The early fine-grained pyrite in the lower thin-bedded section of mineralized Proterozoic shales of the Mount Isa district (Queensland) occurs as densely packed spherical grains in layers paralleling the bedding. Microscopic study of samples from the northern part of the district has shown that the pyrite spheres enclose a fossil microorganism which is rounded to slightly polygonal in shape and composed of nonbirefringent material essentially similar to that of Paleozoic and Mesozoic forms. It is suggested that the microorganisms were probably producers of hydrogen sulfide and may have caused precipitation of iron sulfide from solution.

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