Capabilities of Magnetic Surveying under the Conditions of the Aprelkovo Gold Ore Deposit (Transbaikalia)

V. K. ROZMYSLOV, L. Ya. EROFEEV

(Submitted by the Department of Geophysical Prospecting Methods)

At present, in detailed studies of vein-type gold-ore deposits, geophysical prospecting methods are widely used. Most often, various modifications of the resistivity method are employed, chiefly electrical profiling. It is usually carried out for the purpose of searching for and tracing vein formations and tectonic disturbances. At the same time, the problems of mapping individual rock masses differing in specific resistivity are solved along the way. At deposits where the ore bodies contain a significant amount of sulfides, along with the resistivity method, the induced-potential method and surveying of the natural electric field are used to search for places of the greatest concentration of these minerals. More rarely, the same problem is solved with the aid of methods based on the study of artificially generated alternating electromagnetic fields. The effectiveness of all these methods at different gold deposits varies. Especially unfavorable for the application of electrical prospecting are gold deposits localized in masses of intrusive rocks and represented by low-thickness quartz-sulfide veins. With electrical profiling under such conditions it is possible to establish only the position of major tectonic disturbances.

Gold-bearing veins, dikes, and rock masses differing in petrographic composition do not create a noticeable anomalous effect in volume fields of direct current. Conditions for direct electrometric methods of searching for sulfides here are likewise unfavorable. A typical example in this respect may be the Aprelkovo gold deposit, the greater part of whose quartz-sulfide veins is localized in a diorite mass. The results of experimental-methodological work carried out here in 1958 showed that electrical prospecting methods, at best, can be used for tracing the most extensive ore veins.

At deposits of this type, as follows from the experience of a number of investigations, it is more expedient to use detailed magnetic surveys [1, 2]. We undertook an attempt to determine the capabilities of this method at the Aprelkovo deposit. As a result it was established that magnetometry can be used to solve a wide range of problems connected with the study of this ore field.

1. Thus, with the aid of magnetometry it is possible successfully to search for and trace gold-ore veins, all varieties of dike formations, and tectonic disturbances of various orders. All these forma-

zation are accompanied in the magnetic field by distinct, linearly elongated anomalous zones. The gold-bearing quartz–sulfide veins create negative anomalies (relative to a conventional zero). Despite the fact that the thickness of the veins here is small (centimeters, the first tens of centimeters), the intensity of the anomaly is considerable. The averaged anomaly, for example, obtained from the results of measurements over 50 known veins of the deposit, has an amplitude of 500 gammas and a half-maximum width of 30 m (Fig. 1, c). This is due to the fact that an anomalous

Fig. 1. a — diagram of the orientation of isoanomalies over diorites; b — over metamorphic rocks; c — averaged form of the magnetic anomaly over the ore veins of Aprelkovo.

effect in this area is produced not only by the rocks directly composing the ore body, but also by part of the contact-altered enclosing rocks. Since the thickness of the Aprelkovo ore veins does not remain constant, the anomalies caused by them have a substantially unstable form of variation in intensity along the strike. The same anomalous zones in the magnetic field accompany unmineralized tectonic disturbances and dike bodies, with the sole difference that the form of their anomaly along the strike axis is more stable. All of them, except for diorite-porphyry dikes, are marked by negative linearly elongated zones. Diorite-porphyry dikes are accompanied by positive values of intensity.

The structure of the magnetic field of the Aprelkovo deposit is such that anomalies caused by veins, dikes, and tectonic disturbances in a single section have parameters similar to variations in intensity caused by other magnetic inhomogeneities, which are usually classified as “noise.” In other words, on a single profile, from intensity-measurement data under the conditions of Aprelkovo, it is impossible to indicate unambiguously the sought anomalies; that is, it may be considered that the signal-to-noise ratio here is close to unity. In such a situation, the question of selecting a survey grid when searching for vein formations and tectonic disturbances by means of magnetic measurements becomes essential. For the averaged anomaly we calculated the grid by the “ideal observer” method [3]. According to the calculation, the survey spacing should be in the interval of 4–6 m, and the distance between survey profiles 20–30 m. The validity of the recommended measurement grid was checked by us in the most studied part of the ore field (the area of the Chertovaya vein) of the Aprelkovo deposit, over an area of 1.5 sq. km. From the measurement data it was possible not only to confidently outline the location of the previously known veins of the “ore cluster” of the Chertovaya vein,

but also to obtain some information on the morphology of the disturbing bodies from the nature of the change in the form of the anomaly along their strike. Especially clear results in this respect were obtained for the Chertovaya vein. Here all known blowouts are marked by intense changes in intensity; at constrictions the anomalies are less pronounced.

1. Within the ore field of the deposit, gold-bearing veins and most dikes are distributed unevenly. They are localized in small areas, called by some investigators “ore nodes.” The magnetic field in the area of such “nodes” has a character substantially different from the fields of “barren” rocks. In general, this difference is manifested in the fact that the graph of the vertical component of the magnetic field over ore nodes is more “dissected” than over the enclosing rocks. Consequently, by the degree of dissection of the graphs of the vertical component of the magnetic field, prospective areas can be distinguished. This feature of the function can be quantitatively assessed by calculating the sample variance (Fig. 2). Essential in determining it is the choice of the calculation interval. According to our data, the optimal interval at the Aprelkovo deposit is a segment equal to 250–300 m. All this makes it possible to recommend magnetic surveying for the search for “ore nodes” within such deposits. In this case, the distance between survey profiles must obviously be commensurate with the area of the nodes, and the measurement spacing with the degree of dissection of the fields. In the Aprelkovo ore field, searches for “nodes” can be carried out by means of magnetic measurements performed on a 5×100 m grid.

2. Studies of the magnetic field of the deposit also showed that magnetometry can provide substantial assistance in mapping individual rock bodies; in particular, it can be used to determine the position of the contact between intrusive and metamorphic rocks. Earlier, in 1958, attempts were made here to use geophysical methods for these purposes. As a result, it was established that these formations cannot be separated by electrical profiling. The contact is also not noted from profile magnetic-survey data. The latter is likewise confirmed by our investigations. The mean value of the magnetic-field intensity over bodies of diorites and over metamorphic rocks of various composition in this area is the same.

A detailed study of the structure of the magnetic field in the area of the deposit, however, made it possible to note that the magnetic fields over different rocks of the area, having the same susceptibility, possess different anisotropy in the change of intensity. Moreover, this difference is so substantial that, by this field parameter, the mapping problem can be solved with confidence. This was established from analysis of the fields of micromagnetic measurements carried out on 34 plots (20×20 m). The results of statistical processing of the direction of isolines for typical plots for intrusive and metamorphic rocks, in the form of rose diagrams, are given in Fig. 1, a, b. As is evident from the figure, plots located within metamorphic rocks are characterized by one clearly expressed direction of isolines (Fig. 1, b), whereas the diorites have three such maxima in the diagram of isoline directions (Fig. 1, a).

1. In comparing the data on determining the anisotropy of the magnetic field with the geological structure of the deposit, it was established that the anisotropy of the field is closely connected with its fracture and joint tectonics. This makes it possible to recommend the use of micromagnetic surveying for the study of these elements. However, it should be borne in mind here that, with the aid of micromeasurements under the conditions of Aprelkovo, it is possible

Fig. 2. 1 — ore veins; 2 — diorites; 3 — observed \(\Delta Z\) curve; 4 — sample-variance curve; 5 — diabase dikes.

Axis labels visible in the figure: \(\Delta Z\), m; \(D\). Scale marks: \(0, 750, 1500\); \(150 \cdot 10^3\). Profile marks: \(200, 400, 600, 800, 1000\).

to delineate only the principal strike directions of the fracturing; however, it does not appear possible to assess the intensity of its development over a considerable area, since the thickness of the cover of unconsolidated deposits in the ore-field area is variable.

Reliable information on the relative intensity of development of fracturing in one direction or another can be obtained only on small sites, the dimensions of which should not exceed several hundred square meters.

REFERENCES

1. L. Ya. Erofeev. Application of magnetic surveying in studying the structure of ore fields. In: Proceedings of the Scientific Seminar on the Study of Gold-Ore Deposits of Siberia, Tomsk, 1966.

2. L. Ya. Erofeev. On the study of fracture and disjunctive tectonics of one gold-ore deposit in the Mariinsk taiga. Izv. TPI, vol. 151, Tomsk State University Press, 1966.

3. A. A. Nikitin et al. On assessing the reliability of delineating weak geophysical anomalies by the inverse-probability method. Exploration Geophysics, No. 15, Gosgeoltekhizdat, 1966.