ARSENIC WATERS OF THE KHARADZHUL–BUTRAKHTA ORE FIELD (WESTERN SAYAN)

N. M. RASSKAZOV

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

In analyzing the results of hydrogeochemical work carried out in 1958–1959 in the interfluve of the Abakan and Tashtyp rivers within the area of the Kharadzhul–Butrakhta ore field [1], it was established that, for a whole series of water points (surface watercourses, springs, flowing boreholes, adits), the amount of arsenic was elevated—from hundredths of a milligram per liter to 5 mg/l. This fact deserves attention, since waters containing more than 0.6 mg/l of this element are medicinal mineral waters; on the other hand, the presence of arsenic in amounts greater than 0.05 mg/l does not permit the use of natural waters for drinking purposes.

Arsenic waters are distributed over an area of about 40 km² (Fig. 1). In origin they are connected with the sulfo-arsenide ores of the Kharadzhul and Butrakhta deposits, the principal component of which consists of copper sulfides and cobalt and nickel arsenides. The arsenic waters are formed mainly through the infiltration of atmospheric precipitation. This is indicated by the magnitude of their dry residue (not more than 300 mg/l) and by the results of regime observations [1]. As the waters move toward local natural drains, they become enriched in chemical elements through processes of leaching and dissolution, promoted by the dissected relief and by the predominance of atmospheric precipitation over evaporation. In terms of pH (6.4–7.4), the waters of the area are close to neutral; in overall chemical composition they are mixed, with predominance of the hydrocarbonate ion. Their enrichment in arsenic may occur chiefly through the following processes: 1) ordinary oxidation (by oxygen) of arsenides and dissolution of the resulting products; 2) electrochemical dissolution of mixtures of arsenides; 3) electrochemical dissolution of mixtures of arsenides and sulfides.

1) In the ordinary oxidation of arsenides, the course of the reactions may be represented as follows [4]:

\[ 4\mathrm{CoAsS} + 13\mathrm{O}_2 + 6\mathrm{H}_2\mathrm{O} = 4\mathrm{CoSO}_4 + 4\mathrm{H}_3\mathrm{AsO}_4; \]

or

\[ 3\mathrm{CoAs}_2 + 9\mathrm{O}_2 + 14\mathrm{H}_2\mathrm{O} = \mathrm{Co}_3\mathrm{AsO}_8 \cdot 8\mathrm{H}_2\mathrm{O} + 4\mathrm{H}_3\mathrm{AsO}_4. \]

The arsenate and sulfate formed in this process have different solubilities; erythrite is more stable than the sulfate; however, it too is partially destroyed by natural waters.

Fig. 1. 1–4—arsenic content in water, in µg/l: 1—1–10, 2—10–100, 3—100–600, 4—600; 5—sampling points of surface watercourses and their numbers; 6—springs and their numbers; 7—sampling of wells and their numbers; 8—flowing well and its number; 9—adits with numbers of hydrogeochemical sampling points; 10—Quaternary alluvium; 11—Abakan Formation, lower horizon: plagiophyres, quartz porphyries, amygdaloidal porphyries and their tuffs; 12—Tashtyp Formation: marls, limestones; at the base of the formation, marls with interbeds of tuffs; 13—Golochkovskaya Formation: sandstones and siltstones with interbeds of gravelites, marls, lenses of labradorite porphyrite and diabase porphyrite, tuffs, tuff breccias and amygdaloidal porphyries; 14—Kharadzhul Formation: labradorite porphyries and their tuffs, plagioclase porphyries, tuff breccias of amygdaloidal varieties; 15—Upper Monok Formation: porphyries and their tuffs, slates of varying composition, siltstones, sandstones, tuffaceous sandstones, conglomerates, tuffaceous conglomerates, bodies of reef limestones; 16—Lower Monok Formation: spilites, diabases, keratophyres, keratospilites, metaspilites, metakeratophyres, rarely their tuffs, lenses of jasper, siliceous slates and their hornfelses; 17—Akkol intrusion: gabbro, diorites, gabbro-diorites; 18—geological boundaries; 19—tectonic disturbances.

2) The electrochemical dissolution of mixtures of arsenides was studied by A. S. Dadson, who established the fact that, as a result of this process, aqueous solutions are enriched in the elements composing the minerals under investigation. He also measured the magnitude of the electrode potentials of the latter in a sulfate medium. Later, L. K. Yakhontova [5] established facts showing an intensification of the processes of ordinary (oxygen-driven) oxidation of certain arsenides in the presence of their mixtures, as compared with monomineralic samples. L. K. Yakhontova explains the change in oxidation intensity by the influence of electrochemical processes.

3) Enrichment of natural waters with arsenic may also occur during the electrochemical dissolution of mixtures of arsenides and sulfides. The possibility of this process was established by the author as a result of experimental investigations carried out under the supervision of Associate Professors P. A. Udodov and I. P. Onufrienok. The experimental work performed may be divided into two stages. During the first of these, the electrode potentials of arsenides (smaltite, nickeline) were determined with an LP-58 potentiometer in a normal potassium chloride solution, according to the procedure proposed by G. B. Sveshnikov [2]. The results of the measurements showed that the values of the potentials depend on the time the mineral remains in the solution. Therefore, on the recommendation of G. B. Sveshnikov [3], daily values were taken as standards (Table 1).

Table 1

Results of measurements of the electrode potentials of arsenides in a normal potassium chloride solution

At the second stage the samples of the arsenides studied were crushed; from fractions with a grain size of 0.1–0.25 mm, a 10 g sample was taken, which was washed free of carbonates and oxidation products in a solution of acetic acid and in distilled water. These samples were then mixed in pairs with similarly treated powders of pyrite, galena, and sphalerite and placed in liter vessels with distilled water that had first been boiled and cooled without access of air. After this, the vessels were tightly sealed and left for one month, during which the water samples were regularly stirred. In order to take account of the amount of oxygen entering the water when the mineral powders were introduced, some of the vessels were not sealed, and the content of this gas in them was determined. It proved to be insignificant—less than 1 mg/l; this determination makes it possible to establish what quantity of elements passes into the water as a result of ordinary oxidation and dissolution. After the one-month period had elapsed, the solutions were separated from the powders by siphoning, and a concentrate was obtained from them by the TPI method; it was then quantitatively analyzed by spectral means.

The results of the analysis are given in Table 2. On examining it one can see that, in the case of equality of the electrode potentials of the arsenide and the sulfide (experiment 3), approximately equal amounts of zinc and cobalt with nickel passed into the water. Evidently, here the usual oxidation of the minerals occurred at the expense of oxygen that entered the water when the powders were introduced. A different picture is observed for minerals with different electrode potentials, for example in the smaltite–pyrite mixture. Here the predominance in the solution of the sum of cobalt and nickel over iron is clearly visible, as is the low content of arsenic in comparison with the first two elements. This phenomenon can be explained fairly easily on the basis of the theory of electrochemical dissolution. In the present case the reactions taking place may be represented schematically as follows:

\[ \text{at the anode (smaltite)} \ — \ (\mathrm{Co}, \mathrm{Ni}) \ \mathrm{As}_2 - 6e \to \mathrm{Co}^{3+}, \ \mathrm{Ni}^{3+} + \mathrm{As}^0; \]

\[ \text{at the cathode (pyrite)} \ — \ \mathrm{MeS} + 2e \to \mathrm{Me}^0 + \mathrm{S}^{2-}. \]

Table 2

Content of chemical elements in water after a month’s presence in it of mixtures of arsenides and sulfides

Content of elements in μg/l.

Cobalt, nickel, and sulfur ions enter the solution, while arsenic and iron remain, respectively, at the anode and cathode. Therefore, in experiment 1 we have a high content of the sum nickel + cobalt and low contents of arsenic and iron. The presence of the latter two elements is due, as mentioned above, to the effect of oxygen that entered the water when the mineral mixtures were poured in. If the electrode potential of the sulfide proves to be lower than that of the arsenide, enrichment of the solution with arsenic and with the metal entering into the composition of the sulfide will be observed. In this case the waters will also be enriched in arsenic at the expense of electrochemical dissolution of the arsenide–sulfide mixture. The experiments carried out showed the possibility of electrochemical dissolution processes occurring in mixtures of arsenides and sulfides, which makes it possible to use the hydrogeochemical method in prospecting for concealed sulfo-arsenide mineralization.

The establishment of the fact of an elevated arsenic content in the waters of the area described makes it possible to raise the question of carrying out more detailed work here for a final evaluation of the balneological properties of the arsenic waters. The need for such work is also prompted by the fact that mineral arsenic waters have not been identified within Western Siberia, although possibilities for their discovery exist—sulfo-arsenide deposits are not uncommon here, and arsenide deposits also occur. In a detailed study of arsenic waters, it is first of all necessary to study their regime, to determine carefully the quantitative content of arsenic in the waters, and to designate sites for the construction of captage-

facilities. If favorable results are obtained, such work may serve as the basis for creating, in this readily accessible, picturesque mid-mountain area covered with cedar forest, a health resort of special purpose.

REFERENCES

1. N. M. Rasskazov. Experience in conducting regime observations in hydrogeochemical prospecting. Proceedings of the Interdepartmental Conference on the Hydrogeochemical Method of Prospecting for Ore Deposits, Tomsk State University Press, 1962.

2. G. B. Sveshnikov, S. L. Dobychin. Galvanic dissolution of sulfides and the dispersion halo of heavy metals. Geochemistry, No. 4, 1956.

3. G. B. Sveshnikov. Electrochemical dissolution of sulfide ores and its role in the formation of aqueous dispersion halos of heavy metals. Geological Results of Applied Geochemistry and Geophysics, Section 1. Geochemistry. Reports of Soviet Geologists, International Geological Congress, XXI Session, Gosgeoltekhizdat, 1960.

4. S. S. Smirnov. The oxidation zone of sulfide deposits. Publishing House of the Academy of Sciences of the USSR, 1955.

5. L. K. Yakhontova, P. A. Raudonis. On the oxidizability of certain arsenides of cobalt, nickel, and iron, as well as their binary mixtures. Geochemistry, No. 8, 1956.