Archean Iron Formations Research Articles

The composition of hornblende, grunerite, and garnet in Archean iron formation of the Itchen Lake area, District of Mackenzie, Canada

The compositions of seven pairs of coexisting hornblende and grunerite and five assemblages of coexisting hornblende, grunerite, and garnet from Archean silicate iron formation of low and medium metamorphic grade have been obtained by electron microprobe analysis. Important factors affecting the composition of the amphiboles are: (1) the Mg/Fe ratio of the iron-formation beds, which controls the gross Mg/Fe ratio of the amphiboles; (2) the alumina content of the beds, which affects the degree of alumina substitution in hornblende thereby altering the distribution of Mg and Fe in the coexisting amphiboles; and (3) the occurrence of iron-rich garnet, which produces higher Mg/Fe ratios in both amphiboles. A fourth potentially important factor, the oxygen fugacity, cannot be satisfactorily assessed with these data, but has not obscured the effects of the other three. Temperature of crystallization of the amphiboles was an important factor mainly insofar as it affected the crystallization of garnet in the alumina-rich rocks.Four coexisting hornblende–cummingtonite pairs from metatuffs show similar control of Mg–Fe fractionation by alumina substitution in hornblende.

Carbon and sulfur isotope abundances in Archean iron-formations and early Precambrian life

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.

Archean Iron-Formations and Tectonic basins of the Canadian Shield

Archean Iron-Formations and Tectonic basins of the Canadian Shield

Microfossils and Precambrian Iron-Formations

Petrographic studies reveal that structures of possible organic origin and those believed to be relict microfossils are extremely abundant in many Precambrian iron-formations in the United States, Canada, Australia, and South Africa. The structures are mainly spheroids and ellipsoids that range from about 5–40μ in diameter. Particular forms, however, usually have a narrower size range. Some of the structures appear to be similar to certain forms reported by Barghoorn and Tyler (1965) from the Gunflint chert; others presumably are new forms not previously reported. Particular types of structures are present in widely separated districts, indicating a wide geographic distribution. Certain distinctive forms occur in several Proterozoic iron-formations in North America and in an Archean iron-formation in Australia. Other forms occur in iron-formations from the Mesabi Range, Minnesota, and Belcher Islands and Lake Albanel, Canada.