STRUCTURE OF THE MAGNETIC FIELD OF THE APRELKOVO GOLD-ORE DEPOSIT AND ITS RELATIONSHIP TO THE GEOLOGICAL STRUCTURE OF THE DEPOSIT

(Transbaikalia)

V. K. ROZMYSLOV, L. Ya. EROFEEV (TPI)

At the Aprelkovo gold-ore quartz-sulfide deposit, in whose geological structure various rocks of a Lower Paleozoic metamorphic complex, andesite porphyrites of Upper Jurassic age, intrusions of Variscan diorites, and post-Upper Jurassic dikes of diorite porphyrites and quartz porphyries take part, the special magnetometric work described below was carried out.

To study the magnetic field, the entire deposit was covered by profiles of high-precision magnetic surveying with an M-18 magnetometer, with a 6 m interval along the profile. Measurements were made according to a cyclic scheme, with every 10th point repeated. This made it possible to perform measurements with a root-mean-square error of 4–6 gammas.

The measurement data were processed on the Minsk-1 electronic computer of the TPI computing center. As a result, autocorrelation functions and the spectral density of the variance of changes in magnetic-field intensity were obtained for each profile.

At the ore nodes of the deposit, micromagnetic measurements were carried out on areas of 20 × 20 m, on a 2 × 2 m grid, with an accuracy of 2 gammas. Plans of the isodynamic lines of the sites were processed by the method of angular statistics and are presented in the form of rose diagrams of the orientation of the isodynamic lines. In addition, the ore nodes were investigated by several profiles of high-precision measurements with a 2 m interval and an accuracy of 2–3 gammas.

Above the known veins of the deposit, measurements of the vertical component of the magnetic field were made along a number of profiles.

The average length of the profiles was 50 m, the distance between them 5 m, and the measurement interval 2 m.

This made it possible to assess the shape and dimensions of the change in field intensity above the veins of the deposit.

Using the geological map of the ore field of the deposit, a statistical processing was carried out of the orientation of the identified quartz veins, dikes, and tectonic disturbances.

Analyzing the material obtained and the geological setting at the deposit, the following was established:

1. The structure of the magnetic field of the Aprelkovo deposit is closely related to the geological structure and depends to a lesser degree on the composition and age of the rocks composing this deposit.

2. The magnetic field within the deposit is characterized by very pronounced anisotropy, since most of the anomalies in the magnetic field have linearly elongated forms and an ordered orientation.

3. In the magnetic field of Aprelkovo, anomalies of three types can be distinguished:

a) first-order anomalies with widths from 900 to 1500 m and intensities of 5–6 thousand gammas, caused by the block structure of the deposit;

b) against the background of the first-order anomaly, second-order anomalies are observed, with widths up to 200 m and intensities from several hundred gammas to several thousand gammas. These anomalies are caused by areas of contact-altered rocks, enclosing veins, dikes, and tectonic disturbances;

c) the “background” of the magnetic field is caused by anomalies whose average width is in the first tens of meters, with intensity differences from several tens of gammas to several hundred gammas. Anomalies of this type may be caused by heterogeneity in the distribution of magnetically effective material within the rock mass and by small areas of rocks altered by hydrothermal and tectonic activity, or by buried microrelief.

1. The vein formations of the deposit are marked by linearly elongated negative anomalies, varying in width from several meters to several tens of meters and in amplitude from several hundred gammas to the first thousands of gammas. The width of the anomalies over the veins of the deposit does not agree with the thickness of the quartz veins of the deposit. The thickness of the veins ranges from 20 cm to 3 m, whereas the most probable anomaly could be caused only by a bed 20 m thick. This discrepancy is explained by the fact that the shape of the magnetic anomaly is influenced not only by the vein material but also by part of the near-contact altered rocks.

2. Ore shoots, in comparison with other parts of the deposit, are marked by a greater variability of the vertical component of the magnetic field.

As a result of studying the structure of the magnetic field of the Aprelkovo deposit and its relationship to the geological structure, it was established that magnetic surveying under the conditions of this deposit can be applied to solve the following tasks:

1. With the aid of magnetic surveying, it is possible successfully to search for and trace vein formations. According to calculated data tested under the conditions of the deposit in the area of the ore shoot of the Chertovaya vein, the optimal network for observing the magnetic field is a \(5 \times 20\) m grid.

2. Magnetic surveying can be applied to study the fracturing of the rocks of the deposit. In this case, field surveying may be conducted on separate plots measuring \(20 \times 20\) m, on a \(2 \times 2\) m grid.

3. By measuring the vertical intensity of the field along profiles spaced from one another at distances not exceeding the dimensions of the ore shoots, it is possible to search for these shoots.

4. High-precision magnetic surveys may also be used to study the morphology of individual veins. In solving this problem, the survey-

surveying should be conducted on short profiles oriented across the strike of the vein.

1. Under conditions similar to those of the Aprelkovo deposit, magnetic surveying can be used to map rock masses that differ in petrographic composition. In this case, the parameter by which rock boundaries will be outlined in the magnetic field should not be the mean values of intensity, but the features of anisotropy, since the rocks of the deposit differ in the magnetic field only by this characteristic.

In interpreting the survey results, special attention should be paid to the nature of the local relief. The latter has a noticeable distorting effect on changes in the field. These distortions are expressed, first, in the fact that all geological heterogeneities appear more distinctly in the magnetic field on elevations and are smoothed in lowered areas, and, second, valleys are usually marked by reduced intensity values, while elevations are marked by increased ones.