H. L. James
1954
<p><span>The </span>sedimentary<span> </span>iron<span>-formations of Precambrian age in the Lake Superior region can be divided on the basis of the dominant original </span>iron<span> mineral into four principal </span>facies<span>: sulfide, carbonate, oxide, and silicate. As chemical sediments, these rocks reflect certain aspects of the chemistry of the depositional environments. The major control, at least for the sulfide, carbonate, and oxide types, probably was the oxidation potential. The evidence indicates that deposition took place in restricted basins, which were separated from the open sea by thresholds that inhibited free circulation and permitted development of abnormalities in oxidation potential and water composition. The sporadic distribution of metamorphism and of later oxidation permits description of the primary </span>facies<span> on the basis of unoxidized, essentially unmetamorphosed material. The sulfide </span>facies<span> is represented by black slates in which pyrite may make up as much as 40 percent of the rock. The free-carbon content of these rocks typically ranges from 5 to 15 percent, indicating that ultra-stagnant conditions prevailed during deposition. Locally, the pyritic rocks contain layers of </span>iron<span>-rich carbonate. The carbonate </span>facies<span> consists, in its purer form, of interbedded </span>iron<span>-rich carbonate and chert. It is a product of an environment in which oxygen concentration was sufficiently high to destroy most of the organic material but not high enough to permit </span>formation<span> of ferric compounds. The oxide </span>facies<span> is found as two principal types, one characterized by magnetite and the other by hematite. Both minerals appear to be of primary origin. The magnetite-banded rock is one of the dominant lithologies in the region; it consists typically of magnetite interlayered with chert, carbonate, or </span>iron<span> silicate, or combinations of the three. Its mineralogy and association suggest origin under weakly oxidizing to moderately reducing conditions, but the mode of precipitation of magnetite is not clearly understood. The hematite-banded rocks consist of finely crystalline hematite interlayered with chert or jasper. Oolitic structure is common. This </span>facies<span> doubtless accumulated in a strongly oxidizing, probably near-shore, environment similar to that in which younger hema-titic ironstones such as the Clinton oolite were deposited. The silicate </span>facies<span> contains one or more of the hydrous ferrous silicates (greenalite, minnesotaite, stilpnomelane, chlorite) as a major constituent. Granule structure, similar to that of glauconite, is typical of some varieties; others are nongranular and finely laminated. The most common association of the silicate rocks is with either carbonate- or magnetite-bearing rocks, which suggests that the optimum conditions for deposition ranged from slightly oxidizing to slightly reducing. The relationship between the </span>iron<span>-rich rocks and volcanism, stressed by many authors, is considered by the writer to be structural, not chemical: in the Lake Superior region both </span>iron<span>-deposition and volcanism are believed to be related to geosynclinal development during Huronian time. In Michigan, the lower Huronian rocks are </span>iron<span>-poor quartzite and dolomite-typical "stable-shelf" deposits; much of the upper Huronian consists of </span>iron<span>-poor graywacke and slate with associated volcanic rocks -a typical "geosynclinal" assemblage. Thus the </span>iron<span>-rich beds of the middle Huronian and lower part of the upper Huronian were deposited during a transitional stage in structural history. The major environmental requirement for deposition of </span>iron<span>-</span>formation<span> is the closed or restricted basin; this requirement coincides in time with what would be a normal stage in evolution of the geosyncline: namely, structural development of offshore buckles or swells that subsequently develop into island arcs characterized by volcanism. </span></p>
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10.2113/gsecongeo.49.3.235
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Society of Economic Geologists
Sedimentary facies of iron-formation
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