Structure of the Magnetic Field of the Darasun Gold-Ore Deposit (Transbaikalia)

L. Ya. Erofeev

(Submitted by the Department of Geophysical Methods of Exploration)

At present, for the Darasun gold deposit there is a contour map of increments of the vertical component of magnetic-field intensity at a scale of 1:5000, compiled from materials of magnetic surveys of individual areas surveyed at different times. In a purely qualitative study of this map it may appear that the pattern of the deposit’s magnetic field is extremely complex and has no regular relation to its geological structure, as all previous investigators invariably pointed out. This, in fact, exhausted the characterization of the magnetic field of Darasun. We have attempted to distinguish, within this complex pattern, relatively homogeneous fields, to assess their internal structure, and to give an idea of the forms of variation in the magnetic-field intensity of the deposit. For this purpose, special measurements of the vertical component of magnetic-field intensity were carried out, together with the corresponding processing of the survey results.

The measurements were made within the studied part of the ore field along separate profiles several kilometers long, with a profile spacing of 2–3 m. The measurement error was ±3 gammas.

The observational data were processed according to the following scheme. Considering the magnetic field of the deposit as an area or system subject to study, the measurement results on each profile may be regarded as a random realization, and the set of profiles as a sample. The study profiles at the deposit were laid out so that they crossed the principal rock varieties composing it, all areas with characteristic structural features, and were oriented in different directions, which makes it possible to regard this sample as representative.

From the measurement data for the magnetic-field intensity on each profile, autocorrelation functions were computed, and from them the spectral density of the variance was computed. The spectral-density functions of the variance for each profile were then considered as separate realizations of a random process, for which the corresponding characteristics were determined.

The normalized autocorrelation functions obtained for all profiles of the high-precision magnetic survey can conventionally be divided into four groups, or four types of fields can be distinguished.

Figure 1 presents plots of the functions that most clearly reflect the principal features of the variation in magnetic-field intensity for each group.

The first group includes fields characterized by the form of the autocorrelation function shown in Fig. 1a. A characteristic feature of this type of field is the slow, relatively smooth decrease of the function, which indicates a close statistical relationship among changes in magnetic-field intensity along the profile.

For the second type of field at the deposit, a sharp decrease of the autocorrelation function almost to zero at small comparison intervals is characteristic, followed by an irregular monotonic change as the comparison interval is further increased (Fig. 1b). This form of autocorrelation function is found in distributions that include changes with different frequencies; moreover, these changes have commensurable parameters and are present in the distribution in equal shares. This can be observed more distinctly in the spectral-density graphs (Fig. 2a).

In analyzing the graphs of autocorrelation functions of the third type (Fig. 1c), attention is drawn to the presence, at certain distances, of negative values of the correlation functions. This indicates that the field structure of this type contains a definite element of periodicity; accordingly, at a distance approximately equal to half the period of the principal oscillations, a negative correlation is observed between the field-intensity values—positive deviations from the mean in one section correspond to negative deviations after a certain interval, and vice versa. The principal frequency of the intensity variation is clearly expressed in the spectral-density graph, Fig. 2b.

An idea of the fourth group of variations in the intensity of the vertical component of the deposit’s magnetic field can be obtained by studying the graphs of the normalized correlation functions of this type of field (Fig. 1d). Here, too, negative values of the correlation function are observed at certain distances. Evidently, in this case as well there is a definite element of periodicity in the field variations. However, in the behavior of the autocorrelation functions in this case there is also a significant distinction, namely: against the background of periodic oscillations of the first order, clearly expressed periodic variations of intensity of the second order are observed, whose oscillation period is 2–3 times smaller than that of the principal intensity variations. This is also noticeably reflected in the spectral density of the process dispersion (Fig. 2c).

The division of the field of the deposit into types in this manner is undoubtedly conditional in character. It is clear that there can be no sharp boundary between the fields distinguished. Some profiles can confidently be assigned to one group or another, others only with difficulty, and some profiles may with equal justification be assigned to two groups. Nevertheless, this division still shows that within the deposit there are relatively homogeneous fields that differ qualitatively from one another.

An idea of what variations in the intensity of the vertical component of the magnetic field predominate at this deposit can be obtained from the form of the curve of the spectral density of dispersion shown in Fig. 3.

Four maxima are clearly indicated on the spectral-density graph. This indicates that, at the Darasun deposit, variations in the intensity of the vertical component of the magnetic field tend to be dominated by variations grouped around oscillations with wavelengths at the maxima of 330, 77, 36, and 17 m.

Low-frequency variations in the intensity of the magnetic field of the depos—

Fig. 1. Plots of normalized autocorrelation functions.

Fig. 2. Plots of the spectral density of dispersion.

deposits, as follows from the spectrum presented, are the most frequently encountered at the deposit. The width of these anomalies in the adopted understanding is several hundred meters; the most frequently encountered anomalies of this group have a width on the order of 150–200 m. The intensity difference, or, equivalently, their double amplitude, fluctuates within a fairly broad interval, the value of which is 12–15 thousand gammas. The latter is easily established from the plan in isodynams. However, the mean amplitude of the largest anomalies is only 1500 gammas. The appearance of these anomalies at the Darasun deposit is due to its

Fig. 3. Generalized graph of the spectral density of variance.

block structure. The same group includes part of the variations in magnetic-field intensity caused by the most massive sections of ore veins and by individual sections of rocks intensely enriched in ferromagnetic minerals.

The second-largest variations in intensity at this deposit are anomalies having a width of 50–150 m and an amplitude of several hundred gammas. Variations in intensity of this order characterize the saturation of the deposit area with quartz-sulfide veins, dikes, and faults. It should be emphasized that the anomalies caused by these formations, in comparison with the other variations in intensity distinguished in the spectrum of the deposit field, have a larger range of variation in the parameters that characterize them.

Very characteristic variations in the field intensity of the deposit are anomalies grouped near oscillations whose wavelength most often amounts to the first tens of meters, and whose amplitude is the first hundreds of gammas. They can be clearly observed on the overwhelming majority of profiles of the detailed magnetic survey. They constitute, as it were, the main background of the magnetic field and appear as a “sawtooth” pattern. These variations at Darasun are caused mainly by the presence of numerous feathering fractures accompanying large faults.

Higher-frequency variations in the magnetic field of the deposit, with amplitudes from several tens to the first hundreds of gammas and anomaly widths of several meters, are a substantial component of the background. They are marked in the spectrum by the fourth maximum. They can also be easily observed on graphs of intensity variations along all profiles of the high-precision magnetic survey. The main cause of variations in magnetic-field intensity of this order is the nonuniformity of the magnetic properties of the rocks composing the ore field of the deposit.

Until now we have been discussing disturbances in the magnetic field recorded on magnetic-survey profiles, which represent it in a single section. Therefore, the question naturally arises as to what the identified forms of anomalies look like in three dimensions. An idea of the forms of the anomalies, in particular of the largest ones, can be obtained from analysis of the isodynamic map. The complete functional representation on it includes all those forms of variation in the magnetic-field intensity whose oscillation period is not less than 3–5 times the distance between the observation points (the map was constructed from measurements on a 40 × 20 m grid), i.e., mainly anomalies of the first and partly of the second order. All other variations are represented on this map by changes of a random character. They have a statistical connection with the map pattern. This connection is the weaker, the smaller the oscillation period of the intensity. An objective quantitative characteristic of the forms of anomalies can be obtained by studying the character of the course of the isodynamics.

Such an assessment can be made from the results of statistical processing of the isodynamic directions, the data for which are given in the form of a rose diagram in Fig. 4. The form of the diagram of isodynamic directions convincingly indicates that, at least, the first- and second-order anomalies of the Darasun deposit have linearly elongated forms, and in most cases they are oriented in two directions—northeast and northwest. In other words, the variation in the intensity of the vertical component of the magnetic field of the deposit in this frequency range possesses anisotropy. The anisotropy of the field is determined by the structural features of the deposit. This is readily verified by comparing the rose diagram of isodynamic directions (Fig. 4) with the rose diagram of the directions of veins, dikes, and tectonic dislocations of this deposit (Fig. 5).

Fig. 4. Rose diagram of isodynamic directions, $\Delta Z$.

Taking into account the nature of the occurrence of higher-order anomalies in the ore field of the deposit, it may be assumed that these intensity anomalies also most often have irregular forms and ordered orientation. Their axes are oriented in the same directions as the axes of first-order anomalies, since the sharply expressed structures of the deposit (NE and NW directions) are almost perpendicular to one another.

Fig. 5. Rose diagram of the directions of veins, dikes, and tectonic dislocations.

to one another (Fig. 5), and the magnetic inhomogeneities associated with them have linearly elongated forms and subparallel strike.

With this representation of the anomaly shapes, the maxima in the spectrum of variation of the magnetic-field intensity correspond to doubled mean values of the cross section of the dominant anomalies; the values to the left of the maxima correspond to the largest axes, and those to the right to the smallest; the ratio of the major axis of the anomalies to the minor one is 2–5 units.

Thus, in the complex magnetic field of the Darasun deposit there is a clearly pronounced predominance of four types of variations in magnetic-field intensity, and these anomalies have an anisometric shape and an ordered orientation.

It should be noted that in the present work the discussion concerned that range of frequencies of intensity variation which is usually studied when magnetometric work is carried out within the ore fields of gold deposits.