ON THE STUDY OF FRACTURE AND DISJUNCTIVE TECTONICS AT A GOLD-ORE DEPOSIT IN THE MARIINSK TAIGA BY MICROMAGNETIC SURVEYING

L. Ya. Erofeev

(Presented by Prof. D. S. Mikov)

The gold-ore deposit in question has been studied in detail. The ore bodies of the deposit are gold-bearing quartz veins. The enclosing rock is granodiorite. The veins within the ore field are distributed unevenly. This is explained by the fact that they are confined mainly to tectonic disturbances. The latter are concentrated in the form of belts, forming weakened zones.

At the deposit, three groups of tectonic disturbances are distinguished according to strike direction: northeast, northwest, and sublatitudinal. Their dip angle is 50–70°. Thickness varies from several centimeters to two or three meters, very rarely reaching 15–20 m.

The tectonic disturbances are accompanied by fracturing in the enclosing granodiorites. In weakened tectonic zones and near disturbances, the granodiorite is intensely fractured. Moreover, in these parts of the massif, fractures with an orientation consistent with the strike of the tectonic disturbance or tectonic zone are predominantly developed.

Six fracture systems have been established in the granodiorite massif. The first two systems have the same northeastern strike and differ from one another in intensity and dip angle. On the fracture diagram, the first is characterized by a clear maximum, combining up to 25% of the fractures. The attitude elements of this system are: dip azimuth 130–200°, dip angle 30–80°. The second system has the opposite dip at the same angle. The contour line of maximum value for this system combines points on which 5–10% of the fractures fall. The third and fourth fracture systems likewise have the same strike, 15–35°, with a dip angle of 40–85°. The third system dips to the north-northwest, the fourth to the south-southeast. The third system is expressed less distinctly than the first, but more clearly than the second and fourth. The fifth fracture system has a dip azimuth of 225–250° and a dip angle of 55–90°, while the sixth has a dip azimuth of 35–65° and a dip angle of 70–85°. These systems are expressed more weakly than the preceding ones.

At the deposit, among other geophysical investigations, magnetic measurements of various scales were carried out, including micromagnetic observations on plots measuring 20×20 m, on a 2×2 m grid and with a root-mean-square error of 1.8 gamma. In all, 180 such plots were investigated. Forty percent of the plots were opened by trenches. As a result of carrying out these works it was established that

With the aid of micromagnetic surveying it is possible to solve a number of problems associated with the study of fracture and disjunctive tectonics of the deposit.

First of all, we note that the above-mentioned tectonic disturbances are clearly recorded by micromagnetic surveying. On isoline maps they appear as bands of reduced values of \(\Delta Z\). As a rule, the contacts of tectonic disturbances correspond to areas of increased gradient.

It was established that the structure of the magnetic field of sites, if they are not located directly above an ore dike or above a tectonic disturbance as a whole, is determined by the fracturing of the bedrock, provided that the cover of loose deposits is homogeneous and of equal thickness throughout the entire site.

Figure 1 shows an isoline map of \(\Delta Z\) for one such site. The structure of the magnetic field of this site is rather complex. Isolines—

Fig. 1. Isoline map of \(\Delta Z\) in gammas.
1 — isolines, 2 — visible fractures with traces of tectonic clay, 3 — tectonic clay, 4 — granodiorite.

—of \(\Delta Z\) within the site in its northern part change their direction by almost \(90^\circ\). In the southern part of the site, deviations of the isolines from the latitudinal direction toward the southeast are observed. The isolines of meridional strike, clearly distinguished in the northern part, completely change their direction to latitudinal, without being traceable

in the southern part of the site. As a result of comparing the magnetic plan with the geological structure, it turned out that here there is an intersection of two clearly expressed systems of fractures of latitudinal and northeastern strike. Moreover, the northeastern-striking fractures wedge out within the site, while the latitudinal fractures are traced along the entire site. From the rose diagram of the orientation of the isolines it is also evident, as from the trench sections, that the fractures of latitudinal strike are most strongly developed at this site. The rose also rather clearly marks directions of isolines that do not coincide with the two principal ones. This is quite understandable—at the intersection of two systems of fractures, the rocks are destroyed in all directions.

As already noted, six systems of fractures are distinguished at the deposit, taking into account their dip angle and strike. It is obvious that, with the aid of a micromagnetic survey, fractures with the same strike but with different dip angles cannot be separated. Therefore, when studying the fracturing of the granodiorite massif by means of magnetic measurements, it should be borne in mind that at the deposit there are only three principal systems of fractures, differing from one another in strike angle and degree of development.

Indicative from this point of view is the result of statistical processing of 117 sites. The data of these studies are presented in the form of a general diagram of the orientation of isolines in Fig. 2. In

Fig. 2. Rose diagram of the orientation of isolines.

this diagram the northeastern direction is clearly distinguished. Along this direction at the deposit the first four systems of fractures and, correspondingly, tectonic disturbances are developed. In general, in the northeastern direction two maxima are quite distinctly outlined: the first of them, most strongly expressed, corresponds to the direction of the first and second systems of fractures. At the same time, a large range of angles is noted within which the strike azimuth of the fractures of the first

and second systems. The third and fourth fracture systems also found their expression in this diagram. Their principal strike direction coincides with a fairly clearly expressed maximum. However, as can be seen from the diagram, no sharp separation is observed between the first—second and third—fourth fracture systems. Evidently, in most cases it is possible only conditionally to establish to which system fractures belong if they have a northeastern strike. The northwestern direction, corresponding to the fifth and sixth fracture systems, is easily distinguished in the diagram shown. The degree of development of fractures of this strike is evidently much less than that of those described above. In addition, the diagram has a rather high “background,” probably due to the fact that at the deposit the granodiorite massif has fractures of all directions.

The intensity of rock fracturing from isoanomaly plans can be judged only if the depth of occurrence of the bedrock is known. As the thickness of loose deposits increases, as is known, the anomalous effect caused by fracturing decreases, and when the cover of loose deposits is 10–15 m thick at this deposit it disappears entirely. Fig. 3 gives an iso-dynams plan of an area on which, in addition to magnetic measurements, cross-shaped VES were set out.

Fig. 3. Iso-dynam plan of $\Delta Z$ in gammas.
1—observation points, 2—isodynams.

The sounding was carried out in order to determine the depth of occurrence of the bedrock, which proved to be 9–12 m. If the iso-dynam plan of this area is compared with the plan of the area cited above in Fig. 1, it is not difficult to note the sharp difference between the character of the fields of these areas. If in the area, Fig. 1, clearly

two principal directions of the isolines and a sharp gradient are observed in the magnetic field, then on the site (Fig. 3) it is not possible to judge the principal direction of fracturing, let alone its intensity. The example given also shows that loose deposits, in magnetic terms, constitute a homogeneous medium. At the deposit, the loose deposits are almost everywhere represented by a very homogeneous grus of granodiorite. Within the site, on gentle landforms, their thickness is practically the same and for the most part amounts to three to five meters. Under such conditions, the use of micromagnetic surveying can provide exceptionally valuable material for studying the fracture tectonics of the deposit. This, in fact, is the only method by which the fracturing of the granodiorite massif can be studied from the surface at this deposit.