraphy most likely corresponded to ecological change with evolution of hydrothermal system. On the other hand, sulfur isotope composition (δ34S) of massive sulfides indicated that the sulfur originated from submarine hydrothermal activity. These findings demonstrate that hydrothermal activity was already active in the depositional environment before the formation of BIF and played a significant role in influencing the characteristics of the sediments and the surrounding environment.