M. K. Korovin

THE KUDARA DISTRICT

of Northern Mongolia,

ITS GEOLOGICAL STRUCTURE

and

CONDITIONS OF GOLD MINERALIZATION.

Diploma thesis of a student of the Mining Department,
Mikhail Korovin.

Tomsk.
Partnership “Printing House of S. P. Yakovlev,” Makarovsky Lane, own building No. 4.
1915.

The Kudara District of Northern Mongolia, Its Geological Structure and Conditions of Gold Mineralization

In the summer of 1913, the expedition of Prof. M. A. Usov, at the invitation of the “Society for Ore Work in Mongolia,” carried out geological investigations in the concessions of this society. Over the course of two months, a fairly considerable area of the Tushetu Khan aimag—one of the three principalities constituting Khalkha—was covered. The present article gives an outline of the Kudara placer district nearest to the Russian border, which constitutes only part of the surveyed locality, but nevertheless occupies more than 100 square versts.

The author took part in the expedition as a collector, carried out this work at the suggestion and under the direction of M. A. Usov, and takes this opportunity to express his gratitude to him here.

The rocks collected in the Kudara district made up a collection of 370 specimens.

The processing of the collections was begun with a macroscopic description of the rocks. The description of each specimen was entered on one side of the corresponding label, and, for convenience, a template was worked out that made it possible quickly to systematize the data of the macroscopic study of the rock. The example of a description shown in Table 1 gives a clear idea of this template and of the practical importance it has in the statistical processing of a large body of material.

Table 1.

As is evident from this scheme, the macroscopic description consisted first of all in indicating the features that determine the general appearance of the rock; to this was added a brief description of each separate mineral species entering into the composition of the rock, as well as the secondary changes to which it had been subjected after its formation and, finally, the location of the specimen and the conditions in which the rock occurs.

As regards microscopic investigations, the study under the microscope of all 370 specimens of the collection would have been useless work, since very many of them either completely ...

essentially identical or stand very close to one another. Therefore all the rocks first had to be classified, at least approximately, based partly on summer observations in the field, partly on the macroscopic description of samples already made, and only after that were the most typical specimens selected from the separate groups of rocks. The largest number of thin sections was prepared from granites, since their study was the principal task of the work. Of all 135 thin sections, only 20 were of sedimentary and contact rocks.

The microscopic investigations themselves consisted primarily in determining the simple constants of the individual components, their mutual relations, and also the general microstructure of the rock. The results of the observations were recorded on the reverse side of the sheet with the corresponding macroscopic description, in a special order, illustrated by the example of Table 2.

Table 2.

Structure: Holocrystalline hypidiomorphic.
Order of crystallization: apatite, hornblende, biotite, plagioclase, orthoclase, quartz.

Simple microscopic observations made it possible to classify the rocks more definitely and, instead of this, to select from them only a few types for further detailed study. The latter was based on the application of the Fedorov or universal-optical method. On the Fedorov stage a total of 19 thin sections were studied, and the investigations concerned only the components of the different types of granite, which is almost the only igneous formation of the region and with which the gold deposits of the latter are associated.

The processing of the collection was completed with a chemical analysis of the granite. In view of the uniformity of the granite magma of the district, only one typical specimen of this rock was subjected to analysis. (As for the sedimentary rocks, they were not subjected to analysis, since their detailed description did not fall within the aims of the present work.)

Orographic sketch.

The Kudara placer district is situated in the system of the Kudara River, a left tributary of the Chikoy River, which flows into the Selenga River, and represents a rather strongly dissected mountainous country. The highest point visited by the expedition—the pass between the Amodzhé River and the border river Katantsa—has an absolute elevation of 1300 meters, while the lowest measured point—the mouth of the Belta brook, a left tributary of the Kudara River—is 700 meters. The difference between the greatest and least elevations is, therefore, 600 meters, over a distance between these two points of only 30 versts.

In connection with the considerable difference in elevations, there is a steep fall of the rivers. Most of the valleys, with such a fall, are distinguished by sharp outlines: the slopes are steep, though seldom precipitous; the valleys themselves, especially those of small brooks and the upper courses of larger rivers, are narrow, with a characteristic transverse section. A special place among the river valleys is occupied by the valley of the Kudara River, the oldest in the district. Erosional activity here has already gone far: the fall of the river is comparatively small, the slopes in most cases are strongly smoothed, and the valley itself has attained a considerable width—about a verst even in its middle course. For the Kudara River, which collects the waters of the district, all this is, of course, natural.

The apparently accidental direction of the river valleys, obeying no regularity, in connection with the ruggedness of the relief, determined by the degree of resistance of the rocks to the destructive force of denudation, indicates that all the river valleys of the district are essentially not of tectonic but of erosional origin, and that the sculpture of the mountainous country itself almost does not reflect the internal structure of the latter.

The large number of rivers and the even distribution of abundant atmospheric precipitation over the seasons create rich vegetation in the district. In the river valleys and on parts of the slopes it is represented by mixed forest; all the elevated places—passes, ridges, and steep slopes—are covered with coniferous, more often cedar or pine, forest. More significant

the small rivers in their lower courses, while the Kudara River, even in its middle course, are distinguished by rich grasses.

In general the Kudara district has a severe taiga character. The usual, owing to the abundance of clayey products of the destruction of rocks, swampiness of the valleys, which greatly hampers movement along them and often makes them altogether inaccessible; steep slopes; continuous thickets in the swampy valleys and forests difficult to traverse, with a mass of windfall in the elevated places; finally, the complete absence of any roads whatever, with the exception of animal tracks and occasional hunting trails—all these are characteristic features of the topography of the Kudara district. If to this is added the complete absence of population, then the difficulties which this district presents to the geologist in his field work will be understandable.

The difficulty of the work is further aggravated by the comparatively scanty exposure of bedrock on the slopes of the valleys: everything is covered by products of weathering and by the rich vegetation that has developed upon them. In the valleys, therefore, one often has to be guided by river pebbles, in the best case by stony talus and scree; and on the elevated places the guiding thread is everywhere and throughout supplied by products of weathering, such as granitic grus, schistose rubble, and so forth.

General geological survey.

Geologically, the Kudara district represents an extensive field of graywackes and graywacke schists, which have undergone plicative dislocation and are penetrated by numerous massifs and stocks of granite, with its acidic and more basic facies.

After the Algonkian period, to which the fossil-free rocks of the region may approximately be assigned, the latter, right down to the present epoch, remained dry land, continuously subjected to the action of destructive denudational forces.

The dissected character of the surface of the district, described above, seems as though in contradiction with this ancient age of the rocks and with continuous denudation, whose work should already have ended by smoothing all irregularities of the relief and producing what in geology is called a pene-plain (almost-plain). This contradiction, however, is easily removed if one bears in mind the powerful disjunctive processes that occurred at different times and produced a series of

lines of faults and horsts in the neighboring mountainous region of Transbaikalia *). One of the horsts indicated by V. A. Obruchev lies to the southeast of Kyakhta, also encompassing the Kudara district. These disjunctive processes interrupted the normal development of the river valleys; new cycles of erosion began, giving rise to new river valleys as well, and the work of denudation continued with renewed force.

Over long periods of continuous work, denudation carried away enormous thicknesses of sedimentary rocks, exposing before us a large granite massif in the lower reaches of the Kudara River, evidently uncovered in its central part, and a large number of stocks and stock-like bodies.

The granite outcrops are accompanied by contact-formation belts of varying thickness, which gradually merge into unaltered slates and graywackes.

Such, in general outline, is the relationship among the rocks of the district.

Turning to the individual rocks, we shall first of all direct our attention to the graywacke formation, which is characteristic of northeastern Mongolia **). The most typical member of this formation is a medium-grained, often fine-grained graywacke of gray, sometimes greenish, to dark-gray color. Among its constituent parts one can distinguish irregular gray and greenish fragments of quartz, very small flakes of primary white mica, and often interlayers of pelitic limestone; under the microscope, grains of feldspars and rarely fragments of some dense rocks are also added to them. The irregular network of cracks, sharply manifested in outcrops of the rock, is usually filled with secondary white quartz and yellowish calcite. These secondary veinlets of quartz and calcite are a very characteristic phenomenon in graywackes. Plicative processes, under conditions of increased pressure, caused the transition of graywacke—especially its fine-grained varieties—into graywacke slates.

The comparatively low hardness of the graywackes, and especially of the slates, accounts for their very easy weathering and the almost complete absence of outcrops of these rocks; on very smoothed

) V. A. Obrutschew. Orographie und Tektonik Transbaikaliens*. Verh. d. VII. Internat. Geographen-Kongresses in Berlin. 1899; 201.

) J. Ahlburg. Die neueren Fortschritte in der Erforschung der Goldlagerstätten Sibiriens—Zeitschrift für praktische Geologie. 1913; 112–113.

On the slopes of the valleys only small fragments of graywacke and very fine schistose rubble are encountered.

The primary bedding of the graywacke formation, in the majority of cases coinciding with the secondary schistosity, has a general strike for the district of NE: 60°–70° and a steep SE dip; and near the granitic massifs the schists often prove to be sharply platy, which indicates the considerable volumetric contraction of the sedimentary rocks that resulted from the granitic intrusions.

The igneous rocks of the district are represented chiefly by granite, which cuts through the graywacke-schist formation in the form of numerous separate massifs and stocks. The most significant intrusive bodies are shown on the appended geological map; among them the Lower Kudara massif is especially prominent, its outcrop on the surface of the earth measuring several tens of versts across. In addition, there is also a mass of very small stocks in the district, which it was impossible to record.

In general, granite occupies a considerable part of the district in separate numerous bodies: with such force did the granitic magma break through the graywacke and schist cover that restrained it!

In almost all stocks the granite has the same appearance: fresh—it is colored gray or light gray, sometimes with a greenish, more rarely with a bluish tint; it is somewhat porphyritic, with an average coarseness of grain in the main mass; it is rich in quartz, with a small quantity of dark-colored components, represented chiefly by biotite. Among the eluvium there are often melanocratic schlieren, consisting either simply of the same granite enriched with dark-colored elements, or sometimes of a more basic facies of the granite—diorite. Also common in the granite placers are its vein varieties, denser and fine-grained, which are the result of an additional intrusion of magma; and rare and acid products of differentiation: aplites, dense fine-grained rocks deprived of dark-colored elements, as well as pegmatites—the product of differentiation of the magma in the last phase of solidification, often with white mica and tourmaline. Characteristic of the Kudara granites is the almost complete absence of pure quartz veins; at least under the conditions of observation described above, they were encountered only as an exception.

Among the principal products of the differentiation of granitic magma one should indicate diorite, which often forms veins in granite and sometimes composes entire stocks, for example, in the lower courses of the Budun and Belte streams. Quartz takes a noticeable part in the composition of the diorite, and among the dark-colored elements, alongside hornblende, biotite plays an essential role; thus the rock properly belongs to the quartz diorites.

As the periphery of the granite stocks is approached, the number of vein facies increases, the granite gradually acquires an ever more distinct porphyritic structure, and near the contact the porphyritic granite approaches porphyry-granite, as a product of endogenous contact metamorphism (*).

Outcrops of granite are encountered rather rarely; the rock in them is characterized by coarse mattress-like, more rarely thin slab-like jointing. More common on the slopes are thick granitic taluses, sometimes passing into veritable seas of rocks, for example, on the right slope of the Tsagan-Udzur stream. At the same time the size of the fragments and the thickness of the talus deposits increase toward the margins of the stocks and are especially grandiose in the belts of development of porphyry-granites.

Passing into the contact zone, we see here in outcrops and taluses a massive development of hornfelses, at first in association with the granite, predominantly with its vein facies. In general, in crossing the contact belt, one can almost feel how the granitic magma forced its way into the thickness of the graywacke rocks.

Within the sphere of influence of the granite stock the graywacke rocks become strongly compacted; the schistose varieties, contracting in volume under the pressure of the magma, sometimes acquire a characteristic platy structure, their color darkens noticeably, and the graywacke is transformed into biotite hornfels with a typical hornfelsic structure under the microscope. The principal constituent parts of the hornfels—quartz, feldspars, and micas—mutually intergrow with one another, owing to which the rock acquires the cellular structure characteristic of hornfelses. But in the case under consideration recrystallization did not proceed so far as to conceal the primary bedding of the rock, which is usually well preserved.

Closer to the granite we ordinarily find a banded gneiss-like rock, into which granitic material was injected along the finest cracks and planes of schistosity, trans—

() M. A. Usov. Borderland Dzungaria*. Vol. II, issue 1. Description of rocks. 1911, p. 84.

the primary rock into metagneiss. Here the non-bedded grauwacke is penetrated in all directions by veins of granite, which become ever more frequent; finally, the banded gneiss and injected hornfels pass into granite, containing hornfels material only in the form of separate fragments. These fragments often prove to have been completely remelted, thus forming so-called solution schlieren with imperceptible transitions through metagranite (*) into true granite.

It remains only to note the occurrence in the district of two small outcrops of effusive rocks: albitophyre on the right bank of the Kudara River at the beginning of the large granite massif, and quartz porphyry on the right bank of the Budun stream, below the Kharganta stream. These rocks are of comparatively young age, representing remnants of more extensive lava effusions that occurred in other districts of Northern Mongolia.

Thus, the district investigated consists essentially of a grauwacke formation and granites. Since the latter are the more regular formations, and since the gold deposits of interest to us are associated with them, these rocks are subjected below to a detailed description.

Orthoclase biotite–hornblende granite.

Granite of this type occurs in three stocks of unequal size: the large stock of the middle and upper course of the Tsagan-Udzur stream, which also embraces the headwaters of the Dzerlik and Khota Katangdinskii streams; the small stock of the lower course of the Tsagan-Udzur stream, and the Upper Kudara stock.

In fresh condition the granite of the central parts of the stocks is a rock of gray color with a faint greenish, sometimes bluish tint (in weathered parts—yellowish), in most specimens somewhat porphyritic—with a medium-grained, more rarely fine-grained groundmass and larger porphyritic segregations of feldspar. Under the microscope the rock reveals a holocrystalline, hypidiomorphic structure, with the following constituent parts in the order of their separation: apatite, zircon, titanite, magnetite, hornblende, biotite, oligoclase, orthoclase, quartz. Of the minerals listed, the principal constituent parts are only quartz, orthoclase, oligoclase, and biotite. Horn-

() M. A. Usov, Border Dzungaria*, etc., p. 371.

hornblende occurs only in insignificant quantity and not even in all specimens. Apatite, zircon, and ores are admixtures; of these, only magnetite is common, while the others occur rarely. The above order in which the components of the rock are set apart is given as the most commonly encountered; many granite specimens show more or less considerable deviations, of which mention will be made below.

Quartz occurs in the form of gray, irregular, small and larger grains. Under the microscope the larger grains prove to be irregularly rounded and always have a more or less strongly expressed cloudy extinction; the small grains are more often isometric and extinguish quietly. Both often form polysomes (Fig. 1). The fine quartz material usually forms a cement between large segregations of other minerals; sometimes, however, these small grains form, in the latter, inclusions, being as it were automorphic, which, however, is readily explained by phenomena of resorption.

Large and small segregations of quartz evidently belong to two different phases of crystallization. In the interval between these phases there was pressure, which caused cloudy extinction only in the earlier, large crystals.

Orthoclase is the most characteristic component of the feldspar group, distinguishing this granite from other rocks in the region; its content varies rather strongly, and sometimes it is even almost entirely absent. This mineral forms partly large porphyritic, partly small prismatic crystals, but more often occurs in the form of large irregular grains.

Under the microscope the orthoclase appears strongly kaolinized, the decomposition products, unlike those of plagioclase, covering the mineral as though with a thin film of yellowish color, in places considerably thickening. Most individuals of orthoclase are simple twins and contain intergrowths of albite; the latter have the form of elongated lenses, the long axes of which are arranged in one direction. The size of these lenses varies in cross-section from 0.03 to 0.01 mm.

Large individuals of orthoclase, to a greater or lesser degree, show cloudy extinction and everywhere, side by side, prove to be corroded by residual magma. The fine-grained quartz–feldspar mass formed during the solidification of the latter constitutes, as it were, a cement between the large individuals of orthoclase, while separate grains entered through winding channels into these individuals, having the appearance of inclusions.

Study of the orthoclase on the Fedorov stage, after many measurements, gave an angle between the optical axes of \(2V = (-)64^\circ\). The absence in the mineral of the cross-hatched twinning structure characteristic of microcline shows that here we have a potassium–sodium feldspar of the monoclinic series, corresponding in the diagram*) to No. 10.

Plagioclase is represented in the granite by finely striated oligoclase and, like orthoclase, occurs either as large crystals of porphyritic segregations or as small grains and prisms of the main mass of the rock. The relation between the small and large segregations of plagioclase testifies to the same phenomenon of corrosion or resorption that has already been noted for orthoclase and quartz: as there, the fine material occurs either as inclusions or forms a cement between large individuals of other minerals. The primary pressure, the result of which was in part this resorption, is especially apparent in the curvature of the mineral’s twin striation (Fig. 2). A property characteristic of the plagioclase of this granite is slight zoning.

Fig. 2.

The phenomenon of zoning, together with strong kaolinization of the mineral, makes its microscopic study extremely difficult. The opaque alteration products, in the form of fine dust uniformly covering often the entire section of the crystal and especially the central part of the latter, strongly obscure the fine twin striation, sometimes completely concealing it; often only the character of these weathering products makes it possible to distinguish the mineral from other components of the feldspar group.

Determination of the plagioclases on the Fedorov stage was based chiefly on the measurement of angles, which form—

) M. Usov. The Fedorov or universal-optical method for the study of rock-forming minerals, especially feldspars. Izv. Tomsk Techn. Inst.*, XXII, 1911; fig. 46.

...the twin axis with the axes of elasticity of the mineral. Such a determination, which not infrequently gives a dual solution to the question, was checked by measuring the magnitude and sign of the angle between the optical axes and, as far as possible, by establishing some cleavage plane. The former, in connection with the latter, determined the plagioclase number and the law of twinning; the latter made it possible only to check the composition of the mineral.

A series of such determinations makes it possible to regard the plagioclase of this granite as close to No. 26, with the angle between the optical axes \(2V=(-)82^\circ\), and with angles of the axes of elasticity with the twin axis

for intergrowth according to the Albite law:

\[ \angle B_{1,2} \begin{cases} n_g=-7^\circ,\\ n_m=83^\circ \ \text{and}\\ n_p=90^\circ \end{cases} \]

for intergrowth according to the Manebach law:

\[ \angle B_{1,2} \begin{cases} n_g=80^\circ,\\ n_m=-10^\circ\ *),\\ n_p=88^\circ \end{cases} \]

Checking these determinations with the aid of the cleavage planes along the 3rd pinacoid \((001)\) gave the following results:

\[ \angle B_{001} \begin{cases} n_g=80^\circ,\\ n_m=-10^\circ,\\ n_p=88^\circ \end{cases} \]

which on the curve of the Manebach law corresponds precisely to No. 26. The same measurements were also made for the cleavage plane along the 2nd pinacoid \((010)\):

\[ \angle B_{010} \begin{cases} n_g=6^\circ,\\ n_m=84^\circ,\\ n_p=90^\circ \end{cases} \]

which corresponds to No. 25.

Thus, the plagioclase of this granite has the formula \(\sim \mathrm{Ab}_4 \mathrm{An}_1\), i.e. it must be assigned to the oligoclase series, whose formula ranges from \(\mathrm{Ab}_6 \mathrm{An}_1\) to \(\mathrm{Ab}_3 \mathrm{An}_1\).

In the order of crystallization of the principal components of the granite—quartz, orthoclase, and plagioclase—a rather complex relationship is observed. Each of these minerals appears now idiomorphic, now xenomorphic with respect to the two other components. The study of a large number of thin sections, however, makes it possible to note for them the following order: plagioclase began to separate first, giving the greatest number of idiomorphic crystals; parallel with it, after some time, orthoclase began to separate, being partly xenomorphic with respect to the former, but sometimes forming mutual intergrowths with it. In some cases, where quartz is present in sufficient quantity, the latter takes the place of orthoclase in the order of separation; with

*) Ibidem. Table X.

In this, naturally, there was also such a moment when all these three minerals separated out simultaneously. This moment corresponded, apparently, to the eutectic state of the complex solution: quartz, orthoclase, plagioclase. Such an assumption should not be contradicted by the absence of mutual intergrowth of the components, since a solidifying eutectic may also fail to produce a pegmatitic structure.*).

Biotite, forming not very numerous black and dark-brown spangles and flakes, is distributed in the rock more or less evenly, although it is sometimes also encountered in the form of accumulations—stacks.

Under the microscope the biotite only rarely appears fresh: along the periphery and along the cleavage planes of the mineral there are usually visible black ores, often covering the entire section of the grain with a fine dust and replacing it wholly. False isotropy of biotite is sometimes also observed in fresh flakes, in places where they form accumulations and stacks, when the flakes, overlapping one another, do not have a parallel crystallographic orientation.

Fig. 3.

In fresh individuals, more often encountered as inclusions in feldspar and quartz, biotite is represented by elongate flakes, as if jagged at the ends, often irregularly bent, with good cleavage and distinct pleochroism from a light yellowish-brown to a dark greenish-brown color.

The separation of biotite belongs to the very first phase of the solidification of the magma. The occurrence of inclusions in the biotite of small quartz grains, as if testifying to the separation of the former in the last phase of the solidification of the magma, may be explained by the already mentioned phenomenon of resorption.

Hornblende, encountered considerably more rarely than biotite and by no means in all granite specimens, forms in most cases short prismatic crystals and columns, irregularly bounded at the ends, more rarely irregular grains of black-brown color, sometimes with a characteristic silky luster.

For the most part, the segregations of hornblende have decomposed so strongly, and the secondary products—chlorite and especially ores—so often darken these individuals that to measure, for example,

*) M. A. Usov. “Laws of Physical Chemistry as Applied to Petrography”—Journal of the Society of Siberian Engineers, 1913, pp. 22–25.

the extinction angle of the mineral, varying within the limits of 15°–18°, could be determined only after prolonged effort. The pleochroism of hornblende, weakly distinguishable for the same reasons, has shades from greenish-brown to dark-brown; moreover, the mineral’s own coloration darkens the polarization colors, and between crossed nicols the mineral appears almost isotropic; only the bluish areas of chlorite are fairly clear.

As regards the time of separation of the hornblende, its crystallization together with biotite took place in the very earliest phase of solidification of the magma, after the separation of the ores. In relation to biotite, hornblende is most often idiomorphic and, consequently, separated earlier.

Magnetite is the principal accessory in the granite and, occurring in almost every thin section, forms both regular isolated forms and often very characteristic complex intergrowths. Having separated in the very first phase of solidification of the magma, magnetite occurs as inclusions in all the constituent parts of the rock, whereby it differs sharply from those ores that are observed around the dark-colored components—biotite and hornblende—and are secondary products of the decomposition of the latter.

Apatite occurs much more rarely and in the form of inclusions, most often in biotite, forming very small needles and columns, often distinguishable only under high magnification, with characteristic transverse jointing.

Zircon also occurs as inclusions in biotite and hornblende and is distinguished by sharp outlines and bright polarization colors; it is rather rare.

Titanite sometimes forms rather large brown grains and is characterized by a sharp marginal border and relief, by coarse cracks irregularly cutting across the grain, and by indistinct gray polarization colors.

To clarify the relation between the mineralogical and chemical composition of the granite described, a quantitative chemical analysis was carried out, the data of which, together with the necessary calculations, are given in Table 3 (see p. 17).

Magmatic formulas according to Levinson-Lessing: 1.14 $\overline{R0}$

$R_2O_3\ 7.29\ SiO_2;\quad \frac{R_2O}{RO}=3.2:1;\quad \alpha=3,\ 52;\quad \beta=29,\ 4.$

Table 3.

The data of the same analysis were recalculated into standard minerals according to the method of the American petrographers (*) (Table 4).

Table 4.

This recalculation makes it possible to determine the place of the given rock in the American classification (Table 5).

*) Cross, Iddings, Pirsson, Washington. Quantitative Classification of Igneous Rocks.—Journ. of Geol. 1903.

Table 5.

Let us now turn to the description of the structural modifications of the granite. As one approaches from the center of the intrusive body toward the periphery, the orthoclase granite gradually changes its appearance: the porphyritic character, only weakly expressed in the center of the massif, becomes more and more noticeable, and near the contact zone the granite acquires a sharply expressed and macroscopically porphyry-granite structure. Here it can be established especially well that the three principal salic components occur both in the phenocrysts and in the groundmass; it is necessary to dwell on the explanation of this interesting phenomenon.

The presence of porphyritic phenocrysts of quartz—the latter being the last component, in the order of separation of the constituent parts of our granite—indicates that crystallization in the magma took place in periods, since otherwise the main fine-grained mass of the porphyry-granite would consist only of quartz. But since grains of feldspar are also present in the latter, one must either admit the aforementioned periodicity in crystallization, or suppose that, under the influence of resorption, there occurred a partial dissolution of the first-generation phenocrysts, which supplied material for the formation of feldspars in the principal mass of the rock.

Taking into consideration the sequence in the order of separations that was outlined above, we can give this phenomenon yet another explanation. During the solidification of the granitic magma there was a moment when feldspars—orthoclase and plagioclase—and quartz were separating simultaneously. It was probably at precisely this moment that the intrusion occurred. With such an explanation, both the porphyritic character of the feldspars and quartz and the presence of the former in the principal fine-grained mass of the rock are understandable.

Under conditions of deep-seated existence of the magma, during the first, quiet phase of crystallization, only part of the feldspars and quartz had time to separate out; the remaining part of the feldspar and quartz magma completed its crystallization under new conditions of pressure and temperature, producing the fine-grained groundmass of the granite.

Among the porphyritic segregations of the porphyry-granite, orthoclase attracts attention; it is sometimes also found in the adjacent porphyritic prisms, and in the latter case is distinguished by a poikilitic structure, readily discernible even by the unaided eye. Many individuals have been so strongly penetrated by other constituent parts that they are easily confused with the general mass of the rock, and only perfect cleavage makes it possible to distinguish these crystals. Under the microscope the poikilitic structure of the orthoclase presents a very beautiful picture: within the mass of the orthoclase there seem to float small grains and crystals of almost all the constituent parts of the granite, distinguished by a freshness that they do not possess under other conditions of segregation (Fig. 4).

Fig. 4.

It is rather difficult to explain the origin of such a structure in the orthoclase. It is characteristic that it is peculiar only to orthoclase and has never been observed in segregations of plagioclase. If one takes into account that the prisms of the latter have undergone resorption here, whereas in the massive granite with clearly resorbed orthoclase and quartz the plagioclase was free from any corrosion whatever, then it may be thought that the poikilitic structure represents the strongest manifestation of resorption; it has been found only in the porphyry-granite, where plagioclase proved to have been resorbed and was especially stable.

Study of the feldspars of the porphyry-granite on the Fedorov stage showed that these components of the marginal facies of the granite hardly change their composition, and only in plagioclase is the content of CaO somewhat increased. Thus, in one case the measurements gave plagioclase No. 29 with the following angles:

the axes of the Manebach twin with the axes of elasticity: \(\angle B_{1,2}\)

\[ \begin{cases} n_g=-78^\circ\\ n_m=13^\circ\\ n_p=87^\circ \end{cases} \]

In another case, measurement of a plagioclase triplet gave the following results:

\[ \angle B_{1,2} \begin{cases} n_g=77^\circ\\ n_m=14^\circ\\ n_p=84^\circ \end{cases} \qquad \angle B_{2,3} \begin{cases} n_g=80^\circ\\ n_m=34^\circ\\ n_p=56^\circ \end{cases} \]

Here the intergrowth of individuals \((1,2)\) is subject to the Manebach law, corresponding to No. 27 of plagioclase; \((2,3)\) are connected according to the Carlsbad law and are close to No. 32.

In addition to the macroscopically noticeable porphyritic character of the salic components and the poikilitic structure of the orthoclase, the endogenous metamorphism of the marginal facies of the granite is also expressed in a certain change in the microstructure of the rock. This change is reflected in the sharp manifestation of all those phenomena of primary pressure which were also noted in the massive granite. These include: the polysomatic structure of quartz segregations (Fig. 5), the very strong cloudy extinction of minerals of the first generation—porphyritic feldspars and quartz—and the especially sharply expressed phenomenon of resorption (Figs. 5–8).

Fig. 5.
Fig. 6.

Fig. 7.
Fig. 8.

The pressure that caused the cloudy extinction of the minerals and the polysomatic structure of the quartz took place chiefly between the first phase of crystallization, which produced the porphyritic segregations, and the second, which produced the main mass. This is evident from the circumstance that only the large segregations are distinguished by strong cloudy extinction, whereas the small ones extinguish quite calmly.

As for resorption, it is the result, in general, of a sharp change in the physicochemical conditions of crystallization that occurred in connection with the intrusion of the magma.

As a feature of the marginal facies one may finally point to the numerous schlieren in the granite, observed especially often near the contact zone. Comparing such a schliere with the granite under the microscope, it is easy to note a sharp difference in the composition and structure of these formations: in the composition of the schliere dark-colored components predominate; in its fine-grained structure it approaches hornfelses, while the constituent parts have a xenomorphic character. Evidently these schlieren are not constitutional, since they are common in the peripheral part of the massif, where inclusions of hornfels material are frequent in the granite; it is more correct to suppose that these are schlieren of dissolution.

The described biotite–hornblende granite is accompanied by various facies varieties: vein granite, aplite, and pegmatite.

The composition of the feldspars, which are essential constituent parts of these varieties, is in places subject to some change in comparison with the granite of the massif. Thus, in one vein fine-grained granite the plagioclase has the following optical constants: \(2V = (+)\,75^\circ\);

\[ \angle B_{1,2} \begin{cases} n_g = 83^\circ \\ n_m = 64^\circ \\ n_p = 26.5^\circ \end{cases} \]

According to these constants, the plagioclase, whose twin is composed according to the law of de l’Esterel, corresponds to No. 0, i.e., to pure albite.

But in the same thin section of vein granite, among the generally fine-grained mass of the rock, there is a large grain of ordinary plagioclase. Examination of the latter revealed that the grain consists of several individuals connected with one another according to different twinning laws, with

\[ \angle B_{1,2} \begin{cases} n_g = 86^\circ \\ n_m = 26^\circ \\ n_p = 64^\circ \end{cases} \]

\[ \angle B_{2,3} \begin{cases} n_g = 78^\circ \\ n_m = 12^\circ \\ n_p = 90^\circ \end{cases} \quad \text{and} \quad \angle B_{1,3} \begin{cases} n_g = 81^\circ \\ n_m = 63^\circ \\ n_p = 24^\circ \end{cases} \]

The combination (1,2) corresponds to No. 25 of plagioclase with intergrowth according to the Carlsbad law, and (2,3) to No. 27 according to the Manebach law; the intergrowth of individuals 1 and 3 is not twinned. Thus, the plagioclase of these segregations corresponds to the plagioclase of the principal granite of the massif.

It is interesting that, in aplite, plagioclase likewise belongs to No. 25—7.

As regards the potassium-sodium feldspar, its changes observed in the pegmatites are of interest. In pegmatites with a predominance of quartz and with feldspar grains of medium size, the latter proved to be orthoclase No. 10, with the angle between the optic axes \(2V=(-)64^\circ\). In pegmatites, however, with very large feldspar grains and a decisive predominance of this mineral, measurement on the Fedorov stage gave for the latter an angle between the optic axes of \(2V=(-)83^\circ\), which corresponds to the same potassium-sodium feldspar No. 10, but belonging to the triclinic series; this is also in agreement with the presence of a fine twin-lamellar grating in some sections of the mineral.

Thus, the qualitative mineralogical composition of the various facies of the granite described, which forms several stocks, remains approximately the same.

The description given above of the orthoclase biotite-hornblende granite permits us to outline the principal moments in the life of this rock. In its formation one may suppose two main periods. The first was a period of quiet solidification of the magma under normal conditions of deep-seated existence, at a sufficiently high temperature and uniform pressure; in this phase the porphyritic formations of feldspars and, in part, quartz separated out.

The second period, when the intrusion of the magma took place, is characterized by sharply changing increased lateral pressure and a greatly lowered temperature. Under the new physico-chemical conditions the formations of the first generation were partly crushed, partly remelted; a new complex solution was obtained, which, together with the residual part of the magma, gave rise to formations of the second generation, in the form, as it were, of cement along the periphery of the first crystals and along zones of crushing, or in the form of inclusions that penetrated along cleavage planes and channels of separation of the minerals of the first generation (resorption or corrosion).

At the end of this period, along the crushing zones, still partly filled with liquid magma, gaseous emanations rich in mineralizers penetrated. These gaseous emanations, as the final product of the differentiation of the magma, may explain the appearance of tourmaline, connected with the above-mentioned crushing zones in some rare specimens of granite. The emanational origin of tourmaline in the very last phase of solidif—

the movement of the magma is confirmed by its intrusion into all component parts of the granite, and also by the fact that beside quartz, which is distinguished by very strong clouded extinction, tourmaline becomes extinct quite calmly.

After the final solidification of the magma, additional intrusions occurred more than once, giving rise to vein varieties of granite, aplites, and pegmatites. That the additional intrusions came from the same magmatic basin is shown here by the presence of large segregations of feldspars in the vein granite, as indicated above, which proved to be identical with the feldspars of the massive granite. These segregations were evidently preserved in the residual part of the differentiated magma, from which they were subsequently brought out.

It is only difficult to explain the formation of porphyritic prisms of orthoclase, which occur in all varieties of the granite and are distinguished from the usual porphyritic segregations of porphyry-granites by their size and more regular forms. We do not have sufficient material for a full clarification of this difficult question in the petrography of granites; observations seem to show that all these segregations belong to formations of the same order, and if orthoclase often occurs in especially large, regular individuals, this is probably conditioned by certain crystallization properties of it.

As for the depth of the horizon at which the intrusion of the described granite ended, one may think that this was the extreme peripheral part of the lithosphere. This is indicated by the porphyry-granitic structure of the rock, characteristic not only in the marginal facies but also in the very center of the massifs, and especially by the very fine grain of the groundmass in the marginal facies, testifying to a rapid and strong lowering of temperature after intrusion, as well as to low-temperature conditions in the surrounding rocks.

Microcline biotite granite.

This type of granite in the Kudara district has the greatest distribution here. It composes all the stocks in the western part of the district along the rivers Budunu, Shulogunu, Belte, and along the middle course of the Kudara River; the same granite forms a small stock on the left slope of the headwaters of the Dzerlika River and a number of stocks in the NE of the district in the headwaters of the Kyrysy River.

This microcline granite, both macro- and microscopically, in composition and structure closely adjoins the orthoclase granite, and therefore its description will be purely comparative.

The general appearance of the granite is characterized by a more sharply expressed porphyritic character; at the same time the porphyritic segregations are represented only by feldspar, and only in the peripheral parts of the stocks does porphyritic quartz join them. The dark-colored elements in this granite are represented by biotite alone; the small amount of the latter determines the almost leucocratic character of the rock. The general macrostructure of the rock is holocrystalline, partly porphyritic, with the average grain size of the groundmass being hypidiomorphic.

The composition of the granite, in the order of crystallization, is as follows: apatite, zircon, titanite, magnetite, biotite, microcline, plagioclase, quartz. In comparison with the granite described above, orthoclase is absent here, but a new mineral of the feldspar group appears—microcline.

Among the individual segregations, quartz displays no special features: the same cloudy extinction, the same polysomes and resorption. But all these phenomena are expressed here considerably more sharply. The microcline granite evidently underwent still greater fluctuations of temperature and pressure than the orthoclase granite.

The lime-sodium feldspars show a complete analogy with those in the orthoclase granite. Their investigation with the ordinary polarizing microscope and by the universal-optical method on Fedorov’s stage showed their kinship both in structure and in composition.

Microcline—a new component in the composition of our granites—noticeably predominates over plagioclase and, by rough determination under the microscope, makes up not less than \(^{2}/_{5}\) of the total volume of the rock. It forms irregularly bounded prisms; between crossed nicols it is characterized in most cases by a distinct lattice or by discontinuous, indistinct twin striation, and it is always distinguished from the other feldspars by its freshness. As in orthoclase, it commonly contains microperthitic intergrowths of albite, the amount of which is often very considerable.

Investigation of the microcline on Fedorov’s stage gave an angle between the optical axes ranging from \(2V = (-)\ 84^\circ\) to \(2V = (-)\ 86^\circ\).

Thus, the potassium-sodium feldspar of the microcline granite, in its composition, was identical with the orthoclase corresponding to the same No. 10.

Microscopic examination of a series of thin sections showed that the microcline underwent strong primary pressure, which is expressed in various ways. Almost all the crystals of microcline of the first generation, forming porphyritic segregations in the rock, are distinguished by strong cloudy extinction; in some individuals this is accompanied by intense crumpling of the mineral, expressed in the disturbance of the regular striation, with the twin bands proving to be irregularly bent, as though

Fig. 9.

shifted in one direction; other crystals appear shattered, and only the general orientation of the striation or of the albite bands in individual fragments indicates that the small fragments belong to one large grain (Fig. 9).

This pressure, as in the orthoclase granite, evidently occurred immediately after the crystallization of the large prisms of the mineral. Under the influence, probably, of movement of the magma together with strong lateral pressure, the conditions of crystallization changed sharply, and, in addition to the purely mechanical crumpling and shattering of the first segregations, their corrosion and partial remelting took place. It is characteristic that a particularly sharply expressed phenomenon of resorption is observed in the porphyritic segregations of microcline (Figs. 10 and 11); in the plagioclases, however, it is either very weak or not manifested at all.

Fig. 10.

Fig. 11.

Thus, the formation of the microcline granite evidently took place in the same two phases described above. The uniform pressure and high temperature of the first phase promoted the sp—

final crystallization of large individuals of feldspar and quartz; the increased pressure, caused, probably, by the intrusion of magma, and the new physicochemical conditions of the second phase, together with the crumpling and crushing of the minerals formed earlier, conditioned the formation of fine material, which was deposited along cracks and zones of crumpling and along the periphery of the first crystals.

It is necessary to note that the porphyritic segregations of microcline in this type of granite, in most of their properties, are analogous to the ordinary porphyritic formations of intrusive rocks.

Gneiss-granite.

The third, last type of the Kudara granites is represented by microcline gneiss-granite, or microcline orthogneiss. This granite forms a stock in the upper and middle course of the Khota Katantsinsky River; it also composes a considerable part of the large granite massif in the northern part of the district, along the lower course of the Kudara River and between the Kyrsa and Chikoy Rivers.

Being closest to microcline granite, the gneiss-granite of these intrusive bodies in places assumes the composition of an orthoclase granite; from both types it differs substantially in texture and partly in composition.

The dark-colored elements, being arranged along more or less parallel planes, as is seen in a transverse section of the rock, condition the characteristic gneiss-like texture, which, however, disappears even under weak magnification of the microscope. In the composition of the granite, the only new mineral is muscovite, and in the stock along the Khota Katantsinsky River red garnet is also added to the latter.

Muscovite forms colorless, elongated leaflets with good cleavage, is distinguished by bright polarization colors, and occurs in the form of inclusions in all the constituent parts of the rock. This latter circumstance indicates its undoubtedly primary origin and its separation in the very earliest phase of the solidification of the magma. In addition, macroscopically the muscovite is arranged together with biotite along parallel planes, causing the gneiss-like character of the rock.

Garnet is characteristic of the gneiss-granite of the Khota Katantsinsky River and occurs in rather large, uniformly distributed in the rock, well-bounded crystals with

clearly distinguishable and unaided-eye facets. The color of the garnet is red, sometimes pale yellow; the large quantity of yellow and red garnet crystals accounts for the noticeable yellowish tint of some parts of the granite. Under the microscope the garnet is colorless, is distinguished by sharp relief and irregular coarse cracks; it is isotropic.

In the interrelations of the remaining constituent parts of the gneisso-granite, everything that was noted above is repeated. Somewhat new is only the relationship between quartz and microcline. Always xenomorphic with respect to the other constituent parts, these two minerals often mutually intergrow with one another, and, consequently, their separation, which occurred at the end of the solidification of the magma, continued simultaneously for some time. However, the separation of microcline continued also after quartz, as is indicated by the presence in the microcline of inclusions of large quartz grains—even in those cases when no signs of corrosion in the feldspar are manifested.

Hence the order of separation of the components of the most typical gneisso-granite may be outlined as follows: apatite, zircon, titanite, magnetite, garnet, muscovite, biotite, plagioclase, quartz, microcline.

As was already noted above, in some parts of the stocks composed of the orthogneiss described, the potassium-sodium feldspar is expressed by a monoclinic variety; moreover, the presence of one or another kind of feldspar appears to be connected with the relative amount of aluminosilicate. Where this amount is insignificant, orthoclase appears; where, however, the potassium-sodium feldspar decidedly predominates over the lime-sodium feldspar, microcline is separated, and the gneisso-granite approaches the microcline variety.

Investigation of the potassium-sodium feldspars on the Fedorov stage confirmed the closeness of the gneisso-granite to the microcline and orthoclase varieties of the granites of the district. Indeed, the angle between the optical axes of the potassium-sodium feldspars of the triclinic series ranges from \(2V = (-)84^\circ\) to \(2V = (-)86^\circ\); and for the monoclinic series, \(2V = (-)64^\circ\). Thus in both cases we have No. 10, established for the potassium-sodium feldspars of the described granite types.

Facies varieties—vein granites, aplites, and pegmatites—present no particular features either in the microcline granite or in the gneisso-granite, and therefore there is no need to consider them separately.

As regards the formation of gneisso-granite, the above makes one think that this granite underwent only one phase of solidification. From the very beginning of crystallization, the solidification of the magma here took place under strong pressure, which caused, among the first components of the granite to separate, the appearance of muscovite and garnet—minerals of small molecular volume and high specific gravity—and created that gneissic character of the rock which distinguishes it from other granites. In this phase, apparently, all the magma had time to solidify, as is shown by the almost uniform grain of the gneisso-granite.

General Conclusions

The closeness of the qualitative mineralogical composition and structure of the granites described compels one to think that the parent magma of all three types is common, and that only the difference in the physico-chemical conditions under which solidification took place caused some difference in their structure and partly in their mineralogical composition. The influence of physico-chemical conditions on the structure of the granites was discussed in sufficient detail in the description of each individual type. It is not so easy to explain the differences in the mineralogical composition of the rocks—especially the different manifestation of the potassium-sodium feldspars.

A comparison of the conditions of separation of the latter minerals in the gneissic and orthoclase varieties of granite inevitably suggests the thought whether the separation of these feldspars is not connected with their relative amount. In orthoclase granite, where the amount of potassium-sodium feldspar is in general insignificant, and in those pegmatites where this component is likewise present in a subordinate quantity, only orthoclase separates; in microcline granite, however, in which the potassium-sodium feldspar constitutes no less than \(1/3\) of the total volume of the rock, and also in those pegmatites which represent, as it were, a continuous grain of feldspar with only small inclusions of quartz—in these cases microcline appears.

It is possible that mineralizers play an essential role here, toward which the representatives of the triclinic and monoclinic series behave differently. Whereas orthoclase is common in volcanic rocks and was fairly easily obtained artificially under laboratory conditions (the experiments of Fouqué and M. Lévy*), microcline in effusive rocks is extremely

* K. Bogdanovich. Ore Deposits. I, 1912, p. 24.

it has not yet been possible to obtain by artificial means; since, for its formation, the presence of a large quantity of mineralizers is apparently necessary. The probable connection of these two factors seems to be confirmed by the predominant separation of microcline in especially coarse-grained pegmatites, which, of course, were formed from magma very rich in mineralizers, and which at the same time consist essentially of feldspar. In all probability, the magma that yielded granites with a large quantity of potassic-sodic feldspar was also rich in these mineralizers; the latter here separated out in the form of microcline.

Of course, the difference in the general mineralogical and chemical composition of the Kudara granites is a consequence of the differentiation of the primary magma, and with respect to this process our granites are distributed in the following manner. The least differentiated is the orthoclase granite, which has an average granodioritic composition: the small quantity of potassic-sodic feldspars and hornblende among the dark-colored components bring it close to the diorite group. The microcline granite is already considerably more differentiated: as the dark-colored constituent this granite has only biotite; the quantity of potassic-sodic feldspars increases at the expense of the plagioclases, and the very outcrops of the granite are cut by numerous veins and are accompanied by entire stocks of diorite.

On the basis of such a difference in the degree of differentiation of the primary magma, we may outline the probable sequence in which the intrusion of the described granites took place. Apparently, the least differentiated orthoclase granite of the Tsagan-Udzur type was the first to intrude; the microcline varieties, as more strongly differentiated, intruded considerably later, when the processes of disintegration in the parent magma had already reached the separation of the more acidic fractions of the granite.

In deciding the question of the sequence of intrusion of the Kudara granites, however, we do not possess the most reliable criterion—the phenomenon of inclusion or mutual intersection. Nevertheless, this sequence would not be difficult to establish on the assumption that the lateral pressure connected with the plicative processes and acting, as we have seen, upon the solidifying magma had only one phase.

If the latter assumption is correct, the sequence of intrusions of the granites proves to be precisely the reverse of that just indicated. In this case the gneisso-granite must have been intruded first; its intrusion occurred at the very beginning of the crystallization of the magma and of the manifestation of lateral pressure; the magma undoubtedly came to rest at deep horizons of the lithosphere, as is indicated by the uniform and relatively coarse grain of the gneisso-granite. Next in time the microcline granite was intruded; the orthoclase granite of the Tsagan-Udzur type is the latest formation. Moreover, the magma of the latter granite solidified at a comparatively shallow depth, under low-temperature conditions in the surrounding rocks, since the granite in general has a porphyritic structure with a rather fine-grained groundmass.

The younger age of the granite of the Tsagan-Udzur type is also confirmed by the sharply zonal plagioclase observed in it¹, which is either very weak or entirely absent in the microcline varieties of the intrusive rock: the heterogeneity of the mineral, caused by changes in physicochemical conditions during its formation under the influence of diffusion currents, had already in these granites managed to become smoothed out, whereas in the Tsagan-Udzur granite this heterogeneity was still well preserved. The microperthitic intergrowth of the potassium-sodium feldspars by albite also points to the same thing; it is sharpest in the microcline types and less noticeable in the orthoclase variety of the granite². However, the difference in the structure of the feldspars of the granites being compared may also be explained in part by a certain quenching of the components of the granite of the Tsagan-Udzur type during the rapid solidification of the corresponding magma.

Thus, the sequence of intrusions of the Kudara granites cannot be established with sufficient precision; one can only say that all these intrusions belong to a single geological period.

The study of the geological structure of the Kudara district makes it possible to make a general observation on the conditions of its gold-bearing character.

The gold placers at present being worked along the Tsagan-Udzur River and at the mouth of the latter along the Kudara River give grounds for considering undoubtedly gold-bearing the orthoclase biotite-hornblende-

hornblende granite, since there are no other igneous rocks here with which primary gold deposits are associated; moreover, the gold-bearing character of this granite is proved directly by the occurrence in it of gold-bearing quartz veinlets.

If this is so, then other varieties of granites should also be gold-bearing, as having originated from one magma and from a common magmatic source with the orthoclase granite—only intruded at different times and under different conditions. And it must be said that the broadly organized exploration work in the Kudara district, as well as the work that has been carried out for placer gold in the upper reaches of the Kyrysa River and is now being conducted along the Ulan-Shabortue River, fully confirms the theoretical conclusion regarding the gold-bearing character of the microcline types of granite.

In general, exploration for placer gold, which in recent years has covered almost all of the more or less significant brooks and small rivers of the district, has shown that all of them are, to a greater or lesser degree, gold-bearing. At the same time, the characteristic circumstance is that small rivers cutting through significant granite stocks reveal only indications of gold (the Budun River in its middle course, the Khot-Katantsinsky River); whereas small rivers associated with small stocks or covering only the peripheral parts of more significant stocks (the Ulan-Shabortue River, the Tsagan-Udzur River, the Kudara River) often reveal rich commercial gold. The richness of the gold placers here is determined by some factor apparently not dependent on the nature of the granite.

Such a distribution of the gold placers can be explained only in the following way. Gold-bearing emanations, like all other emanations, on leaving the magma solidifying under pressure, were intercepted by the country rocks—especially by the upper contact belt, where these emanations were chiefly directed; on the other hand, the upper contact belt is always characterized by the presence of small intrusive bodies, as offshoots of the main massif. If we now take into account that the placers arise at the expense of destroyed primary gold deposits, then their connection with small granite stocks and, in general, with the contact zones of the massifs is natural.

The appearance of the placer of the Ulan-Shabortue River, flowing in the lateral contact belt of the large Kyrysa granite massif and in the upper belt of another subterranean stock, is then quite understandable. Also understandable is the distribution, strange at first glance, of the placers—

the occurrence of gold in the Tsagan-Udzur valley, especially rich in its middle and lower reaches—in that intermediate part of the contact zone where gold-bearing emanations were captured from three granite stocks, probably converging at a shallow depth into one common massif.

The valley of the Katanginsky Khoto River appears to be in similar conditions, passing between two large granite stocks; but the general conclusions drawn above concerning the geological structure of the region suggest that here these conditions are not entirely favorable from the standpoint of gold content. Indeed, the two large stocks of this valley belong to two extreme types of granite, the intrusions of which are separated by a considerable interval of time; therefore there is no reason to suppose that they converge at a shallow depth, and the lateral contacts are comparatively weakly gold-bearing.

From the latter point of view, none of the river valleys connected with the large Lower Kudara massif will yield industrial gold. The hopelessness of these valleys as regards gold content is also confirmed by another consideration. The comparatively slight contact alteration of the surrounding rocks and the weak manifestation of granite injection in the schists compel one to assume that what is exposed here is the lower part of the massif—precisely that part where the development of gold-bearing emanations can least be expected. The upper parts of the massif, in which these emanations might have been retained, have been removed by denudation.

The indicated conditions of gold content in the Kudara region, determined by such factors as the character of the granite, the size of the granite stocks, the degree of contact alteration of the schists, and even the relative deep horizons exposed by denudation, make it necessary to say that the promise of each valley here can be finally clarified only after careful geological investigations, each time of a special character.

13 April 1914.

M. Korovin.

Geological Map of the Kudara Gold-Bearing District of Northern Mongolia

Conventional signs:

• Orthoclase granite
• Gneiss-granite
• Microcline granite
• Diorite
• Graywacke

Scale: 5 versts to 1 English inch.

1. M. A. Usov. Laws of physical chemistry as applied to petrography. Journal of the Society of Siberian Engineers, 1913, p. 11. ↩

2. Ibidem, p. 8. ↩