Ovoid Albitites from the “Znamenity” Mine Site in the Kuznetsk Alatau

S. S. Ilyenok

During mine-geological investigations in 1936–38, my attention was drawn to peculiar ovoid albitite rocks occurring at the “Znamenity” mine site in the Kuznetsk Alatau. It is of interest that these rocks are accompanied by quartz veins with rich ore mineralization. Similar formations were described by N. I. Borodaevskii [1] from the area of the “Maly Koran” mine, Uchaly district of the Bashkir ASSR, in the Southern Urals. Closely related rocks were described by B. N. Kupletskii [2] from the area of the Maminskoe gold-ore deposit in the Middle Urals.

Ovoid albitites form relatively small patches with diffuse, indistinct boundaries in small bodies of albitic diorites, among which hornblendites appear in the marginal parts of the bodies, and, in association with the former rocks, chlorite-epidote albitites. In addition, they are of great interest for elucidating the genesis of the rocks of the veins of albitic “aplites” observed among the albitites.

The bodies of albitic diorites occur in the exocontact zone of the Bel’skii pluton, which is of quartz-monzonite composition. The age of the intrusion, as well as that of the enclosing rocks—quartz porphyries, aphanitic and plagioclase porphyrites, and diabases with their accompanying conglomerates and limestone lenses—is assigned to the Proterozoic. A series of exposures of albitic diorites borders a tongue of this pluton, which is elongated in the direction NE:30° and is associated with a tectonic zone into which the magma was drawn.

Along the continuation of the tongue of the Bel’skii pluton, at 480–540 m from the contact, two small bodies of albitic diorites are observed, with outcrop areas of 1200 m² and 240 m² (Fig. 1). They are elongated in the same direction as the tongue of the Bel’skii pluton intrusion, and resemble, as it were, its offshoots or satellites. However, to the east of them there is a series of small bodies of the same type, which already have a different, namely latitudinal, strike. The dimensions of the bodies in cross section are small, on average 15–30 m in length and 8–15 m in thickness, and sometimes are measured only in units of meters. In plan, the contours of these bodies have an elongated or elliptical form. A characteristic feature is the subordination of the bodies to zones of crumpling, in which they are also situated. One zone has a latitudinal strike, and the other a northeastern to meridional strike. The largest bodies are located at the junction of these zones. The later quartz ore veins, which here have an en échelon structure, are also subordinated to the same regularity. Quartz veins in the bodies of albitic diorites alternate with or replace one another along strike; at the same time, each new vein appears on the footwall side in the direction toward the place where the crumpling zones join.

Tracing the small bodies of the indicated rocks to depth along underground workings driven along the ore veins, it may be observed that, with depth, some bodies disappear and others appear. In any case, many bodies have a rootless character. At the same time, local coalescences of separate bodies were observed. In general, no significant increase in thickness ...

bodies is not observed with depth. The dip of the bodies is steep and reaches 80°. In addition, a slight easterly inclination is manifested.

The following features of these bodies should be noted. Their contacts with the country rock are often quite imperceptible and gradual. The enclosing rocks in the area are represented chiefly by plagioclase porphyrites. These rocks here are strongly decomposed, especially at their boundary with the albitic diorites and albitites. The plagioclase in them passes into albite, sericite, and epidote. In the groundmass, besides the minerals indicated, chlorite and calcite occur, often intergrown with one another. Accessory minerals are represented by magnetite, apatite, and leucoxene.

In the endocontact zone a gneiss-like texture is sometimes observed, and then a decrease in grain size toward the contact is noticeable in a narrow belt several cm thick. Albitic segregations in the form of thin syngenetic veinlets are often manifested in the rocks, for their composition is in complete agreement with the composition of the enclosing rock mass. In gneiss-like areas they follow the textural directions, and in massive specimens they form an irregular network.

It is of interest to trace the features of the rocks, their mineralogical composition, and the character of the occurrence of minerals in the varieties that make up the bodies described in the area.

Hornblendites

The rocks occur comparatively rarely and form either narrow vein-like bodies or compose the marginal parts of individual bodies of albitic diorites, with gradual transitions into them.

The rocks have a dark-green color, a massive texture, and are characterized by medium- and coarse-crystalline composition. The latter is characteristic even of small vein-like bodies. The grain size is uniform throughout the rock.

The microscope shows a panidiomorphic-granular structure with well-developed forms of hornblende. The crystals are elongated along “c” by 4–5 times relative to their transverse dimensions. All prism faces (110), pinacoids (010) and (010), as well as terminal faces, are well developed. In specimens containing albite, the latter is present in the spaces between hornblende crystals and is xenomorphic with respect to it. Undoubtedly, here we have albitized plagioclase, for the grains of this mineral contain abundant lumps of epidote and flakes of sericite. Epidote is concentrated at grain boundaries, which is connected with recrystallization and purification of the albite crystal from foreign inclusions. In places a polysynthetic twinning structure is clearly displayed, which made it possible, for example, to determine the following orientation of the twin axis relative to the ellipsoid with the aid of Fedorov’s stage: Ng = 74.5, Nm = 17, Np = 82.5; 2V = +82. Twin axis [001]. Albite No. 4 (specimen 102 g).

In a crystal lacking twinning structure, the following position of the ellipsoid relative to cleavage along (001) was determined: Ng = 78°, Nm = 24, Np = 69; 2V = +80. Albite No. 6 ± 2.

It is interesting that this albitized plagioclase, containing inclusions of epidote and sericite, is overgrown by radial-fibrous albite, to which small xenomorphic grains of quartz are confined.

Hornblende, as shown by the microscope, has a zonal structure. Its groundmass is brown and toward the margins gradually passes into greenish brown. The properties of the mineral (specimen 102-g) in the central part and at the margins are as follows (Table 1).

Table 1

The properties given indicate the closeness of the mineral to brown hornblende no. 48 of Larsen and Berman [3]. The change in color at the margins of the grains is apparently connected with a certain loss of titanium, as was shown by Deer [4] for the hornblendes of the Glen Tilt complex, and perhaps with a change in the character of the ions of this element. In any case, further decomposition of the mineral, with the appearance of chlorite, leads to the precipitation of titanite. The increase in the angle $c : N_g$ along the grain periphery also corresponds to this.

Hornblende is replaced by chlorite. In this process the boundary of chlorite veins is at first rimmed by colorless hornblende, and irregular grains of titanite are observed within the chlorite. In other cases, a rim of colorless hornblende appears where the green-brown hornblende is overgrown by calcite (Fig. 2). Sometimes chlorite is associated with calcite, replacing hornblende from the side of the central part, so that only narrow marginal zones remain of it. The properties of the areas of decolorized hornblende are as follows:

Fig. 2. Appearance of colorless hornblende at the expense of the green variety in places where it is corroded by chlorite or overgrown by calcite. Thin section 102. Magn. 24.

Pleochroism: $N_g$—pale bluish-green, $N_m$—pale green, $N_p$—colorless; $c : N_g = 21^\circ$; $N_g - N_p = 0.029$; $2V = -60^\circ$.

When green hornblende is replaced by chlorite from its marginal zone, Mg and Ti are removed, going into the formation of chlorite and titanite; therefore, colorless hornblende with the properties of actinolite appears in the marginal zone. Further decomposition of the mineral leads to the appearance of calcite.

Thus, the hornblendites, especially varieties transitional to diorites, apparently contained plagioclase, which wholly passed into albite, accompanied by epidote and sericite.

It must be noted that the increased quartz content is accompanied by intense decomposition of hornblende, which is replaced by chlo-

and carbonate (91-6). The latter mineral is of a ferruginous type, for when it is etched with hydrochloric acid a brown insoluble residue remains. In addition, it is accompanied by smears of brown iron ore. Chlorite is represented by large, well-formed elongated grains with lavender-gray interference colors, and in individual fibers by anomalous violet coloration.

Quartz forms granophyric intergrowths with albite, as well as larger grains of mosaic outline. But among the latter, individual faces of a crystal of hexagonal habit sometimes appear.

In the rock, individual bands of crumpling were observed, along which quartz of pavement structure appears, as well as loose calcite and separate grains of pyrite.

The composition of one typical specimen (91-6) of such decomposed rock is as follows: albite-quartz granophyre—32%, chlorite—25%, ferruginous carbonate—30%, sericite—6%, quartz—7%.

Albite microdiorites

The rocks make up the largest bodies, especially in the area of the central part of the latitudinal zone, where at the depth of the 5th level of underground workings a body about 50 m thick has been intersected. They have a grayish-green color, fine or medium grain, and a massive texture. However, strong cataclasis is often observed; chlorite penetrates along thin subparallel cracks, and the rock assumes a schistose appearance. The presence of carbonates is readily detected by weak, and in schistose rocks rather sharp, effervescence under the action of dilute hydrochloric acid.

The microscope shows a rock consisting essentially of hornblende and albite; quartz and chlorite are of secondary importance, still less so epidote and calcite. Accessory minerals are apatite, titanite, and magnetite. In medium-grained varieties the structure is inequigranular, since hornblende forms isometric, rather large crystals with well-formed faces, which are situated among a fine-grained mass of albite and other subordinate minerals. Fine-grained varieties of the rock show a grain size that is uniform in magnitude, but hornblende takes the form of sharp, thin prisms or needles among xenomorphic albite, quartz, and other minerals. The structure in places recalls ophitic structure, but with the reverse content. The place of idiomorphic plagioclase here is occupied by hornblende, and in the interstices between grains of the latter there is not a dark-colored component, as in ophitic structure, but albite.

Hornblende has well-expressed prism faces, the second pinacoid, and also terminal faces, the determination of which in thin section is difficult. In the medium-grained varieties, transitions are clearly manifested from brown hornblende, forming the basis of the grains, to a pale bluish-green, and also to a grass-green variety. Usually the brown hornblende is penetrated in spots and pegmatite-like fashion by the pale bluish-green variety. At the same time titanite is precipitated in the form of small shapeless lumps, and loose calcite penetrates the pale ocher hornblende.

The grass-green variety forms rims at the ends of grains along their elongation and, more rarely, small patches along submicroscopic cracks (Fig. 3). The optical properties of all three varieties of the mineral are summarized in Table 2.

Table 2

The first variety belongs to brown titaniferous hornblende. The colorless variety belongs to actinolite, while the grass-green variety, slightly enriched in sodium, is probably due to the action of residual alkaline solutions from which the albite crystallized.

Hornblende is overgrown and corroded from the side of chlorite (Fig. 4), which evidently formed later than epidote, since it fills cracks in the cataclastic grains of the latter. It is interesting that epidote also forms rims on hornblende crystals (Fig. 5). The boundaries between the two minerals in places have an uneven character, with embayments into the hornblende.

Fig. 3. Green hornblende grows pegmatite-like through the colorless variety. At the ends of the grains and near cracks, grass-green hornblende forms. Thin section 101-6. Magn. 40.

Fig. 4. Replacement of hornblende by chlorite. Specimen 101. Magn. 35.

Albite, as indicated, is xenomorphic in relation to hornblende. It forms isometric, slightly lathlike grains. In places it includes large grains of epidote and flakes of sericite. Determination of the orientation of the optical ellipsoid on the Fedorov stage showed: $N_g = 83$, $N_m = 64$, $N_p = 27$. Twin axis [100]. Albite No. 1.

Large albite grains are overgrown by a rim of a radially fibrous variety in the form of fir-tree-like aggregates or crystals resembling in shape a rocket blade. The angle of maximum extinction with $N_p$ in the zone $(010) = 18^\circ$, which corresponds to pure albite. Usually quartz appears on the outer side of these fir-tree aggregates, sometimes forming a diabasic texture in intergrowth with albite. In places it appears as larger xenomorphic grains corroding the albite (Fig. 6).

Albite in fine-grained rocks sometimes contains inclusions of hornblende, epidote, apatite, and then a pattern appears that resembles the sieve-like structure of metamorphic rocks.

Chlorite is represented by a radial-fibrous variety of pennine, having \(Ng - Np = 0.002\), optically positive, with a very small angle between the optical axes. Sphene is associated with chlorite in the form of small droplet-like grains.

Fig. 5. Overgrowth of green hornblende by epidote. Specimen 106. Magn. 40.

Fig. 6. Growth of quartz on the ends of grains of acicular albite, positioned in turn in lath-shaped albite. Specimen 101-v. Magn. 35.

Chlorite sometimes forms vein-like impregnations in large grains of calcite.

Of the accessory minerals, apatite is the most typical. It forms abundant, thin, and often very long needles. Another mineral of this group is magnetite. It forms comparatively rare grains.

The quantitative mineralogical composition given below shows variable amounts of hornblende and albite. The amount of secondary and accessory minerals changes insignificantly (Table 3).

Table 3

The rocks should be assigned to albite rocks and defined as albitic microdiorites.

Chlorite-epidote albitites

The rocks occur among albitic microdiorites and represent a further stage in the decomposition of the latter under the influence of low-temperature residual magmatic solutions. Here hornblende is unstable and is completely replaced by chlorite, calcite with an admixture of tita-

…a titanium mineral, which here is represented by dark-brown rutile. The boundary of the rocks with the albite microdiorites is quite irregular and difficult to discern with the naked eye.

Macroscopically the rocks are almost indistinguishable from the albite diorites. A strong chloritization of the rock is noticeable, as is the appearance of scale-like segregations of chlorite. In addition, a fine porosity is visible owing to the leaching of carbonate.

The microscope shows that the rock consists essentially of albite with subordinate amounts of chlorite, quartz, calcite, and epidote. Apatite, rutile, magnetite, and pyrite are present in insignificant quantities. The texture is heteroblastic. Albite forms larger crystals than the other components of the rock. It has the form of tablets or slightly elongated lath-like grains with a clear polysynthetic twinning structure. Within the grains, epidote often appears as large skeletal crystals, and sericite as separate scales or in compact clusters. Calcite forms only fracture fillings.

Some albite grains are completely free of inclusions of epidote, which is situated in the spaces between the former.

Determination of the orientation of the ellipsoid by the Fedorov stage showed: 1. Ng—73, Nm—17, Np—87. Twin axis [001]. Albite No. 0 (thin section 101-3); 2. Ng—17.5, Nm—72.5, Np—88. Twin axis [010]. Albite No. 2 (thin section 101-3).

The albite tablets are in places overgrown by a radial-fibrous variety of the same mineral.

Chlorite forms pseudomorphs after hornblende, but has the same radial-fibrous character and optical properties as in the albite microdiorites. In contrast to the latter, it is accompanied not by titanite, but by abundant fine needles of dark-brown rutile, which forms intergrowths (“hedgehogs”).

Quartz is rare and occurs as small xenomorphic grains, usually with radial-fibrous albite.

Calcite intergrows with chlorite and, in general, apparently migrates readily, penetrating along submicroscopic cracks into albite, quartz, and epidote.

The accessory apatite and magnetite possess the same features as in the rocks described above. Apatite appears as abundant fine needles, while magnetite forms comparatively large but rare grains. Pyrite is present sporadically; its large grains are usually bordered by films of brown iron ore.

It should be noted that epidote in places contains a fine dust, apparently products of its decomposition, and then becomes dark gray (a process to some extent analogous to the saussuritization of plagioclase).

Ovoid albitites

They are located in the areas of greatest cataclasis and shattering among albitophyre rocks of intrusive aspect, and therefore have the form of indistinctly expressed bands. Often they occupy the endocontact zone.

With the assistance of stress, which caused the formation of crushed zones of crumpling and the appearance of a dense network of pathways for low-temperature metamorphosing solutions, formations appeared with new, distinctive features of texture and mineralogical composition.

What first catches the eye is the appearance of ovoids of pink feldspar, which impart a pinkish-gray color to the rock. The ovoids usually have a rounded or ellipsoidal form, and sometimes an irregular one with uneven sinuous boundaries. It is interesting that they have different sizes and form now a disorderly arrangement (Fig. 7), now extend in parallel chains (Fig. 8), almost at right angles to the direc—

schistosity. In this case the ovoids are deformed in the direction of the latter. The sizes of these formations average 4–5 mm, but reach more than 10 mm in diameter. The density of distribution of the ovoids varies. They are sometimes sparse and appear as chains, and sometimes abundant and almost touch one another.

Even in the course of field work, attention was drawn to the structure of the ovoids. The fact is that pink albite in the inner part contains a crust of quartz, which forms druses with the terminal faces of the crystals ending in the central part of the ovoid in a field of ferrous carbonate

Fig. 7. Rock with abundant albite ovoids. Black—chlorite. Specimen 31 in natural size.

Fig. 8. Elongation of albite ovoids into chains that do not coincide with the direction of schistosity. Specimen 109.

nate (Fig. 9). Ferrous carbonate appeared at the latest stage, after cataclasis of the rock, and is manifested in it along fine cracks.

The microscope shows a radial-fibrous structure of the albite of the ovoids. It forms either spherulites, the core of which consists of a fine-grained hornfels-like aggregate of albite, or crusts of fibrous albite forming a closed shape. The fibers form broom-like shapes, which terminate in quartz growing on them in the form of a brush of euhedral crystals (Fig. 10). The quartz has a zonal structure, which is characteristic of a mineral formed under conditions of free space [5]. Often the inner part of the ovoids is filled with fine-grained chlorite.

Fig. 9. Albite ovoid containing quartz and ferrous carbonate (black) in the central part. Specimen 48-g. Enlarged 3×.

The main mass of the rock consists of isometric or lath-like grains of albite, either free of inclusions or containing sericite and, only in rare cases, gray epidote from fine dusty inclusions, which here is apparently unstable. Measurements on the Fedorov stage showed the following orientation of the ellipsoid: 1. Ng—73, Nm—17, Np—87, 2V = +82. Double axis [001]. Albite No. 0. Slide 101-b—2.

1. \(Ng = 83,\ Nm = 63,\ Np = 28,\ 2V = +84\). Twin axis \([100]\). Albite No. 1.
   Thin section 101-b—3.

Prismatic albite, judging from the character of the inclusions, formed through the albitization of plagioclase. Along the elongation of its grains there grow rosette-like aggregates of new radial-fibrous albite (Fig. 11). In the interstices of the grains, xenomorphic grains of quartz are observed, passing at their boundary with the latter into thin intergrowths (granophyre).

The rock in which spotted and vein-like albite segregations occur (101-b—2) shows replacement of prismatic albite and earlier quartz by granophyre (Fig. 12). In places, fragments of quartz are noticeable, engulfed and to a considerable degree resorbed from the side of the granophyre. This clearly indicates that before the appearance of the granophyre cataclasis took place. The granophyre is cut by thin veinlets of quartz, which merge and die out simultaneously with the outer rim of quartz encircling the former.

Chlorite has the same character as in the albite rocks and is accompanied by aggregates of fine needles of dark-brown rutile. Calcite replaces

Fig. 10. Part of an ovoid consists of hornblende albite, upon which broom-like albite has grown, and upon the latter—crusts of quartz with well-formed crystals directed into the central field, composed of chlorite. Thin section 109-g. Magn. 40.

Fig. 11. Growth of broom-like albite on the ends of grains of prismatic albite. Specimen 101-3. Magn. 40.

Fig. 12. Replacement of albite and quartz from the side of granophyre. Specimen 101-b. Magn. 40.

chlorite and easily penetrates all minerals along the finest cracks. Sericite, unlike in the previously described rocks, is scattered everywhere, even occurs in quartz, and has the appearance of large scales.

The accessories—apatite, magnetite, and sometimes pyrite—have the same character as in the albite-chlorite-epidote rocks.

In addition to the minerals described in the main mass of the rock, there is superposition of two new stages of carbonates. Veinlets of ferruginous carbonate appear, which readily replaces chlorite and forms in it crystals of rhombic outline in the section of the thin section. The carbonate belongs to the same generation of mineral as that manifested in the central part of the ovoids. The ferruginous carbonate, in turn, is cut, like the other minerals, by veinlets of calcite.

In conclusion it should be noted that formations may be found in which albite proves to be decomposed. This is accompanied by the appearance of mica and abundant quartz and marks the beginning of the stage with which the formation of quartz ore veins is directly connected. Moreover, these rocks are confined to those narrow zones in which the ore bodies formed, and are called “altered wall rocks of the ore bodies.”

Albitic “aplites”

Albitic diorites contain, in places, abundant veinlets of pink “aplite,” which have the character of injections. Their thickness ranges from 4–5 mm to 6–7 cm. At the same time, the thickest veinlets have indistinct, diffuse boundaries and become mixed with the material of the country rock. Macroscopically, among the fine-grained mass of pink feldspar with a grain size of 0.4–0.6 mm, an admixture of finely disseminated chlorite is noticeable.

The microscope shows that the material of the veinlets consists chiefly of albite and quartz. Isometric or lath-like crystals of albite, containing within themselves abundant large flakes of sericite, are bordered everywhere by broad rims of broom-like albite. The latter, in the outer part of these rims, passes into granophyre intergrown with quartz. In places, broom-like aggregates of albitic granophyre form independent grains surrounded by quartz, which usually extinguishes as a single crystal. Thin thread-like veinlets of quartz are widespread; they cut the broom-like albite and extinguish simultaneously with the outer quartz rim (Fig. 13). It may be thought that the veinlets are contemporaneous with the outer quartz rim surrounding the granophyre grains. However, we know of the ability of quartz, when filling fine cracks in a quartz medium, to assume the optical orientation of the enclosing crystals. This feature of quartz was repeatedly noted even in older works on the microscopic study of quartz from ore veins [5, 14], and is often manifested in quartz veins of our district. Therefore, judging from the morphological features of these veinlets, it may be assumed that they formed later than the quartz of the outer rim of the granophyre, following cataclasis of such complex grains. Small, slightly elongated crystals of pure albite with a polysynthetic twin structure occur. They differ from the grains of lath-like albite in the form of the grains and in their size, in the absence of sericite inclusions, and in transitions into the broom-like form of albite. Such a transition is more often manifested along the elongation of the grain.

Fig. 13. Albitic granophyre (in the center) is cut by a thread-like veinlet of quartz, which has one optical orientation with the quartz of the outer rim. Specimen 101-d.

As the dark-colored component, green chlorite occurs, forming felt-like aggregates. The image in crossed nicols shows a fine-grained radial-fibrous structure and an iron-gray, weak interference color. These morphological features, usually characteristic of minerals recrystallized from gels, apparently indicate the low-temperature character of the mineral.

Apatite, occurring as an accessory mineral, is very abundant here. It forms characteristic long, thin needles, usually cataclased and broken into segments.

The ferruginous carbonate present in the rock is associated with the superimposed stage of mineralization, for it cuts the mineral aggregate described above in the form of veinlets. In areas where chlorite appears, it readily replaces the latter, forming within it densely arranged aggregates of rhombohedral crystals.

In sample 31-g, a granoblastic aggregate of albite with a subordinate amount of quartz is manifested. Thin flakes of sericite are included in many albite grains. In addition, calcite is also visible here in the form of felt-like aggregates, as well as fine-grained radial chlorite in the form of smears and diffuse spots.

In sample 113, among the albite aggregate, in the central part of the veinlet, quartz is separated out. In the quartz, fine needles of a mineral resembling rutile are observed. Among the albite aggregate, individual grains of magnetite are visible. Pyrite is present, confined to the quartz, as well as secondary limonite associated with the former.

Thus, the structure and mineral composition of the veinlets show that here we have “aplites” of hydrothermal origin. The principal constituent is albite; quartz is present in subordinate amount and was precipitated later than albite, with a transition into spatially separated areas. The colored mineral is low-temperature chlorite. The secondary minerals are calcite and sericite. The accessories are apatite, magnetite, and pyrite. The superimposed stage of mineral formation is represented by ferruginous carbonate.

Origin of the albitic rocks

The association of lath-shaped albite grains in the examined rock varieties with epidote and sericite, in all the fine details of the arrangement of these minerals, undoubtedly indicates their appearance as a result of desanorthitization of plagioclase. Consequently, initially the rocks had the composition of diorites and of more leucocratic rock types—quartz–hornblende monzonites, widespread near this area, as well as of rarer hornblendites. It is possible that hornblende, as in the quartz–hornblende monzonites of the Bel’sk pluton, also appeared later than the principal rock-forming minerals, during the desanorthitization of plagioclase. This is indicated by its perfect crystal outlines and good preservation in rocks containing epidote. This may also explain the separation of hornblendites in the form of vein-like bodies.

Considering the series of rocks from hornblendites and albitic diorites to chlorite–epidote albitites and ovoid albitites, one can everywhere note the intense action of alkaline sodium solutions, the alteration of the rock through decomposition of the former minerals and formation of new, lower-temperature assemblages. Decomposition of brown hornblende first yields actinolite, titanite, and calcite, and then chlorite. In the low-temperature assemblages the titanium mineral is represented by rutile, which is situated in chlorite. The appearance of a border of grass-green amphibole on grains of brown-type hornblende, or one passing into actinolite, can be explained by an increase in the sodium content of the solution. If, in the albitic diorites, neoformations of snowstorm-like albite overgrowing the lath-shaped grains—pseudomorphs of albite after plagioclase—are comparatively rare, then in the chlorite–epidote albitites they are abundant, and in the ovoid rocks they predominate. The deposition of snowstorm-like albite is followed by the appearance of quartz and the formation of a fine diablastitic structure from these minerals. Quartz forms rims in the form of complete crystals around the granophyric aggregate or snowstorm-like albite with traces of resorption of the latter.

The features of the mineral composition and structures of the described rocks show that the processes of metamorphism here began with desanorthitization of plagioclase, i.e., under conditions of the greenschist facies at temperature conditions on the order of 500°. The rocks were impregnated by a slowly diffusing solution with a significant content of CO₂, H, OH, Na₂O, and SiO₂. The movement of the solutions was facilitated by a repeatedly manifested—

cataclasis of the rock grains. The degree of metamorphism of the rocks here was not manifested uniformly, and undoubtedly depended on the intensity of cataclasis. The conditions of an unstable synkinematic environment, with the temperature of the solution continuously falling and its composition changing, led, after the albite-diorites, to the formation of albite-chlorite-epidote rocks corresponding to the albite-epidote facies and, finally, of ovoid albite-chlorite rocks, in which epidote is unstable. The latter correspond to a new low-temperature facies—the albite-chlorite facies—with typical stable minerals.

The study of ovoid albitites shows a considerable mobility of the albite molecule, which is concentrated in the ovoids. The appearance of ovoids in the form of chains may be explained either by the presence of cracks in the rock, along which ovoids could grow owing to a more intense influx of solution, or by the formation of water droplets in a quasi-mobile, plastic material. The manifestation of even slight movement of this material could have caused these droplets to be drawn out into a chain. In favor of the latter mode of genesis of the ovoids is the presence of gneissosity with the formation of local flows and eddies. In this case the gneissic rocks occur as spots and are connected with movements of the material from below upward along pipe-like zones. The same mode of formation of the ovoids is also indicated by the direction of growth of the ovoids from the walls toward the center, with the formation of geodes and drusy quartz with zonal structure, which appears under conditions of free crystal growth.

Consequently, it may be assumed that ovoid albitites indicate conditions of separation of droplet-like hydrothermal solutions, leading us to the formation of the quartz stage of mineralization. If cracks are present, it can probably be segregated with the formation of quartz veins. The mineralization of these veins at the site, in the form of ore-mineral deposits, may be regarded as one of the final stages in the evolution of an alkaline sodium solution. It should be noted that the rocks described by N. I. Borodaevskii [1] and B. N. Kupletskii [2] are likewise situated within areas of gold-ore deposits. The albitites from the area of the “Malyi Koran” mine of the Uchalinskii District of the Bashkir ASSR in the Southern Urals, described by N. I. Borodaevskii, are very close in mineral association and structure to the albitites described above. They consist of albite, often dusty, especially in veinlets, as well as quartz, potash feldspar, apatite, rutile, ferruginous carbonate. The transitions of the albitites into the enclosing rocks are gradual. The author connects the genesis of the rocks with the metasomatism of mylonites by alkaline magmatic solutions. The ovoid or spheroidal granitogneisses from the Maminskoe gold-ore deposit in the Middle Urals, described by B. N. Kupletskii, consist of quartz (41.9%), albite (44%), muscovite and chlorite (10%), calcite (3.8%), pyrite (0.2%), and apatite (0.1%). The ovoids are distinguished by an increased amount of quartz with an almost complete absence of calcite. Having examined in detail all possible modes of genesis of the rock, this scholar came to the conclusion that its appearance is connected with processes of granitization of a sedimentary rock of the micaceous-sandstone or sericite-siliceous shale type.

In the area of the former “Znamenity” mine, we explored another ore site, at which the albitite rocks described were also discovered. The close connection of this ore occurrence in the area with albitized rocks and albitites may serve as a valuable prospecting criterion not only in searches for new deposits, but also in the exploration of old ones, both along strike and at depth.

References Cited

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