Some Features of the Structure and Composition of the Bakchar Ore-Bearing Strata

E. A. Babina, A. A. Babysh

(Presented by the scientific seminar of the Department of Petrography)

In regional terms, the Bakchar deposit of oolitic iron ores is located in the zone of junction between the rigid Kolyvan–Tomsk fold arc and the northwestern subsidence of the West Siberian Lowland. The folded basement, occurring at a depth of 2100–2600 m, is composed of Upper Paleozoic rocks. According to N. N. Rostovtsev, the platform cover represents a junction of two regional first-order structures: the Barabinsk–Pikhtovskaya monocline and the Middle Parabel mega-downwarp. The latter are complicated by second- and third-order structures, among which the Parbig swell, the Upper Bakchar and Maksimovka uplift areas, the Semenovka depression, and the Galkino, Krasno-Bakchar, and Gavrilovka local uplifts are clearly expressed.

In the history of geological development of the platform cover, three stages of sediment accumulation are distinguished: 1—the Lower Mesozoic, preceding ore accumulation; 2—the Upper Mesozoic, accompanied by ore deposition; 3—the Early Cenozoic, or post-ore. Below, only the second and, in part, the third stages of sediment accumulation, with which ore formation is associated, are considered.

The ore-bearing strata of the Bakchar deposit differ sharply from the ore-hosting formations of other areas of the West Siberian iron-ore basin. This difference consists in the erosion of the carbonate clays of the Gankino Formation and the absence of deposits of the Slavgorod and Ipatovo Formations. The thickness of the ore-bearing strata of the deposit is 2–3 times less than in the areas of the Middle Ob region. It is characteristic that, owing to the disappearance from the section of the Gankino, Slavgorod, and Ipatovo formations, all the ore horizons of the deposit—and there are four of them—are brought closer together. Thus, between the upper (Chigirin) and the underlying (Bakchar) ore horizons, inter-ore rocks are absent altogether, while between the Bakchar and the underlying Kolpashevo horizons their thickness on the eastern margin of the deposit does not exceed 2–6 m. Moreover, the inter-ore rocks of the eastern margin of the deposit contain iron in amounts of 18–27%. The foregoing clearly distinguishes the Bakchar ore field from other ore fields of the basin.

The circumstances listed made it possible to distinguish a new stratigraphic unit in the legend of the Tobolsk series of geological maps, which we have called the Bakchar ore-bearing strata. In terms of the time of its formation, it occupies the interval from the Turonian through the Eocene inclusive and lies unconformably on the continental sandy-clayey deposits of the Upper Simonovo subformation.

In the Bakchar ore-bearing strata, as was mentioned above, four iron-ore horizons are distinguished. The three lower ones—the Narym, Kolpashevo, and Bakchar horizons—occur in Upper Cretaceous (Senonian) deposits; the fourth (Chigoryn) constitutes the very lowest part of the Lyulinvor Formation of the Eocene. A very important feature is found in the structure of the Bakchar ore-bearing strata, namely its rhythmicity. In general terms, the rhythmic structure is expressed in the regular multiple repetition throughout its section of one and the same three-member complex of deposits, represented by gravelites, sandstones, and oolitic ores. In this, thin beds of gravelites compose the lower part of the rhythm, sandstones the middle part, and oolitic ores the upper part. In the section of the Bakchar ore-bearing strata, four such rhythms are observed. The first corresponds to the Narym horizon; the second, third, and fourth respectively to the Kolpashevo, Bakchar, and Chigoryn ore horizons. The boundaries of the rhythms are sharp and are established by gravelites and gravel pebbles, which in the first and fourth rhythms play the role of basal horizons. The boundaries within individual rhythms, especially between sandstones and ores, are characterized by gradual transitions.

The structure of the rhythms is asymmetrical, with variable thickness. The maximum thickness is characteristic of the Kolpashevo and Bakchar horizons, and the minimum of the Narym and Chigoryn horizons. In all rhythms without exception, the thickness of the lower member is less than the thickness of the upper members. The lower members of the first three rhythms are characterized by the development of phosphorite concretions, and in the fourth by lenses and interbeds of siderite. In general, each rhythm has individual features that are not repeated in other rhythms. Thus, the lower member of the first rhythm is characterized by the development of siderite lenses, absent in the second and third rhythms. In the lower member of the first rhythm, sandstones of predominantly quartz composition are developed; in the second, the sandstones are enriched in glauconite; and in the third, ferruginous chlorites have undergone considerable development. Finally, the upper members of the first three rhythms are characterized by the development of oolitic ores, while the fourth is characterized by non-oolitic glauconite-siderite ores and opoka-like clays. As for the ores, individual features are manifested here as well: in the first rhythm, leptochlorite ores are developed predominantly; in the second, leptochlorite-hydrogoethite ores; and in the third the dominant role belongs to hydrogoethite ores.

In sections of the Bakchar ore-bearing strata, a complete rhythm of iron accumulation is observed, beginning with a weak concentration in the lower horizons, reaching a maximum in the middle and upper horizons, and gradually dying out in the very uppermost part of the section. The distribution of iron and associated elements (vanadium, phosphorus, etc.) in plan also reflects a certain regularity: the content of iron and of the listed elements gradually increases with distance from the shoreline, reaches a maximum in the central part, and then, in the deep-sea zone, their amount gradually decreases. From this it may be concluded that the complex of alternating rocks and ores that determined the rhythmic character of the structure of the Bakchar ore-bearing strata is the result of repeated transgressions and

regressions of the boreal sea, associated with oscillatory movements of the Earth’s crust.

B. I. Marchenko (1965), for rocks poor in \(C_{\text{org}}\) and sulfides, proposed determining the oxidation coefficient by the ratio of ferric iron to ferrous iron. Using this ratio, we calculated the value of the oxidation coefficient and distinguished six geochemical zones. In the transition from shallow-water facies to deep-water facies, the value of the oxidation coefficient decreases. Moreover, if it exceeds unity, the environment has an oxidizing setting; if it is less than unity, reducing conditions predominate (Table 1).

From the table and the data presented above it may be concluded that the formation of the ores and rocks of the Bakchar deposit took place under conditions of a platform marine basin in its shallow-water zone, 20–25 km from the shoreline, and in three principal geochemical zones: oxidizing, oxidizing–reducing, and reducing. The probable form of migration of iron and other elements was colloidal solutions stabilized by humic and peaty substances, as well as highly complex organic iron compounds.

In the rocks and ores of the ore-bearing stratum, more than 40 rock-forming minerals have been identified, including authigenic, detrital, and clay minerals. The authigenic minerals are of greatest interest, most of which compose the ores of the Bakchar deposit. The authigenic minerals—goethite, hydrogoethite, leptochlorites, glauconite, siderite, kerchenite, vivianite, and others—occur in aggregate form as oolites and ooids, as well as fragments and concretions. In both dense and loose varieties of ore, fragments of plant-

... tissues, pieces of wood, and remains of poorly preserved fossil microfauna.

Goethite occurs in subordinate amounts and in close association with hydrogoethite. It forms oolites, ooids, makes up the nuclei and concentric zones of oolites, and also concretions. In some cases the concentric zones consist of hydrogoethite, in others of goethite, but in most cases hydrogoethite zones predominate. Goethite is most widespread in the ores of the Kolpashevo and Bakchar horizons.

In reflected light goethite is grayish-white in color. However, reflectivity and anisotropy are masked by the presence of a large number of light-yellow internal reflections. The microhardness, measured with the PMT-3 instrument and calculated using the tables of M. M. Khrushchev and E. S. Berkovich, is 568 kg/mm²; the specific gravity is 4.2.

Hydrogoethite is the principal ore mineral, occurring in several morphological varieties. It forms pisolites and oolites. Their color is dark brown; the surface is polished, more rarely matte. The forms of the pisolites are flattened, ellipsoidal, and rounded; their dimensions range from 0.1 to 1.2 mm.

Oolites are of subordinate importance. Their form is analogous to that of the pisolites, but their structure is concentrically shell-like. They consist of 3–4 microconcentric layers up to 0.03 mm thick, alternating in hydrogoethitic and leptochloritic composition. The centers of the oolites have different compositions.

In reflected light hydrogoethite is gray, light gray, with yellowish-brown internal reflections. The microhardness is 463 kg/mm²; the specific gravity is from 3.8 to 4.2. Variations in the specific gravity of hydrogoethite are probably caused by its water content.

On the heating curve of hydrogoethite-goethite, three endothermic effects are distinctly expressed, of which one, at 355–360°, corresponds to goethite, while the other two indicate the presence of hydromica impurities.

Leptochlorites occur as oolites and in the cement, also being a constituent part of the ores. Their color is yellow, yellowish-brown, and greenish-black. The form of the oolites is discoidal and spherical. Glauconite and quartz are often found in the nuclei of leptochlorite oolites.

In the leptochlorites of the Narym horizon, heterogeneity of composition has been noted, which is confirmed by optical, chemical, and X-ray structural data. In thin section they have a green coloration with pleochroism from green to dark-green shades, locally to colorless. The refractive indices vary within the following limits: maximum — 1.670 ± 0.002, minimum — 1.662 ± 0.002, with a low birefringence. The increase in the refractive index may be explained by an elevated iron content in the mineral.

According to D. Foster’s classification, chlorite belongs to the chamosite group, while according to D. P. Serdyuchenko it is close to the strigovite series.

The second variety of chlorite differs from the first by somewhat lower refractive indices: maximum — 1.598 ± 0.003, minimum — 1.592 ± 0.003, with pleochroism from yellowish-green to green; the angle of the optical axes is very small, down to (−)2°. The specific gravity is 2.78.

According to D. P. Serdyuchenko’s classification, the chlorite is close to the delessite series. The trivalent iron in the chlorite is explained by subsequent oxidation.

Glauconite is widely developed both in granular glauconite-siderite ores and in oolitic leptochlorite-hydrogoethite ores. Two varieties of glauconite are distinguished in the composition of the ores: autoch-

and redeposited. The former is green in color, with platy isometric forms and a microaggregative structure.

Redeposited glauconite forms the nuclei in oolites.

Glauconite has a green, greenish-yellow coloration with pleochroism from yellowish-green to green. During weathering and oxidation its color becomes yellowish-brownish.

The refractive indices of glauconite vary depending on the iron content: the more iron, the higher the refractive index. The highest refractive index is 1.612–1.638 ± 0.002; the lowest is 1.584–1.610 ± 0.002. Specific gravity = 2.53–2.70.

Gysingerite is a rare mineral, found in the pores of the cement of goethite–hydrogoethite ores of the Bakchar horizon, and it also composes peripheral rims in ooliths. Under the microscope it is orange-brown in color, finely fibrous in structure, isotropic, and of low hardness. Its refractive indices are: highest—1.502 ± 0.001; lowest—1.494 ± 0.001.

X-ray structural analysis confirmed that the mineral belongs to gysingerite.

Ferruginous phosphates are represented by vivianite and kerchenite. These minerals are rare. Fresh vivianite is colorless, and upon oxidation it acquires a bright blue and dark-blue coloration. Kerchenite, for its part, has a blue-green and blue coloration.

The optical properties of these minerals coincide with the data in the literature.

Siderite is the principal mineral in siderite ores, where it forms fine-grained aggregates; in other varieties it cements goethite–hydrogoethite oolites and pisolites, or forms concretions and nodules up to 25 mm in size. X-ray structural and thermal analyses, as well as the optical properties of all varieties of siderite, have shown their identity.

Conclusions

1. The Bakchar ore-bearing strata differ sharply from the ore-bearing deposits of other areas of the basin; they have, as it were, a compressed thickness and a rhythmic structure, with each of the four rhythms having a three-membered structure and its own individual features.

2. Six geochemical zones are recorded in the deposit, each of which is characterized by a distinctive paragenesis of rocks, ores, and minerals peculiar only to it.

3. The ore-forming part of the strata is represented by authigenic minerals, the principal ones being goethite, hydrogoethite, leptochlorites, and siderite, accompanied by glauconite, gysingerite, kerchenite, vivianite, and others.

References

V. I. Marushko. The significance of the oxidation coefficient for facies analysis of marine deposits (with reference to the Neocomian deposits of Kopet-Dag). Lithology and Mineral Resources. 1965, No. 4.

N. N. Rostovtsev. Tectonic scheme of the West Siberian Lowland along the base of the platform Mesozoic–Cenozoic deposits, with elements of paleotectonics. Proceedings of SNIIGGiMS, issue 7, 1961.