THE KANZYBINSKY INTRUSIVE MASSIF

(EASTERN SAYAN)

G. V. Shubin

(Presented by Professor A. M. Kuzmin)

The Kanzybinsky intrusive massif is exposed by erosion in the basins of the Kanzyba, Dzhebi, and Margoz rivers and forms part of the southwestern slope of the Eastern Sayan. The Kanzybinsky massif itself constitutes part of the large Olkhovsko-Chibizheksky intrusion.

In the geological structure of the area within which the intrusive massif under study is situated, rocks of the Lower and Middle Cambrian take part; they are represented by metamorphosed limestones, amphibole-quartz-feldspar schists, quartz sandstones, siltstones, quartz keratophyres, and their tuffs. All these rocks have been intruded and partly assimilated by a granitoid intrusion, the marginal parts of which have acquired a hybrid appearance.

The area attracts the attention of researchers because of the discovery of iron ores.

Within the Kanzybinsky massif, fields are relatively clearly distinguished that are composed essentially of: a) granites—in the central and northeastern parts of the massif; b) granodiorites, quartz diorites, diorites, and gabbro-diorites, representing the marginal facies of the massif; c) aplite-like granites and aplites in the form of spots and fields in the central part of the massif and numerous dikes in the rocks of the marginal facies.

Granites make up a considerable part of the Kanzybinsky massif and are represented by medium-grained and fine-grained rocks, more often with an equigranular and more rarely with a porphyritic texture. Microscopic study of the granites revealed the following minerals in them: a) principal—plagioclase, quartz, orthoclase; b) accessory—biotite, hornblende, magnetite; c) secondary—sericite. The rock texture is typically granitic. Signs of cataclasis are manifested only in weak undulatory extinction of quartz. The quantitative mineralogical composition of the granites is as follows: quartz—47.4%, orthoclase—30.9%, plagioclase—19.0%, mafic minerals—2.7%.

Plagioclase is present in the form of oligoclase (No. 16–24). Its crystals are idiomorphic and have a tabular or short-prismatic form with characteristic fine twinning striation. Quartz often forms large, sometimes porphyritically distinguished grains with weak undulatory extinction and the presence of definitely oriented inclusions. Orthoclase, in quantitative proportion, is second only to quartz.

For the most part, the orthoclase grains are fresh and sharply xenomorphic with respect to plagioclase and quartz. Perthitic formations in orthoclase grains appear as wormlike veinlets, often being confined to cleavage cracks. As a rule, a weakly expressed microcline lattice is manifested in perthitized orthoclase grains. Biotite is represented by idiomorphic plates up to 2 mm in size. The surrounding quartz grains are clearly xenomorphic with respect to it, and in rare cases corrosion of biotite plates by quartz is observed. Hornblende occurs very rarely and, on the whole, is not characteristic of the granites.

The chemical composition of the granites (Table 1) shows that they do not differ from the average Daly granite type.

In a number of cases, small, 5 to 8 cm in diameter, fine-grained dark-gray, almost black schlieren can be found in the granites; these consist of plagioclase No. 35, scanty quartz, green hornblende, and abundant magnetite. The boundary of the schlieren formations is very sharp, which is emphasized by the panidiomorphic-granular structure of the schlieren formations.

Study of the endocontact varieties of granite shows that, in addition to the development of granodioritic and dioritic varieties, there is strong enrichment of the contact zones in alkalies, leading to the formation of distinctive leucocratic rocks. An analogous picture was described, for example, by V. V. Lyakhovich for the Northern Caucasus and by A. I. Aleksandrov for the Taletskii intrusive massif.

Granodiorites, quartz diorites, diorites, and gabbro-diorites constitute the marginal facies of the granites, formed as a result of assimilation of the enclosing rocks, represented by limestones, schists, sandstones, quartz keratophyres, and their tuffs.

The rocks of the marginal facies of the granites are characterized by a fine-grained structure with a slight tendency toward porphyritic texture. Owing to the widespread development of hornblende in the rocks, they may be assigned to hornblende-bearing varieties; biotite-hornblende varieties occur more rarely.

The mineralogical composition of the rocks described is characterized by the following minerals: a) major—plagioclase, hornblende, biotite, quartz; b) subordinate—orthoclase, pyroxene, apatite, sphene, titano-magnetite; c) accessory—zircon, garnet; d) secondary—sericite, epidote, chlorite, leucoxene.

The quantitative mineralogical composition of the rocks of the marginal facies is as follows: a) granodiorite: plagioclase—42%, orthoclase—21.7%, quartz—21.9%, hornblende—12.5%, magnetite—1.9%; b) diorite: plagioclase—58.2%, orthoclase—9.1%, quartz—8.2%, hornblende and magnetite—24.5%; c) gabbro-diorite: plagioclase—62.2%, orthoclase—1.4%, quartz—2.9%, pyroxene—4.2%, hornblende—24.3%, magnetite—5%.

The structure of the rocks of the marginal facies is mainly hypidiomorphic-granular, more rarely cryptic. Signs of cataclasis are manifested in the formation of a mortar structure.

Plagioclase always forms grains of prismatic outline with distinct idiomorphism and is present as andesine-labradorite (No. 30—54). Despite the diversity of hybrid rock types, zonal plagioclases are not characteristic of them. Individual plagioclase grains undergo weak processes of microclinization. Hornblende is distributed rather unevenly in the rocks. Its quantitative increase is observed toward rocks of the dioritic and gabbro-dioritic varieties. It is represented chiefly by xenomorphic grains with respect to plagioclase and, more rarely, by idiomorphic grains with respect to...

to quartz. The refractive indices of the hornblendes are: \(Ng = 1.653\)—1.668, \(Nm = 1.648\), \(Np = 1.635—1.652\), \(cN = 16—25\), \(2V = (-)63—74\). Pleochroism: along \(Ng\), from light green to dark green; along \(Nm\), greenish-yellow to green; along \(Np\), light yellow to light green. Content according to Tröger: \(Mg\)—78.5—83.5%; \((Fe'', Mn, Ti)\)—16.5—21.5%.

Biotite is present in insignificant quantity and is represented by scales of irregular shape and by partly bent flakes. Pleochroism: along \(Np\), brownish-yellow; along \(Ng\), dark, brownish-red. In sections perpendicular to \(Np\) it is colored dark brown and dark brownish-red. Quartz is usually represented by two genetic types: primary (magmatic) and secondary (hydrothermal). Magmatic quartz forms comparatively large grains, more rarely xenomorphic with respect to plagioclase and hornblende. Hydrothermal quartz, as a rule, forms small veinlets consisting of an aggregate of fine grains. The quartz veinlets cut all the rock-forming minerals, including primary quartz. Orthoclase in the rocks of the marginal facies is very scarce; only in the granodiorites does its amount increase to 20%. The quantitative increase of orthoclase is for the most part caused by processes of feldspathization. Pyroxene is most widely represented in rocks of gabbro-dioritic composition; in other varieties it is rare and is almost completely replaced by hornblende. The mineral is colorless or weakly greenish. Extinction angle \(36—39^\circ\), \(2V = 57—60\), \(Ng - Np = 0.027—0.030\). A tendency toward xenomorphism is characteristic of pyroxene. Apatite always forms prismatic crystals with pyramidal terminations. The amount of apatite in the rocks under consideration is undoubtedly elevated. N. V. Bezborodko [3], in particular, notes for the Ukrainian rocks a significant amount of apatite precisely in the hybridized amphibole granites of Ekaterinoslav (now Dnepropetrovsk). Sphene in the overwhelming majority of cases is xenomorphic. As in the case of apatite, an increase in the amount of sphene is observed toward the contact. Titanomagnetite in most cases is evidently a mineral of the same origin as the greater part of the apatite and sphene. A close association of titanomagnetite with sphene and leucoxene is observed. Accessory minerals (zircon and garnet) are present in insignificant amounts. Secondary minerals are above all sericite-like micaceous minerals, to which are added relief grains of minerals of the clinozoisite-epidote group, representing products of alteration of feldspar. In some cases we have a rather typical picture of saussuritization. Chlorite occurs as frequently as epidote. Chloritization of biotite and replacement of pyroxene and hornblende are observed.

The extraordinary similarity of all the rocks, both in mineralogical composition and in structural features, permits the conclusion that they are genetically related. Chemical analyses of the rocks of the marginal facies are given in Table 1.

The formulas of the rocks, derived by the method of T. Barth and used by us subsequently for constructing the diagram of change in the chemical composition of the granitoids, are as follows:

1. Formula of granite:

\[ \mathrm{K}_{40}\mathrm{Na}_{66}\mathrm{Ca}_{22}\mathrm{Mg}_{12}\mathrm{Mn}_{0}\mathrm{Fe}^{+2}_{6}\mathrm{Fe}^{+3}_{6}\mathrm{Al}_{133}\mathrm{Ti}_{2}\mathrm{Si}_{163}\mathrm{P}_{0} \]
\[ \mathrm{O}_{156}\mathrm{OH}_{11}. \]

1. Formula of granodiorite:

\[ \mathrm{K}_{33}\mathrm{Na}_{57}\mathrm{Ca}_{33}\mathrm{Mg}_{37}\mathrm{Mn}_{0}\mathrm{Fe}^{+2}_{25}\mathrm{Fe}^{+3}_{23}\mathrm{Al}_{154}\mathrm{Ti}_{6}\mathrm{Si}_{280} \]
\[ \mathrm{P}_{2}\mathrm{O}_{[[unclear: subscript]]}\mathrm{OH}_{8}. \]

1. Formula of diorite:

\[ \mathrm{K}_{13}\mathrm{Na}_{34}\mathrm{Ca}_{35}\mathrm{Mg}_{21}\mathrm{Mn}_{0}\mathrm{Fe}^{+2}_{7}\mathrm{Fe}^{+3}_{15}\mathrm{Al}_{139}\mathrm{Ti}_{6}\mathrm{Si}_{614} \]
\[ \mathrm{P}_{1}\mathrm{O}_{1786}\mathrm{OH}_{14}. \]

1. Formula of gabbro-diorite: $\mathrm{K}_5 \mathrm{Na}_{60} \mathrm{Ca}_{94} \mathrm{Mg}_{60} \mathrm{Mn}_1 \mathrm{Fe}^{+2}_{45} \mathrm{Fe}^{+3}_{43} \mathrm{Al}_{184} \mathrm{Ti}_{11}$
   $\mathrm{Si}_{490} \mathrm{P}_4 \mathrm{O}_{1583} \mathrm{OH}_{17}$.

Table 1

Recalculation according to A. N. Zavaritsky

The rocks of the marginal facies of the Kanzybinsky massif possess a number of features that allow us to regard their formation as the result of assimilation by the magma of the country rocks. The following speaks in favor of such a conclusion.

1. The presence of uneven grain size in the rocks of the marginal facies is expressed in the fact that not only in outcrops as a whole, but even within a single specimen, one can observe transitions from fine-grained portions of the rocks to coarse-grained ones, often without traces of a gradual transition. One may also note the uneven distribution of mafic components. Particularly noteworthy is the presence of accumulations of mafic minerals, which resemble schlieren formations.

2. The appearance of sphene and other titanium-bearing minerals in the contact zone of the hybrid facies of the intrusive body indicates an introduction of titanium; a similar example and evidence are found in T. M. Broderick [7].

3) The factor of intrusion formation under dynamic stress, as Kh. M. Abdullaev [1] indicates, apparently has general significance—

…that is, an unstable tectonic setting prepared the sedimentary rocks for easier penetration of magmatic melt into them, and thereby favored assimilation processes.

1. The elevated silica content in the rocks of the marginal facies, in comparison with Daly’s average indices for rocks, once again indicates their hybrid origin. The increase in calcium oxide in these rocks leads to an increase in the basicity of the plagioclase (from No. 16 to No. 54).

2. The increase in the content of CaO and MgO in the magma also affected the mafic components, the quantity of which in the basic marginal facies increases sharply, giving them a melanocratic character. Along with the increase in CaO and MgO, FeO, Fe₂O₃, TiO₂, and Al₂O₃ increase, while SiO₂ and especially K₂O decrease, as is clearly seen from the graph (Fig. 1).

Intrusive rocks far from the contact contain hornblende and biotite. As the contact is approached, the amount of hornblende increases, while the amount of biotite decreases. Phenomena of this kind have been described, for example, by Sh. A. Azizbekov [2] for the northeastern part of the Lesser Caucasus.

Fig. 1. Variation diagram of changes in the chemical composition of granitoids from the center of the intrusion to contact with the enclosing rocks.

Fig. 2. Summary diagram of calculation of the quantity, in percent, of the principal minerals of the Kanzyba massif.

The quantitative-mineralogical changes associated with the process of assimilation are shown by us on the graph (Fig. 2). Here one can see a definite relationship: with a decrease in the amount of quartz toward the marginal facies of the massif, the content of plagioclase and mafic minerals in the rocks increases sharply, and the content of orthoclase falls sharply.

1. The sharply differing physicochemical characteristics of the rocks participating in the assimilation process (intrusion, limestones, schists, sandstones, quartz keratophyres) affected the dependence of the type of marginal facies on the character of assimilation and the composition of the assimilated rocks. Rocks of granodioritic, dioritic, and gabbro-dioritic composition are developed, in their main mass, where carbonate-bearing rocks are in contact with them. The sandy-schistose sequence did not play a major role in the assimilation process; intrusive rocks in contact with it usually acquire only a fine-grained texture and a leucocratic appearance (the area of elevation 828 and Mt. Lysaya). However, in a number of cases it is evident how hybrid rocks (quartz diorites) also appear at the contact with the sandy-schistose sequence. These facts may…

may indicate that deep-seated solutions, which enriched the near-contact zones with mafic minerals, also partly participated in the formation of the hybrid rocks.

The circumstances noted above compel us to conclude that, in the process of assimilation, iron, titanium, aluminum, magnesium and, possibly, in part calcium, silica and sodium were introduced into the near-contact zone. Thus, the emergence of the rocks of the marginal facies of the intrusive was governed by assimilation phenomena not only at the site of direct contact between the magma and the country rocks, but also by an additional introduction of a number of elements from the magma.

The observations carried out make it possible to conclude that the original magma had a uniform granitic composition, with certain deviations in one direction or another caused by the development of the source over time. Only in connection with assimilation along the paths of movement and at the place where the magma of the intrusion came to rest did it acquire a more complex character.

To summarize, it may be said that the Kanzybinsky massif has a distinctly zonal structure; noteworthy in this connection is the fact that granites in the present erosion section make up more than 50% of the entire area of the intrusive.

During the emplacement of the granitoid intrusion, the enclosing rocks underwent deep contact-metasomatic alteration, with the formation around the intrusive body of a contact aureole 300–400 m wide. The intrusive formations themselves were also subjected to profound alteration. The transformations of the apical part of the intrusive and of the rocks enclosing it were chiefly metasomatic in character and were expressed in hornfelsing, silicification, biotitization, amphibolization, muscovitization, skarnification, and the formation of magnetite ores. A detailed description of all the contact-metasomatic formations associated with the Kanzybinsky massif was given by the author in an earlier published paper [6].

Among the dike rocks of the region of the Kanzybinsky massif two groups may be distinguished: a) dike rocks of acidic composition (aplites, aplite-porphyries, micro-granophyres, aplite-like granites); b) dike rocks of basic composition (diabases); a brief description of the latter may be found in a paper published by the author [5].

Despite the rather great diversity of the dike rocks of acidic composition, they are all closely connected by mutual transitions and possess a dense, more rarely fine-grained, fine-grained or trachytoid groundmass, composed exclusively or almost exclusively of light-colored minerals. An increased content of mafic minerals is observed in aplite-like granites and in varieties with porphyritic structure. Among the phenocrysts, white, grayish-white orthoclase, completely clogged with pelite, predominates, in an amount of 50–55%; its dimensions range from fractions of a millimeter to one centimeter. More rarely, plagioclase (No. 10–20) and hornblende occur as phenocrysts. In addition to the minerals noted, quartz (in an amount of 30–35%), muscovite, biotite, calcite, magnetite, pyrite, apatite, and sphene are present.

In their composition, spatial position, and common accessory minerals (apatite, sphene, magnetite), the dike rocks of acidic composition fully correspond to the deep-seated rocks, representing a product of the residual crystallization of granitoid magma; only their structures are determined by the peculiarities of crystallization under conditions of intermediate depths.

Quite interesting and important for understanding the entire course of development of the metasomatic processes associated with the Kanzybinsky in-

...intrusive body is the question of the relationship of skarn-ore formations with acidic dike rocks, which is reflected by us in the scheme.

1. Formation of metasomatic hornfelses
        |
        |          |
        |          |
   Strong brecciation
        |          |
        |     2. Formation of skarn
        |        minerals at the Margoz
        |        and Odinochnoe deposits
        |          |
        |     Brecciation
        |          |
3. Formation of dike rocks
   of acidic composition
        |
   Weak brecciation
        |
4. Formation of skarn
   minerals at the
   Rudnyi Kaskad deposit
        |
   Intensive brecciation
        |
5. Magnetite
        |
   Brecciation
        |
6. Sulfides

The following facts speak in support of the scheme presented. At the Margoz deposit, in deep shaft No. 4 it was clearly possible to see how a series of aplite veins cuts garnet-pyroxene and garnet skarns, forming sharp contacts with the latter. In this case the skarns, as a rule, are subjected to not very strong brecciation.

At the Rudnyi Kaskad deposit we see a more complex picture of the relationships between metasomatic formations and dike rocks. Thus, the earlier metasomatic hornfelses, as a rule, are cut by dikes; the later metasomatic formations, the skarns proper, develop along zones of brecciation and fracturing both in the metasomatic hornfelses and in the dike rocks. Consequently, it is necessary to admit the existence of skarns of two ages: earlier skarns, formed before the acidic dike rocks, and later ones—after the intrusion of the dike rocks. We note that these two different-aged skarns are also characterized, respectively, by different compositions. If the first skarn is characterized by salite, diopside, diopside-salite, and garnet of the grossular-andradite series, then the second skarn is characterized by ferrosalite and garnet of the schorlomite type. Naturally, such a distinction is not accidental and reflects definite changes that occurred in the general chain of a single process.

As for the ore-mineralization phase, it was preceded by brecciation of both the skarns and the dike rocks, which is very clearly visible at Rudnyi Kaskad in borehole 16.

Thus, the fractures along which the vein magma penetrated served in some cases also as pathways for the entry of skarn-ore and especially ore solutions from the same source. From this point of view...

there is an undoubted genetic connection between the dikes and the ore deposits, which have a common source with them.

The existence of intra-mineralization dikes, as has been shown by a number of researchers, including in the present article, is a real factor, and it must be taken into account when studying the history of the development of the tectonics and magmatism of certain iron-ore deposits.

In conclusion, we note that the petrographic composition of the Kanzybinsky intrusive body, the character of its occurrence, the far-reaching phenomena of assimilation with the formation of characteristic marginal zones, the presence of contact metamorphism and of vein rocks, as a rule located within the intrusive body, characterize our intrusion as having formed in the mesoabyssal batholith-laccolith facies [4].

LITERATURE

1. Abdullaev Kh. M. Genetic connection of mineralization with granitoid intrusions. Gosgeolizdat, 1945.

2. Azizbekov Sh. A. Geology and petrography of the NE part of the Lesser Caucasus, 1947.

3. Bezborodko N. V. Contact relationships of granites and gneisses. Lubny, 1918.

4. Kuznetsov Yu. A. Facies of magmatic rocks. Problems of the Geology of Asia, vol. 2. Publishing House of the Academy of Sciences of the USSR, 1954.

5. Shubin G. V. Diabases of the middle course of the Kanzyba River. Reports of the 7th Scientific Conference of Tomsk State University, issue 4, 1957.

6. Shubin G. V. Contact-metasomatic formations in the area of iron-ore deposits of the Krasnokamensk group. News of Tomsk Polytechnic Institute, vol. 90, 1958.

7. Broderick T. M. The relation of the Titaniferous of Northern Minnesota to the Duluth Gabbro. Econ. Geol., 12, 1917.