ON SERPENTINE FROM THE SOUTHWESTERN SPURS OF THE EASTERN SAYAN

O. M. GLAZUNOV

(Submitted by Prof. A. M. Kuzmin)

On the territory of the Eastern Sayan, serpentines are widely developed. Everywhere associated with them are such useful minerals as chromites, asbestos, and titanomagnetites.

An extensive literature is devoted to the characterization of serpentines. Especially important are the works of V. N. Lodochnikov [8], M. F. Shestopalov [14], and A. F. Korzhinsky [6, 7]. However, these works describe serpentines genetically related to ultrabasic rocks of the hyperbasite formation proper.

In recent years, on the southwestern margin of the Eastern Sayan, in the area of the upper reaches of the Sisima, Shinda, and Seiba rivers, formations of serpentine have become known that are part of a gabbro–pyroxenite complex, typically basaltic in nature [3]. In comparison with the hyperbasites within this complex, the ultrabasic rocks are spatially sharply subordinate to the feldspathic differentiates. In section, they are closely interbedded with gabbros and pyroxenites, forming linearly elongated massifs. A feature of the geochemistry of the complex is the anomalously high content in them of titanium and iron, their relative poverty in chromium, nickel, and vanadium, and the absence of platinum. Apopyroxenite and, more rarely, apoperidotite serpentinites carry elevated concentrations of titanomagnetite.

In the present article the author describes serpentines from the gabbro–pyroxenite complex, attempts to clarify their place in the process of formation of titanium ores, and notes certain features that distinguish them from serpentines of hyperbasites.

On the basis of a study of serpentine-bearing rocks from the indicated region of the Eastern Sayan, it appears possible to distinguish two varieties of serpentine: antigorite and chrysotile.

Antigorite belongs to the principal rock-forming minerals of serpentinites. It composes the main mass of the rock and includes grains of titanomagnetite. In the form of leaf-like individuals it replaces rhombic pyroxene, more rarely olivine, as a result of which a lattice, platy, and scaly microstructure of the rock arises. Sometimes flakes of antigorite penetrate inclusions of titanomagnetite. The character of the arrangement of serpentine within the outlines of the replaced grains varies and depends on the composition of the primary mineral. When olivine is replaced by antigorite, radial and feathery aggregates are formed. In place of pyroxene, li-

veinlets of antigorite are arranged so as to form a reticulate pattern. As an exception, veinlets of antigorite are observed.

In comparison with antigorite, chrysotile occurs more rarely in serpentinites. It forms a complex looped network only within pseudomorphs of serpentine after olivine (Fig. 1). More often, chrysotile composes veinlets cutting the serpentinites. In their structure the latter are divided into veins of cross-fibrous chrysotile and veins with the fibers arranged diagonally to the selvages. The boundaries of the veins are sharp, always

Fig. 1. Looped replacement structure of olivine by chrysotile, ×65, N ||.

rectilinear or geniculately bent. In a number of veins, stringers of magnetite are established (Fig. 2), relics of titanomagnetite inclusions with corroded margins, and also hairlike aggregates of magnetite finely intergrown with chrysotile. In many chrysotile

Fig. 2. Stringers of magnetite in a chrysotile veinlet, ×20, N ||.

veins, a distinct banded structure is observed by color shade. The genesis of these banded structures has been treated in detail in the works of V. N. Lodochnikov and V. R. Artem'ev; therefore we do not dwell on this question.

Macroscopically, both varieties of serpentine have a dark-green color and a greasy luster; in thin splinters they are translucent at the edges. Aggregates of antigorite are platy, platy-foliated; chrysotile is fine-fibrous with characteristic moiré extinction.

The different character of the antigorite and chrysotile individuals is illustrated by the electron micrographs given below (Fig. 3). It is evident from the figures that both minerals have a distinct crystalline structure; moreover, in antigorite the cleavage cracks are drawn rather clearly. In the opinion of A. M. Kuz’min, the cleavage, which is usually not discernible, is revealed here during slip.

The tubular internal structure of chrysotile, demonstrated by B. Ya. Merenkov, K. I. Tolstikhina, and A. A. Aleksandrova [9], is not confirmed by our investigations.

In transparent thin sections, antigorite and chrysotile are colorless or light green, do not show pleochroism, and interfere in gray tones.

Because the mineral studied lacks definite crystallographic boundaries, and because of the difficulty of selecting the necessary sections, it proved impossible to obtain reliable determinations of the angles of the optical axes. Although the plane of the optical axes of chrysotiles is usually located along the fibrousness, isotropic sections were selected by us for measuring the refractive index in immersion. \(N_g \approx N_m\) of antigorite, according to a series of measurements, is on average equal to 1.568, that is, only 0.005 lower than the corresponding standard values for antigorite [13, 16]. \(N_m\) of the Lysan chrysotile (1.558–1.564) also slightly exceeds the tabulated data.

A chemical characterization was obtained only for vein occurrences of chrysotile, previously purified from the magnetic fraction and other extraneous impurities.

In composition, the analyzed serpentines differ only slightly from one another. This can be verified by comparing the results of the analysis and the crystallochemical formulas given below.

\[ (\mathrm{Mg}_{5.90}\mathrm{Fe}^{2+}_{0.03}\mathrm{Fe}^{3+}_{0.11})[\mathrm{Si}_{3.72}\mathrm{Al}_{0.19}\mathrm{Ti}_{0.02}\mathrm{O}_{9.88}][\mathrm{OH}]_{8.20}. \tag{1} \]

\[ (\mathrm{Mg}_{5.45}\mathrm{Fe}^{2+}_{0.06}\mathrm{Fe}^{3+}_{0.16}\mathrm{Al}_{0.22}\mathrm{Ti}_{0.01})[\mathrm{Si}_{3.58}\mathrm{Al}_{0.42}\mathrm{O}_{10.0}][\mathrm{OH}]_{8.00}, \tag{2} \]

\[ (\mathrm{Mg}_{5.60}\mathrm{Fe}^{2+}_{0.09}\mathrm{Fe}^{3+}_{0.16}\mathrm{Al}_{0.14})[\mathrm{Si}_{3.62}\mathrm{Al}_{0.38}\mathrm{O}_{10.0}][\mathrm{OH}]_{8.05}, \tag{3} \]

\[ (\mathrm{Mg}_{5.60}\mathrm{Fe}^{2+}_{0.05}\mathrm{Fe}^{3+}_{0.18}\mathrm{Al}_{0.12})[\mathrm{Si}_{3.70}\mathrm{Al}_{0.30}\mathrm{O}_{10.0}][\mathrm{OH}]_{8.00}. \tag{4} \]

An almost identical degree of isomorphous substitution of magnesium by iron and a similarity in the tetrahedral layer of the different serpentines are established. At the same time, the incorporation into the mineral of aluminum in an amount up to 1/7 of the silica is noteworthy. Such a ratio is considered not entirely usual for serpentine (Table 1).

According to Hess, Dengo, and Smith [17], aluminum is necessary for the formation of antigorite, whereas it is practically absent in chrysotile.

In contrast to this view, other investigators [18] are inclined to regard alumina as an essential part of all serpentines.

The presence of aluminum in the Lysan chrysotile can be explained by the fact that the mineral develops at the expense of rhombic pyroxenes rather than olivine. Aluminum probably partly replaces magnesium, which is deficient.

The magnesium content in vein serpentinites is 1–2% lower than the theoretical value (Table 1) and at the same time 1.5–2 times higher than in the enclosing serpentinites. Among the impurity elements in serpentine, spectroscopic analysis recorded Ga, Ti, V, Ni, Co, Cr, Zn, Cu, Ba, Sr (Table 2). The maximum content of elements falls on chrysotile. Antigorite is noticeably depleted in impurities, which apparently must be connected with the phenomenon of autolysis of the mineral occurring in the process of recrystallization.

In deciphering the Debyegrams it was found that only the fine-fibrous vein serpentine has complete similarity to chrysotile. In other

Fig. 3a.

Fig. 3b.

Fig. 3c.

Fig. 3. Serpentine under the electron microscope:
a) antigorite, b) antigorite, c) chrysotile. Suspension method, magnification 12000×.

Table 1

Results of chemical analysis of serpentine

1—Vein antigorite. Eastern Sayan. Sample RK-16-10 c.
2, 3, 4—Fine-fibrous, vein chrysotile, same locality; 2—sample 218; 3—sample 219; 4—sample RK-16-9.
5—Antigorite from the eastern part of the Eastern Sayan (after A. F. Korzhinskii, 1959);
6—Chrysotile asbestos of the Aktovrak deposit, Tuva (after V. P. Eremeev, 1959).
7—Ideal composition of serpentine (after A. G. Betekhtin, 1953).

in the samples, along with the diffraction lines of chrysotile, lines characteristic of antigorite are detected. At the same time, no complete analogy is established between these Debye patterns and the reference Debye patterns of antigorites (Table 3).

Consequently, the experimental data of X-ray and chemical analysis obtained by us do not permit definite conclusions to be drawn regarding the compositional features and internal structure of the various modifications of the Lysan serpentines, just as, incidentally, certain analogous measurements [17] do not provide grounds for final conclusions in this direction. As a preliminary formulation of the question, one may suppose that there is no difference in the structure of chrysotile and antigorite.

At the same time it is known that the two modifications of serpentine react differently upon heating (Fig. 4). This feature can be used for their diagnosis. The release of constitutional water in the antigorites studied occurs at a higher temperature (700–750°) and over a longer period of time than in chrysotile,

dehydration of which is confined to the interval 650–700°. In addition, in antigorite, upon heating, the reaction of orthosilicate formation apparently proceeds less actively, as indicated by a weak exothermic effect at a temperature of 900° (Fig. 4).

Turning to a comparison, let us note that the serpentines from the western spurs of the Eastern Sayan have an almost identical composition with analogous

Table 2

Results of spectral analysis of serpentine

1. Antigorite from serpentinites (sample rk-16-7). 2. Vein antigorite (sample 222).
2. The same, sample rk-16-11a. 4, 5, 6, 7—vein chrysotile, samples 218, 219, 220, rk-16-9a.

NOTE: p—whole percentages; tr.—0.001; —element not detected.

minerals of the Union and of foreign countries. At the same time, they possess a certain specificity with respect to the suite of impurity elements, which reflects the general geochemical specialization of the magmatic complex. In particular, in comparison with serpentines from the well-known Bazhenovskoye, Alapaevskoye (Urals), and Aktovrakskoye (Tuva) [1; 4] deposits, the minerals studied bear a higher content of Ti, Fe, Al and a lower content of Si, Mg, Cr, Ni. These same compositional features distinguish the given serpentines from the serpentines of the eastern part of the Eastern Sayan, which belong to the hyperbasite type proper and are usually enriched in chromium, nickel, and magnesium [6].

It is necessary to dwell on the question of the connection of serpentines with the process of formation of titanomagnetite ores. As a result of studying the mineralogy of the deposit it has been established that increased concentrations of titanomagnetite in serpentinites belong to late-magmatic (fusive, according to A. N. Zavaritsky) concentrations, forming after the crystallization of rhombic pyroxene and olivine.

The only ore mineral synchronous with the antigorite of the enclosing rocks is magnetite. It usually appears in the form of thin chains, branching stringlet-veinlets, and in the form of “hairlike” aggregates, as well as spherical accumulations of concentric structure. Magnetite of this generation has mineralogical significance.

According to the views of B. Ya. Merenkov [11] and P. Riordon [19], “fibrous” magnetite forms at the expense of iron released during the period of serpentinization.

In analyzing the features of the localization of titanomagnetite inclusions, however, no direct dependence is observed anywhere between the degree of serpentinization of the rock and its saturation with titanomagnetite. As for the chrysotile veins, they everywhere cut the antigorite

serpentinites with inclusions of titanomagnetite, while exerting no visible effect on the titanomagnetite.

1. Thus, serpentinization of the rocks of the gabbro–pyroxenite complex in the western part of the Eastern Sayan took place in at least two stages separated in time—massive and vein stages.

Fig. 4. Heating curves of serpentinite: 1—chrysotile, Eastern Sayan, sample 218; 2—chrysotile (after Serdyuchenko, 1953); 3—antigorite from serpentinites, Eastern Sayan, sample 12007; 4—antigorite from veins, Eastern Sayan, sample 222; 5—antigorite (after A. F. Korzhinskii, 1959).

1. Massive serpentinization ended with the transformation of bronzitites and peridotites into antigorite serpentinites under conditions of high pressures.

2. Manifestations of vein serpentinization (chrysotile) filled joint cracks that arose in the massif later.

3. The closeness of the chemical composition of different samples of vein chrysotile and antigorite indicates their origin from a single source during the stage of autometamorphism and the hydrothermal process of formation of the intrusion.

1. On the basis of the results of X-ray analysis, it may be assumed that, in structural terms, there is no fundamental difference between antigorite and chrysotile.

2. Serpentine from the gabbro–pyroxenite complex is characterized by a number of specific compositional features (high Al and Ti content, Mg deficiency, low Cr and Ni content), which reflect the geochemical specialization of the complex.

Table 3

Table of interplanar spacings of serpentine

1—Vein antigorite. Eastern Sayan (sample rk-16-10s).
2—Antigorite. New Zealand (Mikheev, 1957)
3, 4—Vein chrysotile. Eastern Sayan. (3—sample 218, 4—sample 219).
5—Chrysotile. Chelmsford (Mikheev, 1957)

1. The process of serpentinization of the rocks is not directly connected in time with the separation of practically interesting localizations of titanomagnetite. The crystallization of the latter apparently occurs independently of serpentinization and precedes it.

These conclusions are in accord with the theoretical concepts of D. S. Korzhinskii [5] and A. G. Betekhtin [2] concerning the course of autometamorphism of ultrabasic rocks.

LITERATURE

1. V. R. Artem’ev. Ultrabasic rocks of the eastern slope of the Southern Urals and their asbestos-bearing character. Author’s abstract of candidate’s dissertation, VSEGEI, 1958.

2. A. G. Betekhtin. On metamorphic rocks formed at the expense of ultrabasic igneous rocks. In: Problems of Mineralogy and Petrography, vol. 1, Publishing House of the Academy of Sciences of the USSR, 1953.

3. O. M. Glazunov. The Lysanskii gabbro–pyroxenite–serpentinite complex in the western part of the Eastern Sayan. Geologiya i Geofizika, No. 3, 1961.

4. V. P. Eremeev. Petrography and genesis of the Aktovrakskoe deposit of chrysotile asbestos. Transactions of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, vol. 31, 1959.

5. D. S. Korzhinskii. Mobility and inertness of components during metasomatism. Izvestiya of the Academy of Sciences of the USSR, Geological Series, No. 1, 1936.

6. A. F. Korzhinskii. Thermo-optical study of some chlorites of Western Siberia. Proceedings of the Siberian Branch of VMO, vol. 1, 1959.

7. A. F. Korzhinskii. Thermo-optical studies of hydrous magnesian silicates of the Ospinskii hyperbasite massif in the Eastern Sayan. Transactions of the Institute of Geology, East Siberian Branch of the Academy of Sciences of the USSR, vol. 8, 1958.

8. V. N. Lodochnikov. Serpentines and serpentinites of Il’chir and others. Transactions of TsNIGRI, vol. 38, 1936.

9. B. Ya. Merenkov, K. I. Tolstikhina, A. L. Aleksandrova. The significance of electron-microscopic study of chrysotile asbestos and serpophite for resolving the question of their genesis. Transactions of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, vol. 31, 1959.

10. V. I. Mikheev. X-ray Determinative Guide to Minerals, 1957.

11. B. Ya. Merenkov. On fibrous vein magnetite from the Bazhenovskoe deposit of chrysotile asbestos. Transactions of the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, vol. 31, 1959.

12. D. A. Serdyuchenko. Chlorites, their chemical constitution and classification. Transactions of the Institute of Geological Sciences, vol. 140, Mineralogical-Geochemical Series, No. 14, 1953.

13. V. E. Treger. Tables for the Optical Determination of Rock-Forming Minerals. Gosgeoltekhizdat, 1958.

14. M. F. Shestopalov. The ultrabasic massif of the Kitoi Alps of the Eastern Sayan and the deposits associated with it. Collection of Works on Precious Stones and Gems, vol. 4, 1938.

15. V. Marmo. Serpentinites of central Sierra hione. Bull. de la commis. geol. de Finl., No. 180, 1959.

16. Niggly. Lehrbuch der Mineralogie, 1926.

17. H. H. Hess, G. Dengo, R. J. Smith. Antigorite from vicinity of Caracas, Venezuela. Amer. Miner., v. 37, No. 1–2, 1952.

18. Roy Della, Roy Rustam. Studies in the system MgO—Al₂O₃—SiO₂—H₂O. Bull. Soc. Amer., v. 63, p. 1294, 1952.

19. P. H. Riordon. The genesis of asbestos in ultrabasic rocks. Econ. Geol., v. 50, No. 1, 1955.