THE ROLE OF GEOCHEMICAL BARRIERS IN THE FORMATION OF AQUEOUS DISPERSION FLOWS OF HEAVY METALS

(A CASE STUDY OF THE KOLYVAN–TOMSK ZONE)

P. A. UDODOV, B. M. LEVASHOV

For areas of abrupt change in the geochemical environment, caused by various factors and leading to changes in the conditions of migration and concentration of elements, A. I. Perelman (1965) proposed the term “geochemical barriers.” Depending on their specificity and the medium in which they occur, they are divided into three main groups: mechanical, physicochemical, and biogeochemical.

The concentration of ore components caused by the action of geochemical barriers took place throughout the entire history of the geological development of the Kolyvan–Tomsk zone. Traces of the existence of ancient physicochemical barriers in its territory are numerous and varied. In addition to mineral formation in the oxidation zones of sulfide ore occurrences, the most striking of these include widely developed infiltration brown iron ores and manganese formations in the weathering-crust profile, interbeds and concretions of phosphorites in the Pachinskaya Formation, interbeds of siderite in the upper parts of the Bolotninskaya Formation, areas of enrichment of terrigenous Paleozoic rocks of the main trough with lead, zinc, copper, and other metals, ferruginous and manganese boboviny in Mesozoic–Cenozoic deposits, etc.

Both ancient and modern geochemical barriers are of great importance for the hydrogeochemical method of prospecting for ore deposits. The former reflect the activity of groundwater in the past. Analysis of epigenetic formations in rocks and tracing the paths by which solutions entered the zone of action of such barriers help reconstruct the hydrogeological conditions of the existence of ancient aqueous dispersion flows of heavy metals and their sources. Study of the latter makes it possible to more fully reveal the picture of the formation of the microcomponent composition of waters and to take a more correct approach to the interpretation of hydrogeochemical survey data.

In the territory of the Kolyvan–Tomsk zone, the geochemical barriers most frequently encountered and important from the prospecting standpoint are those associated with the precipitation from spring waters of travertines and iron hydroxides (degassing–carbonate and iron barriers, according to A. I. Perelman).

Travertines are very widely developed in the region. These formations have the form of bowls, cones, covers, and irregular accumulations at groundwater outlets. Most often they occur in the vicinity of the city of Tomsk (Petukhovo village), near Tutalskaya station, in the basins of the Tugo-

...kovka, Pacha, Inya, Suzun, and in many other areas. Their significance for hydrogeochemical prospecting is due to the fact that, together with them, the microcomponents contained in the waters also precipitate.

To identify the complex of microcomponents that precipitate together with travertines from the waters, hydrochloric-acid extracts were prepared from 67 travertine samples. The calculated average contents of the elements showed that one gram of travertine contains (in micrograms): manganese—447, barium—85, titanium—10, zinc—4, vanadium—5, nickel—2, copper—1, lead—0.5, silver—0.1. In the waters of springs depositing travertines, the corresponding average element contents (in µg/l) are 8.1; 0.5; 0.4; 0.2; 0.1; 0.01; 0.25; 0.1; 0.1.

Arsenic, cobalt, antimony, strontium and, apparently, bismuth, tin, and cadmium are most prone to precipitation by travertines. These elements, with insignificant contents or in the absence of them in the waters, were found in travertines in amounts of 100 µg/l or more. Manganese and barium are recorded both in travertines and in spring waters. Elements such as copper, chromium, zinc, vanadium, lead, nickel, silver, and titanium, at the analyzed points, are recorded either in the waters or in the travertines. When present in travertines, they are always recorded in the waters. The maximum values for copper in water are up to 10 µg/l and in travertines up to 500 µg/l; respectively, for chromium, 10 and 30; for zinc, 30 and 200; for vanadium, 10 and 300; for lead, 5 and 20; for nickel, 7 and 30; for silver, 2 and 4; for titanium, 100 and 1500. The differing behavior of these elements during travertine precipitation is, apparently, determined to a greater extent by the diversity of the forms of their migration in waters.

Another form of manifestation of the degassing–carbonate barrier is the deposition of carbonates of heavy metals in the near-surface part of oxidation zones, associated with the sharp depletion of waters in carbon dioxide at places where they emerge onto the ground surface. Cerussite, hydrozincite, azurite, malachite, as well as secondary calcite and siderite formed under such conditions have been found in the upper parts of the oxidation zone of the Baranovskoye, Ognevozaimskoye, and other ore occurrences. The basic character of the salts (hydrozincite, azurite, malachite) shows that precipitation occurred in an alkaline medium.

A widely distributed geochemical barrier in the territory described is the deposition of iron hydroxides when the conditions of its migration change.

The works of V. I. Vernadsky (1960), N. M. Strakhov (1962), A. I. Perelman (1965), and others show that migration of iron in aqueous solutions may occur in ionic form, in the form of colloids, and as various complex compounds. The diversity of the forms of its migration determines the significance of iron in the geochemistry of some heavy metals. Iron compounds precipitating from aqueous solutions play the role of a geochemical barrier for many microcomponents.

As noted earlier, within the Kolyvan–Tomsk zone, iron is associated with various types of groundwater. It occurs in both divalent and trivalent forms. The total iron content in the waters of individual springs sometimes reaches 16 mg/l. Under conditions of discharge of iron-enriched groundwaters, it precipitates in the form of hydroxides, which sorb a large complex of microcomponents. In 35 samples of iron hydroxides collected from groundwater springs in different parts of the region, spectral analysis detected (in %): manganese (0.06–1.0), titanium (0.001–0.06), barium (0.001–0.03), phosphorus (0.06–0.6), chromium (0.001–0.06), arsenic (0.006–0.03), nickel (traces), copper (traces–0.001), lead (traces), zinc

(0.003–0.006), strontium (0.006–0.01), cobalt (0.001). At the same time, elements such as manganese, titanium, barium, and phosphorus occur in most samples. All the others are present in iron hydroxides collected within the development limits of aqueous dispersion flows of mineralization zones.

Of local significance under the conditions of the Kolyvan–Tomsk zone is the hydrogen sulfide barrier. Isolated springs containing hydrogen sulfide occur only in the northern part of the region (the Ya River, the right bank of the Tom River below the mouth of the Basandaika River). At the latter point, in the waters of a spring containing hydrogen sulfide, in contrast to neighboring hydrogen-sulfide-free springs, divalent iron appears, carbonic acid (up to 67 mg/l), strontium, and especially barium and manganese appear, while lead, zinc, mercury, and silver—found in other springs of this area—disappear. Characteristic of a number of hydrogen sulfide springs is the precipitation, at their outlets, of native sulfur and alum.

The role of limestones as a geochemical barrier is well known: they cause an increase in the pH of waters, in particular the replacement of acidic waters by neutral or alkaline ones. This causes the precipitation of most metals from solutions and their concentration within the zone of action of the barrier. Traces of the former existence of this barrier include the brown iron ores we discovered near the Vlaskovo limestone quarry. They formed as a result of metasomatic replacement of limestone and of the rocks in contact with it by iron hydroxides, and contain inclusions of native copper. Spectral analysis of the brown iron ores revealed lead (traces), zinc (0.2%), silver (0.001%), cadmium (0.001%), mercury (0.001%), barium (0.07%), and other elements. Brown iron ores of analogous origin in the quarry near the village of Verkhnii Koen contain lead (traces), zinc (0.5%), arsenic (0.2%), molybdenum (0.001%), barium (0.05%), as well as cobalt, nickel, beryllium, manganese, and titanium. A common feature of the noted brown iron ores is their development near areas with aqueous dispersion flows of zones of polymetallic mineralization.

The influence of limestones at the present time is manifested in a sharp decrease in the contrast and contents of metals in aqueous dispersion flows of mineralization zones. Thus, for example, for the Baranovka polymetallic ore occurrence, where a hydrogeochemical survey at a scale of 1:25,000 was carried out, the boundaries of the distribution of the aqueous dispersion flow are clearly established, conditioned by the presence of a small interbed of limestones in the sandstone sequence on the eastern outskirts of the village of Baranovo.

The material presented shows that modern geochemical barriers, in the zone of action of which a large complex of elements is precipitated from waters, belong to phenomena that must be taken into account in hydrogeochemical investigations. Therefore, when carrying them out, along with studying the microcomponent composition of waters, it is necessary to perform obligatory analysis of all newly formed deposits (travertine, hydroxides of iron and manganese, and other compounds) observed at spring outlets. On the other hand, great assistance in evaluating the areas under study may be provided by the study of epigenetic formations of springs that operated in the past, changed their position in space, or disappeared completely. A characteristic example of such formations is the travertines discovered on the elevation of the right bank of the Gryaznushka River near its confluence with the Okino River. Here they occur as a continuous cover over an area of about 1 km². Trenching work showed that the thickness of the travertine deposits in the central part exceeds 3 m. In their section, horizons are distinguished,

intensely impregnated with iron and manganese hydroxides, in which carbonate material is of subordinate significance. These horizons are the most enriched in microcomponents and contain (in µg/l): lead—10, copper—72, zinc—110, cobalt—88, nickel—60, silver—1, barium—560, arsenic—10. In addition, mercury (0.1 µg/l) and cadmium in trace amounts are present. The contents of elements in the waters of springs in this area exceed background values by no more than 2–3 times, while such elements as zinc, silver, cadmium, and mercury were not detected in the waters. All this, together with other prospecting indicators—the presence of fragments of brown iron ores, pyritization of rocks, hypergene bleaching of rocks, and others—made it possible to regard the area as promising. Examples of the same kind include the already described brown iron ores developing on limestones, as well as tuffaceous sandstones enriched with iron and manganese hydroxides on the elevation 1 km northwest of the village of Lushnikovo, and manganiferous fragments of quartz veins and sandstones on the slope of the right bank of the Elbash River opposite the village of Elbashi. The manganese hydroxides of the latter area contain more than 1% zinc, nickel, and cobalt.

From what has been said it follows that the role of geochemical barriers in changing the chemical composition of aqueous dispersion flows of mineralization zones and in concentrating ore elements is very great. It is sufficient to note that, under conditions of geochemical barriers that acted for a long time within a limited area in the past, concentrations of ore matter can reach industrial values. Thus, I. Chadeck et al. (1971) believe that the world’s largest uranium deposits (in the USA, FRG, and Japan) were formed through the precipitation of uranium from circulating meteoric waters under favorable oxidation–reduction conditions. The formation of so-called stratiform deposits of lead, zinc, and copper is associated with the activity of geochemical barriers (Konstantinov, 1963, et al.). Conversely, geochemical barriers that are unstable in space and time lead to the formation of disseminated ore mineralization in sedimentary sequences, the weathering crust, soils, and alluvial deposits, both in past epochs and at the present time. This mineralization, while in individual cases serving as a source of heavy metals in waters, is not an object of industrial exploitation. Therefore, the associated aqueous dispersion flows in hydrogeochemistry are conventionally called “non-ore,” in contrast to “ore” flows, whose source is deposits.

Thus, according to the degree and characteristics of their influence on the formation of aqueous dispersion flows of heavy metals, three main groups may be outlined, as a first approximation, among geochemical barriers:

1. Modern geochemical barriers that influence the composition and content of microcomponents in aqueous dispersion flows. A characteristic result of their activity is epigenetic neogenic formations at the outlets of groundwater springs. Analysis of these neogenic formations serves as an additional criterion for evaluating the area under study in hydrogeochemical prospecting.

2. Geochemical barriers of past epochs, acting for a long time within a limited area and leading to the formation of epigenetic ore deposits, are a source of “ore” aqueous dispersion flows of heavy metals.

3. Geochemical barriers of past epochs and of the present time that are unstable in space and time. The disseminated mineralization formed as a result of their activity serves as a source of “non-ore” aqueous dispersion flows.

Assessment of the activity of geochemical barriers is necessary for the correct interpretation of hydrogeochemical data. The accumulation of elements in the zone of action of barriers may indicate, taking into account the direction of water movement in present-day and former aquifers, their primary source. This is especially important when subdividing aqueous dispersion flows into “ore-bearing” and “non-ore-bearing.” The table presents the division of these principal groups of hydrogeochemical anomalies depending on the results of the activity of geochemical barriers and other sources of enrichment of waters with heavy metals.

Principal types of hydrogeochemical anomalies

REFERENCES

V. I. Vernadskii. Selected Works, vol. 1. Publishing House of the Academy of Sciences of the USSR, 1954.

M. M. Konstantinov. Origin of Stratified Lead and Zinc Deposits. Publishing House of the Academy of Sciences of the USSR, 1963.

N. M. Strakhov. Stages in the development of the outer geospheres and sedimentary ore formation. Izv. AN SSSR, Geological Series, No. 12, 1962.

A. I. Perelman. Geochemistry of Epigenetic Processes. Moscow, “Nedra,” 1965.

I. Chadek, M. Maikovskii, Z. Szulc. Geochemical significance of underground waters for the accumulation of ore components. Geology and Geochemistry of Ore Deposits. Proceedings of the XXIII International Geological Congress. Moscow, “Mir,” 1971.

* Hydrogeochemical assessment of anomalies and their assignment to ore-bearing or non-ore-bearing should be carried out taking into account the results of geological, geochemical, paleohydrogeological, paleogeographic, and geophysical investigations.