MAPPING DIKE ROCKS BY THE METHOD OF MAGNETIC SURVEYING: A CASE STUDY FROM THE VICINITY OF THE CITY OF TOMSK

L. I. IVANCHURA, E. V. YAROSHINSKY

(Presented by the scientific seminar of the Department of Geophysical Methods of Prospecting for Mineral Deposits)

The question of the development of dike rocks in the vicinity of the city of Tomsk is once again attracting the attention of geologists.

In the present article, on the basis of the results of a magnetic survey over the area of the valley of the Ushaika River, from the city of Tomsk to the village of B. Protopopovo, the possibility and effectiveness of magnetic prospecting in mapping dike rocks are demonstrated; the methodology of these works is indicated, and their geological results are described, some of which until recently had been unknown.

Geological mapping of dike rocks in the vicinity of the city of Tomsk is of great interest for solving a number of geological questions and for the practical use of these dike rocks as construction materials.

However, until the present time little attention has been paid to the problem of areal mapping of dikes. This is explained mainly by the fact that the usual methodology of geological surveying, based on visual observations of outcrops of dike rocks, did not provide sufficiently complete information about these rocks, which are almost everywhere overlain by Tertiary and Quaternary deposits; and geophysical methods of prospecting, in particular magnetic prospecting, which possesses high resolving power under the given geological conditions, were not applied until 1953.

Dike rocks in the vicinity of the city of Tomsk are widely distributed and are known in the literature under the name “Tomsk diabases.”

Geological information about these rocks is found in the works of M. K. Korovin, K. V. Radugin, M. A. Usov, F. I. Shnel, and a number of other researchers who studied this region.

One of the largest works summarizing a large body of factual material is K. V. Ivanov’s dissertation, “Geology and Petrography of the Lower Carboniferous and Dike Rocks of the Vicinity of the City of Tomsk” (1956).

According to the data of geological study, the area described is composed of a complex of sedimentary rocks of Lower Carboniferous age—clayey shales, siltstones, and sandstones. The magmatic rocks that cut them have vein and dike forms of occurrence.

The dike form of occurrence is predominant and the most typical. In most cases dikes form linearly elongated, steeply dipping bodies, trending in a WNW–ESE direction, almost perpendicular to the orientation of the folded structures of the Lower Carboniferous. The thickness of the dikes varies within considerable limits. More often it is small and ranges from 1 to 2–3 m, but along with such low-thickness dikes, which apparently are not persistent along strike, there occur dikes of medium thickness (5–10 m) and large dikes, with thicknesses from 20 to 50 m, which, unlike the low-thickness ones, are more persistent along strike.

The external appearance, composition, and structure of the dike rocks are quite varied; among them K. V. Ivanov distinguishes a number of varieties, namely: 1) dolerites, 2) porphyritic dolerites and essexite-dolerites, 3) diabases and essexite-diabases, 4) monzonites, and 5) hybridized dike rocks, of which the dike rocks of the second and fourth varieties are the most widespread.

As for the age of the dike rocks, this question, owing to the insufficiency of factual material, is resolved only tentatively. According to K. V. Ivanov, the upper limit of the time of intrusion of the dike rocks may be considered the Upper Triassic or, less probably, the Lower Jurassic.

In 1953, and then in 1955, the first geophysical work was carried out in the area under consideration, including magnetic surveying and electrical prospecting—the methods of symmetrical two-phase electrical profiling and vertical electrical sounding.

The main objective of these works was to identify “diabase” dikes and to determine the thickness of the loose sedimentary formations covering them. The work carried out demonstrated the great possibilities of using magnetic surveying in mapping dikes in the area investigated.

The magnetic survey was carried out with an M-2 magnetometer over a previously laid-out observation grid of 500 × 20 m, with some densification along the profile to 10 m in places where increases in the magnetic field were detected. The area covered by the survey is about 40 km².

One of the results of this work is the composite scheme of $\nabla Z$ graphs shown in Fig. 1, which indicates the presence in the area described of a complex magnetic field created by magnetic differences in the dike rocks. On the same scheme, as a result of processing the field materials with account taken of known geological information, it proved possible to show, approximately, the position of the magnetic axes of the dikes in this territory.

In considering the question of magnetic-prospecting work, we shall briefly characterize the magnetic properties of the rocks distributed in the area.

The dike rocks of the environs of the city of Tomsk are for the most part rocks of basic composition, possessing magnetism to one degree or another. The enclosing sedimentary rocks of the Lower Carboniferous (clayey shales, siltstones, sandstones) are practically nonmagnetic.

A relative determination of the magnetic properties of samples of dike rocks, carried out by the geologist K. V. Ivanov, showed that of 92 hand specimens investigated, 65 displayed relatively strong magnetism, 7 weak magnetism, while the remainder had no noticeable effect on the magnetic needle. Among the weakly magnetic rocks K. V. Ivanov includes dikes of quartz syenite-diorite-porphyry. An example is the dike of the so-called “Tolstyi Mys”-1 (see the scheme). Dikes of essexite-dolerite composition are also classed as weakly magnetic.

Legend

• Graph of $\Delta Z$:
• $\Delta Z$ curve
• Profile position
• Position of dike rocks
• Dike number
• Boundary of detailed-survey area A

Scale: 0 250 500 750 1000 m

Inset A scale: 0 20 40 60 m

North: N

Visible place and river names: Ushaika; Stepanovka; Kutachevo; Zavarzino; Zavarzinsky Garden; Mal. Protopopovo; Bol. Protopopovo.

Fig. 1. Diagram of the magnetic-survey profiles and the position of dike rocks in the area of the Ushaika River valley, from the city of Tomsk to the village of B. Protopopovo.
Compiled by L. I. Ivanchura and E. V. Yaroshinsky (1960).

Direct magnetic observations over the known thick Zavarzino monzonite-2 dike and the Zavarzino sovkhoz-3 dike confirm the magnetic nature of the anomalies. The large intensity values, reaching 2500–3000 gammas over these dikes, testify to the relatively high magnetism of these rocks.

A substantial shortcoming in the study of the magnetic properties of the rocks of the district consists in the absence of data on the absolute value of magnetic susceptibility and on the remanent magnetization of the dike rocks; without knowledge of these, quantitative interpretation of the magnetic-survey results is made more difficult.

Thus, analyzing the available data on the magnetic properties of the rocks, on the conditions of occurrence of the dikes in the enclosing rocks, and on the results of magnetic-prospecting work in the territory described, we come to the conclusion that the magnetic anomalies in the district are apparently produced exclusively by dike bodies, which are traced rather well by zones of elevated \(\Delta Z\) values against the general background of the normal magnetic field existing over the sedimentary rocks of the Lower Carboniferous.

The scheme of the results of the magnetic survey presented in Fig. 1 for the area adjoining the valley of the Ushaika River from the city of Tomsk to the village of B. Protopopovo indicates a considerable saturation of this district with dike bodies, especially the territory adjoining the village of Zavarzino.

On the basis of the qualitative interpretation of the magnetic data that has been carried out, taking into account the known strikes of the dikes from geological observations, it appears possible, as a first approximation, to trace in the investigated area 48 magnetic axes of dikes, whose strike azimuth ranges from WNW 275° to NW 330°; the predominant direction is northwestern.

The most intense magnetic field, of 3000 gammas, was recorded in the area adjoining the Zavarzino sovkhoz-3. Natural outcrops of dike rocks are not known here; apparently, the dike causing this anomaly lies at considerable depth. The character of the magnetic anomaly indicates a vertical or nearly vertical dip of the dike. According to the survey data, the magnetic axis of the dike is approximately traced along an azimuth of WNW 285°. The dike is traced along strike for no less than 1100 m.

Another, likewise intense, magnetic anomaly is noted over the known Zavarzino dike of monzonitic composition-2, which crops out at the surface somewhat northwest of the outskirts of the village of Zavarzino. The thickness of the dike is determined as 25 m; the strike azimuth of its magnetic axis is NW 312°. The dike is traced beneath cover deposits for a distance of 1800 m.

Over the known “Tolstyi Mys”-1 dike, of quartz syenite-diorite-porphyry composition, which is exposed in two quarries near the village of Stepanovka, the magnetic-field intensity does not exceed 500 gammas. The thickness of the dike is about 20 m; its strike azimuth is determined as NW 331°. The dike is traced for a distance of more than 1000 m.

The detailed work carried out on a small scale in the area of the Zavarzino active springs shows the detailed tracing of a dike whose strike here is determined as NW 318°. The dike has been traced for a distance of 1000 m; near it another dike is identified, but it is less intense.

Thus, the magnetic-prospecting work carried out rather clearly shows the possibility of applying magnetic surveying to identify and trace dike rocks in the vicinity of the city of Tomsk; moreover, the costs of these operations are considerably less than the costs of—

costs, provided that such work is carried out by other geological-geophysical investigations. However, in order to obtain a clearer (more detailed) picture of the position and extent of the dike rocks, a denser grid for the magnetic survey is needed than has been used hitherto—namely, \(100 \times 20\) m, with densification along the profile to 10 m in the event that an increase in the magnetic field is detected. The use of such a grid of magnetic observations will not only make it possible to trace confidently the extent of the dike rocks, but also to answer the question of whether intersections of dikes of different ages occur in the area; this may be of practical interest both from the standpoint of sulfide mineralization and for solving a number of current geological problems. That such intersections exist is suggested by the characteristic magnetic field-4 obtained from the survey, where along a number of profiles a gradual transition is clearly expressed from one magnetic-field maximum to two. Unfortunately, the sparse survey grid does not make it possible to answer this question precisely, since to the southeast we do not have such a pattern of the magnetic field.

Calculations show that the cost per \(1\ \mathrm{km}^2\), using the proposed survey grid, for field work in magnetic prospecting will amount to approximately 70–80 rubles.

It should be noted that, according to the results of the magnetic survey, it appears possible to determine the thickness and depth of occurrence of the dike rocks by fairly simple methods of interpretation.