A. I. ALEKSANDROV.

ON THE COMPOSITION AND GENESIS OF THE ORES OF THE KAILAN, DITUR, AND KIMKAN IRON-ORE DEPOSITS OF THE FAR EASTERN TERRITORY.

1. Introduction.

The microscopic investigation is part of the work on the study of the iron ores of the Far Eastern Territory that the Siberian Institute of Metals carried out in 1933. The material for the investigation consisted of specimens of ores and enclosing rocks selected from samples from the Kailan, Ditur, and Kimkan deposits of the FET. The samples from which the specimens were taken were delivered to the Siberian Institute of Metals in the summer of 1933 by S. I. Golosov, a fourth-year student of the Siberian Mining Institute, who, on assignment from the Institute of Metals, visited all the deposits mentioned.

The microscopic investigation was carried out with the aim of determining the mineralogical composition, texture, and structure of the ore and enclosing rocks, the paragenesis of the ore minerals, and, if possible, of clarifying the genesis of the deposits. It must be noted that we did not have at our disposal systematically collected suites; for the most part, the specimens for study were selected from samples in which, along with ore, country rocks also occurred. A better-selected suite was available only from the Kailan deposit, where it had been collected by A. Vladimirov, foreman of the Kailan geological-exploration party. This suite contained specimens of ores from the footwall, hanging wall, and middle of each ore bed of the deposit and, in addition, specimens of the enclosing rocks—siliceous shales and ore-bearing and barren breccias.

The work was carried out in the ore-deposits laboratory of the Siberian Mining Institute. I express my sincere gratitude to F. N. Shakhov, head of the Department of Mineral Resources, for a number of valuable suggestions.

2. Brief information on the geology of the district.

According to the report of S. M. Tkalich¹, the district of the northern part of the Lesser Khingan is composed predominantly of Precambrian and Devonian rocks.

The Precambrian rocks are represented (from bottom to top) by biotite orthogneisses, two-mica orthogneisses, crystalline-biotite schists, graphite schists, limestones, magnesite, dolomite, crystalline paraschists, pyrrhotite quartzites, and quartzites. Within the Precambrian deposits four unconformities are noted. The thickness of these deposits is not less than 1000 m.

Devonian deposits rest unconformably on the Precambrian rocks; their composition includes talcose schists, limestones, siliceous schists, an ore-bearing member, chlorite schists, breccias, dark talcose schists, gray and white limestones, and siliceous schists. The thickness of the Devonian deposits is about 1200 m. The Veldian coal-bearing series (the Kailan series) rests unconformably on the Devonian deposits.

The intrusive rocks of the district are represented by Precambrian biotite granite, Upper Devonian (?) porphyritic granite and porphyries, and granites of the Tertiary period, and, of the same period, quartz porphyries and basalts.

Contact metamorphism of the iron ores of the Kimkan deposit is associated with the intrusion of granite of Tertiary age.

3. Ores and enclosing rocks of the Kailan deposit

The Kailan iron-ore deposit is located in the northern part of the Maly Khingan Range of the Far Eastern Territory. It is the largest and best studied among the other iron-ore deposits of the Maly Khingan¹.

Work carried out in 1932 at the Kailan deposit revealed four ore bodies—“ore beds”: the Upper, with a thickness from 0.14 to 0.7 m; the Nadezhny, with a thickness from 0.7 to 1.6 m; the Tonky, with a thickness from 0.4 to 0.7 m; and the Slozhny, with a total thickness averaging up to 2.4 m.

The ore of the listed beds is identical; therefore, in the following description the separate beds are not distinguished by composition and structure.

Macroscopically the ore is reddish-brown in color and has a banded texture. In coloration the layers differ only slightly from one another (brown, reddish-brown). The thickness of the layers varies from 0.3 to 10 mm, but layers 0.3–2 m thick occur predominantly. The ore is very fine-grained. When struck with a hammer it gives polyhedral fragments with relatively even faces, but sometimes the denser varieties give a conchoidal fracture.

In polished sections the ore consists of hematite, magnetite, limonite, and non-ore minerals, and gold occurs in negligible quantities.

Hematite is the predominant ore mineral in these ores and, by approximate estimate, constitutes about 55–60%, which corresponds to the Fe₂O₃ content given in the report of S. M. Tka-

personally. Part of the iron is probably also contained in carbonates. \(Fe_2O_3\) is distributed throughout the ore in the form of very fine needles, measuring from 0.001 mm in length and up to 0.0002 mm in width. The most characteristic hematite grains, in terms of size, have a length of 0.002–0.004 mm. These grains are scattered irregularly throughout the ore, but more often they accumulate in the form of layers, in which a parallel arrangement of the needles is discernible, giving the ore a banded appearance. Alternation of hematite with non-ore minerals is observed in all ores of the Kailan deposit. Sometimes hematite layers pass uninterrupted through the entire thin section, but usually the interlayering has the form of small lenses; in places the non-ore areas have an irregular angular form. Layers that are macroscopically homogeneous, when studied microscopically, prove to consist of alternating hematite and non-ore areas. Individual layers have, for example, the following dimensions: hematite—0.005, non-ore—0.023, hematite—0.008, non-ore—0.010, hematite—0.006, non-ore—0.03 mm.

Such alternation is most characteristic of the Kailan ores. If the alternation of hematite and non-ore material has the form of lenses, then the latter in the thin section measure from \(0.07 \times 0.03\) to \(0.3 \times 0.05\) mm. In areas of accumulation of non-ore material there are always inclusions of fine hematite needles (photo 1), which give these areas a red internal reflection.

Magnetite is observed very rarely in the ores of the Kailan deposit. It occurs as individual grains ranging in size from \(0.02 \times 0.015\) to \(0.002 \times 0.002\) mm; the average and most prevalent grain size is \(0.01 \times 0.008\) mm.

The magnetite grains have an angular form. Some of them are weakly martitized. Structurally they are etched by fuming HCl within 30 sec.

Limonite occurs in every thin section, either as very thin (0.02 mm) veinlets or as irregularly shaped accumulations. As we shall see below, limonite formed in two generations; that occurring in the form of accumulations formed first of all, even before the development of hematite, whereas the limonite occurring as veinlets formed after the ore deposit had taken shape, the limonite veinlets cutting the ore at various angles to the bedding.

Hydrohematite is present in a few thin sections. It always occurs together with limonite, but differs from the latter in the following properties in reflected light: its color is somewhat lighter, the pleochroism of reflection is more clearly noticeable; when immersed in cedar oil it turns gray (limonite darkens); in crossed nicols it is anisotropic. Its internal reflection is redder than that of limonite.

Gold occurs in the ores of the Kailan deposit in small quantities. It is always spatially associated with limonite of the second stage¹. The size of the gold grains varies within—

¹ S. M. Tkalich indicates that, according to field chemical analyses carried out by engineer Presich, indications of gold were found in the Kailan ores.

in sizes from 0.002 to \(0.008 \times 0.004\) mm. Etching with \(\mathrm{AgNO_3}\) gave negative results, and under the action of \(\mathrm{KCN}\) it darkens slightly.

In polished sections prepared from ores of the Tonkii and Nadezhnyi seams, several grains of pyrite were encountered, with diameters from 0.009 to 0.004 mm.

The non-ore minerals are represented by quartz, carbonates, titanite, and chlorite.

Quartz is distributed throughout the ore in very fine grains. It often occurs as separate accumulations which, in cross section, more often have the form of small lenses and more rarely an irregular angular form. Owing to contamination by limonite, the quartz in these aggregates has a yellowish-brown color both macroscopically and microscopically. Finally, quartz occurs in the form of individual veinlets that cut the entire rock at various angles to the bedding. Among this quartz, accumulations of limonite were not observed.

Carbonates occur in the ore more rarely than quartz. They are represented by separate small grains, sometimes by aggregates having an angular form (ore breccia), and are very rarely observed in the form of veinlets. Along the edges of carbonate aggregates there is almost always a narrow rim, consisting of hematite and limonite, which indicate that iron was more readily concentrated around the carbonates, or, possibly, that it was partly derived at their expense.

Titanite is present in the rock in a small amount; it is always confined to areas of quartz accumulation.

Chlorite is poorly developed; like titanite, it is situated near quartz and carbonates.

Of the enclosing rocks of the Kailan deposit, we have at our disposal only breccias and siliceous shales, as they are identified by S. M. Tkalich¹.

Breccia is macroscopically gray in color with a greenish tint; upon weathering it acquires a yellow coloration. The fragments are represented by carbonate rocks; more often they have the form of rounded pebbles and more rarely an angular form.

Microscopic study determined that the pebbles and fragments composing the breccia consist of carbonates and quartz, while the cement is composed of limonite, chlorite, and a small amount of hematite. Carbonates constitute the greater part of the rock and occur either as separate grains ranging in size from 0.07 to 0.01 mm, or as rounded pebbles of oval form, measuring on average \(1.68 \times 1.05\) mm, and more rarely as angular fragments (phot. 2).

Quartz occurs in small amounts in the form of separate grains, partly rounded and partly angular. It is situated among the calcareous-marly material.

Chlorite is observed throughout the entire polished section; it is situated among

cement and sometimes entirely replaces some mineral. Limonite is often encountered around chlorite.

Limonite and hematite, for the most part, enter into the composition of the cement and, in addition, almost around every carbonate pebble in the more altered conglomerate they form a rim up to 0.07 mm wide (Fig. 3).

In some conglomerates, accumulations of limonite and hematite are not observed around the carbonate pebbles; rather, they are distributed uniformly throughout the cement. The principal mass consists of limonite, while hematite occurs in small quantity. According to the degree of roundness of the carbonate pebbles, in our opinion it would be more correct to call the present rock a conglomeratic rock.

Silicilite is reddish-brown in color, with a faintly noticeable banded texture. The rock is very fine-grained. Thin veinlets of quartz (0.1–0.3 mm) cut the rock in various directions. In thin section the rock consists of quartz, limonite, titanite, and chlorite.

Quartz is the predominant mineral; it occurs chiefly in the form of very fine-grained (microcrystalline) aggregates, which in cross section have the form of small lenses, ellipses, a bent spindle-like shape, and, finally, an angular form. Iron hydroxide is confined to the peripheral part of the quartz accumulations. It is true that this mineral is sometimes also situated inside the quartz aggregates, though in small amounts. In addition, quartz forms very thin veinlets cutting the rock at various angles to the bedding (Photo 4).

Limonite occurs in all thin sections, as well as in the ore. In thin sections prepared from samples from a depth of 80 m, it forms flakes and spots; under surface conditions (depth 1.1 m), in addition to flakes, it occurs in the form of veinlets and penetrates the rock, giving it a brown color. The limonite composing the veinlets apparently formed at the expense of the decomposition of pyrite and carbonates during weathering. Sometimes separate oval fragments of carbonate are encountered, in which small inclusions of limonite have a linear form and intersect at a certain angle, resembling the cleavage of calcite. It is possible that during the weathering of carbonates, limonite is released and forms first of all along their cleavage planes. During metasomatic replacement by quartz, the hydroxide is transformed into hematite.

A small amount of titanite and chlorite is encountered in the thin sections. Titanite is situated among the quartz aggregates, and chlorite around hematite and limonite.

According to the distribution of quartz and other minerals, the rock may be assigned to a conglomerate altered by subsequent processes, in which the carbonate contained in the conglomerate was metasomatically replaced by quartz, while the iron hydroxide along the periphery of the quartz was transformed into hematite (Fig. 4).

Thus, among the country rocks there occur both conglomeratic rocks, in which we observe rounded and angular pebbles (Figs. 2 and 3) with ferruginous cement (substantially li

limonite), and silicified rocks—silicilites (fig. 4), in the cement of which the ore minerals are represented chiefly by hematite. The pebble in these latter rocks is flattened and has irregular forms, but even in this case it is still clearly visible that the silicilites were formed from a conglomeratic rock. In fig. 1 we have a hematite ore in which inclusions of non-ore minerals have the form of small lenses, fragments, etc.

Turning to the question of the primary structure of the ores, we must note the following: 1. The conglomeratic rock with slight development of hematite (fig. 3) does not differ in structure from the rock (fig. 2) in which hematite is not encountered. 2. The essentially hematitic ores (fig. 1) have morphological structural features entirely analogous to those of the strongly silicified rocks (fig. 4). Aggregates of hematite develop at the expense of the cement of the clastic rock, the fragments of which were subsequently deformed and partially replaced by silica. Undoubtedly, silica was also deposited together with iron hydroxide.

Therefore we consider it possible to interpret the ores of this deposit as primary sedimentary rocks, in which fragments of limestone rocks and, more rarely, quartz were present, while limonite and also silica served as the cement.

4. Ores of the Ditur Deposit.

Macroscopically, the ores of the Ditur deposit range in color from reddish-brown to dark brown. The ore having a dark-brown color consists predominantly of fine-grained magnetite. Like the Kailan ores, it has a banded texture, caused by the alternation of ore interbeds with non-ore ones.

In polished sections, magnetite, hematite, limonite, hydrohematite, and non-ore minerals are observed; pyrite, chalcopyrite, and gold occur in small quantities.

Magnetite is developed unevenly in different parts of the deposit. In a sample from the VII prospecting line magnetite predominates, whereas in other samples it occupies a subordinate position relative to hematite. It is distributed as individual grains ranging in size from \(0.22 \times 0.07\) to \(0.002 \times 0.002\) mm. All magnetite grains are martitized; sometimes martitization is developed to such a degree that around grains of \(\mathrm{Fe_3O_4}\) a rim of hematite is formed, resembling a halo in shape. Some magnetite grains are corroded (fig. 5). Structurally it is etched by fuming HCl for 1.0–1.5 minutes.

Hematite is scattered throughout the ore in the form of individual grains having the shape of small needles, from 0.001 to 0.003 mm in length. In areas of accumulation of non-ore material, hematite is present in small quantity, approximately 10–15%, and here too it is arranged in the form of needles of the same size. At higher magnifications (especially with immersion) a very strong brownish-red internal reflection is observed, which obscures the entire polished section.

The content of hematite in the samples is not uniform; for example, in sample No. 1 hematite is predominant; in sample No. 7 it is present in an amount approximately equal to magnetite; and in the sample from exploration line VII it is present in an amount subordinate to magnetite.

Limonite occurs chiefly in the form of very thin veinlets, situated now parallel to the bedding of the rocks, now cutting it at various angles to the bedding. Sometimes limonite is located at the contacts of quartz veinlets with the rock and, finally, in the form of accumulations of irregular shape.

In these accumulations, acicular hematite is sometimes observed.

Hydrohematite is present in all thin sections, although in small quantity. It develops chiefly around limonite veinlets and, more rarely, around limonite arranged in accumulations of irregular shape.

Pyrite and chalcopyrite were encountered as several grains of each. The size of the grains of these minerals ranges from 0.01 × 0.004 to 0.006 × 0.004 mm.

Gold is present in the ores in small quantity. It is always located near limonite, in the form of small grains up to 0.004 mm in size.

The non-ore minerals are represented by quartz and chlorite. Microcrystalline quartz is distributed throughout the ore, now in the form of separate accumulations, now forming interlayers in it. These areas are impregnated with limonite, as a result of which, in transmitted light, the quartz appears colored in yellowish, brownish, and reddish colors. In addition, quartz forms honeycomb aggregates, filling individual cavities, or its grains occur among the ore. Some quartz grains are cut by limonite veinlets. And, finally, quartz forms very thin veinlets that fill cracks in the ore; these veinlets cut the entire rock. Limonite and chlorite often appear at the contacts of the veinlets (fig. 6).

Chlorite is located in the form of separate areas near limonite and metasomatic quartz; sometimes it forms very thin veinlets, in which the chlorite grains are oriented perpendicular to the walls. Some chlorite areas, owing to impregnation with iron oxides, have a yellowish coloration. The non-ore formations have the form of interlayers and small lenses and occur as separate grains. The small lenses and layers have a width from 0.012 to 0.4 mm. These areas also contain hematite and magnetite in small quantities.

The rock is very fractured in various directions, and these disturbances occurred after the formation of the quartz veinlets, since the latter are displaced near the fractures.

In the ores of the Ditur deposit, magnetite is present in greater quantity than in the ores of the Kailan deposit. Chlorite is also present in greater abundance. Consequently, the deposit underwent stronger metamorphism, during which quartz was introduced, chlorite was formed, and iron hydroxide was transformed into hematite and, in smaller quantity, into magnetite.

5. The Kimkan Deposit

a) Ores

The ores of the Kimkan deposit are very fine-grained. They have a banded texture. The color of the ore is heterogeneous: interlayers consisting of magnetite are dark gray, while interlayers consisting of gangue minerals are light gray; the thickness of these interlayers varies within the following limits: gangue, from 0.3 to 5.0 mm; ore, from 0.8 to 20.0 mm.

Sometimes inclusions of gangue minerals have an irregular shape.

I consider it necessary to point out once again that the ores and enclosing rocks for study were taken from the bulk sample of the deposit.

Microscopic study of the Kimkan ores revealed magnetite, hematite, gangue minerals, and, in small amounts, limonite and chalcopyrite.

Magnetite in the Kimkan ores is the predominant mineral, and in some polished sections of ore minerals only magnetite is present. It is light gray in color, with a brownish tint; with fuming $HCl$ and $HF$ it is structurally etched within 3 seconds, the periphery of the magnetite grain being etched more strongly, and the center more weakly. After etching, only traces often remain of the fine grains. The grain size ranges from $0.2 \times 0.008$ to $0.05 \times 0.04$ mm. Despite the fact that the ore samples were taken from small workings, martitization was not observed. The grains are irregular in shape and often form granoblastic and skeletal structures (Fig. 7).

Hematite in the Kimkan ores occurs as separate grains situated between magnetite grains. The grains of $Fe_2O_3$ more often have irregular shapes, and more rarely one can observe the form of elongated plates having dimensions from $0.012 \times 0.005$ to $0.03 \times 0.01$ mm. It constitutes, approximately, up to 5% of the ores.

In one polished section several small grains of chalcopyrite were encountered. In reflected light it has a yellow color. From $AgNO_3$ its grains darkened. Pyrite also occurs in the ore in insignificant quantity.

Limonite was observed very rarely. It was found in a few polished sections, in the form of small areas of irregular shape. In the Kimkan ores limonite, by quantity, does not merit attention.

The gangue minerals in the ores of the Kimkan deposit are represented by quartz, fibrous amphibole, biotite, muscovite, chlorite, garnet, and tourmaline. They form separate interlayers that alternate with the ore layers. As an example, the following characteristic alternation of these interlayers is given, their widths being: ore—3.0, gangue—0.1, ore—0.6, gangue—2.4, ore—0.84, gangue—1.8 mm. In addition, among the gangue minerals there are very few grains of magnetite, and conversely. All interlayers of gangue have a granoblastic structure.

Quartz is the most widespread of the gangue minerals...

valuable. It is present as separate grains among the ore. Most of the barren interlayers consist almost exclusively of quartz, where it has a hornfelsic structure. In some areas the quartz contains such a large quantity of the finest inclusions of magnetite that in transmitted light it becomes cloudy.

Radiating amphibole occurs in the ores in small quantity. It forms small acicular grains, often almost colorless; sometimes they have a greenish coloration. In the latter case a weak pleochroism in yellowish-greenish tones is noticeable. Radiating amphibole occurs predominantly near magnetite and only rarely away from it. At the boundaries between magnetite and quartz it forms interlayers 0.43 to 0.65 mm wide (Fig. 8).

Biotite occurs comparatively rarely. It is situated as small grains among the quartz, and its grains have a hornfelsic structure.

Muscovite occurs here and there among the barren interlayers; in addition, it forms thin veinlets in the magnetite, these veinlets being arranged parallel to the bedding of the rock. Chlorite is present around the muscovite veinlets. Chlorite, furthermore, occurs both among the magnetite and among the barren areas, developing chiefly around radiating amphibole and sometimes along the margins of small quartz lenses bordering on magnetite.

Among the barren minerals, tourmaline occurs in small quantity. It is present as small prisms ranging in size from 0.08 × 0.05 to 0.05 × 0.02 mm. In some parts of the grain the pleochroism is from colorless to cloudy green; in others, from colorless to brownish. The interference color is mottled and appears in colors from grayish to pinkish, yellowish, and blue.

b) Country rocks.

The collection of country rocks from the Kimkan deposit is represented by a few types.

For example, it contains adamelite, actinolite-garnet hornfels, biotite hornfels, and hornfelsed sandstone.

Adamelite is light gray in color. It has a massive texture and is medium-grained. It consists of quartz and white feldspars. The dark-colored components are represented by biotite.

In thin section, quartz, plagioclase, biotite, muscovite, chlorite, magnetite, zoisite, apatite, and sphene are encountered. The structure of the adamelite is hypidiomorphic-granular.

Quartz, in the form of irregularly shaped grains, fills the spaces between the plagioclase grains.

Plagioclase occurs as idiomorphic crystals of prismatic form. The central parts of the grains have undergone greater alteration; in transmitted light they have a grayish color, while the marginal zones are fresher, with no cloudy tones in them. Most of the plagioclase has a zonal structure; measurements of two or three plagioclase grains by the Fedorov method showed that

the plagioclase present in the rock is of intermediate acidity, namely Nos. 36–38. Along cleavage cracks in it, elongated crystals of muscovite (sericite) appear. The development of muscovite grains is more often encountered in the central parts of plagioclase grains, where, as was indicated, it has decomposed more strongly. Muscovite is occasionally also encountered as separate flakes.

Biotite occurs in the thin section in the form of small leaves in the interstices of plagioclase; it is sometimes also found among quartz grains. The edges of the biotite grains are corroded; in places small areas have been preserved. Pleochroism ranges from colorless to brown.

Zoisite is present in the rock in small quantity. Its grains are isometric in form and are situated among the plagioclase. In transmitted light it is colorless or greenish; the interference colors are anomalous. Sometimes it occurs beside biotite, forming small prismatic grains; it is always secondary.

Chlorite is almost always formed at the expense of biotite; it is also observed in places where muscovite develops. In addition, magnetite, apatite, and sphene are present in the rock in small quantities.

The presence in the composition of the rock of andesine and an entirely insignificant quantity of orthoclase defines it as an adamellite. We have at our disposal only one specimen and, probably, from the peripheral part of the intrusive body, where a more basic differentiate of magma is possible, evidently rich in volatile components. As a consequence of this, we have the formation of muscovite, sericite, and zoisite both in the country rocks (hornfelses) and in the ore body (thin veinlets in the ore).

Actinolite-garnet hornfelses are macroscopically grayish-green in color and very fine-grained. Within this fine-grained mass there are inclusions of calcareous material, having the form of a flattened pebble measuring \(2 \times 7\) mm. Among the carbonates calcite is present (effervesces with \(HCl\)). In addition, there are small cavities, the walls of which are coated with yellow ocher.

In thin sections the rock consists of actinolite, quartz, garnet, biotite, chlorite, and magnetite, representing a spotted actinolite-garnet hornfels.

Actinolite is the predominant mineral in the rock; it occurs as small grains having an elongated form; the grains are often isometric. In addition, radiating aggregates are observed in individual areas. Sometimes actinolite forms a veinlet with uneven margins, where it is fresher, whereas in other areas it decomposes with the formation of chlorite.

Quartz is present in these hornfelses in small quantity. It consists of small grains with undulatory extinction.

Garnet has a brownish coloration and is isotropic. It forms separate knots, which determine the porphyroblastic structure of the rock. Around the garnet knots biotite is observed, which rarely forms small leaves; it is more often encountered in irregularly shaped accumulations. It has a brown color, with pleochroism from colorless to brownish.

Chlorite is distributed throughout the entire thin section, and it often develops after actinolite, while around magnetite grains it occurs in the form of small accumulations. In individual portions the rock has preserved the form of a conglomerate structure; here the chlorite is especially well developed in the cement, whereas in the portions where the pebbles of the conglomerate were situated, sericite and actinolite develop in part. These portions are colorless in transmitted light. The aggregates of actinolite and sericite form cross-sectional shapes that are more often oval (the shape of pebbles) and less often angular.

Magnetite occurs in the form of irregular grains scattered throughout the entire thin section.

Biotite hornfels in the collection from the Kimkan deposit is dark brown in color, very fine-grained, with a weakly expressed banded texture. V. A. Pervago, who investigated this deposit in 1932, probably calls it a siliceous shale, but in his report he does not give a description of the rock¹).

Microscopic investigations showed that the rock consists of a granoblastic mixture of quartz, biotite, actinolite, titanite, and epidote.

Quartz constitutes the greater part of the rock. Its grains have an isometric form and produce a hornfelsic structure. A large part of the grains contains very many inclusions of magnetite, which in transmitted light are responsible for their cloudy coloration. In addition, there is quartz that has no inclusions at all.

Biotite is present in very large quantity. It occurs chiefly in the form of flakes, more rarely having irregular patchy forms. Pleochroism ranges from colorless to brown; interference colors are of the second and third orders. Often along the periphery of biotite grains (sometimes penetrating toward the center) epidote begins to form, producing very small grains.

Actinolite is situated chiefly around quartz and does not play an essential role in the formation of the rock. It always occurs in the form of elongated crystals; together with it, grains of titanite are encountered.

In addition to the minerals described, pyrrhotite and “melnikovite”—marcasite occur in the biotite hornfels.

Pyrrhotite is present in small quantity. It occurs in the form of small grains, more often prismatic in form and more rarely isometric. In reflected light it has a pinkish coloration. Reflective pleochroism in air is almost imperceptible; in oil the color changes from light pink to pinkish brown. In crossed nicols it is distinctly anisotropic.

The greater part of the ore mineral in the rock consists of melnikovite-marcasite. It has a heterogeneous coloration: in the center of the grains it is dark gray, sooty, with clearly visible striation, and along the periphery of the grains it is white. In the central parts, pleo—

¹) V. A. Pervago. Preliminary report on the preliminary exploration of the Kimkan iron-ore deposit in 1932. Manuscript.

chroism of reflection, while at the periphery pleochroism is not observed. In exactly the same way, in crossed nicols the central parts of the grains polarize in colors from blue to greenish, while the margins do not polarize. The relation to chemical reagents is different. If the center of the grains effervesces rapidly in \(HNO_3\), then at the periphery the acid leaves only a black spot. From \(KCN\) and \(FeCl_3\) a coating appears in the center of the grains, while at the periphery no changes occur. This mineral forms colloform structures (Fig. 9). In some aggregates of melnikovite-marcasite, preserved remnants of pyrrhotite are still visible. Thus, we have exogenous alteration of pyrrhotite into marcasite-melnikovite.

Finally, limonite is present in the rock in a very small amount.

The next representative of the country rocks is hornfelsed sandstone, which has a massive texture. Its color is grayish-yellow, spotted. The spots, 2 to 3 mm in size, are represented by quartz.

Under microscopic study, quartz, microcline, plagioclase, biotite, muscovite, and limonite were identified in the rock. Quartz forms irregularly shaped grains, producing a granoblastic structure. It constitutes the greater part of the rock. Among the quartz grains, microcline and plagioclase (albite) occur in small amounts; they also form grains similar in shape to the quartz grains. Biotite is situated between the quartz grains; it forms small flakes containing many inclusions of opaque material and zoisite. Muscovite also contains many inclusions; its grains have a lath-like form. In addition, the rock contains a small amount of limonite. Probably this rock is what V. A. Pervago calls aplite-granite. He writes: “granite occurs, forming veins from 0.5 to 1 m thick. These veins follow the bedding of the metamorphic rocks”¹). In our opinion, this is hornfelsed sandstone, in which, under the influence of pneumatolysis from the granite intrusion, biotite, muscovite, microcline, and albite (?) formed, with recrystallization of the sandstone and the formation of a granoblastic structure.

6. Genesis of the Ores.

The question of the genesis of the Kailan, Ditur, and Kimkan deposits of the Maly Khingan district of the Far Eastern Territory has not been resolved up to the present time. Researchers express different opinions on the genesis of these deposits. The geologist Konstantov²), who discovered the Kailan deposit in 1912–15, considers it exogenous—sedimentary. The geologist Semenyuk, who investigated the Kailan deposit in 1931, came to the following conclusion: “Probably, in

¹) V. A. Pervago. Preliminary report on the preliminary prospecting of the Kimkan iron-ore deposit, in 1932. Manuscript, p. 5.
²) E. E. Anert. Mineral Resources of the Far East. Joint-Stock Company “Knizhnoe delo.” Khabarovsk—Vladivostok, 1928, pp. 284–285.

in Lower Paleozoic time this area represented some shallow-water basin, apparently a coastal strip of the sea. On the floor of this sea, i.e., of its coastal strip or bay, hydrates of iron oxides \((+)\) and hydrates of silica \((-)\) were precipitated from colloidal solutions. These hydrates, differently charged, mutually precipitated one another and produced alternating accumulations of iron and silica¹. Further, the engineering geologist S. M. Tkalich², who directed the exploration of the Kailan deposit, also adheres to the hypothesis of sedimentary genesis. In defense of this hypothesis he states: “1) The ore bodies in the region of the Lesser Khingan are confined to originally sedimentary and subsequently weakly regionally metamorphosed deposits. 2) The mineralogy of the Kailan and Ditur deposits speaks for metamorphism of the rocks in the epizone. 3) The Kimkan deposit is contact-metamorphosed. 4) The ore bodies most significant in size approach true beds in appearance. 5) The chlorite-dolomite-limestone breccias, in their structure, most closely resemble typical breccias and conglomerates of collapse or submergence.”

The opposite view on the genesis of the deposit is expressed by the geologist N. N. Pavlov³, who says that the ores of the Lesser Khingan, in relation to the enclosing rocks, are epigenetic (metathetic—A.) and originated through the decomposition of pyrite that impregnated the rock, which during weathering was transformed into a red iron hydroxide.

The microscopic study carried out on the ores and part of the rocks of the deposit shows that the conglomeratic rocks included among the wall rocks contain weakly rounded pebbles and, more rarely, fragments composed of calcareous rocks. The cement of these rocks includes limonite, chlorite, and hematite.

In the ores of these deposits, with a high hematite content (in transmitted light the thin section is opaque), in places it is possible to observe the structure of the enclosing rocks; moreover, the relics consist predominantly of quartz clogged with limonite. In places in the ores of the Kailan and Ditur deposits, flake-like inclusions of limonite are observed, which was deposited from colloidal solutions simultaneously with the rocks enclosing the ore.

The source of the iron was apparently the rocks of the continent in general and, in particular, the dolomites, in which the content of \(\mathrm{Fe_2O_3}\), according to analyses cited in the report of S. M. Tkalich, is equal to 0.10%, and the pyrrhotite schists, upon which Devonian deposits lie transgressively.

The ores of the deposits studied have a banded texture: ore layers alternate with non-ore layers, and macroscopically homogeneous layers, under the microscope, prove to consist of alter-

of still thinner interlayers, whose thickness ranges from 0.03 to 0.006 mm. The formation of microlamination may occur, as shown by the experiments of E. S. Moor and J. S. Maynard¹, as a result of the unequal rate of coagulation of hydrous ferric oxide and silica when they are precipitated from colloidal solutions. The formation of the deposit apparently took place in the coastal zone of a water basin, into which pebbles and fragments of carbonate rocks were brought; these promoted the coagulation of ferric hydroxide and silica from colloidal solutions. The clastic structures of the ores also indicate deposition in a coastal zone.

It is possible that certain parts of the water basin, in their conditions, approached a lagoon or an enclosed water basin.

After the formation of the entire ore-bearing series, the deposits underwent diagenesis and metamorphism (regional and contact), under the influence of which most of the ferric hydroxide was transformed into magnetite and hematite. The latter forms very fine (0.002–0.004 mm) grains, arranged predominantly parallel to the bedding of the rocks, and only in places have flake-like forms of accumulations of hydroxide been preserved. In addition, during these processes part of the hydroxide was grouped around pebbles and fragments of carbonate rocks, forming around them narrow rims of limonite and hematite.

The effect of metamorphism on the rocks and ores of the deposits studied was not the same. The Kailan deposit was metamorphosed most weakly, as a result of which more limonite of flake-like form was preserved in it and a quite insignificant amount of magnetite was formed. The Ditur deposit already underwent stronger metamorphism. In some specimens from this deposit the ore consists half of magnetite (in a sample from the VII reconnaissance line), which was strongly martitized by subsequent weathering processes. In these deposits the principal ore mineral is hematite.

The Kimkan deposit underwent the strongest contact metamorphism, caused by a hypabyssal intrusion of adamellite, under whose action the country rocks were transformed into hornfelses, and the ore minerals into magnetite; the content of hematite, also recrystallized, is insignificant. It is interesting to note that the ore has a granoblastic structure, and sometimes also a skeletal one. Incidentally, a microscopic study of agglomerates of iron ores from the Goroblagodatsk and Kerch deposits, carried out by A. I. Aleksandrov², showed that during recrystallization under appropriate conditions magnetite is obtained chiefly, regardless of the compounds (limonite, hematite, etc.) in which the iron was contained in the original material. For example, an agglomerate obtained from brown iron ores of the Kerch—

deposit, contains predominantly magnetite, with a skeletal structure developing in it. Thus, under natural conditions, with strong contact metamorphism, the formation of magnetite is possible through the recrystallization of limonite, hematite, and other ferruginous compounds.

According to the report of S. M. Tkalich (pp. 126–127), the Kailan deposit contains pyrite, which forms inclusions of individual grains in the rock or even constitutes veinlets cutting both the ore and the country rocks. In the study of samples of ores and rocks from the deposits described, pyrite was encountered in small amounts. On the other hand, veinlets of limonite occur in the rock and ore, apparently formed at the expense of pyrite. In addition, this limonite participates in the formation of rims around pebbles of conglomerates. The limonite veinlets encountered are secondary and formed after the originally deposited iron hydroxide had already been partly transformed into hematite. The development of this limonite should apparently be connected with present-day weathering processes.

7. Conclusion.

The present work sets forth the results of a microscopic study of iron ores and part of the country rocks of the Kailan, Ditur, and Kimkan deposits of the Far Eastern Territory. On the basis of the investigations, the following conclusions may be drawn.

1. The ores of the Kailan deposit are represented chiefly by hematite, and those of Kimkan by magnetite. The ores of the Ditur deposit occupy an intermediate position between them; the magnetite content in samples from this deposit ranges from an insignificant amount in some samples to 50% in others. Thus, in the deposits there is a clearly indicated change in the ores from hematitic to magnetitic.

2. The ores of the deposits studied have a banded texture; the width of individual layers ranges from 0.3 to 20 mm. The ore interbeds in the ores of the Kailan and Ditur deposits consist of hematite, forming very fine (0.002–0.004 mm) needles arranged, in general, parallel to the bedding of the rocks. Magnetite in the Ditur deposit is arranged as individual grains concentrated in separate places—“nodes.”

In the ores of the Kailan and Ditur deposits, as mentioned above, relict clastic structures are often encountered, while the Kimkan ores form granoblastic structures.

1. In the ores of the Kailan and Ditur deposits there are two generations of limonite: (a) one deposited from colloidal solutions and having a flake-like form, and (b) one formed as a result of the decomposition of pyrite and part of the carbonate rocks, occurring chiefly in the form of veinlets that cut the ore and the country rocks.

2. Contact metamorphism of the ores and rocks in the deposits

expressed unevenly—more weakly in the Kailan deposit and most strongly in the Kimkan deposit.

1. The process of ore formation may be represented by the following scheme.

Scheme of formation of the iron ores of the Khingan deposits.

I. Epoch. Pebble rock composed of fragments of carbonate rocks and quartz with ferruginous and possibly chloritic cement. In this same epoch there occurs the introduction and precipitation of iron hydroxides and silica. The possibility is not excluded that a certain amount of sulfides developed during this epoch.

II. Diagenesis (?). In this epoch the formation of hematite at the expense of limonite and the development of chlorite (?) are possible.

III. Metamorphism brought about the formation of magnetite and hematite at the expense of limonite. In this same epoch additional silica was introduced (quartz veins cutting the rocks and the ore), sulfides were introduced, a small amount of gold was introduced, and chlorite was formed.

IV. Exogenous processes are characterized by the development of limonite at the expense of sulfides and possibly partly of carbonates, by the formation of martite, and by the migration of gold.

Microphotographs of ores and rocks from the deposits of the Lesser Khingan, Far Eastern Territory.

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Explanation of the Photographs

Photo 1. Structure of the Kailan ores. Fine-grained hematite (white) with non-ore areas having a lenticular form (black). Among the non-ore minerals very fine needles of hematite are visible.

Kailan deposit, magn. 300 (reflected light).

Photo 2. Conglomeratic rock. Pebbles consisting of carbonates (white and light gray), chlorite-ferruginous cement (gray).

Kailan deposit, magn. 12 (transmitted light).

Photo 3. The same. Around a pebble consisting of carbonates (white), a narrow band of limonite and hematite (black) is visible. In individual parts of the section holes are visible.

Kailan deposit, magn. 32 (transmitted light).

Photo 4. Structure of silicilite. Quartz (white), ore minerals—hematite and limonite (black).

Kailan deposit, magn. 34 (transmitted light).

Photo 5. Among fine-grained hematite (white) there occur martitized grains of magnetite (M), around which rims of hematite are visible; non-ore minerals (black).

Ditur deposit, magn. 350 (reflected light).

Photo 6. Crushed hematite ore. Cracks are filled with quartz (white); non-ore areas are heavily stained with limonite (gray); ore passes through in separate bands (black).

Ditur deposit, magn. 12 (transmitted light).

Photo 7. Granoblastic structure of ore consisting of magnetite (white) and quartz (dark gray).

Kimkan deposit, magn. 350 (reflected light).

Photo 8. Banded texture of ore. Around magnetite (black) actinolite is formed (elongated gray crystals); behind the actinolite quartz of granoblastic structure (Qu) is visible.

Kimkan deposit, magn. 40 (transmitted light).

Photo 9. Colloform structure of “melnikovite”—marcasite (white, gray in the centers of grains) in biotite-actinolite hornfels.

Kimkan deposit, magn. 200 (reflected light).

SUMMARY.

The results of a microscopic study of iron ores and partly of rocks from the Kaylansky, Ditursky and Kimkansky deposits of the Far-Eastern Region are given in this paper. The study allows to draw the following conclusions.

1. The ores of the Kaylansky deposit are represented by hematite, those of the Kimkansky deposit—by magnetite. The ores of the Ditursky deposit occupy a medial position between the first two: the tenor of magnetite in the samples of this deposit ranges from an inconsiderable amount to 50 p. c. The alteration of ores from hematite to magnetite is thus obviously marked.

2. The ores of the investigated deposits have a banded texture, the width of separate layers varies from 0.3 to 20 mm. Ore streaks in ores of the Kaylansky and Ditursky deposits consist of hematite, forming very fine needles (0,002—0,004 mm), disposed generally upon the stratification of rocks. Magnetite in the Ditursky deposit is disposed as separate grains concentrated in certain places—„knots“. In ores of the Kaylansky and Ditursky residual clastic structures are often met with and the Kimkansky ores form granoblastic structures.

3. Two generations of limonite are presented in ores of Kaylansky and Ditursky deposits: a) limonite deposited from colloidal solutions and possessing the form of flakes and b) limonite resulting from decomposition of pyrite and of a part of carbonate rocks, occurring mainly as small veins which intersect the ores and the wallrocks.

4. The contact-metamorphism of ores and rocks in these deposits is expressed differently, being the strongest in the Kimkansky deposit.

5. The process of formation of ores may be represented by the scheme of the table in page 18.

I epoch. Pebble from carbonate rocks and quartz with a ferruginous and possibly chloritic cement. The introduction and the precipitation of hydroxide of iron and of silica occurred at the same epoch. The possibility of development of a certain amount of sulphides at that epoch is not excluded.

II. Diagenesis (?). The formation of hematite from the limonite and the development of chlorite (?) is probable at this epoch.

III. The metamorphism caused the formation of magnetite and hematite at the expence of limonite. At the same epoch silica (veins of quartz intersecting the rocks and the ores) as well as sulphides, a small amount of gold are introduced and chlorite is formed.

IV. The exogenic processes are characterized by the development of limonite at the expence of sulphides and possibly of carbonates too, by the formation of martite with a migration of gold.

Translated by B. Gootovskaya.

1. E. S. Moor and J. S. Maynard. Solution, Transportation and precipitation of iron and silica.—Econ. Geol., Vol. XXIV, 1929, Nos. 3–5. ↩↩↩↩↩

2. Aleksandrov, A. Composition of some agglomerates of iron ores. Tomsk, 1933, manuscript. ↩↩

3. Ibid., p. 122. ↩