F. N. Shakhov.

Relationships and Contacts of Igneous Rocks with Limestones in the Area of the Bago-Borigchen Copper-Ore Deposit in Northern Mongolia

From the Geological Cabinet of Tomsk Technological Institute.

Introduction

In 1913, Professor of the Tomsk Technological Institute M. A. Usov, on assignment from the Board of the Mining Society of the Tushetukhan and Tsetsenkhan aimaks in Mongolia, carried out geological investigations in the area of the indicated aimaks. On the return journey, on the road to the town of Kyakhta, M. A. Usov stopped at the Karnakovka wintering station on the Iro River and examined the nearby Bago-Borigchen copper-ore deposit. A single traverse was made, illuminating the geological setting of the mine and covering a large tract of the adjoining locality. The material, consisting of a petrographic collection of 47 specimens and a diary, served as the basis for the present work.

Microscopic determinations were carried out by means of the universal-optical method of studying rock-forming minerals. As far as possible, all minerals were determined—both salic and femic. Summaries of the chemical analysis of the rocks were obtained by applying Rosiwal’s microscopic method, which was convenient, since all the rocks are granular, and their principal components—plagioclases and, in part, potash-soda feldspars—were determined by means of the Fedorov method (13 and 6); for the dark-colored components it was necessary to use the analyses appended by Iddings in his book Igneous Rocks, Vol. I (21). In the description of each rock the number of the specimen is indicated, as marked in the diary.

The insufficiency of both the literature and the field geological material, quite understandable in the case of an incidental traverse, in some cases does not make it possible to elucidate categorically one question or another. It is therefore necessary to confine oneself to presenting the factual material and to indicate questions requiring further investigations.

General Geological Sketch

The Tushetukhan and Tsetsenkhan aimaks of Outer Mongolia lie between 50° and 43° north latitude and 117° and 103° east longitude. The Karnakovka wintering station is situated in the northern part of the Tushetukhan aimak on the Iro River, 10 kilometers below the mouth of the Bain-gol River into it (Fig. 1), on the placer road 70 kilometers south of the town of Kyakhta¹. Eight kilometers west of the wintering station is the Bago-Borigchen copper-ore deposit, declared by the Mongols to be a holy place and therefore not being worked.

The geological investigations carried out by Professor M. A. Usov in 1913–1914 are not closely connected with the district of the deposit, but they approach it rather closely (11). On the basis of the works of M. A. Usov (10 and 11) and of the route made by V. A. Obruchev in 1892 from Kyakhta—

to Kalgan (7), plotted on a card of the area of the Karnakovka homestead: scant geological material (Fig. 1).

The entire sedimentary sequence composing the area of the Kentei Range was assigned by Prof. M. A. Usov (10, 919–929) to the Agnostozoic and divided into two formations: 1, the Barkhin Formation, corresponding to the Archean system, and 2, the Grauwacke Formation—the Algonkian system. In lithological composition the two formations have many similar features and are difficult to distinguish from one another. Their principal difference is the exclusive development of grauwacke and clayey rocks and the almost complete absence of limestones in the upper formation (10, 928). In the northern part of the area studied by Prof. M. A. Usov, only the grauwacke formation occurs.

The route of V. A. Obruchev takes in a narrow belt of granite from Kyakhta to the town of Urga. From the Shibeta Iro to the Kuitun River (7, 8) there are outcrops of coarse-grained granite, syenite, and veins of diorite. Along the right bank of the Sharin-gol River there are outcrops of coarse-grained aplite, syenite, and quartz diorite (?)¹ and, finally, nearer the Urmukhtu picket, “a high rocky mountain on the same bank consists of sheer and steeply dipping beds (strike 58°) of gray-green clayey shale, locally abounding in impressions of 2 genera of bryozoans (one close to Fenestella) and of one coral, very similar to Lophophyllum proliferum M’Ch. var.? saurdiens White?, figured by Puxthofen and found by him in the Upper Carboniferous deposits of Lo-ping.”²

The area of the Karnakovka homestead is composed of limestones, replaced to the south by an intrusion of aplite-like granite (I have only one specimen of it). This granite is exposed in small outcrops in low hills to the left of Prof. M. A. Usov’s route, about 4 kilometers southwest of the homestead (Fig. 2). To the right of the route, on the southern side of the following, higher hills, there is a whole series of outcrops and talus accumulations of rocks. The main element here is the same granite, among the debris of which fragments of rocks (Nos. 923 a–f) of dioritic appearance begin to occur; they partly form veins (No. 923 f and No. 923 e), and partly, judging from the schlieren formations encountered here (No. 923 d), inclusions in the main granite massif. Farther on, on the eastern slope of meridionally elongated hills, in the spoil heaps of mole burrows, calcareous contact rocks (Nos. 925 a–e) were found, replaced by gabbro (Nos. 924 a–f), which is exposed at the summit of one hill. The remaining part of the route, as far as the ore deposit, is characterized by limestones cut by veins of aplites and quartz. Finally, in the dumps, and also in the walls of the small shafts of the mine itself, syenitic, lamprophyric, and aplitic rocks were recorded, composing small intrusive bodies of hypabyssal type.

The chief components of the sedimentary sequence in this area are limestones, everywhere altered into marbles (No. 936), and calcareous shales. The marbles nowhere preserve signs of bedding; at least, elements of their attitude are not recorded in the material available to me, but in Prof. M. A. Usov’s description of the area (10, 929) it is stated that both formations composing the Kentei Range dip irregularly and were repeatedly subjected to powerful plicative dislocations. The age of the limestones remains undetermined. On the one hand, the presence of a broad area of the grauwacke formation to the south, at a distance of some 15–18 kilometers from this limestone, gives considerable grounds for assigning the limestones to the upper part of the Agnostozoic. But on the other hand, the presence of thick, well-developed limestones is not characteristic of the grauwacke formation (10, 929). Moreover, 50 kilometers to the southwest, near the Urmukhtu picket, there occur, according to the determ—

¹ The question was raised by V. A. Obruchev (7, 8).

² The text in quotation marks is cited verbatim from the sketch by V. A. Obruchev: “A Brief Geological Sketch of the Caravan Route from Kyakhta to Kalgan.” — St. Petersburg, 1893, pp. 8–9.

According to the data of V. A. Obruchev (7), the sedimentary rocks containing Paleozoic fossils; however, M. A. Usov (11, 22) does not exclude the possibility of assigning the Borigchen limestones to a suite of sedimentary rocks younger than those composing the Kentey Range.

Intrusive Rocks.

Aplite-like granite (No. 922). This rock forms, on the left bank of the Iro River, 4 km southwest of the Karpakovka wintering place, a series of low, small hills (Fig. 2). Its deluvial fragments apparently cover the entire more level area of the district south and east of the route, and only nearer the limestone hills do they become enriched with scree of more basic rocks. Macroscopically this is a holocrystalline, even- and medium-grained rock of pink color with a slight grayish tint, representing a mixture of pink feldspar with grayish quartz. A small amount of an indistinct dark-colored component gives it an aplite-like appearance. Under the microscope the following minerals are found: microperthitic microcline, oligoclase, quartz, and admixtures of magnetite, rare aggregates of biotite flakes, muscovite, and zircon.

Microcline—with microperthitic intergrowths of albite and with characteristic twin striation. Sometimes the latter is not manifested, being obscured by irregularly branching intergrowths of albitic material, and then the grain has a distinctly irregular structure, expressed in disorderly extinction, which spreads as if in patchy knots throughout the grain. In the mineral, cleavage is generally poorly expressed,\(^1\) for which reason it was necessary to be limited to the constant \(2V\), which, when observed in several grains directly of both optical axes, gives a persistently maintained value \(2V = -85^\circ\). Birefringence: \(n_g - n_p = 0.0062\), \(n_m - n_p = 0.0027\); the angle of the optical axes, by calculation, is equal to \(-82^\circ\).

Quartz—in large quantity, fractured, and in most cases with clouded extinction. Grains intergrowing with one another, of irregular outline, are sometimes encountered.

Oligoclase—in subordinate quantity relative to microcline, with very fine twin striation and at times with distinctly manifested zoning. It is usually sericitized and has a strongly developed clayey coating. Cleavage is more readily manifested than in microcline. Destruction is also much more strongly manifested than in the alkali feldspar. Twins according to the Albite and Manebach laws are observed. In \(\perp (010)\) \(n_g = 10^\circ\); in \(\perp (010)\) \(n_m = 79^\circ\); in \(\perp (010)\) \(n_p = 89^\circ\); in \(\perp (001)\) \(n_g = 83^\circ\), in \(\perp (001)\) \(n_m = 4.5^\circ\), in \(\perp (001)\) \(n_p = 77^\circ\). Plagioclase No. 13. The composition plane in both cases is (010). \(n_g - n_p = 0.0077\); \(n_m - n_p = 0.0037\); \(2V = +88^\circ\).

Biotite—a very small quantity of thin flakes, gathered into complex aggregate-like clusters, irregularly arranged and bent. Sometimes it fills thin cracks in the microcline, being secondary (Fig. 3), probably connected with possible contact metamorphism of the rock under the influence of later-injected gabbro-syenite rocks.

Muscovite fills a fairly large veinlet (in the thin section) among feldspar grains (Fig. 4), in bent leaves, being, in all probability, a product of post-volcanic processes.

\(^1\) It is possible that the reluctant manifestation of cleavage in microcline is a consequence of the mineral’s incomplete homogeneity, in view of intergrowths of albite and thin twin lamellae in two mutually perpendicular directions, penetrating one another and thus reducing the difference in resistance along different directions (6, 374).

Chemical analysis of the rock was carried out by the method of Rosival.

	Microcl.	Quartz	Biotite	Plagioclase	Plagioclase	Magnet.	Total
$\sum$ sections	1993	2788	252	1213	1213	51	6327
% by volume	31.52	44.07	3.98	19.63	19.63	0.80	100%
Sp. gr.	2.57	2.65	3	2.68	2.68	5.17
				Ab. 19.77	An.
% by weight	30.39	43.81	4.48	17.20	2.57	1.55	100%
SiO$_2$	19.75	43.81	1.63	11.87	1.10	—	78.16
Al$_2$O$_3$	5.47	—	0.85	3.27	0.95	—	10.54
Fe$_2$O$_3$	—	—	0.25	—	—	1.07	1.32
FeO	—	—	0.67	—	—	0.48	1.15
CaO	—	—	0.03	—	0.52	—	0.55
MgO	—	—	0.44	—	—	—	0.44
Na$_2$O	—	—	0.02	2.06	—	—	2.08
K$_2$O	5.17	—	0.42	—	—	—	5.59
H$_2$O	—	—	0.12	—	—	—	0.12
TiO$_2$	—	—	0.01	—	—	—	0.01
MnO	—	—	0.01	—	—	—	0.01
Total							100%

Magmatic formulae according to Levinson-Lessing:

$$ 1.18\,\overline{RO}\cdot R_2O_3.\ 11.8\,SiO_2 $$

$$ \alpha = 5.65,\qquad \beta = 18.46,\qquad \gamma = 5.41, $$

$$ R_2O : RO = 2.5 : 1. $$

According to the chemical classification the rock approaches alaskites (4). The high content of SiO$_2$ is unusual for analyses of ordinary granites, for the most acid granite in the literature material at my disposal is Rapakivi granite (18, 12), containing 77.71% SiO$_2$, while in Rosenbusch (23, 8) the quartz content in granites does not exceed 76.44%; however, for granite-aplites there are figures suitable for my analysis (23, 43). Neglecting the small amount of dark-colored components (increasing at their expense the percentage content of plagioclase), we obtain, in round numbers: quartz 44%, plagioclase 26%, and microcline 30%. Plotting the obtained ratio on Foth’s diagram of the ternary system quartz—plagioclase—orthoclase (30, 45), we obtain a place (point a) for our rock in the field characteristic for distinguishing, in the first instance,

quartz, then orthoclase, and only then the ternary eutectic (Fig. 5). Since, in view of the perthitic structure of the microcline, the content of Na—and consequently of plagioclase—cannot be taken into account by Rozival’s method, point a should in fact move to the right, closer to point e, and the question of the order of crystallization of the feldspars becomes indeterminate. Quartz is in a different position. On the basis of the quantitative ratios of the rock-forming minerals, it should have crystallized first and therefore possess a certain idiomorphism. In reality, neither in hand specimen nor in thin section, despite all the care taken in observation, were such phenomena noticed.

This contradiction, seemingly rather sharp, may be smoothed out by the following considerations. In order for the figurative point a to reach the eutectic line \(E_{Qu-Or}\), the quartz content must be reduced from 44% (approximately) to 33%, i.e., by 11%. Consequently, only 25% of all the quartz can claim idiomorphism, i.e., the remaining part must have crystallized simultaneously with the other components. If to this one adds that, in connection with the precipitation of the eutectic ternary mixture, corrosion of the faces of quartz crystals is possible and natural, then the absence of crystallographic boundaries in this mineral in rocks close in composition to the ternary eutectic will not be surprising. Of course, macroscopically the quartz should have manifested itself, but for this observations are needed in situ at an outcrop, where there is a large amount of material. Incidentally, in Prof. M. A. Usov’s Description of the Rocks of the Border Dzungaria (12, 11), in the description of the granite of Kozhurtas, there is a mention that, despite the magnificent quartz crystals occurring in the rock, under the microscope the idiomorphism of this mineral is considerably obscured by corrosion of the faces from the feldspathic substance.

Recently an article by N. L. Bowen appeared in the literature (16, 193), in which the author demonstrates the probability of the absence of eutectic relations during the crystallization of magma. Considering the eutectic as a possible special case of crystallization, Bowen points out that the latter must proceed chiefly according to the type, as he expresses it, of “continuous reaction series,” i.e., by means of a continuous reaction between the precipitating solid and the remaining liquid phases, so often manifested in silicates in view of their tendency to form solid solutions (e.g., albite—anorthite). As for quartz, its order of crystallization, in Bowen’s opinion, must depend on the role it plays as a mineral in the primary composition of the magma. Namely, if, in the incipient mineralogical composition of the rock in the magma, it plays the role of a normal primary mineral, then, in accordance with its sufficiently high melting temperature, it crystallizes first; if, however, it is, in Bowen’s expression, a “released-mineral,” i.e., a mineral formed from the residues left by the crystallization of the other minerals of the rock, then, naturally, it fills the spaces between the previously formed mineral components. Such a position, when applied to the order of crystallization of minerals in rocks, readily provides a theoretical explanation of Rosenbusch’s empirical scale, developed by a purely statistical method and expressing the most widespread actual order of crystallization of minerals.

Applying the view set forth above to our rock, it would be necessary to explain the xenomorphic outlines of its quartz by its role as a “released mineral” during the crystallization of the granite; but, taking into consideration the excessively high percentage of quartz in the rock—44%—it is hardly possible to suppose that it played the role of a residual mineral. Most likely, crystallizing first (independently of Bowen’s principle or of the eutectic) in a limited amount, the quartz lost its idiomorphism owing to corrosion of its faces, which occurred during the crystallization of the subsequent components of the rock.

Closer to the periphery of the intrusive body, rocks of a more basic dioritic aspect are encountered (Nos. 923 a–f). According to the diary these are dike rocks, but probably in part also marginal facies of the same aplite-like granite.

Biotite–hornblende quartz diorite (No. 923a). A thin section of a strongly weathered, dark greenish-gray, somewhat mottled rock. Macroscopically, very few porphyritic segregations of plagioclase of a dirty-green color are visible. The texture is medium-grained, hypidiomorphic. The chief components of the rock are plagioclase, quartz, biotite, and a very small amount of hornblende. Accessory minerals: apatite, magnetite, and titanite.

Plagioclase has fine twinning striation, in very zonal, slightly columnar crystals. The smaller grains show a stronger tendency to elongate, approaching lath-like forms. A strongly developed clayey coating did not make it possible to determine precisely the number of the plagioclase; but judging by the angle between the optical axes—\(2V = +88^\circ\)—and taking into account the fine twinning striation, the composition of the mineral may be assigned to oligoclase-andesine. The mineral is strongly kaolinized and sericitized.

Quartz occurs in small grains of jagged form and fills the interstices between the feldspars. Sometimes, as in the granite, intergrowth of individual grains with one another is observed. Individuals of larger size also occur; in this case they lose their serrated form and have a capriciously sinuous, resorbed character (Fig. 6) in their outlines. Always with cloudy extinction.

Biotite. Characteristically ragged individuals, with a large number of inclusions of magnetite and apatite. It often assumes a greenish coloration. It also occurs in the form of aggregate accumulations of small scales.

Hornblende is strongly altered and in places has completely passed into chlorite. In the thin section, in grains, a brownish shade predominates. \(2V = -58^\circ\). The extinction angle \(c n_g = 21^\circ\). The angle between the cleavage planes \(\varphi = 60^\circ\). Pleochroism is weak, from dark brown to light yellow with a slightly greenish tint. Along the cleavage cracks an accumulation of magnetite is noticeable.

Deviations from normal properties in the minerals: biotite acquires a greenish coloration, with weakening of the polarization colors, and also contains chlorite and small ore grains along the cleavage cracks; hornblende is decolorized and likewise has ore deposited along the cleavage cracks; the feldspars are sericitized and have a strongly developed clayey coating; here and there grains of zoisite and tremolite are visible.

In addition to this rock there are samples (No. 923b, No. 923c) which apparently represent transitional members from the diorite described above to more basic formations. Both are marked by finer grain, are compacted, dirty green with products of destruction. They contain a large quantity of sulfides. In the first, a rock with coarser grain (No. 923b), the principal background is secondary acidic plagioclase. The primary plagioclase in most cases has passed into a mixture of epidote with actinolitic hornblende and chlorite. The dark-colored, indistinct component has entirely passed into uralite and has strongly indented contours. There is much titaniferous iron ore in large grains and small inclusions. The second rock (923c) has a finer grain, is of an aphanitic aspect, and under the microscope represents a confused-fibrous mass of secondary hornblende. In places saussurite-like products of destruction show through, resolving into secondary plagioclase and zoisite. There is an exceedingly large amount of titaniferous magnetite and sulfides.

The rocks are so destroyed that it is difficult to restore their primary physiognomy. It must be thought that the destruction occurred under the influence of postvolcanic processes, so natural in the apical-peripheral part of an intrusive body (2, 52–70). Here, together with these rocks, there are encountered—

is a typical example of schlieren formation, once again emphasizing the facial character of these rocks (2, 60–63).

Dissolution schlieren (No. 923 d) is a dark-gray, with a greenish basic groundmass, rock, as though filled in places with medium-grained pink microcline with a small amount of quartz. Here and there occur black patches of a dark-colored component (biotite). The texture is hypidiomorphic. The principal components are: oligoclase, microcline-microperthite, quartz, aggregates of biotite, and some other dark-colored component, which are products of fragments of the continent that were not fully digested in the magma.

Oligoclase is idiomorphic, sericitized, with a considerable clayey film. In almost all grains a sharp zonal structure and fine twinning striation are manifested. Bng = 13°, Bnm = 78°, Bnp = 83°. The coordinates of the twin axis give an ambiguous solution: 1) under the twin law ⟂ (010)—No. 10–13 or No. 28–30; 2) under the twin law [010]—No. 4–5 or No. 28–30. The twin plane along (010) causes the natural exclusion of twinning according to the pericline law. 2V = +85, determined by direct observation of both optical axes, gives on diagram (13) No. 13 of plagioclase. Thus, plagioclase No. 13 forms twin intergrowth according to the albite law, which is very characteristic for fine twinning striation.

Microcline-microperthite—fresh grains of irregular outline. Sometimes the lattice characteristic of microcline is clearly manifested. It is always xenomorphic with respect to plagioclase and is, as it were, clamped between the grains of the latter. 2V = −83° for an area without microperthitic intergrowths. The latter are arranged in a definite direction, close to the plane (100), since they appear most sharply when this plane coincides with the plane of symmetry of the microscope. Cleavage is not manifested.

Quartz—xenomorphic grains with cloudy extinction, strongly fissured; sometimes breaking up into small grains, creating the impression of crush belts. It is definitely xenomorphic not only with respect to plagioclase, but also to the alkali feldspar. It is interesting to note an original picture: a fissure in microperthitic microcline is filled by splinters from two quartz grains, differently optically oriented (Fig. 7).

Aggregates of flakes of biotite, bleached and in places altered to chlorite, mixed with some unclear component, ore grains, and at times epidote, indicate the schlieren character of the rock. Accessories are magnetite, zircon, and pyrite. The latter, in all probability, is a product of post-volcanic processes. The strong sericitization of the feldspars, as well as the appearance of epidote and chlorite in the dark-colored components of both this and the preceding rocks, must partly be ascribed to the influence of these same processes. It is interesting that all grains of plagioclase, however strongly affected by destructive processes, preserve an untouched external zonal, apparently albite-like, rim of slightly fringed outline.

Among the rocks described there occur representatives of the vein facies of the granitic intrusion: aplite (No. 923 e) and biotite granodiorite-porphyry (No. 923 f). The latter rock, occurring in the marginal zone of the granitic body, has a clearly expressed relation between the plagioclases of the porphyritic phenocrysts and those of the groundmass (the external zonal rim of the plagioclases of the porphyritic phenocrysts has one and the same number as the plagioclase of the groundmass), indicating that crystallization took place at a lowered temperature without assimilation of foreign material. Probably this rock composes a vein. Therefore, while not excluding, of course, the possibility that the granodiorite-porphyry may be part of the peripheral facies, I, owing to the absence of elements of occurrence, conditionally refer its description to the vein rocks.

Farther along the route, on the summit of the nearest hill lying to the north, basic rocks of the gabbro type crop out (Nos. 924 a–f), surrounded by a contact belt of limestone formations. In the material at my disposal there are no elements indicating the mode of occurrence of this body, so that it is not possible, on the basis of these data, to determine its relation to the granite intrusion. But if one takes into account the character of the intrusions of the Kentei Range (10, 233–242) and their composition, where such basic products of differentiation are not encountered on the periphery of intrusive granite bodies, and also the mechanical contact, described below, of the granite batholith with limestones including gabbroic rocks, then it will be natural to regard the latter, together with the syenites, as constituting an independent, more juvenile formation of abyssal-type igneous rocks.

Hornblende–diopside gabbro is represented in the collection I have by dark-gray rocks, of various grain size (from coarse- to medium-grained), consisting of a mixture of plagioclase with diopside and hornblende, poikilitically intergrowing with one another and varying in quantitative proportion. In parts nearer the contact pyroxene predominates; in those farther away, hornblende; moreover, in immediate proximity to the contact zone pyroxene completely displaces hornblende, while plagioclase, increasing its basicity, remains in very few grains poikilitically intergrown by pyroxene. The poikilitic intergrowth of the minerals is extremely characteristic of this rock, especially with respect to the dark-colored components, as was also emphasized by Rosenbusch (26, 345) in describing hornblende gabbro. Among the accessories, pyrite is very widespread, often passing into brown iron ore. It is of interest that the coarsest-grained varieties contain the greatest amount of it.

Plagioclase occurs in large grains, which in the overwhelming majority of cases have a poorly expressed prismatic character; it must be said that crystallographic boundaries in general are poorly developed in the components of the rock. It often forms complex twins according to the pericline and Manebach laws (Fig. 8), giving the impression of a coarse oblique lattice. Twins according to the Manebach law are more common. The composition plane in the Manebach law is always (010); in the pericline law it is close to (001). The angle between the optic axes fluctuates from \(+83^\circ\) to \(+85^\circ\). The plagioclase number is 74–77.

Hornblende is more often brown, sometimes greenish, with sharp cleavage cracks, in very large grains poikilitically intergrown by diopside. Crystallographic boundaries are never observed. In its constants it approaches uralite hornblende from hornblende gabbro (from the Urals), also containing colorless diopsidic pyroxene and analyzed by Rosenbusch (26, 346). \(2V = -74^\circ\); extinction angle—\(C n_g = 15^\circ\); angle between cleavage planes \(\varphi = 60^\circ\). Pleochroism: along \(n_g\)—yellowish-green, \(n_m\)—greenish-brown, and along \(n_p\)—light yellow.

Diopside occurs mostly in xenomorphic grains of elliptical outline, poikilitically intergrowing feldspar and hornblende. Occasionally grains of prismatic appearance occur, with frequent and sharp cleavage cracks. \(2V = 54^\circ\); extinction angle \(C n_g = 40^\circ\); angle between cleavage planes \(\varphi = 86^\circ\). In its constants it is closer to diopsidic augite (25, 209).

None of the components has precedence with respect to crystallographic boundaries, which creates the typical gabbroid structure.

Secondary alterations of plagioclase (kaolinization and sericitization) become more intense with decreasing grain size of the rock. This phenomenon involuntarily suggests the effect on the rock of metasomatic processes in connection with contact metamorphism, which, in all probability, were the cause of the deposition of a large quantity of sulfides in the rocks.

The analysis of the rock was carried out by the method of Roziwal.

	Plagioclase An	Plagioclase Ab	Diopside	Hornblende	Sum
$\Sigma$ sections	1649	1649	785	440	2874
% by volume	57.38	57.38	27.31	15.31	100.00%
Sp. gr.	2.68	2.68	3.3	3.2	—
% by weight	52.50	52.50	30.77	16.72	100.00%
	An.	Ab.
SiO$_2$	17.38	8.33	16.18	7.71	49.50
Al$_2$O$_3$	14.95	2.30	0.41	1.76	18.42
Fe$_2$O$_3$	—	—	1.80	0.47	2.27
FeO	—	—	0.99	1.39	2.38
MgO	—	—	3.80	2.41	6.21
CaO	8.09	—	5.69	2.11	15.80
Na$_2$O	—	1.45	1.17	0.27	2.89
K$_2$O	—	—	0.32	0.06	0.38
H$_2$O	—	—	—	0.33	0.33
TiO$_2$	—	—	0.67	0.13	0.80
MnO	—	—	0.05	0.02	0.07
P$_2$O$_5$	—	—	—	0.03	0.03
Total . . .					100.00%

Magmatic formulas according to Levinson-Lessing:

\[ 2.66\, \overline{RO}. \ R_2O_3 : 4.23\, SiO_2 \]

\[ \alpha = 1.49, \qquad \beta = 86.67, \qquad \gamma = 1.24. \]

\[ R_2O : RO = 1 : 9.5. \]

According to the chemical classification, the rock lies between norite and gabbro (4). It is interesting to note that, when the chemical composition of this rock is compared with compilations of analyses of igneous rocks (24, 25; 18, 55—65; 15, 11—16), it fits equally well with gabbro, norite, amphibolites, and even with analyses of certain diabases.

In the dumps of the copper-mine shafts, syenitic rocks have been found. There are no direct outcrops or exposures of these rocks, i.e. it is not possible to establish their mode of occurrence. However, their freshness and the absence of manifestations of dynamometamorphism in the rocks give grounds to suppose that both the syenites and the gabbro described above constitute an independent formation of intrusive bodies in the area, younger than the aplite-like granite and the diorites.

The chief representative of this family is a biotite-hornblende-pyroxene syenite (No. 931 a) of monzonitic aspect. This rock, less than medium-grained, is a slightly viscous mixture of light-yellow, at times pink feldspar, biotite, and dark-green aggregates of pyroxene and hornblende. The texture is hypidiomorphic. The principal components are: potash-soda feldspar, plagioclase, hornblende, pyroxene, and biotite. Accessory components: titanite and apatite.

Potash-soda feldspar is the predominant component of the rock. It often occurs in very large grains with weak perthitization and cleavage cracks along (001). The twinning lattice characteristic of microcline is completely absent. It sometimes contains inclusions of idiomorphic plagioclase grains (fig. 9). Very rarely there occur individuals elongated in the plane (010), forming twins according to the de l’Esterel law (fig. 10); plane of intergrowth—(010). Twin axis—[100]. Coordinates of the twin axis: \(B_{[100]} np = 24^\circ\), \(B_{[100]} nm = 66.5^\circ\), \(B_{[100]} ng = 87^\circ\). \(2V = -58^\circ\).

The feldspar belongs to the triclinic system. The small angle between the optical axes brings it close to the anorthoclase series.

Plagioclase—in large idiomorphic grains, in subordinate amount relative to anorthoclase. According to the rock analysis, anorthoclase constitutes 48.8% of the total mineral composition of the rock, whereas plagioclase accounts for only 27.1%, and, incidentally, still more than the content of this mineral in the aplitoid granite (19.8%). In measuring plagioclase twins by the Fedorov method, a twinned intergrowth (fig. 11) was found according to the very rare complex twin law la Roc—Tourné—\(\perp [001]\ \parallel (010)\).

\[ \begin{aligned} \mathrm{I}\quad &1.\ 104^\circ\ \text{incl. }40^\circ\ np^{1} \\ &2.\ 135^\circ\ \text{right }47^\circ\ nm^{1} \\ &\qquad A' \text{ from itself }20^\circ \\ \mathrm{II}\quad &1.\ 290^\circ\ \text{incl. }11^\circ\ np^{3} \\ &2.\ 16^\circ\ \text{right }19^\circ\ ng^{2} \end{aligned} \qquad \begin{aligned} 2V &= 82^\circ \\ B\left\{ \begin{aligned} ng &= 84^\circ\\ nm &= 26^\circ\\ np &= 63^\circ \end{aligned} \right. \end{aligned} \]

Possible cases: 1) la Roc—Tourné—No. 27 and 2) De l’Esterel—No. 2 or No. 50. Plane of intergrowth—(010), twin plane close to (100). Since the twin axis in the la Roc—Tourné law is a line very close to [100], it is extremely difficult to distinguish these two laws. It would be more likely to suppose that the twinned intergrowth occurs according to the de l’Esterel law, as the simpler one, rather than the complex la Roc—Tourné. But the number of the plagioclase in measuring the second grain refutes this supposition: twin plane—(001); plane of intergrowth—(010); \(B_{[001]} ng = 80.5^\circ\), \(B_{[001]} nm = 29^\circ\), \(B_{[001]} np = 64^\circ\); Carlsbad law No. 28.

Hornblende is dark green in color, with stepped, irregular cleavage cracks; it always forms aggregate clusters together with pyroxene and biotite amid the feldspathic mass of the rock (fig. 12). \(2V = -74\); extinction angle—\(Cng = 14^\circ\); angle between cleavage planes \(\varphi = 120^\circ\).

Pyroxene—in small, slightly elongated grains of aegirine-augite type. Birefringence high, pleochroism colors bright green. It is almost always overgrown by hornblende (fig. 13). \(2V = +52^\circ\), extinction angle \(Cng - 48^\circ\), angle between cleavage planes \(\varphi = 86^\circ\). Pleochroism colors:

    along \(np\)—dark green,
    » \(nm\)—light greenish,
    » \(ng\)—yellow.

Biotite—in characteristically ragged grains, for the most part altered with the separation of chlorite with a greenish tint. A small quantity

Chemical analysis of the rock was carried out by the Rosiwal method.

	Anorthoclase	Plagioclase Ab.	Plagioclase An.	Hornblende	Aegirine-augite	Biotite	Titanite	Apatite	Sum
Σ segments	3100	1654	1654	999	101	38	26	61	5979
% by volume	51.85	27.66	27.66	16.71	1.69	0.63	0.44	1.02	100.00%
Sp. wt.	2.57	2.68	2.68	3.2	3.3	3	3.5	3.2	—
% by weight	48.81	27.13	27.13	19.58	2.04	0.69	0.56	1.19	100.00%
		Ab.	An.
		19.81	7.32
SiO₂	31.73	13.61	3.16	9.01	1.04	0.25	0.17	—	59.97
Al₂O₃	8.79	3.86	2.69	1.57	0.02	0.09	—	—	17.02
Fe₂O₃	—	—	—	1.76	0.06	0.13	—	—	1.95
FeO	—	—	—	2.94	0.16	0.10	—	—	3.20
MgO	—	—	—	0.98	0.26	0.01	—	—	1.25
CaO	—	—	1.47	1.96	0.44	—	0.16	0.68	4.71
Na₂O	4.31	2.34	—	0.59	0.02	0.02	—	—	7.28
K₂O	3.98	—	—	0.20	0.02	0.05	—	—	4.21
H₂O′	—	—	—	0.37	—	0.03	—	—	0.40
TiO₂	—	—	—	0.20	0.02	—	0.23	—	0.45
MnO	—	—	—	—	—	—	—	—	—
P₂O₅	—	—	—	—	—	—	—	0.51	0.51
Total . . .									100.00%

Magmatic formulas according to Levinson-Lessing:

\[ 1.7\,\overline{RO}.\ R_{2}O_{3}.\ 5.5\,SiO_{2} \]

\[ \alpha = 2.36,\qquad \beta = 48.75,\qquad \gamma = 2.05. \]

\[ R_{2}O : RO = 1.1 : 1. \]

In its chemical composition the rock occupies an intermediate position between normal and alkaline syenites (23, 17, and 100), which is fully consistent with the clearly alkaline tendency of the pyroxene, occupying a middle position between diopside and aegirine-augite, and with the distinctly manifested features of a monzonitic structure in the relationships between plagioclase and the alkaline feldspar.

The described varieties also delimit the series of deep-seated rocks of the district. Both by their mineralogical composition and by the relationships revealed in the synthesis of the feldspar material, it is possible to divide these rocks into two separate groups: a granodiorite group on the one hand and a gabbro-syenite group on the other.

The affinity of the granite with the dioritic rocks, apart from their joint occurrence in the field, is established quite firmly by similar features of their mineralogical composition, and also by the clearly expressed traces of dynamometamorphism to which the rocks of this complex were subjected.

The relationship between gabbro and syenites is not entirely clear; nevertheless, if one takes into account the alkaline character of the syenites, their relative freshness, which is also manifested in the gabbro rocks; the weak manifestation, or complete absence, in the complex of these rocks of signs of dynamometamorphism, which stand out so sharply in the granodiorites; the separation of the limestones containing gabbro and syenites from the granodiorite body by a mylonite zone; and, finally, the character of the alkaline feldspars, indicating complete decomposition of the solid phase of the mineral (microcline-microperthite) in the granite and an only incipient decomposition (weak perthitization) in the syenite¹,—then there accumulates a sufficient basis for the possibility of separating the group of gabbro-syenite rocks from the granites, and assigning them to a separate, younger formation. The latter assumption becomes still more probable if one takes into account that the Paleozoic deposits discovered by V. A. Obruchev on the bank of the Sharangol River near the Urmukhtu picket were subjected to strong dislocation, as a result of which injections of igneous rocks are quite possible in this region.

Vein rocks.

The vein rocks of the granodiorite complex are very few in number, which is quite natural in view of the great denudation of this intrusive body². They all bear traces of great pressure, manifested chiefly in strong fracturing and cloudy extinction of quartz, in the filling of cracks in feldspars with secondary mica, and even sometimes in the formation of displacements along these cracks, which is especially convenient to observe in plagioclases, owing to their fine twinning structure (Fig. 14).

Biotite granodiorite-porphyry (No. 923 f) is a light-gray rock with a granite-porphyry structure. The porphyritic segregations are represented by oligoclase No. 13. The plagioclase of the groundmass, however, is almost pure albite. Mineralogical composition: plagioclase, microcline, quartz, biotite, and magnetite. Under the microscope the rock shows a strong development of numerous cracks filled with colorless mica (Fig. 14).

Plagioclase in the porphyritic segregations is represented by large idiomorphic crystals of prismatic form, always strongly sericitized, with fine twin striation according to the Albite law. Its number is the same as in the aplitoid granite—No. 13. In \(\perp(010)\), \(N_g = 12.5^\circ\), \(Bnm = 78^\circ\), \(Bnp = 85.5^\circ\). \(2V = +85^\circ\) (by observation). \(n_g - n_p = 0.0076\), \(n_m - n_p = 0.0036\), \(n_g - n_m = 0.0040\); \(2V = +87^\circ\) (by calculation). The plagioclase in the groundmass is albite—almost always fresh, without any signs of alteration—in small short tabular grains. Twins according to the Albite law. In \(\perp(010)\), \(N_g = 17.5^\circ\), \(Bnm = 72^\circ\), \(Bnp = 87^\circ\). Plane of intergrowth (010). Twin plane—(010). \(2V = +77^\circ\). Plagioclase No. 3.

Microcline-microperthite—entirely fresh xenomorphic grains, as if pinched between large prisms of plagioclase. When the grain is set in a plane close to \((10\bar{1})\), the characteristic twin lattice always stands out clearly; \(2V = +82^\circ\).

Quartz occurs in large, strongly fractured grains, always with cloudy extinction.

Biotite—green, almost completely altered to chlorite, in most cases forms aggregates of brownish-green flakes.

In addition to this rock, in the area of the granite outcrops there occur fine-grained pale-pink aplites (No. 923e), with an insignificant content of hornblende and, like the preceding rock, with traces of pressure that it experienced.

Far more conspicuous and numerous are the rocks of the vein facies of the gabbro-syenite magma, whose acidic derivatives constitute a whole series of transitions from monzonitic to alkaline rocks, while the basic ones are represented by compositionally variable lamprophyric rocks belonging to the kersantite family.

Most rocks of the acidic series have, as an essential constituent, potassic-sodic feldspar, always with a greater or lesser quantity of microperthitic intergrowths, which appear more distinctly in connection with the strengthening of the alkaline character of the rock. Dark-colored components are always present in very small quantity; moreover, the hornblende of monzonitic aplite is replaced, as the alkaline properties of the rock increase, by aegirine-augite and then simply by aegirine.

Quartz monzonite-aplite (No. 930v). A light-gray, slightly yellowish, medium- to fine-grained rock, representing a hypidiomorphic mixture of feldspar and quartz with a weakly expressed porphyritic structure, so characteristic of this type of rock (26, 598), and detectable throughout the entire thin section only under the microscope. The feldspars—plagioclase and microperthitic potassic-sodic feldspar—are represented in the rock in almost equal amounts. There is very little hornblende and a large amount of quartz.

Plagioclase appears either in large, somewhat resorbed idiomorphic grains, representing as it were porphyritic segregations, or in smaller grains regularly overgrown by potassic-sodic feldspar, which constitute an essential part of what is, as it were, the groundmass of the rock (Fig. 15).

The plagioclase of the porphyritic segregations is usually extremely zonal, with numerous cleavage cracks. With the aid of the Fedorov stage, the internal (v) and external (a) parts of such a zonal crystal were measured (Fig. 15). External part (a): cleavage along (010); B⊥(010) \(n_g = 2^\circ\), B⊥(010) \(n_m = 88^\circ\), B⊥(010) \(n_p = 90^\circ\); Nos. 19–20. Internal part (v): cleavage along (010); B⊥(010) \(n_g = 18^\circ\), B⊥(010) \(n_m = 72^\circ\), B⊥(010) \(n_p = 86^\circ\); No. 36. As was to be expected, the plagioclase of the internal part is considerably more basic than that of the external. The plagioclase in the groundmass forms twins according to the Carlsbad law. Composition plane—(010). B(001) \(n_g = 88^\circ\), B[001] \(n_m = 17.5^\circ\), B[001] \(n_p = 73^\circ\). \(2V = -86^\circ\) (by observation). \(n_g - n_p = 0.0063\), \(n_g - n_m = 0.0028\), \(n_m - n_p = 0.0035\). \(2V = -84^\circ\) (by calculation). Plagioclase No. 20. Thus, the plagioclase of the groundmass is identical in composition with the outer part of the porphyritic segregations.

Microperthitic potassic-sodic feldspar, crystallizing after plagioclase, often regularly overgrows the latter, forming, in accordance with it, twins likewise according to the Carlsbad law (Fig. 15). B[001] \(n_g = 78^\circ\), B[001] \(n_m = 20^\circ\), B[001] \(n_p = 75^\circ\). \(2V = -56^\circ\) (by observation). Composition plane (010). \(n_g - n_p = 0.0048\), \(n_g - n_m = 0.0010\), \(n_m - n_p = 0.0036\). \(2V = -55^\circ\) (by calculation). The mineral always occurs in xenomorphic grains, with a barely perceptible patchiness. A twinned microcline lattice is not observed. In contrast to plagioclase, cleavage cracks are almost not manifested.

Hornblende—in very small quantity, with brown tones of pleochroism; it approximately preserves the prismatic character of its crystallographic boundaries. Extinction angle \(C n_g = 24^\circ\); angle between cleavage planes \(\varphi = 124^\circ\). \(2V = -89^\circ\). Pleochroism weak, from yellowish-brown to gre—

hornblende. In optical constants it represents a transitional form from green to ordinary hornblende (25, 232).

Quartz—in numerous xenomorphic grains, always lacking undulatory extinction.

Fragments of the rock described are found halfway between the outcrops of the granodiorite rocks and the abandoned copper mine (Fig. 2), in the walls and dumps of the small shafts of which syenite vein rocks with a more strongly expressed alkaline character have been discovered.

Pyroxene syenite-aplite (No. 931 b) is a light-pink, fine- to medium-grained rock with a hypidiomorphic texture. It consists essentially of microperthitic potassium-sodium feldspar with a small amount of a rare dark-colored component of aegirine-augite type, giving the rock a dirty tint. Among accessory minerals titanite occurs.

Microperthitic potassium-sodium feldspar, with a very well-developed clayey coating and at times with distinctly expressed cleavage; usually in xenomorphic grains with irregular, owing to weak perthitization, patchy extinction. Rarely there occur grains of prismatic outline, usually forming twins according to the de l’Esterel law (Fig. 16), with composition plane along (010). \(B_{[100]} n_g = 85.5^\circ\), \(B_{[100]} n_m = 69^\circ\), \(B_{[100]} n_p = 17.5^\circ\), \(2V = -62^\circ\). Weak perthitization and the large angle between the optical axes bring the mineral closer to the more pure potassic feldspars, which is partly confirmed by its pink coloration, giving the dominant tone to the whole rock. Twins according to the Manebach law with composition plane along (001) also occur. \(B \perp (001) n_g = 84^\circ\), \(B \perp (001) n_m = 10^\circ\text{—}11^\circ\), \(B \perp (001) n_p = 81^\circ\). \(2V = -63^\circ\). The coordinates of the twin axis approach those of microcline (13, 129).

Rare pyroxene of aegirine-augite character, in lath-like elongated prisms. Pleochroic colors are strongly weakened by intense chloritization. Rarely, broad-tabular grains of diopsidic character are encountered. \(2V = +66^\circ\). Extinction angle \(C n_g = 44^\circ\). Angle between cleavage planes \(\varphi = 88^\circ\).

Titanite in well-formed crystals with characteristic pleochroism.

Alkaline quartz syenite-aplite (No. 934 c). A gray, fine-grained, hard, dense rock rich in quartz. The texture is panidiomorphic-granular. Rock-forming minerals: quartz, microperthitic potassium-sodium feldspar, aegirine, and titanite.

Potassium-sodium feldspar, with extremely well-developed microperthitization (Fig. 17), gives the irregular patchy extinction usual for this mineral. Well-expressed cleavage along (010) made it possible to make the following measurements on the Fedorov stage. Principal part (b) of the mineral: \(B \perp (010) n_g = 23^\circ\), \(B \perp (010) n_m = 79^\circ\), \(B \perp (010) n_p = 73^\circ\); \(2V \leq -70^\circ\) (by observation); \(n_g - n_p = 0.0061\), \(n_m - n_p = 0.0039\), \(n_g - n_m = 0.0022\). \(2V = -59^\circ\) (by calculation). Perthitic intergrowths (c): \(2V = +86^\circ\) (by observation), \(n_g - n_p = 0.0093\).

Aegirine is represented by densely colored, elongated, prismatic crystals with slight pleochroism, darkened owing to a large amount of ore deposited chiefly along cleavage cracks. \(2V = -40^\circ\). Extinction angle \(C n_p = 9^\circ\text{—}8^\circ\). Angle between cleavage planes \(\varphi = 87^\circ\).

Quartz—always xenomorphic, sometimes with inclusions of feldspar, without undulatory extinction. Microperthitic replacement of it by potassium-sodium feldspar is observed (Fig. 18).

A somewhat unclear position among the described vein formations is occupied by a bostoniitic-type rock encountered near the gabbro stock (No. 928 c), constituting one of the selvages of the quartz vein passing here.

veins. Strongly altered—poisitized—it is a light greenish-yellow, dense rock with a siliceous fracture, consisting essentially of laths of plagioclase of bostonite habit and secondary minerals—albite, poisite, and quartz. Apparently it forms, like all syenitic rocks, a small body of hypabyssal type.

Plagioclase, in thin laths elongated along \(np\), forms twins according to the albite law. \(B \perp (010)ng = 19^\circ,\ B \perp (010)nm = 72^\circ,\ B \perp (010)np = 87.5^\circ.\) \(2V = +86^\circ.\) Two possible solutions are obtained: No. 37 or No. 0. By measuring the cleavage plane according to \((001)\), the zero number of the plagioclase is excluded: \(B \perp (001)ng = 67^\circ,\ B \perp (001)nm = 24^\circ,\ B \perp (001)np = 85^\circ.\) No. 37.

Poisite, in colorless grains of elliptical outline, is gathered into numerous aggregates, representing, apparently, a product of destruction of feldspars and, perhaps, partly of an original dark-colored component showing through here and there in these accumulations, owing to high interference colors. Perfect cleavage on \((010)\). \(2V = +57^\circ.\) Strong dispersion. Besides the described vein formations, in the area of the gabbro-syenites there occur rocks of lamprophyre type, of kersantitic appearance.

One specimen (No. 931 c) of such a rock was found in the dumps of the small shafts of the Bago-Borigchen copper mine, together with syenitic rocks. It is a dark-gray, brownish, fine-grained rock, consisting essentially of plagioclase, biotite, and diopside, with a small admixture of hornblende. In thin section a large quantity of iron ores is observed, and a slightly porphyritic character of part of the dark-colored components (diopside and biotite) in relation to the plagioclase, which forms the chief background of the principal panidiomorphic-granular mass.

Hornblende occurs only in the groundmass and in very small quantity.

Plagioclase is extremely zonal, with fine twin striation in moderately elongated lath-like prisms. Cleavage along \((001)\) is sometimes manifested. The finely zonal structure of the mineral makes it very difficult to establish precisely the number of the plagioclase by Fedorov’s method. Repeated measurements made it possible to reduce the optical constants to the following form. Twins according to the Carlsbad law: \(B[001]ng = 83^\circ,\ B[001]nm = 25^\circ,\ B[001]np = 67^\circ.\) \(2V = -86^\circ.\) No. 25.

Biotite occurs in large quantity, in characteristically ragged plates with numerous inclusions of apatite and magnetite.

Pyroxene-diopside is sometimes in rather large grains with inclusions of ores and apatite, colorless or with a faint greenish tint, and with a large extinction angle \((Cng = 40^\circ)\), with the usual value \((86^\circ)\) of the angle between the cleavage planes on the prism.

In all respects the rock very closely approaches the typical kersantites described by Rosenbusch (26, 667—671).

The second specimen (No. 927) of lamprophyre rock, taken near the gabbro outcrops, is in external appearance very close to the rock just described, differing from it upon attentive examination only by the absence of a large quantity of biotite. But when the thin section is studied under the microscope, substantial differences between these rocks are established. The structure is hypidiomorphic-granular. The chief essential components of the rock are plagioclase and hornblende, present in almost equal quantity. There is a large content of titaniferous iron ore. In addition, in the thin section there are observed here and there rare rounded small grains of quartz, large grains of uralitic hornblende, developed, probably, at the expense of primary pyroxene, and a little biotite.

Plagioclase, in yellow, slightly elongated prisms, sometimes with fine twin striation, forms twins according to the Carlsbad law.

Cleavage plane (010). In \([001]\) \(ng = 86.5^\circ\), in \([001]\) \(nm = 22.5^\circ\), in \([001]\) \(np = 68^\circ\);
\(2V = -86^\circ\). No. 23. \(nm - np = 0.004;\ ng - np = 0.007.\)

Primary hornblende, dark green, almost with a bluish tint, abounds in inclusions of ore and apatite. Extinction angle \(Cng = 17^\circ\). The angle between cleavage planes \(\varphi = 126^\circ\). Pleochroism:

\[ \begin{aligned} \text{along } ng &\text{ — bluish grass-green,}\\ \text{along } nm &\text{ — light greenish-yellow,}\\ \text{along } np &\text{ — pale yellow.} \end{aligned} \]

Quartz—in small, not very numerous grains, without signs of undulatory extinction.

Thus, in comparison with the preceding rock, this one is distinguished by: 1) replacement of biotite by hornblende, which has developed to the degree of the principal component; 2) the appearance of quartz; and 3) a hypidiomorphic texture, recalling malchitic rocks by the last two factors. The plagioclase remains of the same composition.

An intermediate position between the two described rocks is occupied by lamprophyre, exposed in a pit near the mine adits (No. 935a). This rock, possessing a porphyritic texture, consists essentially of plagioclase, hornblende, and biotite. Among the accessory minerals are apatite and magnetite. Secondary formations: sericite, chlorite, epidote, zoisite, and magnetite usually accompanying them; the latter was probably deposited by metasomatic processes, in the zone of whose formations this rock occurs (Fig. 25).

Plagioclase is contained in two generations, as phenocrysts and in the groundmass, where it occurs in short-columnar, sometimes lath-like elongated prisms. The phenocrysts are well-outlined crystals of prismatic form, with sharp, moderate twin striation and with strong development of sericite, usually in the middle part of the grain; \(2V = +86^\circ\). Twins according to the albite law: \(B \perp (010)\ ng = 29^\circ\), \(B \perp (010)\ nm = 65.5^\circ\), \(B \perp (010)\ np = 75^\circ\). No. 57. The plagioclase of the groundmass is completely fresh, in small grains, with fine zoning, which affects the precision of measurements. Twins according to the Carlsbad law: in \([001]\) \(ng = 72.5^\circ\), in \([001]\) \(nm = 42.5^\circ\), in \([001]\) \(np = 53^\circ\). No. 37. Measurements on the cleavage plane (010) give: \(B \perp (010)\ ng = 24.5^\circ\). No. 48. On average the plagioclase may be considered No. 42, which is considerably more acid than that of the phenocrysts.

Hornblende in the phenocrysts is often poikilitic, intergrown with plagioclase of the groundmass. It occupies an intermediate place between basaltic and ordinary hornblende. Extinction angle: \(Cng = 14^\circ\). \(2V = (-)76^\circ\text{–}78^\circ\). It occurs in two generations.

Colorless to pale green, in thin, not very numerous grains, is secondary zoisite.

A large amount of fine biotite.

Apatite in large quantity is grouped around the hornblende.

Magnetite is partly primary; in the greater part of its mass, as is characteristic of metasomatic deposits, it is accompanied by the occurrence of secondary micas, chlorite, and epidote.

In the character of its occurrence the rock approaches the kersantites of the Lower Gartsa, described by M. Koch (22, 53). It partly resembles diorite-porphyrites.

It must be said that, according to Rosenbusch’s summary (26, 667), lamprophyric rocks in general vary greatly in mineralogical composition and texture; in particular, in kersantites there are not infrequently observed: 1) a hypidiomorphic texture (26, 667); 2) enrichment in hornblende and the presence of quartz (26, 669); and, finally, 3) sometimes, in transitional members toward spessartites, in the so-called hornblende kersantites, a decrease in the content of biotite to its complete disappearance is observed (26, 681). Na-

Finally, Weinschenk describes an altered vein kersantite rock, which, adjacent to the contact zone, assumes a malchite structure with quartz content in the groundmass (31, 328—331). All these considerations—taking into account the features of occurrence, the clearly vein-like character of the rocks, their external similarity, and also the absence of traces of pressure (the quartz is completely pure, without any signs of undulatory extinction)—compel us, by uniting these rocks into one family, to assign them to the vein products of gabbro–syenite magma, especially since in the literature there are indications of cases of occurrence of kersantites in gabbroic rocks (26, 671).

In conclusion it is necessary to mention a fine-grained, essentially feldspathic, dirty greenish-gray rock found in the dumps of the mine shafts, with parallel, very narrow films of fine, pale-brown biotite, giving it a slightly gneiss-like appearance (No. 931 d). The mineralogical composition is very simple, usual for syenitic rocks. The main mass consists, judging by \(2V = - (56^\circ—58^\circ)\), of potassium-sodium feldspar, close to anorthoclase, in small grains with a slightly clayey coating, with a barely perceptible irregularity in structure, revealed in undulatory extinction.

Among this feldspathic mass are densely scattered weakly greenish, almost colorless grains of diopside, lacking pleochroism, often grouped into aggregates with crystals of pleochroic titanite, of rhombic section; and here and there, in ribbon-like fashion, characteristically torn leaflets of biotite are arranged in fine dust.

The presence of such a rock indicates that, apparently, the crystallization of the syenitic magma, at any rate in the last moments of its solidification, was accompanied by a certain pressure in individual zones.

Description of the contacts of igneous rocks with limestones.

1. Granite.

Exposed in a series of low hills (Fig. 2), far from the limestones of Bago-Borigchen, the normal granite consists of microcline-microperthite, oligoclase, quartz, and a very small quantity of biotite. In the western part of the intrusive body, nearer to the higher hills, among the granite debris occur rocks of dioritic and shlieric character (No. 923). It must be said that formations of this type, while preserving in the porphyritic segregations the composition of the minerals of the granitic magma, alter it in the groundmass, which, in comparison with granite, is distinguished by greater basicity and smaller grain size, indicating crystallization under lower-temperature conditions. Especially characteristic of these rocks are aggregate accumulations of dark-colored components of indistinct form and composition, represented in the diorites by thin flakes of biotite and small grains of hornblende, and assuming in the shlieren of dissolution—the typical sample of hybrid forms (19, 340)—the clear character of fragments of country rock not digested by the magma. Since formations of this kind were found only as detached blocks, it is not possible to connect them spatially with the main intrusive body; but, as was indicated above, in particular taking into account the similarity of mineralogical composition, their kinship with granite is undoubted.

Closer examination of the contacting rocks leads to the conclusion that the contact of the limestones with the granite is purely mechanical, formed as a result of a disjunctive disturbance manifested here, the influence of which is reflected both in the granite itself and in all rocks derived from it.

In the granite, still farther from the line of disturbance, there appear sericitization and kaolinization of the feldspars and cloudy extinction of quartz, which, like the feldspars, possesses a large number of cracks filled with secondary mica.

Rocks of dioritic composition, all compacted, acquire a green diabase coloration owing to the strong development of chlorite and sericite and, finally, under the microscope reveal strong crushing of quartz and feldspars. Of particular interest in this respect is granodiorite-porphyry (No. 923 f), which contains a very small amount of dark-colored components and nevertheless assumes a greenish diabase shade of color. In addition, as a result of the pressure experienced by this rock, the filling of cracks in the feldspar by secondary mica is noted in it, and sometimes even displacements along them, well observed in the plagioclases because of the strong development in them of polysynthetic twins (Fig. 14).

At the immediate contact of the limestones with the granites, in the latter the phenomenon of crushing reaches its highest limit, expressed in the complete milonitization of the rock. Granitmilonite (No. 929) changes its pink color to light yellow,^[1] owing to the complete comminution—grinding—of the feldspars and quartz. Many dark greenish-gray streaks appear, creating the impression of indistinct schistosity and connected with zones of crumpling, along which the dark-colored component has been ground down. The composition of the rock is the same as that of the aplite-like granite, with the exception of rare secondary epidote.

Quartz—in an exceedingly large amount, sometimes in very large grains, strongly crushed and sometimes transformed into aggregate accumulations of angular fragments cemented by fine dust of feldspathic material. Always with cloudy extinction.

Feldspars in the rock were preserved only in a few cases. Microcline—in xenomorphic grains with low interference colors. Perthitization has been carried to unusually large dimensions. Entire albitic areas—patches—are observed among the microcline potassic substance. The twin lattice is almost not manifested, being replaced by nodularity under extinction in connection with the strong perthitization of the mineral. In places there occur twin formations according to the Carlsbad law: \(B[001]n_g = 87.5^\circ,\ B[001]n_m = 69^\circ,\ B[001]n_p = 22^\circ.\ 2V = -84^\circ,\ n_g - n_p = 0.0057.\)

Plagioclase is preserved in very few cases, with a vaguely appearing fine twin striation, which is very strongly reflected in the measurement of its optical constants. Repeated measurements reduce to the following figures: \(2V = +(82^\circ—86^\circ),\ Bn_m = 16^\circ,\ Bn_g = 84^\circ,\ Bn_p = 73^\circ;\) the plane of intergrowth is \((010)\). There is the possibility of the formation of twins according to two laws (13): the Carlsbad—\([001]\) No. 18 or XI—\(\perp [010]\ 11(100)\) No. 13. Taking into consideration the magnitude of the angle between the optical axes, it is necessary, despite the complexity, and therefore the rarity, of law XI, to regard No. 13 of the plagioclase as more probable, i.e., exactly the same as in the aplite-like granite.

Epidote is the only well-preserved mineral, in view of its secondary origin at the expense of the rare dark-colored component of the rock, which is not manifested in its primary form. Collected in sparse aggregates, the mineral possesses characteristic pleochroism from colorless to greenish-yellow shades. The interference colors are very high. Perfect cleavage along the plane \((001)\). The plane of the optical axes is perpendicular to the cleavage lines (Fig. 19) and coincides with the plane of symmetry of the mineral. \(2V = -66^\circ.\) Pleochroism: along \(n_p\)—yellow, along \(n_m\)—the same, along \(n_g\)—colorless.

^[1] Such a phenomenon is usual for milonites (12, 13).

Mylonites of this character were encountered at two points along the route, which made it possible to plot (Fig. 2) the approximate strike of the line of disturbance, measured also during field work (NE: 55°).

The second hand specimen of mylonite in the collection (No. 926 b) is essentially the same picture, differing only in still greater crushing and compaction, since the rock was taken at the site of direct contact with the limestones.

Limestone (No. 926 a) at this locality is represented by a fine-grained, marbled rock, dark gray, almost black in color, finely banded, slightly friable, with a shining fracture. The banding of the rock is cut at an acute angle by veinlets of calcite, apparently deposited later. Under the microscope there are numerous, irregularly scattered grains of graphite, sometimes gathered in aggregate-like accumulations, and veinlets of yellow¹ isometric grains of garnet—apparently grossular—which were deposited, in all probability, under the influence of the injection of gabbro-syenite magma that followed the dislocation. Incidentally, this limestone directly adjoins a skarn rock (No. 925), formed as a result of the contact action of gabbro on the limestones.

Taking into account 1) the direct transition of the above-described metamorphosed limestone into calcareous contact-metasomatic rocks composing the contact belt of the gabbro stock, 2) the small distance (Fig. 2) separating these rocks, and 3) the presence of epidote in the mylonite, a mineral in most cases connected, according to the observations of Goldschmidt (17, 44), with the contact metamorphism of igneous rocks of various formations, it is easy to conclude that in the present case we are dealing with the outer contact zone of gabbroid rocks.

2. Gabbro.

As the gabbro is approached from the limestone rock described above, the limestones first of all lose their primary dark color, owing to the presence of organic formations, passing into dolomitized, ocher-colored contact-metasomatic formations with a greater or lesser amount of contact minerals (garnet and pyroxene) and ores. In the dumps of mole burrows on the slope of the mountain, at the summit of which the gabbro is exposed, the following rocks were encountered.

Yellowish-white, marble-like, dolomitized dense limestone (No. 925 a) with grains slightly protruding, like porphyritic segregations, in a yellowish groundmass. This structure of the rock is sharply expressed in specimens (Nos. 925 b–d), from which, incidentally, thin sections were prepared. Large grains of calcite are irregularly distributed in the light-yellow, fine-grained mass of the rock, with black specks of ore. The quantity of clear calcite crystals varies in the hand specimens up to their complete coalescence into a dense, impure marble (No. 925 c) with black specks of ore and yellow spots of serpentine. Under the microscope the groundmass of the rocks consists of rare grains of garnet, ores, and serpentine, which preserves a characteristic mesh structure (Fig. 20) and probably formed at the expense of pyroxene; no primary substance is observed at all, and only the outlines of the serpentinous formations recall the tabular forms of diopside. In places, between the fibers of the serpentinous substance, brownish grains appear, partly of secondary ferruginous formations, which evidently originated at the expense of the iron contained in the pyroxene. The garnet-andradite, black in the hand specimen, proves to be slightly reddish in the thin section. No anomalous phenomena are observed. Among the ores, magnetite and pyrites are noted.

It is very possible that the decomposition of the primary pyroxene mineral took place at the moment of ore deposition; at least such a phenomenon has been observed in the hedenbergite skarn of the Bago-Borigchen ore deposit (see below), and the serpentine formations in principle differ in no way from the famous Eozoonstruktur encountered in the Klara mine of the Pitkäranta deposit (29, 244–251).

Here, too, an original skarn rock was encountered (No. 925 e), consisting essentially of light bluish scapolite with dark spots of aggregate accumulations of andradite, diopside, vesuvianite, albite, apatite, and rare flakes of sodium mica.

Scapolite composes the principal mass of the rock. In thin section the cleavage is not sharply expressed, having the appearance of frequent, slightly curved cracks that do not maintain a definite direction and create the impression of a fibrous structure of the mineral. The polarization colors are considerably lower than in diopside, but somewhat higher than quartz—\(n_m - n_p = 0.012\), which makes it possible to suppose (4, 56) a high content of the marialite molecule. Uniaxial, optically negative. Simple twins are observed. No crystallographic boundaries are manifested. It is always xenomorphic with respect to the other components of the rock. It was deposited last.

Diopside—pale green; fine-grained aggregates in scapolite. In thin section completely colorless, with a characteristic system of cleavage cracks almost at right angles and with high polarization colors. Occasionally an indistinct cleavage along (100) is observed—Fig. 21. \(2V = 59^\circ\); \(n_g - n_p = 0.029\). The angle between the cleavage planes \(\varphi = 87^\circ\). Extinction angle—\(C n_g = 37^\circ\). Judging by the extinction angle, it approaches the extreme series of iron-free pyroxenes (20, 1020). The mineral usually occurs as inclusions in scapolite, vesuvianite (Fig. 22), and garnet, always having corroded external outlines. Undoubtedly, it separated first.

Vesuvianite—in an amount equal to garnet; when observed with a hand lens it appears as excellently bounded crystals of pale green color. In thin section it preserves signs of crystallographic boundaries, manifested in the rectilinear outlines of individual grains (Figs. 22 and 23). It has very high relief and low interference colors. Between crossed nicols, an extremely irregular structure of the mineral is observed, manifested in indistinct patchy extinction (Fig. 22). Weak zoning. When measured on the Fedorov stage, some areas prove to be anomalously biaxial. It is interesting to note the presence of sharp twins, manifested as a chain of regularly outlined rhombs (Fig. 23). Unfortunately, the extremely strong dispersion and complex structure of the mineral did not make it possible to determine precisely the angle between the twin axes, which greatly complicates establishing the twin law, probably according to one of the pyramids. Optically negative, uniaxial. From the point of view of Goldschmidt (17, 195), vesuvianite is the most characteristic contact mineral for rocks of class 10 that form from limestones. However, for its formation, according to Goldschmidt’s equation¹, the presence of water vapor is required, i.e., in essence a metasomatic process must take place—with introduction of material—which somewhat contradicts Goldschmidt’s basic views on the processes of contact metamorphism. It separates earlier than garnet, since inclusions of small grains of vesuvianite are subsequently found in the latter.

Garnet-andradite—with a faint pinkish color in thin section and almost black in the hand specimen. No anomalies are observed, which, according to Goldschmidt’s observations, is most characteristic of the end members of the isomorphous

grossular–andradite series (17, 384). Considering andradite to be a metasomatic mineral, Goldschmidt gives the following scheme for its formation:

\[ \underset{\text{wollast.}}{3\,\mathrm{CaSiO_3}} + \underset{\text{iron mica}}{\mathrm{Fe_2O_3}} = \underset{\text{andradite}}{\mathrm{Ca_3Fe_2Si_3O_{12}}} \]

Thus, for the formation of andradite, in addition to other oxides, an introduction of iron is required. It must be said that, if this construction is correct, the introduction of iron must occur after the formation of pyroxene, since in its composition it is almost salite. This latter supposition agrees with the order of crystallization of the minerals—diopside, vesuvianite, and garnet—of which andradite is formed last (Fig. 22).

Albite occurs in an amount subordinate to scapolite, sometimes including small grains of vesuvianite, rare apatite, and flakes of sodic mica (Fig. 24).

Measurement by means of the Fedorov stage gave the following constants: \(2V = +80^\circ\), cleavage on \((001)\); \(B \perp (001) n_g = 73^\circ\), \(B \perp (001) n_m = 18^\circ\), \(B \perp (001) n_p = 74.5^\circ\); Nos. 7–8. Large crystals of albite often, as already mentioned above, contain inclusions of vesuvianite grains. Thus, according to observations, the crystallization of albite falls between vesuvianite and scapolite; but, taking into account the related composition of albite and scapolite (Na content), as well as the observations of Goldschmidt, who indicates that albite crystallizes after the iron-bearing minerals (17, 303), it is necessary to place it, in the order of crystallization of the minerals, after andradite.

As a result of observations in thin section, by comparing the crystallographic interrelations of the minerals, the order of their crystallization is established as follows: 1) diopside, 2) vesuvianite, 3) garnet, 4) albite (apatite and sodic mica), and 5) scapolite. At the same time, bearing in mind that the appearance of apatite is associated with the formation of albite and scapolite, one must suppose in its composition the presence not of fluorine, but of chlorine. Incidentally, the relations of diopside, vesuvianite, and garnet fully coincide with the observations of A. K. Meister (5, 489).

From the field material at my disposal it is impossible to establish exactly the relation of this skarn rock to the gabbro, since it was taken from the dumps of exploratory pits in limestones. It can only be assumed that it constitutes, in all probability, a small vein in the metamorphosed limestone and in no case represents a direct contact of gabbro with limestones, which excludes the possibility of the formation of scapolite at the expense of plagioclase (26, 385). Nor is there manifested in the scapolite a process of its alteration into anorthite (3, 253), which, in essence, is very simply explained by the predominance in the mineral of the marialite sodium molecule over the calcium—meionite—one. The scheme proposed by Goldschmidt (17, 313), in which the appearance of this mineral is connected with emanations of ferric chloride reacting together with water vapors upon limestones, comes closest to explaining the formation of scapolite under these conditions.

Thus, the appearance of diopside and vesuvianite occurs before the introduction of ferruginous material. Their formation is accompanied by an influx of \(\mathrm{SiO_2}\), \(\mathrm{Al_2O_3}\), \(\mathrm{OH}\), and, perhaps, Mg. The later introduction of Na and iron, in all probability in the form of \(\mathrm{FeCl_3}\), supplies material for the formation of andradite, albite, scapolite, and other skarn minerals. Thus all the minerals of the skarn are clearly metasomatic in character, which is not entirely in agreement with the views of Goldschmidt (17, 228), who assigns diopside and vesuvianite to purely contact formations at the expense of recrystallization of rocks belonging to classes 9 and 10 (impure limestones). Incidentally, following Goldschmidt and developing his views, Svitalsky (8, No. 1—28) even assigns ferruginous garnet to contact formations without introduction of material.

Among the gabbro specimens in the collection, as was already noted in describing these formations, there is a gradual transition from coarse-grained diopside–hornblende gabbros to a diopside–plagioclase endocontact rock (No. 924 f) through a gradual diminution in its composition of plagioclase, distinguished by greater basicity, and the replacement of hornblende by diopside. This endocontact formation is a dark greenish-gray, dense, tough, medium-grained rock with rare dirty-white patches, consisting essentially of diopside and basic plagioclase. There is a large amount of apatite and titanite¹. The structure of the rock resembles that of gabbro—all components have irregular xenomorphic boundaries. One observes, as it were, an intergrowth of feldspar with pyroxene.

Plagioclase—in large grains filled with pyroxene ingrowths. The coarse polysynthetic twin striation, due to the frequently intergrown individuals, partly recalls oligoclases. Zoning is not manifested. According to the optical constants, the mineral approaches anorthite. Twins according to the Pericline law. In [010] \(n_g = 51^\circ\), in [010] \(n_m = 58^\circ\), in [010] \(n_p = 55^\circ\). \(2V = -82^\circ\). The composition plane is close to (001).

Pyroxene—differs somewhat from the diopside-augite of normal gabbros, approaching diopside. In thin section it is slightly greenish, at times in very large grains; according to measurements it has the following constants: \(2V = +60^\circ\), extinction angle \(C n_g = 37^\circ\), angle between cleavage planes \(\varphi = 84^\circ\).

Apatite—always xenomorphic, in rounded grains, sometimes reaching considerable dimensions.

Titanite—in individuals of characteristic rhombic outline, with pleochroism usual for this mineral.

The rock extremely resembles the “contact porphyrites” described by Fedorov (14, 54–55).

3. Quartz vein.

Not far from the gabbro stock, in a southwesterly direction from it (Fig. 2), among the limestones there is exposed a stock-like vein of quartz more than 4 meters in diameter.

Quartz is white, massive, slightly transparent; there are also specimens of a coarse-grained glassy mineral.

In the selvage of this vein there occurs, with a thickness of about 0.75 m, an altered (see vein rocks), poikilitized vein rock of bostonite character (No. 928 c). Judging by the general appearance of the sienites, it apparently represents one of the vein varieties of these hypabyssal rocks, altered by the fluids that followed its injection and deposited the given quartz vein.

The limestones in contact with this igneous rock become marbled, dense, possibly silicified.

4. Quartz monzonite-aplite (No. 930 c).

On the way from the mylonitic rocks (No. 929) to the copper mine (Fig. 2), in the valley, fragments of the above-described quartz monzonite-aplite are found in association with surface outcrops of limestone hornfels (No. 930 a). The latter is a dirty yellowish-gray, somewhat banded, fine-grained rock with a hornfelsic fracture, representing the highest degree of silicification,

probably under the influence of powerful metasomatic processes. Under the microscope there appears a typical bridge structure with parallel veinlets of the largest irregular quartz grains.

5. Contact-metasomatic phenomena at the Bago-Borigchen copper-ore deposit

When this deposit was examined, the exploration shafts proved to have caved in; consequently the material available in the collection was taken either from dumps or from a section (Fig. 25).

The pit of the first shaft \((a_1)\) is occupied by a diopside-garnet skarn (No. 932), impregnated with chalcopyrite. This is a dark, dense, medium- to coarse-grained rock; it consists chiefly of iron-bearing diopside with rare veinlets of calcite and copper oxides. There is a small amount of garnet and ores. The unevenness of texture, generally characteristic of this type of rock, is observed with respect to grain size.

In thin section only one diopside was observed, occurring in aggregates of now large, now small grains; macroscopically dirty-green in color, in thin section it completely loses its color. \(2V = +65^\circ\). Extinction angle \(Cng = 43^\circ\). The angle between cleavage planes \(\varphi = 86^\circ\).

Farther on, at the beginning of the trench, the skarn is replaced by a greenish-gray igneous rock (No. 933 b), extremely compacted, consisting almost exclusively of aggregate clusters of epidote grains in a sericite-like, slightly fibrous mass, which apparently formed at the expense of feldspathic material during deposition by the solutions of pyroxene skarn.

This rock is rather abruptly replaced by a band up to 4 meters thick of mineralized gray, sometimes yellowish-gray, dense rock with a splintery fracture (No. 933 c), containing irregular disseminations of black ores and, here and there, colored by coatings of copper oxides. Under the microscope the following minerals are found: radiating hornblende, for the most part altered to serpentine and talc, and a very large quantity of iron ores (magnetite), with a small admixture of chalcopyrite.

Serpentine, possessing a typical mesh structure, makes up the greater part of the thin section, preserving here and there within its meshes relics of the hornblende mineral.

Hornblende in some cases has been converted into talc, which forms pseudomorphs after this mineral. The few preserved grains of it, when measured on the Fedorov stage, give the following constants: \(2V = -87^\circ\) and extinction angle \(Cng = 17^\circ\).

Magnetite is contained in the rock in an extremely large amount, making up approximately 0.5 of the rock. The highly irregular outlines of the grains of this mineral closely resemble (Fig. 26) the usual pictures of replacement of skarn material by deposition of ores, which are commonly accompanied by serpentinization and talcification of the minerals formed earlier (24, 638—640).

Among the rock described above there occur areas (No. 933 c) of black color, with greenish veinlets of serpentine, very compacted, with a content of chalcopyrite and a small amount of garnet. Under the microscope the general background is a pyroxene of hedenbergite character, occurring in aggregates of now large, now small grains. \(2V = +60^\circ\). Extinction angle \(Cng = 44^\circ\). The angle between cleavage planes \(\varphi = 88^\circ\). It is interesting to note the presence of polysynthetic twins, intergrown along the plane (100) and resembling parting in diallage. Cleavage along (001) often appears. The plane of the optical axes is situated in the plane (010).

Among the hedenbergite, under the microscope, there is noted a large quantity of sulfides, characteristic of metasomatic formations, appearing as sulfides, and

sometimes also magnetite, surrounded by patches of serpentine, recalling analogous formations noted in the description of the limestone contact rocks near the gabbro stock.

Here and there there is always found the garnet accompanying the ores, slightly anomalous, yellowish in thin section, apparently representing an intermediate member of the andradite–grossular series.

Further on, these rocks pass imperceptibly into a white, slightly yellowish, dolomitized medium-grained m a r b l e, which in its pure form extends for about 2 m, being replaced in turn by dark-gray fine-grained varieties of the same rock with a small graphite content. These same marbles are excellently exposed in the southern wall (Fig. 27) of the prospecting trench, in the floor of which shaft No. 2 was sunk.

D o l o m i t i z e d m a r b l e (No. 934 a) is essentially the same rock as that in the trench (No. 933 d) (cf. Figs. 27 and 25). In comparing the specimens, the whole difference consists in the stronger dolomitization of the former rock (No. 934 a). Among them occur nodule-like patches (No. 934 b), representing varieties of this marble little affected by the processes of dolomitization.

The principal ore mass (No. 934 d) is represented by a fine-grained, dark-gray rock with a bluish tint, passing at times into a dirty yellowish-greenish rock (No. 934 e), containing small veinlets and disseminations of copper pyrite. Small nests of dark andradite occur. Under the microscope a large amount of actinolite and iron-bearing diopside—hedenbergite—is found. Disseminations of magnetite and sulfides are accompanied by grains of garnet and by the presence of serpentine. Here and there rare grains of calcite occur.

The coarser-grained and serpentinized portions in this rock constitute h e d e n b e r g i t e s k a r n with garnet, calcite, and ores. Actinolite is absent. Serpentine usually accompanies the portions containing magnetite and copper pyrite. In ore content it is inferior to the actinolite skarn.

This complex of skarn formations (Nos. 934 d and 934 e), in outward appearance, extremely closely resembles the above-described rocks discovered in the trench (Nos. 933 b and 933 c), differing from them by a lesser degree of destruction (weak serpentinization and absence of talcification) and, perhaps, by a smaller ore content. They probably belong to one ore belt.

Near shaft No. 2, in a deep pit (Fig. 25), there is exposed a stock-like mass of the above-described lamprophyre (No. 935 a) rock, bordered by a thick zone (about 1.5 m) of a u g i t e - g a r n e t s k a r n with a high content of copper pyrite. This skarn is a green, medium-grained rock with a dirty tint, consisting chiefly of green pyroxene (salite), brownish-red garnet, rare grains of calcite, and a large amount of sulfide ore (chalcopyrite).

Pyroxene, in irregular grains typical of skarn rocks, occurs now in small, now in large, almost porphyritic segregations. In its constants it approaches diopsides with a low iron content (20, 1021); \(2V = +59^\circ\), extinction angle \(Cng = 40^\circ\).

Here and there, calcite is found in thin section, appearing sometimes as large grains enclosing diopside, and sometimes itself forming inclusions in diopside crystals. This phenomenon is readily explained by the role of calcite in the formation of the rock, being apparently the principal material replaced in places by pyroxene matter.

Garnet, apparently, judging by its reddish coloration, stands closer to andradite than to grossular.

In concluding the description of the skarn formations, it is necessary to note the following two main points:

1) metasomatic zones are not always confined to the contacts of igneous rocks with limestones; and

2) the introduction of iron ores and the sulfides that follow them is always accompanied by the formation in the skarn of serpentine and talc, thus marking the time of deposition of the ore minerals.

The first point is vividly illustrated in the section cited above (Fig. 27): the alkaline allite rock, in direct contact with the limestone, produces no special new formations, whereas the lamprophyre rock is surrounded by a zone of augite-garnet skarn. Moreover, farther on, the igneous rock (No. 933 v), entering the zone of metasomatic formations, itself bears signs of the influence of the therms, consisting in epidotization and sericitization of its constituent minerals. It follows from this that the injection of gabbrosyenitic hypabyssal-type rocks was not accompanied, at the moment of their solidification, by the formation of skarn material; only later, perhaps soon afterward, metasomatic processes took place along the limestone zones weakened by the cooling of the intrusion, being confined in particular also to the contacts of the igneous rocks with the limestones¹.

The second point, noted in the study of thin sections of almost all skarn rocks, being taken as the basis for the order of separation of minerals in them, makes it possible to compose the scheme below and, consequently, to outline the order of introduction of the mineral and ore materials.

Scheme

of the order of separation of minerals in skarn rocks:

1. Light-green or green diopside, possibly containing aluminum.
2. Vesuvianite.
3. Black or dark-green hedenbergite.
4. Fibrous hornblende.
5. Andradite.
6. Apatite (and sodium mica).
7. Albite.
8. Scapolite.

a) magnetite
b) sulfides
} Serpentine and talc (formed at the expense of pyroxenes and hornblendes).

I have deliberately not included two minerals in the general scheme: grossular and epidote. The first, as an iron-free mineral, should be placed next to vesuvianite, but, for lack of material, I could not establish these relations by the microscopic method. The second occurs in the material at my disposal only in altered igneous rocks. Whether it is a metasomatic formation or a product of the decomposition of the primary substance of the rocks under the influence of therms—nothing definite can be said because of the insufficiency of factual material. If, however, one admits the possibility of the existence of epidote as a metasomatic mineral, which, generally speaking, is quite probable, then its separation should be expected, as an iron-bearing mineral, somewhere between albite and hedenbergite. As for the order of separation of magnetite and sulfides, since I did not have at hand means for determining opaque minerals (at least in reflected light), I made use of the literature data available on this question for the Dolores deposit (28, 481 and 27, 485—

492), the order of separation of minerals in the metamorphic formations of which in principle differs in no way from the scheme presented above.

Taking into account the established order of formation of skarn and ore minerals, I represent the process of introduction by emanations of various substances in the following form: 1) first come emanations containing Al₂O₃, SiO₂, OH and probably Mg; 2) then their composition is enriched in iron and, probably, very soon in sodium; and 3) finally, as the silicates become saturated with iron, magnetite and sulfides are deposited, possibly coinciding with the formation of sodium compounds and always connected with the appearance of serpentine and talc, formed at the expense of pyroxene and hornblende materials.

General conclusions

Thus, on the basis of the treatment of the petrographic material and of the literature data concerning the area described, one may arrive at the following principal propositions:

1. In the area of the ore deposit two formations of intrusive rocks are represented:

a) a granodiorite formation, situated in the southern part, composing a batholithic-type intrusive body and represented in its main mass by alaskite-like granite, containing in places diorite and shlieren rocks;

b) a gabbro-syenite formation, represented by a large number of hypabyssal bodies, very diverse in composition, partly alkaline in the acidic varieties.

1. The granodiorites probably belong to the Agnostozoic, entering into the composition of the graywacke formation that composes the Kentei Range.

2. The gabbro-syenite rocks are associated with limestones younger than the granite, most probably of Paleozoic age.

3. These sedimentary formations have been brought, owing to faulting, into mechanical contact. It is not without interest to note that the strike (NE 58°) of the Paleozoic clay shales discovered by V. A. Obruchev near the Urmukhtu picket coincides with the strike of the fault (NE 55°) that brought the limestones and granites into mechanical contact. Such a coincidence involuntarily suggests that the formation of the disjunctive disturbance and the folding of the Paleozoic belong to one phase of dislocation, after which there was intrusion of hypabyssal gabbro-syenite bodies.

4. The injection of the gabbro-syenites took place before the beginning of formation of the skarn rocks.

5. The formation of skarn material took place only after the last intrusions had solidified, and is not always connected with the contacts of igneous rocks, being confined to cracks and, in general, to weakened zones in the limestones; thus the presence of igneous rock in metasomatic formations is only a particular case of deposition of the latter.

6. Ore minerals are usually associated with pyroxene and hornblende skarns.

7. The deposition of magnetite and sulfides is commonly accompanied by serpentinization and talcification of the skarn minerals, thereby indicating their later origin in time.

8. The minerals of the ore-bearing formations, in the order of their separation, are: diopside, hedenbergite, fibrous hornblende, and garnet.

9. The ore minerals are magnetite and sulfides (copper pyrite).

Plate 1.

Provisional Geological Sketch Map

of the Area of the Karnakovskaya Zaimka, Northern Mongolia

For the article by F. N. Shakhov, “Relationships and Contacts of Igneous Rocks with Limestones.”

S. Mongolia

Map labels visible on the plate:

• R. Onon
• R. Shirin-Gol
• Road from Urga to Kyakhta
• R. Kurun
• P. Ir.
• Ir.
• Ibitsykh. [[unclear: place name]]
• Bal-Borigchen
• Road from the mines to Kyakhta
• Karnakovy Zaimka
• Bayan-Gol
• [[unclear: Kholonas Ziazn.]]
• R. Ulukhuy
• R. Iro
• Iro
• N

Legend:

• Granites and other igneous rocks
• Clay shales
• Limestones
• Boundary of the surveyed part of the Karnakovy prospect
• Roads

Scale: 10 versts to the inch.
1 : 420,000

Regarding the article by F. N. Shakhov: “Relationships and Contacts of Igneous Rocks with Limestones”

Fig. 2.

N
S

SCHEMATIC
ROUTE CARD

r. Bago

Borigchen.

Lyuta.

r. Shro.

s. Karnakovka.

r. Bago Borigchen.

Legend:

— — line of disturbance.
- - - route line.
Nos. indicate samples.

Fig. 3.

a
b

Fig. 4.

a

To the article by F. N. Shakhov: “Relationships and contacts of igneous rocks with limestones.”

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

To the article by F. N. Shakhov: “Relationships and contacts of igneous rocks with limestones.”

Fig. 12.

Visible labels: \(A_r\), \(A_{mp}\), \(T_1\), \(\beta_1\).

Fig. 13.

Visible labels: \(b\), \(b\), \(a\), \(a\), \(a\).

Fig. 14.

Visible labels: \(a\), \(c\), \(b\), \(d\), \(c\), \(e\).

Fig. 16.

Fig. 17.

Visible labels: \(a\), \(b\), \(c\), \(c\).

Fig. 18.

Visible labels: \(a\), \(b\), \(c\), \(c\), \(a\), \(a\), \(c\), \(c\), \(a\), \(a\), \(a\), \(a\).

Fig. 19.

Visible labels: \([[unclear: vertical-axis label]]\), \([[unclear: horizontal-axis label]]\), \([[unclear: diagonal-axis label]]\).

To the article by F. N. Shakhov: “Relationships and Contacts of Igneous Rocks with Limestones.”

Fig. 15.

Fig. 20.

Fig. 21.

Fig. 22.

Fig. 26.

Fig. 24.

To the article by F. N. Shakhov: “Relationships and contacts of igneous rocks with limestones.”

Fig. 23.

Fig. 27.

Trench.

References

1. D. S. Belyankin. On the question of the mutual relationships of alkaline feldspars.—Izv. Petrograd Polytechnical Institute, 1918.

2. R. Daly. Igneous Rocks and Their Origin, Part II.—Petrograd. Inst. “Lithogaea,” 1920.

3. B. Z. Kolenko. Petrographic sketches.—Bulletin of the Moscow Mining Academy, 1922, vol. I, No. 2.

4. F. Yu. Levinson-Lessing and D. S. Belyankin. Petrographic tables.—Petrograd, 1915.

5. A. K. Meister. Igneous rocks and the conditions of gold-bearing capacity in the southern part of the Yenisei district.—S. P. B. Izv. Geolog. Komit., 1910.

6. V. V. Nikitin. Fedorov’s universal method.—Petrograd, 1915.

7. V. A. Obruchev. A brief geological outline of the caravan route from Kyakhta to Kalgan.—S. P. B., 1893.

8. N. I. Svitalskii. On the question of the classification of crystalline schists.—Geol. Vestn., 1915, vol. I, Nos. 1, 3 and 5.

9. P. P. Sushchinskii. Materials for the study of contacts of deep-seated igneous rocks with limestones.—Trudy S. P. B. Society of Naturalists, XXXVI, issue 5, 1912.

10. M. A. Usov. Orography and geology of the Kentei Range in Mongolia.—Izv. Geol. Kom., vol. XXXIV, 1916.

11. M. A. Usov. Districts of the mines of the Society for Ore Mining of the Tushetukhan and Tsetsenkhan aimaks in Mongolia.—Mining and Gold-Industry News, 1914, Nos. 14–18.

12. M. A. Usov. Description of mountain rocks (Boundary Dzungaria).—Izv. T. T. I., 1911, vol. II, issue 1.

13. M. A. Usov. Fedorov’s, or universal-optical, method for the study of rock-forming minerals, especially feldspars.—Izv. T. T. I., 1911, vol. II, issue 1.

14. E. S. Fedorov and E. D. Straganovich. The genesis of aviatite-granite rocks according to new data.—Zap. Gorn. Inst. named for Empress Catherine II. S. P. B., 1909, vol. II, issue 1.

15. Franc D. Adams. On the Origin of the Amphibolites of the Laurentian Area of Canada.—The Journal of Geology, 1909, Vol. XVII, No. 1.

16. N. L. Bowen. The reaction principle in petrogenesis.—The Journal of Geology, 1922, Vol. XXX, No. 3.

17. V. M. Goldschmidt. Die Kontaktmetamorphose im Kristiania gebiet.—Kristiania, 1911.

18. V. Hackmann. Die chemische Beschaffenheit von Eruptivgesteinen Finnlands und der Halbinsel Kola im Lichte des neuen amerikanischen Systemes.—Bull. de la Comm. Geol. de Finlande, 1905, No. 15.

19. A. Harker. The Natural History of Igneous Rocks.—London, 1909.

20. C. Hintze. Handbuch der Mineralogie.—Leipzig, 1897.

21. Iddings. Igneous Rocks, Vol. I, 1909.

22. Max Koch. Die Keratite des Unterharzes.—Jahr. der K. Pr. geol. Land. und Bergakad., 1880.

23. R. Reinisch. Petrographisches Praktikum II.—Berlin, 1912.

24. David H. Newland. Zinc-mining district near Edwards N. Y.—Econ. Geol., 1916, Vol. XI, No. 7.

25. H. Rosenbusch. Mikroskopische Physiographie der petrographisch wichtigen Mineralien.—Stuttgart, I, 1905.

26. H. Rosenbusch. Mikroskopische Physiographie der massigen Gesteine.—Stuttgart, II, 1907.

27. J. E. Spurr. Theory of ore deposition.—Econom. Geology, 1912, Vol. VII, No. 5.

28. J. E. Spurr, G. H. Garrey and C. N. Fenner. A contact-metamorphic ore deposit.—Economic Geol., 1912, Vol. VII, No. [[unclear: issue number]].

29. Otto Trüstedt. Die Erzlagerstätten von Pitkäranta am Ladoga-See.—Bull. de la Comm. Geol. de Finl., No. 19, 1907.

30. I. H. L. Vogt. Über anchi-monomineralische und anchieutektische Eruptivgesteine.—Christiania, 1908.

31. E. Weinschenk. Ganggestein aus dem Habachtal Oberpinzgau.—T. Min. und Petr. Mitth., 1891, Vol. XII.

Explanation of the figures

1. Geological sketch map of the area of the Karnakovskaya deposit.
2. Schematic sketch map of the route (copy from the field diary).
3. a—microcline grain; b—aggregatic clusters of biotite.
4. a—microcline; b—oligoclase; c—muscovite.
5. a—figurative point of aplitoid granite; e—ternary eutectic point;
   E—binary eutectic points;
   Or—orthoclase; Qu—quartz; An—anorthite;
   Ab—albite.
6. Quartz in biotite diorite. Qu—quartz; B—biotite; shaded plagioclase groundmass, entirely altered into a clayey mass with flakes of sericite.
7. Qu—quartz; Mcr—microcline; Pg—plagioclase.
8. Pg—plagioclase; Dps.—diopside.
9. a—potassium feldspar; b—plagioclase; cc—perthitic intergrowths.
10. Twin of potassium feldspar according to the de l’Esterel law [weak perthitization, approximately in section (100)].
11. Plagioclase with twins according to a complex law—La Roc-Tourné.
12. Ag′—aegirine-augite; Amp.—hornblende; Bt—biotite; Ap—apatite; Ti—titanite.
13. a—pyroxene of aegirine-augite type; b—hornblende.
14. aa—veinlet filled with colorless mica; b—plagioclase; cc—fractures along which displacements occurred in the plagioclase.
15. a—outer shell of zonal plagioclase; b—its inner part; c—plagioclase of the groundmass; d—potassium feldspar; ee—perthitic intergrowths.
16. Perthitic intergrowths in potassium feldspar forming a twin according to the de l’Esterel law [drawing in the plane (100)].
17. a—potassium feldspar; b—pyroxene; c—perthitic intergrowths.
18. a—quartz; b—potassium feldspar; cc—perthitic intergrowths; dd—cleavage cracks—(001). Drawing in section (100).
19. Epidote grain: ng, np, and nm—axes of the ellipsoid of optical elasticity.
20. a—calcite; b—serpentine with grains of magnetite; c—chalcopyrite.
21. Pyroxene grain: aa, bb, cc—cleavage cracks.
22. a—diopside; b—vesuvianite; c—garnet.
23. Vesuvianite crystal with twins; anomalies (unequally extinguishing areas are hatched differently).
24. a—scapolite; b—albite.
25. Plan of exploration workings at the Bago-Borigchen mine: a₁—small shaft No. 1; a₂—small shaft No. 2; c—adit; d—pit; the numbers 932, 933, etc., indicate the Nos. and places of the rocks taken; ef—southern wall of the adit.
26. Se—serpentine; T—talc; Tr—tremolite; Mg—magnetite.
27. a—main mass of dolomitized marble (No. 934 a); b—nodule-like areas of pure marble (No. 934 B); d—serpentinized actinolite skarn (934 d); e—areas of strongly serpentinized coarse-grained hedenbergite skarn rock.

1. Such isolation of metasomatic formations, noted by Fedorov (14, 59), at one time led him to interpret them as primary igneous rocks. ↩↩↩↩↩↩

2. Specimens of these rocks were encountered only among granite rubble. ↩