HYDROGEOCHEMICAL CHARACTERISTICS OF THE SEMILUZHENSKOE ORE OCCURRENCE

P. A. UDODOV, N. M. SHVARTSEVA, A. I. SKOGOREV

The hydrogeochemical method of prospecting for mineral resources is becoming increasingly widely included in the set of not only geological-survey works, but also prospecting and exploration works, because it is distinguished by the greatest depth penetration among all geochemical methods.

In the summers of 1968 and 1970, detailed hydrogeochemical investigations were carried out at the Semiluzhenskoe site by a team from the Tomsk Polytechnic Institute jointly with the Tomsk Integrated Expedition, with the aim of identifying anomalous aqueous dispersion flows of antimony and mercury on the basis of the application of the latest precise methods. Antimony was determined by the amalgam-polarographic method, whose sensitivity is 0.01 µg/l; mercury was determined by a chemical method with a sensitivity of 0.5 µg/l. As a result of the field and laboratory investigations carried out, the geological and hydrogeochemical features of the Semiluzhenskoe antimony ore occurrence were refined.

In 1959, P. A. Udodov and V. M. Matusevich [1965] conducted hydrogeochemical investigations in this area; their target task was to assess the prospects of the area with respect to ore mineralization. The authors identified sites with elevated contents in the waters of microcomponents of the complex: lead, copper, zinc, antimony, mercury, silver, and barium. The Kolarovo–Semiluzhenskaya zone was traced by the hydrogeochemical method. In the waters of the antimony mineralization, the presence of antimony and an elevated zinc content of up to 0.24 mg/l were found at only a few points. The limited occurrence of antimony in the waters can be explained by the low sensitivity of the spectral analysis employed. It is interesting to note that all points with the presence of antimony, arsenic, mercury, and with elevated zinc content were assigned by the authors to zones of tectonic disturbances.

The Semiluzhenskoe ore occurrence is located in the northern part of the Kolyvan–Tomsk zone, for which a two-tier structure is characteristic.

The lower tier is composed of Upper Paleozoic formations, the upper one of loose Cenozoic sediments lying horizontally.

The ore-hosting rocks are Tournaisian deposits of the lower tier. Lithologically, these are aleurite-clayey shales and siltstones with interbeds of sandstones and limestones. In most cases the sequence has undergone carbonatization and ferruginization and is penetrated by a large number of quartz and carbonate veinlets ranging in thickness from 1–2 mm to 20–25 cm.

The deposits of the Paleozoic basement are crumpled into latitudinal folds with a northeastern trend of the axes and are complicated by second- and third-order folding. Structurally, the antimony ore occurrence is associated with the Kolarovo–Semiluzhenskoe crushing zone, which was first distinguished by M. P. Nagorskii. Within this zone there is intense fragmentation of the shale sequence, mylonitization, flexure-like bends of the schistosity, and graphitization.

The area of the Semiluzhenskoe ore occurrence is composed of rocks of the Tournaisian stage of the Lower Carboniferous, forming an anticlinal fold whose axis trends in a direction close to northeastern. Where the axial part of the anticline is intersected by sublatitudinal crushing zones, quartz veins and veinlets appear; these carry stibnite and form complex stockworks with an elevated antimony content.

The complex geological structure determines the distinctive nature of the hydrogeological conditions. Depending on the geological structure, the character of movement, and the chemical composition of the waters, we have distinguished two aquifer complexes:

1. The aquifer complex of loose Cenozoic deposits.
2. The aquifer complex of Paleozoic Carboniferous deposits.

The first aquifer complex, in turn, is subdivided into two types: perched water, groundwater of the alluvial deposits of river floodplains, and groundwater of above-floodplain terraces and cover deposits.

The second aquifer complex, according to conditions of occurrence, is likewise subdivided into two types of waters: fissure waters of Paleozoic deposits and fissure pressure waters of zones of tectonic disturbances.

Perched water, reflecting the specificity of the physicogeographical conditions and geological structure of the study area, constitutes a zonal type of water. It is widely distributed on watersheds, in near-slope parts of valleys, and in places where groundwater springs emerge. This is favored by the excess of atmospheric precipitation over evaporation and by the presence of poorly permeable rocks of Quaternary formations, represented mainly by loams and, more rarely, sandy loams. Perched waters occur at depths of 0.5–1.5 m, usually in sandy lenses, veinlets, and pockets. The regime of these waters depends entirely on atmospheric precipitation. In chemical composition, perched water belongs to the bicarbonate calcium type, with mineralization of 0.05–0.1 g/l, pH 6.5–6.7, oxygen content 6–10 mg/l, \(C_{\text{org}}\) 8–10 mg/l, \(CO_2\) 8–10 mg/l, and chlorine 4–5 mg/l. The slightly acidic reaction of the waters, low mineralization, fairly high oxygen content, somewhat elevated organic content, and the presence of dissolved carbon dioxide and chlorine characterize the aggressive capacity of the water, which over a short period becomes enriched, from the soil horizon, loams, and sandy loams of Quaternary deposits, with microcomponents in significant quantities. Thus, spectral analysis revealed in these waters (in µg/l): copper up to 5.3; zinc up to 21.5; nickel up to 7.5; manganese up to 323. In addition to these elements, eleven more microcomponents were determined in the perched waters, including, at individual sampling points, ...

specific elements were found: antimony and mercury in trace amounts. The source of enrichment of the waters with these elements is not yet entirely clear, but it may be supposed that they entered the water directly from the upper soil horizon, rich in humus of plant origin. The influence of soils on the chemical composition of waters remains very poorly studied; yet V. I. Vernadsky [1960] already drew attention to the fact that the role of soil waters in forming the chemical composition of groundwater is great.

Groundwaters of the Quaternary deposits, as already noted, are subdivided into the sporadically distributed waters of the Taiginka Formation, of the cover deposits and of the above-floodplain terraces; and the waters of the alluvial deposits of the river floodplain. The water-bearing rocks are silty loams and brown polymictic sands with interbeds of clays and loams. Waters of the first type occur in separate sandy, low-thickness horizons, sandy lenses, veinlets, and nests. Their discharge takes place in the form of low-yield, scattered descending springs along river valleys and ravines. Their regime is variable, being affected by atmospheric precipitation. Recharge of these waters is predominantly local. They are characterized by a bicarbonate magnesium-calcium composition of waters with a neutral reaction, are enriched in oxygen (9–10 and more mg/l), have a total mineralization of 0.2–0.4 g/l, CO₂ 7–10 mg/l, and chlorine 4.5 mg/l. The water temperature in the summer period is 15–23°C. The waters contain a complex assemblage of microcomponents, which reflects the composition of the enclosing rocks. The contents of the elements are low. At individual points, elevated contents of antimony, arsenic, and mercury were found. This is explained by the presence of antimony-mercury mineralization or by recharge from Paleozoic fracture waters enriched in these elements.

Of interest are the waters of the alluvial deposits of the river floodplain, which is composed of clayey-silty formations with sand at the base. We established that the Kirgizka and Kamenka rivers, and partly the tributaries Malka and Mezhovka, even in summer are supplied predominantly by groundwater, owing to fracture waters of the Paleozoic and confined waters of zones of tectonic disturbances. This is indicated by the almost complete absence of oxygen in the water, the low content of organic carbon (up to 3 mg/l), the frequent presence of sulfate ion up to 10 mg/l, the presence of free carbonic acid from 7.5 to 18 mg/l, and the density of hydroisogyps lines. The water is characterized by a bicarbonate calcium-magnesium composition with mineralization of 0.4–0.6 g/l and a slightly alkaline reaction. The microcomponent composition of the waters is very complex: antimony, arsenic, mercury, zinc, lead, copper, silver, etc. The contents of antimony, mercury, and arsenic are presented in Table 1, from which it is evident that antimony is present in these waters over a wide range from 0.5 to 50 μg/l, and mercury and arsenic in smaller quantities. The contents of zinc, copper, lead, and silver in these waters range from the first units to the first tens of μg/l. Background values for the leading elements are as follows: antimony—3.8 μg/l, arsenic—3.0 μg/l, mercury—1.0 μg/l. Anomalous antimony contents in river waters (points 126, 135, Table 1) indicate its direct connection with antimony mineralization. This is confirmed by the results of the study of samples at point 1 (Table 1), which was taken directly at the site of the antimony ore occurrence on the Kirgizka River. High mercury contents in the waters also indicate a connection of these waters with ore mineralization. At the same time, it is necessary to take into account that the summer of 1970, when the data were obtained, was very rainy, and, of course, dilution of the waters by atmospheric precipitation occurred. Nevertheless, we obtained anomalous contents of antimony and mercury in the waters.

Table 1

Content of antimony, mercury, and arsenic in the waters of the antimony ore occurrence

Of particular interest from the prospecting standpoint are the fracture waters of the Paleozoic and the artesian fracture waters of zones of tectonic disturbances. A characteristic feature of this type of water is the more complex composition of the microcomponent assemblage, which reflects the complex conditions of their recharge and movement. In the process of discharge these waters mix with waters of overlying horizons, which often differ in the composition of macro- and microcomponents.

Groundwaters of Paleozoic deposits are widespread and are confined to the regional fine fracturing of the weathering zone of clay shales, siltstones, and sandstones of the Lower Carboniferous. These waters discharge along the valleys of the Kirgizka and Kamenka rivers in the form of individual ascending-type springs with a flow rate of 0.01–0.2 l/sec. The water temperature is generally 5°C. These waters have been penetrated by a large number of boreholes. The aquifer is overlain by Quaternary deposits or by the weathering crust of the bedrock. The depth of occurrence varies from several to 30 and more meters. The direction of movement of the groundwater flow is mainly toward the valleys of the Kirgizka and Kamenka (Fig. 1). The most intense discharge occurs from the watershed part of the Kirgizka and Kamenka rivers. On this basis two sectors are distinguished (I and II, Fig. 1), which differ in the magnitude of the hydraulic gradient ($I = 0.04—0.06$, $I = 0.0083—0.02$) and in the modulus of underground runoff. The fracture waters of the Paleozoic almost everywhere have a head of from 3 to 10 and more meters; in a number of boreholes nonartesian waters were encountered. In chemical composition the fracture waters of the Paleozoic are mainly calcium bicarbonate waters with mineralization of 0.4–0.6 g/l, neutral (pH 6.8–7.2), with sulfate ion present from traces to 60 mg/l. These waters, as a rule, are practically without oxygen, with free carbonic acid content of 15–32 mg/l and chloride ion of 6–14 mg/l. They contain a complex assemblage of elements similar to the assemblage in alluvial waters of river floodplains. It should be noted that the content of zinc, lead, copper, and silver in the latter waters is lower than in the Paleozoic waters. In the fracture waters of the Paleozoic, antimony was detected at 80% of the sampling points, with contents from traces to 30 and more µg/l, and mercury at 50% of the points, with contents from 0.5 to 5.0 µg/l (Table 1, spring 2, borehole 2). With a contrast coefficient of 10 and higher, it may be assumed that the waters considered are enriched in antimony only at the expense of the antimony mineralization of the Paleozoic formations. It is necessary once again to note that antimony was not found in all Paleozoic springs.

Fracture artesian waters are confined to zones of deep-seated rupture disturbances of northeastern strike. The Paleozoic rocks, as noted above, are more often silicified, hornfelsed, with an abundance of syngenetic pyrite, and are strongly crushed.

Waters of the disturbance zones were penetrated by boreholes 80–150 m deep. All the waters are artesian; specific flow rates were 0.45–0.66 l/sec. Most often these waters discharge along river valleys in the form of ascending springs with a flow rate of 0.02–0.5 l/sec. The spring regime is constant, with a water temperature of 4–5°C. The recharge area of these waters lies beyond the limits of the study area (the Tom–Ob watershed). The waters of the tectonic disturbance zones are distinguished among other types of water by an increased total mineralization (0.6–1.0 and more g/l) and absence of oxygen, elevated values of free carbonic acid (50–66 mg/l), chloride ion (74–92), sulfate ion (30–100 mg/l), and the presence of ferrous iron and ammonium ion. Frequently around springs connected with zones of tectonic disturbances, carbonate travertines (calcareous tuffs) are deposited in the form of cones and covers. They have various

Fig. 1. Schematic map of the distribution of antimony and mercury in natural waters of the Semiluzhenskoe ore occurrence: 1 — Lower Carboniferous, Tournaisian stage, clayey and aleurite-clayey shales; 2 — Lower Carboniferous, Lagerno-Sadskaya Formation, clayey and aleurite-clayey shales; 3 — Lower Carboniferous, Basandaika Formation, sandstones and clayey shales. Antimony content in µg/l: 1970; 4 — from 1 to 3.5; 5 — from 3.5 to 5; 6 — from 5 to 10; 1959; 7 — from 0.0005 to 0.05. Mercury content in µg/l: 8 — from 1 to 3; 9 — from 5 to 10 and more; 10 — hydrogeochemical sectors with elevated antimony and mercury contents; 11 — sectors with differing water abundance. 12 — direction of movement of the groundwater flow with \(J_I = 0.04—0.06\), 13 — direction of movement of the groundwater flow with \(J_{II} = 0.0083—0.02\); 14 — conventional boundaries of the distinguished hydrogeochemical sectors; 15 — conventional boundaries of hydrodynamic zones.

in composition, complexes of microcomponents, which makes it possible to use them in prospecting.

The formation of travertines around springs is clear evidence of the manifestation of a geochemical barrier.

The artesian waters of the disturbance zones have a bicarbonate sodium-calcium composition. Tectonic disturbances provide the greatest depth of penetration of gases dissolved in the water, especially oxygen, which promotes the oxidation of sulfides at considerable depths (\(> 150\) m). Owing to deep oxidative pro-

favorable conditions are created for the enrichment of the waters with metals.

In these waters a complex of microcomponents has been found that is analogous in composition to the fissure waters of the Paleozoic. The occurrence of antimony in the waters is 100%, of mercury 50%, and of arsenic 10%. Some points characterizing the waters of zones of tectonic disturbances are presented in Table 1 (points 3, 31, 137, 1 r. and 28). The tabulated data indicate a wide range of antimony concentrations in the waters, from 1.0 to 46 µg/l.

Using the data for borehole 28 as an example (Table 1), it is evident that with depth the presence of the sulfate ion decreases, while, conversely, the contents increase: total mineralization from 0.65 to 0.95 g/l, arsenic from 0 to 9.46 µg/l, antimony from 1.9 to 16 µg/l, and zinc from 2.3 to 473 µg/l. The elevated contents at a depth of 100 m of antimony, zinc, and arsenic are evidently associated with inflow from waters of the fault zone, which intersects the borehole at the same depth.

The fissure waters of the Paleozoic have a contrast of more than 10, which indicates the possibility of deep occurrence of antimony mineralization.

From all the foregoing it may be concluded that antimony, mercury, and arsenic are widely distributed in the waters of the antimony ore occurrence. Elevated contents of antimony and mercury are noted in the fissure waters of the Paleozoic. Figure 1 presents a scheme of the distribution of mercury and antimony in the natural waters of the antimony ore occurrence, on which three areas with anomalous values are distinguished. In these areas it is necessary to continue comprehensive geological and hydrogeochemical investigations.

REFERENCES

1. Vernadsky V. I. Selected Works. Vol. 4, Book 2. Publishing House of the Academy of Sciences of the USSR, Moscow, 1952.

2. Udodov P. A., Matusevich V. M., Grigor’ev N. V. Hydrogeochemical Prospecting under Conditions of Semi-Closed Geological Structures of the Tom–Yaya Interfluve. TSU Publishing House, Tomsk, 1965.