SOME ASPECTS OF THE HYDROGEOCHEMISTRY OF THE KHARADZHUL-BUTRAKHTA ORE FIELD (WESTERN SAYAN)

N. M. RASSKAZOV

(Presented by the scientific seminar of the Department of Hydrogeology and Engineering Geology)

The study of the hydrogeochemical conditions of the Kharadzhul-Butrakhta ore field was carried out in 1958 by a detachment of the Research Sector of the Tomsk Polytechnic Institute under the direction of the author, with the aim of establishing prospecting hydrogeochemical criteria for this area and identifying new sectors promising with respect to sulfide mineralization. In the course of these investigations, a hydrogeochemical survey was carried out over an area of 200 km², as well as a more detailed survey over an area of 20 km² and, in individual sectors, a metallometric survey of bedrock. During the hydrogeochemical surveys, outlets of underground and ground waters and surface watercourses were sampled (at intervals of 200–500 m). In addition, water samples were taken from prospecting pits, underground mine workings, and boreholes. Simultaneously with the collection of hydrochemical and metallometric samples, geological and geomorphological observations were conducted.

At the observation points, whose total number was 310, the discharge of the water point, the temperature of the air and water, its pH, the content of sulfate ion in it, and the sum of metals by dithizone were determined; an aqueous concentrate of metals was taken by the TPI method [12]. Concurrently with these investigations, 160 water samples were taken for a shortened chemical analysis and 420 rock samples in order to determine the principal chemical composition of the waters and to establish the chemical activity of the rocks according to the TPI method [13]. At three points, observations were carried out on the regime of waters of various types.

The hydrogeochemical features of any region are determined by a large number of factors, which may be combined into the following groups:

I. Factors conditioned by the peculiarities of the physical-geographical conditions of a given region (climate, orography, and hydrography).

II. Factors conditioned by the geological-hydrogeological conditions of the region (tectonics, metallogeny, water-bearing capacity of rocks, and types of waters).

III. Factors that depend on the geochemical properties of the elements.

The physical-geographical conditions of the Kharadzhul-Butrakhta ore field are determined by its position in the northeastern part of the Kirsa spur of the Western Sayan, extending along the Abakan River in the southern part of Khakassia.

The region is a mountainous, elevated, dissected terrain with a symmetrical transverse profile. The part of it facing toward ...

toward the system of the Abakan River are occupied by sectors of the Kharadzhul deposit; on the area facing the system of the Tashtyp River are located sectors of the Butrakhta deposit.

Absolute elevations in the area reach 1250 m; the watersheds rise 200–300 m above the valley floors. The densely branched river network belongs to the systems of the Abakan River and its tributary, the Tashtyp River. Its recharge is mainly local, from atmospheric precipitation, the average annual amount of which is 410 mm. Approximately half of it falls in the summer months.

The geological structure of this ore field and of the adjoining area is determined by their position in the zone of junction of the Western Sayan and the Minusinsk Basin. The rocks composing the first structure within this region are Lower Cambrian in age; the rocks of the second structure in this region are Lower and Middle Devonian. The Cambrian rocks of the Western Sayan occupy the extreme southwestern part of the ore field and are represented by effusive-sedimentary formations of the Lower and Upper Monok formations. The Devonian of the Minusinsk Basin in this region is composed of sedimentary and effusive rocks of the Chilan (Kharadzhul), Tolochkov, Tashtyp, and Abakan formations.

Intrusive formations are of subordinate importance. According to Yu. A. Kuznetsov, M. I. Yudina, and A. G. Sivov, they are represented by rocks of the Middle Devonian diorite-diabase intrusion and the Middle Devonian intrusion of quartz albitophyres [8]. Within the ore field there occur dikes of keratophyres, diabases, and diabase porphyrites. The effusive-sedimentary formations of the Cambrian and Devonian are broken by tectonic dislocations, which have mainly sublatitudinal and northwestern trends. The ore bodies of the Kharadzhul and Butrakhta deposits are confined to these dislocations; their genesis has been defined as hydrothermal.

The mineralogical composition of the ores is very complex. According to A. I. Kruglov, the oxidation zone in different sectors of the deposits is developed extremely unevenly. This is due mainly to the heterogeneity of the chemical activity of the rocks within the deposits and to the varying thickness of the ore bodies.

The distinctive geological structure of the region, associated with its position in the zone of junction of the Western Sayan and the Minusinsk Basin, also leaves its imprint on the hydrogeological conditions. Most of the area belongs to a region transitional from the Western Sayan, with its exposed structures and active water exchange, to the closed structure of the Minusinsk Basin with slow water exchange.

The following types of water are distinguished here: 1—surface waters; 2—pore waters of alluvial and deluvial-alluvial formations; 3—fissure-karst and fissure-stratal waters of the sedimentary Devonian formations—the Tolochkov and Tashtyp; 4—fissure waters of effusive-sedimentary rocks of the Chilan and Abakan formations of the Devonian and of the Upper and Lower Monok formations of the Cambrian; 5—waters of zones of tectonic dislocations.

The recharge area of the waters is mainly local; recharge occurs through the falling atmospheric precipitation. The water yield of the rocks is low. Test pumping within the Kharadzhul and Butrakhta deposits established that the specific yields of wells, at drawdowns from 2 to 55 m, do not exceed 0.09 l/sec.

The hydrogeochemical investigations that were carried out showed that pH, which exerts a great influence on the migration of metals in natural waters [11, 7, 5, 2, 1], varies within this region from 5.4 to 7.7. Acid waters (pH 5.4–6.0) occupy very small areas. They were encountered in only 10 points, 8 of which are confined to [[unclear: continuation cut off at bottom of page]].

waterlogged areas and only 2—to zones of sulfide mineralization. Slightly acidic waters (pH 6.1–6.8) are distributed over a much larger area than acidic waters and are in most cases confined to watershed areas. Such a distribution is not accidental and is due to two causes: 1) the confinement of the outcrops of the ore bodies of the Kharadzhul and Butrakhta deposits mainly to watershed areas; 2) the presence of waterlogged areas on watersheds and watershed slopes.

The predominant type is neutral waters (pH 6.9–7.1), occupying the greater part of the area of the district.

Slightly alkaline waters (pH 7.2–7.7) have areal distribution only in the upper reaches of the Kharadzhul River. They are chiefly characteristic of boreholes and mine workings at the deposits and are recorded at individual points of the district.

Attention is drawn to the extremely limited distribution of acidic waters, despite the sulfide character of the mineralization of the ore field. As I have established, this phenomenon is explained by the high chemical activity of the rocks enclosing the ore bodies. Moreover, the content of pyrite and other disulfides in the ore bodies is generally insignificant; the sulfuric acid formed as a result of their oxidation is rapidly neutralized by the carbonates of the surrounding rocks, and therefore no increase in the acidity of the waters is observed.

The theoretical proposition concerning the influence of the geochemical environment on the migration of elements in natural waters is well known [14, 15, 8]. A. V. Shcherbakov [14] distinguishes oxidizing, oxidation–reduction, and reducing environments in natural waters. In carrying out hydrogeochemical investigations over the area of the Kharadzhul–Butrakhta ore field, in order to determine the geochemical environment at one point or another, the ratio

\[ \frac{\mathrm{Fe}^{\cdot\cdot}}{\mathrm{Fe}^{\cdot\cdot\cdot}} \]

and the content of the nitrite ion were determined. The anion \(\mathrm{NO}_2^{-}\) occurs in waters extremely rarely and in small quantities, and therefore it was not used as a criterion for determining the type of geochemical environment. Determinations of the ratio

\[ \frac{\mathrm{Fe}^{\cdot\cdot}}{\mathrm{Fe}^{\cdot\cdot\cdot}} \]

showed that shallow groundwaters are characterized by an oxidizing environment, which, at a depth of approximately 4 m from the water table, passes into an oxidation–reduction environment: the ratio

\[ \frac{\mathrm{Fe}^{\cdot\cdot}}{\mathrm{Fe}^{\cdot\cdot\cdot}} \]

increases with depth (Fig. 2).

Special attention was paid to the behavior of individual metals in the waters of boreholes at varying distances from the water table, depending on changes in the geochemical environment. It was established that, in accordance with the change in the geochemical environment, the content of metals in the waters also changes. If in an oxidizing environment (Fig. 1), according to spectral analysis of aqueous concentrates from borehole No. 34 of the Butrakhta deposit, Pb, Cu, Zn, Ni, Ag, and Sn were detected in an amount of 0.1523 mg/l, then at a depth of 100 m only Cu and Zn were encountered, in an amount of 0.009 mg/l. Beginning at a depth of 100 m, the metal content in the water of the borehole again increases somewhat. Evidently, at this depth the influence of the sulfide mineralization, found in the interval 201–205 m, begins to be felt. Enrichment of waters with metals at these depths under conditions of an oxidation–reduction environment occurs, as the investigations of G. B. Sveshnikov in the Rudny Altai [10] have shown, mainly as a result of processes of electrochemical dissolution of sulfide minerals.

The factual data presented above show that the water dispersion halos of mineralization zones under oxidizing–reducing and reducing conditions will, for most metals, be considerably smaller than under oxidizing conditions.

Fig. 1. Graph of changes in the content of metals in water from borehole No. 34 of the Butrakhta deposit

1 — alluvial–deluvial formations with sulfide mineralization; 2 — pyroxene–plagioclase porphyrites; 3 — andesitic porphyrites; 4 — volcanic breccia; 5 — rocks with sulfide mineralization; 6 — water-bearing zones; 7–10 — presence of metals in waters: 7 — Cu, 8 — Zn, 9 — As, 10 — Sn.

Determinations carried out in the field unit with the aid of the laboratory of A. A. Reznikov [9] established the heterogeneity of the waters of the district in chemical composition. Hydrocarbonate–calcium–magnesium–sodium waters are widespread. In areas of development of the Abakan, Chilan, and Monok formations, hydrocarbonate–sodium–calcium waters are not infrequently encountered. A very variegated chemical composition of waters is observed in areas with sulfide mineralization. Here, in addition to the three types described above, hydrocarbonate–chlorid-

… sulfate–sodium–calcium–magnesium and bicarbonate–sulfate–calcium–sodium–magnesium. Thus, the presence, in separate small areas, of waters diverse in their overall chemical composition may be regarded as one of the prospecting indicators of sulfide mineralization zones. The sulfate ion should also be assigned to such indicators, since no gypsified rocks have been encountered. Contents of this anion in the waters of the region in amounts greater than 10 mg/l are always confined to zones of sulfide mineralization.

Spectral analysis of aqueous concentrates revealed 19 elements (Table 1). What is noteworthy is the rather low copper content, in comparison with other ore regions, in springs and in waters from mine workings and boreholes confined to deposit areas. This fact can be explained by the fairly high pH values of the waters in these areas, which in turn are determined mainly by the chemical activity of the rocks. As indicated above, acidic waters are encountered here only extremely rarely; neutral waters are the most widespread, and, in addition, slightly alkaline waters have been noted.

Fig. 2. Graph of metal contents in natural waters of the Kharadzhul–Butrakhta ore field.

alkaline. Studies of the dependence of copper content on the pH value of the solution [5, 7] show that in slightly acidic and neutral solutions it cannot occur in large amounts.

From analysis of the hydrogeochemical map it may be concluded that the waters confined to areas of sulfide mineralization zones in the given region are characterized by a complex suite of elements. Along with elements widely distributed in the waters of the ore field, such as Cu, Pb, Zn, Ni, As, Co, Sn, Mo, Sb, Bi also occur here. Therefore, when evaluating

Table 1

The total number of concentrates collected was 342. Of these, 57 were collected from mine workings and exploratory boreholes, and 285 from sources of natural waters and surface watercourses.

the prospects of any point or site, according to hydrogeochemical investigations, attention should be paid first of all to the complex of elements found in the waters. Of great importance here, undoubtedly, is also the quantitative content of individual metals. Therefore, for those of them that were encountered outside areas with zones of mineralization, the background content and the content in an aqueous dispersion train have been distinguished (Fig. 2).

The data obtained made it possible to establish hydrogeochemical criteria for the district, on the basis of which areas promising with respect to sulfide mineralization can be identified. Such criteria may be considered:

a) contents of Pb, Cu, Ni, Ag, Co, Ba of more than 0.001 mg/l; contents of Zn and As of more than 0.003 mg/l;

b) the presence in the waters of Sb, Bi, Sn, Mo;

c) the presence, over small areas, of waters complex in their general chemical composition (the simultaneous presence of several types);

d) a sulfate-ion content in the waters of more than 10 mg/l.

The pH values, as already noted, because of the high chemical activity of the rocks, cannot serve as a reliable prospecting indicator for areas with a small thickness of ore bodies. The same may be said of swampy areas.

The criteria outlined made it possible to identify, in the region studied, dispersion trains of mineralization zones in natural waters. Some of them were not previously known and require additional investigations. In one such newly discovered site along the Prjamomu spring, in the same year 1958, the author found porphyrites with copper mineralization and established a zone of disturbance of northwestern trend.

REFERENCES

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