Metamorphism silicate-carbonate

Fig. 82. Diagrams of mineral equilibria in silicate-carbonate iron-formations (isothermal sections in coordinates of log /coj /h o graphite buffer). Fields of actual pressures in metamorphism are hatched I = phase boundaries 2 = isobars of fluid pressure, in kbar isobars of partial fugacity (log/j, in bar) 3 = O2 4 - H2 5 - CO 6 = CH4.

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

Ferry J. M. (1996) Prograde and retrograde fluid flow during contact metamorphism of siliceous carbonate rocks from the Ballachuhsh aureole, Scotland. Contrib. Mineral. Petrol. 124, 235-254. 

The oxidation of sulfides preferentially dissolves carbonates, rather than silicates, because the rate of carbonate dissolution is orders of magnitude faster. For example, Haut Glacier d Arolla has a bedrock which is composed of metamorphic silicate rocks. Carbonates and sulfides are present in trace quantities in bedrock samples (0.00-0.58% and <0.005-0.71%, respectively). There are also occasional carbonate veins present in the schistose granite. Despite the bedrock being... 

Carbon dioxide is frequently evolved with H2O during metamorphism of siliceous carbonates and marls. Large volumes of rock can be volatilized (up to 40%) with the potential for significant isotopic shifts. 

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. 

The rock types distinguished hardly ever occur in pure form, for the oxide facies always contains a certain amount of carbonate in addition to hematite and magnetite, and the iron silicate rocks usually contain carbonate, sometimes magnetite. Sustained mineralogical composition is observed only in thin bands on the whole, in conditions of low and medium degrees of metamorphism the pile of BIF looks like a layer cake . The composition of highly metamorphosed rocks is more uniform, which can be explained both by the disappearance of carbonate rocks due to thermal dissociation and by more intensive diffusion. 

Metamorphism of silicate BIF containing no excess free carbon is characterized by successive dehydration reactions ... 

Thus metamorphic transformation of hematite into magnetite is possible only in the presence of a reducing agent (free carbon, gaseous CO, H2, or CH4), but it is expedient to consider the oxide-reducing reactions that take place when we analyze mineral equilibria in silicate and carbonate rocks. 

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. 

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. 

Similar ideas have also been expressed recently by other investigators of the BIF. Klein (1973) pointed out that in the Labrador Trough some parts of the section of the BIF, of quartz-carbonate composition, may have been involved in reactions which produced silicates of the metamorphic series, while in other places only recrystallization of the quartz-carbonate associations with or without magnetite occurred. In the author s opinion this indicates that the chemical potential of COj varied from place to place in the course of metamorphism and, consequently, COj could not have been an ideally mobile component at any moment of time. Perry et al. (1973) believe that in the BIF of Biwabik, Minnesota, individual bands behaved as isolated subsystems and that different minerals formed in each band depending on the composition of the original unmetamorphosed rocks. 

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