Microscopic observations show obvious relationships between pyrite grain morphology and iron isotope ratio. Most pyrite grains with positive ¦Ä56Fe values show hexagonal, rectangular, and parallelogram shapes, which may replace former iron-oxide crystal systems: hematite, magnetite, and goethite, respectively. In contrast, more than half the pyrite grains with negative ¦Ä56Fe values show irregular forms. The correlation allows the possibility to solve the origin and the formation process of each grain of pyrite. The positive ¦Ä56Fe values suggest the partial oxidation of iron in an oxygen-limited environment. Some pyrites show very lower ¦Ä56Fe values below ?2.2¡ë suggesting a biological origin, probably due to microbial iron reduction.
On the other hand, the pyrite is accompanied by isotopically very light organic carbon (¦Ä13Corg: ?51.8¡ë to ?40¡ë), which indicates aerobic or anaerobic methanotrophy. The coexistence of the low ¦Ä56Fe values and low ¦Ä13C values in the some rocks suggests anoxic oxidation of methane by iron-reduction (AOM/IR). The iron and carbon isotopes demonstrate the metabolic variations of microorganisms in a Late Archean shallow marine environment.