Abstract

The Precambrian iron-formations in the European part of the Soviet Union occur mainly along a trend which can be followed between the Azov Sea coastal region in the south and Kola Peninsula in the north. The deposits of Krivoy Rog and Kremenchug, within the Ukrainian Shield, and those of the Kursk magnetic anomalies (Voronezh plate) form a gigantic metallogenic province. Precambrian banded ores also occur in the Urals and the Asiatic part of the Soviet Union in the cores of Caledonian and Hercynian orogenic belts.Lithologically the iron-formations consist of intercalated, rather continuous bands of silica and iron oxides, or in rare cases, of iron carbonates (carbonate facies). Separate bands range in thickness from a fraction of a millimeter to several centimeters. The bands in turn are composed of microbands. The average iron content of the iron-formations is about 35 percent. The iron-formations are part of a marine geosynclinal sequence and typically are represented by magnetite and hematite facies, the latter being considered by most Russian geologists as the product of sedimentation in relatively deep water far offshore. They are part of the transgressive series. Most of the iron-formations of the Russian platform are thought to be correlative, but the detailed stratigraphy, such as the number of banded ore members, differs from one locality to another. The iron-formation in Krivoy Rog consists of seven members having a total thickness of 1,300 m, whereas that of Kursk consists of five members having a thickness of 500 m.The iron-formations of the Russian platform are underlain by a formation consisting mainly of arkose, phyllite, schist, and quartzite, whereas they are overlain by a succession consisting mainly of carbonaceous shale or schist and dolomite.Reconstruction of the ancient environments in which the iron-formations were formed suggests deposition under both miogeosynclinal and eugeosynclinal conditions. The iron was concentrated in the middle formation of the sequence, corresponding to a sedimentary metallogenic epoch, which probably can be correlated throughout the gigantic metallogenic province of the Russian platform. The major iron-formations in the province are between 2,000 m.y. and 2,600 m.y. old. Some of the smaller iron-formations in other areas are apparently either older or younger than the main ones.Five hypotheses have been proposed to explain the origin of the Precambrian iron-formations and the banding. Among these, hypotheses for a sedimentary origin and a volcanic-sedimentary origin appear to be most realistic and best substantiated.

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