S. S. Goldich
H. Lepp
1964
<p><span>A statistical study of the chemical composition of the </span>Precambrian<span> </span>iron<span> </span>formations<span> of the Canadian Shield affords a new approach to the </span>origin<span> of these unusual </span>formations<span>. The average total </span>iron<span> content of 2,200 samples from the literature and from unpublished mining company analyses is 26.7 percent Fe. The average Fe content for 16 </span>iron<span> </span>formations<span> in the United States and Canada ranges from 24.5 to 34.1 percent. Low contents of A1203, Ti02, P2O5, and CaO characterize the </span>Precambrian<span> </span>iron<span> </span>formations<span> compared to the relatively large amounts of these constituents in the post-</span>Precambrian<span> </span>iron<span>-bearing sediments. The chemical data emphasize that whereas </span>iron<span>, manganese, and silica were transported and deposited together in the cherty </span>iron<span> </span>formations<span> of the </span>Precambrian<span>, these same elements were chemically differentiated in younger geological time in large but separate deposits of </span>iron<span> and silica. Isotopic age determinations indicate that cherty </span>iron<span> </span>formations<span> were deposited during a long interval of geologic time from approximately 1,700 to 3,000 million years ago. A model is proposed to explain the </span>origin<span> of the </span>iron<span> </span>formations<span> of the Lake Superior type based on the absence or marked deficiency of free oxygen in the atmosphere prior to the Late </span>Precambrian<span>. Lateritic weathering under these conditions permitted the transport of </span>iron<span> and manganese together with silica. The weathered mantle effectively retained aluminum, titanium, phosphorus, and colloidal clay. Graphitic material of biogenic </span>origin<span> is closely associated with the </span>Precambrian<span> </span>iron<span> </span>formations<span>. Although it is uncertain whether </span>iron<span> was precipitated directly through biologic processes, the removal of C02 and the liberation of oxygen to the sea water through photosynthesis of primitive plants undoubtedly influenced the energy relationships among the </span>iron<span> minerals. As a result of the variable conditions the </span>iron<span> </span>formations<span> commonly are characterized by nonequilibrium mineral assemblages. In Late </span>Precambrian<span> time a critical level of free oxygen in the atmosphere was attained permitting a marked acceleration in plant growth and in accretion of oxygen. This stage in the development of an oxygenated atmosphere was reached at least 1,200 million years ago and effectively curtailed the development of cherty </span>iron<span> </span>formations<span> of the Lake Superior type.</span></p>
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10.2113/gsecongeo.59.6.1025
en
Society of Economic Geologists
Origin of precambrian iron formations
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