STRUCTURE AND MINERALIZATION PROSPECTS OF THE NORTHWESTERN VICINITY OF A GOLD DEPOSIT

(BASED ON MAGNETIC SURVEY DATA)

L. Ya. EROFEEV, Z. A. MYSHKO, G. G. NOMOKONOVA, L. V. LUKASHKINA

(Presented by the Department of Geophysical Prospecting Methods)

The area of the medium-temperature hydrothermal deposit of the gold-sulfide formation is composed of various intrusive rocks, among which seven phases are distinguished, intruded in the following sequence: gabbro-peridotites, diorites, granodiorites, porphyritic granites, syenites, leucocratic granites, and granite-porphyries. The ore bodies are represented by veins. The composition of the ore veins, owing to the development of several stages of mineralization, is complex. The deposit is located in a large junction of intersection of structures of different ages with northeastern and northwestern strikes.

A study of the nature of the relationship between the magnetic field and the geological structure of this deposit made it possible to establish that, in the area, magnetic surveying can be used for a detailed investigation of the structural framework of the areas and for assessing their mineralization prospects. For this purpose, an areal magnetic survey was carried out in the northwestern part of the ore field of the deposit (Fig. 1).

Within the surveyed area, the magnetic field very clearly showed characteristic changes in the vertical component of intensity, represented on contour maps by linearly elongated anomalous zones. Among them, by their linear dimensions, three anomalies are readily distinguished (1, 2, 3, Fig. 1). They constitute, as it were, a framework around which smaller, accompanying anomalies are formed. At the same time, zones 2 and 3 extend almost parallel to one another, having a strike azimuth of about 60°. Anomalous zone 1, extending at an azimuth of 40°, cuts anomalies 2 and 3. All three zones are traceable over distances of several kilometers.

In addition to these three large, nearly rectilinear zones and their accompanying anomalies with sublatitudinal strike, anomalous zones with northwestern strike are observed in the area. The linear dimensions of the latter are intermediate between the largest anomalies and anomalies having a nearly latitudinal strike and small extent (the first hundreds of meters).

According to the form of the increase in intensity and the character of its variation along the strike of the anomalies, all these zones may be divided into two groups. Some are characterized by relative stability in the law of change of intensity along the strike of the anomalies. On the contour map they are represented, to a first approximation, by trough-shaped negative (relative to the conventional zero) anomalies. The correlation coefficient of the form of intensity variation along the axis of the anomaly

— Quartz-sulfide veins

--- Axes of magnetic inhomogeneities

▟ — Survey-area outline

Fig. 1. Layout of the quartz-sulfide veins of the deposit and the axes of the main magnetic inhomogeneities.

remain substantially constant (greater than 0.6) over a comparison distance equal to several hundred meters. The decrease in intensity in this case does not exceed 500 gammas (Fig. 2a).

Another group of anomalous zones is characterized by a more complex law of intensity variation. The same anomalous zone of this group, on different profiles, may have any curve shape of intensity variation among the curves shown in Fig. 2b. The graph of the increment of anomaly intensity is represented in one limiting case by a curve with three extreme points—two identical maxima and one minimum; in the other, the curve has two extreme points (a maximum and a minimum). This intensity variation may also be represented by a transitional curve. The latter curve, like the first, has three extreme points, but its maxima are not identical. When moving along the axis of such an anomalous zone, a transition from one curve shape to another is observed, so that only the anomaly minimum remains relatively constant. The change in the shape of the curve in this case occurs rapidly. Thus, when comparing curves of the change in the intensity increment obtained on profiles cutting across the strike of the anomaly and spaced from one another by a distance equal to the first hundreds of meters, substantial differences in their configuration are revealed (the correlation coefficient for this distance is less than 0.5). Anomalies of this type differ substantially from anomalies of the first group not only in the complex character of the intensity variation, but also in the magnitude of its variation. The intensity drop here is enormous—several thousand gammas; the width of the anomalies is several tens, or the first hundreds, of meters. This group includes mainly anomalies of northeastern and sublatitudinal strike.

A noteworthy feature of the magnetic field of the study area is also that the area of development of linearly elongated anomalous zones of both groups is clearly marked by the degree of “dissection” of the field.

At present there is scant information on the geological structure of the northwestern part of the ore field of the deposit. In this connection, when analyzing the structural features of the structure of this area from magnetometry data, we shall rely on the best-studied sector both from the geological point of view and with respect to the relationship between the character of the field and the geological structure. Such a sector here is the ore field of this deposit.

When the area of localization of the axes of the main anomalous zones of the survey area is compared with the ore field (Fig. 1), it is not difficult to note the striking similarity of the tectonic structure of the deposit to the pattern of distribution of the main magnetic inhomogeneities. This similarity is indicated by a set of features.

1. At the deposit there are three principal strike directions of tectonic structures. These are northeastern—the principal direction; then structures of northwestern and, finally, sublatitudinal strike.

The same is observed in the strike of the axes of linearly elongated anomalous zones, and in the same proportions.

1. The deposit is characterized by the presence of several major northeast-trending faults, which, by their linear dimensions, stand out sharply in the ore field. Such here are the tectonic faults enclosing veins C, HK, and G. In the study area, anomalous zones of northeastern strike are likewise dominant.

2. The tectonic structures of the deposit that strike in a northwestern direction are most often bounded by northeast-trending faults. The same pattern can be

Fig. 2. Graphs of $\Delta Z$: a — from anomalous zone 1; b — from anomalous zone 2.

Visible labels: prof. 39; prof. 12; prof. 43; prof. 17. Scale bars: 20 m; 300 γ; 1000 γ.

be seen also in the character of the interrelationship of the linearly elongated anomalous zones within the survey area.

1. The numerous anomalous zones of sublatitudinal strike, for the most part short in extent, correspond in the ore field of the deposit to tectonic disturbances (also numerous and often mineralized) of the same direction and of slight extent.

2. When the area of development of the linearly elongated anomalous zones is compared with the ore field, a very important detail is noted, as it were linking these areas. This is represented by tectonic structures of submeridional direction. In the ore field and in the study area there are few tectonic disturbances and anomalous zones with such a strike. In particular, within the deposit the SE fault has a nearly meridional strike (Fig. 1). If it is mentally extended beyond the limits of the studied part of the ore field, it coincides with anomaly 7, which farther along anomalous zone 8 may be connected with anomaly 4. These three anomalies and the SE fault form a zone crossing both areas. There are no other anomalies of such strike or well-expressed submeridional structures within the ore field of the studied area.

Thus, the character of the arrangement and interrelationship of the anomalous axes in the studied area has many features in common with the tectonic structure of the ore field of the deposit.

It was noted above that, according to the form and intensity of the change in intensity, the magnetic anomalous zones can be divided into two qualitatively different parts. This indicates the different nature of the occurrence of the anomalies. Linearly elongated negative anomalies (relative to the conventional zero), as follows from the experience of previous studies, are accompanied in the magnetic field of the deposit by tectonic disturbances without noticeable signs of hydrothermal activity. The disturbing object here consists of crushed, intensely kaolinized rocks filling the rupture disturbance and part of the intact enclosing wall rocks, which, owing to dynamic stresses previously applied to them, have become practically nonmagnetic. The same anomalies may accompany dikes of nonmagnetic rocks and low-sulfide quartz veins. To these may be assigned the disturbing objects responsible for anomaly 1, the southwestern flank of anomaly 2, and entirely anomalous zones 6, 7, 8, 9, 10, 13, 18, 19, 20, 21, 22 (Fig. 1).

Anomalies of another type, occurring in the study area within the limits of the deposit, are caused in the magnetic field by thick quartz-sulfide veins. As an example, Fig. 3 gives graphs of the change in the vertical component of the magnetic-field intensity over the quartz-sulfide veins NK and YuK. The probable scheme of the disturbing formations in this case is as follows. It is assumed that an anomalous zone with two maxima and a minimum between them (Fig. 2b) represents the sum of three anomalies. The minimum of the anomaly is caused by practically nonmagnetic rocks directly composing the ore body. The two lateral positive anomalies are caused by hydrothermally altered enclosing rocks. For this deposit, as indeed for other gold-ore deposits, it is known that the character and degree of alteration of the wall rocks along the strike of an ore vein do not remain constant. This accounts for the pattern of change in intensity that is observed when studying the form and intensity of the anomaly along its strike.

In these conditions, contacts of basic rocks with acid...

Fig. 3. Graphs of $\Delta Z$ over the YuK and NK veins.

Visible annotations in the figure: profiles 1–12; scale $1000\gamma$, 20 m / 20 m; “YuK”; “vein NK”; “zh s” [[unclear: handwritten abbreviation]].

... The variability in the form and intensity of the anomaly along its axis may, in this case, be caused by an abrupt change of rocks in the contact zone, owing to the uneven development of hybrid formations.

Anomaly 3, most of the anomalies that abut it with sublatitudinal strike, and also the northeastern flanks of anomalies 3 and 2 may be assigned to this group of anomalies within the study area. Quantitative interpretation carried out for some anomalies of this type showed that the disturbing bodies here occur at a shallow depth, equal to 5–10 m. Their dip angle is 50–60°, and their thickness is in the first tens of meters.

In assessing the prospects of the studied area for the discovery of mineralization here, and in summing up all that has been set out above, the following may be noted.

The distinctive structure of the magnetic field of the area undoubtedly indicates that this area is situated in a major junction of intersecting tectonic disturbances of different ages, with northeastern, northwestern, and nearly latitudinal strikes.

The tectonic junction identified has many features in common with the structural setting of the ore field of the deposit, and the character of the manifestation of the elements of this junction in the magnetic field is closely connected with the peculiarities of changes in intensity caused by vein formations and tectonic disturbances of this deposit.

All this indicates that the studied area of the northwestern environs of the ore field is highly promising for the discovery of large quartz-sulfide veins.