Ukrainian shield

The largest areas where Precambrian iron-formations are developed are the East European platform (the Krivoy Rog, Kremenchug, Belozerka, and other districts of the Ukrainian shield, the Voronezh, Baltic, and Central Kazakhstan shields), the Lake Superior area on the North American platform, Minas Gerais in Brazil, Hamersley in Australia, Singhbum in India, etc. 

BIF have been found in the Precambrian beginning with the oldest strata, 3500 m.y. old. The rocks of the Konka-Belozerka zone of the Ukrainian shield and of the Pilbara Block of the Austrahan platform have an age of 2700-3500 m.y. 

An age of 2000-2700 m.y. has been established for the iron cherts of the Bazavluk zone of the Ukrainian shield, the Olenogorsk deposit on the Baltic shield, the Singhbum deposit in India, and the Algoma-type BIF of the Lake Superior area. 

In the Ukrainian shield, BIF are developed in a number of synclinorium zones in different structural levels of the Precambrian. They are spatially unconnected, which makes it difficult to correlate them and establish their common genetic regularities. Usually several zones of development of BIF are distinguished (Semenenko, 1973), differing in location (Fig. 1) and time of formation (Fig. 2). 

Fig. 2. Comparative schematic sections of the cherty ironformations of the Ukrainian shield. Symbols 1 = schists and gneisses, carbonate rocks of the upper suite 2 = sandy suites (sandstones, conglomerates) 3 = schist suites 4 = cherty iron-schist suites 5 = schist-sandy suites (arkoses, phyllites) 6 = schist-keratophyre suites (keratophyres, tuffs, schists) 7 = schist-ultramafic suites (talc, talc-magnesite, and green schists) 8 = cherty iron-metabasite suites 9 = quartzite-sandstones in metabasites 10 = metabasite suites.

In the Mikhaylovka group, which is correlated with the Verkhovtsevo series of the Ukrainian shield (Zaytsev, 1966), iron cherts are interbedded with keratophyres and their tuffs and orthoschists, and are underlain by orthoamphibolites. 

The association of BIF with basic volcanic rocks is typical of the first type it can be correlated with the jaspilite formation of the Algoma series in the Lake Superior district, which was formed in the greenstone formation. The rocks of the Verkhovtsevo group of the Ukrainian shield are also considered analogs. 

Semenenko et al. (1959, 1967) assigned a large role in the formation of the iron cherts of the Ukrainian shield to processes of volcanism. According to his formational scheme, a direct relationship to submarine volcanism is established for the CIM, CIU, and CIK, from the paragenetic association of iron cherts with volcanic rocks. 

Fig. 15. Variations in iron content of greenalite, minnesotaite, amphibole, pyroxene, and olivine in silicate rocks of the BIF and iron-rich shales (schists). Slightly metamorphosed BIF (Biwabik, Minnesota Gunflint, Ontario and Sokoman, Labrador Trough Brockman and Roper River, Australia) 1 = greenalite 2 = minnesotaite (Klein, 1974). Moderately and highly metamorphosed BIF i = Krivoy Rog district 4 = Mariupol district 5 = other districts of the Ukrainian shield (Mel nik, 1975) 6 = Tobacco Root Mountains, Montana (Immega and Klein, 1976).

Orthopyroxenes, found only in highly metamorphosed rocks, have an iron content of 60 to 88% the latter value is close to the maximum iron content of natural pyroxenes of the ferrosilite series (Fig. 15). In the BIF of the Ukrainian shield the content of minor components in the pyroxenes is not high AI2O3 —0.15-1.60 (0.78) CaO—0.49-2.11 (1.15) MnO—0.07-1.56 (0.35) Na20 + KjO—0.08-0.47 (0.26) TiOj —0.05-0.19 (0.10) (average of 12 analyses in parentheses). Pyroxenes from other areas where BIF are developed have a low content of impurities except for the manganiferous varieties (up to 3-6% MnO) found in the Tobacco Root Mountains and in the Carter Creek area of the Ruby Mountains, Montana (Immega and Klein, 1976) the iron content of these pyroxenes is within 60-62%. 

The carbon contents in the schists underlying the BIF and in the overlying schists (analogs of the schists of the lower and upper suites of the Saksagan group) are about the same as in the BIF of the Ukrainian shield. 

Characteristics of bitumens from graphite gneisses of the Ukrainian shield (after A. and S. Sidorenko)... 

Magnetite-siderite rocks are very common in the section of the iron-ore sequence of the Krivbass (Krivoy Rog basin) and of numerous magnetic anomalies of the Ukrainian shield. Such rocks also occur in the KM A, in the Lake Superior and other areas of slightly metamorphosed BIF. The presence of excess magnetite does not affect further metamorphic transformations of siderite, which proceed as in pure carbonate iron-formations. 

When olivine and pyroxene are oxidized the same regularities are observed as in the oxidation of carbonates. In particular, it was established in a study of the rich and unique olivine-magnetite iron ores of the Volodarsk deposit (Ukrainian shield) that the magnesium content of the silicates increases as the magnetite content of the ore increases (Mel nik and Yaroshchuk, 1966). In this case the most likely oxidants may be water and carbon dioxide, the main components of the fluids causing metasomatic reworking of the olivine-and pyroxene-bearing iron formations. 

Belevtiiev, Ya.N., 1964. Mctallogeny of the Prccambrian geo.synclinc of the Ukrainian Shield. Izv. Akad. 

Mel nik, Yu.P. and Yaroshchuk, M.A., 1966. A thermodynamic analysis of the conditions of formation of the olivine-magnetite rocks and ores of the Volodarsk district of the Ukrainian shield. In Problemy teorii i eksperimenta v rudoobrazovanii (Problems of Theory and Experiment in Ore Deposition). Izd. Naukova Dumka, Kiev, pp. 98-113 (in Russian). Geochem. Int. 1966, 3 1218-1229 (in English). 

Semenenko, N.P., 1973. The iron-chert formations of the Ukrainian shield. In Genesis of Precambrian Iron and Manganese Deposits. Unesco, Paris, pp. 135-142. 

Semenenko, N.P., Boyko, V.L., Borgunov, I.N., Ladiyeva, V.D, and Makukhina, A.A., 1967. Geology of Sedimentary-volcanogenic Formations of the Ukrainian Shield. Izd. Naukova Dumka, Kiev, 378 pp. (in Russian). 

Kalyaev G. I. Mode of albitite distribution in zones of the Ukrainian Shield. In Albitized uranium deposits Arashov A. transl. Open-File Rep. U.S. Dep. Energy GJBX-193(80), 1980, 13 p. 

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