SOME FACIES AND GEOCHEMICAL FEATURES OF THE FORMATION OF THE IRON-ORE STRATA OF THE BAKCHAR DEPOSIT

A. N. KONDAKOV

(Presented by Prof. A. M. Kuzmin)

The Bakchar deposit of oolitic iron ores of Upper Cretaceous–Lower Paleogene age is located in the southeastern part of the West Siberian iron-ore basin. In the structure of the ore-bearing strata of the deposit, four rhythms of sediment accumulation have been distinguished. The upper iron-bearing parts of the rhythms are combined into iron-ore horizons (from bottom to top): the Narym, Kolpashevo, Chigara, and Tym–Bakchar horizons. This article considers the facies conditions of accumulation of the iron-ore horizons and analyzes the relationship of facies distribution to the relief of the sediment-accumulation floor and the activity of wave-surf processes.

1. Narym horizon. Within the deposits of this horizon, ores have a subordinate significance. The thickness of the horizon deposits varies from 4 to 10 m, while the thickness of ore bodies with an iron content of \(>25\%\) ranges from 1–3 to 5 m. In the deposits of the horizon the following are distinguished (Fig. 1): 1—oolitic coarse-grained brown ores and sandstones with basal chlorite, siderite, and chamosite cement, more rarely chloritolites; 2—oolitic fine-grained greenish sandstones with interbeds of ores; the cement is chloritic, clayey, sideritic; glauconite, vivianite, and very rarely pyrite have been established; 3—clayey-sandy chloritolites with pyrite and burrows of silt-eaters; the sandy ingredients are fine-grained and small-grained; 4—fine-grained quartz sandstones containing glauconite. In the southwestern and northeastern parts of the deposit the oolitic material is predominantly goethite–hydrogoethite; in the remaining part of the area it is hydrogoethite–leptochloritic. The structural surfaces of the floors of the horizons (Figs. 2, 4, 6, 8) have been constructed with the introduction of a correction to the modern hypsometric position of \(+0.08\) m per 1 km east of well 54, taking into account the post-ore inclination of the structural plan in an easterly direction. As the ancient level of the horizon, in calculating the correction, the basal surface of the Chegan horizontal-bedded clays was used. In the relief of the floor of accumulation of the Narym horizon, two depression structures are outlined, separated by a submeridional swell-like uplift. The indicated structures are consedimentation structures, since maximum sediment thicknesses have been established in the axial parts of the depressions and minimum ones on the elevated areas. Relief slopes amount to 2–5 m per 1 km. According to the studies of V. P. Zenkovich, O. K. Leontiev [3], I. A. Shamray [5], A. A. Aksenov et al. [1],

Fig. 1. Lithologic-facies scheme of the deposits of the Narym horizon. Explanations in the text. 19—boundaries of lithologic-facies zones, 20—present-day boundary of the distribution of deposits of the horizon, 21—core-drilling boreholes and their numbers.

Fig. 2. Structural map on the base of the deposits of the Narym horizon.

Under shallow-water conditions, sediments heterogeneous in grain size are sorted by wave activity, with coarse or heavy particles concentrating on the steep parts of the equilibrium profile and fine particles on the gentle ones. In the zone of abrupt wave breaking and in the beach zone, on the slopes of underwater bars and on their crests, the deposits are washed and sharply enriched with heavy minerals or with firm and heavy oolitic bodies [1]. P. A. Volkov has experimentally demonstrated the wave transport of heavy minerals upslope, up to \(2/3\) of its face [2]. The coarsest-grained oolite-enriched sediments (Figs. 1, 2) are found in the axial parts of elevated areas, which, in our view, is explained by the concentration of oolitic material by wave activity during stages of shallowing of the basin and the removal of sediments into the zone of agitation. The metasomatic character of the iron hydroxides in the oolites confirms this view. The groups of deposits distinguished above correspond to the following facies zones (Fig. 1): 1—deposits of underwater bars and coastal shallow water; 2—shallow-water deposits in the zone of weak and periodic wave influence; 3—deposits of a bay isolated from active wave action; 4—shallow-water deposits influenced by a coastal current bending around the territory of the deposit.

II. Kolpashevsky horizon. The thickness of the horizon’s deposits varies from 2 to 12 m; in the eastern part of the deposit the ores constitute 90–100% of the complete section of the horizon. In terms of petrographic composition, the following are distinguished (Fig. 3): 5—coarse-grained oolitic brown ores with thin interbeds of sandstones with chlorite, siderite, and chamosite cementation; 6—oolitic greenish sandstones with interbeds of brown ores, the cement being chlorite, siderite, and clayey; pyrite and vivianite are noted in the cement; 7—varigrained clayey sandstones and sandy clays with burrows of mud-eaters, vivianite, and phosphate concretions. In the central and eastern parts of the area, hydrogoethite oolites predominate in the deposits; in the northwestern and western directions, ferruginous chlorites assume the leading role in the composition of the oolitic material, while on the northern and western flank the oolites are replaced by glauconite. The structural plan inherits the main details of the relief of the depositional floor of the Narym horizon (Figs. 4, 2). The shallowest-water sediments, enriched in oolites, accumulated on elevated parts of the relief in the zone influenced by active wave action (Fig. 4б). The northern part and the extreme western flanks are composed of sediments of an open basin. In the southwestern part, the deposits formed under conditions of a quiet bay, protected from waves, with a predominantly clayey composition of sediments.

III. Chigarinsky horizon. The ores of the horizon are of secondary importance because of the insignificant thickness of the ore beds and the limited area of their distribution. The following are distinguished in the deposits: 8—oolitic clayey-chloritic sandstones with ore interbeds; 9—chloritolites and ores with an increasing amount of oolitic material in the rock composition eastward, containing siderite and phosphorite concretions; 10—sandy clays with glauconite and an insignificant admixture of oolites; 11—clayey siltstones and fine-grained sandstones; 12—quartz-glauconite varigrained sandstones. The ovoid-oolitic material in the central part of the area and on the eastern flank consists mainly of hydrogoethite; in the southeast—of chlorites and hydrogoethite; westward the oolites are replaced by glauconite ovoids. In the structural pattern, a series of ridge-like elevations and depressions is discernible, the position of which coincides with the arrangement of the corresponding structures in the underlying hori-

zones (Figs. 6, 4, 2). The shallowest-water and most reworked deposits accumulated in the axial part and on the flanks of positive structures, in the zone affected by wave breaking (Figs. 5, 8, 9), and in the territory of the southwestern depression (Figs. 6, 5)—under shallow-water conditions isolated from the influence of

Fig. 3. Lithologic-facies scheme of the deposits of the Kolpashevo horizon. Explanations in the text.

Fig. 4. Structural map along the base of the deposits of the Kolpashevo horizon.

wave activity in the bay. In the western part of the deposit, a zone of intensive alongshore current is outlined (Fig. 5₁₂).

IV. Tymsko-Bakchar Horizon. The deposits of the horizon, together with the Kolpashevo horizon, include the main industrial

iron-ore beds up to 15 m thick. Within the deposits of the horizon the following rock types have been distinguished (Fig. 7): 13—loose oolitic limonite ores, in the upper half of the section cemented with siderite cement;

Fig. 5. Lithologic-facies scheme of the deposits of the Chigarin horizon. Explanations in the text.

Fig. 6. Structural map of the base of the deposits of the Chigarin horizon

siderite cement; 14—loose oolitic ores and ores with siderite-chlorite and chlorite cement; 15—oolitic ores with chlorite and siderite cement; 16—oolitic ores, passing in the upper part of the section into glauconite-quartz sandstones; 17—glau-

conite–quartz sandstones with oolitic ores at the base of the section;
18 — glauconite–quartz sandstones. In the northeastern and central parts of the area, limonitic oolites predominate in the deposits; southward

Fig. 7. Lithologic-facies scheme of the deposits of the Tym–Bakchar horizon. Explanations in the text

Fig. 8. Structural map for the base of the deposits of the Tym–Bakchar horizon

and westward they contain oolites of hydrogoethite–chlorite composition. In the deposits of the extreme western flank there are ovoids of glauconite, predominantly unoxidized. The structural plan reflects the structural architecture of the Chigara horizon (Fig. 8, b). A distinctive feature of the dis-

A distinguishing feature of the deposits in the horizon, in contrast to the underlying deposits, is the absence of ores in the area of the axial part of the Bakchar swell and the sharp reduction in the thickness of the lower part of the section. Wells 30 and 31, in the basal part of the section, penetrated an interbed enriched in quartz material. On this basis we have concluded that an island uplift temporarily existed in the area of the Bakchar swell. Loose ores on the western and eastern downwarps of the swell formed under beach conditions. On the submerged slopes of positive relief forms, shallow-water deposits accumulated, bearing the imprint, to one degree or another, of wave activity (Fig. 7_{14, 15}). On the western flank, deposits of the alongshore-current zone were formed (Fig. 7_{16}). Toward the end of accumulation of the horizon’s deposits, the zone influenced by currents expanded (Fig. 7_{16, 17}), and individual branches of the currents broke through into the central part of the area, causing the sandy–glauconitic composition of the deposits in the upper part of the horizon.

Conclusions

1. The ore deposits of the horizons accumulated in a shallow-water basin isolated from the influx of terrigenous material transported by alongshore currents and by a system of submarine swells.

2. The deepest-water glauconite-bearing sediments formed at depths of \(200 \pm 100\) m [4]. The increase in depth took place in a northwestern direction.

3. Sediments enriched in limonitic ooid–oolitic material show a close connection with positive structural elements of a consedimentation nature. This connection was caused by the intensification of wave activity during periods of shoaling, when sorting, washing, and excavation of ooid–oolitic material upslope took place.

LITERATURE

1. A. A. Aksenov, E. N. Neveskii, Yu. A. Pavlidis, F. A. Shcherbakov. Problems of the formation of nearshore-marine placers. In: Transport of Sediments and the Genesis of Placers of Heavy Minerals in the Coastal Zone of the Sea. Transactions of the Institute of Oceanology, vol. 76, 1965.

2. P. A. Volkov. Experimental study of the mechanical sorting of heavy minerals. In: Transport of Sediments and the Genesis of Placers of Heavy Minerals in the Coastal Zone of the Sea. Transactions of the Institute of Oceanology, vol. 76, 1965.

3. O. K. Leont’ev. Geomorphology of Sea Coasts and the Sea Floor. Moscow University Press, 1955.

4. N. M. Strakhov. Details of reconstruction of the physico-geographical environment in ancient marine basins. In: Methods for Studying Sedimentary Rocks, vol. 2. Gosgeoltekhizdat, 1957.

5. I. A. Shamrai. The mechanical sorting activity of the sea as a factor of marine ore accumulation. In: Problems of the Mineralogy of Sedimentary Formations, books 3–4. Lviv University Press, 1956.