A Few Remarks on Iron Ores (Siderites) among the Jurassic Coal-Bearing Deposits of the Southeastern Margin of the Chulym-Yenisei Coal Basin

M. P. Nagorsky

Among the Jurassic coal-bearing deposits of the Chulym-Yenisei basin, siderites are widespread, often closely associated with coal-bearing horizons. The first to raise the question of the significance of these siderites were A. R. Anan’ev, D. A. Vasil’ev, and I. V. Lebedev¹. They pointed out several deposits of this high-quality iron ore, gave a detailed description of its external and microscopic features, etc.

Usually the siderites from the Jurassic deposits are dense, almost non-granular formations with a high specific gravity. They are greenish-gray, gray, sometimes very light, and not infrequently densely colored. They are colored, especially in beds, uniformly. They are very dense and of homogeneous composition. Only under high magnification under the microscope is it visible that the rocks are composed of submicroscopic grains of siderite with a slight content of clayey material and extremely rare, somewhat larger grains of quartz. Their hardness is very characteristic. Some varieties, most often the light-colored ones (grayish- and greenish-white), have a hardness between 5 and 6 on the Mohs scale. Such siderites do not effervesce with acid even in powder. The more densely colored varieties effervesce very weakly with acid.

Within the area of the southeastern margin of the Chulym-Yenisei basin there are a number of deposits or, more precisely, occurrences of this ore. The most important of them are as follows.

The Orlovka siderite deposit is located on the right bank of the Bobrovka River, 3 km southeast of the village of Orlovka, B. Murtinsky District. Here, in the lower part of the slope, from beneath a thickness of micaceous argillites softened by weathering, there emerges a bed of greenish-brown and dark-brown siderite with coaly spots and carbonized plant remains. The thickness of the bed is from 0.35 to 1 m. The siderite is very hard. Its blocks obstruct the river channel and can be traced for more than 1 km downstream. Chemical analysis of this siderite shows its high quality. It contains $\mathrm{Fe_2O_3 + FeO}$—59.7% (Table 1).

The Rozhdestvenskoe deposit is located 5.5 km north of the village of Rozhdestvenskoe, Kazachinsky District, on the left slope of the Temnaya River. Here,

among white and dirty-white sandy clays, there lies a bed of disseminated ore. The thickness of the bed varies greatly (from 0.2 to 1.5 m). Often the bed pinches out along strike and then reappears. The bed has been traced along the slope for 200 m. The bed consists of separate blocks of siderite in yellowish enclosing clays. Along strike it is replaced by bean ore, which represents the result of oxidation of siderite. The bean ore consists of very small and regular, densely packed pellets of brown and gray color, cemented by a white clayey-calcareous material with a large amount of white mica (kaolinite?). The siderites contain 45.38% ferric oxide, while the bean ore contains 43.61% ferric hydroxides; for it a high iron content is characteristic (Table 1).

The Novo-Nikolaevka deposit is located near the village of Novo-Nikolaevka in Sukhobuzimsky District. In the area of the mentioned village, situated on the Tel’ River (a right tributary of the Yenisei River), a sandy-clayey sequence with small lenses of brown coal is developed. Quartz-feldspar, usually light-colored sands predominate; they sometimes contain fine pebble and quartz gravel. At 350 m to the NW of the village of Novo-Nikolaevka, at the foot of a slope, there is an abandoned prospecting pit which, as was learned from questioning local residents, was sunk in 1900. The pit is caved in and heavily overgrown with woody vegetation. In the dumps of the pit a large amount of iron ore is observed, ranging from brown to reddish-brown in color. Pieces of strongly oxidized siderite occur. At 400 m to the NW, in the lower part of a gentle slope facing the Tel’ River, two old workings were encountered: a pit and a trench, as well as traces of other collapsed pits and small excavations. The ore has a nest-like and bedded mode of occurrence among Jurassic sands with interbeds of greenish clays (argillites) and thin beds of sooty brown coal. The ore lies at a depth of 10–15 m from the ground surface. Part of the siderites is oxidized. Chemical analyses (Table 1) show the very high quality of the ore: $\mathrm{Fe_2O_3} + \mathrm{FeO}$ up to 77.93%. Some pieces of ore contain imprints of flora, such as, for example, Desmiophyllum sibirica Chachl, which determines their Jurassic age. The ore is distributed over a considerable area.

The Barabanovka deposit of siderites is located immediately above the village of Barabanova, Sukhobuzimsky District, on the left bank of the Yenisei River. The left bank is a broad 45-meter terrace, the base of which rises 8–10 m above water level and is composed of Jurassic argillites softened by weathering. Above lies a 35-meter sequence of terrace gravels, sands, and loams. The gravels, lying on the eroded surface of the argillites, are highly water-bearing. Associated with the argillites are rather large (up to 1 m across), but comparatively rare, loaf-shaped concretions of siderites. The deposit is not entirely convenient for exploratory work, owing to the abundance of groundwater and the rare content of siderite concretions in the argillites.

The Kubekovo siderites are confined to the contact between the lower coal-bearing and the middle non-coal-bearing horizon. They are noted on the left bank of the Yenisei River. They have the form of concretions of various sizes, comparatively small. However, individual horizons contain rare concretions up to 0.5 m in diameter. Chemical analyses (Table 1) show a high content of ferric oxide in these siderites. The reserves of the Kubekovo siderites are probably not large, since the concretions are comparatively rare. However, the possibility is not excluded that in individual areas siderite concretions may be encountered in abundant quantity.

The Gadalovo siderite occurrence is located on the left bank of the Yenisei River, 8 km below the city of Krasnoyarsk. A siderite bed up to 0.8–0.9 m thick

Table 1.

Chemical analyses of siderites.

gently dips downstream to the NE. The roof of the bed consists of sands, and the floor of argillites. These rocks are part of the lower horizons of the Krasnoyarsk Jurassic sequence. The upper part of the bed is cut off by the pebbles of a 45-meter terrace; the bed goes below the water level of the Yenisei River. Along the dip it has been traced for 50–60 m. The siderites yield Fe₂O₃ + FeO up to 65%.

Among the Jurassic deposits on the left bank of the Yenisei River, below the city of Krasnoyarsk, siderite concretions are often encountered, usually associated with brown-coal seams. Their size ranges from 0.1 to 0.4 m in cross-section. During the driving of an adit along a coal seam above the village of Korkina, spherical siderite concretions were observed. Judging by the amount of rock and coal removed, they occur rarely. Similar concretions were found in Jurassic conglomerates that lie on the eroded surface of the variegated Devonian of the Kachinskaya suite.

The Bolshoe-Balchugskoe siderite deposit was described by A. R. Ananiev, D. A. Vasiliev, and I. V. Lebedeva. These authors characterize this deposit as follows. “On the B. Kemchug River, 0.5 km from B. Kemchug, a siderite bed 1.1 m thick crops out. Its roof and floor are diagonally bedded light-gray sands with small lenses of pebbles and with horizons of Jurassic flora, represented by several species of the genera Cladophlebis sp. and Phoenicopsis angustifolia Heer. The thickness of the overburden is 7 m. The bed is sharply separated from the roof and floor. The bed has been traced for 150 m downstream. It dips northward at an angle of 7–8° (p. 25).”

In the northern part of the basin, within the right bank of the Yenisei River in the Yeniseisk district, Jurassic coal-bearing deposits of the Zyryanka depression are developed. The Jurassic of the Zyryanka depression was described in detail by E. M. Velikovskaya, who carried out geological work here¹. She determined the structure and outlined the area of the Jurassic deposits. As it turned out, nests of siderites, in no way differing from those described above, occur quite often among the sediments of the Jurassic. E. M. Velikovskaya notes the following siderite deposits.

Siderites in the basin of the Chernaya River occur directly above basal conglomerates. They have the form of nests. Above, they are overlain by white and gray clays, greasy to the touch and slightly sandy, which contain beds and lenses of gray, heavy raspberry-red—owing to the onset of oxidation—siderite and sideritic sandstone with imprints of Jurassic flora.

Siderites in the basin of the Zyryanka River have the character of lenticular beds among Jurassic clays. A bed of siderite was discovered at the head of the right slope of the Cherny spring (a right tributary of the Zyryanka River), and was later exposed by a clearing on one of the tributaries of the Sinyaya River. E. M. Velikovskaya gives the following section here.

1. Bluish-gray, yellow, and other clays . . . . . . . . . . . . . . . . . . . . 0.9 m.
2. Siderite. Has the appearance of large blocks, the intervals between which are filled with gray and yellow clay. On the surface the blocks of siderite are somewhat oxidized and are often transformed into brown iron ore. In the lower part of the bed lies a tree trunk, transformed into iron-rich siderite . . . . . 0.7 m.
3. Dark-gray clay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.5 m.
4. Brown, strongly weathered coal . . . . . . . . . . . . . . . . . . . . . . 1.5 m.
5. Gray, brown, and other clays . . . . . . . . . . . . . . . . . . . . . . . 1.4 m.

Chemical analyses of the siderite (Table 1) indicate a high iron content.

On the right bank, at the head of the slope of the Cherny spring (2–3 km from its mouth at its confluence with the Zyryanka River), the entire slope is pitted with holes, now completely overgrown. About 100 years ago siderite was mined here by the local population, and iron was smelted from it. Similar traces of large workings have been found on the slopes of the Sinyaya River, the Dyadina River, and others. The ore apparently occurs in nests. According to information from old inhabitants, at the foot of the slopes of the Chernaya River “orts” were driven, from which large loaf-shaped ore concretions were extracted.

In the opinion of E. M. Velikovskaya, the siderites in the basin of the Zyryanka River are confined to the upper horizons of the Jurassic series and occur within a horizon that is very persistent along strike. The latter circumstance indicates a broad area of distribution of the Jurassic siderites. Additional investigations carried out by us make it possible to say that siderites are also encountered in the lower horizons of the Jurassic of the Zyryanka River basin.

In addition to the deposits indicated here, there is a whole series of numerous occurrences of siderites, for example along the Buzim River, the Mingul River, and so forth. These siderites have the form of concretions and do not form large accumulations. They are sporadically scattered among the rocks of the Jurassic series.

From the facts set out above it is evident that the siderite deposits known at present are substantially confined to the marginal parts of the enormous Chulym–Yenisei coal-bearing basin. A particularly abundant quantity of sideritic concretions, beds, and nests is observed in those bays which the Jurassic basin formed in the gneissic basement of the Yenisei Ridge.

Geological investigations of the southwestern margin of the Chulym–Yenisei basin make it possible to divide the Jurassic deposits into three series (from below): the lower, or Krasnoyarsk, the Kubekovo or Badalyk, and the Tolstomysskaya. Each series consists of several horizons. The lower series, according to Prof. M. K. Korovin¹, forms the Korkino trough, elongated in a NW direction². The series is composed of conglomerates, arkosic sands,

¹ Korovin, M. K., Prof.—The Chulym–Yenisei Coal-Bearing Basin—Mineral Resources of Krasnoyarsk Territory, Krasnoyarsk, 1928.

² Gagorsky, M. P.—Materials on the Geology of the Eastern Margin of the Chulym–Yenisei Brown-Coal Basin—Materials on the Geology of Krasnoyarsk Territory, issue 4, 1938.

often coarse-grained, with allochthonous wood, argillites with lenses not persistent along strike, most often of allochthonous brown coals. The entire sequence is characterized by a remarkable instability of facies, which fully corresponds to the conditions of its formation in the Krasnoyarsk region near the shoreline of an ancient lake. And only the upper coal-bearing (Korkino) horizon of this sequence is characterized by constancy of facies, calmer conditions of sediment accumulation, and thick autochthonous seams of brown coal. The Kubekovo sequence, especially its upper horizons, is characterized by a remarkable constancy of facies. It is composed of two horizons: the lower is represented by sands and calcareous sandstones, while the upper—coal-bearing—by argillites, sometimes sandy, containing interbeds and concretions of hard marl, carbonaceous argillites, seams of sapropelite and brown coal. The Kubekovo sequence rests unconformably on the lower, or Krasnoyarsk, one. This is clearly seen not only in fluctuations in the mineralogical composition of the heavy fractions (Table 2), but also in the dislocated nature of the deposits. Thus, for example, rocks of the lower sequence at the boundary with the Devonian have dips of the argillites of up to 20–25°. At the very same time, the argillites of the Badalyk horizon usually have a dip of 4–5°. The upper, or Tolstyi Mys, sequence of the Jurassic is composed of arkosic sands, argillites, coals, etc. It represents a strong contraction of the Jurassic basin.

The sequences characterized above are subdivided into a whole series of horizons. The observations of Prof. M. K. Korovin, the author, and other investigators make it possible to distinguish the following formations and, within them, horizons of the Jurassic sequence on the southeastern margin of the Chulym-Yenisei basin, from below:

With respect to coal-bearing capacity, the Korkino horizon, saturated with autochthonous seams of brown coal of excellent quality, deserves close attention. Other coal-bearing horizons also contain seams of good brown coal, sapropelites, etc.

Table 2.

Comparative mineralogical composition of the heavy fractions from various sequences of the Chulym-Yenisei basin.

For the lower, or Krasnoyarsk, sequence of the Jurassic, according to our observations, accumulations not only of siderite but also of pyrite and

marcasite, forming small concretions or sometimes literally filling carbonaceous argillites. In the latter case pyrite and marcasite have the appearance of microscopic spherical globules—an circumstance indicating that their formation was simultaneous with the accumulation of the sediment. Among these formations gypsum is very often encountered; it sometimes cuts, in the form of veinlets, some pyrite–marcasite concretions. The gypsum is of secondary origin. The amount of pyrite and marcasite taken together (including also their fine microscopic grains, often densely scattered in the sediments) is undoubtedly greater than the amount of siderite. This distinguishes (as does gypsum) the lower, or Krasnoyarsk, series.

The same numerous observations show that for the Kubekovo series, on the contrary, among the syngenetic minerals a predominance of siderite is characteristic, along with a low content of gypsum and a high content of carbonates, which displace gypsum (Table 3). Finally, the Tolstomyssovo series is characterized by a predominance of carbonates precipitated from solutions, limonite, etc.

Thus, the study of the group of syngenetic and secondary minerals makes it possible to distinguish the series and also, as our later observations show, to correlate them. The study of the group of syngenetic minerals quite evidently testifies to a shifting

Table 3

Ratio of syngenetic minerals in the Jurassic series

of the geochemical facies of the Jurassic deposits of the Chulym-Yenisei basin. This shifting took place in the direction (from below): marcasite + pyrite → siderite → carbonates. It represents a shifting of climatic zones; the enormous areas occupied by Jurassic deposits are subject to this general regularity. Thus the pyrite–marcasite, siderite, and carbonate Jurassic continental geochemical facies may be briefly characterized.

It is quite obvious that the above-noted shifting of facies is in reality more complex, if only because individual series, as was shown above, consist of separate horizons, between which, in turn, facies shifting takes place. However, in the general case, on the scale of individual Jurassic series, such a process of shifting remains characteristic. It is absolutely necessary to note that the pyrite–marcasite and siderite facies require more or less identical conditions for their formation. The difference will consist only in the fact that for the pyrite–marcasite or hydrogen-sulfide facies all processes will be emphasized especially sharply, i.e., here an especially abundant quantity of hydrogen sulfide is necessary, an almost complete absence of oxygen¹, etc., i.e., for the accumulation—

¹ Pustovalov L. V.—Geochemical facies and their significance in general and applied geology—Problems of Soviet Geology, No. 1, 1933.

for the formation of pyrite and marcasite, especially “sealed-off” conditions are needed for those processes that took place during the accumulation of sediments. In this case the boundary between the reducing and oxidizing environments passes high above the sediment.

All three facies (hydrogen-sulfide, siderite, and carbonate) require a warm climate for their formation. However, if humidification during the formation of the first two facies may be considerable and even excessive, then for the carbonate facies—the salt facies—this humidification must be extremely slight, inconstant, periodic. The carbonate facies is characterized by a hot and dry climate, humid in periods. In continental basins under the conditions of such a climate, considerable quantities of carbonates could accumulate, chiefly in the form of calcite. Under conditions of another climate (for example, with excessive humidification), carbonates and others would have been carried much farther owing to their easy solubility, mobility, etc. The hydrogen-sulfide facies is correlated with a humid and warm climate; slight cooling is characteristic of the Kubekovo series, among which there are sediments of the hydrogen-sulfide facies. Some other data also make it possible to judge a considerable cooling during the time of deposition of the upper coal-bearing horizon of this series, such as, for example, unquestionable indications of the seasons of the year (winter and summer), which may be observed in studying the bedding of the sediments in the upper part of the bluffs below the village of Kubekovo on the Yenisei River, and the study of bedding and, especially, inclusions near the village of Podsopochnaya in the Sukhobuzim district. In broad outline, the change of climate from the lower series to the upper proceeded in the following order: warm, strongly humid → temperate → warm, dry. However, if the corresponding relationship is traced by individual horizons, it will prove more complex.

It must be emphasized that practical possibilities also rest upon these conclusions: the solution, or rather the main direction, of certain practical problems. Indeed, as is evident from Table 3, the most considerable quantity of siderite is confined to the Kubekovo, and then to the lower or Krasnoyarsk series. The siderite facies, like the pyrite-marcasite facies, is essentially confined to the marginal parts of the basin, since it is precisely under such conditions that the setting for the accumulation of siderite will be most favorable (proximity of ground and surface waters bringing iron compounds, depth of the basin, organic matter, and numerous other conditions). This regularity is expressed with surprising clarity on the southeastern margin of the Chulym-Yenisei basin. The greatest number of siderite deposits is confined to the periphery of the Jurassic basin and especially to those bays that penetrated into the ancient gneiss basement of the Yenisei Ridge. With distance from the latter, with distance from the shoreline of the chain of Jurassic lakes, the number of siderite occurrences decreases on the one hand, while on the other—in the sediments ever smaller concretions begin to predominate. Thus, for example, in the western part of the Bolshaya Murta district, siderite forms very rare large concretions and more frequent small ones; beds are entirely absent. It is natural to expect that in this direction the siderite facies may be replaced by another facies. It is quite natural that searches for siderite deposits should be carried out primarily within the above-mentioned areas—within the Jurassic bays. The lower (Krasnoyarsk) series of the Jurassic is also undoubtedly promising in terms of possible discoveries of siderite accumulations.

In the opinion of A. R. Ananyev, D. A. Vasiliev, and I. V. Lebedev, during certain intervals of Jurassic time siderites formed especially in areas that, in Jurassic time, were located near sources supplying lake waters with iron. Evidently, in the authors’ opinion,

these will be areas situated near the Kuznetsk Alatau, the Arga Range, etc. There is undoubtedly a grain of truth in what has been said: the distribution of siderite deposits and the decrease in the saturation of Jurassic deposits of the Kubekovo sequence with them as one moves away from the ancient mountain structures speaks in favor of such a point of view.

It is quite obvious that, during certain epochs of Jurassic time, especially significant quantities of iron salts were brought into the lakes by ground and surface waters; these precipitated on the bottoms of the water bodies from colloidal solutions under the appropriate environmental conditions. For the Krasnoyarsk sequence, such intervals will be the time of formation of the lager horizon; for the Kubekovo sequence—the time of deposition of the lower horizons of the non-coal-bearing suite and the time of deposition of the lower horizons of the coal-bearing suite. Thus, among the Jurassic sequences one can distinguish no fewer than three siderite-bearing horizons. In very rare cases these horizons can be observed in a single section, because the ancient shoreline was not something entirely constant; it was subject to considerable fluctuations. The study and delineation, therefore, of the ancient shoreline of the Jurassic basin is a very important matter.

The necessary conditions of the water bodies in which siderite accumulated were: an abundance of organic matter, an excess of carbon dioxide, a sharp deficiency of oxygen, etc. Owing to the greater mobility of iron-oxide compounds, they precipitated considerably earlier than, for example, compounds of aluminum, silica, and others possessing greater mobility. The latter were carried farther away. As an example one may cite the occurrence of Jurassic sandstone with a silica cement in the western part of the Sukhobuzimo district. The processes of “petrification” of siderites into very dense and strong formations, as well as the processes of replacement of some calcareous sandstones and others, are connected, like the finest recrystallization, with subsequent time.

Thus we obtain quite distinct data on the areas of possible distribution of siderites. These data directly indicate that the most favorable areas for discovering siderite ores will be the regions that formed part of Jurassic bays. In this respect the Zyryanka, Novo-Nikolaevka, and Kantat depressions attract particular attention. The Zyryanka depression is characterized by interesting siderite deposits that were worked in the past. These deposits must be explored in order to delineate them and establish their industrial value. It is also necessary to carry out prospecting and exploration work in the Novo-Nikolaevka depression. These works must resolve the question of how widely siderites are developed in this depression, and also clarify the question of their industrial value. From an economic point of view, the Novo-Nikolaevka siderites are very convenient for exploitation, since they are located near the Yenisei River.

It is absolutely necessary to carry out prospecting work along the left bank of the Yenisei River below the city of Krasnoyarsk and, chiefly, to trace by exploration workings the siderites of the Godalovo deposit.

In addition to the deposits described here and their possible areas of distribution, siderites among the Jurassic coal-bearing deposits are also very widely distributed in other regions.

1. E. M. Velikovskaya—Jurassic coal-bearing deposits of the Yenisei Ridge—Transactions of the All-Union Scientific Research Institute of Mineral Raw Materials, issue III, vol. 1, bauxites. 1936. ↩↩