Belsky Intrusive Complex

S. S. Il’enok

The Belsky intrusive massif appears on the eastern slope of the Kuznetsk Alatau, occupying part of the interfluve of the Cherny Iyus and its small right tributary, the Izekiyul. The area attracted the attention of researchers in connection with gold mining, and in the last decade—thanks to discoveries of tungsten ores.

The geological formations of the area are ancient, considerably dislocated deposits, which form a large synclinal structure cut in its central part by the Izekiyul River. The structure is complicated by small flexures, and its axial part plunges to the northeast. The most ancient formation of rocks, in the form of marbled limestones, occurs along the boundary of the ridge with the Tapchinsko-Chebakovskaya depression, within which the broad valley of the Cherny Iyus River is situated. The marbled limestones underlie a formation of quartz porphyries with a total thickness of about 1000 m, consisting of acid effusives in the form of sheets and sills interbedded with tuffs, conglomerates, sandstones, shales, and black limestones. Above lies a formation of diabase porphyries with abundant sheets of pillow lava, beds of agglomerates and limestones, cut by numerous dikes of uralite, plagioclase porphyries, and olivine diabases. In the upper horizons the formation contains a graywacke suite. The total thickness of the formation reaches 2400 m.

All three of the indicated formations have been provisionally assigned to the Proterozoic on the basis of their similarity to the faunistically characterized Precambrian formations of the neighboring Saralinsky district.

Among the formation of diabase porphyries, and also partly of quartz porphyries, occurs the intrusive massif of the Belsky pluton (Fig. 1). The rocks of this pluton compose the highest points of the watershed area—the Belsky and Nikolsky goltsy. The age of the intrusion, likewise to a considerable degree provisionally, is assigned to the Precambrian.

It must be noted that farther south, in immediate proximity to the outcrops of the Belsky pluton, occurs the Spassky intrusive massif, close in age to the former but belonging to an early phase of the intrusive cycle. The Spassky intrusive massif occupies the high areas of the Spassky and Kuznetsovsky goltsy and extends northward in the form of small bodies. It is composed of quartz and quartz-free pyroxene-hornblende and hornblende diorites, microdiorite-diabases, which are accompanied by veins and dikes of plagioaplites, malchites, and diorite-porphyrites.

General Information

The intrusive rocks within the area under consideration form two large bodies, accompanied by small outcrops situated to the north of them. The first body composes the high Belsky golets and is hidden

is overlain by greenstone porphyries on the middle and lower parts of the slopes. The area of its outcrop is 1.2 km². It has a more or less isometric form with two tongues in the northern part. To the northeast

Fig. 1. Schematic geological structure of the left bank of the Izekyul River.
1—quartz pyroxene-hornblende and quartz hornblende monzonites, quartz gelsenkytes; 2—quartz pyroxene and hornblende diorites; 3—hornfels; 4—garnet-pyroxene and magnetite skarns; 5—Grauvakka sequence; 6—diabase-porphyrite formation; 7—quartz-porphyry formation; 8—formation of ancient marbled limestones.

there occurs a small intrusive body with an area of 0.125 km², and still farther on, quite small pipe-like outcrops of the same intrusion. Thus, a more or less broad plunge of the magmatic massif in this direction is clearly indicated.

As regards the southern part of the body under consideration, here a projection of intrusive rocks is observed toward Nikolsky Golets. Within the limits of the latter there is a second large intrusive body, which, in turn, has a projection toward the first body. Thus, the two bodies are separated at the surface only by a small field of enclosing rocks. The petrographic similarity of the rocks of both outcrops and the elements of occurrence of the contacts indicate that, at depth, these projections of the intrusion merge. At the same time, this makes it possible to identify the boundary of the intrusive body in the form of a concave surface with a steep dip to the northeast. The area of the second body is 0.625 km². The total area of the intrusion outcrops reaches 2.0 km, and with the areas on the right bank of the Izekiul River that extend beyond the limits of our district—about 4 km².

The boundaries of the intrusive massif are quite clear and sharp in the western and eastern parts, but are poorly expressed in the area of the northern and eastern parts of the body, where abundant hybrid xenolithic formations occur. This must be natural, since here we have the roof of the intrusion, dipping to the northeast. Of great interest is the configuration of the contacts in connection with the need to determine the form of the pluton. The dip of the northern contact of the first body is directed northward and northeastward at 45–50°. We have the same steepness of contact along the eastern boundary of the body. However, when this boundary is traced southwestward, a sharp flattening of the boundary plane can be observed, which explains the embayment of intrusive-rock outcrops seen in plan from the first body toward the second body. Here the contact dips at an angle of 10–15° to the southeast. At the same time, a certain convexity of the boundary of the intrusive body toward the southeast is explained by the fact that the flattened contact is exposed by the valley of the stream.

The western contact has been studied comparatively well because a large zone of crumpling is pressed against it, accompanied by ore bodies that have been subjected to prospecting and exploitation. Here it is clearly established that the contact plane dips at angles of 60–75° to the southeast, i.e., beneath the intrusive body. The southern contact is steep, but the direction of its dip is unclear. The “cold” character of the contact, with weak development of hornfelses, suggests a possible dip to the north.

As for the small northern tongue of the intrusion, it is closely connected with the zone of crumpling manifested here and represents a magmatic mass drawn into the tectonic zone.

In the extreme northern part this tongue passes into a series of small bodies elongated in two directions. Some of them have a northeastern strike in the direction of the intrusive tongue, while others have a strike close to latitudinal. These latter bodies are subordinated to an additional system of fractures, which has a regular connection with the first northeastern zone. The morphological features of the contact are manifested as follows. Northward, in the direction of elongation of the intrusive tongue, the contact at first dips very gently—at an angle of 5–10°. The cover of wall rocks has been denuded, and only small remnants of it remain on the surface of the intrusion. Farther north, after 200–300 m, the angle of dip of the surface of the intrusion increases to 25°. The intrusion disappears beneath diabase porphyrites, dipping gently for a distance of 400 m; here a broad field of contact-metamorphic formations appears. Then the intrusion dips steeply, giving abundant apophyses in the form of small bodies in the zone of crumpling.

The second field of intrusive rocks, cropping out on Nikolsky Golets and along the stream of the same name, shows a gentle dip of the contacts in the northern and northwestern part, where we see an embayment toward the Belsky intrusive field. The gentle occurrence of the contact is emphasized by a very broad field of well-developed contact-metamorphic formations here.

in the form of hornfelses and, in part, skarns. In the southwestern corner of the intrusive field, a steep dip of the contact is observed to the northwest at an angle of 80° to vertical. The southern boundary of the body is characterized by narrow embayments along the secondary folded structure of the country rocks, with a northeastern strike and a general plunge of the roof of the intrusion to the south.

The uneven boundary lines of a small isolated intrusive field projecting to the northeast from the first body are explained by their subordination to small folds of northeastern strike in the enclosing rock mass.

Thus, in general form the pluton has the shape of an etmolith, projecting in the trough of a syncline and plunging in a northeastern direction, toward the plunge of the fold axis. The outcrops of intrusive formations of the Spasskii complex are subordinate to the same tectonic structure and occur in the southeastern limb of the syncline. The close petrographic features of both intrusive complexes indicate that here we have, if not coeval, then apparently comagmatic formations.

In size, relative to the mountain system, the pluton belongs to the category of small ones. There are as yet no data to speak of it as a direct branch of another, larger pluton, although similar intrusions are widely distributed in the Kuznetsk Alatau.

The relation of the intrusive to the tectonic structure of the area shows that here we have a synclinal pluton, subordinate to later tectonic movements with respect to the folded structure and therefore partly cutting the latter.

As for tectonic facies, it is necessary to note the absence of true gneissic textures. Along the western margin of the massif, cataclasis and schistosity are observed, associated with the zone of crumpling manifested here. In its main mass the intrusive bears traces of contamination, with the development of hybrid taxitic rocks; therefore it may be assigned to the type of granite plutons.

The internal tectonics of the pluton is difficult to study, all the more so because in the area of well-exposed contact sections it is masked by schistosity and cataclasis. The arrangement of xenoliths indicates rather the absence of autonomous tectonics and parallelism with the contact boundary.

The relation of the contact to the tectonics of the surrounding rocks has been clarified fairly well. The pluton is concordant with the general structure, but has discordant boundaries in connection with local bends of the country rocks, especially in its southern part.

The relation of the internal tectonics to the contact is conformable. This is well emphasized by the concordant arrangement of xenoliths. At the same time, one must recognize, in general, the conformable position of the internal tectonics in relation to the external.

As for the distribution of rocks in the body of the pluton and their mutual relations, the following features should be noted: the abundance of hybrid formations is striking; these occur in almost all parts of the massif. It should be noted that xenoliths predominate in the northern areas. In the marginal zones near the western contacts, more basic rock varieties are observed, in the form of quartz pyroxene–hornblende monzonites and diorites. The principal part of the body is usually composed of quartz monzonites. In the eastern part of the pluton, more leucocratic varieties are observed, transitional to quartz syenites.

Among the rocks of the intrusive field, especially in the area of the Nikol’skii golets, quartz felsinkites occur as small patches. Their dimensions range from 30–40 m to 5–6 m in diameter. In plan the patches have an elongated or isometric form. The transitions from the enclosing rocks, which here ...

represented by quartz monzonites—gradual, as is generally the case for the rocks mentioned above.

Rocks of the principal intrusive field

Hornblende–microcline diorites

The rocks occur in the Annensky area and have been exposed by an adit that cuts at right angles across the boundaries of the intrusive bodies and the complex of quartz veins running parallel to the latter. The diorite body is about 30 m thick and is elongated in a northeasterly direction along the trend of the contact of the Belsky intrusive field. It directly adjoins the quartz monzonites of the Belsky field and dips beneath them, conformably with the contact of the latter. Along the boundary between the quartz monzonites and the diorites runs the Annenskaya quartz vein. It is accompanied by intense alteration of the wall rocks, which masks the character of the transition between the two rock types.

The rock has the character of a coarse, dark-green kakirite and, when struck, breaks into roughly parallel plates. The texture of the rock is medium-grained; it is preserved almost up to the contact with diabase porphyries. Near the latter, at a distance of 3–5 m, there is a noticeable decrease in grain size, strong cataclasis of the rock, and its replacement by calcite. The usual grain size is 2.5–4 mm. Plagioclase grains have a greenish-gray color, are blurred, and show themselves poorly. The dark-green, almost black amphibole is better preserved. It has the form of slightly elongated thick prisms without terminal faces. The main mass of amphibole crystals is elongated along the plane of strike–dip of the intrusive body.

The microscope shows a monzonitic texture of the rock, consisting of plagioclase, green hornblende, microcline, and microperthite. Plagioclase and green hornblende, which may have developed at the expense of pyroxene, possess the same degree of idiomorphism. Microcline is xenomorphic with respect to them.

Plagioclase forms broad tablets with zonal structure. Everywhere it is strongly decomposed and filled with fine dark-brown saussuritic dust. Only in one case was it possible to measure $\perp (001)\,Ng—57;\ \perp (001)\,Nm—40$ and $\perp (001)\,Np—70$, which indicates acid labradorite No. 52 (37 e—3). It is interesting that plagioclase is regularly overgrown by microcline, brown from abundant pelitic products. Nevertheless, one can discern spindle-shaped twins running parallel to the elongation or the direction of cleavage of the plagioclase.

Microcline in places shows straight boundary lines, especially with microperthite, which apparently indicates that the appearance of quartz is connected with the end of the crystallization period of K-feldspar. Microperthite occupies the spaces between crystals of the latter and forms apophyses into it. At the same time, in microcline one can observe separate sprouts of quartz. Microcline contains a fine perthitic network with an albite content of about 40%. Measurement of the orientation of the ellipsoid relative to $(001)—P$ showed: $\perp PNg—87;\ \perp PNm—18;\ \perp PNp—72;\ 2V=+78$. This confirms the microcline character of the K-feldspar.

Green hornblende has the appearance either of compact grains or of grains replaced in the central part by platy chlorite-serpentine (bastite), so that in the marginal zones there remain, as it were, needle-like relics of hornblende split along cleavage, bordering the chlorite-serpentine in a thin band. The compact grains contain, in the central part, a field finely dusted with ore matter, analogous to the fine impregnation of plagioclases with saussurite. At the same time, the hornblende contains larger-

...a different strength of magnetite, which is distributed noticeably more densely in the central parts of the grains than at the margins. Such fine impregnation of hornblende is connected with its transition into chlorite-serpentine.

The compact grains of hornblende, moreover, contain ingrowths of a fine-needle variety, which is sometimes arranged across the elongation of the host grain. This fine-needle variety differs appreciably from the compact hornblende and in places resembles separated fibers of the latter. The optical properties of the hornblende, measured on the Fedorov stage, show the following features of the mineral.

Pleochroism: Ng—light green; there are spots colored more densely, to the color of withered grass; Nm—bluish green; Np—pale green. Angle c: Ng = 19°; Ng − Np = 0.038; 2 V = −74.

The characteristically high birefringence, and in part the character of absorption, set the mineral apart in the direction of iron-rich grunerite.

Among the further changes in the rock, mention should be made of the presence of a fine network of epidote veinlets observed in the plagioclase. It turns out that these veinlets cut all the minerals of the rock, including potash feldspar and quartz, but have a widely branched system only in the lime-soda feldspar.

Mineralogical composition of the rock: plagioclase 39%, hornblende 35%, microcline 10%, microperthite 16%.

The more basic character of the plagioclase than in typical diorites brings the rock closer to hornblende gabbro.

Pyroxene-hornblende-sossurite monzonites

They occur on the summit and western slope of Nikolsky Golets. Macroscopically they are indistinguishable from hornblende monzonites, since the pyroxene is to a considerable extent replaced by hornblende. The transitions between the rocks are gradual.

The rocks have a grayish-green color, a massive structure, and an even, medium-sized grain. They consist of pale-greenish sossuritized basic plagioclase, idiomorphic crystals of pyroxene, which has passed at the margins into amphibole, of potash feldspar and quartz, which form micropegmatitic intergrowths. Accessories are ilmenite and apatite.

The microscope shows a monzonitic structure with idiomorphism of plagioclase over potash feldspar. The plagioclase has the form of tablets, usually entirely replaced by sossurite. The latter consists of a close aggregate of sericite scales, in which small prisms of clinozoisite are scattered. In the marginal parts of some grains there appear rounded, larger grains of the latter. They are colored a denser yellow and have a spotted, supernormal interference color characteristic of epidote. Albite forms the main framework of the grain, preserving the forms of the plagioclase grains. Without this property of albite—to form pseudomorphs after plagioclase—the shape of the grains of the latter apparently could not have been preserved. The arrangement of the sossurite aggregate indicates the zonal structure of the plagioclase. In rare cases, polysynthetic twinning of the feldspar is faintly revealed, which makes it possible to determine the coordinates of the twin axis (Table 1).

The pyroxene has the form of short prisms with clearly expressed cleavage. On the outside it passes into a rim of pale-green hornblende. This latter usually penetrates the entire pyroxene crystal in the form of thin stalks, clearly visible when the mineral is set at extinction. [[unclear: word cut off at page bottom]]

Table 1

The color of the pyroxene is slightly greenish, and it changes hardly at all along the axes. Other properties are as follows: \(c:Ng = 37^\circ\); \(Ng - Np = 0.028\); \(2V = +57^\circ\).

On the basis of these properties the mineral is readily identified as diopside. It should be noted that the diopside contains rounded grains of titanite and a fine dusting of magnetite and, possibly, formed through alteration of titanitic augite.

The pale-green hornblende in places has clearly replaced the pyroxene, forming uralite proper. It has either a compact structure or passes into a fibrous variety. Its color is pale green, with weak pleochroism of the following character: \(Ng\)—light green, \(Nm\)—yellowish green, \(Np\)—pale green. Other properties: \(Ng - Np = 0.025\), \(c:Ng = 16\); \(2V = -80\) indicate that we are dealing with actinolite. This mineral is replaced by chlorite, which is arranged along its fibers.

K-feldspar almost everywhere proves to be densely intergrown with quartz, which forms beautiful regular granophyric patterns. In addition, it is strongly pelitized and therefore has a brownish color in transmitted light, which masks the internal structure of the mineral. However, the orientation of the ellipsoid with respect to the cleavage plane on (001) indicates that here we have microcline: \(Ng—87\); \(Nm—18\); \(Np—72\); \(2V = +74\).

It is of interest that quartz shows a tendency toward the perfect development of faces, and in one case it was even possible to find a euhedral grain.

The accessory minerals are represented by fine-needle apatite and ilmenite. The latter is always decomposed and forms a network of plates of ore mineral in a titanite envelope.

The quantitative mineralogical composition of the rock is given in Table 2.

Table 2

NOTE: I. Sample 66. Nikolsky golets.
II. Monzonite from Tröger’s work (1935).

For comparison, the composition of a monzonite according to Tröger is given. Our rock differs in its high content of mafic minerals, mainly at the expense of a decrease in orthoclase.

According to Tröger (1935), owing to the presence of albitized plagioclase, the rocks may be assigned to albite monzonites, but the presence of abundant scapolite compels us to call them pyroxene–hornblende–scapolite monzonites.

Quartz hornblende monzonites

The rocks are the most widespread and make up the massif of Belsky Golets. On the eastern and northern slopes of the latter, near the contact, they pass into varieties of generally the same composition, but containing abundant small xenoliths.

The color of the rocks is pinkish gray and lighter than that of the quartz pyroxene–hornblende–scapolite monzonites. A characteristic feature is the presence of a taxitic texture in the form of fine-grained aggregates scattered everywhere among the medium-grained groundmass. In them there is little pink potash feldspar and an increased amount of dark-green hornblende. In places, aggregates consisting almost of a single mafic component appear. The dimensions of such schlieren spots are small, for the most part the size of a pea.

Macroscopically, pale-green scapolitized plagioclase, pink potash feldspar, dark-green hornblende, and a small amount of quartz are readily distinguishable in the composition of the rock. Plagioclase forms lath-like forms measuring \(1.5—2 \times 3\) mm. In specimens with limited development of scapolite it has a pale-green color and vitreous luster. In areas of intense scapolitization the mineral is dull and has denser light-green colors. Potash feldspar is xenomorphic with respect to plagioclase and is represented as isometric light-pink and pink grains measuring \(1 \times 2.5\) mm and less. Usually it is surrounded by a fringe of microperthite. Hornblende has the appearance of elongated grains with moderate development along the third axis. The grain size is \(2.0—2.2 \times 0.8—1.1\) mm. The side faces are indistinctly developed, the terminal faces are completely absent, which is characteristic of the uralite type. Among the pink potash feldspar, xenomorphic quartz grains occur, reaching 1 mm in diameter. Individual grains of epidote are also noticeable to the naked eye. The microscope shows a monzonitic texture. Plagioclase is sharply idiomorphic in relation to potash feldspar. It has the form of tablets and laths and contains either very fine-grained scapolite or sericite together with loose lumps of clinozoisite, and sometimes larger grains of epidote. Judging from observations on the scapolitized monzonites of Nikolsky Golets, where epidote occurs as larger grains among the marginal parts of plagioclase, while clinozoisite is located in the rest of the mass, it must be assumed that this mineral appears at a later stage of scapolitization.

Plagioclase forms polysynthetic twin intergrowths, more often according to the albite and pericline laws, more rarely according to the Carlsbad and Manebach laws. Determination of the coordinates of the twin axis on the Fedorov stage showed the composition of acid andesine. \(Ng = 75;\ Nm - 15.5;\ Np—85.5;\ 2V = +84.\) Pericline law. Plag. No. 31 (specimen 37a). However, in many cases plagioclase contains no scapolite, while that having inclusions of large epidote grains proves to be albite:

1) \(Ng = 75;\ Nm — 17;\ Np — 81;\ 2V = +81;\) twin axis \([001]\); plag. No. 5 (specimen 38–1).

2) \(Ng — 82.5;\ Nm — 69;\ Np — 22;\ 2V = +80;\) twin axis \([100]\); plag. No. 4 (specimen 38–1).

Epidote inclusions, the absence of saussurite dust, and the clean character of the crystals indicate that the mineral appeared as a result of albitization of plagioclase. This process followed saussuritization and, probably, was accompanied by the dissolution of fine mica.

Potash feldspar is strongly pelitized and appears gray. It is of interest that calcite veinlets cutting the rock near the Annenskaya 1st vein are accompanied by complete cleansing of the potash feldspar from pelitic substance (Fig. 2). In this case the pelitic material does not migrate toward the calcite or in the opposite direction, but apparently dissolves in the potash feldspar. The bands of renewed potash feldspar are glassy-transparent and reveal a distinct twinned structure of microcline. This phenomenon, in my opinion, proves that the pelitization of potash feldspar is earlier in age than the ore veins, or at least than the carbonate stage of mineralization. It becomes clear that the pelitization of potash feldspar, like the saussuritization of plagioclase, is a widespread autometamorphic process. At the same time this proves the low-temperature character of the process—below the temperature interval of carbonate solutions.

Fig. 2

Pelitized potash feldspar shows a change in optical properties from orthoclase with a small angle of optical axes toward microcline. This is also indicated by the simultaneous appearance in places of weakly expressed twin lamellae.

The change in the properties of potash feldspar in sample 38 is as follows:

Coordinates of the axes of the ellipsoid to the pole of cleavage (001)

1) Ng — 88; Nm — 5; Np — 85; 2V = −66. Orthoclase;

2) Ng — 86; Nm — 6; Np — 86; 2V = −77. Slight deviation toward microcline;

3) Ng — 90 ± 3; Nm = 14 ± 0; Np = 76 ± 1; 2V = −84. Considerable deviation toward microcline.

In other areas (sample 37a) typical microcline with a lattice twinning structure is manifested.

The mineral also contains irregular patches of albite, sometimes of rather large size, occupying on average 40% of the area. Judging by the forms of the albite intergrowths, it most closely corresponds to replacement microperthites and, apparently, is connected with the same process as the albitization of plagioclase.

It is curious that potash feldspar rims plagioclase in the form of broad margins. In areas with development of microcline, the spindle-shaped twins of this mineral coincide with the direction of the twin bands of plagioclase. At the boundary between plagioclase and potash feldspar, coarse myrmekite is developed. Abundant worm-like grains of quartz grow into plagioclase and simultaneously into potash feldspar. Separate isometric crystals of potash feldspar are rimmed by micropegmatite; in this case quartz in the intergrowths is often arranged radially or feather-like, and the grain size regularly increases toward the periphery. This clearly indicates the earlier appearance of potash feldspar relative to the micropegmatite and, at the same time, the eutectic character of the latter. In the presence of replacement microperthite, apparently the entire potash feldspar would have been subjected to the process of pegmatitization.

In places, separate xenomorphic grains of quartz appear with transition to pegmatite, which apparently is connected with the separation of a small amount of excess quartz relative to the eutectic.

Hornblende has a green coloration, and also has either a compact or a fibrous habit. Pleochroism is sharply expressed in the following tones: Ng—grass-green, Nm—yellowish-green, Np—yellow. Its other properties: c:Ng = 16°; Ng—Np = 0.024; 2V = −78° indicate that we are dealing with ordinary green hornblende, passing into uralite. In addition to large crystals of fibrous habit there are transitions into a felted fine-grained aggregate of the same dense-green actinolite. During granulation the optical properties of the mineral do not change appreciably.

Large hornblende crystals contain titanite in the form of small grains situated along the mineral’s cleavage. Both coarse-grained and felted fine-grained hornblende are associated with epidote. In this case the coarse-grained hornblende is penetrated by the latter, while the fine-grained variety overgrows epidote in the form of a brush of acicular crystals. The later appearance of epidote relative to the coarse-grained hornblende is demonstrated by cutting veinlets of epidote of the same generation.

Fig. 3

A very interesting feature is the connection of compact hornblende with leuchtenbergite and the appearance of apatite. Hornblende crystals are accompanied by an accumulation of apatite. In this case the apatite needles are oriented preferentially according to the direction of elongation of its crystals, especially in the areas of elongation (Fig. 3).

Compact hornblende, as well as fine-grained hornblende of actinolite type, is replaced by green chlorite. It is more often fine-grained, but has transitions into larger grains, where pleochroism is clearly manifested: Ng—pale yellowish-green, Nm = Np—green. Extinction is straight. Elongation—Optically + with a small angle of the optical axes. The interference color is anomalous, pale violet. All this indicates optically positive pennine. It is interesting that chlorite readily appears in plagioclase together with saussurite. Not all the plagioclase of the rock undergoes such replacement, but only some of its grains.

Accessory minerals are represented by altered ilmenite, titanite, apatite, and zircon. The first of these forms aggregates of small shapeless grains of magnetite in a titanite envelope. Titanite forms independent grains without regular outlines, with pleochroism in brownish-yellow tones. It has transitions to titanite developed over green hornblende and, apparently, is connected with the decomposition of an earlier titanitic amphibole. Apatite is abundant in the rock and gives thin long needles, larger spear-shaped grains, and little prisms. Zircon is occasionally encountered, forming small idiomorphic crystals in chlorite with dense brown pleochroic rims.

It should be noted that there is a variety of the rock distinguished by a number of features, chiefly by the absence of micropegmatite, instead of which there are independently developed grains of K-feldspar and quartz; by the manifestation of pseudomorphs of chlorite and epidote after a mineral with fine cleavage, apparently biotite (specimens 63, 64, 66 d). The rock has direct transitions to the pyroxene–hornblende saussuritic monzonites of Nikolsky Golets and occurs on the eastern slope of the latter.

of it. The texture of the rock is monzonitic, with a successive series of idiomorphism of the light-colored minerals: plagioclase, K-feldspar, quartz. However, the dark-colored component, in the form of green hornblende, displays remarkable features. It develops metasomatically in plagioclase and K-feldspar (sample 64—1) and even penetrates into quartz (Fig. 4). The mesostatic development of hornblende is typical. Its grains are not scattered uniformly through the rock, but are grouped in clusters with mutual intergrowths of crystals (Fig. 5).

Fig. 4

Fig. 5

The quantitative mineralogical composition of the rocks (sample 37a) is as follows: plagioclase 31%, K-feldspar 27%, hornblende 19%, quartz 18%, impurities 5%.

The rocks are characterized by a ratio of plagioclase to K-feldspar close to that in monzonites, which, together with the texture and the idiomorphism of plagioclase relative to K-feldspar, provides a criterion for assigning them to the group of quartz hornblende monzonites.

Xenolithic quartz hornblende monzonites

The rocks are widely distributed in the marginal parts of the outcrops of the intrusion.

The xenoliths occur in the typical quartz hornblende monzonite described above, and there is no need to repeat its description. The size of the xenoliths is 4–7 cm, rarely larger. Their form is angular, but they usually pass into rounded schlieren. The xenoliths consist of material of a diabase hornfels. This is a dark-green, fine-grained rock with an intergranular texture. It consists of plagioclase (45–50%), completely replaced by a fine-grained saussuritic aggregate; green hornblende (40–50%), xenomorphic in relation to it; a subordinate amount of quartz (5–7%); and accessory magnetite and apatite (2%). The hornblende forms grains as fragments and spots of an intergrown aggregate, i.e., it bears features of a metamorphic mineral. It has a dense green coloration with pleochroism: \(Ng\)—deep green, \(Nm\)—green, \(Np\)—yellow. Its other properties are: \(c:Ng = 16^\circ\); \(Ng - Np = 0.021\); \(2V\)—large.

Quartz forms xenomorphic grains as fillings of angular interstices between plagioclase grains.

Among the features of the occurrence of accessory minerals, one should note the uneven development of magnetite and the irregular form of its grains, as well as the constant presence of apatite in the form of thin needles.

In mineralogical composition the rock should be classified as a plagioclase–hornblende hornfels.

Schlieren areas show disintegration of the xenolith with the formation of a curious microtextural picture. Plagioclase appears in the form of group intergrowths, within which xenomorphic green hornblende is developed. The latter develops metasomatically, partly corroding the plagioclase. Such plagioclase intergrowths, or “hedgehogs,” are situated in the ordinary mass of the quartz hornblende monzonite, with idiomorphic tablets and prisms of saussuritized plagioclase (Fig. 6). The arrangement of saussurite in the tabular grains clearly indicates broad zoning, but the plagioclase here, as also in the intergrowths, owing to complete decomposition, is less definite. At the boundary with the intergrowths, microperthitic orthoclase is noticeably concentrated in the form of small rectangles and bars. At the same time they are overgrown by micropegmatite with a graphic arrangement of quartz and an increase in grain size toward the periphery. In other parts of the plagioclase intergrowths, windows with K-feldspar and micropegmatite appear, apparently indicating further disintegration.

Fig. 6

Schlieren quartz hornblende monzonites pass into taxitic rocks with the development of small, pea-sized, fine-grained areas among the medium-grained main mass. In such cases the microscope shows complete equalization of composition between the main part and the fine-grained areas of the rock. The difference is manifested only in the textural pattern and grain size; characteristic is the behavior of green hornblende, which forms intergrowths or clusters of grains. The crystals of this mineral already have a subhedral appearance.

It should be pointed out that the green hornblende appeared not only later than the plagioclase, but also later than the K-feldspar and quartz, for it penetrates these minerals along fine cracks (73a—2,73a—3). The mineral is associated with chlorite and titanite, with the formation of pegmatite-like intergrowths. Idioblasts of fresh hornblende are often developed in the chlorite. The chlorite belongs to an optically positive pennine. Epidote is present in the rock both as individual grains and in intergrowth with hornblende. Among the accessory minerals abundant apatite and ilmenite are observed in the form of decomposed grains with a typical network of magnetite plates at an angle of 120° in a leucoxene shell or in chlorite.

Curious threadlike thin veinlets should be noted, caused by the circulation of low-temperature hydrothermal solutions along cracks. The veinlets are filled with local material. Where they cut hornblende they are composed of chlorite; among K-feldspar they contain a fine-grained sericite-like mineral; within the plagioclase field they are filled with epidote; among micropegmatite they are composed of quartz having the same optical orientation as the enclosing crystal.

From these data it is clear that the taxitic texture of the quartz hornblende diorites is connected with the disintegration of xenoliths having the composition of plagioclase–hornblende hornfelses. The disintegration is connected with the deposition of K-feldspar and quartz and the development of mesostatic hornblende. The rocks belong to typical hybrid formations.

Quartz helsinkites

Within the field of quartz monzonites of the Belsky and Nikolsky goltsy, spots of light-colored rocks of various sizes occur. Thus, on the watershed of Mt. Nikolsky and Bezymyanny, the quartz hornblende monzonites eastward give way to grayish-white rocks occupying an area of 60–85 m across and slightly elongated to the northeast. Farther south, spots of small size, 10–25 m, of rounded form are observed. Closely spaced spots merge with one another. Study of the mineral composition and structural features of the rocks makes it possible to assign them to quartz helsinkites.

Macroscopically the rocks are light gray or grayish-white, and in transitional varieties light pink in color. In fresh condition they are massive. On the weathered surface a fine porosity is noticeable. Weak effervescence at individual points indicates the presence of carbonate.

The rocks have a medium-grained structure and consist of colorless potassium feldspar, 2–3 and 4–5 mm in size; a subordinate amount of dull, slightly greenish albitized plagioclase of the same size; pistachio-green epidote, 4–10 mm in diameter; and quartz.

The microscope shows a hypidiomorphic structure with variable grain size; namely, in places segregations of a fine-grained subhedral aggregate of albite and quartz appear. At the same time, the freshness and purity of the rock-forming minerals, in contrast to the rocks described above, are striking.

In transitional varieties the albitized plagioclase still contains raw saussurite, but in the typical rock epidote migrates outward, forming independent grains, and partly crystallizes inside with the formation of enlarged grains. Sericite is less mobile and occurs only in feldspar in the form of small leaves or scales. Determination of the optical constants of the albitized plagioclase showed the composition of almost pure albite (Table 3).

Table 3

Potassium feldspar is xenomorphic in relation to the albitized plagioclase; in transmitted light it is slightly brownish owing to the presence of pelite. In the potassium feldspar from a typical helsinkite, a microcline lattice is clearly manifested in the form of crossed twins according to the albite and pericline laws. In addition, the orientation of the ellipsoid to the pole of cleavage (010) = M or (001) = P indicates that the mineral belongs to microcline.

1) ⟂PNg = 83, ⟂PNm = 13, ⟂PNp = 80; 2V = +76 (sample 58–2),

2) ⟂PNg = 80, ⟂PNm = 13, ⟂PNp = 81; 2V = −70 (sample 55–3),

3) ⟂MNg = 12, ⟂MNm = 80, ⟂MNp = 86°; 2V = −86 (sample 58–1).

Microcline is perthitized to varying degrees. Grains with vein-like perthite are observed, as well as the development of albite in the form of dense branching spots, clearly indicating replacement perthite. The margins—

the outer parts of the microcline grains are more strongly subject to replacement by albite than the central parts. In most large grains of this mineral the content of the latter is as follows—in the marginal portions—40–60%, in the central portions—15%.

Quartz is developed in subordinate amount and forms intergrowths in microcline from the margins of grains, and also along the suture of Carlsbad twins.

Epidote is observed as large idiomorphic grains, often of skeletal form. The color is pale, lemon-yellow. Pleochroism: \(Ng\)—pale lemon-yellow, \(Nm\)—pale yellow, \(Np\)—colorless. Other properties: \(Ng - Np - 0.012\); \(c : Ng = 3^\circ\); \(2V = -80^\circ\), which corresponds to a mineral with a content of about 15% \(Fe_2O_3\).

It is interesting that the rock contains no hornblende, while chlorite is widely developed in association with titanite and calcite. The chlorite has a pale yellowish-green color with pleochroism in the following tones: \(Ng\)—pale yellow, \(Nm = Np\)—pale green. Extinction is straight. The interference color is gray with a pale-yellow tint. Optically positive, with a small angle of the optic axes. Radial-fibrous aggregates are common. The mineral belongs to pennine. Its association with calcite is manifested in close pegmatite-like intergrowth. Sphene is spatially confined to chlorite, forming small grains without regular boundaries, which have a brown color and pleochroism in yellow-brown tones.

The accessory minerals are represented by apatite and ilmenite. Apatite is widely distributed as small and thin needles included in all minerals of the rock. Ilmenite forms rather large grains, but is decomposed and shows a typical pattern in the form of plates of the ore mineral intersecting at an angle of \(120^\circ\) in the mass of titanite, often together with chlorite.

The quantitative mineralogical composition, in weight percent, for a typical rock (sample 38) is: albitized plagioclase and albite 38, microcline 20, quartz 23, epidote 11, chlorite 3, calcite and others 5.

Berezites

On the eastern slope of Goltsy Nikol’skii, within the field of intrusive rocks, mainly among their helsinkite facies, light patches of rocks with white mica appear. The patches of bleached rocks are small, 1.5 to 4–5 m in diameter, and occur in groups. Macroscopically, quartz is clearly visible; it merges into a continuous cementing mass, owing to which the rock acquires the appearance of quartzite. In addition, white feldspar and sericite are visible, both as grains about 1 mm in size.

The microscope shows the development of large complex quartz grains, which occupy more than half of the entire area of the thin section. Among the quartz, locally in the form of poikilitic inclusions, albite with wavy twin lamellae is seen. The albite has been resorbed from the side of the quartz and has uneven boundaries with embayments from the latter into its grains. The coordinates of the twinning axis confirm the determination of the mineral: \(Ng = 83\), \(Nm = 68\), \(Np = 23\); \(2V = +76\); twinning axis \([100]\); plagioclase No. 4.

Sericite forms large flakes, sometimes as rosette-like intergrowths. There is more of it among the albite and less in the quartz. In both the quartz and the albite, inclusions of apatite are observed in the form of well-formed acicular crystals. Small loose accumulations of calcite are confined to the sericite.

The quantitative mineralogical composition of the rock is shown in Table 4.

Table 4

I. Berezite of the Nikolsky massif. II. Berezite according to Rose.

The following circumstances are noteworthy: a) the presence of berezites not along zones of crushing or tectonic fractures, but in the form of patches in massive rocks without traces of fracturing; b) the occurrence of scheelite and pyrite as sporadic grains.

All this indicates the formation of the berezites as a result of deep autometamorphism of the intrusive rocks under the action of residual solutions of hydrothermal character.

Vein rocks

Rocks belonging to the vein facies have a limited distribution within the area. Within the intrusive field they were not observed, but were found in the exocontact zone. An increased amount of such rocks was encountered along the western zone of the exocontact of the Belsky intrusion. Here there is a deluvium of quartz monzonite-porphyries, and only in rare cases is it possible to record individual bedrock exposures of these rocks, in which a cutting contact with the enclosing greenstone effusives is visible. However, the full outlines of the bodies remain unclear because the remaining parts are covered by deluvium.

The points with discovered outcrops of quartz monzonite-porphyries are arranged in a linear direction and apparently emphasize some northeastern fracture zone. Along this direction these rocks were found even in the southern part of the area, where they include xenoliths of coarse-grained quartz pyroxene-hornblende diorite of the Spassky intrusion.

In addition to unsplit varieties of vein rocks in the form of quartz monzonite-porphyries, which vary in composition up to syenites, products of magma splitting occur in the form of granite-porphyries and aplites. Granite-porphyries were found at two points, and one of them is on the right bank of the Izekyul River, i.e., outside the area described, while aplites are observed in the rocks of the intrusive field, where they form small veinlets.

Quartz monzonite-porphyries

These rocks have a greenish-pink or brick-red color, massive fabric, and porphyritic texture. Macroscopically, phenocrysts of hazel-yellow plagioclase, 2–4 mm in size, and grass-green amphibole are distinguished, sometimes with serial transitions from grains 2–3 mm in size to vanishingly small sizes. The segregations are situated in a fine- to very fine-grained, essentially feldspathic groundmass.

The microscope shows a porphyritic texture consisting of segregations of plagioclase and green amphibole in the principal, crystalline-granular groundmass. The plagioclase has a distinct, though comparatively weak, zoning.

Determinations on the Fedorov stage showed (Table 5) the following composition by zones in one grain (sample 314):

Table 5

The mineral is considerably decomposed and contains flakes of sericite and small grains of zoisite. In places it is replaced by epidote, which is accompanied by chlorite (pennine).

Amphibole forms elongate-prismatic grains with pale coloration and pleochroism of the following character: Ng—light green, Nm—pale yellow-green, Np—pale yellow. The high birefringence, about 0.029, and the extinction angle \(c:Ng = 20^\circ\) indicate that the mineral belongs to actinolite. Characteristically, an aggregate of small flakes of green biotite is developed along the amphibole. The latter has pleochroism from pale yellow to deep green according to the biotite absorption scheme, and also a high birefringence typical of micas.

The main mass of the rock has a micro-monzonite structure, with idiomorphism of plagioclase relative to K-feldspar and quartz. The composition of the plagioclase, determined by the zonal method, is close to the composition of the peripheral zones of the phenocrysts of this mineral and corresponds to No. 15–17. The lath-shaped grains of K-feldspar are clogged with dense brown dust of secondary clayey products and are less readily identifiable. In the interstices between them, xenomorphic grains of quartz are observed, sometimes reaching large sizes. Intergrowths of K-feldspar by quartz occur, with the formation of a micropegmatite structure.

Among the accessory minerals, magnetite, apatite, and rare grains of zircon are observed. Magnetite is noticeably confined to the flakes of biotite.

Quartz syenite-porphyries

A whole series of transitional varieties is observed from monzonite-porphyries to rocks in which, owing to a decrease in the amount of plagioclase, the content of K-feldspar and partly quartz increases. Porphyritic phenocrysts are rare or occur sporadically and are represented by light-green saussuritized plagioclase, and also, in some cases, by green amphibole. Their distribution is uneven.

The microscope shows strong decomposition of the primary minerals of the rock. Plagioclase is usually replaced by a dense mass of saussurite. In fresh specimens of the rock (sample 298) it is possible to determine the following properties of this mineral: \(BNg—57\), \(BNm—75\), \(BNp—36\); law \(\perp (110)\); plagioclase No. 30. The phenocrysts serially decrease to the size of the grains of the principal mass of the rock. The plagioclase content does not exceed 34% by area. Rare fragments of amphibole belong to light-green actinolite of the type described above.

The principal mass of the rock consists of brown pelitized K-feldspar, occupying about 50% of the area of the thin section. Despite the abundance of secondary products, it is possible to note the lattice structure characteristic of microcline. This is also indicated by the coordinates of the pole of the cleavage \((010)\)—\(Ng—20\), \(Nm—72\), \(Np—72\). Irregular patches of perthitic albite are noticeable in the K-feldspar. Quartz forms xenomorphic grains, distri-

located between the crystals of K-feldspar. Its content is usually about 15%, although in one case, in a rock of similar composition (sample 156), an increased amount of quartz, up to 30%, was observed.

The accessory minerals are represented by magnetite and apatite. The former forms rare, but fairly large grains, while the latter is observed as small, separate prismatic crystals.

Granite porphyries

They are encountered as small dike bodies.

The rocks have a pinkish-gray color and usually a coarsely porphyritic structure. Large quartz grains of various sizes are readily distinguished macroscopically. They reach 0.8–1.0 cm in diameter and have a rounded, resorbed shape. In addition, small gray plagioclase segregations are visible as tablets measuring 0.5–0.8 mm × 0.7–1.4 mm. All this is set in a light-pink fine-grained groundmass, which has a rough surface in fracture.

The microscope shows a porphyritic structure consisting of quartz and plagioclase phenocrysts within a fine-grained groundmass. Quartz phenocrysts are abundant, noticeably resorbed, sometimes even with narrow embayments of the groundmass into the crystal. A number of grains prove to be cracked and to have angular boundaries, which, apparently, is connected with rapid cooling during crystallization of the magma. The plagioclase phenocrysts prove to be twinned according to the albite, Manebach, or complex laws. Determination of them by the zonal method shows a composition corresponding to No. 16, No. 18 (sample 333). The plagioclase contains a rare, fine dusting of sericite.

The groundmass has variable grain size, even within a single thin section. Among the fine-grained to microfelsitic mass, areas composed of coarser grains are observed. They consist of closely spaced crystals of plagioclase No. 13–15 and isometric quartz grains. Transitions between such areas with different grain size are gradual. The fine-grained areas of the thin section have the same quartz-plagioclase composition, with a quartz content of about 30–35%. As accessory minerals, rare grains of magnetite and apatite are observed. Thin flakes of sericite are either evenly dispersed or become concentrated, forming fibrous lace-like patterns.

On the basis of the significant quartz content and the composition of the plagioclase, the rocks have been assigned to granite porphyries.

Aplites

They are encountered only as thin veinlets 4–5 cm thick, more often 1–2 cm. Such veinlets are observed among the intrusion body near the contact. Many aplite veinlets have been found among quartz monzonites on the northern slope of Nikolsky Golets.

The rock has a pink color, owing to the predominance of K-feldspar, and a fine-grained structure. In addition to K-feldspar, there is much xenomorphic quartz, as well as grains of greenish saussuritized plagioclase. The boundaries with the enclosing rock are sharp, although penetration of K-feldspar into it is noticeable in the form of injections and vein-like masses.

The microscope shows a structure of the rock resembling hypidiomorphic-granular rather than aplitic, owing to the presence of crystal faces on most of the constituent parts. Plagioclase forms smaller crystals than K-feldspar and quartz, with an isometric or short-prismatic habit. The mineral is replaced here ...

with that coarse scattering of epidote which in the central parts of the grains coalesces into a continuous aggregate. At the same time, the composition of the plagioclase decreases to pure albite, as shown by measurements of the optical constants on the Fedorov stage: BNg—73, BNm—17, BNp—85, optic axis [001], which corresponds to albite, No. 2.

Potash feldspar forms elongated, bar-like grains, the angular intervals between which are filled with quartz. The mineral is strongly pelitized and is rather indeterminate. Dense perthitic mottling is noticeable.

Of the accessory minerals, apatite occurs in the form of rare needles, and also titanite, confined to chlorite and, apparently, genetically connected with it. Chlorite has the form of isometric crystals with the properties of clinochlore. It forms intergrowths with epidote and contains within itself a scattering of ore mineral. Apparently, the association of these secondary minerals indicates their formation from a titanium-bearing amphibole.

The mineralogical composition of the rock, calculated planimetrically: plagioclase 18%, potash feldspar 32%, quartz 25%, impurities 5%.

It is necessary to note the appreciable content of albitized plagioclase in the composition of the rock. The considerable decomposition of the feldspars, of the same character as in the quartz monzonites, indicates that the aplites are earlier in age relative to the processes of autometamorphism. This sharply distinguishes them from the albitic aplites from the area of the Znamenitinskoe gold-ore deposit, which appeared there at the expense of solutions that caused autometamorphism.

Contact-Metamorphic Aureole of the Belsky Intrusion

As we already know, the Belsky intrusive massif is situated mainly among a greenstone effusive sequence, and also borders on sandy-marly rocks, which occur in the trough of the syncline. Both these and other rocks in the contact area underwent significant alteration with the formation of hornfelses. In this process the sandy-marly rocks were transformed into dense, banded, albitic hornfelses, while the effusives form certain varieties of metamorphic formations described by us as diabase hornfelses. Incidentally, albitic hornfelses occur to a limited extent also among the latter, being associated with zones of brecciated rocks.

The width of the belt of hornfelsic rocks varies for different parts of the intrusion. Along the western and southern contacts the width of the belt of hornfelsed rocks is small, usually 30–50 m. In the area of the northern contact it is considerably greater and reaches 400–600 m. As for the area of sandy-clayey rocks situated between the first field of intrusive rocks of the Belsky goltsy and the second field of the Nikolsky goltsy in the trough of the syncline, these rocks, lying on the gently inclined surface of the intrusion, underwent strong contact metamorphism. Everywhere these rocks prove to be hornfelsed to one degree or another. Skarns are of limited distribution, possibly owing to the absence of limestones. They have been encountered as patches on the left slope of the Nikolsky spring, and there they were subjected to a small prospecting survey for iron; on the watershed of the Pravy Kamenisty and Nikolsky springs; and on the left slope of Shpilinsky spring (“Hematite Knoll”).

Diabase Hornfelses

These are gray-green rocks possessing a blastoporphyritic structure, which, however, is clearly noticeable macroscopically but is difficult to distinguish under the microscope because of the complete decomposition of the phenocrysts,

represented by plagioclase. Within the fine-grained saussuritic mass there are schlieren-like segregations of medium- and fine-grained material, with now nematoblastic, now porphyroblastic structure, with the development of dark-green amphibole up to 4 mm in diameter amid the grayish-white or greenish-gray mass of saussurite. The amphibole also forms veinlets 1–3 cm thick and less, cutting the rock, or lens-shaped segregations at the boundary of schlieren-like portions with the enclosing mass.

The schlieren-like segregations are more often rounded or ellipsoidal, and sometimes vein-like (sample 70b).

The microscope shows a blastoporphyritic structure. The main mass of the rock consists of relicts of porphyritic plagioclase phenocrysts, situated in a fine-grained saussuritic aggregate, among which small xenomorphic grains of pale-green actinolitic hornblende are scattered. The plagioclase phenocrysts are replaced by a fine aggregate of clinozoisite and albite, with the content of the latter about 40%. Here, too, rounded lemon-yellow grains of epidote appear, either near the grain boundary or in the form of rare chains passing into the cutting veinlets. Thus, the later appearance of epidote is established. In places, among the saussurite, a fine and rather abundant aggregate of sericite is visible.

Among the saussuritic groundmass, separate small albite grains with uneven boundaries are observed. The angle \(Np\) with \((010)\) is \(13^\circ\), which corresponds to plagioclase No. 7. In one case it was possible to measure the coordinates of an albite twin on the Fedorov stage, which proved to be: \(Ng = 15\), \(Nm = 75\), \(Np = 90\), indicating plagioclase No. 6 (sample 71v—1).

The schlieren-like segregations are characterized by the development of porphyroblasts of pale-green actinolite. In places the rock is broken into fragments of saussuritic mass, separated by veinlets of actinolite aggregate with a dense dissemination of magnetite (saussuritic aggregate 30%, actinolite 40–50%, magnetite 18–30%). It is curious that in some parts of the thin section the actinolite is not accompanied by magnetite, while in others the latter rims the actinolite veinlets along the selvages.

Actinolite, in the areas where it is developed, forms an interwoven aggregate. It has the following properties: pleochroism—\(Ng\)—pale blue, \(Nm\)—yellowish-bluish, \(Np\)—pale yellow; \(c : Ng - 24^\circ\); \(Ng - Np = 0.028\); \(2V = -86\).

The porphyroblasts have a sieve-like structure, since they include fragments of the saussuritic aggregate of the groundmass. In individual porphyroblasts, cores and small grains with higher interference colors and an extinction angle equal to \(36^\circ\) relative to the actinolite cleavage were noted, which evidently indicates relicts of diopside.

Coarse-grained actinolite passes abruptly into a fine-grained interwoven aggregate of the mineral, with the same properties. In rare cases calcite can be observed as a fine intergrowth in the actinolite.

The described type of hornfels, developed at the expense of plagioclase porphyrite, in the Annensky area is replaced by a rock of breccia-like texture with light-pink vein-like segregations of albite, as well as lenses and veinlets of dark-green pyroxene. At the same time the pyroxene veinlets cut the pink albite aggregate and have grains from small to medium size. The albite aggregate is very fine-grained and dense, with a porcelain-like fracture.

The microscope shows a blastoporphyritic structure, with the rock consisting of decomposed plagioclase phenocrysts, which are scattered in a loose saussuritic aggregate of albite, sericite, and epidote. The phenocrysts are replaced by the same aggregate of saussurite, but in places also contain calcite.

The macroscopically distinct veinlets of pink, dense material under the microscope prove to consist of a very fine aggregate of albite, filled with disseminated sericite and epidote.

Pyroxene stands out in spots and in the form of veinlets, accompanied by brown sphene, magnetite, and epidote. Usually the pyroxene aggregate, resembling the panidiomorphic texture of eruptive rocks, contains small grains of sphene with pleochroism in brownish-yellow tones. Here and there magnetite appears as large composite grains. But the main mass of this ore mineral is observed in the form of an outer rim in association with fine-grained epidote. These rims encircle the pyroxene veinlets. At the same time, separate spots of fine-grained epidote with a dense dissemination of magnetite can be observed in the rock. Thus, as in the albite-actinolite hornfels described above, the later appearance of magnetite relative to the dark-colored component is manifested. The pyroxene has a barely noticeable greenish coloration, not changing along the axes. Other properties: \(c:Ng = 40^\circ\); \(Nm - Np = 0.029\); \(2V - 60^\circ\), indicate that we are dealing with diopside.

On the outer side of the contact-metamorphic aureole, the diabase hornfels pass into rocks of the effusive series that have been altered to a considerable degree. The primary minerals of the rock prove to be decomposed to one degree or another. Plagioclase is replaced by a dense dissemination of sosurite and is often overgrown by needles of radiating albite, passing into micropegmatite. Pyroxene is replaced by actinolite hornblende. In the groundmass of the rock, new formations in the form of small grains are often manifested. Actinolite, in turn, is penetrated by epidote and calcite. Both minerals form numerous veinlets cutting the sosuritized plagioclase; in this case, the calcite veinlets intersect the epidote ones. The catalyzing agent for the metamorphic reactions in the rocks was stress, which caused repeated manifestations of cataclasis, and often the formation of kakirites and slightly discemented formations. The myriads of pathways formed for magmatic fluids and solutions made possible their free circulation and the wide spread of the “greenstone” alteration of the effusives.

Albite Hornfels

In the watershed area of the Nikolsky and Kamenisty springs, fine-grained sandstones and sandy-marly rocks crop out, transformed to one degree or another into hornfels. As for the sandy rocks, they are better preserved and contain a clearly expressed clastic structure. But the fragments, and especially the cement, have undergone considerable recrystallization. Thus, grains of plagioclase are decomposed with transition into sosurite; the alteration of this mineral proceeds in the form of indentations, grooves from the periphery toward the center. Sometimes only remnants of plagioclase grains remain among an amorphous, fine-grained sosuritic mass.

Fragments of effusives preserve the primary porphyritic texture with a pilotaxitic groundmass. Plagioclase is likewise sosuritized, and chlorite is located among the microlites of the groundmass. The fragments retain traces of rounding and contain traces of resorption. On the outside they prove to be overgrown with a thin film of quartz, which produces an uneven surface of the grains.

The cement of the rock is recrystallized with the formation of a granoblastic texture composed of mosaic, very small grains of albite of spheroidal structure, rarely quartz, rosette-like intergrowths of radial-fibrous epidote, and also chlorite and calcite. Chlorite is arranged in the form of un-

large indistinct spots with fine grains, into which euhedral epidote crystals readily grow. Calcite forms lacy, friable grains, more often confined to the boundaries between clasts and cement.

Sandy-marly rocks form typical hornfelses, in which no traces of clastic structure are detected. However, in the fresh state these rocks possess fine lamination. In the metamorphosed rocks this lamination is preserved, but is altered to a considerable degree. Here there appears an alternation of white or pale-pink bands with green and bottle-green “interbeds.” The former consist essentially of albite, and the second and third of albitized sandstone material.

All of them have a fine-grained, dense fabric. It is curious that the dark-colored areas reveal metamorphic differentiation of the material with the formation of rounded ellipsoidal segregations, in which the central part consists essentially of epidote, while the marginal part is impoverished in it and correspondingly enriched in albite. Such cushions are situated in a green, essentially chlorite-albite mass. White or pale-pink bands cut across the relict lamination (Fig. 7), showing considerable mobility of the albitic material. To the characterization of the textural pattern of the rock one should add the presence of parallel siliceous interbeds consisting of fine-grained grayish-white quartz.

Fig. 7

The banded texture is also well expressed under the microscope. The light areas are composed of a granoblastic aggregate of albite (70%), sericite (18%), epidote (5%), quartz (3%), and chlorite (4%). Albite forms aggregates of rounded grains, which at high magnification show a clear radial-fibrous structure (69a, 69e). Their refractive index is lower than that of Canada balsam. In places an increase in grain size appears, accompanied by the formation of mosaic albite grains with polysynthetic twinning. Then, from the extinction angle with \(Np\) of the albite twins, equal to \(-16^\circ\), the composition of albite No. 4 is determined. Sericite has the appearance either of fine dust or of small, evenly scattered flakes. The smoky-gray, thin lumps in transmitted light prove to be epidote. Quartz occurs as separate mosaic grains. The greenish films are formed by chlorite.

Among this essentially albitic mass, bands and lenses of epidote appear. In the marginal parts of the accumulations it resembles fish roe, owing to the spherical shape of the grain. Here and there annular crystals or hollow crystals of hexagonal form are distinguished within. In the central part of the accumulations, crystals of prismatic habit are developed, associated with calcite. Where calcite appears, epidote forms well-developed prisms growing into the calcite grains. Epidote, in turn, is overgrown by radial-fibrous rosette-like sericite. In individual areas the epidote accumulations are enriched in calcite, which forms a uniform sprinkling. Such bands, consisting essentially of epidote with subordinate amounts of calcite and sericite, evidently correspond to interbeds of the primary rock enriched in lime.

Bands of green color contain a continuous friable aggregate of chlorite, which impregnates the same fabric of granoblastic structure consisting of albite, epidote, and, to a lesser degree, sericite and quartz.

Cushion-like or ellipsoidal concretions of bottle-green epidote under the microscope show a fine granoblastic or porphyroblas-

... a cellular structure. In general, the latter mineral is apparently more mobile than albite and sericite, and is concentrated in the inner part of these concretions, merging into a continuous mass.

The albite hornfelses, occurring among fields of diabase hornfelses in the form of narrow bands, have a mottled coloration in the form of pink varieties of dense, fine-grained albite material in a greenish-gray mass containing, in addition to albite, epidote, chlorite, calcite, and also quartz and pyrite. The fracture is dense, porcelain-like. A palimpsest amygdaloidal texture is observed, containing flamboyant quartz in the segregations. In hornfelses formed at the expense of quartz porphyries, resorbed xenoblasts of quartz are present. In places a blastoporphyritic structure is preserved (Fig. 35), owing to remnants of shattered porphyritic segregations of plagioclase. In the most recrystallized varieties (Fig. 32), a porphyroblastic structure appears, with a hornfelsic groundmass of slightly elongated albite grains with serrated or mosaic boundaries. The porphyroblasts are represented by albite and have serial transitions to the groundmass. Many crystals have a sieve-like character and include small grains of epidote. Sericite dust is noticeable only at high magnification. Measurement of the angle between $Np$ and $(010)$ in twins according to the albite law, which is equal to $13^\circ$, indicates an albite composition of No. 7. Among the albite there are epidote and small ovoid grains of brown sphene with pleochroism from brown to pale yellow. As for epidote, it is characterized by an uneven distribution in the rock mass and the ability to form rounded concretions. Between the albite grains, calcite appears in the form of sieve-like crystals. It also commonly rims grains and accumulations of epidote.

The albite hornfelses developed at the expense of sedimentary rocks, as already indicated, show significant mobility of the albite material, which forms characteristic intersections. In addition to simple recrystallization, a considerable introduction of sodium also takes place here. After the formation of albite aggregates, epidote still had considerable mobility; it produced concentric concretions and layered veinlets. The rocks bear the typical features of adinoles.

Skarns

They are observed as small patches among diabase and albite hornfelses.

The magnetite skarns contain an admixture of iron pyrite, locally in considerable quantity. The microscope shows that pyrite appeared later than magnetite, replaces the latter, and in places forms veinlets that cut it. Among the ore mass there are small windows consisting of corrugated packets of sericite, colored yellowish by iron oxides. The skarns are bordered in the western part of the area by dark-green rock consisting of a fine-grained aggregate of grains of green hornblende, serpentine, epidote, ore dust, and remnants of diopside. The latter is tightly intergrown with green hornblende, which in the marginal parts of pyroxene grains forms broad rims. Diopside is colorless, has a distinct cleavage along $(100)$, and has the following properties: $c:Ng = 41^\circ$; $Ng - Np = 0.030$; $2V = +61$. A constant content of fine magnetite dust in the mineral is characteristic. Green hornblende has sharply expressed pleochroism: along $Ng$—grass-green, $Nm$—bluish-green, $Np$—pale green. Other properties: $c:Ng = 17^\circ$; $Ng - Np = 0.023$; $2V = -62$.

Serpentine either forms large grains after pyroxene (bastite) or replaces it together with epidote and green hornblende. There are ...

the mineral is colored in pale tones: \(N_g\)—pale green, \(N_m=N_p\)—pale greenish-yellow. The birefringence \(N_g-N_p=0.004\). The sign of the principal zone is positive, which, together with the platy character of the grains, permits it to be assigned to antigorite.

Epidote is often developed as large grains having a radial-fibrous structure. Among the ore minerals there is magnetite in the form of rare grains, as well as a fine dusting in diopside, and pyrite, forming irregularly disseminated small nests \(2—6 \times 8—10\) mm in size.

The rock is cut by veinlets filled with asbestos together with epidote. Their thickness is from 1 to \(6—8\) mm.

Hornblende–epidote–serpentine rock is observed over a strike length of 18 m and passes in thickness into light-gray albitic hornfels with a palimpsest porphyritic structure. On the basis of relics of uralite and plagioclase it is possible to determine the primary rock as belonging to a pyroxene–plagioclase porphyrite, which also occurs near the site.

In the eastern part of the site a series of narrow crush zones is found, trending \(47^\circ\) with a vertical dip. They are situated at the boundary of the skarns with the plagioclase-uralite porphyrites and within the latter. In the crush zone the effusives have been transformed into chlorite-sericite schists. At the same time, sulfidization and hydrothermal decomposition of the skarns with the formation of serpentine are associated with them. Apparently, the crush zones served as paths for the movement of hydrothermal solutions.

The skarns of the upper part of the Nikolsky klippe consist of a granular aggregate of garnet, the interstices between which are filled with calcite and quartz. The garnet skarn occurs as bands or zones along the structural trends of the enclosing mass. In individual areas magnetite appears, which sometimes completely displaces the garnet, forming massive ores. The dimensions of such magnetite bodies are insignificant—usually about 20 cm in thickness and several meters in length.

It is necessary to note the change in the composition of the skarns in the northern parts of the area. Along the left bank of the Shipilinsky klippe, among plagioclase porphyrites, small patches of light-green actinolite skarn are observed, within which peculiar cockades of an aggregate of gray-yellow andradite and larger nests of garnet–iron skarn appear.

The actinolite skarn consists of large actinolite crystals up to 2 cm long, gathered into bundles, between which fine-grained magnetite and hematite are situated, as well as grains of andradite. The latter readily groups itself in the form of nests or cockades up to \(2—3\) cm in diameter, surrounded by a rim of hematite. The central part of the larger cockades is hollow or filled with calcite.

The microscope shows a nematoblastic structure and considerable replacement of actinolite by chlorite and calcite. Garnet has anomalous birefringence with weak interference colors. Magnetite forms isometric idiomorphic crystals, while specular hematite is scattered in the form of long plates, sometimes as though growing on the first mineral. In areas of garnet–iron skarn, magnetite is poorly developed. Here specular hematite is widespread, as is later red hydrohematite. The rock is cut by thin carbonate veinlets of symmetrical structure. Their central part is composed of ferruginous carbonate, and the marginal selvages of calcite. The boundary between them is sharp.

Thus, the iron skarns of the northern part of the area have an essentially hematitic composition and a somewhat lower-temperature character. It should be noted that the quartz veins of this site also dif-

are distinguished by the presence of iron luster, together with ferruginous carbonate.

On the question of the genesis of the hybrids of the Belsky massif

The mineral composition, structures, and textures of the xenolith quartz hornblende monzonites studied, which occur in the northern part of the intrusive field or in the upper horizons of the intrusion, definitely indicate their hybrid origin as a result of the assimilation by magma of the country rocks—diabase porphyrites. Indeed, a characteristic feature of these rocks is a peculiar taxitic texture, with the appearance of small, fine-grained schlieren-like areas in the medium-grained material of the main mass. The transitions from xenoliths to schlieren and, finally, to taxitic areas show the genesis of these formations. The mineral composition of the xenoliths is characterized by the absence of potash feldspar, the appearance of a small amount of quartz, and the development of secondary green hornblende. The plagioclase of the porphyrites is close in composition to the plagioclase of the freshest rocks of the quartz-monzonite type. In any case, saussuritization obscures many features of the alteration of plagioclase in this process.

The disintegration of the xenolith is connected with the introduction of mobile alkaline material, which caused the deposition of orthoclase, microperthite, and possibly plagioclase of acid andesine composition. This was followed by the formation of green hornblende, as well as titanite and chlorite, at the expense of the dark-colored components of the diabase porphyrites. Otherwise it is difficult to explain their disappearance from the composition of the hornfelsed rocks of the effusive sequence.

Apparently, the introduction of granitizing solutions was accompanied by the transition into solution, or into a mobile state, and by the partial removal of iron–magnesium–calcium aluminosilicates, and by the appearance of amphibole after the formation of orthoclase and microperthite. Thus, the invading magma had the composition of aplite or quartz syenite. At the same time, the final product of crystallization of this magma was sharply enriched in sodium. The disintegration of the hornfels mass at first proceeds through the appearance of numerous veinlets and small veins of quartz-syenite composition, preferentially along the structural directions of the rock. The veinlets do not have cutting boundaries, but have the character of injections. Subsequently the number and thickness of the veinlets increase, and the rock assumes the character of an intrusive breccia. Intensification of the process in the same direction leads to a new division of the pieces of hornfels into parts, and we pass to xenoliths. It is characteristic that in the Belsky intrusion the xenoliths do not attain large dimensions.

The disintegration of xenoliths leads to a peculiar “clotted” structure, with the formation of disunited clots, groups, or intergrowths of plagioclase crystals. Macroscopically, a schlieren texture is thereby manifested.

The microscope shows, however, that the schlieren have only a smaller grain size and a greater development of plagioclase intergrowths, while having the same mineralogical composition as the main mass. A new stage in the formation of the rock leads to a taxitic texture, through the resorption and crushing of the schlieren under conditions in which the mass has acquired considerable mobility. These conditions are possible, probably, even with a small admixture of liquid. A mobile mass, consisting of crystals and liquid under hypabyssal and tectonically unstable conditions, can move. Even slight local movements may lead to the appearance of a sharp contact with immobile rock. The main mass of the quartz hornblende monzonites has the character of such mixed rocks with a fabric that gives a fine taxitic pattern.

The diversity of rock types of the intrusive massif and the character of their occurrence are due to the peculiar conditions of magmatic differentiation in steeply inclined plutons. This peculiarity is connected with the slow, age-long migration of mobile solutions that continuously changed their composition. The quartz hornblende monzonites of the Belsky complex appeared as a result of the separation of aplite-syenite magma in the upper horizons of the pluton and its injection into the mass of altered diabase porphyrites. This is also indicated by the composition of the vein rocks, among which rocks of quartz-syenite composition predominate. The process of autometamorphism is later and was superimposed on the hybrid rocks with considerable intensity.

Mineral facies of the Belsky complex

From consideration of the Belsky intrusive complex, the variability of the rocks as a result of autometamorphic processes is clearly evident. Field observations show gradual transitions of rocks of the following series: quartz pyroxene-hornblende monzonite—quartz hornblende monzonite—quartz helsinkite—beresite. The sizes of the areas occupied by such rocks progressively decrease, and the last members of this series have the form of pipes. The mineralogical composition of the rocks shows progressive enrichment of the residual solutions in Na₂O, SiO₂, partly Al₂O₃, and also H₂O and CO₂. Thus, these residual solutions give rise to abundant albite, followed by the precipitation of excess silica. It is noteworthy that the appearance of such aplitic solutions can be divided into two stages. The first stage is connected with the appearance of a granophyric residue after the formation of plagioclase and pyroxene. Such a process is well expressed in the Spassky intrusive complex, which is closely adjacent to the gabbro. It also appears sharply in the process of formation of pyroxene-bearing monzonites. The second stage is observed during the deep autometamorphic transformation of rocks corresponding to the helsinkite facies. This facies is characterized by physicochemical conditions under which even actinolitic amphiboles are unstable. It is noteworthy that the second stage of activity of aplitic solutions is preceded by a stage of mobility of Fe, Mg, and Ca, with manifestation of iron-magnesium metasomatism.

The manifestation of iron-magnesium and partly calcium metasomatism embraces chiefly the conditions of the amphibolite facies and, to a lesser degree, the epidote-amphibolite and actinolite–greenstone facies. Under the conditions of the greenstone facies, amphiboles, which during progressive alteration of the rock are the principal representatives of the iron-magnesian minerals in the overlying epidote-amphibolite facies, are unstable and pass entirely into chlorites. In terms of the character of the main typomorphic minerals, the greenstone and helsinkite mineral facies are analogues. As field observations, supported by microscopic studies, show, the helsinkite facies of autometamorphism is characterized by a “lightening” of the rock owing to the removal of mobile chlorites. This process is so characteristic that it is conspicuous even in field observations. The simultaneous introduction of Na₂O, Al₂O₃, SiO₂ causes the appearance of albite and excess silica, forming granophyric intergrowths with the former, and creates a rock of syenitic composition which, by virtue of its peculiarity, should be assigned to the helsinkite facies.

It is necessary to note the increasing mobility of the epidote molecule, beginning with the epidote-amphibolite and actinolite–greenstone facies. As already indicated above, epidote here begins to appear in the marginal zones of plagioclase, replaced by a fine powder of clinozoisite and sericite.

and albite. Then veinlet-like segregations of epidote arise, taking over the entire crystal of saussuritized plagioclase. Further, during the development of albite in place of plagioclase, epidote migrates, being deposited at the boundary between potash feldspar crystals. Owing to the weak crystallization force of albite, large skeletal grains of epidote are often formed in the central parts of its crystals. Despite such inclusions, the albite crystals look perfectly fresh. The crystallization of albite is accompanied by dissolution of fine sericitic mica. Mesostatic development of epidote is noted by N. A. Eliseev for Altai helsinkites¹. The aggregate crystallization of this mineral leads to the formation of radial-fibrous intergrowths or spherulites. Such spherulites are common in quartz helsinkites of our area and are noted by various authors for helsinkites in other areas, including the Altai.

The considerable mobility of epidote is apparently indicated by the blast-like accumulations of epidote (60%) and quartz noted by N. A. Eliseev among the helsinkites of the Altai. Such formations have the shape of small rounded bodies 10–12 m in diameter. Judging by its optical properties, the epidote here is less ferruginous than in other rocks.

It should be emphasized that the metamorphism of the rocks probably takes place at continuously falling temperatures. Connected with this is a whole series of phenomena of decomposition of early minerals, as well as the alteration of minerals toward the formation of low-temperature stable compounds. Thus, the presence of diopside in quartz pyroxene–hornblende saussuritic monzonite, studded with fine grains of magnetite and sphene, apparently indicates the formation of this mineral at the expense of titaniferous augite. Further alteration of diopside leads to the paragenesis—green hornblende + magnetite + sphene. Progressive alteration of these dark-colored components gives the following paragenetic series, which is observed in the rocks studied:

\[ \begin{array}{ccccc} \begin{array}{c} \text{Titaniferous}\\ \text{augite (theore-}\\ \text{tically)} \end{array} & \longrightarrow & \left\{ \begin{array}{l} \text{Diopside}\\ +\,\text{magnetite}\\ +\,\text{sphene} \end{array} \right. & \longrightarrow & \left\{ \begin{array}{l} \text{Green hornblende}\\ +\,\text{magnetite}\\ +\,\text{sphene} \end{array} \right. \longrightarrow \\[2.0em] \longrightarrow & \left\{ \begin{array}{l} \text{Actinolite}\\ +\,\text{chlorite (pennine)}\\ +\,\text{magnetite}\\ +\,\text{sphene}\\ \pm\,\text{epidote} \end{array} \right. & \longrightarrow & \left\{ \begin{array}{l} \text{Chlorite}\\ +\,\text{calcite}\\ +\,\text{magnetite}\\ +\,\text{sphene}\\ \pm\,\text{epidote} \end{array} \right. & \longrightarrow \left\{ \begin{array}{l} \text{Chlorite}\\ +\,\text{calcite}\\ +\,\text{magnetite}\\ +\,\text{sphene}\\ +\,\text{ferruginous carbonate} \end{array} \right. \end{array} \]

In areas of manifestation of hydrothermal mineralization, in zones of crushing, chlorites give rise to abundant iron-magnesian, weakly manganiferous carbonates.

The regular change in the mineral composition of the rocks makes it possible to think of a progressive change in physicochemical conditions in connection with the evolution of residual magmatic solutions and to distinguish a number of mineral facies. The basis for distinguishing them is provided by mineral complexes united in the concept of one rock or another. These complexes have a close genetic relationship and mutual transitions and are shown in Table 6.

Here one should note a very important fact: in the successive differentiation of magmatic fluids, a stage is observed characterized by an abundance of volatile components, namely H₂O, F, and B. The appearance of abundant apatite is associated with the green hornblende of the monzonites, which appeared later than potash feldspar and quartz. Apparently, such an element as

Table 6

NOTE. Arrows indicate transitions from an unstable mineral to a stable one. Crosses connect associations or groups of minerals that appeared at the expense of the primary mineral.

F took a significant part also in the formation of hornblende, which, as is known, is connected with the activity of volatiles. As for boron, which is part of the axinite developed in the exocontact zone, its deposition is connected with the conditions of the helsinkite facies. This facies is characterized by the splitting off of the anorthite molecule and its removal into the exocontact zone in the form of iron-poor epidote. The prelude to the mass appearance of epidote is the deposition of axinite in the form of veinlets and the metasomatic replacement of plagioclase in the country rocks. It is noteworthy that axinite is observed not only in the exocontact zone of the quartz hornblende monzonites of the Belsky complex, but especially strongly in the exocontact of the quartz pyroxene-hornblende diorites of the Spassky complex. This emphasizes the kinship of the two intrusions.

The widely manifested precipitation of albite, completed by albite granophyre, indicates enrichment of the residual solutions in Na₂O, SiO₂, and partly Al₂O₃, which give rise to a special sodium branch of ore solutions.

Conclusion

The paper has examined the character of the intrusive rocks of the Belsky massif, exposed on the eastern slope of the Kuznetsk Alatau, in the area of the watershed of the Chernaya Iyus River and its right tributary, the Izekiyul River. The body has the form of a small synclinal pluton, generally subordinate to the structure of the enclosing sequence and plunging from southwest to northeast in accordance with the line of the fold axis. The age of the intrusion is probably Precambrian.

The rocks of the intrusive massif prove to have been very strongly altered by autometamorphic processes and have a composition that varies from hornblende–microcline diorites and quartz pyroxene–hornblende cossyrite monzonites in the lower levels of the massif to quartz hornblende monzonites, with transitions to quartz syenites, in the upper parts of the body. The transitions between these types of rocks are gradual. The separation of syenitic magma during the formation of the massif is also indicated by the composition of the rocks of the vein facies, among which quartz syenite porphyries predominate. In the upper levels of the massif, hybrid formations appear in the form of xenolithic and schlieren quartz hornblende monzonites. The hybrids are formed through the intrusion of mobile syenite–aplite magma into diabase hornfelses, and through the disintegration and assimilation of the material of the xenoliths.

The quartz pyroxene–hornblende monzonites have close features of similarity with the quartz pyroxene–hornblende diorites of Spassky Golets, which are probably comagmatic formations. The rocks of both complexes show signs of origin from gabbro. These signs include: 1) development of the structure in the direction ophitic → monzonitic; 2) relics of basic plagioclase of labradorite composition; 3) relics of diopside in close association with titanite and magnetite dust, which indicates its origin at the expense of titanaugite; 4) a noticeable quantity of large grains of ilmenite.

The disappearance of pyroxene, together with an increase in the amount of granophyre and the mobile behavior of the latter in the xenolithic formations, makes it necessary to suppose that the beginning of the autometamorphic processes is connected with the appearance of this granophyric or quartz–potash-feldspar material. The processes of autometamorphism began with the decomposition of plagioclase and pyroxene. Together with the decomposition of pyroxene, the femic components acquired considerable mobility. In rocks with an increased content of quartz–potash-feldspar material, pyroxene disappears altogether, and green hornblende develops mesostatically, in places penetrates into quartz along protoclase cracks, or replaces cossyritized plagioclase. A later process of pelitization became regionally developed and led to a reddish alteration of the potash feldspars. The low-temperature character of this latter process is determined by the fact that pelite dissolves in potash feldspar at the contact with carbonate veinlets connected with quartz hydrothermal veins. At the same time, it is also confirmed that pelitization occurred earlier than the appearance of the hydrothermal carbonate veinlets, i.e., the clearly autometamorphic character of this process is revealed.

Along with the described processes of autometamorphism, which affected the entire field of development of the intrusive rocks, more intensive manifestations of this process, having a local character, are observed. Thus, in the form of separate small pipe-shaped bodies there occur light syenitic rocks belonging to quartz helsinkites. The autometamorphic origin of these rocks is beyond doubt, since the rocks have no intrusive contacts and gradual transitions are traced from quartz hornblende monzonites to quartz helsinkites. Alterations of the first type of formations and the transition to the second type of rocks are manifested in the overgrowth of cossyritization of plagioclase by its albitization. In place of plagioclase there remains albite, while the anorthite molecule enters into the composition of epidote, which develops in the form of large skeletal crystals either inside the albite grain or, most often, mesostatically. Hornblende proves unstable; in the transitional rocks it is replaced by chlorite and is removed, causing the formation of a zone of rocks enriched in chlorite around the quartz helsinkites, in which, in this way, the mafic components disappear and the rock acquires—

acquires an almost white or light-gray color and a syenitic composition. The pelitized pink potassium feldspar of the quartz hornblende monzonites is here replaced by fresh, glassy-transparent microcline with a clearly expressed twinning lattice and with transitions into chessboard albite. Finally, among the quartz syenites and quartz hornblende monzonites there occur small patches of fine-grained beresites up to 5 m across. In them the potassium feldspar proves to be decomposed into quartz and mica. Albite is corroded from the side of the quartz and is replaced by the same minerals as the potassium feldspar. To resolve the question of the genesis of the beresites, additional studies of their chemical composition and, in particular, of the character of the mica are needed. It is highly probable that these rocks could have appeared here through the action of the same alkaline sodium solutions.

Thus, the products of autometamorphism of the Belsky pluton show the evolution of magmatic solutions under conditions of falling temperature and changes in the concentration of carbon dioxide, alkalis, and the hydroxyl group in the solution, as well as of other elements. This caused the appearance of mineral facies of the following series, corresponding to mineral associations: amphibolite, epidote-amphibolite, actinolite–greenschist, syenite (greenschist), and chlorite–albite. It should be noted that the mineral facies in the intrusive massif correspond to the mineral facies of the contact aureole. Diabase hornfelses with diopside and the hornblende rim replacing it border the pyroxene–hornblende dioritic monzonites and hornblende–microcline diorites. In areas where quartz hornblende monzonites are developed, diopside among such hornfelses may occur as a relatively rare relict mineral, whereas amphibole is widely developed.

The study of autometamorphism processes is also of purely practical interest, since the most profoundly altered rocks are associated with the occurrences of quartz ore bodies containing sulfide-gold and scheelite mineralization.

1. Eliseev N. A. On the question of the genesis of helsinkites. Questions of the Geology of Siberia, vol. 1, 1945. ↩