HYDROGEOCHEMICAL CONDITIONS OF THE ZONE OF GENTLE BRACHYSTRUCTURES OF KUZBASS

THE CASE OF THE ERUNAKOVSKY COAL-BEARING REGION

D. S. Pokrovsky, G. A. Plevako

The Erunakovsky region is located in the central part of the Kuznetsk Basin, at a considerable distance from the surrounding mountain structures, at the junction of the forest-steppe and taiga zones, which are sufficiently humid.

Fig. 1. Schematic hydrogeochemical map of the Erunakovsky region of Kuzbass: 1 — sandy-clayey deposits of the conglomerate series of the Jurassic; 2 — sandy-clayey deposits of the Maltsev series of the Triassic; 3 — coal-bearing sandy-clayey deposits of the Kolchugino series of Permian age; 4 — landscape zones: I — forest-steppe, II — taiga; 5 — areas of development of ultrafresh waters of the hydrocarbonate-chloride and chloride sodium types; 6 — areas with anomalous contents of microcomponents in groundwater. In the circle—the element index and the contrast of its content relative to the background; 7 — individual sampling points with anomalous contents of microcomponents in groundwater. In the circle—the element and the contrast of its content; 8 — stratigraphic boundaries; 9 — boundary of landscape zones; 10 — zones of tectonic disturbances; 11 — outcrops of coal seams beneath loose Quaternary deposits.

The geological structure of the region is characterized by the wide distribution of continental formations of the Kolchugino Series of Paleozoic age. In the northern part of the region, within the Kyrgai-Georgievka syncline, Paleozoic rocks are overlain by Triassic deposits, on which deposits of the conglomerate suite of Jurassic age rest with angular and stratigraphic unconformity. Both the Paleozoic and Mesozoic formations are overlain by an almost continuous cover of Quaternary deposits.

The Kolchugino Series is represented by rhythmically interbedded sandstones, siltstones, and argillites with coal seams. An upward increase in the scale of sedimentation rhythms and in the sandiness of the rocks is observed in the section.

Metal contents in groundwater

Deposits of Triassic age are represented by rocks of the type of tuffaceous siltstones and sandstones, among which interbeds of normal sedimentary varieties occur. Jurassic formations have a sandy-clayey composition. Sandstones, from fine-grained to gravelites, often calcareously cemented, occur as thick beds that are well sustained along strike.

Quaternary deposits are represented by loams and clays, and, within the limits of the development of river terraces, by gravel-pebble material.

Tectonically, the western part of the region belongs to the Prisalaïr zone of linear folding and to the subzone of gentle linear folds; the eastern part belongs to the central zone of gentle folds and domal uplifts (Belitskii, Pakh, Tyzhnov, 1970).

Plicative tectonics is characterized by a series of synclinal and anticlinal brachystructures oriented in various directions (Fig. 1). The synclinal folds have distinct outlines in plan, with broad, trough-shaped, very gentle trough parts, and are separated by weakly expressed anticlines. Disjunctive tectonics is widely developed. Its largest elements are the Sokolovskii, Iganinskii, and Uspenskii thrust faults, genetically close to the marginal faults separating the Kuznetsk Basin from the surrounding mountain structures. There are many smaller disturbances of the type of concordant

throws and overthrusts. Maximum disturbance is characteristic of the crestal parts of anticlines. In the area, regularities in the water abundance of rocks common to all of Kuzbass are clearly established. The Jurassic deposits are characterized by maximum water saturation; specific well yields of up to 8.6 l/s have been recorded there. The specific yields of wells that penetrated deposits of the Kolchugino Series do not exceed 2.6 l/s. The upper fractured zone, to depths of 100–150 m, is the most water-bearing, especially in river valleys and relief depressions. With depth, the fracturing of the rocks decreases, and their water abundance decreases as well.

Within the zone of intense fracturing, groundwater of calcium bicarbonate and calcium–magnesium composition predominates, with mineralization up to 0.7 g/l. On hypsometrically elevated—

Table 1

[[unclear: beginning of table title]] in waters of the Erunakovsky region

Metals, μg/l

—well-washed sectors in the northeastern part of the area contain weakly mineralized (0.07–0.2 g/l) waters of sodium bicarbonate–chloride composition, similar in character to waters of atmospheric precipitation.

With depth, the mineralization of groundwater increases to 2–2.5 g/l, and its type changes to sodium bicarbonate. Within individual anticlinal structures (the Naryk anticline), sodium bicarbonate–chloride waters occur with mineralization up to 3.5–5.5 g/l.

In terms of metal content, the groundwater of individual aquifer complexes differs somewhat from one another.

In the zone of intensive water exchange, the groundwater of Quaternary deposits is characterized by the broadest complex of metals. Here, copper, zinc, barium, titanium, manganese, and chromium are recorded in most samples. Silver, molybdenum, and tin are specific; they are absent in the groundwater of other aquifer complexes or occur only in isolated samples (Table 1). It should be noted that most of the samples belong to groundwater associated with the pebbles of the terraces of the Tom River and its tributaries. In comparison with groundwater belonging to eluvial–deluvial deposits and studied by us within the Osinovka area, they show a clearly noticeable decrease in the occurrence of a number of microcomponents, especially me-

of the polymetallic complex. The occurrence of nickel and tin decreases. Antimony and strontium are noted very rarely.

In terms of the microcomponent complex, the groundwaters of Mesozoic and Permian deposits are quite close to one another. The differences are most clearly observed in the percentages of occurrence and, to a lesser extent, in the contents of metals. Under the conditions of the Erunakovsky district of Kuzbass, where zones of anomalously elevated metal contents and zones of ore occurrence and mineralization are absent in the rocks, when elucidating the conditions of groundwater formation the most indicative value, in addition to occurrence, is the “mode,” which characterizes contents with a five-to-ten-percent probability of occurrence; whereas the criterion of the “upper boundary of the background,” usually used for purposes of prospecting hydrogeochemistry, is of little applicability because the anomalous concentration values of individual metals belong to areas where the factors affecting them act predominantly in one direction. At the same time, we also considered areas with anomalous metal contents. As the latter, values corresponding to the one-sigma limit were adopted (probability of occurrence 33%), relative to which contrast was assessed.

In distinguishing the indicated characteristics we used the method of rectified distribution diagrams, applied at the Tomsk Polytechnic Institute (Udodov, Shestakov, 1964). The method is based on assumptions of a logarithmic-normal law of the general distribution (Razumovsky, 1948) and a normal distribution within the background (Smirnov, 1963). The validity of the logarithmic-normal distribution of microcomponents in natural waters was confirmed in studies in many regions of Siberia (Udodov et al., 1962). From Fig. 2 and Tables 1, 2, where the results of determinations are given, it is evident that most metals show increased values of the “mode” for groundwaters of the Paleozoic aquifer complex, and a large part of them (copper, zinc, vanadium, manganese, barium) are characterized by maximum content values in groundwaters exposed by boreholes. The noted regularity indicates that, with depth, the contents of metals in groundwaters increase. This is also confirmed by analysis of hydrogeochemical profiles constructed across the strike of the structures. An increase is established—

Average (background) and anomalous contents of metals

Labels in the figure: Contents of metals in groundwater of aquifer complexes: springs; wells; Paleozoic; Mesozoic; Quaternary deposits. Scatter intervals of values; “mode” value; limits. Metal content, µg/l. Cu, Pb, Zn, Ni, V, Ti, Mn, Cr, Ba.

Fig. 2. Metal contents in the groundwater of the Erunakovsky district

…of metal-bearing capacity at points associated with anticlinal structures, along which groundwater from deep horizons is discharged.

An increase in the metal-bearing capacity of groundwater from deep horizons has also been noted in adjacent districts of Kuzbass, in particular at the Osinovskoye deposit, where underground hydrogeological surveys in mines established an increase in the concentrations and occurrence of copper, zinc, silver, barium, beryllium, vanadium, and chromium. When examining fluctuations in the contents of metals in groundwater (Fig. 2), a decrease is found in the intervals of dispersion of values from Quaternary deposits to the Paleozoic aquifer complex, which is clearly seen in the example of copper, nickel, vanadium, titanium, manganese, and others.

In the distribution of metals within the area of development of the Paleozoic aquifer complex, certain regularities have emerged. In spring—

Table 2

in the groundwater of the Erunakovsky district

In springs associated with the crest parts of anticlinal structures, copper, zinc, and lead were noted in most samples. The low percentage of lead occurrences in the groundwater of the complex should be taken into account. Thus, almost all occurrences of the complete polymetallic series of metals belong to these areas. Characteristic examples are the core of the Zhernovo-Nikolskaya anticline, the Narykskaya anticline, and the western limb of the Kyrgaiskaya anticline, truncated by the Sokolovsky upthrow. Major tectonic disturbances, as a rule closed and clay-filled, have a low occurrence of the indicated elements; in areas intensely broken by small, numerous disturbances, however, their occurrence increases, and in individual cases springs with elevated contents of copper, lead, and zinc rather clearly reflect the disturbed zones. The waters usually have a reaction close to neutral (pH 6.8–7.2). A considerable part of the samples in which nickel and barium were encountered also belongs to these same areas. In addition to the above, it should be noted that barium, when associated with copper and zinc at points where lead is absent, also tends toward fault zones, whereas nickel is distinguished by a comparatively uniform distribution in the area, with some tendency toward the mountain-taiga landscape zone.

Cobalt, the presence of which in groundwater is generally uncharacteristic, was recorded in three closely localized areas—on the disturbed northeastern limb of the Zhernovskaya syncline, in the area of the disturbed crest part of the Markino-Nikolskaya anticline, and on the northeastern limb of the Narykskaya anticline. In most samples it was recorded in traces.

The regularities in the distribution of metals in groundwater that appear within the different landscape zones of the area are of interest. First of all, when the factual material is analyzed, the breadth of the complex of anomalously high metal contents within the territory of taiga-landscape development attracts attention; moreover, the set of elements regularly increases from west to east, from forest-steppe to taiga landscape (Fig. 1). If, within forest-steppe territories, as a rule, 1–2 elements appear as anomalous, then in the taiga zone their number increases to 5–6.

The metals occurring with anomalous contents are zinc, nickel, chromium, barium, and, more rarely, titanium, copper, and manganese.

Silver is encountered in a considerable number of the sampled springs in the western part of the area, in the forest-steppe landscape zone. Its concentrations are small and usually amount to 0.1–0.6 µg/l, reaching 4.5–4.8 µg/l only at individual points. In the taiga part of the area, silver was not recorded in groundwater.

Main conclusions:

1. The macrocomponent composition of groundwater in the zone of intensive water exchange of the area of gentle brachystructures is relatively homogeneous within the development of individual lithologic-stratigraphic rock varieties. The clearest differences appear when analyzing their microcomponent composition.

2. The coal-bearing Paleozoic deposits are characterized by increased occurrence and contents of metals in groundwater. The broadest complex of metals with elevated contents is characteristic of groundwater in zones of tectonic disturbances.

3. With depth, the contents of metals in groundwater increase, while the scatter of their values decreases.

4. Groundwater of the taiga zone is characterized by more extensive—

with a set of metals and their elevated contents compared with the waters of the forest-steppe landscape zone.

The indicated regularities must be taken into account when processing materials from hydrogeological investigations.

REFERENCES

1. A. A. Belitskii, E. M. Pakh, A. V. Tyzhnov. Tectonics. — In: Geology of Hard-Coal Deposits of the USSR, vol. VIII, 1970.

2. N. K. Razumovskii. The logarithmic law of distribution of matter and its properties. Proceedings of the Leningrad Mining Institute, 1948.

3. S. I. Smirnov. Probabilistic-statistical assessment of the geochemical background in prospecting for mineral deposits. Geochemistry, 1963, No. 3.

4. P. A. Udodov, I. P. Onufrienko, Yu. S. Parilov. Experience of hydrogeochemical investigations in Siberia. Vysshaya Shkola Publishing House, 1962.

5. P. A. Udodov, B. N. Shestakov. On the question of determining the background in hydrogeochemical investigations. Izvestiya TPI, vol. 112, 1964.