Abstract

The volcanically enclosed Michipicoten and Woman River iron-formations, both approximately 2.75 b.y. old, in Superior Province of the Canadian Shield lie at lithologically similar stratigraphic contacts between underlying felsic pyroclastics and overlying mafic lava flows. Both iron-formations display similar lateral transitions of, from west to east, oxide, carbonate, and sulfide facies, a common facies pattern attributed to chemical deposition of volcanically derived components upon eastward-inclined slopes forming parts of local volcano-tectonic basins of deposition. Both iron-formations contain numerous, thin graphitic units.Six samples of Michipicoten graphitic carbon give delta 13 C values ranging from --20.4ppm to --27.7ppm with a mean of -- 24.7ppm. A sample of Michipicoten inorganic carbon in the form of massive siderite gave a delta 13 C value of +2.4ppm, which is similar to Phanerozoic marine carbonates. These data indicate extensive biological activities in the Archean basin during deposition of iron-formation.Sulfur isotope abundances in 62 samples of Michipicoten iron-formation taken across live main stratigraphic sections have a mean delta 34 S value of +0.02ppm with a range of 20.6ppm (--10.5 to +10.1ppm) and in 33 samples of Woman River iron-formation taken across three sections have a mean delta 34 S value of --1.3ppm with a range of 15.0ppm (--6.8 to +8.2ppm). The delta 34 S distribution patterns, which show large fluctuations over short distances, in the sediments are essentially the same for the different iron ranges and for the different members, chert, siderite, and sulfide (pyrite), within the Michipicoten iron-formations. The results strongly indicate isotopic fractionation in the biological reduction of sulfate under anaerobic conditions from a restricted reservoir at low sulfate concentrations.The sulfur and carbon isotope data provide strong evidence for the existence of autotrophic organisms and reducing bacteria in Archean times, but taken together with geological considerations as to the physical and chemical environment of deposition, the evidence appears overwhelming.Further detailed sampling and isotope analysis of Archean iron-formation is under way in order to advance the understanding of biogenic activities and basin deposition during accumulation of these ancient Precambrian chemical sediments.

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