A. I. Aleksandrov.

Mineragenic Studies of the Irbinsky Iron-Ore Deposit

I. Preface.

In 1932, while completing postgraduate study at SGI, I received the topic: “Mineragenic studies of the Irbinsky iron-ore deposit.” In the summer of that same year, over the course of 2½ months, I worked at the deposit and collected the rock material necessary for the study. I was able to begin laboratory processing of the collected material only in 1934, since for a year I was engaged in administrative work.

During the collection of material in the summer of 1932, mining engineer A. A. Mesyaninov acquainted me with the geological structure of the Irbinsky deposit. He also placed at my disposal graphic material on the deposit and allocated part of the rock material from his collections (cores from boreholes). During the execution of the work, my immediate supervisor was the head of the Department of Mineral Resources, F. N. Shakhov, who gave me sufficient attention. I also made use of the advice of Doctor of Geological Sciences, Professor M. A. Usov. I take this opportunity to express my sincere gratitude to them.

II. Introduction.

The historic decision of the Party and the government on the creation of a coal-and-metallurgical base in the east—the creation of the Ural–Kuznetsk Combine—set before geological-exploration organizations the task of expanding the iron-ore base of the region. Looking over the reserves of iron ores in the region, we see that, in connection with the development of geological-exploration work for iron, a rapid growth of reserves began: whereas as of 1.I.1931 the reserves of all categories were expressed in the amount of 91,744 thousand tons, by 1.I.1934 they were 434,662 thousand tons; that is, reserves had increased 4.74-fold.

Along with the general growth of iron-ore reserves in the region, the appraisal of the Irbinsky iron-ore deposit also changed. P. K. Yavorovsky in 1893 estimated the reserves of rich and lean ores of the deposit at about 8 million tons. In 1926, V. A. Obruchev (48³⁶) determined the ore reserves in 4 large stocks down to a depth of 21 m at 115 million tons. In 1931, A. A. Mesyaninov, on the basis of exploration and magnetometric data, gave ore reserves of categories C₁ + C₂ equal to 42.4 million tons. The same reserve figure (42 million tons) is also cited by K. S. Filatov (34). Finally, after exploration carried out in 1932, the ore reserves are somewhat reduced—to 32.6 million tons (14), while as a result of exploration work the reserves of the higher categories increased, for example: A₂ = 1689.5; B = 639.4; C₁ = 16925.65; C₂ = 13345.85; total 32600.4 thousand tons. Thus, the total ore reserves at 8 million.

tons in 1893, had risen to 32.6 million tons by the present time.

For the individual mountains, the ore reserves are distributed, according to Mesyaninov (14), as shown in the following table.

Table of ore reserves by individual mountains and categories.

Names of mountains	A₂	B	C₁	C₂	Total
Bezymyannaya	—	—	146.9	—	146.0
Zheleznaya	1689.5	639.4	2802.7	1226.0	6357.6
Svintsovaya	—	—	2937.35	1960.85	4898.2
Rudnaya	—	—	657.6	—	657.6
Granatovaya	—	—	10026.0	10027.0	20053.0
Total	1689.5	639.4	16925.65	13345.85	32600.4

As of 1932, the reserves figure was determined (50³¹⁶) as 33,704 thousand tons, of which category A₂—1,689.5 thousand tons, category B—639.4 thousand tons, and C₁ + C₂—31,375.1 thousand tons. It is quite possible that, with further exploration, the reserves will increase, since many bodies are only just being uncovered by denudation and, in addition, ore outcrops occur at various hypsometric elevations.

Along with the rapid growth of reserves, the problem of a comprehensive study of iron-ore deposits comes to the fore. The Department of Mineral Resources of SGI includes in its plan and, under an agreement with ZSGRT, in 1938 organizes the first two works on mineragenic investigation of the iron-ore deposits of the West Siberian Territory. One of these works is being carried out at the Irbinsky iron-ore deposit.

The Irbinsky deposit is located 100 km northeast of the town of Minusinsk, in the Kuragino District, on the steep left bank of the Bolshaya Irba River—the right tributary of the Tuba River, which flows into the Yenisei from the right. It lies 1–3 km from the village of B. Irba and is confined to a chain of low mountains, elongated in a meridional direction for approximately 6 km. The absolute elevations of the mountains are 530–560 m, and their relative height is no more than 250 m.

By the time the material was collected for mineragenic investigations, surface work had been carried out at the deposit (1930–1932), three boreholes had been drilled to depths of 104 to 121 m, two twenty-five-meter shafts had been sunk, with crosscuts from them on Mount Zheleznaya, and on Mount Granatovaya an adit 49 m long had been driven. Thus, we had the opportunity to collect rock material not only from the surface but also from a certain depth, for all the cores from the drilled boreholes, together with the logs and sketches of the mine workings, were made available to us by the technician of the Irbinsky geological exploration party, A. A. Mesyaninov.

In the investigation, the following principal tasks were set: a) clarification of the mineralogical composition of the ores and the enclosing rocks; b) study of the textures and structures of the ore and their relation to those of the enclosing rocks; and c) genesis of the deposit. The investigation was carried out primarily by the method of microscopic analysis. The results of chemical analyses of the ores were taken from the report of A. A. Mesyaninov. To compile the geological card of the deposit, the graphic material accumulated as a result of exploration work at the deposit was used.

III. History of Previous Investigations

Small pits on the Zheleznaya and Svintsovaya mountains with preserved charcoal and slags from the smelting of iron ores indicate that, already in ancient times, iron ore was being processed at the Irbinsky deposit. According to P. K. Yavorovsky (5), the population of the Minusinsk region became acquainted with iron in the Copper Age.

In 1734, the first small ironworks in Siberia had already been built on the ores of this deposit. A mention of the operating Irbinsky ironworks is found in the works of the well-known traveler Gmelin (2³⁰⁴), and in the second half of the eighteenth century the traveler Pallas, who traveled through Siberia on assignment from the Academy of Sciences, wrote: “Further, likewise, the iron needed for the Kolyvan mine and works was brought from the Irbinsky works along the Yenisei to this village (the village of Yanovaya—A.), where it was unloaded, and in winter transported overland across the Yus..... so that during high water it could be carried away on vessels built there toward the Chulym, to the Ob” (3).

In 1770 it was decided to build the Tomsk ironworks, which began to supply the Kolyvan mines and works with iron. The Irbinsky works operated periodically and in 1829 ceased to exist. The cessation of the works’ activity was caused by a whole series of economic reasons (4). At the present time, only traces remain of the former works, in the form of several exploitation workings and remnants of a dam on the B. Irba River.

After the cessation of operations at the Irbinsky works, the deposit was not investigated at all for a long time. Only in 1889 was the deposit visited by the mine surveyor of the Tomsk Mining Administration, mining engineer V. Kotsovsky, who in his evaluation gave a favorable opinion of it.

In 1893, mining engineer P. K. Yavorovsky (6²⁷⁵) carried out a geological survey of the Irbinsky mining-works allotment and conducted a fairly extensive surface exploration of the Irbinsky iron-ore deposit itself. As a result of his investigation he came to the conclusion that the iron-ore deposit was of magmatic origin and was connected with the formation of felsite and telleflint. He assigned a passive role to the granites. In the geological structure of the Irbinsky iron-ore deposit, a large role is assigned to telleflint. By telleflint he meant a dense, apparently siliceous (lithoid) rock possessing banding, due to the alternation of thin dark, light and gray, or light-gray and green stripes. This same rock, in which banding is absent and which is situated closer to the contact with granite, he called felsite. Here it has a gray color and a massive texture.

In 1916 the Irbinsky deposit was examined by the geologist A. N. Churakov (7³⁰), who came to the conclusion that a magnetometric survey needed to be carried out. He believed that telleflint and felsite were not as widely distributed as P. K. Yavorovsky had supposed, and that feldspar porphyrites and dark-green hornfelses, which are closely associated with the ore segregations, have the principal development.

In 1920, under the direction of engineer I. M. Bakhurin (8), a magnetometric survey was conducted at the deposit.

In 1924 the deposit was examined by the geologist Ya. S. Edelstein, who believed that the deposit belonged among the very large ones in the Minusinsk Territory and in the future would play a major industrial role. He expressed the idea of the need to carry out detailed geological surveying, since the geological characterization given by P. K. Yavorovsky required complete revision. A. Ya. Bulynnikov, who conducted geological investigations in the Nizhne-Kizir district, in his work touches upon the geological structure of the district adjacent to the Irbinsky deposits. Among the effusive rocks of the district he considers felsite porphyries and orthophyres to be the most widespread; in the contact zones near the Irbinsky deposit these have been epidotized, with garnet and clinozoisite developed in them.

South of the Irbinsky area he indicates the occurrence of metamorphosed acidic effusives, represented by brownish porphyries with rare segregations of yellowish feldspars, and by purple-gray, strongly epidotized felsites and lenticular porphyries. A. Ya. Bulynnikov assigns the intrusion of quartz syenites to the Devonian age, since he observed that the quartz syenites cut and metamorphose Devonian effusives.

In 1927 A. G. Vologdin (11), during geological investigations in the Tubinsko-Sisimsky district, also examined the area of the Irbinsky iron-ore deposit. He assigned the deposit to the contact-metasomatic type and noted that it is confined to the line of contacts between pure Middle Cambrian marbled limestones and ancient felsites, with a hypabyssal intrusion of reddish alkaline granite. With regard to the marbles and felsite porphyries he writes: “apparently, before they were injected by granite, the marbles were cut by a series of veins of rocks of the felsite-porphyry type, which at the contacts underwent noticeable hornfelsing, epidotization, and other alterations under the influence of the granite, up to the formation of ore bodies. P. K. Yavorovsky called some of these rocks felsites, others—halleflintas” (37). He relates the intrusive rocks of the district to the Neocaledonian folding. Other geologists considered the intrusive rocks of this district to be of Hercynian age. For example, V. A. Obruchev (47³⁷) attributes the formation of the Irlinsky deposit to intrusions of the Hercynian epoch. I. A. Molchanov (49⁴⁶) also ascribes a Hercynian age to the granite intrusion with which he connects the formation of the deposit. In a personal conversation he stated that some facts on the age of the intrusion were published in the literature after his article had been submitted for printing. Therefore he assigns the syenite intrusion to the Neocaledonian age ($D_1$). According to data of the party of the West Siberian Geological-Prospecting Trust, in the area of the Irdzhinsky iron-ore deposit, in red-colored sandstones and conglomerates lying below the definite Middle Devonian ($D_2$) and probably belonging to the uppermost $D_1$, pebbles of the acidic Irdzhinsky intrusion, which is analogous to the Irbinsky intrusion, occur in large quantity.

In 1930 the WSGPT, in the area of the Irbinsky deposits, carried out exploration, magnetometric, and topographic work by a party of the West Siberian Geological-Prospecting Trust. As a result of these works the character of the magnetic anomalies was clarified, a number of small bodies were discovered, and a geological survey of the deposit was carried out. In the same year an iron-ore deposit on Mount Granatovaya was discovered, which had not previously been known. The depos—

The birth of this mountain was named “Novaya Irba.” Incidentally, the largest ore bodies in terms of iron reserves are the deposits of Mount Granatovaya.

In 1931, in the area of the deposits, geologist V. D. Tomashpolskaya (13) carried out a geological survey at a scale of 1:42000. At the same time, under the direction of D. I. Savvateev, a square magnetometric survey was being conducted, and engineer A. A. Mesyaninov directed the exploration work that by this time had been deployed at the deposit. As a result of geological exploration, the outlines of the ore bodies were refined; some bodies were explored at depth by drill holes and small shafts (Mount Zheleznaya), and on Mount Granatovaya one adit was driven, which by 1932, by the time the work was discontinued, had reached a length of 50 m. Concepts of the geology of the deposit also changed in part. For example, in the reports of geologists Mesyaninov and Tomashpolskaya, helleflinta is not mentioned at all. The hornfels occurring in the area of the deposit (near the village of Panchevaya), in their opinion, were formed at the expense of limestones. They write: “The oldest rocks of the area are Cambrian limestones, partly marbled, partly transformed into hornfels”; and further—“As for the genesis of the hornfels, they are evidently the product of metasomatic replacement of limestones” (7—12).

In September 1932, exploration work at the deposit, owing to a reduction of credits, was discontinued and has not been resumed to the present time, while the deposit has remained insufficiently explored.

IV. General data on the geology of the district.

On the basis of geological investigations carried out by A. Ya. Bulynnikov (10), A. G. Vologdin (11), A. A. Mesyaninov and V. D. Tomashpolskaya (13), the geology of the district may be presented as follows.

The oldest rocks of the district are mute limestones, light- and dark-gray in color. They have been encountered in the area of the Peschano-Poperechny Spring (14), along the Bezymyanka River, the right tributary of the Chebizhek River, in the area of the Olkhovsky mine (10), in the area of the village of Beloyarskoye, the village of Kameshki, and the village of Shadrinskoye (11), and occur among effusive rocks, here and there in contact with intrusions, in the form of separate islands and patches. The age of these limestones is determined as Precambrian (10). The grauwacke sequence encountered in the northern part of the sheet investigated by A. Ya. Bulynnikov (10) has been assigned to the Middle Cambrian (?) age.

Upon them lie Middle Cambrian light-gray and gray crystalline limestones containing a fauna of archaeocyaths and trilobites. These limestones, in the form of separate patches and islands situated in contact with the intrusion or among felsite-porphyries, also occur in the area of the Irbinsky iron-ore deposits (11).

The next formation is represented by metamorphic schists and rests unconformably on $Cm_2$ ($10^{32}$). In lithological composition this formation is very diverse. It includes dark-green chloritic, clayey, calcareous-clayey, and calcareous schists, epidotized amphibolites, metamorphic sandstones, and conglomerates. The conglomerates occur in the lower parts of the formation (10, 11). The most typical rocks of the formation are developed along the Kaspa and Tarbatka rivers, in the upper reaches of the Syda River, and in the area of the village of B. Knyshi. The age of the metamorphic—

...thickness Vologdin and Bulynnikov define as Cambrian-Silurian. According to data from the work of the ZSGTT geological parties in 1934, this thickness is assigned an age of \(S_1\).

Effusive rocks overlie the metamorphic thickness; they are assigned to the upper Silurian—lower Devonian. They are especially widely developed in the area of the rivers Syda, Khabyk, Idra, Shush, Irba, Burluk, and occur as separate patches in the NW corner of A. G. Vologdin’s map sheet. The effusive rocks include diabases, porphyrites, orthophyres, felsite-porphyries, felsites, banded porphyries, quartz porphyries, and albitophyres. In direct contact with syenites, the effusive rocks underwent hornfelsing and skarnification. In the contact zones of the Tubinsky syenite massifs, A. Ya. Bulynnikov (10, 20) describes hornfelses and metasomatic rocks formed from porphyrites. Microscopic study of hornfelses located in the Irba area shows the development in them of plagioclase and quartz with a hornfelsic structure. In addition, relict structures of effusive rocks occur in the hornfelses, which makes it possible to suppose that the hornfelses formed at the expense of felsite-porphyries and albitophyres. A more detailed characterization of the hornfelses will be given below in the description of the geology of the deposit.

In the NW part of the map sheet of the work of V. D. Tomashpolskaya (13), red-colored sandstones crop out; in composition they are identical to the sandstones of the Abakan Formation (\(13^{15}\)), whose age was determined by Ya. S. Edelshtein (\(16^{63}\)) as Lower Devonian. At the base of the Abakan Formation (\(11^{99}\)) lie conglomerates encountered in the vicinity of the village of Panachevaya. The pebbles of these conglomerates consist mainly of limestones and hornfelses, but microscopic study reveals small fragments of effusive rocks in them. The cement of the conglomerates is a lime-clayey substance or volcanic material (\(13^{13}\)). A. G. Vologdin (\(11^{93-45}\)) notes the presence of conglomerates in various parts of the area he studied (the village of Otrok, the villages of Kameshki and Moiseevsky, the Sasimsky area, Mount Sarallyk, etc.). In individual areas (Mounts Sarallyk and Khamei) the conglomerates contain pebbles consisting of alkaline granites and syenites. The red granites whose pebbles occur in the conglomerates of Mount Khamei, A. G. Vologdin considers related to the granites of the Irba area and Mount Ural. In the Irba area, the pebbles of conglomerate \(D_1\) contain no intrusive rocks, and it is possible that the pebbles of such rocks described by Vologdin (11) belong to another intrusion. Consequently, we may assume that their age is not older than \(D_1\).

In the region described, intrusive rocks are very extensively developed. In composition they are characterized by great diversity. A. Ya. Bulynnikov (10) distinguishes two intrusions: an intrusion of granodiorites (adamellites and banatites) and an intrusion of quartz syenites. The rocks of the granodiorite intrusion are distributed along the rivers B. Burluk and Kizyr, in the area from the Mishkin mine to the Zolotoy spring. In the southwestern part (the area of the Kizyr River) the granodiorite intrusion is separated from the quartz-syenite one by a small distance (1.6 km). Quartz syenites in the Irba area are widely distributed. They are developed from the village of Irba northward to the village of Znamenka, northwestward to the village of Panachevaya, and southwestward, with an interruption, to the village of Gryaznukha. Farther on, as separate patches, they are encountered north of the village of M. Irba and in the area of the iron-ore deposits as far as the village of Uspenka. They have a flesh-red color and are coarse-grained. They consist of K-Na feldspar...

with an admixture of plagioclase, as well as residual quartz. The dark-colored components are represented by biotite, hornblende, and augite (10, 14). Along with coarse-grained quartz syenites, flesh-red medium-grained rocks are developed, in which graphic textures are often observed. The marginal zones of the syenite intrusion are represented by gray and pinkish-gray rocks, which in composition are close to monzonites. The varied mineralogical composition of the intrusive rock and the narrow zone of contact metamorphism indicate the hypabyssal character of the intrusion (17).

A. Ya. Bulyannikov (10¹⁵) observed that the intrusion of quartz syenites cuts through and metamorphoses the Devonian effusives. Consequently, its age should not be older than the Lower Devonian. A. G. Vologdin (11⁹⁰) arrived at the same conclusion regarding the age of this intrusion. He assigns the formation of the magmatic rocks developed in the area of his work to the upper Silurian and partly to the lower Devonian. The age of the quartz-syenite intrusion, as the younger one, can evidently be considered Lower Devonian. M. A. Usov (33) and Yu. A. Kuznetsov (40) also assign it to the Lower Devonian.

The vein rocks associated with the syenite intrusion may be divided into two groups: a) basic and b) acidic. To the group of basic rocks we assign rocks represented by dark-greenish, grayish-greenish granular varieties, which are united under the general name diabases. The group of acidic vein rocks includes quartz syenite-porphyries, monzonite-porphyries, and albite-porphyries. These vein rocks cut effusives, skarns, ore bodies, and also intrusive rocks. It is necessary to note that calcite is developed in the vein rocks, and that it is metasomatic.

The Neo-Caledonian intrusion completes the formation of the igneous rocks of the area adjacent to the Irbinsky deposit.

In later epochs we observe here only deposits of loess and river alluvium. The loess occurs as separate patches near the deposit itself, for example on Mount Svintsovaya, or in the depressions of Mount Granatovaya and in other areas.

V. Description of the Rocks of the Deposit

The geological formations of the Irbinsky iron-ore deposit include limestones; effusive rocks, represented by porphyries, felsite-porphyries, and orthophyres; intrusive rocks, represented by quartz monzonites; vein rocks (syenite-porphyries, monzonite-porphyries, albite-porphyries, and diabases); and skarns.

1. Limestones. The oldest rocks of the deposit are light- and dark-gray medium-grained limestones—marbles. Their age, by analogy with the limestones distributed near the village of M. Irba and containing fauna, is determined as Middle Cambrian (11, 13). Massive light-gray medium-grained limestones extend along the western foot of Mount Rudnaya and Mount Svintsovaya. Then, as a narrow band over a distance of 2.5 km, they reach the northern end of Mount Granatovaya. In the northern part of this band, where there is a strip of breccia-like rocks, a fluorite deposit occurs. In addition, on Mount Svintsovaya, along the 3rd exploration line, an outcrop of marbles was encountered. On Mount Zheleznaya there are separate outcrops of skarnized limestones, for example on the southern slope of the mountain in contact with the ore body on the right side of Zhelezny ravine, in the passage—

In the adit and trench No. 26, coarse-crystalline limestones of a light-gray color occur. These limestones have been partially subjected to skarnification, with the formation of garnet.

In the form of separate small islands, pure light-gray marbles have been encountered (sample 52) at the very summit of the western slope of Bezymyannaya Mountain. Closer to the contact with the intrusive rocks, green bands and irregularly shaped spots appear in these limestones, consisting of garnet. Under the microscope it is evident that the garnet replaces the limestones, spreading through them in the form of separate spots and veinlets. Dark-gray marbled limestones have been encountered in trenches 34 and 69 of Bezymyannaya Mountain (Fig. 1), where the limestones, together with the ore body, are cut by a diabase dike.

Consequently, limestones occur in the area of the deposit itself in the form of separate small islands or spots, and are sometimes situated near the ore bodies or even in contact with them. Under the action of the intrusion, the limestones have been marbled and skarnified, with the formation of garnet, diopside, and magnetite. In thin sections the garnet has a yellowish-green color, is isotropic, and is distributed in the marble as separate spots and veinlets. Magnetite occurs sporadically in the marble and is always formed near garnet. In addition, some areas are replaced by quartz, which, along with the formation of individual grains among the limestone, forms thin veinlets in the garnet. Colorless mica occurs here and there in the limestones. It should be noted that individual cracks are filled with calcite; possibly, local regrouping of the calcareous material is manifested here.

2. Felsite porphyries occur in the area of the deposit chiefly on the western slope of Zheleznaya Mountain. Here they extend from shaft No. 72 to the southeast as far as exploration line XVIII, and westward to the foot of the mountain, i.e., to the Bolshaya Irba River. The name felsite porphyries is given to strongly altered dense rocks. They have a banded texture caused by heterogeneity of coloration, which varies from lilac-brown to dark violet, sometimes gray. These rocks are very dense, with a conchoidal fracture, fine-grained, and some varieties of them resemble hornfels, while where the banded texture is more strongly expressed they resemble halleflinta (according to P. K. Yavorovsky). Microscopic study has established that the rock consists of plagioclase, quartz, pyroxene, epidote, garnet, sphene, magnetite, chlorite, and limonite. The structure of the rock is hornfelsic. In some thin sections heterogeneity of coloration was observed; differently colored areas are arranged as spots, either light-colored or having a darker coloration. The light-colored areas consist of coarse-grained aggregates of quartz and plagioclase, together with which epidote and light-colored garnet are present in them, while the areas colored gray consist of fine-grained material—namely quartz and feldspar contaminated by chlorite—and have a replacement structure. At the same time, it is sometimes observed that the finer-grained areas are, as it were, surrounded by a ring of coarser-grained material. Apparently, the rocks originally consisted of clastic material of varying grain size and composition, or perhaps were broken up after their formation. The differently colored areas are more often arranged as separate bands.

Plagioclase occurs as small (0.12–0.6 mm) corroded grains; sometimes they have fine polysynthetic striation. More often the plagioclase has decomposed to such an extent that it is difficult to determine in

in transmitted light it becomes brownish—being filled with iron oxides. In places, sericite develops on the plagioclase. All feldspar grains are corroded and have a hornfelsic structure.

Quartz fills the interstices between the plagioclase grains. It is distributed as small grains with a cross-sectional area from 0.002 to 0.005 mm² and is residual. In addition, the rock contains quartz of a second generation; it occurs in the form of veinlets cutting the rock in various directions.

Diopside is observed in all thin sections in the form of small grains that corrode the plagioclase grains. Diopside is distributed unevenly: in rocks situated near the contact with the monzonites it is observed in large amounts, forming pyroxene skarns, while in rocks occurring far from the contact its content is insignificant.

In thin sections 2176 and 2177, colorless (rarely brownish) isotropic garnet is observed, occurring as separate grains; in places it forms bands, and sometimes forms veinlets of small thickness. Garnet more readily replaces plagioclases. Then epidote appears in the rock, occurring as separate accumulations. In some thin sections (2176), in areas consisting of plagioclase, quartz, and epidote, very fine-grained biotite is encountered. In transmitted light it has a greenish-yellow color.

At the boundaries between coarse- and fine-grained areas, sphene develops, occurring as separate grains. In the finer-grained areas, a small amount of magnetite is observed. Finally, chlorite and sericite are rather well developed in the rock. In addition, the rock is in places impregnated with iron oxides, formed, evidently, at the expense of the destruction of sulfides.

The sequence of mineral formation may be represented by the following scheme.

Minerals	Primary rock	Metamorphic processes	Exogenous processes
1. Plagioclase (albite)	—
2. Quartz	—	—
3. Diopside		—
4. Epidote		—
5. Garnet		—
6. Magnetite		—
7. Biotite		—
8. Chlorite-sericite		—
9. Limonite			—
10. Sericite		—

From the scheme it is evident that only plagioclase and quartz have been preserved from the primary rock; at present they have a hornfelsic structure. The dark-colored components of the primary rock were not encountered. In the first stage of metamorphism, caused by the intrusion of quartz monzonite, the rock underwent hornfelsing, and subsequently skarnization, with the formation of diopside, garnet, magnetite, quartz, epidote, biotite, and chlorite-sericite. It is possible that part of the biotite formed during the hornfelsing stage. In the exogenous stage the rock is filled with limonite, which sometimes forms veinlets. Thus it may be considered that the described rock belongs to the effu-

mass, which has been altered under the action of contact metamorphism. These felsite-porphyries may be compared with the banded porphyries located in the lower reaches of the Buruluk River and along the Cheresh-tyky key eastward to Mount Izyk, and described by A. Ya. Bulynnikov (10).

On Mounts Svintsovaya, Rudnaya, and Zheleznaya there occur garnet skarns of banded structure. In places among them relict structures of felsite-porphyries can be observed. Taking into account that the felsite-porphyries possess a banded texture and are comparatively readily skarnized, and that near the ore bodies garnets develop in them predominantly, we may assume that the skarns with a striped texture were formed by replacement of banded felsite-porphyries.

The question of the age of the effusives remains unresolved, but, by analogy with the felsite-porphyries of the district, we may assign them to $D_1$ (?).

3. Porphyrites occur on Mount Granatovaya. They compose the southeastern and southwestern slopes of the mountain, bending around the ore bodies and the skarn field from the southern side. Separate outcrops of porphyrites, together with calcitized sandstones, were encountered on the western slope of the mountain at its very foot. Macroscopically, the porphyrites have a dark-gray color with a greenish tint; the texture is massive, medium-grained. Around the skarn fields they acquire a dense structure and contain sulfides. Under the microscope, the porphyrites show plagioclase, alkali feldspar, hornblende, pyroxene, epidote, chlorite, carbonates, sphene, sericite, leucoxene, magnetite, and sulfides. Plagioclase forms chiefly elongated laths, more rarely short-prismatic ones, arranged at various angles to one another, sometimes parallel. In individual grains polysynthetic twinning is visible and weak zoning is noticeable; the plagioclase grains are strongly corroded and in places even torn apart. Sericite develops after plagioclase and affects all the plagioclase grains. Alkali feldspar is also very well developed; it corrodes the plagioclase grains, and in the case of a porphyritic structure fills the interstices between the plagioclase laths and more rarely forms porphyritic segregations. In porphyrites from the western slope of the mountain, greenish actinolitic hornblende is observed, sometimes forming large grains or entering into the composition of the groundmass, where it has fine grains. Closer to the skarnized areas, diopside develops in the porphyrites, forming small grains in the rock. Chlorite develops after diopside and actinolite and in places clogs the rock. Sphene, epidote, leucoxene, and carbonates occur in insignificant amounts, the latter being represented by calcite and siderite (?). The carbonates are a later formation, and they corrode (in individual areas) all the minerals composing the rock. In the porphyrites, magnetite and sulfides (chalcopyrite and pyrite) are observed. Magnetite is more often located in the interstices of plagioclase and is rarely formed around the dark-colored components.

The structure of the porphyrites varies rather strongly. In the studied samples we observed an andesitic structure, a structure close to bostonitic, where feldspars with serrated edges are arranged irregularly and magnetite occurs in their interstices, and, finally, a porphyritic structure. In the latter case, plagioclase and hornblende occur in the porphyritic segregations, and alkali feldspar rarely. In the groundmass there are the same minerals, only fine-grained.

Taking into account the variability of the rock structures, one may assume that the porphyrites, or more precisely the trachyandesites, were formed under conditions close to hypabyssal ones, i.e., their formation took place at some depth below the surface.

4. Sandstones were encountered on Mount Bezymyannaya (k. 54) and at the NW foot of Mount Granatovaya. In their composition they differ sharply. On Mount Bezymyannaya the sandstones have a variegated coloration. The greater part of the rock consists of a greenish felsite-like mass, in which there are inclusions of a reddish color, probably consisting of feldspars; some parts of the rocks have a mylonitized appearance.

Under the microscope we observe angular fragments of feldspar and quartz; fragments with hornfelsic structure are common, and here and there graphic structures occur in them, formed by intergrowths of feldspar and quartz. Weakly corroded fragments are observed rarely. The plagioclase composing the fragments has fine polysynthetic twinning and probably belongs to albite-oligoclase. Quartz occurs as separate small grains situated among the cement; sometimes it enters, in small quantity, into the composition of the cement. The cement of the sandstone is represented by a microfelsitic mass, green in transmitted light. Apparently it contains a large amount of chlorite. The rock also contains a small amount of hornblende, which is arranged in the form of small grains and is apparently metasomatic. Magnetite grains occur in some thin sections. Epidote is developed in small quantity in the sandstones. It occurs as individual grains. Around the fragments a brown substance—limonite—often accumulates, forming narrow rims. In addition, limonite impregnates the cement of the sandstone, giving it a yellowish color.

At the western foot of Mount Granatovaya, in the direction toward the B. Irba River, an outcrop of sandstone containing a large amount of calcite was encountered. Macroscopically, the color of the rock is heterogeneous—spotted. The main mass of the rock is colored greenish, and within it there are separate inclusions of white calcite. The texture is breccia-like. Microscopic study established that the fragments consist of plagioclase, calcite, and alkali feldspar; the cement is represented by calcareous material and chlorite. Accessory minerals include apatite, limonite, ilmenite, and leucoxene.

Plagioclase occurs as separate grains having fine polysynthetic twinning. All plagioclase grains are corroded by calcite, which constitutes the cement of the rock. Along with plagioclase, a small amount of alkali feldspar occurs, brownish in transmitted light. Calcite occurs as separate aggregates of grains and makes up a significant part of the rock; moreover, the part of the calcite that enters into the rock fragments is coarser-grained, while the calcite forming the cement is fine-grained. It, together with the plagioclases, cuts by thin veinlets both fragments consisting of calcite and [[unclear: continuation cut off at bottom of page]]. Chlorite forms part of the cement and is developed rather widely in the rock. It sometimes forms separate accumulations, among which radial-fibrous chlorite occurs. In places chlorite forms veinlets in calcite fragments. Near plagioclase, 2–3 elongated grains of apatite were encountered. Limonite surrounds some fragments of plagioclase and calcite with a narrow rim, and

also enters into the composition of the cement, where it occurs in insignificant quantity. In addition, ilmenite and leucoxene occur in the sandstone.

The sandstones described above have been metasomatically altered. In the area of the deposit they are not very widespread and occur, as already noted, only on Granatovaya and Bezymyannaya mountains. On Bezymyannaya Mountain they form a small area enclosed within a branching ore body.

5. Hornfels among the rocks of the deposit are of minor extent. They occur in the contact zones of intrusive rocks, i.e., quartz monzonites, and form a narrow belt whose thickness varies, approximately, from several tens to 200–300 m. The coloration of the hornfels is heterogeneous—dark gray with a greenish tint, greenish, pinkish, and red. Sometimes the hornfels have a banded texture, caused by heterogeneity of color and composition. They are dense, fine-grained. Inclusions of sulfides, epidote, garnet, and magnetite are observed. Under microscopic study the hornfels were found to contain plagioclase, quartz, actinolite, sphene, epidote, chlorite, calcite, garnet, apatite, sericite, ilmenite, magnetite, and sulfides.

Plagioclase occurs in the form of very small grains having a granoblastic structure. In some grains a fine polysynthetic striation is visible, but more often polysynthetic twinning is not observed. In transmitted light it has a grayish color. In quantities subordinate to plagioclase, alkali feldspar occurs in the hornfels, having a brownish color in transmitted light. Quartz occurs in the hornfels in varying amounts. Sometimes it makes up a large part of the thin section, but more often it is observed in small quantities and predominantly has a granoblastic structure. In addition, quartz forming thin veinlets was observed in the thin sections. Diopside occurs in all thin sections as irregularly shaped grains, whose size on average is 0.09 mm; it should be noted that the grain size of the rock, varying from 0.2 to 0.40 mm, does not affect the size of the diopside grains. It forms more readily around grains of feldspar, swelling the latter. Sometimes short-prismatic grains of diopside are observed, which in most cases are replaced by hornblende—actinolite. Evidently, this pyroxene belonged to the primary rock. Actinolite is relatively weakly developed. It forms along the periphery of diopside grains, more rarely replacing such grains completely. Epidote is relatively weakly developed in the hornfels. It occurs in almost all thin sections and is arranged as individual grains, chiefly around diopside, garnet, and calcite. In some places it was observed that pyroxene, amphibole, and epidote form short veinlets in the rock. Calcite occurs rarely; it sometimes surrounds feldspars in the form of a narrow border, sometimes replaces them, and more rarely forms veinlets. Along feldspars arranged in the form of laths, sericite occasionally develops. In several thin sections (borehole No. 3 and Svinцovaya Mountain No. 209) garnet was encountered; in transmitted light it has a brownish color and is isotropic. In addition, apatite, sphene, ilmenite, magnetite, and sulfides occur in small amounts in the rock. Separate portions of the hornfels are chloritized.

The sequence of mineral formation may be represented by the scheme on page 13.

Consequently, the process of formation of the hornfels may be represented as follows. The composition of the igneous rock (probably,

Scheme of mineral formation in hornfelses.

Minerals	I. Primary rock	II. Hornfelsing process	III. Skarnification process	IV. Exogenous processes
Feldspars	—	Recrystallization of feldspars and quartz.	—
Quartz	—	Recrystallization of feldspars and quartz.	—
Pyroxene	— ? —	Recrystallization of feldspars and quartz.	—
Actinolite		Recrystallization of feldspars and quartz.	—
Sphene		Recrystallization of feldspars and quartz.	- - - - -
Epidote		Recrystallization of feldspars and quartz.	—
Chlorite		Recrystallization of feldspars and quartz.	—
Calcite		Recrystallization of feldspars and quartz.	—
Garnet		Recrystallization of feldspars and quartz.	- -
Apatite	—	Recrystallization of feldspars and quartz.	— ? —
Sericite		Recrystallization of feldspars and quartz.	—
Ilmenite		Recrystallization of feldspars and quartz.	—
Magnetite	—	Recrystallization of feldspars and quartz.	—
Sulfides		Recrystallization of feldspars and quartz.	—
Limonite		Recrystallization of feldspars and quartz.		—

the effusive rock) included plagioclase, alkali feldspar, quartz and, possibly, pyroxene. The latter was observed here and there in the form of very disintegrated grains, for the most part replaced by actinolite. This phenomenon compels one to suppose that pyroxene was present in the original rock.

Under the influence of contact metamorphism the rock underwent hornfelsing. In this stage the formation of new minerals did not take place; there occurred only recrystallization of the minerals, with the formation of a granoblastic and allotriomorphic structure.

In the following period, when recrystallization had already been completed, the hornfelses underwent skarnification, with the formation of diopside, garnet, actinolite, epidote, apatite, ilmenite, quartz and, in the final stage of skarnification, chlorite, sericite, calcite, and sulfides. The time of formation of sphene could not be established, but its formation probably also belongs to the stage of skarnification.

Quartz and calcite, which make up veinlets in the rock, and also, in part, chlorite and sulfides, were apparently formed during the formation of the porphyry dikes, which are accompanied by the removal of silica and carbonates.

In the exogenous stage limonite is formed; in the sections studied it was encountered in a very insignificant amount.

6. Quartz monzonites in the area of the deposit are widely distributed and constitute the marginal part of the intrusion of quartz syenites, which do not occur within the limits of the deposit and are not described here. The coloration of the monzonites varies from reddish to gray, and near the contact a greenish tint appears, apparently due to the presence of epidote. The texture of the rock is massive; rarely they have a weakly expressed banding, which is manifested in the heterogeneous coloration of individual bands. At the contacts there is almost always observed a narrow border (30–40 cm) of rock having a felsite-like appearance. The monzonites include gray to flesh-red feldspar, a small amount of quartz; the dark-colored components are represented by pyroxene. In areas having a greenish tint, epidote is noticeable.

Microscopic study established that the rock consists of plagioclase, alkali feldspar, quartz, augite, and accessory minerals—apatite, ilmenite, zoisite. The structure is panidio-

morphic. Plagioclase, with fine polysynthetic twinning, occurs as idiomorphic short-columnar prisms ranging in size from 2.6 to 0.25 mm. Individual plagioclase grains are strongly corroded by alkali feldspar and quartz. In places the plagioclase has undergone strong decomposition with the formation of sericite, and, as an exception, the formation of zoisite is observed. The central parts of the grains have been more strongly decomposed and the peripheral parts more weakly; moreover, in some thin sections the peripheral parts of the plagioclase grains have remained entirely fresh. Measurement of several plagioclase grains on the Fedorov stage gave Nos. 5–13. Consequently, the plagioclase belongs to albite and albite-oligoclase. At the contact of the intrusion with the country rocks, plagioclase grains often assume an oval form. In quartz monzonite, alkali feldspar is present in considerable quantity and often corrodes the plagioclase. Alkali feldspar is not infrequently found intergrown with quartz and produces micropegmatitic structures, which form more readily in the reddish varieties of monzonite, for example, a monzonite sample from the right bank of the B. Irba River.

Pyroxene occurs in small amounts. Its grains have idiomorphic outlines and reach sizes of up to 1.8 mm. In transmitted light its color is pale greenish to colorless. Sometimes pyroxene fills the interstices between plagioclase grains; in places its grains appear corroded by plagioclase. Determination by the Fedorov method gave \(2V = +6^\circ\); \(cNg = 42^\circ\). Consequently, the pyroxene belongs to iron-bearing diopside. Amphibole forms along the periphery of diopside grains.

Together with diopside, ordinary hornblende occurs in the monzonites; in transmitted light it has a greenish color and pleochroism in greenish tones. Its grains are often destroyed with the formation of chlorite. Measurements of the grain give \(2V = 72^\circ\); \(cNg = 15\text{--}21^\circ\), which identifies it as ordinary hornblende.

After hornblende, and sometimes also in the feldspars, a mineral develops that has a light brownish color. Its pleochroism is in brownish tones. The mineral’s adsorption is biotitic. The cleavage of the mineral is expressed in one direction; extinction is straight. It polarizes in colors of the first and second orders; the interference color is heterogeneous. This mineral is rarely found in crystals; more often it forms veinlets in plagioclase. In hornblende it creates an irregular pattern and gives the impression that it is a relic of some replaced mineral. The mineral described evidently belongs to iron-bearing chlorite and formed at a low-temperature stage by replacement of biotite, which had begun to decompose and is not encountered in the quartz monzonites of the contact zone¹.

Quartz occurs in varying amounts: in some thin sections it is present in insignificant quantity, situated in the interstices of plagioclase; in others, in addition to filling the spaces, it produces graphic structures, intergrowing predominantly with alkali feldspars. In addition, there is quartz of a second generation, with irregularly shaped grains and forming, in places, veinlets that cut the rock in various directions. Among accessory minerals in the rock, apatite develops in the form of fine needles, and ilmenite, along the periphery of which leucoxene forms. In addition, isolated small grains of sphene occur.

Near the ore bodies, the quartz monzonites are weakly hornfelsed and skarnified. The skarnification of the monzonites is expressed in the development of garnet, pyroxene, actinolite, magnetite, epidote, quartz, and calcite. Garnet selectively replaces feldspars, forming bays, inlets, etc., within them. Sometimes only small remnants remain of the feldspar grains, arranged as a rim around garnet grains; for example, Mount Svintsovaya No. 391 (Fig. 2). Selective replacement of feldspars by garnets was observed by A. N. Zavaritskii (38²¹) in his study of copper-ore deposits in the Urals. He writes that garnet develops in the place of plagioclase, while augite remains unreplaced. Consequently, in igneous rocks garnet preferentially develops after feldspars. In transmitted light the garnet has a brownish color and is isotropic. Rarely, anomalous garnet is also encountered; in transmitted light it has a pale-greenish color. Pyroxene is distributed in the form of small grains, which often have an oval shape. It is colorless; in crossed nicols it gives interference colors up to yellow. Measurement on the Fedorov stage gave \(2V = +62\); \(cNg = 48\), which corresponds to diopside-hedenbergite. Epidote appears in the form of separate flakes scattered throughout the rock, but often it develops along the periphery of plagioclase grains and, more rarely, forms thin veinlets. Magnetite is encountered in small quantity. It forms near garnet, sometimes penetrating into the latter along cracks. In the rock there is a radiating amphibole, which develops after pyroxenes and is often associated with magnetite. Incidentally, near magnetite the amphibole is always lighter than away from it. Along with the formation of quartz during skarnification, a considerable amount of calcite appears; it corrodes feldspars, sometimes replacing them entirely, and forms veinlets cutting the rock in various directions. Calcite veinlets cutting quartz have uneven margins and, consequently, are replacement veinlets. The development of actinolite is associated with the time of calcite formation. Chlorite forms last; it is situated near dark-colored components and magnetite, and more rarely near quartz veinlets.

The order of mineral formation may be represented by the following scheme.

Scheme of mineral formation in quartz monzonite.

Minerals	I epoch, magmatic	II epoch, emanational	III epoch, exogenic
1. Plagioclase	———
2. K-Na feldspar	———
3. Diopside	———
4. Diopside-hedenbergite		———
5. Ordinary hornblende	---
6. Actinolite		———
7. Quartz	---	———
8. Epidote		———
9. Chlorite		———
10. Apatite	—
11. Ilmenite	—
12. Leucoxene		---	---
13. Sphene	---
14. Magnetite		———
15. Calcite		———	--------
16. Zoisite	—
17. Garnet		———
18. Sericite		---

— In the magmatic stage, the formation of the country rock—the quartz monzonite—took place. In the emanation stage, the principal phase, simultaneously with the formation of the ore deposits, was the skarnization of the monzonite, expressed in the formation of garnet, diopside, magnetite, epidote, actinolite, quartz, and calcite. In this same epoch chlorite, leucoxene, and sericite are formed. It should be noted that the development of calcite and, in part, quartz and epidote is probably connected with the formation of porphyry dikes. In addition, calcite formation also continues into the exogenous stage.

Vein formations of diabases and porphyries are associated with the intrusion of the quartz monzonites; they occur in the area of the iron-ore deposits and are especially strongly developed on Mount Svintsovaya, where all the ore bodies are cut by dikes. Here 23 dikes are counted, of which 14 are diabase and 9 are porphyry. On the other mountains the number of dikes is smaller; for example, on Mount Zheleznaya there are 10 dikes, Bezymyannaya—4, Rudnaya—4, Granatovaya—3. The thickness of the diabase dikes varies from 0.7 to 19 m (Zheleznaya and Svintsovaya) and from 1.2 to 3.6 m on Mount Granatovaya. The thickness of the porphyry dikes ranges from 0.4 to 24 m.

7. Diabases have the form of dikes of varying thickness from 0.3 to 12 m; their strike is in various directions, but predominantly NW. Macroscopically the diabases have a greenish, dark-gray color with a greenish tint and, in the case of strongly weathered varieties, a greenish color with a yellow tint. The texture of the diabases is massive, fine-grained, with porphyritic segregations of plagioclases. On disintegration they give spheroidal jointing, more rarely columnar. In some diabase dikes of Mount Zheleznaya (298°) and Granatovaya (213°), inclusions of pyrite crystals are observed. In contacts with porphyries, the diabases acquire a denser structure and become partly lighter. Microscopic study of the diabases has shown that they consist of plagioclase, pyroxene, quartz, apatite, ilmenite, magnetite, epidote, pyrite, calcite, chlorite, and limonite.

Plagioclase is arranged in the form of laths ranging in size from 0.2 to 1.2 mm. The laths are arranged at certain angles to one another and resemble an ophitic structure. The plagioclase grains have polysynthetic twins; distinct zoning is often observed. Almost all plagioclase grains have undergone some destruction, the result of which is the development of sericite. Some of the plagioclase grains are bent and torn into separate segments. In the interstices of the plagioclase is pyroxene, which in the greater part of the rock is destroyed with the formation of chlorite. But in places grains of diopside have been preserved. In transmitted light they are colorless, pleochroism is imperceptible, and interference colors are up to the second order. Quartz is also located in the interstices of the plagioclase. It occurs in small quantity and has an irregular grain shape.

In the diabase, chlorite is highly developed. It often fills all the spaces between plagioclase, apparently replacing the dark-colored components. Judging by the preserved remnants of grains located in the center of the chlorite, the dark-colored components were represented by diopside. In some thin sections chlorite with a heterogeneous interference color is observed, having good cleavage in one direction; it is possible that here grains of hornblende have been replaced by chlorite. In places chlorite has a yellowish and yellow-brown color and various interference colors (variegated). This heterogeneity is probably caused by contamination with its oxides.

iron. Sometimes chlorite forms narrow rims around plagioclase grains. In thin sections apatite is occasionally encountered, arranged in the form of very fine needles. Epidote is observed in small quantity. It develops near sulfide grains and apparently formed simultaneously with the latter, possibly during the formation of porphyry dikes. A certain amount of sulfides is present (\(\mathrm{FeS_2}\) and \(\mathrm{CuFeS_2}\)). Around them limonite forms, which spreads through the rock and contaminates it. Calcite, which develops along plagioclase, is observed very rarely in thin sections. Finally, magnetite and ilmenite and the decomposition product of the latter—leucoxene—are encountered in thin sections.

Magnetite has a granoblastic structure, and in some thin sections it is distributed throughout the entire section in the form of a fine sprinkling. Sometimes ilmenite is encountered among plagioclase in the form of elongated grains.

The order of mineral separation may be represented by the following scheme.

Scheme of mineral formation in diabase.

Minerals	Magmatic stage	Emanation stage	Exogenous processes
1. Plagioclase	———
2. Quartz	———	— —
3. Pyroxene	———
4. Calcite		—	— — — —
5. Epidote		———
6. Chlorite		—
7. Apatite	—
8. Sulfides		—
9. Magnetite	—	— ? —
10. Limonite			————
11. Ilmenite	—

From the scheme it is evident that in the magmatic stage plagioclase, pyroxene, quartz, apatite, ilmenite, and magnetite were formed. In the emanation stage magnetite is formed, especially that part of it which has a granoblastic structure. In this same stage epidote and chlorite appear. The appearance of quartz and carbonates is not entirely clear. It is possible that these minerals formed during the formation of porphyry dikes, with whose formation the alteration of the rocks is connected, expressed in silicification, carbonatization, and, as we shall see in describing the porphyries, in the appearance of chlorite. The sulfides, apparently, formed partly in the emanation stage of the formation of the diabases and partly, especially near the contacts, during the appearance of the porphyry dikes.

In the exogenous stage limonite is formed and, in small quantity, calcite, which here and there penetrates along fractures.

8. Porphyries occur throughout the entire ore-deposit area in the form of dikes, except for Mount Granatovaya, where porphyries have not been encountered. Macroscopically, the porphyries are represented by grayish and reddish rocks having a massive texture, more rarely a banded one. The bulk of the porphyries is dense, fine-grained, and sometimes assumes a felsitic appearance (especially at the margins of the dikes). The porphyritic segregations are represented by red feldspars. Some dikes experienced pressure, traces of which are expressed in the form of parallel fractures. Along the fractures thin (about \(1\ \mathrm{mm}\)) veinlets of quartz form. The dikes

porphyries in general have a NE, often nearly latitudinal, strike. Microscopic study has established that the rock structure is porphyritic. The porphyritic segregations are represented by plagioclase and, rarely, by alkaline feldspar. The size of the feldspar laths ranges from 1.7 to 0.17 mm. Most of the plagioclase laths possess fine polysynthetic twinning; in some grains a weak zoning is noticeable. The plagioclase composing the porphyritic segregations, according to measurements on the Fedorov stage, belongs to albite-oligoclase and has No. 6—13. Many plagioclase grains are sericitized, corroded by quartz, chlorite, epidote, and calcite. The greater part of the groundmass of the porphyries consists of plagioclase, alkaline feldspar, quartz, and, in smaller quantities, apatite, sphene, epidote, pyroxene, amphibole, chlorite, magnetite, sulfides, carbonate, ilmenite, limonite, and orthite.

Plagioclase has elongated laths measuring from 0.05 to 0.01 mm, arranged at various angles to one another, and the interstices between them are filled with quartz, which has distinctly xenomorphic grain forms. Quartz, in addition, occasionally forms thin veinlets in the porphyries. It corrodes the plagioclases, of which only small remnants sometimes remain. Alkaline feldspar is less widespread than plagioclase. It sometimes forms porphyritic segregations, but occurs chiefly in the interstices of the plagioclases and enters into the composition of the groundmass as small grains. In transmitted light it has a brownish coloration. The grain form of the alkaline feldspar is irregular—in relation to plagioclase it is xenomorphic. In several thin sections small relics of pyroxene and actinolite (?) grains were encountered. Most of the dark-colored components have been destroyed and converted into chlorite, which is very widespread in the porphyries. It replaces entirely some dark-colored components. Chlorite develops in the form of flakes; in certain parts of the rock it corrodes plagioclases and sometimes forms short veinlets. Epidote is developed to a comparatively minor degree; it also eats into the plagioclases and appears, evidently, simultaneously with the quartz that forms veinlets in the porphyry. In almost every thin section apatite is encountered, forming small needles. In some places it is developed in large quantities. For example, in the dike on Mount Svinцovaya (273), apatite is arranged as small needles in the groundmass and, in addition, occurs in large elongated grains. Sphene is encountered here and there, forming small grains of irregular shape with well-expressed pleochroism. In the porphyry dikes studied, calcite is widely developed. It is a later formation and occurs in the form of irregular grains that corrode plagioclase, alkaline feldspars, and other minerals. Sometimes it forms thin veinlets. In some areas the plagioclase grains are entirely replaced by calcite. Further, ilmenite, leucoxene, magnetite, and sulfides occur in the dikes as accessory constituents.

In the quarry near trench No. 5 on Mount Svinцovaya, a porphyry dike of heterogeneous structure was encountered. In the center of the dike the porphyry is reddish in color, fine-grained, with porphyritic segregations of plagioclases. In the marginal parts (approximately 15 cm wide) of the dike, the groundmass takes on a felsite-like appearance, the coloration becomes dark gray, and the porphyritic segregations are represented by plagioclase. In the composition of the dike, along with a high content of apatite, orthite appears. Orthite has high relief and a mottled coloration, which

gray varies in color from brown to colorless; pleochroism is fairly pronounced. The interference colors are also variegated. Some grains of orthite pass into epidote, with a gradual transition between them.

In the country rocks, in particular in quartz monzonites, the porphyry dikes cause a certain alteration, expressed in the formation of epidote, silicification, and calcitization. The alteration of the country rocks occurs at the contacts themselves and forms a narrow selvage.

On the basis of their composition, the porphyry dikes described should be assigned to syenite porphyries and monzonite porphyries. We assign to the syenite porphyries those dikes in which the porphyritic segregations are represented by albite-oligoclase and grains of alkali feldspar; in the groundmass there are orthoclase, plagioclase, and quartz. We assign to the monzonite porphyries dikes in which the porphyritic segregations are represented predominantly by plagioclase. In their groundmass plagioclase and alkali feldspar predominate; the quartz content is small. Dikes of syenite porphyries are widespread in the area of the deposit. In addition, individual dikes on Zheleznaya and Svintsovaya mountains, in composition and structure, are albitite porphyries.

The scheme given below illustrates the order of mineral separation in the porphyries.

Scheme of mineral formation in the porphyries.

Minerals	Magmatic stage	Emanation stage	Exogenous changes
1. Plagioclase	———
2. Quartz	——	—
3. Pyroxene	– –
4. Amphibole	– –	—	–
5. Epidote		–
6. Apatite	—
7. Sphene		—
8. Calcite		—
9. Chlorite		—	– –
10. Ilmenite	—
11. Leucoxene		– – –
12. Magnetite	—
13. Sulfides		– – –
14. Sericite		—
15. Limonite			———
16. Orthite		– –
17. Fluorite		– – –

Thus, from the diagram it is evident that, after their formation, the porphyries underwent alteration under the action of emanations, with which are associated the formation of quartz, epidote, sericite, chlorite, the transformation of pyroxene into actinolite, and the formation of leucoxene and orthite. The formation of sulfides is possible; they occur in small quantity and in the scheme have been placed in the emanation stage conditionally. The formation of calcite also proceeded in this same emanation stage. In separate parts of the porphyries, calcite is very strongly developed. It attacks all the feldspars, corrodes quartz and other minerals. At contacts near the dikes, calcitization of the country rocks is observed, sometimes the formation of epidote and, more rarely, silicification. In time, the formation of ... is probably connected with these same emanations.

sulfides in the ore bodies of the deposit and possibly fluorite, which sometimes forms veinlets in the porphyries.

In the exogenous stage, limonite is formed, which stains the rock or forms veinlets, filling cracks, and possibly chlorite is also formed in this same stage.

9. Skarns. The coloration of the skarns varies within wide limits. Skarns of dark-gray, greenish, dark-greenish coloration occur, variegated, sometimes banded. Their texture is massive, banded; drusy texture occurs here and there. Garnet grows on the walls of cavities in magnetite, and cavities in garnet skarn are filled with quartz or calcite. By composition the skarns may be divided into garnet, pyroxene-garnet, and pyroxene-amphibole skarns. There are all transitions between them; moreover, individual parts of the skarns (especially of Mount Zheleznaya) are enriched in epidote, up to the formation of epidote skarns—epidosites. As a result of microscopic study it has been established that the skarns include garnet, pyroxene, amphibole, quartz, epidote, magnetite, apatite, chlorite, limonite, albite, calcite, sphene, hematite, leucoxene, chlorite, serpentine, zoisite, and sulfides.

Of the minerals listed, garnet is the most highly developed. Under the microscope it has greenish and brownish colors to colorless. Greenish and brownish garnet is isotropic and very rarely shows anomalies, whereas colorless garnet is in most cases anomalous and rarely isotropic. Sometimes one can observe how colorless, anomalous garnet grows on grains of brownish garnet (Fig. 3), forming growth zones. Here and there colorless garnet forms veinlets in the skarn and fills cracks in the brownish garnet. In individual thin sections (Mounts Granatovaya, Zheleznaya, Rudnaya) the rock contains chiefly garnet with only a small amount of epidote, calcite, and pyroxene. Brownish, and then colorless, garnet is formed in places where silicate rocks are skarnized. For example, on Mount Svintsovaya one could observe how brownish garnet develops in quartz monzonite, replacing, first of all, the plagioclases (Fig. 2). The same occurs on Mounts Granatovaya, Zheleznaya, and Rudnaya. Garnet having a greenish color develops in skarns situated near limestones, and, in particular, on Mounts Bezymyannaya (Nos. 13, 14) and Zheleznaya (No. 147) we observed skarnized marbles in which garnet forms separate grains, spots, and veinlets (Fig. 4). Under the microscope this garnet has a greenish color, and in the study it was not observed to be accompanied by colorless garnet. The garnet-bearing areas in the limestones macroscopically have a pale greenish color and a felsitic appearance.

It should be noted that sometimes drusy structures occur in ore skarns. A cavity is formed in magnetite, on the walls of which anomalous garnet grows, while the central part is filled with quartz (Fig. 5).

Pyroxene is widespread among the skarn minerals. It occurs in various quantities in almost all skarns, and in places pyroxene skarns are formed. It is usually scattered throughout the rock in the form of small grains (0.02–0.06 mm in size) with indented edges (Fig. 6). Fragments of diopside grains are often located within garnet, resembling a poikilitic structure, and sometimes it was observed how garnet forms veinlets in pyroxene or swells it. Measurement of several pyroxene grains on the Fedorov stage gave \(2V = +62\), \(CNg = 48\), which op-

defines it as diopside-hedenbergite. In some thin sections, very fresh pyroxene grains are encountered, which formed later than garnet, since there are areas where garnet is replaced by pyroxene.

Actinolite develops along pyroxene, sometimes replacing pyroxene grains entirely, sometimes forming only rims along the periphery. In amphibole skarns (Mount Zheleznaya, Mine No. 2, and the northern slope of Mount Svintsovaya), actinolite develops predominantly. It has a fibrous structure and, in transmitted light, a green color with clearly expressed pleochroism. On Mount Zheleznaya, amphibole-garnet skarns develop around the ore body (Mine No. 2), but in most cases, directly in contact with the ore body, pyroxene-garnet skarns develop. It is true that on the hanging wall of the ore body of Mine No. 2, at the very contact, amphibole-pyroxene skarn is developed, and the amphibole developed predominantly at the expense of pyroxene. Sometimes individual fibers of hornblende are arranged in quartz, forming comet-like intergrowth forms. On Mount Svintsovaya, amphibole skarns are also encountered, and the amphibole mainly has a radial-fibrous structure and develops within a felt-like chlorite-serpentine mass. In these skarns, diopside occurs in quite negligible quantities and is located for the most part in the centers of amphibole grains. In individual thin sections, a small number of strongly corroded grains of ordinary hornblende were encountered; for the most part this hornblende has been replaced by chlorite. Apparently, these hornblende grains were preserved from the primary rock. In addition, the skarns contain uralite hornblende (Mount Zheleznaya), which forms pseudomorphs after pyroxene.

Epidote is widely distributed in the skarns, except for the skarns of Mount Granatovaya, where it is encountered relatively rarely. Epidote occurs in the skarns in the form of individual areas, spots, and veinlets. Its formation took place simultaneously with the appearance of quartz; moreover, epidote is often observed situated at the boundary of quartz grains or veinlets with feldspars. Garnet and diopside grains are corroded by epidote.

Quartz in the skarnized rocks and skarns belongs to two generations. Quartz belonging to the skarnized rock, where it has a granoblastic structure, is a mineral of the first generation; and quartz that forms veinlets in the skarns or fills cavities is of the second generation. It is widely developed.

Magnetite is comparatively not very widely distributed in the skarns described. It can be observed in the form of separate small areas of irregular shape, situated around garnet grains. In individual thin sections, an alternation of growth rings of garnet and magnetite was observed. Consequently, magnetite began to form simultaneously with garnet, which has a brownish color. In addition, magnetite was observed in skarnized hornfels, where it occurs in the form of fine dissemination.

In several thin sections from Mounts Granatovaya and Zheleznaya, hematite was encountered. It is situated in separate spots of irregular shape. Within these spots, hematite needles are visible, elongated along the cleavage of diopside and actinolite grains. In transmitted light, at high magnifications, a brownish color is noticeable. In crossed nicols, the interference color is reddish-brown and is confined mainly to the peripheral parts of the hematite needles. Sometimes it is located in calcite, developing along its cleavage.

Calcite occurs in varying amounts in all skarns of the deposit. As already mentioned above, calcite differs in its mode of formation. Part of the calcite belongs to the remnants of marbles that underwent skarnization. Residual calcite in the skarns is not widely distributed (excluding weakly skarnized marbles); its grains are corroded; in places a weak development of magnetite is observed. A large amount of calcite formed after ore formation. This calcite is very strongly developed. It affects all minerals, swelling them and filling individual cracks and cavities present in the skarns. Calcite is especially strongly developed near porphyry dikes, with which, probably, the low-temperature emanation currents that gave rise to the calcite are associated. In drusy cavities calcite is often encountered, filling them and forming fairly large crystals. Calcite forms veinlets in pyroxene grains, which undoubtedly indicates replacement of pyroxene by calcite. Sometimes these calcite veinlets grow into irregular metasomes. Such phenomena are also observed in feldspar.

Feldspars are present in the skarns in small amounts. They have been preserved as separate areas of skarnized silicate rocks (effusive and intrusive), and their grains are always corroded; in some thin sections only small fragments remain, located predominantly among garnet grains. Sericite develops on the feldspars, but it likewise is not strongly developed.

Chlorite is distributed in the skarns in varying amounts. In skarns formed at the expense of limestones, chlorite is weakly developed. Here it forms mainly along the periphery of ore minerals and garnet. In skarns formed at the expense of silicate rocks, however, chlorite is sometimes rather strongly developed. Especially much chlorite occurs in amphibole-pyroxene and amphibole-garnet skarns. Usually chlorite is arranged in the form of scales, and only in a few thin sections of amphibole skarns was it found in the form of radially fibrous aggregates. Chlorite develops on pyroxene, amphibole, and garnets. It sometimes wholly replaces amphibole grains; and in the skarns of Mount Granatovaya it was observed that chlorite had developed strongly on garnet, forming in it swellings, veinlets, and, in general, irregularly shaped spots. In many thin sections it was possible to observe chlorite forming veinlets in the skarns. If the rock is impregnated with limonite, then the chlorite acquires a yellowish color. Limonite itself, however, very rarely forms thin veinlets along fractures in the skarns.

In the skarns there occur sphene, leucoxene, and apatite. The latter is developed predominantly in pyroxene-garnet and magnetite-garnet skarns. Sulfides ($\mathrm{CuFeS_2}$ and $\mathrm{FeS_2}$) were rarely encountered in the skarns. Finally, in skarns formed at the expense of limestones, several grains of colorless mica were found. It has low relief; in thin section it is completely colorless; the interference colors are high. Its development is confined to places where the limestones have been silicified.

The order of formation of minerals in the skarns may be represented in the scheme on page 23.

In the scheme presented, the formation of minerals in the skarns is shown independently of the composition of the rock being skarnized. Therefore, in the column “minerals of the rocks being skarnized,” alongside plagioclase, calcite is listed; this means that plagioclase belongs to silicate rocks, i.e., to effusive and intrusive rocks, while calcite belongs to lime-

Scheme of mineral formation in skarns.

Name of minerals	Minerals of the skarn-forming rocks	Stage of skarn formation	Formation of porphyry dikes	Exogenous processes
1. Garnet		—
2. Pyroxene	—	—
3. Magnetite		—
4. Apatite	—	—
5. Epidote			—
6. Quartz		—	—
7. Actinolite	—	—
8. Sphene	—
9. Zoisite
10. Feldspars	—
11. Leucoxene		—
12. Calcite	—		—
13. Chlorite	—	—
14. Chlorite-serpentine			—	—
15. Limonite
16. Sericite	—
17. Hematite		— ? —	—
18. Sulfides ($\mathrm{CuFeS_2}$, $\mathrm{FeS_2}$, etc.)		— ? —	—
19. Vesuvianite (?)
20. Colorless mica		—

contraction. Pyroxene in the skarns formed at the expense of quartz monzonites has been partly preserved. The preserved pyroxene can be observed in the centers of actinolite grains. In the extrusive rocks, apparently even before skarnization, actinolite, sericite, and chlorite developed. The skarns apparently contain a small amount of apatite and sphene, which formed during the magmatic stage, but the resolution of this question is uncertain. It is possible that the observed sphene and apatite formed exclusively during the stage of skarn formation.

During the stage of skarn formation, garnet and pyroxene are formed in larger quantities, and both of these minerals form with certain interruptions. Pyroxene is the first to form; after it, almost simultaneously, brownish garnet begins to form in the silicate rocks and greenish garnet in the limestones. This brownish garnet is in places replaced by pyroxene. Pyroxene and brownish garnet are replaced by colorless garnet, which grows over the brownish garnet in the form of separate zones. At the end of the deposition of brownish garnet, a certain amount of magnetite forms (the formation of magnetite will be described in more detail in the section on ore). Before magnetite, and partly before garnet, apatite formed; in certain parts of the skarn it is fairly widespread (Mount Granatovaya, etc.).

Before the appearance of quartz, epidote and actinolite begin to form; the latter develops chiefly at the expense of pyroxene. Along with epidote, several grains of zoisite were encountered, which apparently formed at the expense of feldspars. At the end of the stage of skarnization, leucoxene and chlorite develop in small amounts; apparently calcite appears in part, and, to an insignificant extent, chlorite-serpentine, which was observed mainly in amphibole skarns.

During the stage of formation of porphyry dikes, quartz is formed, cutting the skarns in thin veinlets and filling cavities formed in the skarns. Along with the formation of quartz, there appear

epidote and actinolite. In this stage a large amount of calcite is formed, which affects the skarns, fills cavities in them, and replaces a number of minerals. Simultaneously with it are formed chlorite, chlorite-serpentine, and, possibly, hematite, which in the observed polished sections was located in calcite or near it. The hematite replacing pyroxene was formed, apparently, at the same time. Finally, sulfides are formed. In the exogenous stage limonite is formed, contaminating the skarns, and a certain amount of chlorite.

The diagram does not show the time of recrystallization of the rocks. Observations show that hornfelsing of the silicate rocks (effusive rocks) and the transformation of limestones into marbles occurred before the beginning of skarnization. In rocks with a hornfelsic structure, epidote, pyroxene, and garnet later begin to form; these develop, forming skarns of various composition. In addition, the banded textures of hornfelsed felsitophyres are also preserved in the skarns formed at the expense of these rocks.

The composition of the skarns of the deposits of Mount Bezymyannaya, Rudnaya, and Granatovaya is predominantly pyroxene-garnet and even garnet (Mount Granatovaya). On Mount Zheleznaya and Mount Svintsovaya, besides pyroxene-garnet skarns, there are amphibole-pyroxene, amphibole, and epidosite skarns.

The magnetite content in the skarns increases with approach to the ore bodies, which are separated from the skarns by indistinct boundaries.

VI. Description of the Ores

The ore bodies of the Irbinsky iron-ore deposit are situated along a chain of low mountains, elongated in a meridional direction along the left bank of the B. Irba River. They occur as a narrow belt over a distance of 6 km. The ores are hosted mainly among igneous rocks—in Devonian effusives and in sandstones (sandstone was encountered only near one ore body), as well as in quartz monzonite. The ore bodies are confined chiefly to the contact of the quartz monzonite, extending from it for short distances. Some ore bodies (Mount Bezymyannaya, prospect pit 34, and Mount Zheleznaya No. 147) occur in contact with limestones. All ore bodies, regardless of the rocks in which they occur, are surrounded by a zone of skarns of varying width (from 5 to 50 m), beyond which follow skarnized rocks that are comparatively widespread in the area of the deposit.

The ore bodies have lenticular and stock-like forms, more rarely irregular and plate-like forms. Their strike is close to meridional, as is the general direction of the mineralized zone. Most have a westerly dip. In places there are deviations in the strike of the ore bodies. For example, on Mount Zheleznaya their general disposition is meridional. But in the southwestern part of the mountain individual bodies are elongated along the right bank of Zhelezny Creek and have a northwest trend; the angle between the lines of these trends is about 60°. In this case the strike of the lenses is also northwestern, and the dip is southwestern. Such a disposition of the bodies is evidently subordinate to the tectonic structures of the enclosing rocks. (All lenticular bodies have a steep dip, predominantly to the west.)

The ores of the Irbinsky deposit are characterized by considerable diversity in mineralogical composition and in the quantitative proportions of individual minerals. Along with rich ores, whose content ...

iron in which, according to Filatov (34), reaches up to 70% (Granatovaya Mountain); on average, the iron content in the ores of the deposit is about 50%; poor ores occur, in which the presence of iron is expressed in insignificant percentages. These ores gradually pass into garnet-magnetite or pyroxene-magnetite skarns.

A large part of the ore bodies has been exposed only by small mine workings (trenches, shallow prospect pits), which makes it difficult to resolve the question of harmful impurities; however, in ores taken from boreholes, small shafts (Zheleznaya Mountain), and adits of Granatovaya Mountain, the content of harmful impurities is not uniform. According to the data given in the report of A. A. Mesyaninov (14, tables), the sulfur content in the ores from boreholes Nos. 1 and 2 of Zheleznaya Mountain fluctuates, for individual samples, from 0.14 to 5.56%, with the average sulfur content in borehole No. 1 being 2.66%, and in borehole No. 2—0.35%. It should be noted that the percentage of sulfur is higher in the hanging wall (borehole No. 1—5.56%); it gradually decreases toward the footwall, where its content is 0.44–1.28%. The sulfur content in borehole No. 3 averages 0.35%. Silica averages 15.52% in borehole No. 1, 15.68% in borehole No. 2, and 25.46% in borehole No. 3. The phosphorus content is negligible; most often only traces are present, and only rarely fractions of a percent. In some samples the alumina content is indicated, expressed by the following figures: borehole No. 1—0.16; borehole No. 2—3.61; borehole No. 3—6.94; borehole No. 5—4.00%. The silica content in the upper parts of the ore bodies cut by boreholes increases somewhat; for example, in borehole No. 1—26.26% at the surface, as against 17.43% at a depth of 50.64–80.71 m. In other workings it remains almost unchanged. The sulfur content in the upper parts of the ore bodies is relatively reduced, and in trenches along the line of boreholes Nos. 1 and 3 it is expressed as 0.08 and 0.21%, as against, respectively, 2.66% and 0.35%.

Macroscopically, the ores have a dark-gray color, often with a greenish tint. Owing to the presence of non-ore minerals (garnet, quartz, calcite, etc.), the ores acquire a mottled coloration. This is especially clearly expressed in poor ores, where the content of non-ore minerals is high. At the surface the color of the ores becomes reddish. The texture of the ores is also heterogeneous. Three varieties may be distinguished: a) ores with massive texture, b) ores with banded texture, and c) ores with drusy texture. The most widespread are ores with massive texture. They have a dense structure, are fine- and medium-grained, with a relatively small content of non-ore minerals. The sulfide inclusions in them have the form of irregularly shaped aggregates, more rarely veinlets, and here and there the form of a “mesh,” reminiscent of plant remains in sandstones (fig. 7). In addition, apparently, along the walls of the short veinlets of sulfides there is a polishing of magnetite, as a result of which, in fractures, it seems to possess cleavage planes and resembles zinc blende.

Ores of banded texture (fig. 8) occur more often on Zheleznaya Mountain (trench No. 7) and Granatovaya Mountain (adit 174), more rarely on Svintsovaya Mountain, where banded textures are more widely developed among the skarns. The banding of ores and skarns can be explained by the manner in which the mineralizers penetrated into layered rocks (46^625), with preservation of the texture of the replaced rocks. In these ores the sulfides have predominantly the form of veinlets and are elongated along the banding, which emphasizes still more clearly the banded structure of the ores (borehole No. 3); of course, here too there are sulfide veinlets cutting the ore at various angles to the bedding-

…of density. In addition, it should be noted that among the ores there was observed a phenomenon in which ore of originally massive texture, upon weathering, took on the appearance of burunduchnaia. Thin bands of ore (about 0.1 mm wide) are separated from one another by silicate material, which has a brownish coloration; moreover, the ore bands appear continuous. Examination of thin sections established that the gangue substance is represented by chlorite and quartz, which fill the gaps between the ore minerals with some discontinuity and give the ore the appearance of banding (Fig. 9).

Drusy textures in the ores are of minor distribution. They were encountered in the ores of Zheleznaya, Granatovaya, and Rudnaya mountains. Among the magnetite, cavities of various size are formed (from 0.5 mm to 2 cm), on whose walls garnet (of the second generation) is encrusted, sometimes quartz, while the center is filled with quartz or calcite (Fig. 5). The formation of such textures is not entirely clear, and one may suppose that the deposition of material during metasomatism proceeds more slowly than its dissolution and removal; under such conditions, naturally, cavities may form, which will later be filled by minerals, with the formation of drusy textures.

The ores of the entire deposit were subjected to microscopic study. In this connection it is necessary to note that the ores of Bezymyannaya, Svintsovaya, and Rudnaya mountains were exclusively from the oxidation zone, or rather from the very upper parts of the ore bodies, and therefore, naturally, they were all oxidized. Primary ores at our disposal were available only from the drill holes of Zheleznaya Mountain and the adit of Granatovaya Mountain. In the microscopic study the ores were divided into two groups: a) ores occurring among igneous rocks, and b) ores located among marbles. Unfortunately, the ores located among the marbles are exposed only by surface workings, and therefore it proved possible to study exclusively oxidized ores. The primary ores were not studied, and comparing them with the ores occurring in igneous rocks is very difficult, although, as we shall see below, certain distinctive properties can be observed.

a) Microscopic study established that the ores of the Irbinsky deposit occurring among igneous rocks include magnetite, hematite, pyrite, chalcopyrite, marcasite, arsenopyrite, sphalerite, covellite, limonite, and gangue minerals—garnet, quartz, calcite, chlorite, chlorite-serpentine, hornblende, pyroxene, apatite, epidote.

Magnetite in the ores of the Irbinsky deposit is predominant in abundance in comparison with the other minerals. It has a gray color with a brownish tint; light-gray magnetite is rarely encountered. When immersed in cedar oil it darkens somewhat, but remains gray. Reflection pleochroism is not noticeable; it is isotropic. With fuming HCl it is structurally etched, on average, over 5 seconds, with slight deviations (some areas are etched in 3 seconds, others in 8). In some thin sections the magnetite grains show zoning; moreover, their central parts are etched more quickly, while the peripheral parts are etched more slowly, often remaining unchanged. The structure of the magnetite is allotriomorphic, sometimes granoblastic. As a result of etching, individual magnetite grains (boreholes Nos. 1 and 3, adit of Granatovaya Mountain) show zoning; in some thin sections magnetite is observed forming elongated grains in which twins are visible. In thin section No. 325 from Bezymyannaya Mountain, the elongated magnetite grains are arranged at a certain angle to one another, resem…

... the location of plagioclase grains. It is possible that this structure is due to the fact that magnetite first of all replaces the plagioclases. When etched with HCl for 8 seconds, twins are observed in such grains. Rarely, magnetite with regular crystallographic outlines is observed in the ores. In thin sections Nos. 228 and 226, prepared from ores of Mount Svintsovaya, rather original structures are encountered in the magnetite. The magnetite grains are deformed and have the appearance of elongated, often spindle-shaped hexagonal crystals, which are arranged almost perpendicular to the cracks present in the ore (Fig. 10). Here also occur magnetite grains elongated in one direction and gathered into small folds; moreover, among the fine folding there are magnetite grains that appear to have been rubbed inward (Fig. 11). The formation of the described forms is possible under strong lateral pressure, during which the ore was deformed. In general, ores with such structures may be regarded as tectonites. In addition to Mount Svintsovaya, schistose ores, also with magnetite grains abraded within them, were encountered in borehole No. 3 of Mount Zheleznaya.

The size of the magnetite grains ranges from 0.4 to 0.02 mm; where the magnetite grains have the form of elongated laths, their size ranges from 2.1 × 0.3 to 6.18 × 1.54 mm. Aggregates of magnetite grains form irregularly shaped clusters and often fill cracks in garnet that has a brownish coloration. In almost all thin sections the magnetite is martitized. Martitization develops from the periphery toward the center, with hematite in the form of fine needles arranged along the planes of the octahedron. An exception to this is thin section No. 170 from mine No. 2 of Mount Zheleznaya. In this thin section the magnetite grains have an allotriomorphic structure. A large part of the grains are martitized, with oxidation proceeding from the center toward the periphery. For example, many grains are martitized, and only along the margins is a very narrow rim of magnetite preserved, having a brownish coloration, in which martitization is not observed (Fig. 12). The magnetite rims preserved from martitization always border non-ore minerals, evidently calcite (it effervesces when etched). In the thin section from borehole No. 1, from a depth of 76 m, magnetite with a brownish coloration grows on the faces of pyrite, and therefore formed later than it. Finally, in thin section No. 166, prepared from the ore of mine No. 2 of Mount Zheleznaya, magnetite of two generations is encountered. Among the magnetite there is a crack filled with magnetite that forms a veinlet with uneven edges, which, according to Graton, Bastin, Lindgren, Newhouse, Schwartz, and Short (18), indicates a replacement process. This magnetite veinlet cuts magnetite of the first generation and non-ore minerals (Fig. 13). The magnetite composing the veinlet is finer-grained (0.01–0.15 mm), is structurally etched by fuming HCl for 3 seconds, and by SnCl₂ for 20 seconds (normal magnetite is etched by SnCl₂ for 1 minute), i.e., faster than normal magnetite of the Irbinsky ores. It is also martitized.

In thin sections prepared from ores exposed by small mine workings, the magnetite is very strongly oxidized. It has a brownish coloration and is etched normally by fuming HCl. Often its grains have been almost entirely replaced by hematite, and only relics of magnetite remain, located in the centers of the hematite grains.

On the basis of structural features, it may be considered that magnetite began to separate after the beginning of the formation of brownish garnet, but its formation proceeded for some time simultaneously with the garnet.

This is established rather definitely, for in garnet we sometimes observed veinlets of magnetite and, moreover, the garnet has idiomorphic boundaries and is sometimes corroded by magnetite. The magnetite that I have assigned to the second generation formed after the separation of pyrite, and its formation was evidently caused by a local transfer of iron.

Hematite occurs in the ores of the Irbinsky deposit in varying amounts. It is of hypogene and supergene origin. Supergene hematite is very widely developed, especially in ores from the surface. Hypogene hematite was encountered in small amounts in ores from borehole No. 3, adit No. 2 of Zheleznaya Mountain, and also in one polished section (No. 240) from Svintsovaya Mountain. It forms elongated grains that are situated among quartz and calcite, and sometimes fill the spaces between grains of magnetite, forming veinlets here and there. It is possible that it was deposited during the final stage of skarn formation and of the formation of the ore bodies. Hematite is white in color, has high reflectivity, and a hardness higher than that of magnetite. At high magnifications, especially with immersion, a red internal reflection is visible. It polarizes in colors from pale yellow to bluish. Twins were observed in some grains (polished section No. 240). Chemically it is inert—all the chemical reagents we used do not affect it.

Supergene hematite is somewhat more widely developed, if the ores from surface workings are taken into account. In ores from borehole No. 1 from depths below 60 m, supergene hematite occurs in entirely negligible amounts. It develops as a very rare network along crystallographic directions in magnetite. In ores from small shafts from a depth of 25 m, hematite is already present in larger quantity; here it sometimes forms a dense network along the periphery of magnetite grains. The same is true of ores from the adit on Granatovaya Mountain: in ores down to about 20 m hematite is present in considerable quantity, whereas farther on, i.e., from 25 to 50 m, it occurs very rarely. An entirely different picture is observed in ores from small mine workings, especially of Rudnaya Mountain, the southern part of Svintsovaya Mountain, and, in part, other mountains. In these ores hematite is the predominant mineral. It develops after magnetite in the form of small needles arranged along the planes of the octahedron. Often the grains of magnetite are almost completely replaced by hematite, but at high magnifications (with immersion) the lattice of magnetite is visible. According to the experiments of J. Gruner (19) and L. Twenhofel (20), artificial oxidation of magnetite to hematite begins at a temperature of about 150°; the magnetite lattice is preserved if its replacement by hematite occurs at a temperature not exceeding 500° C. The formation of hematite after magnetite always proceeds from the periphery toward the center, except for the cases noted in the description of magnetite, where a narrow rim of magnetite is situated along the edges of martitized grains. Supergene hematite also has a white color, high hardness, and the usual reagents do not act on it. With strong development of hematite, the ore macroscopically assumes a brownish-red color.

Pyrite occurs in the ores in varying amounts. It is often included as separate crystals, not infrequently forms veinlets in the ore, and sometimes constitutes irregularly shaped inclusions (Granatovaya and Zheleznaya mountains). Under the microscope it forms separate inclusions among non-ore minerals; it is often observed in magnetite in the form of veinlets and separate small pools and pockets. Sometimes pyrite replaces

magnetite grains almost entirely, leaving only small relics of the latter. The color is light yellow; reflectance is high. Isotropic. Etching with $\mathrm{KMnO_4}+\mathrm{H_2SO_4}$ reveals structures that vary from allotriomorphic to colloform, encountered in separate sections (Mount Granatovaya No. 212³ and Zheleznaya No. 300). Weak zoning is rarely noticeable in pyrite grains. The grain size ranges from $4.1 \times 0.3$ to $0.9 \times 0.3$ mm. Sometimes small grains are as small as 0.2 mm. Around some of its grains a narrow rim of quartz is observed (section 378—Mount Granatovaya), and, finally, in the section from borehole No. 1, brownish magnetite grows over pyrite grains (Fig. 14). It should be noted that in ore bodies intersected by borehole No. 1 and shaft No. 2, pyrite is more widespread in the hanging wall than in the footwall. If in the hanging wall it occurs in considerable quantity, sometimes even in the form of accumulations of irregular shape, then in the footwall it is observed much more rarely. The same regularity in the distribution of sulfides was noted by Hickok (32) in the ore bodies of Cornwall. In ores taken from small mine workings, pyrite occurs rarely and in insignificant quantity. Here we observe cavities and limonite that formed as a result of the leaching of sulfides.

Around pyrite in some ores of the Irbinsky deposit, marcasite is observed. It is arranged as small inclusions around pyrite grains, sometimes forming concentric forms; sometimes small marcasite grains are scattered irregularly. Such forms resemble a porphyroblastic structure (Fig. 15). In section No. 212—from the adit of Mount Granatovaya—it was observed how marcasite forms thin veinlets with uneven margins in pyrite, which indicates a replacement process (Fig. 16). In reflected light marcasite has a color lighter than pyrite; it polishes poorly—worse than pyrite; reflectance is high. No reflection pleochroism was observed, but marcasite polarizes in colors from blue to pinkish. From $\mathrm{HNO_3}$ a brownish coating forms, which is removed with difficulty; $\mathrm{KMnO_4}+\mathrm{H_2SO_4}$ reveals the structure, and it is etched by potassium permanganate more rapidly than pyrite. All other commonly used reagents have no effect.

Marcasite in the contact-metamorphic deposit of the Pita district in Arizona was described by V. Webber (21), who believes that marcasite could have formed in a special phase, when the environment was acidic and the temperature no higher than $450^\circ$ C. In our case, marcasite formed in the final stages of formation of the deposit, since it forms thin veinlets in pyrite. According to the criteria for recognizing hypogene and supergene minerals given in the work of G. M. Schwartz (41), replacement veinlets and irregular embayments into earlier minerals (Fig. 16) indicate a hypogene origin of the marcasite. In addition, marcasite is distributed in the zone of primary ores. Consequently, marcasite formed by a hypogene route in the final stages of formation of the deposit and, apparently, at a comparatively low temperature.

Chalcopyrite occurs in the ores in insignificant quantity. It is almost always subordinate to pyrite. Sometimes it forms veinlets in pyrite; in places pyrite grains are corroded by chalcopyrite. In the form of irregularly shaped aggregates or individual grains it is observed in gangue minerals. In sphalerite it forms emulsion inclusions. In ore from borehole No. 3 it was observed that chalcopyrite is located along the cleavage of radiating hornblende.

blende (Fig. 17). If it occurs in considerable quantity (borehole No. 3), it forms veinlets in the magnetite and makes it swell. Sometimes relics remain from the magnetite grains, enclosed among the chalcopyrite. In some polished sections (168, mine No. 2), chalcopyrite grains are surrounded by limonite, which has a colloform structure (Fig. 18). The color of chalcopyrite is yellow; it polishes well; its hardness is lower than that of pyrite; it turns slightly brown from HNO₃. The time of deposition of chalcopyrite relative to marcasite could not be established, but it is quite evident that both marcasite and chalcopyrite were deposited after pyrite. In oxidized ores chalcopyrite occurs extremely rarely. It was observed in several polished sections prepared from ores of Mount Svintsovaya and Mount Zheleznaya.

In ores from the adit of Mount Granatovaya, arsenopyrite occurs in insignificant quantity. It is situated near pyrite, and its aggregates are formed of elongated prismatic grains. Its color is white, with a faintly noticeable greenish tint in contrast to pyrite. Its hardness is high. It is anisotropic. In places its prismatic aggregates are situated among pyrite, sometimes among gangue minerals. Its formation took place simultaneously with pyrite and even somewhat earlier, since in some places its replacement by pyrite is observed. In these same ores, sphalerite and hydrohematite occur in a wholly negligible quantity.

Sphalerite is situated among the gangue minerals, mainly calcite, together with chalcopyrite, with which it forms emulsion structures. Sometimes zinc blende is situated around chalcopyrite grains, in which case emulsion structures are not observed.

Hydrohematite was encountered in a wholly negligible quantity. In polished section No. 378, made from ores of Mount Granatovaya, hydrohematite is situated in the interstices of idiomorphic magnetite grains; it also surrounds, sometimes corroding, grains of pyrite and chalcopyrite (Fig. 19).

Its color is white with a bluish tint (lighter than limonite and darker than hematite); when immersed in cedar oil it becomes somewhat gray; the internal reflection is reddish, a little browner than that of hematite. Its hardness is lower than that of magnetite. Fuming HCl has no effect for 2 minutes; other commonly used reagents likewise have no effect. Hydrohematite forms after pyrite and chalcopyrite and corresponds, in time of deposition, to calcite. According to the work of F. N. Shakhov (42, 25), the formation of hydrohematite may be connected with ascending solutions rich in H₂O, which at high temperatures possess, like CO₂, the properties of a strong oxidizer. This proposition is probable, since the polished section containing hydrohematite was made from ores taken from 22.5 m of the adit, where supergene processes are poorly developed.

Among the secondary ore minerals, limonite and covellite occur.

Covellite was observed in ores taken from prospect pits Nos. 1 and 2, and also from borehole No. 1 of Mount Zheleznaya, where it is always situated together with chalcopyrite, sometimes surrounding it. Its distribution in the ores of the deposit is insignificant. In ores from borehole No. 3 at a depth of 56–88 m, covellite was not encountered; nor was it observed on Mount Granatovaya. The presence of covellite in the ores of Mount Zheleznaya is apparently explained by the fact that they contain more chalcopyrite.

Limonite occurs in almost all ores of the deposit. It forms veinlets and fills cavities, forming colloform structures in places. It should be noted that in ores from depth, limonite is not observed. For example, in the ores of prospect pit No. 1, at a depth of 25 m,

limonite was not encountered, while in prospect pit No. 2, likewise from a depth of 25 m, it was encountered only in one thin section, in a wholly insignificant quantity. In the adit of Mount Granatovaya, limonite was observed over a distance of 18 m, and farther along the adit it was not encountered in the thin sections. Occasionally grains of magnetite, hematite, and small grains of pyrite occur in the limonite. The color of the limonite is gray, with a reddish-brown internal reflection. The reagents we used do not affect it.

The ores of the deposit contain a considerable quantity of non-ore minerals. They are disseminated throughout the ore in the form of small inclusions. Sometimes, in the middle of the ore body, areas of non-ore minerals are encountered. These areas have irregular forms. The boundaries of the ore bodies with the enclosing rocks are not sharp: the ore bodies predominantly rich in iron gradually give way to poor ores, and the latter likewise gradually pass into ore skarns. Therefore, the content of non-ore minerals in the ores, in its diversity, is close to the minerals described in the section on skarns, and here we must dwell chiefly on the time of separation of the individual minerals.

Garnet occurs rather frequently in the ores; it is especially developed in the ores of Mount Granatovaya. It is also often encountered on other mountains. Garnet is observed in two generations. Garnet of the first generation has a brownish coloration and in most cases is isotropic; it is separated before magnetite, since magnetite in places forms veinlets in garnet grains, and its grains are partly corroded by other minerals, in particular by calcite and more rarely by magnetite. Chlorite develops rather strongly after this garnet. Garnet of the second generation is colorless in transmitted light and anomalous. Growth zones and twins are often observed in it. It sometimes grows over brown garnet, and also fills drusy cavities, growing on magnetite. In the ores of Mount Granatovaya, garnet of the second generation is very widespread.

Pyroxene is developed in the ores in a somewhat smaller quantity than garnet. It occurs in the form of large grains that are corroded by magnetite, garnet, and other minerals and, consequently, was formed first among the skarn minerals. This pyroxene is present in the ores in small quantity. It is true that there are separate thin sections from the ores of Mount Zheleznaya (borehole No. 1) where pyroxene constitutes a large part of the rock. Alongside the pyroxene described, which has large grains, pyroxene also occurs in the form of small grains. It develops in magnetite and garnet, corroding them and even eating into the edges of grains of first-generation pyroxene. The pyroxene described forms aggregates of small grains, or else these grains are scattered among other minerals. In transmitted light it is colorless; interference colors reach yellow of the second order. Its formation takes place at the end of magnetite formation and even somewhat later.

In some thin sections (borehole No. 3 and others), radiating amphibole occurs in small quantity. The development of amphibole is confined mainly to the footwall side of the ore bodies, as was already noted in the description of the skarns. Radiating amphibole in places develops in chlorite-serpentine, sometimes together with quartz, as if floating in the latter. It corrodes grains of magnetite. The color of the amphibole is greenish; pleochroism is expressed rather sharply. It apparently forms simultaneously with quartz. Here in the ores there also occurs hornblende formed after pyroxene.

Chlorite is rather widely developed in the ores; it forms after dark-colored components and garnet. It often fills the spaces between

with magnetite and other minerals; in addition, it forms veinlets in the ore.

Epidote is not widely distributed in the ores. It occurs chiefly together with pyroxene and, mainly, is developed in the skarns at some distance from the ore bodies.

In the poor ores from Mount Zheleznaya and Mount Svintsovaya, chlorite-serpentine was observed; it occurs in small quantities.

Apatite occurs comparatively often in the ores of the deposit. It is present as small elongated grains, and large grains are very rarely formed. It is somewhat more strongly developed in the ores of Mount Granatovaya. At times apatite is corroded by garnet and calcite. In addition, cracks develop in it, which are filled with chlorite and calcite.

Quartz occurs in small quantities in the ores of the Irbinsky deposit. It is observed more in the ores of Mounts Zheleznaya and Granatovaya, but macroscopically it can be seen comparatively rarely—mainly as crystals in drusy cavities. Under the microscope, some cavities are filled with quartz with the formation of drusy structures. Occasionally quartz was observed in the form of separate aggregates, especially in non-ore areas enclosed among the ore bodies. In places where pyroxene and fibrous hornblende are developed, quartz occurs in greater quantity than in other parts of the ore; in addition, quartz veinlets are occasionally encountered, cutting the ore in various directions. It is somewhat more developed in skarns located at some distance from the ore bodies, and especially in hornfelses or hornfelsed rocks.

Calcite is widely developed in the ores, occurring in all ore bodies of the deposit. Macroscopically it is white to pinkish in color, more often coarse-grained. In the ores, calcite is present as separate accumulations and forms veinlets in them. In ores from the adit of Mount Granatovaya it is more developed than in the other ore bodies. (Here the ores located near limestones are excluded.) In polished sections it is present as separate patches and veinlets; the latter cut the ore in various directions. Calcite corrodes all minerals and, in terms of time of formation, is the latest. Sometimes it forms narrow rims around grains of magnetite and pyrite, and fills cracks in garnet. Calcite belongs to two generations. Calcite of the first generation forms grains and corrodes the minerals present in the polished section, while that of the second forms veinlets. In ores from the oxidation zone, calcite is observed predominantly only in the form of veinlets.

b) Ore bodies occurring among limestones have been exposed only by trenches; they were encountered on Mount Bezymyannaya (southern body) and Mount Zheleznaya. Macroscopically the ores are represented by dark-gray fine- to medium-grained magnetite with a greenish tint and brownish coloration. In some ore samples from Mount Zheleznaya, dodecahedra and, more rarely, octahedra of magnetite are clearly visible. Under the microscope, magnetite, hematite, sulfides, and non-ore minerals are observed in the ores.

Magnetite is present in the form of irregularly shaped aggregates and constitutes the greater part of the polished section. It has a light-gray color, rarely with a faint brownish tint, but is always lighter than the magnetite in igneous rocks. It is etched by fuming HCl within 8–10 seconds. Grain size ranges from 0.05 to 1.3 mm. The grains are corroded by non-ore minerals, chiefly calcite. Along the periphery of the grains and along the edges of cracks, hematite develops in the form of small needles arranged along crystallographic directions. In one polished section (No. 148a)

from the ores of Mount Zheleznaya, two very small grains of chalcopyrite were encountered. The assemblage of minerals in the primary ores, apparently, will be supplemented by a number of other minerals that are absent in the oxidation zone.

Among the non-ore minerals present are garnet, pyroxene, calcite, quartz, and chlorite. The garnet has a greenish color. It is isotropic. Here a colorless anomalous garnet is encountered very rarely. Diopside is formed in comparatively small quantity; in time of formation it precedes garnet.

Quartz in some thin sections occurs in considerable quantity. It forms aggregates of grains between relics of limestone and ore minerals; sometimes a border is observed consisting of quartz with hornfelsic structure and garnet. In the latter, small grains of magnetite occur (Fig. 20). Where cavities are present in the ore, quartz forms druses, situated in the center of the geode and surrounded along the periphery by garnet.

Calcite is often encountered in the ores; it is represented either by relics of limestone or by newly formed crystals. In it, ore minerals included in the form of dust are observed in places (apparently magnetite). Around ore minerals and garnet, chlorite sometimes develops in small quantities.

Thus, magnetite in limestones has a lighter color than magnetite in igneous rocks. Garnet in transmitted light is greenish, whereas in the igneous rocks the garnet has a brownish coloration. We were unable to establish distinctive features in the morphology of the other minerals.

The order of mineralization may be represented by the following scheme.

Scheme of the order of mineralization in the ores of the Irbinsky deposit:

Minerals	Skarnization stage	Long break	Formation of dikes and sulfides	Long break	Exogenous processes
Pyroxene	— —
Garnet	— —
Apatite	—
Magnetite	———		— — —
Arsenopyrite			———
Epidote	— —		———
Quartz	— —		———
Hornblende	———		———
Hematite	— ? —				— — — — —
Pyrite			—
Chalcopyrite			—
Marcasite			—
Sphalerite			—
Calcite			—		— — —
Covellite					———
Limonite					———
Chlorite	———		———

From the scheme it is evident that during the skarnization stage pyroxene, garnet, apatite, magnetite, epidote, quartz, and hornblende are formed; the latter partly develops after pyroxene, and, apparently, chlorite as well. Then there is a break in mineralization and movements occur. After the break, diabase and porphyry dikes are formed, which cut the ore bodies and skarns. The formation of arsenopyrite, quartz (veinlets and quartz in geodes), hornblende, and hematite, which forms after quartz, is connected in time with the formation of the dikes (the relation of hematite to sulf—

it was not possible to determine [[unclear: beginning of word]]), pyrite, chalcopyrite, marcasite, sphalerite, calcite, and chlorite. The formation of sulfides is assigned to this stage because they cut the ore bodies and skarns and extend in the latter for considerable distances. In addition, sulfides occur in dikes of diabases and porphyries, which indicates their connection with vein formations. Finally, in the fluorite ores, together with galena, sulfides are also encountered; their formation undoubtedly occurred after the formation of the iron-ore deposits and is temporally connected with the porphyry dikes. In the latter case, barite occurs together with fluorite and Iceland spar. After the formation of the dikes and of the minerals described, there follows a long interval during which, under the influence of exogenous processes, limonite, covellite, and martite are formed.

VI. Genesis of the Deposit

The considerable development of skarns, the structure and composition of the ore, the form of the ore bodies, and their relation to the surrounding rocks all argue quite convincingly that the Irbinsky deposit, along with the Telbessky deposit studied by Prof. Usov (22), the Abakansky deposit studied by Usov (23), Kuznetsov (24), Filatov and Yatsenko (25), and the deposits of the Kondoma group studied by a number of geologists, in particular Batov (26), belongs to the contact-metasomatic type. The same conclusion regarding the genesis of the deposit was reached by previous investigators of the Irbinsky deposit itself (9, 10, 11, 13), with the exception of P. K. Yavorovsky (6), who assigned it to the magmatic type and believed that the ore bodies formed as a result of differentiation of the magma of porphyries and felsites. They are likewise assigned to the magmatic type in Bogdanovich’s course on ore deposits (27, 207).

It must be noted that the Abakansky deposit, as well as the Tashtagolsky and Kochurinsky deposits, occur far from intrusive bodies, and therefore their enclosing rocks are only weakly altered (chloritization, epidotization, sericitization, actinolitization, silicification, etc.), although crushed xenoliths of labradorite porphyries in the Abakansky deposit, as Kuznetsov notes (24, 34), have been transformed into skarns. The Irbinsky deposits are not similar to the deposits of the group just mentioned, since they are characterized by ore bodies occurring near an intrusive body among intensely skarned rocks.

The processes of skarn formation, the formation of ore bodies, and the later sulfide jets, as we shall see below, fit quite well into the scheme for the formation of contact deposits given by Professor Usov in his works (28, 22), if certain local deviations in the order of mineralization are not taken into account. For example, magnetite in the Telbessky iron-ore deposit forms second, following pyroxene (22, 58), whereas in the Irbinsky deposit magnetite formation takes place after the separation of brown garnet, which I have assigned to the first generation (garnet of two generations), i.e., it forms third: pyroxene, garnet, magnetite. Below we shall examine the connection of the deposits with the intrusion and the magmatic processes, as well as the forms and dimensions of the ore bodies.

1. Connection with the Intrusion and Magmatic Processes

The ore bodies of the Irbinsky iron-ore deposit are located in the western contact of a large intrusive body, represent-

... by quartz syenite, the peripheral parts of which have the composition of monzonites. In the apical parts of this large intrusive body there is an enormous remnant of the enclosing rocks, represented by Cambrian limestones and Devonian effusives. This remnant is divided into a number of smaller bodies, in the form of patches or inclusions of another shape.

The ore bodies are situated along the contact of the quartz monzonites and are elongated in the meridional direction for about 6 km. They occur among effusive rocks, partly in contact with limestones and, finally, within the intrusive body itself, in its peripheral parts. The intrusion of quartz monzonites hornfels the effusives, while the limestones, under the influence of contact metamorphism, are transformed into marbles. In the subsequent stage of cooling of the intrusives, all the rocks are skarned. The peripheral zones of the intrusive body itself are also partly skarned. Earlier investigators, on the basis of the petrographic composition of the intrusive, the variable composition of the rocks of the marginal zones, and also the comparatively narrow belt of hornfelses and skarns, quite correctly assigned the intrusion of quartz syenites to the hypabyssal facies (10, 13, 14).

The location of the ore bodies along the contact of the intrusive body, and partly within the intrusive itself, the skarning of the enclosing rocks and of the peripheral zones of the intrusive (skarn zones around the ore bodies), indicate that the formation of the deposit is connected with the intrusion of quartz syenites (monzonites along the periphery). In view of the foregoing (the occurrence of ore bodies in the peripheral parts of the intrusive rocks and the skarning of effusive rocks), it is obvious that the formation of magnetite cannot be connected with the magma of felsites and porphyries, as Yavorovsky supposed.

The ore bodies are cut by dikes of diabases and porphyries, which are derivatives of the syenite-intrusion magma. These same dikes also cut the peripheral parts of the intrusive itself, which had crystallized by the time the dikes formed.

The magmatic activity of the intrusion may be divided into three stages: a) hornfelsing, b) skarning, and c) formation of dikes of diabases and porphyries.

a) The process of recrystallization of the country rocks and the formation of hornfelses belongs to the first period in the development of the intrusion. For contact metamorphism, the most important factor is the considerable rise in temperature caused by the intrusion. According to the time-temperature curves given in the works of Grubenmann and Niggli (29, 159) and Schneiderhöhn (30, 471), the maximum temperatures at the contacts of the enclosing rocks develop in the first periods of magmatic activity of the intrusion, whereas the formation of hydrothermal deposits takes place considerably later. From these curves it is evident that a sharp rise in temperature occurs near the contact, while at a distance the high temperature (absolutely lower) is transmitted more slowly. If the rocks of the contact zone contain SiO₂, Al₂O₃, and carbonates, recrystallization begins already in dry melts at a temperature of not less than 500°C (29, 162). Consequently, the initial temperature of recrystallization of the rocks in contact with the intrusion was above 500°C; and according to Vogt’s work (35, 232), the temperature at which syenite begins to crystallize is approximately 1100°C. During this period the release of volatiles almost does not occur, unless one counts the distillation of gases and vapors into the country rocks, which, according to Grubenmann and Niggli, do not enter...

enter into the mineral composition of the recrystallizing rocks, although a regrouping of material within the latter probably does take place.

In contact with the intrusion of quartz syenites, the effusive rocks of acid composition underwent hornfelsing. Whether new minerals were formed in this process through the introduction of material could not be determined from our material, since the subsequent processes of skarn formation considerably obscure the picture. Under the microscope it is clearly seen that, first of all, recrystallization of the rock occurred, with the formation of a hornfels structure and with some regrouping of the material. For example, the iron contained in the rock proved to be dispersed among the quartz grains or also arranged as a fine dust along their periphery. In weakly skarned hornfelses, besides the fine dusting of magnetite, only diopside is encountered. Actinolite develops after the diopside in these rocks. Owing to weak skarn formation and hornfelsing, relict structures can often be observed in them (Fig. 21). Limestones in contact with the intrusion, before skarn formation, were transformed into marbles.

The process of recrystallization was apparently of short duration and occurred in the first stages of formation of the intrusive body, at the time when the temperature reached its maximum. It is possible that in individual areas, near the forming ore bodies, recrystallization of the rocks partly also took place during skarn formation.

b) The process of skarn formation is longer in time; it follows hornfelsing and begins at the time when the temperature, after the maximum, begins to fall. Between the stages of hornfelsing and skarn formation there was an interval during which emanations accumulated, causing skarn formation in the subsequent period. Such an interval is also indicated by the circumstance that the peripheral parts of the intrusion, by the time of skarn formation, had already recrystallized and themselves underwent skarn formation.

In the first period of skarn formation, both in the erupted rocks and in the limestones, diopside-hedenbergite is formed, which is developed comparatively weakly in the skarns. Even before the end of the formation of diopside-hedenbergite, garnet begins to separate out, having a brownish coloration. From analyses of garnets made in the laboratory of the Siberian Mining Institute by analyst A. K. Kal’e, it is evident that the garnets separating out earlier than magnetite contain 1.99% ferrous iron in their composition, whereas in garnets having an anomalous structure and separating out after the formation of magnetite, the content of ferrous iron is 0.36% (see the table of analyses), and the total iron content is somewhat greater, which indicates its assimilation from the magnetite formed earlier, or that the precipitation of garnet coincided with an intensified removal of iron by emanations.

Composition of garnets according to the data of the analysis performed by analyst A. K. Kal’e.

Time of garnet formation	SiO₂	Al₂O₃	FeO	Fe₂O₃	TiO₂	MgO	CaO	MnO₂	Cr₂O₃	Total
1. Garnet separated before the formation of magnetite	38.72	13.36	1.99	12.93	0.58	0.32	30.59	1.29	—	99.78
2. Garnet separated after the formation of magnetite	35.82	14.25	0.36	21.09	0.01	0.35	28.47	0.02	—	100.37

Consequently, in the first stages of skarnification at high temperature, ferrous oxide is formed, which later serves as the basis for the formation of magnetite, obtained if the solutions contain ferrous iron (28,75). The next mineral to begin separating is magnetite; simultaneously with it, pyroxene of the diopside type, arranged in small grains, continues to form. Incidentally, let us note that this same pyroxene is developed in the skarnified hornfelses. After the formation of magnetite is completed, partly simultaneously with it, garnet of the second generation begins to form; it is anomalous and in its composition contains almost no ferrous oxide. Apparently, toward the end of magnetite formation the temperature is already rather low, and ferrous oxide cannot be formed. For the formation of ferrous oxide it is necessary that it be transported in the compound FeCl₂ (FeF₂), which, according to the work of C. Lasky (31), is volatile only up to a temperature of 699°C, while the critical temperature of this compound (FeCl₂), given in the work of Interson (43), is 1450°C. Consequently, from the beginning of formation of the intrusive body down to a temperature of 699°C, the compound FeCl₂ can exist in the emanations and ferrous oxide can be formed, according to the reaction FeCl₂ + H₂O = FeO + 2HCl. With a decrease in temperature, iron is transported in the compound FeCl₃, from which ferric oxide is already formed by the reaction 2FeCl₃ + 3H₂O = Fe₂O₃ + 6HCl; the latter, combining with ferrous oxide, forms magnetite. If the environment is oxidizing, then at low temperatures hematite precipitates from the solutions, according to the reaction given above.

It therefore seems sufficiently clear that magnetite in contact deposits must form as one of the first minerals. At the end of the skarnification stage, epidote, chlorite, and actinolite are formed, whose composition includes chiefly Fe₂O₃. It must also be noted here that near magnetite the pyroxenes and hornblendes are somewhat more weakly colored than away from it. Evidently, minerals with a lower iron content form around magnetite.

c) After a long interruption, when the ore bodies had already been formed, dikes of diabases and porphyries were being formed. These dikes cut the ore bodies, skarns, felsite-porphyries, and marginal zones of the intrusive rocks. Simultaneously with the formation of the diabase and porphyry dikes, sulfides (pyrite, chalcopyrite, partly arsenopyrite and sphalerite) were deposited in the ore bodies, partly in the skarns and even hornfelses; these contaminated the ore with harmful impurities. In this same stage the rocks were partly silicified and, in addition, epidote, chlorite and, apparently, calcite formed. Even magnetite was deposited, forming veinlets in the magnetite of the first generation and growing on the faces of pyrite grains (Fig. 14). The formation of this magnetite apparently occurred at the expense of the dissolution and redeposition of normal magnetite. Toward the end of emanation activity, hydrohematite and marcasite were deposited in the ores. After a long interruption, as a result of weathering processes, limonite and covellite began to form.

The course of the process of cooling of the intrusion and the order of mineralization are given in the diagram in the description of the skarns and ores.

2. Composition of the Emanations

The order of mineralization in the skarns and ores and the character of alteration of the replaced rocks make it possible to outline the direction of change in the composition of the emanations during cooling of the intrusive body. Here, first of all, it must be noted that in the formation of minerals ig-

temperature played a large role. As was already noted above, in the first stage of intrusion emplacement in the country rocks there occurred a process of recrystallization and some regrouping of material within the rock itself, without any substantial introduction (?) of material from the magma.

In the stage of skarnification, which was separated from the stage of recrystallization by a certain interval, a large quantity of emanations entered the country rocks. The interval between these stages, apart from the skarnification of the peripheral zones of the intrusion, is also determined by the fact that, regardless of the uneven grain size of the hornfels even within a single hand specimen, the size of the grains of the skarn mineral—diopside—remains constant. Sometimes diopside forms in cracks of the hornfels. It is quite obvious that the diopside was deposited after the recrystallization of the rock had already ended. Since pyroxene is, in time of deposition, one of the first skarn minerals, it may be considered that an interval exists between crystallization and skarnification. During the recrystallization of the country rocks, heat was being given off from the magma and crystallization of the intrusive body itself was taking place, and, consequently, an accumulation of volatiles was occurring, whose removal from the magma caused skarnification and ore formation.

Ore formation took place in separate parts of the contact, apparently those most accessible to the penetration of emanations.

In the first stages of skarnification, FeO and MgO were carried out of the magma; reacting with feldspars, they produced pyroxene with leaching from the feldspathic rocks of K₂O, Na₂O, and Al₂O₃. Alumina, leached from the feldspathic rocks and incorporated into the emanations, is consumed in the formation of garnet, which under these conditions forms with the introduction from the magma of FeO, CaO, MnO₂, partly Fe₂O₃, and Al₂O₃. If the formation of pyroxene and garnet takes place in limestones, then Mg, Fe, Al, and SiO₂ must be brought in from the magma and enter into compounds, with leaching and removal of CO₂. It is possible that CO₂ oxidized FeO into magnetite, which could have been deposited simultaneously with pyroxene and garnet, or even somewhat later. The work of S. Lasky (31, 488) established that magnetite forms in limestones through the oxidation of FeO by means of CO₂ and replaces limestone volume for volume at a temperature of about 620°C. It may be assumed that part of the magnetite in the limestones was formed by oxidation of FeO by means of CO₂ in the first stages of skarnification.

Under the conditions of the Irbinsky deposits, all ore bodies occur in skarns; many of them are overlain by skarns from the surface as well. The skarn zone around the ore bodies exists independently of the composition of the replaced rocks. True, sometimes it forms a very narrow rim—for example, in the hanging wall of the ore body on Mount Zheleznaya (mine No. 2), where the montzonite is very weakly skarnified.

It may be said that the ore bodies are interbedded with skarns, while the country rocks are always bordered by skarns of varying composition, and magnetite is concentrated within them. The same position of ore bodies within the skarn zone is also characteristic of other contact-metasomatic iron-ore deposits. For example, the ore bodies of Mount Magnitnaya, according to Zavaritsky (44³²), are interbedded with skarns, and Derviz (44¹⁴⁵⁵) describes the deposit of Mount Blagodat, where likewise the greater part of the ore bodies occurs within skarns.

Without deciding the question of the aggregate state of the emanations, we think that the process of formation of skarn minerals, in particular the formation of magnetite, may be represented as follows. Ema-

...nations with which the formation of ore deposits is associated contained a large amount of iron. When they left the intrusive body for the enclosing rocks, or passed among the recrystallized peripheral zones of the intrusive body itself, the compounds of magnesium (MgO) and ferrous iron (FeO) were the first to enter into reaction, with absorption of carbon dioxide and oxygen from the enclosing rocks. During this period diopside-hedenbergite and garnet were formed, in which the iron is contained chiefly in the ferrous form. Consequently, at this time reducing processes were taking place, and the formation of minerals occurred at high temperature.

For the formation of pyroxene and garnet silicates from the emanations, $Al_2O_3$, MgO, $SiO_2$, and part of the iron are consumed first of all. Undoubtedly, in this process the emanations would become relatively enriched in iron. The material of the replacing rocks and the temperatures existing at that time apparently favored primarily the formation of silicates. In addition, it must be assumed that the emanations changed their composition with time and, possibly, became increasingly enriched in iron. The indistinct boundaries of the ore bodies with the enclosing rocks and the gradual decrease in iron content from the ore bodies toward the country rocks (industrial ore, “poor ore,” ore skarn, “pyroxene- or amphibole-garnet skarns,” enclosing rocks) indicate that, in the weakened zones, silicates were deposited first, while the emanations became richer in iron, which, when the temperature decreased (necessary for the formation of $Fe_2O_3$), began to precipitate in the form of magnetite, forming separate accumulations within the skarn zones. A. N. Zavaritsky ($46^{568}$) describes the gradual transition from skarns to magnetite ores on Mount Magnitnaya; moreover, he points to skarns containing a large amount of iron, which there received the name “podrudki” or “mineralized skarns.” Indistinct boundaries of ore bodies with the enclosing rocks are also characteristic of the Irbinsky deposit. Emanations containing iron penetrated along pores and fractures into the skarn zones, in which separate grains and veinlets of magnetite were formed. The presence of such features indicates that, during the formation of magnetite, the emanations were in an extremely mobile state and penetrated along small fractures and, upon interacting with the country rocks, deposited magnetite. Such evidence is presented in the work of Uglow (39) on the genesis of magnetite deposits, where he objects to Clapp’s hypothesis, according to which a magnetite deposit formed as a result of the intrusion of “almost magnetite magma” into the contact zone. The presence of colloform structures in magnetite (Fig. 22) suggests that, toward the end of the skarnization stage, individual portions of the emanations became more concentrated and, probably, less mobile, since the formation of such structures is characteristic of minerals precipitating from colloidal solutions. It should also be noted that the formation of drusy textures is possible when deposition proceeds more slowly than dissolution.

During the crystallization of magnetite, the emanations become depleted in iron, and the next mineral to begin forming is garnet, containing ferric oxide and an insignificant amount of ferrous iron; i.e., it forms at lower temperatures. At this time oxidizing processes take place. Anomalous garnet often grows on the walls of cavities in magnetite and impoverishes the ore (Fig. 5). Finally, during this stage $SiO_2$ was removed, which at the end of the epoch precipitated in the form of free quartz.

After the formation of metasomatic quartz there follows a prolonged hiatus, which is succeeded by a new manifestation of magmatic activity of the intrusion, expressed in the formation of dikes of diabases and porphyries and in the mineralization of the rocks.

In this phase the first mineral to be deposited is epidote, which is distributed more widely than the minerals of the preceding stage. For the formation of epidote among feldspathic rocks, an introduction of H₂O, Fe₂O₃, and partly CaO is necessary, with leaching of Na₂O.

Along with the formation of epidote, SiO₂ is introduced, simultaneously with which hornblende is formed. And, finally, CaO is introduced, with which the deposition of calcite is connected. Iron here occurs only in the form of oxide or in sulfur compounds, and enters into the composition of hematite, pyrite, arsenopyrite, chalcopyrite, and marcasite. The silicates formed during this stage contain chiefly ferric oxide, i.e., oxidative processes are taking place. As an exception, during this stage magnetite is dissolved and redeposited.

After a long hiatus comes the supergene stage, during which processes of oxidation and leaching proceed. Sulfides (FeS₂, CuFeS₂, etc.) decompose, and the iron released from them migrates in the form of solutions and is deposited as limonite in the cracks and pores of the rocks through which it penetrates.

3. Transport of iron.

According to the data of Clarke and Washington (36), the average composition of magmatic rocks of the accessible parts of the lithosphere contains ferric oxide (Fe₂O₃)—3.08% and ferrous oxide (FeO)—3.80%. Recalculated as metallic iron, the content of the latter is equal to 5.08%. A significant iron content is also present in sedimentary rocks, except limestones, in which it is found in negligible quantities. A study of the enclosing rocks of the Irbinsky deposit showed that iron occurs in them in the form of very fine grains of magnetite (polished sections were prepared from a number of rocks), and also enters into the composition of the dark-colored components of the rocks. After recrystallization of the rocks, iron is also observed in them in small quantity, being situated as a fine sprinkling among quartz grains. Without chemical analyses it is difficult to decide the question of the quantitative change in the iron content, but on the basis of microscopic observations it may be said that it remained almost in the same quantities, or even somewhat greater. Thus there are no grounds for asserting that emanations capture a large quantity of iron from the upper parts of this section of the earth’s crust. True, the dark-colored components around magnetite are in places more lightly colored and, evidently, contain less iron than at a distance from it. Here it must be taken into account that these silicates formed either before the magnetite under the action of the same emanations, or even simultaneously with it. Therefore there is no reason to speak of extraction of iron from them by emanations.

Thus, for the formation of the ore bodies of the deposit it is necessary to admit the removal of iron from the magma by means of emanations. In the composition of the latter, the most widespread elements were, apparently, F and OH, with which the iron was transported in the form of volatile compounds.

The presence of fluorine in the emanations is indicated by the formation in the skarns and ore bodies of apatite, into whose composition it enters in small quantity. In addition, the later emanations carried a large

amount of fluorine, as a result of which we have the formation of a fluorite ($\mathrm{CaF_2}$) deposit on Mount Svintsovaya. This deposit, according to Onosovskaya (37), is undoubtedly connected with the later emanational activity of the same syenite intrusion, whose emanations produced the magnetite iron-ore bodies. Along with iron, the emanations carried silica, magnesium, and other elements, and it is possible that part of the iron was transported in the form of siliceous compounds.

In the first stages of emanational outflows there existed a reducing environment, and iron in the form of ferrous oxide entered into the composition of anhydrous aluminosilicates, in particular pyroxene and garnet. With a decrease in temperature, the transition of a reducing environment into an oxidizing one may have been caused by the conversion of OH into water, with the release of oxygen according to the reaction $4\mathrm{OH} = 2\mathrm{H_2O} + \mathrm{O_2}$. The liberated oxygen is a strong oxidizer, and the transport of iron at this stage probably occurred in the form of $\mathrm{FeCl_3}$ ($\mathrm{FeF_3}$). The latter, upon reaction with water, yield $\mathrm{Fe_2O_3}$, whose presence in the emanations accounts for the precipitation of iron in the form of magnetite. Toward the end of magnetite formation, garnet begins to form, containing predominantly ferric oxide, which in part indicates an oxidizing environment of the process.

From an analysis of the emanational process it is evident that the greater part of the magnetite was deposited at a relatively high temperature, which apparently was somewhat above the critical temperature of water, for the formation of magnetite proceeds simultaneously with the precipitation of anhydrous aluminosilicates (garnet, pyroxene). Of course, magnetite also formed at lower temperatures, when minerals containing water were partially formed.

With a further lowering of temperature, under oxidizing conditions, ferrous oxide no longer forms (of course, the composition of the emanations also changes), and iron precipitates in the form of hematite, enters into the composition of sulfur compounds ($\mathrm{FeS_2}$, $\mathrm{CuFeS_2}$, etc.), or is contained in epidote, chlorite, actinolite, and other minerals. It is true that during the influx of calcite (?) there occurs corrosion of magnetite and its deposition in new areas (overgrowth on pyrite, etc.), but here only a local regrouping of iron takes place.

4. Forms and sizes of the ore bodies.

It has already been mentioned above that the ore bodies of the Irbinsky deposit have lenticular, plate-like, and stock-like forms. This form of theirs is apparently due to the structure of the enclosing rocks. For example, the deviation in strike of individual ore bodies on Mount Zheleznaya (instead of a meridional strike, 2–3 lenses have a NE strike) can be explained by the structure of the enclosing rocks. In contact with limestones on Mount Bezymyannaya, the ore body also has a stock-like form. In size they are very varied, and the contoured areas of individual bodies range from 65 to 8660 sq. m. The largest ore bodies are located on Mounts Zheleznaya and Granatovaya. On Mount Zheleznaya they range from 1470 to 5475 sq. m, while on Mount Granatovaya four large ore bodies have an area of about 19,000 m². Smaller bodies (on average about 450 m²) have been exposed on Mounts Bezymyannaya and Rudnaya. Mount Svintsovaya stands apart, since here the ore bodies are broken up by numerous dikes of diabase and porphyries. Incidentally, dikes of vein rocks are distributed on all the mountains (except Mount Granatovaya) and break the ore bodies into separate sectors, which will hinder exploita-

of the deposit. For example, A. Mesyaninov (13) gives a calculation of the area of the ore bodies and writes that the total area of all the bodies is about 54,750 sq. m, of which 914 sq. m are ores beneath skarns; the area of dikes cutting the ore bodies is 425 sq. m. Thus it is evident that the area of the dikes constitutes a rather large percentage and will undoubtedly have an adverse effect on exploitation.

In view of the fact that only small bodies have been exposed on Bezymyannaya and Rudnaya mountains, these mountains will not have independent significance in exploitation.

VI. General Conclusions

The present work sets forth the results of field observations, optical investigations, and literature data on the Irbinsky iron-ore deposit. The main conclusions of this work may be presented in the following propositions.

1. The deposit is located in the Kuragino District of the West Siberian Territory, near the villages of Bolshaya and Malaya Irba, to the NE of the town of Minusinsk at a distance of about 100 km, with which it is connected by a good motor road. On the ores of the Irbinsky deposit, the first iron-smelting plant in Siberia was built in 1734. The deposit was poorly explored, and until 1930 ore bodies on Zheleznaya, Svintsovaya, and Rudnaya mountains and old workings on Bezymyannaya Mountain—“Staraya Irba”—were known.

Detailed study of the deposit began in 1930, and in 1932 the work was suspended and has not been resumed up to the present time, although the deposit remained insufficiently explored. In 1930, detailed geological and magnetometric surveying established the presence of ore on Granatovaya Mountain (“Novaya Irba”). Further exploration showed that, of the entire deposit, Granatovaya Mountain possesses the largest reserves of iron ores.

By the time of the investigation the deposit had been explored by surface workings; on Zheleznaya Mountain three core boreholes had been drilled (in addition, two boreholes were laid out and not completed by drilling), and on Granatovaya Mountain an adit 49 m long had been driven.

1. The ore bodies of the deposit occur in Cambrian limestones, acidic and intermediate effusive rocks, and also in the peripheral parts of quartz monzonite, and are situated along its contact. They are elongated in a comparatively narrow belt in a meridional direction along the left slope of the valley of the Bolshaya Irba River and occur on the mountains (from north to south) Bezymyannaya, Zheleznaya, Svintsovaya, Rudnaya, and Granatovaya, with a total strike length of about 6 km.

2. The iron-ore deposit was formed as a result of the emanational activity of an intrusion of quartz syenites. This intrusion is widespread in the Irba region. The variability of the mineralogical composition in the marginal zones, and the narrow belt of hornfelsing and skarnization around the intrusion, make it possible to assign it to the hypabyssal facies.

3. The emanational activity of the intrusion of quartz syenites may be divided into three stages: (a) the stage of hornfelsing (contact metamorphism), (b) the stage of skarnization, and (c) the stage of formation of dikes and sulfides. The stage of hornfelsing corresponds to the period of emplacement of the intrusion and the first phase of its cooling. At this time heating and recrystallization of the country rocks took place, with regrouping of the material within them. After an interval, during which

…after which emanations accumulated in the intrusion, the stage of skarn formation began. The latter was accompanied by the formation of pyroxene, garnet, magnetite, apatite, and other minerals, for the formation of which the material was undoubtedly brought in by emanations from the magma.

The stage of skarn formation was followed by a long interruption, after which diabase and porphyry dikes were formed. Among them, the diabases were formed first, while the porphyries are later. The dikes cut the ore bodies, the skarns, and the peripheral parts of the quartz massif. The formation of sulfides (pyrite, chalcopyrite, arsenopyrite, zinc blende, etc.), silicification, the formation of calcite, and partly epidote and chlorite are connected in time with the formation of the dikes.

The order of mineral formation is evident from the following scheme.

Minerals	Endogenic processes: Hornfelsing	Endogenic processes: Skarn formation	Endogenic processes: Formation of dikes and sulfides	Supergene processes
Apatite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption; —	long interruption; —
Pyroxene	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption	long interruption
Garnet	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption	long interruption
Magnetite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption	long interruption
Arsenopyrite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —; ?	long interruption
Epidote	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption; —	long interruption
Quartz	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; —	long interruption; —	long interruption
Hornblende	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —; ?	long interruption
Hematite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption; —
Pyrite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption
Chalcopyrite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption
Marcasite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption
Sphalerite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption; —
Calcite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption
Covellite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption	long interruption; —
Limonite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds	long interruption; —	long interruption; —
Chlorite	During the stage of hornfelsing (contact metamorphism), recrystallization of the rocks takes place with regrouping of matter within them.	short interruption; accumulation of emanations proceeds; ?	long interruption; —	long interruption

1. The order of mineral formation (see the scheme) makes it possible to establish that the composition of the emanations changed over time. In the first, high-temperature stages, pyroxene, garnet, and other anhydrous aluminosilicates were formed. For their formation the emanations carried from the magma FeO, MgO, CaO, SiO₂, Al₂O₃ (in the feldspathic rocks, part of the Al₂O₃ and SiO₂ is taken from the replaced rocks). Then the emanations apparently became richer in iron, and, with a decrease in temperature, under the corresponding physicochemical conditions, magnetite was formed from them. Toward the end of magnetite formation, oxidative processes began, and actinolite, epidote, chlorite, and other minerals were formed, in which water is contained, while iron (except in actinolite) enters chiefly in the form of oxide. Finally, the temperature became low, and iron could no longer form oxides, but was deposited in the form of sulfides.

2. Iron was carried out from the magma, probably, in the form of fluorine compounds. The high fluorine content in the emanations is indicated by apatite, developed in the skarns and partly in the ores, and by the fluorite occurrence (Svinцovaya Mountain), which is a product of the emanation activity of the same intrusion. It is possible that part of the iron was also transported in siliceous compounds. In the removal of iron, OH undoubtedly played a major role.

1. Regardless of the composition of the enclosing rocks, around the ore bodies there is a zone of skarns or skarnified rocks, probably caused by the action of emanations that deposited magnetite. The boundary of the ore bodies with the wall rocks is usually not sharp; rather, a gradual transition is observed from concentrated magnetite deposits to ore skarns and enclosing rocks.

Between garnets occurring in limestones and similar garnets located in igneous rocks, there are certain distinctive properties. For example, garnets formed in limestones have a greenish coloration both macroscopically and in transmitted light under the microscope. They are rarely anomalous, and are predominantly isotropic. If garnets formed in igneous rocks, then they have a brown coloration, both macroscopically and under the microscope. Finally, in skarns located among igneous rocks, anomalous garnets—grossular—are developed.

1. The texture of the ores is mainly massive, more rarely banded and drusy. The banded textures of the ores resulted from the replacement of similarly banded rocks, i.e., the texture is relict. Drusy textures form because dissolution of the material proceeds faster than its deposition. In ores of massive texture, after weathering, banding appears, as a result of which they sometimes acquire a botryoidal form of texture: here, chlorite and quartz are located in the intervals between the ore bands. The formation of such textures may also be explained by the relict texture of the replaced rocks.

2. The ores of the deposit consist predominantly of magnetite with an insignificant content of hematite and with admixtures of sulfides (beneficiation of the ores during exploitation is necessary), which are represented by pyrite, chalcopyrite, marcasite, sphalerite, and arsenopyrite. The sulfides are listed in decreasing order.

Magnetite in reflected light has a gray color with a brownish tint. It is etched by fuming HCl on average within 5 seconds. It has an allotriomorphic, more rarely granoblastic structure. In certain areas, deformation structures were encountered, which apparently formed as a result of movements and pressure that occurred in the area of the deposit.

Hematite has predominantly supergene origin. It develops in the form of small needles and forms martite. Alongside it, hypogene hematite is encountered, which is arranged in the form of separate elongated grains and replaces magnetite or is observed between its grains.

1. Marcasite around large pyrite grains is observed in the form of a fine sprinkling, giving a porphyroblastic structure, or it forms thin veinlets within it. The presence of marcasite veinlets in pyrite and the corrosion by them of the edges of chalcopyrite grains prove that it separated in the last stage of the emanational activity of the intrusion. The occurrence of marcasite in primary ores and the structural interrelations indicate its hypogene formation.

2. At the surface, the ores underwent oxidation, as a result of which hematite developed after magnetite, forming in it a fine network. The hematite needles are arranged along the crystallographic directions of magnetite. In some areas (the Svintsovaya and Rudnaya mountains), at the surface, magnetite is replaced almost entirely by hematite, or only relicts of it remain. The oxidized ore has a brownish coloration. The sulfides here are also leached out, with the formation at their expense of limonite and, in places, covellite. As a result of this, the ores from the surface are purer.

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15. V. A. Obruchev.—Metallogenic epochs and provinces of Siberia. Transactions of the Institute of Applied Mineralogy and Metallurgy, issue 21, 1926.

16. N. V. Shishkin; A. N. Zavaritsky, V. A. Obruchev, et al.—Manganese and iron ores of the USSR. Transactions of the Commission on Metal under Gosplan of the USSR, 1926.

17. I. A. Molchanov.—Essays on the geology of Siberia. Eastern Sayan. Publishing House of the Academy of Sciences of the USSR, 1934.

18. The principal iron-ore deposits of the USSR. TsNIGRI, volume II, 1934, p. 316.

Explanation of the Figures

Fig. 1. A marble area in skarns near the ore body. Mount Bezymyannaya, trench No. 34 (sketch).

Fig. 2. Selective replacement of feldspars (1) of quartz monzonite by garnet (2). Fragments of pyroxene have been preserved (3). Mount Svintsovaya, trench 78. (Sketch with drawing eyepiece), transmitted light.

Fig. 3. Overgrowth of anomalous (2) garnet on brown (3). The spaces between grains are filled with calcite (4). A remnant of a pyroxene grain (1) is visible. Mount Zheleznaya, transmitted light, magn. 38.

Fig. 4. Skarnification of marble. Garnet (black) develops among marble (light gray and gray) in the form of veinlets and individual grains. Mount Bezymyannaya, transmitted light, crossed nicols, magn. 70.

Fig. 5. Drusy texture. On the walls of magnetite (black), anomalous garnet (gray) grows; the center is filled with quartz (white). Mount Zheleznaya, transmitted light, magn. 70.

Fig. 6. Pyroxene grains (light) are corroded by garnet (dark). Transmitted light, magn. 70.

Fig. 7. Arrangement of sulfides (white) in magnetite (gray). The sulfides have the form of elongated grains. Mount Granatovaya, adit, polished surface of a specimen, natural size.

Fig. 8. Banded ores of Mount Zheleznaya. Polished specimen, magn. 1.7.

Fig. 9. Layered banded magnetite ore (chip-like texture). Mount Granatovaya, polished surface at natural size.

Fig. 10-a. Deformed grains of magnetite (dark-light gray). Fine needles of hematite (white) are visible. Mount Svintsovaya. Reflected light, magn. 110.

Fig. 10-b. Same, magn. 75.

Fig. 11. Deformed grains of zonal magnetite (gray) with grains of the same magnetite trapped within them (white). Reflected light, magn. 75.

Fig. 12. The central parts of magnetite grains are martitized (lattice); along the margins magnetite is preserved (gray); the black field is non-ore minerals. Reflected light, magn. 420.

Fig. 13. Fine-grained magnetite forms a veinlet among normal magnetite (gray) and non-ore minerals (black). Reflected light, magn. 120.

Fig. 14. Magnetite (gray) grows on pyrite grains (white). The black field is non-ore minerals. Reflected light, magn. 260.

Fig. 15. Porphyroblasts of pyrite (gray), situated among marcasite (gray fine grains). The pyrite grains are surrounded by quartz and calcite (black). Reflected light, magn. 105.

Fig. 16. Veinlets of marcasite (2) in pyrite (3); non-ore minerals (1); reflected light, magn. 420. (Exact copy from a photograph.)

Fig. 17. Chalcopyrite (black), situated along the cleavage of actinolite (gray). Transmitted light, magn. 30.

Fig. 18. Colloform structure of limonite (4) around chalcopyrite (3). Magnetite (2) occupies the greater part of the section. Non-ore minerals (1). (Sketch.)

Fig. 19. Among grains of magnetite (dark gray) is hydrohematite (gray), which penetrates inward and corrodes grains of pyrite (white); reflected light, magn. 260.

Fig. 20. Magnetite (black) is separated from marble (gray) by a band of garnet and quartz (dark gray). Transmitted light, magn. 70.

Fig. 21. Relict grains of plagioclase among hornfelsed effusive rock. Transmitted light, magn. 30.

Fig. 22. Colloform structure of magnetite. Reflected light, magn. 160.

Mt. Bezymyannaya
trench No. 34

Fig. 1.
1. Ore. 2. Skarn. 3. Limestone. 4. Diabase.
Scale 1:100.

Fig. 2.
1. Plagioclase. 2. Garnet. 3. Pyroxene.

Fig. 3.
1. Pyroxene. 2. Anomalous garnet. 3. Brown garnet. 4. Calcite.

Fig. 4.

Fig. 5.

Fig. 6.

Table 1

Plate II

Fig. 8

Fig. 7

Fig. 10-a

Fig. 9

Fig. 11

Fig. 10-b.

Plate III.

Fig. 14.

Fig. 17.

Fig. 13.

Fig. 16. 1. Non-ore minerals. 2. Marcasite. 3. Pyrite.

Fig. 12.

Fig. 15.

Table IV.

Fig. 18. 1. Non-ore minerals. 2. Magnetite. 3. Chalcopyrite. 4. Limonite.

Fig. 19.

Fig. 20.

Fig. 21.

Fig. 22.

Geological Map of the Irbinsky Iron-Ore District

Scale: 0 — 100 — 200 — 300 — 400 m

Legend

• Quartz monzonite
• Porphyrites and their tuffs
• Felsite-porphyries and orthophyres
• Limestones
• Ore bodies
• Skarns
• Diabase dikes
• Porphyry dikes
• Less altered granites
• Alluvial deposits

Map labels visible

• Bezymyannaya
• G. Zheleznaya
• G. Solntsevaya
• G. Rudnaya
• G. Granatovaya
• M. Zhelezny
• M. Mednitsky
• B. Irba
• Uspenka
• R. Bulan
• R. Bol. Irba

Schematic geological sections through the ore bodies of Mount Zheleznaya

Along the line of borehole No. 1

Along the line of borehole No. 3

Scale 1:1000

Legend:

• Quartz monzonite.
• Ore bodies.
• Skarns and skarnified hornfels.
• Diabase dikes.
• Porphyry dikes.
• Overburden.

1. A. Ya. Bulynnikov, in the composition of quartz syenites, indicates the presence of biotite. ↩