THE ROLE OF MONITORING OBSERVATIONS IN HYDROGEOCHEMICAL EXPLORATION

P. A. Udodov, I. P. Onufrienok

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

The hydrogeochemical method of prospecting for ore deposits is becoming widely developed in geological exploration work. Experience from field investigations has shown that this method is entirely applicable to various regions of Western and Eastern Siberia. In the zone of permafrost and talik it also yields positive results [8].

At present, as a result of thematic field investigations by VSEGINGEO, VIMS, the Academy of Sciences of the USSR, Tomsk Polytechnic Institute, and other scientific-research institutes and geological administrations, a methodology for field work has been developed. Unfortunately, it has so far been insufficiently covered in print, which to a considerable extent hinders the introduction of this method into geological-production organizations. Some questions of the methodology of field investigations remain still poorly studied and almost unreported in the literature.

Thus, for example, until now the influence of physico-geographical conditions on the content of heavy metals in surface and ground waters has remained almost unstudied.

The authors’ investigations in Rudny Altai (Zyryanovsk district) in 1955 showed that during the period from August 8 to 31 the metal content in ground waters changed from 0.16 to 0.04 mg/l, while in 1956, at one spring of the Zmeinogorsk mine, the content of heavy metals in the water on July 3 was 7.1703 mg/l, and on July 30 it was 0.183 mg/l.

Similar data are given by J. S. Webb and A. P. Millman [2] for surface waters in studies in West Africa. According to these investigations, in two areas of lead-zinc mineralization in Nigeria it was established that the highest content of heavy metals in surface waters was observed after heavy rains.

A. I. Perelman and A. A. Saukov [6] indicate that the content of chemical elements in the waters of rivers and springs changes depending on climatic factors. Therefore, they recommend carrying out hydrochemical prospecting at one and the same time or introducing corrections for the season of the year.

In 1957, at an enlarged meeting of the chemical section of VSEGINGEO, A. A. Brodsky, in his report, drew attention to the need to study changes in metals in natural waters during field hydrogeochemical investigations in order to introduce corrections when interpreting the results.

V. I. Krasnikov [4], in elucidating the question of the comprehensive study of a hydrographic network in prospecting, indicates that the extent of the dispersion halo is determined by a number of causes, among which a very important role is played by the intensity of the process by which waters of the dispersion flow are diluted by waters of background composition. Consequently, in studying the dimensions of dispersion flows, monitoring observations are absolutely necessary.

I. I. Ginzburg [3], as a result of an analysis of data from hydrogeochemical investigations, recommends conducting hydrogeochemical prospecting for metals at that time of year when river waters more fully reflect the metal content in ore-bearing areas.

V. G. Melkov and L. Ch. Pukhalskii [5] pay serious attention to the regime of natural waters in interpreting data from hydrogeochemical investigations in prospecting for uranium deposits.

Over the course of four years, all hydrogeochemical parties working in the territory of Western Siberia and Transbaikalia carried out monitoring observations during the period of field investigations. The observational data show that changes in the content of heavy metals in natural waters depend on a number of factors (climatic, hydrogeological, soil, geobotanical, and others). Without dwelling on consideration of all the causes, we shall give only some remarks on the influence of climatic and geological-hydrogeological conditions.

As is known, the regime of surface and ground waters is determined to a considerable degree by physicogeographical and geological factors. For mineralization areas confined to the zone of active drainage of ground waters, the process of ore oxidation is characteristic. As a result of this process sulfates are formed, most of which are readily soluble in natural waters.

We studied changes in the content of metals in ground waters in the Rudnyi Altai at the site of the Zmeinogorsk mine, at spring No. 1, on the Mashinka River, in the area of the Zyryanovsk mine, and at other points. Spring No. 1 is located at a distance of 250–300 m from the Zmeinogorsk open pit, where ore was formerly mined. Its recharge area is apparently located within the open-pit area; therefore the spring water should be assigned to the waters of the mineralization zone. The regime of the spring is characterized by great instability, which is determined by its recharge from the local area. The results of monitoring observations at spring No. 1, presented in Fig. 1, show that the content of metals in the water changes almost 40-fold over the course of 27 days. This is explained by the fact that when precipitation falls, the groundwater level rises, as a result of which new portions of oxidation products, formed in the rocks and ores during the period without atmospheric precipitation, enter the migration zone. In addition, atmospheric waters contain increased amounts of oxygen and carbon dioxide, which accelerate the process of oxidation of sulfides and the transfer of the resulting sulfates into solution. As a result, the content of heavy metals in natural waters increases. In a dry period, on the contrary, a decrease in the content of heavy metals in natural waters and an increase in the pH value of the waters are observed.

The study of changes in the content of metals in waters assumes special importance in mountainous regions, where structures are characterized by intensive water exchange as a result of increased infiltration and drainage of atmospheric precipitation.

To check the influence of atmospheric precipitation on changes in the content of metals in natural waters, we collected additional concentrates from spring No. 1 before precipitation fell and after it fell—

of their precipitation. Water concentrates were collected by the TPI method [9]. The concentrate collected before precipitation on 17/VIII-56 showed the presence of metals in the waters at 0.369 mg/l, while in the concentrate collected after the rainfall on 18/VIII-56 the presence of metals was observed at 0.8820 mg/l (Fig. 1).

Fig. 1. Graph of changes in metal content in natural waters from spring No. 1
in Zmeinogorsk
1—atmospheric precipitation in mm, 2—copper, 3—zinc, 4—lead, 5—barium, 6—titanium.

The longest continuous regime observations of natural waters in hydrogeochemical investigations were carried out in the area of the city of Tomsk in 1958–59. The results of the investigations show that the metal content in waters changes sharply both for soil waters and for groundwaters. The metal content in groundwaters during the summer period changed within the range from 0.001 to 0.015 mg/l (Fig. 2).

Sharper fluctuations were noted in the winter period. Thus, for the spring (Fig. 2), changes from 0.002 to 0.25 mg/l were established. It is interesting that different elements respond differently to changes in the regime. The most sensitive in this respect is zinc; copper and a number of other elements respond less. For zinc in the water of the spring (Fig. 2), a change in its content from 0.0005 (summer period) to 0.24 mg/l (winter period) was established. Approximately analogous changes in zinc content in natural waters were observed, according to data of N. M. Rasskazov, for the Western Sayan (Kharadzhul deposit) and for the eastern slopes of the Salair and Kuzbass, according to data of G. M. Rogov [7].

Some researchers assign the group of metals that sharply change their content in natural waters to the category of accumulating elements in groundwaters. Thus, for example, A. A. Brodsky [1] classifies the sulfate ion and zinc as accumulating elements in groundwaters. Unfortunately, he does not reveal the causes that determine this process.

The results of hydrogeochemical investigations in the area of the city of Tomsk suggest very complex relationships between the total mineralization of waters,

the presence of metals in them and the discharge of springs. It may be assumed that, in addition to the reasons given above, certain geochemical characteristics of individual elements must also be taken into account.

Fig. 2. Graph of changes in the content of metals in natural waters in the area of the city of Tomsk

Curves of change: 1—water temperature, 2—air temperature, 3—free carbon dioxide content, 4—Eh of the water, 5—pH of the water, 6—content of metals determined by dithizone, 7—total mineralization of the water; indices of elements contained in the water: 8—zinc, 9—manganese, 10—barium.

Experience has shown that interpretation of the results of hydrogeochemical investigations carried out without monitoring observations is often difficult. Thus, for example, at the beginning of the 1956 field season on the Krutishka River (Rudny Altai), an increased content of metals in the water was noted (more than 0.1 mg/l), with a large complex of metals (9 elements) characteristic of zones of polymetallic mineralization. Geologically, the site is associated with a productive ore-bearing sequence. On the basis of all the data presented, it was assigned to areas where prospecting and exploration work was planned. During control sampling one month later, the content of metals in the water sharply decreased (to 0.006 mg/l), and the complex of elements also became small (5 elements), with the most typical elements of zones of polymetallic mineralization absent. Such sharply differing results did not permit an assessment of the metallogeny of this point. We repeatedly encountered similar difficulties in other areas as well. This circumstance served as the reason for setting up monitoring observations in the course of field work and for introducing the corresponding corrections to metal contents when compiling hydrogeochemical maps. Corrections for the maps are obtained from graphs of monitoring observations for all types of natural waters.

The compilation of maps with the introduction of corrections for the entire complex of metals (on average 9–12 elements) is in practice very difficult. Therefore, it is most expedient to compile maps with corrections for individual

elements. Thus, for example, for polymetallic mineralization zones a correction map is usually compiled only for zinc. Comparison of hydrogeochemical maps compiled without introducing regime corrections and with corrections shows that the configurations of the dispersion flows of mineralization zones differ significantly from one another. The dispersion flows on the map compiled with account taken of regime observations correspond most closely to the presence of mineralization zones in the rocks.

On the hydrogeochemical map taking regime observations into account, one of the sectors in an area of a zirconium-titanium placer was assessed more correctly; on the map without the introduction of corrections it manifested itself rather weakly.

The examples given show that the organization of regime observations and the compilation of maps with the introduction of corrections based on them are absolutely necessary when conducting hydrogeochemical surveys.

LITERATURE

1. Brodsky A. A. Hydrochemical methods for prospecting ore deposits. Proceedings of the First All-Union Conference on Geochemical Methods for Prospecting Ore Deposits, Gosgeolizdat, 1957.

2. Webb J. S. and Millman A. P. Heavy metals in natural waters as a prospecting indicator (preliminary investigations in West Africa). Geochemical Methods for Prospecting Ore Deposits, IL, 1954.

3. Ginzburg I. I. Experience in developing the theoretical foundations of geochemical prospecting methods. Gosgeoltekhizdat, 1957.

4. Krasnikov V. I. Integrated studies of the hydrographic network in prospecting. Exploration and Protection of Mineral Resources, 12, 1957.

5. Melkov V. G. and Shukhalsky L. G. Prospecting for uranium deposits. Gosgeolizdat, 1957.

6. Perelman A. I. and Saukov A. A. Geochemical foundations of prospecting for ore deposits. Proceedings of the First All-Union Conference on Geochemical Methods for Prospecting Ore Deposits. Gosgeolizdat, 1957.

7. Rogov G. M. Groundwater of the Belovo district of Kuzbass. Kuzbassuglegeologiya archives, 1959.

8. Udodov P. A. and Onufrienok I. P. Experience in hydrogeochemical investigations in a permafrost zone. Proceedings of the Second Conference on Groundwater and Engineering Geology of Eastern Siberia, issue IV, Irkutsk, 1960, pp. 118–125.

9. Udodov P. A. and Onufrienok I. P. On the question of the hydrogeochemical method of prospecting for nonferrous metals and certain rare elements. Izv. TPI, vol. 90, 1958.