Metamorphism iron-formations

French, B.M., 1973. Mineral assemblages in diagenetic and low-grade metamorphic iron-formation. Econ. Geol., 68 1063-1074. 

Klein, C. Jr., 1966b. Petrology of a metamorphic iron formation in Southwestern Labrador, Canada. Geol. Soc. Am. Spec. Pap., 87 88-89. 

Kranck, S.H., 1961. A study of phase equilibria in a metamorphic iron formation. J. Petrol., 2 137-184. Krause, A. and Lezuchowska, J., 1962. Die Alterung des rontgenamorphen Eisen-lII-hydroxid-Trockengels. Kolloid-Z. Poly mere, 181 69. 

Mel nik, Yu.P, 1966c. Theoretical and experimental study of the conditions of formation and stability of magnetite in the metamorphic iron formations and ores of the Krivoy Rog deposits. In Problemy teorii i eksperimenta v rudoobrazovanii (Problems of Theory and Experiment in Ore Deposition). Izd. Naukova Dumka, Kiev, pp. 58-98 (in Russian). 

Zheng Y-F, Simon K (1991) Oxygen isotope fractionation in hematite and magnetite A theoretical calculation and application to geothermometry of metamorphic iron-formations. Eur J Mineral 3 877-886... 

Kaufman AJ (1996) Geochemical and mineralogic effects of contact metamorphism on banded iron-formation an example from the Transvaal Basin, South Africa. Precam Res 79 171-194... 

Klein C (1974) Greenalite, stilpnomelane, minnesotaite, crocidolite, and carbonates in a very low-grade metamorphic Precambrian iron-formation. Can Min 12 475-498 Klein C (1978) Regional metamorphism of Proterozoic iron-formation, Labrador Trough, Canada. Am Min 63 898-912... 

Models for the formation of Precambrian sediments suggest that the chemical sediments (such as cherts) of the Isua supracrustal belt have formed as shallow water deposits. This is in agreement with structures locally preserved in the metacherts of the sequence. After deposition, the supracrustals were folded and metamorphosed. Finally, the metamorphism reached lower amphibolite facies and in consequence, most of the primary minerals became recrystallized. As a result all chert now appears as quartzite. But apparently metacherts, magnetite iron formation and quartz carbonate rocks have retained their major element chemistry largely unaltered during metamorphism (Nutman et al., 1984) 119). 

On the basis of these data, fairly complete and reliable information has been obtained which makes it possible to demonstrate certain general regularities and particulars of the chemistry of the essential minerals, and also to distinguish the main mineral associations in iron-formations of different rank of metamorphism. 

Carbonates are extremely widespread in the banded iron-formations and schists of low-rank metamorphism. Sideroplesites predominate in the com-... 

Comparison of the data given leaves no doubt as to the extreme similarity of the chemistry of the carbonates and silicates of typical low-alumina iron-formations regardless of the area of occurrence, age, rank of metamorphism, and formation to which the BIF belong. 

On an AFM diagram (Fig. 16) all the minerals from iron-formations proper fall into a narrow and clearly defined field, while the rock-forming minerals of the associated iron-rich shales from BIF of Superior type (chamosite, ripidolite and other chlorites, garnets and some biotites) form another broader and less clear-cut field. These two fields do not overlap, which indicates the isochemical character of metamorphism and the limited mobility of the main rock-forming minerals, at least within individual layers and bands of the iron-formations and shales. The chemical composition of stilpnomelane occupies an intermediate position both in iron content and in alumina content. However, a high content of potassium (up to 2.2% K2O), an element not typical of most BIF, is necessary for the formation of this mica. 

Physicochemical conditions of metamorphism of the rocks of the Precambrian cherty iron-formations... 

Particulars of metamorphism of iron-formations of various types... 

Oxide-carbonate iron-formations. Hematite-siderite rocks are relatively rarely encountered, which apparently is explained not by the exceptional nature of this association in sediments, but by metamorphic transformation into magnetite rocks at relatively low temperatures ... 

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. 

Therefore the formation of magnetite in that way could hardly be of essential importance in the metamorphism of iron-formations, and martitiza-tion is still less hkely. However, in deposits of other genetic types, for instance skam deposits, oxidation of iron silicates to magnetite at the contact with large masses of carbonate rocks (dolomite, magnesite) can be considered an ore-forming process. The last conclusion is still feasible because the carbon dioxide released in the dissociation of carbonates probably had an undisturbed CO O2 ratio. 

Fig. 81. Diagrams of mineral equilibria in oxide-carbonate iron-formations (isothermal sections in coordinates of log /co l S /hjO magnetite-hematite buffer) 1 = isobars of total fluid pressure (Ff = - — vertical thin straight lines—isobars of log/y 3 = isobars of log/cc Fields of actual pressures in metamorphism are hatched.

If there is no constant influx of fluid of a certain composition, decomposition of magnetite ceases. The limiting case is a dry system closed to CO2. By analogy with systems closed to water, in such a system with constant pressure P — Pf = const) the fluid phase disappears entirely, and the Mgt + Sid + Hem association (system Fe-C-O) becomes bivariant and can exist stably below the P-T curve (see Fig. 77) in the stability field of the Sid -1- Hem (+ fluid) association. From these considerations the Mgt -I- Sid + Hem association cannot be used to judge the low-temperature limit of mineral formation the upper limit is fixed quite definitely inasmuch as removal of CO2 begins at P P and the reaction proceeds irreversibly to the right. The extensive occurrence of magnetite in oxide-carbonate iron-formations of low-rank metamorphism apparently indicates the absence of equilibrium or even a deficiency of COj and special dry conditions. 

Fig. 85. P-T curves of monovariant equilibria of decarbonation and dehydration reactions in the metamorphism of silicate-carbonate iron-formations containing graphite.

The established regularities in the evolution of the composition of fluids also explains some particulars of the metamorphism of iron-formations of other t) es. In particular, the frequently observed disappearance of hematite in oxide rocks metamorphosed in amphibolite and granulite facies conditions can be caused by reduction by volatiles, especially by easily diffused hydrogen. Acceleration of diffusion with rise in temperature and increase in concentration gradients of volatiles leads to the appearance of more uniform mineral associations in place of formerly extremely variegated banded sequences. 

Metamorphism of carbonate iron-formations in the presence of aqueous fluids leads to the formation of cummingtonite. When equilibrium is reached, reactions of the type ... 

In the low-temperature region pyroxene and olivine can be retained only in dry rocks containing no volatiles. In the presence of a fluid phase the appearance of cummingtonite should be expected in the case of excess water, or carbonates in the case of excess carbon dioxide, which happens in hydrothermal metasomatism or even in progressive metamorphism of silicate or carbonate iron formations, respectively. 

Thus the conditions of metamorphic transformation of iron-formations could have varied widely, from low- to high-temperature, even within short distances. 

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