ON THE METHODOLOGY OF MAGNETIC STUDIES OF THE STRUCTURE OF THE ORE FIELD OF A GOLD-ORE DEPOSIT IN THE MARIINSK TAIGA

L. Ya. EROFEEV

(Presented by Prof. D. S. Mikov)

The gold-ore deposit under study is located in a granodiorite massif. Throughout its entire area, dikes of aplite and spessartite, as well as quartz and pegmatite veins, are widely developed.

The tectonics of the deposit is complex. The deposit abounds in disjunctive disturbances of varying thickness and spatial orientation.

The distribution of quartz veins—both gold-bearing and with low metal content—within the deposit is uneven. Most of the quartz veins are localized in so-called “ore nodes.” The best-known and most clearly expressed of these nodes adjoin operating shafts. A characteristic feature of their structure is that here quartz veins of latitudinal and northeastern strike intersect. The area of these “nodes” does not exceed 0.1–0.2 sq. km.

The thickness of quartz veins, dikes, and tectonic disturbances varies from several centimeters to 2–3 m, rarely reaching 5–10 m. Moreover, the thickness along strike of individual veins, dikes, and disturbances does not remain constant.

Loose deposits are developed everywhere. The depth of occurrence of the bedrock varies within the limits of 3–12 m. The thickness of the loose deposits is often consistent with the relief of the terrain.

Magnetic surveys of various scales were carried out at the deposit: an areal survey at scales of 1:50000, 1:1000, and 1:200 (microsurvey), and a profile survey with an observation interval along the profile of 2 m.

The measurement error of the small-scale work is 8–30 gammas, and that of the micro-measurements is 1.5–2 gammas.

As a result of carrying out these works, in particular, it was established that quartz veins, dikes, and tectonic disturbances create rather distinct negative anomalies in the magnetic field of the deposit. The intensity of the anomalies fluctuates within the limits of 15–150 gammas, and the width of the anomalies is 1–15 m. With rare exceptions, very thick faults and dikes are distinguished by an anomaly on the order of 200–300 gammas and have a width of up to two hundred meters.

It should be emphasized that at this deposit the unevenness in the thickness of the cover of loose deposits creates anomalies with an intensity of 15–500 gammas and a width from several meters to several hundred meters, i.e., of the same order as the anomalies caused by the geological structure.

With such a relationship of anomalies, it is very important to determine a sufficiently effective observation method, both for searching for and tracing individual geological formations and for studying the structure of the deposit.

To select the most rational method for magnetometric work, investigations were carried out along 10 profiles in one of the areas of the deposit. The distance between profiles was 200 meters; observation points along a profile were spaced every 2 m. Measurements were made according to the scheme: initial point—10 working points—initial point. This made it possible to carry out the work with a mean-square error of 1.8 γ. The M-18 instrument was used for the observations.

Figure 1 shows graphs of \(\Delta Z\) of the magnetic field for two profiles relative to an arbitrary zero. It is clear from the figure that on individual profiles quite distinct anomalous sectors are distinguished. The magnitude and shape of the anomalies vary. No regular relationship is observed between the anomalies of one profile and those of the others.

Fig. 1. Graphs of \(\Delta Z\). 1—graph of \(\Delta Z\), 2—terrain relief, 3—micro-survey site, its number, trench and number.

Changes in \(\Delta Z\) within the field of an individual profile can be divided into two main groups: the first includes anomalies 50–150 m wide and more; the second, anomalies with a width on the order of 10–50 m. At the same time, the first group has a larger difference in intensity.

In interpreting the nature of the anomalies, a careful study of the terrain was carried out, with the relief of the terrain plotted along the profile. By comparing these data with the magnetic graphs, it proved possible to classify a number of fluctuations as noise. Some of them were caused by dumps of bedrock from old mine workings (shafts, trenches), and some by outcrops of bedrock at the surface.

When the magnetic graphs were compared with the relief of the profile, it turned out, as can be seen from Fig. 1, that positive forms of relief correspond to a general increase in the intensity of the magnetic field, and negative forms to a decrease.

It is evident that the large anomalies on these graphs are caused by variations in the thickness of the loose deposits.

However, anomalous sections remained on the profiles whose nature could not be explained from the positions indicated above.

To determine the nature of these anomalies, areal micromagnetic measurements were carried out on the anomalous sections of the profiles (a 20×20 m site). Some of these sites were opened by trenches, as a result of which it was established that these disturbances are caused by dikes, veins, and tectonic faults.

As an example, Fig. 2 gives an isoline plan of sites 137 and 138. This plan is divided into two parts by a belt of increased gradient. From experience of previous years it is known that the left part of the sites is composed of more

Fig. 2. Isoline plan of \(\Delta Z\). 1 — granodiorite; 2 — intensely altered granodiorite; 3 — tectonic clay; 4 — isodynamics; 5 — spessartite.

intensely fractured rocks, which are accompanied by veins and dikes. Trench 4420, driven for verification purposes, exposed weathered granodiorites, in places altered to clays; in the left part a spessartite dike is noted.

The structure of the magnetic fields of sites 142, 172, 192, 189, and 135 is analogous to the field of sites 137 and 138. In the trenches on these sites, either dikes or tectonic disturbances were encountered. Similar fields, but with a less clearly expressed anomalous effect, are observed at sites 141, 136, and 177.

The field of sites 133 and 134 is caused by an old mine working (a shaft), not noticed during the survey of the locality. The isolines on these sites clearly delineate the location of the shaft collar and its dumps.

From the example given, important recommendations can be made concerning the methodology of applying magnetic surveying to the study of the structure of a deposit field.

An important point in investigations is the survey spacing. It is known that, for the reliable delineation of an anomaly, it is necessary to have several points characterizing it. Changes in the magnetic field caused by a vein, a dike, or a fault zone have an average width of 15–30 m.

It is clear that, to delineate such anomalies, observations must be made every 5–10 m.

Thus, at the deposit, an areal magnetic survey was carried out over two fairly large areas on a 20 × 10 m grid, with an accuracy of 7–10 γ.

We do not present here the isodynamic plans for these areas, noting only that, as a result of these investigations, only a thick dike (apparent thickness, together with the zone of intensely crushed rocks, 10 m) and a major tectonic disturbance (apparent thickness about 20 m) were found. These formations, especially the fault zone, stand out quite clearly on the isodynamic plan. The structure of the magnetic plans of these areas as a whole, however, is very complex. It abounds in bands of increased gradient, differing in shape and intensity.

When many anomalous areas on these plans were checked, it turned out that the structure of the magnetic field of these areas is determined mainly by variations in the thickness of loose deposits. Nevertheless, the veins, dikes, and disturbances that are undoubtedly present in these areas (with the exception of the two mentioned above) found no reflection on the isodynamic plans, solely because the distance between the observation profiles was relatively large and, on the other hand, the accuracy of the survey by no means ensured the detection of “small” anomalies.

It was noted above that anomalies on one profile cannot be traced on adjacent profiles. This is explained by the fact that anomalies caused by local disturbances, naturally, should not correlate; and changes \(\Delta Z\) in the magnetic field associated with variations in the thickness of loose deposits also do not correlate, because the relief line along each profile is not the same. Anomalies caused by geological formations likewise are not repeated on the profiles. The reason for this is mainly the comparatively large (200 m) distance between profiles, at times exceeding the length of the dike or vein, or the structure of the object sought, along strike.

On the basis of these results, the following methodology is recommended for conducting magnetometric work at this deposit.

When carrying out investigations in unexplored areas, it is more rational to conduct the survey along profiles spaced 4–6 m apart, to place observation points along the profile every 2–3 m, and, in anomalous areas, to carry out detailing either by a series of short profiles (2–4 profiles) or by micro-survey sites.

Anomalous areas on the profiles must be selected with allowance for the relief of the terrain and the data of a careful visual study of the locality.

The survey should be carried out during the period when there is no snow cover (in places it reaches up to 12 m), since it introduces a strong distortion into the character of the observed field and will make the study of the locality difficult.

The tracing of individual quartz veins and dikes may be carried out by short profiles (20–30 m) with a spacing of 1–3 m.

The study of the structure of ore nodes and the morphology of individual quartz veins requires more detailed work. Observations should be conducted in “strips” 20–30 m wide, or even by continuous surveying on a grid from \(2 \times 2\) to \(4 \times 4\) m.

The distance between individual strips should be taken as 15–30 m. The accuracy of the survey in all cases must be as high as possible. The root-mean-square error should not exceed 2–3 gammas. To ensure such high-precision work, it is recommended that measurements be made with a return after every 10–15 points.

Areal micromagnetic measurements should be carried out with the selection of a KT for each area, followed by the tying together of the KTs of the areas.