CARBON ISOTOPIC COMPOSITION OF CARBONATE FORMATIONS OF THE ALTAI MOUNTAINS AND THE BATENEVSKY RIDGE

N. K. Grigor'ev, V. L. Kokunov, N. F. Stolbova, V. A. Shititsin

(Presented by Professor A. V. Aksarina)

In connection with the study of the isotopic composition of carbon in carbonate rocks of the Mesozoic section of Western Siberia, an attempt was made to assess the isotopic composition of carbon in carbonate formations of more ancient age from areas of the southeastern margin of Western Siberia. It was precisely these formations that subsequently served as the original material for the formation of the carbonate-bearing rocks of the Mesozoic platform cover of Western Siberia, which undoubtedly affected their isotopic composition.

These are mainly marbles, normally sedimentary limestones, marbled limestones, and calcites from carbonatized dikes, nests, lenses, as well as thick veins and their selvages.

A brief literature review on the magnitude of the isotopic composition of carbon in rocks and the causes of its variation is given below. Studies of the isotopic composition of carbon in marine limestones are the works of A. Nier and E. Gulbransen (1939), B. Murphy (1941), A. V. Timofeev (1952), K. Marks (1954), H. Craig (1954), Landengren (1954), P. Burch (1951, 1957), P. Jeffrey (1955), H. Lowenstam and S. Epstein (1957), D. Weber (1969), E. M. Galimov (1968, 1971), and others.

These investigations encompassed modern oolitic limestones of the Atlantic Ocean and reef coral limestones of the Indian Ocean, as well as fossil limestones of demonstrably marine origin from almost all geological age subdivisions.

According to foreign data, the isotopic composition of carbon in modern marine carbonates varies within the limits from \(+0.08\) to \(+0.57\%\), and its average value is \(+0.32\%\). A more significant range of variation in \(\sigma C^{13}\) is noted by both Soviet and foreign researchers for fossil marine limestones. Thus, according to data of P. Burch (1957), P. Jeffrey (1955), M. Keith (1964), and E. M. Galimov (1968), \(\sigma C^{13}\) of fossil marine limestones varies within the limits from \(-0.81\%\) to \(+0.52\%\), and its average value is close to zero \((\sigma C^{13} = -0.013\%)\).

Variations in the isotopic composition of carbon in marine limestones are a consequence of the thermodynamic isotope effect in the system \(CO_2—HCO_3—CO_3—\), with the final result of fractionation...

... is affected by the salt composition of the water, the depth of the basin, the intensity of water circulation and of its gas exchange with the atmosphere, etc.

The point is that, during the precipitation of carbonates, they become enriched, to one degree or another, in the heavy isotope of carbon as compared with atmospheric carbon dioxide and water bicarbonate. The greatest separation of isotopes in the above-mentioned system, and hence the greatest enrichment of carbonates in the isotope \(C^{13}\), is attained under conditions of equilibrium isotope exchange. This requires the conditions of a well-aerated basin with good water circulation. This means that the environment in the basin of deposition of isotopically heavy carbonates is necessarily oxidizing, or else requires the obligatory presence of additional cations that promote an increase in the isotope-separation constant. Such carbonates have a carbon isotopic composition characterized by positive values of \(\sigma C^{13}\).

Negative values of \(\sigma C^{13}\) are characteristic of carbonates deposited in basins with a stagnant regime and, most often, a reducing environment. Under such conditions the rate of carbon isotope exchange will be insignificant; isotope-exchange equilibrium is not attained, which is one of the reasons for the increased concentration in such carbonates of the light isotope \(C^{12}\). Another extremely important reason for the lightening of the carbon isotopic composition of such carbonates is that, in basins of this type, a considerable amount of isotopically light organogenic carbon dioxide accumulates (the mean value according to E. M. Galimov is \(\sigma C^{13} = -2.5\%\)), formed as a result of the decomposition of organic matter. Exchange of the carbon of organic acid with the carbon of precipitating carbonates leads to an increase in them of the concentration of the light isotope, forming a short closed isotope-exchange cycle \(CO_2\) org.—\(HCO_3^{1}\)—\(CaCO_3\).

As for organogenic limestones, the carbon isotopic composition in them, according to the data of the review work by E. M. Galimov (1968), depends on what kind of invertebrates participated in their formation. The isotopic composition of reef limestones ranges from \(-0.06\) to \(+0.22\%\), with a mean value of \(+0.14\%\). These are reefogenic limestones of Late Jurassic age from the Mountain Crimea. As can be seen, their isotopic composition is shifted toward a higher content of \(C^{13}\) as compared with the mean value of \(\sigma C^{13}\) for chemogenic marine limestones (\(+0.05\%\)). Modern corals usually originate on a shallow, firm, and clean bottom under conditions of intense aeration of the water, which creates very favorable prerequisites for a sufficiently complete exchange of atmospheric \(CO_2\) with \(HCO_3\) of seawater; this predetermines the heavier isotopic composition of reef limestones, except in those cases where the salinity of the water proves to be lowered by the influence of vadose fresh waters (M. Keith and J. Weber, 1964, 1965).

Foraminiferal and brachiopod limestones are also somewhat enriched in the heavy carbon isotope \(C^{13}\) as compared with the isotopic composition of chemogenic limestones, as indicated by the studies of H. Craig (1954), W. Compston (1960), C. Ross and S. Ona (1961), M. Gross (1964), and E. M. Galimov (1968). The mean value of \(\sigma C^{13}\) for foraminifers is \(+0.14\%\); for brachiopods it reaches \(+0.5\%\).

Gastropod and pelecypod limestones are characterized by broader variations in isotopic composition (from \(-0.7\) to \(+0.22\%\)), which is explained by the broader area of their distribution, from coastal waters rich in organic matter to conditions of great depths of the open sea.

Thus, the results of isotopic analysis of carbon in organogenic marine limestones show that the mechanism of isotope fractionation during the formation of skeletons and shells by marine organisms cannot be identified with a simple isotope-exchange reaction; on the whole, however, the content of C¹³ in organogenic marine limestones is very close to the content of C¹³ in carbonates formed in the inorganic system CO₂—HCO₃—CO₃.

In contrast to marine limestones, carbonate sediments associated with freshwater bodies are, as a rule, substantially enriched in the light isotope of carbon. Judging from published sources (J. Vogel, 1959; K. Munnich and J. Vogel, 1959; R. Clayton and E. Degens, 1959; M. Keith and J. Weber, 1964; R. M. Lloyd, 1964; M. Keith and R. Parker, 1966; J. Weber et al., 1965), the mean value of $\sigma$ C¹³ of freshwater limestones is shifted by approximately 0.6% toward a lighter isotopic composition in comparison with the mean isotopic composition of marine limestones. This is quite natural, since in the atmosphere—ocean—carbonate system there are no additional sources of isotopically light carbon dioxide. An entirely different matter is a freshwater basin, tending toward land with the isotopically light carbon dioxide of organic residues of terrestrial vegetation. It should be noted that, according to Academician K. I. Lukashev (1970), who studied the conditions of present-day carbonate accumulation in the lake basins of Belorussia, their formation, dissolution or precipitation, and stabilization in the solid phase of carbonates are connected with the state of the calcium-carbonate system of natural waters.

Under these conditions, the greatest importance for shifting the reaction in one direction or another is possessed by stable supersaturation of the water mass with calcium carbonate, the absence or presence of aggressive carbon dioxide, and also the absence or presence of water-soluble organic substances. This predetermines the accumulation of isotopically light carbonates and, indeed, the ground waters of intracontinental basins, according to J. Vogel, contain bicarbonate with a carbon isotopic composition from −1.29 to −1.76%.

According to E. M. Galimov (1968), along with enrichment in the light isotope, freshwater limestones are also characterized by a broader range of variations in isotopic composition than marine limestones. In terms of depositional conditions, coastal sediments of marginal seas, bays, marine lagoons, etc., are similar to freshwater limestones.

In the Altai Mountains, the sampling area is located in its northeastern part, in the upper reaches of the Mrass-su River and the Lebed River, and is confined to the Lebed skarn zone. The river valleys here are broad, trough-shaped, and in places swampy. The slopes descending to the rivers are steep and have a large number of outcrops and kurums.

Within the Lebed deposit, three principal types of rocks are widespread: the oldest—effusive-sedimentary rocks of Cambrian age; intrusive formations of the Lebed magmatic series; and red-colored sedimentary formations of the Devonian. The Lebed magmatic series has a tonalite–granodiorite–plagiogranite composition. It is a batholith-like body called the Lebed pluton.

V. I. Yarygin (1967) believes that the granitoid magma of the Lebed pluton was formed by magmatic replacement, caused by residual solutions of basaltic magma under conditions of intense folding accompanied by high pressures and temperature. The magma was thereby subjected to local and deep contamination by the basic material of the surrounding rocks. The skarn bodies of the Lebed—

vein formations accompany the skarn zone: quartz–feldspar, quartz–sulfide, and carbonate–sulfide–oxide.

The carbonate class is widely distributed in the Lebed skarn zone. Of these, calcite is the most widespread; malachite occurs in minor amounts.

At the N. F. Stolbovaya deposit (1970), three generations of calcite are distinguished. Calcite of the first generation is known among skarn rocks: wollastonite, wollastonite–garnet, and pyroxene–epidote rocks. Here it forms patchy accumulations and, in the form of individual grains, develops along the interstices of skarn minerals, probably forming somewhat later than them. The amounts of calcite-1 are small.

Calcite of the second generation forms a thick vein passing through the deposit, as well as a series of small veinlets forming a stockwork. It also appears in the intense carbonatization of rocks. In addition, calcite forms lenses and nests within the skarn rocks. Usually this is a white or slightly brownish mineral, owing to admixture of hematite and biotite-like chlorite. In relation to the skarn minerals, calcite-2 is a later mineral.

Calcite of the third generation is very limited in distribution. It usually forms veinlets confined to the selvages of large calcite veins. Calcite-3 is characterized by an admixture of 34% dolomite–ankerite.

When considering the isotopic characteristics of the rocks, it turns out that samples Nos. 1, 2, 3, having an absolute value of $\delta C^{13}$ less than 1, are sedimentary rocks (in the limestones represented by sample No. 1, trilobite fossils were found), whereas the remaining rocks are products of a complex and not fully clear magmatic and postmagmatic process.

Comparison of the isotopic composition of samples Nos. 255/68, 1411.235 with the average values of the carbon isotopic composition of marine limestones (+0.05%) shows that the rocks studied have a comparatively low content of the isotope $C^{13}$ (average −0.99%).

The decrease in the content of the isotope $C^{13}$ in the studied samples Nos. 1, 2, 3 may be connected with the following features of their formation.

The formation of these rocks may have taken place in the coastal zone, where carbon of organic origin played a substantial role.

The primary carbon of carbonate sediments may have been displaced to a significant degree by carbon arriving in the form of $CO_2$ from the intrusion.

Indeed, under conditions of the coastal zone, lagoons, and river deltas, carbon dioxide formed as a result of the decomposition of organisms and plants plays an important role. This gas is substantially depleted in the isotope $C^{13}$ (Galimov, 1968); therefore carbonate sediments formed under such conditions will also be depleted in the isotope $C^{13}$.

Studies of the isotopic composition of $CO_2$, which is a product of magmatism, have shown that its carbon is also depleted in the isotope $C^{13}$. The value of $\delta C^{31}$ lies within the range from −0.3 to −1.5% (Galimov, 1968). Therefore rocks subjected to carbon dioxide metasomatism are enriched in the light isotope of carbon. The spatial proximity of the carbonate rocks studied to the intrusion and to zones of intense postmagmatic and tectonic activity indicates the possibility of such a process. Probably, during the formation of the rocks studied, both of the processes described above were manifested.

Let us now consider the results of measurements of the isotopic composition of carbon in samples Nos. 2165, K-9/14, 9/13, K-55, P-4, K-11, 28-7.

Table

Carbon isotopic composition of carbonate rocks of the Altai Mountains and the Batenevsky Ridge of the Kuznetsk Alatau

As is evident from the table, all these minerals have carbon substantially depleted in the isotope C¹³. In the present case the depletion cannot be explained by the influence of carbon dioxide of organic origin, since their formation is closely connected with magmatic and postmagmatic activity. The depletion of the studied minerals in C¹³ is due exclusively to magmatic carbon.

The observed variability of the isotopic composition of carbon is related to the variable contribution to these rocks and minerals of carbon of magmatic and sedimentary origin. This conclusion is confirmed by the fact that the carbon of the carbonate vein (sample No. 9), forming in immediate proximity to a major tectonic disturbance, is depleted in C¹³ to the greatest degree. Probably, the formation of this vein proceeds exclusively at the expense of carbon dioxide of magmatic origin.

It is interesting to note that the constant (K), characterizing the magnitude of carbon isotope separation in an isotope-exchange reaction

\[ \mathrm{C}^{13}\mathrm{O}_2+\mathrm{C}^{12}\mathrm{O}_3^{=} \rightleftarrows \mathrm{C}^{12}\mathrm{O}_2+\mathrm{C}^{13}\mathrm{O}_3^{=} \]

does not remain constant when the temperature changes. At a temperature of 25° C, \(K=1.012\); at 130° C, \(K=1.000\); and at 425° C, \(K=0.996\) (Thode et al., 1965). Thus, at low temperatures the carbonate rock is enriched in the heavy isotope, whereas at high temperatures the carbon dioxide gas is enriched. The dependence of the separation constant on temperature (within the indicated interval) is rather smooth. With a change in temperature of 10°, the isotopic composition of carbon changes by 0.15%.

The samples we studied, Nos. 9 and 10, are represented by calcite from one and the same vein, but were taken at different depths. As was shown above, the difference in the isotopic composition of carbon in these samples may be due to different temperatures of their formation. In a hydrothermal process, isotope-exchange equilibrium is usually not attained; therefore the difference in the temperatures of formation of samples Nos. 9 and 10 may amount to 30 degrees or more.

The Lebed’ magmatic complex, and consequently the metasomatic formations genetically associated with it, formed during the Salairian tectogenesis at early, but not initial, stages in the development of the geosyncline.

Resolving the question of the relative time of formation of the deposit is of great importance in clarifying the source of the incoming solutions. As S. S. Smirnov (1969) showed, deposits associated with the early stages of development of the geosynclinal cycle are characterized by a juvenile group of sources of matter.

Thus, studies of the isotopic composition of carbon make it possible to conclude that the formation of the carbonate mineralization of the Lebed’ skarn zone, which is the latest in the postmagmatic process, took place under conditions of an intense input of carbon from a magmatic source, which explains the light isotopic composition of carbon in these formations.

If attention is paid to the distribution of the isotopic composition of carbon in calcites of different generations, distinguished on the basis of petrographic studies and the study of the chemical composition by N. F. Stolbova (1970), it turns out that the distinguished generations also have different isotopic compositions of carbon. Thus, the average isotopic composition of carbon in limestones and marbles is \(-0.99\%\), the average isotopic composition of carbon in calcites of skarns and carbonatized dikes is \(-1.38\%\), and the average isotopic composition of carbon in calcite of the thick carbonate vein that cuts the entire deposit is \(-1.63\%\). Thus, three generations of calcites are confirmed by the isotope method.

What is most important for us in the study carried out is that the average isotopic composition of carbon in the carbonates investigated is characterized by \(\delta \mathrm{C}^{13}=-1.34\%\), that is, it is considerably isotopically lighter than normal marine limestones. It was precisely carbon that was “light” in this isotopic respect that subsequently entered the sedimentation areas of interest to us from the direction of the Gorny Altai.

The second part of the samples in the table was collected in the territory of the Batenevsky Ridge, on the northwestern slope of Mount Peshchernaya (eastern spurs of the Kuznetsk Alatau). This area of the Kuznetsk Alatau is widely known for a number of manganese deposits associated with carbonate formations. The manganese-bearing sequence of the Sladkokorenevskaya Formation of the Amginian Stage of the Cambrian, more than 100 meters thick, rests unconformably on carbonate deposits of the Bogoyul’skaya Formation of the Lower Cambrian—

...Cambrian and is unconformably overlain by graywacke sandstones of the unnamed suite of the Middle Cambrian. The age of the Sladkokorenevskaya suite is determined by finds in it of trilobites and epiphyton reef-building algae. The manganocalcite limestones make up a single bed occurring in the lower part of the Sladkokorenevskaya suite and are represented by two varieties of limestone: organogenic–clastic and biohermal.

Organogenic–clastic limestones consist, to 70–80%, of detrital algal material and, to a lesser extent, of lumps of microgranular calcite.

The algae are represented by two varieties: filamentous epiphytons and tubular Volodynella. The filamentous algae have a stunted form; their bushes are low-growing, rounded, and the filaments are thickened. The limestone has undergone recrystallization. The centers of recrystallization were probably pores that existed in the primary sedimentary clastic material and were filled during diagenesis by carbonate. The limestone is cut by numerous veinlets of later calcite, along the margins of which segregations of brownish-red oxides of Mn and Fe are often observed.

Biohermal limestones consist, to 60–70%, of reef-building algae (with a sharp predominance of epiphytons), buried in life position. In other respects the biohermal limestones are completely identical to the organogenic–clastic limestones described above.

A characteristic feature of the insoluble residue of both varieties of limestone is the abundance of organic matter, which colors the insoluble residue resinous black.

All the rocks composing the Sladkokorenevskaya suite are marine facies.

The reef limestones studied formed in the coastal zone of a marine deep-water basin. Indicators of shallow depths are the red algae, which, according to V. P. Maslov (1962), grow at depths from 0 to 150 meters, but are most widely distributed at depths from 0 to 10 m.

From the data presented in the table it is evident that the carbonate of all the limestones sampled both in the Ust-Peshcherny sector and in the Yuliyevsky sector of the reef has a negative value of $\delta C^{13}$ (average $= -0.93\text{‰}$), with the isotopically lightest carbon sample taken at the margin of the reef on the lagoon side (sample No. 69802). In the center of the reef the heaviest isotopic composition is recorded (sample No. 69811). From the other margin of the reef, toward the open sea, an intermediate isotopic composition of carbon is found (samples Nos. 69808a, 69806, 10-69-3). Judging from the data obtained, we have here the same pattern of isotope distribution as was obtained by J. Weber and P. Woodhead (1969) in studying the carbonates of Heron Island of the Great Barrier Reef, Australia.

The lightness of the isotopic composition of the reefogenic limestones in this case is explained by the presence of a reducing environment, recorded by the presence of pyrite in the limestones and by the stunted forms of reef-building algae, as already mentioned above.

One further indication of an environment not entirely normal for the life of reef-building organisms in the area studied is the comparatively small thickness of the reef body. In the opinion of G. A. Ivankin, I. I. Koptev, and V. A. Shipitsin (1970), who studied the manganese content of the Middle Cambrian of the eastern part of the Kuznetsk Alatau, the presence...

manganese in the limestones of the Sladkokornevskaya Formation is explained by its introduction into the marine basin in the form of volcanic ash, since ash material is abundant in reef breccias, as is recorded in microscopic observations. The increase in manganese concentration specifically in reef limestones should apparently be attributed to the activity of crimson algae, which absorbed manganese together with carbon dioxide. This is supported by the presence, in thin sections, of Mn oxides completely replacing clusters of epiphytic algae.

Finally, according to L. P. Listova (1961), precipitation of the rhodochrosite molecule occurs at pH = 7.4, whereas the greatest amount of carbonates precipitates at pH = 8.0–8.5.

The most important point among the foregoing is the fact that here, too, we are dealing with isotopically light carbonates.

Thus, in the southeastern margins of the West Siberian Lowland, which were one of the principal source areas for terrigenous material later deposited in the form of the platform cover, the initial carbon-bearing material was predominantly isotopically light, from −0.93% to −1.34%.

REFERENCES

E. M. Galimov. Geochemistry of Stable Carbon Isotopes. “Nedra,” 1968.

G. A. Ivankin, I. I. Koptev, V. A. Shipitsin. Manganese content of the Middle Cambrian in part of the Kuznetsk Alatau. In: Problems of the Geology of Exogenous Mineral Raw Materials of Krasnoyarsk Krai. Krasnoyarsk, 1970.

L. P. Listova. Physicochemical Investigations of the Conditions of Formation of Oxide and Carbonate Manganese Ores. Publishing House of the Academy of Sciences of the USSR, 1961.

K. I. Lukashev, A. L. Zhukhovitskaya, A. A. Khomich. Conditions of modern carbonate accumulation in lake basins of Belorussia. Doklady of the Academy of Sciences of the USSR, Geological Series, vol. 193, No. 2, 1970.

K. E. Mars. Preliminary investigation of the relative content of carbon isotopes in rocks of Sweden. In: Isotopes in Geology. Publishing House of Foreign Literature, 1954.

V. P. Maslov. Fossil crimson algae of the USSR and their relation to facies. Transactions of the Geological Institute, Academy of Sciences of the USSR, issue 53, 1962.

V. I. Smirnov. On the sources of material of endogenous deposits. Izvestiya of the Academy of Sciences of the USSR, Geological Series, No. 3, 1969.

N. D. Stolbova. Mineralogy and genesis of the Lebed gold-bearing skarn zone. Dissertation for the degree of Candidate of Geological-Mineralogical Sciences, 1970.

V. I. Yarygin. Petrochemical features of rocks of the Lebed pluton (Gorny Altai). Izvestiya TPI, vol. 167, 1967.