THE ROLE OF HYDROGEOCHEMISTRY IN INTEGRATED GEOLOGICAL PROSPECTING BASED ON RESEARCH RESULTS IN TOMSK OBLAST

V. M. MATUSEVICH

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

Hydrogeochemical investigations in Tomsk Oblast have been carried out by the Problem Geological Laboratory of the Tomsk Polytechnic Institute in cooperation with the Tomsk Integrated Expedition since 1956, in combination with the following geological-prospecting and geophysical methods: a) route geological surveying, b) mapping and exploratory drilling, c) the heavy-mineral concentrate method, d) metallometry, e) geological prospecting work (digging of trenches and test pits), f) magnetic surveying and electrical profiling.

The set of hydrogeochemical investigations included the following types of work: a) route hydrogeochemical sampling with a geological-hydrogeological description of the point, b) collection of aqueous concentrates by the TPI method, c) determination of the total heavy metals with dithizone, d) metallometry of rocks and bottom sediments, e) study of the chemical activity of rocks, f) study of the chemical composition of natural waters with the aid of Reznikov’s field hydrogeochemical laboratory, g) regime observations of certain types of waters, h) hydrogeochemical sampling of mapping and exploratory boreholes.

Verification of hydrogeochemical data is carried out by comparing them with the geological-prospecting methods listed above. The use of rapid analyses, as well as the timely performance of spectral analysis of aqueous concentrates, rocks, and silts, makes it possible to verify hydrogeochemical data to a considerable extent during the field period. In the most interesting areas, exploratory boreholes up to 150–200 meters deep are planned.

The application of hydrogeochemistry in combination with other geological-prospecting work has made it possible to solve a number of questions of both theoretical and practical character.

In 1956, based on the results of hydrogeochemical investigations carried out in the area of the Tom–Ya interfluve, sites with anomalously high titanium content in the waters were identified. In 1957, ilmenite placers were discovered in the immediate vicinity of these sites.

Hydrogeochemical investigations conducted in the area of the placers in 1958, and experimental work, made it possible to solve, in prin-

the question of the form of migration of titanium in waters, to outline the principal hydrogeochemical criteria for identifying areas promising for ilmenite. In the natural waters of placers, elevated contents of titanium, chromium, phosphorus, and sometimes nickel, tin, and silver are noted.

On the basis of the results of hydrogeochemical work in 1958, several areas promising for ilmenite were delineated. Verification by drilling of one of the areas gave positive results.

The application of the hydrogeochemical method in combination with geophysical investigations is very effective in identifying and tracing zones of tectonic disturbances concealed beneath a thickness of unconsolidated deposits. Thus, comparison of hydrogeochemical data with the results of magnetometric investigations in the area of the Tom–Yaya interfluve makes it possible to outline a whole series of tectonic zones extending across the structure of the Tom–Kolyvan folded zone. On the basis of drilling work and hydrogeochemical investigations in the area of the village of Semiluzhki, a large fault zone has been traced, which is outlined in a southwesterly direction. The criteria for identifying tectonic zones concealed beneath a loose cover are deposits of carbonate travertines by spring waters, elevated contents in the waters of zinc, lead, silver, antimony, etc., a somewhat increased value of the dry residue and a complex assemblage of macrocomponents, the presence of specific elements (zinc, lead, and cobalt) in bottom sediments, etc.

Interesting data are obtained by comparing the heavy-mineral and hydrogeochemical methods for zirconium–titanium mineralization. In areas where the weathering crust is being eroded, a considerable quantity of titanium and its associated elements—chromium, vanadium, etc.—enters natural waters in the form of colloids and suspended particles. In water concentrates the content of titanium reaches 1.0 mg/l, chromium—0.1 mg/l, vanadium—0.07 mg/l, tin—0.005 mg/l. In the heavy-mineral concentrates here a high content of ilmenite, rutile, etc., has been established.

The use of metallometry of bottom sediments in combination with hydrogeochemical investigations makes it possible to characterize a given area more fully, and sometimes provides additional data for determining tectonic zones. Thus, according to work in 1958 south of the village of Semiluzhki, along the Sukhaya and Gogolevskaya rivers, a tectonic zone is indicated by magnetometric data. By water concentrates this zone is distinguished only along two rivers, while along the Berezovaya River, situated between them, the water concentrates showed an absence or background content of metals characteristic of waters of zones of tectonic disturbances. Nevertheless, in silt samples from the Berezovaya River, at two points, an elevated content of many metals was obtained, including elements specific to silts—cobalt and zinc. The collection of water concentrates here was carried out during a period of rains, i.e., during a period of strong dilution by atmospheric precipitation, which apparently also affected the content of metals in the waters.

The results of hydrogeochemical investigations are widely used by geologists of the Tomsk Integrated Expedition in geological mapping and accompanying prospecting for sulfide mineralization. According to data from hydrogeochemical work in 1959, areas with elevated contents in the waters of zinc, antimony, barium, and other elements, confined to the Semiluzhki fault zone, were outlined. Drilling in this area in 1960 exposed rocks with dense disseminations of pyrite, pyrrhotite, more rarely chalcopyrite and sphalerite.

In 1960, in the area of the Tom–Yaya watershed, hydrogeochemical investigations traced dispersion streams of lead, zinc, silver, antimony, arsenic, barium, and strontium, confined to the Shcherbak-

zone of crumpling. In these areas, ditches exposed brown iron ores of the iron-cap type with a high content of lead, zinc, barium, etc. In addition, near one of the areas, blocks of porphyrites with disseminated pyrite, chalcopyrite, and sphalerite were found in dumps.

Because of the concealed nature of the geological structure of the area, hydrogeochemical sampling of boreholes deserves special attention, especially in watershed areas. It consists of intensive flushing of the borehole with clean water, gelonization of three to four volumes of the water column, followed by interval sampling of water to determine macro- and microcomponents. Directly at the borehole, indirect indicators of the geochemical environment are determined (ions of ferric and ferrous iron, pH, and oxygen). The sampling intervals are set in agreement with geologists and on the basis of the borehole section.

In the course of hydrogeochemical investigations, more than 500 chemical analyses of water samples were carried out. This made it possible to subdivide natural waters not only by microcomponents but also by macrocomponents. Study of the chemical composition of the waters showed that waters of zones of tectonic disturbances are characterized, as already noted, by a somewhat elevated total mineralization (sometimes up to 1.0 g/l), by the predominance in a number of cases of the magnesium ion over calcium, by an elevated content of the chloride ion, by the presence of divalent iron, etc. Surface waters, perched waters, and groundwaters, under conditions of isolation from deeper waters, are characterized by an extremely low dry-residue value (sometimes less than 50 mg/l). Where these waters are recharged by deeper fissure waters, their composition does not differ substantially from the latter.

The application of hydrogeochemistry in combination with other geological prospecting methods makes it possible to carry out hydrogeochemical zoning of the Tom–Yaya interfluve. Here two principal types of mineralization may be distinguished:

1. Sedimentary syngenetic (complex in waters: titanium, chromium, vanadium, sometimes nickel and tin). This mineralization is confined to the framing of the Tomsk swell, to the sandy-shaly deposits of the Tom–Kolyvan folded zone, and to the ancient weathering crust.

Sometimes sedimentary syngenetic mineralization is represented by an iron–manganese complex: iron, manganese, titanium, chromium, nickel, more rarely vanadium.

1. Postmagmatic mineralization of the zones of tectonic disturbances of the Tom–Kolyvan folded zone. It should be noted that there is a certain difference in the element complexes by zones of disturbances located in the sedimentary deposits of the Upper Devonian and Lower Carboniferous and by zones in the effusive-tuffaceous sequence. In the first case the metal complex in the waters is lead, copper, zinc, and antimony, with zinc and antimony being the predominant elements here. The second case is characterized by a more complex metal assemblage (lead, zinc, silver, antimony, arsenic, barium) and by a considerably higher content of the indicated microcomponents in the waters than in the first. In addition, such specific elements in the waters as cadmium and mercury have been noted here.

As a result of checking hydrogeochemical anomalies with mapping boreholes in this area, ore occurrences of certain metals have been exposed in bedrock occurrence, and a belt promising for detailed prospecting for ore deposits has been outlined.