Precambrian BIF

Nearly all the metasedimentary rocks entering into the make-up of the BIF contain free carbonaceous matter, and its typical association with the argillaceous (shaly) component of the rocks is observed. Let us examine some typical examples. 

Carbonaceous matter occurs in the rocks of all three suites of the sedimentary cherty iron-formation. 

In the Middle suite (K,) carbonaceous matter is characteristic of clastic shaly horizons, layers, and individual bands within the cherty iron-formation. The chemogenic rocks (oxide and carbonate facies) are practically devoid of carbonaceous matter and the content in them usually is not more than a few hundredths of a percent. 

In the unoxidized rocks of the Middle (iron-ore) suite of the Saksagan district of the Krivoy Rog the content of is 0.29% and of COj, 4.10%. These figures were obtained from 109 analyses of composite samples (the total number of original samples was more than 1000) of core from eleven boreholes. In the calculations it was assumed that the ratio of the masses of shaly and iron horizons was 2 1 and the total thickness of the middle suite, 1000 m. 

The distribution of and CO2 in the stratigraphic section is shown in Fig. 29. The average contents of and CO2 in the iron horizons are 0.11 and 4.06% respectively, and in the shaly, 0.38 and 4.13%. Here it should be kept in mind that the iron horizons, together with the chemogenic iron-formations, always contain layers and intercalations of schist, and the shaly horizons are characterized by extensive development of barren and low-grade cherts and carbonate-chert rocks. The lack of correlation between the 

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Deposits of Precambrian BIF, the rich iron ores of Cerro Bolivar in Venezuela (Ruckmick, 1963) and in the states of Para and Minas Gerais in Brazil (Dorr, 1969 Tolbert et al., 1973), are known. In the Sierra das Carajas area in Para, iron cherts were found not long ago in the rather inaccessible forests of the Amazon basin, and are not well known. 

Vinogradov (1964) divides the history of the atmosphere into three phases (Table 1) ancient (water vapor), transitional (nitrogen atmosphere), and present (oxysphere). Apparently the Precambrian BIF were deposited at the boundary of the transitional atmosphere and the oxysphere. Therefore, it is of particular interest to examine the evolution of the nitrogen atmosphere and its individual components, mainly nitrogen and carbon. 

Vinogradov has pointed out that with the appearance of the biosphere somewhere on the verge of 3-10 yr ago, there was a major upheaval in the evolution of the Earth. Oxidizing processes were abruptly accelerated, a nitrogen atmosphere arose in which carbon dioxide predominated over methane, and free carbon was oxidized to CO2. After the carbon was oxidized or at the same time as that process, there began oxidation of divalent iron (at — 10 ), which led to subsequent wholesale deposition of the sediments of the Precambrian BIF. Free carbon in equilibrium with the atmosphere appeared only after complete oxidation of ferrous iron compounds in the hydrosphere and on the land surface. 

The deposition of greenalite and its interrelationships with siderite are controlled by reactions (2.16) and (2.17), considered earlier. The line separating the field of deposition of Sid and Gree (Fig. 24) in the first approximation characterizes the conditions of joint deposition of Fe-Mg carbonates and silicates of variable composition. From the diagram it is seen that as the content of Fe in solution decreases and, apparently, the magnesium content of the minerals deposited increases somewhat, the buffer value of also decreases several times from 7- 10 to (l-2)- 10 bar. Inasmuch as in most Precambrian BIF magnesian silicates occur relatively rarely (the finding of talc in the unoxidized oxide facies of the BIF of the... 

Results of study of Precambrian BIF in the U.S.A., Canada, Australia, and South America are summarized in a work by La Berge (1967). Numerous spherical structures, easily perceptible if there is pigment in them, have been found in cherts. The author distinguishes seven types (varieties) of structures. 

In conclusion, experimental and theoretical thermodynamic and geochemical data are compared and possible factors causing the formation of concentrations of iron and silica sediments in various conditions, including the specific setting of the Precambrian, are examined. The main purpose of this was not to create any universal hypothesis of the formation of the Precambrian BIF, but to determine the conditions which, when fulfilled, make the ore-forming process possible. 

The particulars of the metamorphism of Precambrian BIF usually have been established on the basis of study of the mineral associations, interrelationships of minerals, sequence of mineral formation and paragenetic analysis. Thermodynamic calculations and experimental data have been used to a lesser extent. 

As has already been mentioned, iron-rich rocks characterized by uniform chemical composition and very diverse mineral associations, textures, and structures are of decisive importance in the composition of the Precambrian BIF. A general feature of these rocks is similar silica and total iron contents the other essential components occur in subordinate amounts, although some of them, such as MgO and AI2O3, greatly influenced mineral formation. 

Fig. 94. Variations in the oxygen isotopic composition of the ore minerals of Precambrian BIF I = magnetite 2 = hematite 3 = siderite.

Comparing the data in Fig. 94, it can be suggested that the spread of of the ore minerals of Precambrian BIF was caused by substantial differences in the original oxygen isotopic composition, which makes it possible to use the results obtained to reconstruct the conditions of sedimentation and determine the composition of the iron sediments and the origin of the main ore mineral, magnetite. 

Additional evidence of the existence of microorganisms in the Precambrian BIF s is provided by the occurrence of stromatolites within the iron formations. 

The masses of laminated chert characteristic of all Precambrian BIF s occur in a wide variety of external forms comprising just about every form type described for modem and ancient stromatolites. Hofmann (1969) devised a complex form taxonomy of stromatolites from the Gunflint Formation of the Lake Superior region, and has tied this classification into those of Logan et al. (1964), Rezak (1957), and Soviet geologists (e.g. Korolyuk, 1960 Krylov, 1963 Komar, 1966) which are based mainly on carbonate stromatolites. 

The origin of the earth s atmosphere is a subject of such enormous complexity, and has so many implications and ramifications that it almost defies synthesis. Fortunately, the subject has been eloquently summarized at several different levels by Preston Cloud and his associates (e.g. Cloud, 1968, 1973, 1974 Cloud and Gibor, 1970). Our main concern here is that the Precambrian BIF s represent the very first evidence of oxygen in the hydrosphere. 

In this chapter, we have attempted to relate current ideas on the roles of microorganisms to the genesis of two very different types of iron deposits — one very ancient (Precambrian BIF s) and one modem (ferromanganese nodules and crusts) one comprising the most extensive resource of iron ore on land, and the other comprising the most extensive mineral deposit in the oceans. We can conclude that organisms were almost certainly involved, directly or indirectly, in the formation of both types of deposits, but exactly how and to what extent they are involved remain unanswered questions even for modem deposits. 

By far the most important ores of iron come from Precambrian banded iron formations (BIF), which are essentially chemical sediments of alternating siliceous and iron-rich bands. The most notable occurrences are those at Hamersley in Australia, Lake Superior in USA and Canada, Transvaal in South Africa, and Bihar and Karnataka in India. The important manganese deposits of the world are associated with sedimentary deposits the manganese nodules on the ocean floor are also chemically precipitated from solutions. Phosphorites, the main source of phosphates, are special types of sedimentary deposits formed under marine conditions. Bedded iron sulfide deposits are formed by sulfate reducing bacteria in sedimentary environments. Similarly uranium-vanadium in sandstone-type uranium deposits and stratiform lead and zinc concentrations associated with carbonate rocks owe their origin to syngenetic chemical precipitation. 

During conversion of goethite to hematite only small fractionation effects seem to occur, because most of the oxygen remains in the solid (Yapp 1987). Thus, in principle it should be possible to reconstruct the sedimentary environment of iron oxides from Precambrian banded iron formations (BIF). By analyzing the least metamorphosed BlFs, Hoefs (1992) concluded, however, that the situation is not so simple. Infiltration of external fluids during diagenesis and/or low temperature metamor-... 

Data are from Saito et al. (1984) unless otherwise noted. Noble gas amounts are in cm3STPg 1 and are representative values. ( ) Markov et al. (1990) magnetites were separated from Precambrian banded iron formation (BIF) in Russia. 

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. 

Younger BIF are also known, including post-Precambrian ones, related chiefly to volcanogenic rocks (Kalugin, 1969 Novokhatsy, 1973). 

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). 

In some works a tendency toward convergence of the volcanogenicsedimentary and clastic-sedimentary hypotheses is noted. Belevtsev et al. (1966), who considered mainly the clastic-sedimentary hypothesis, postulate the extensive occurrence of acid waters in the Precambrian hydrosphere as the result of intensive volcanic activity. Tyapkin and Fomenko (1969) believe that the main source of iron and silica in the Precambrian was the basic rocks which were the chief constituent of the Earth s crust at that time, but that some was also derived from basaltic rocks erupted along abyssal faults and other products of basic volcanism. In this case it is impossible to deny the possibility that part of the iron and silica was supplied to the sea basins along with products of volcanic activity. In this scheme the role of volcanic activity in the formation of the BIF comes down chiefly to the creation of acid environments which promoted the leaching of iron compounds from basic rocks and its transport and subsequent accumulation. The primary banding is explained by periodic revival and extinction of volcanic activity, as a result of which the pH of the water basin varied, which ultimately led to deposition of iron or cherty sediments in turn. The periodicity of those cycles might have been of the order of several hundred years. 

The question of the distribution of free carbon in Precambrian sedimentary rocks also needs special consideration. Ronov and Migdisov (1970) show that the content in Lower and Upper Proterozoic complexes remained at the same level, but at the boundary of the Proterozoic and Paleozoic there is observed a two-fold increase in the carbon content. However, quantitative data on the carbon content in different types of rocks, including the BIF, are very few. 

Table IV gives the composition of the Precambrian ocean after neutralization of HCl, calculated under these assumptions. The pH values correspond to the end of congruent solution of the original rocks of the Earth s crust and beginning of deposition of the carbonate and silicate facies of the BIF (amorphous sediments). The values of correspond to the beginning of deposition of FeCOj (first value) and the beginning of deposition of Fe3Si205(OH)4 (second value). The values of the sum of carbonate ions in solution (2C02. ) are consistent with the values of co, in the atmosphere. Questions of the proportion of the masses of CO2 m the atmosphere, hydrosphere, and carbonate sediments will be considered in more detail later.

To complete the picture, let us also examine the conditions of deposition of magnesian silicate in neutral and slightly alkaline environments at low COj although these processes, theoretically possible after deposition of the BIF, would hardly have been extensive in the Precambrian, yielding intensive accumulation of carbonate strata as in the Phanerozoic. 

The relatively low abundance of the sulfide facies of BIF and its spatial and genetic restriction to volcanogenic sequences indicates that sulfur concentrations were low in the Precambrian ocean. In reducing conditions practically all the sulfur was fixed in pyrite. 

Thus available data on the variations in the carbon isotopic composition in Precambrian carbonates, graphitites, and organic matter confirm the important role of organic processes in the formation of the BIF, especially in the Proterozoic. In the Archean, apparently, organic life was more limited and processes of chemical deposition of carbonates prevailed. 

The regularities in the distribution of free carbon in the rocks of the BIF of the KMA have been studied in detail by Plaksenko (1966). The data obtained were fundamental for reconstructing the processes of sedimentation in the Precambrian (Strakhov, 1960) and the formation of ore mineral parageneses. 

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