Abstract

In four unmetamorphosed, North American iron formations that display arenitic (granular) textures, much of the chert and megaquartz occupies former intergranular pore space and exhibits textures typical of cements. Iron-bearing phases are very rare in these cements, excluding replacive crystals. Cracks and more irregular cavities that formed in these arenites soon after deposition (probably via the shrinkage of gelatinous silica) are filled with similar chert and megaquartz cements. Silica-cemented intraclastic pebbles of arenite, local occurrences of cross-stratified, cherty internal sediments resting on the cements, and numerous zones where the abundance of cement equals the porosity of comparable modern sands indicate the cementation commenced near the sediment-water interface. Opaline silica, fibrous silica polymorphs, and megaquartz druses are inferred to have been the most voluminous initial precipitates, and they are very inhomogeneously distributed. Silica concentrations in the pore waters of newly deposited iron-formation sands must therefore have been high but variable. The lack of evidence for subaerial desiccation or normal saline deposits renders an evaporative origin for either the cements, or iron formations as a whole, highly unlikely. Steeper geothermal gradients are suggested as a more likely cause of rapid silica cementation, and silica cements from geothermal areas show very similar textures. Most of the cements in iron-formation arenites were affected by neomorphism, replacement, mechanical compaction, and/or pressure solution. Regardless of their origin, the cements are noteworthy for the following reasons: they indicate that modern iron formations are enriched in silica relative to their precursor sediments; they provide new evidence that iron formations are not replaced limestones or dolomites; and the iron minerals they are associated with are probably the closest approaches to primary ferruginous phases in iron-formation arenites. Early silica cements also appear to be widespread in lutitic or banded iron formations in the form of chert pods.

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