Forms of Magnetic Anomalies over Ore Veins of the Darasun Deposit and Their Nature

L. Ya. Erofeev

(Presented by Professor D. S. Mikov)

As a result of studying the structure of the magnetic field at the Darasun deposit, it was established that quartz–sulfide veins are marked by reduced values of the vertical intensity. These anomalies are variable in form and intensity. Their amplitude and width vary over wide limits even for one and the same quartz–sulfide vein.

We made an attempt to determine the limits and the nature of the variation of these parameters both within a single vein and for a number of veins of the deposit. To this end, across the strike of most of the known Darasun veins, profiles of high-precision magnetic measurements were laid out (spacing 2 m, accuracy 2 gammas). Some curves over the veins are shown in Fig. 1. Data of this kind were regarded as a representative sample. Its representativeness is based on the fact that, in determining the places for laying out profiles across the strike of the veins under study, a 1:5000-scale isodynamic map was used, and the investigation profiles were located in various and typical places of the anomalies.

Another criterion by which the representativeness of the sample can to some extent be judged was the dip angle of the veins of the deposit, determined, on the one hand, from geological data and, on the other, from those anomalies that were obtained over the veins. In particular, the dip angle of the vein formations, determined from the most probable form of the variation in intensity (Fig. 2), which was obtained in processing the above-mentioned measurements, coincides (to within several degrees) with the angle that represents the arithmetic mean of the dip angles of all veins of the deposit over which measurements were made.

For this sample of fields over the veins, in addition to the most probable form of the variation in intensity, the probability of the occurrence of anomalies with one or another amplitude (Fig. 3) and width at half-maximum (Fig. 4) was also calculated.

From this factual material it is evident that the width of the anomalies over the veins of Darasun varies from the first tens of meters to the first hundreds of meters, and the amplitude—from the first hundreds of gammas to several thousand gammas. However, with all this, it should be especially emphasized that although the parameters characterizing the anomalies accompanying the veins vary over wide limits, most of them are close to the most probable form of the variation in intensity with the parameters: amplitude—800 gammas, width at half-maximum 38 m. The arithmetic mean value of the standard

deviations in amplitude amount to 250 gammas, and in width at half-maximum to 17 m.

In studying this material, it is not difficult to note a clear discrepancy between the intensity and width of the anomalies and the thickness of the quartz veins. According to calculated data, the most probable anomaly, for example, may be produced by a bed 27.2 m thick, occurring at a depth of 9.2 m and having a dip angle of \(65^\circ 20'\). Yet the thickness of the overwhelming majority of quartz veins at this deposit does not exceed 2–5 cm. Consequently, the form of the disturbance in the magnetic field accompanying quartz veins is governed not only by the vein material itself, but also by part of the near-contact rocks enclosing the vein.

Fig. 1. Forms of variation of \(\Delta Z\) over the veins of Darasun.

From the physicochemical point of view, the nature of the appearance of such anomalies over veins is generally known. As a result of hydrothermal and tectonic activity, the rocks directly in contact with vein formations are altered in such a way that their magnetic susceptibility is, for the most part, significantly reduced in comparison with unaltered rocks. The decrease in the magnetic susceptibility of the enclosing rocks occurs through the processes of chloritization, sericitization, and hypergenesis (the formation of zones of bleached rocks near ore bodies). During chloritization and sericitization, iron and magnesium are intensively removed from the enclosing rocks and replaced by sericite. A considerable part of the iron and magnesium goes into the formation of sulfide ores. At the same time, the divalent iron present in magnetite (accessory magnetite) is converted into trivalent iron (hematite, limonite) [1, 2, 3].

Fig. 2. Most probable form of variation of \(\Delta Z\) over the veins of Darasun.

On the other hand, the decrease in the magnetization of the rocks enclosing the veins occurs through mechanical stresses that took place during the formation of the vein fracture and especially during subsequent tectonic movements of rock blocks along these fractures [1].

A significant role in the formation of anomalies is also played by the fact that the rocks filling the fractures are themselves practically nonmagnetic. As a result of the study of the magnetic properties of the rocks and ores of the deposit

E. V. Kuzmicheva established that, within Darasun, the complex of gabbroid intrusions composing the western half of the ore field belongs to strongly magnetic rocks, whereas the eastern part, represented mainly by granites and syenites, belongs to magnetic rocks. To assess the magnetic susceptibility of rocks altered by hydrothermal and tectonic activity, determinations were made of the magnetic properties of samples taken along the Novo-Kuznetsovskaya and Glavnaya veins, in the Kapitalnaya adit, and in numerous boreholes. As a result, it was established that there is a direct statistical relationship between the degree of alteration of the rocks and the decrease in their magnetic susceptibility; the polar position in this relationship is occupied by the ores of the deposit and by practically unaltered rocks.

Therefore it is clear that, against the background of magnetic and strongly magnetic rocks, such formations as quartz-sulfide veins—which, together with the altered rocks, as a whole constitute a practically nonmagnetic body—are marked by reduced magnetic fields. At the same time, it should not be assumed that the form of the magnetic anomaly, since it is caused mainly by altered enclosing rocks, has no connection with the morphology of the ore veins themselves. On the contrary, the form of the disturbances and their intensity are, in the general case, closely connected with the morphology of the vein body, since the thickness of the altered rocks is consistent with the thickness of the vein formation.

Fig. 3. Distribution of the probability of the amplitude \(\Delta Z\) above the veins of Darasun.

Analysis of the materials of magnetic measurements carried out in the area

Fig. 4. Distribution of the probability of the width at the half-maximum of \(\Delta Z\) anomalies above the veins of Darasun.

well known at the deposit—the Glavnaya, Novo-Kuznetsovskaya, Lebedevskaya I, Oktyabrskaya, and Vodorazdelnaya veins—shows that, in the general case, the width and amplitude of the anomalies are consistent with the thickness of the ore bodies. In particular, in places where the veins have blowouts and are enriched in sulfides, where hydrothermal processes were manifested most clearly, the magnetic anomalies over the veins have considerable width and amplitude.

The character of the change in the amplitude of the anomalies, their form, and their intensity along the strike of the veins at Darasun was assessed for the five veins listed above. This characteristic was obtained from the results of magnetic-field measurements along profiles spaced 3–10 m apart and laid across the strike of these veins. Thus, the correlation coefficient for the selected characteristic forms of variation in intensity for each vein does not change appreciably (greater than 0.6) over a distance of 50–60 m along the strike of the vein; in places where there is a sharp change in the form of the intensity, it becomes very small (less than 0.3) for a comparison distance of several meters. The characteristic form was taken to be those changes in intensity at the vein which, over some segment of it (not more than 3–5 m), remained relatively constant.

Many researchers, analyzing the 1:5000 isodynamic-contour plan of this deposit, noted a characteristic feature in the structure of the magnetic field of Darasun—the presence of negative (relative to the conventional zero) magnetic anomalies spatially coinciding with separate sections of quartz-sulfide veins. Taking into account the comparatively sparse survey grid from whose results the isodynamic-contour plan of the deposit was compiled, the accuracy of the measurements (30–80 gamma), and the information presented above, it is not difficult to explain the reason for the appearance of these negative, isolated, and relatively isometric magnetic anomalies. They are caused by sections of veins intensely enriched in sulfides. These anomalies are separate parts of more extended, linearly elongated anomalies which, owing to the filtering property of the survey grid and to errors, were not fully recorded on this isodynamic-contour plan; as a result, an impression was created of separate intense negative anomalies. Some of these anomalies spatially coincide with disturbances of the natural electric field, which led some researchers to an incorrect interpretation of the nature of these anomalies [3], explaining the reason for their appearance by the action of natural electric currents that arise as a result of oxidation-reduction processes on the surface of sulfide bodies.

It is indisputable that magnetic fields created by natural current exist in nature and in this specific case as well. But their intensity is very small, and in practice these fields do not exert a strong influence on the form of the magnetic anomalies accompanying the veins. The presence of anomalies can be fully explained if calculations are made taking into account the change in the magnetic properties of the rocks and ores of the deposit.

REFERENCES

1. M. A. Grabovskii. Change in magnetic properties under the action of mechanical stresses. Izv. AN SSSR. Series of Geography and Geophysics, No. 2, 1949.

2. L. P. Zhogolev, I. T. Zhogoleva. Relationship of the statistical characteristics of the physical properties of rocks with processes of rock alteration. In: Problems of Ore Geophysics in Kazakhstan, Alma-Ata, 1966.

3. L. A. Zashchinskii. On the question of the magnetic effect of constant natural electric currents. In: Methods, Techniques, and Results of Geophysical Prospecting, Nedra, 1967.

4. O. D. Ivanov. Application of magnetic prospecting for outlining halos of wall-rock alteration of rocks. ONTI Bulletin, No. 3 (53), Nedra, 1964.