EXPERIENCE IN APPLYING MICROMAGNETIC SURVEYING TO STUDY THE STRUCTURE OF THE MAGNETIC FIELD OF A GOLD ORE DEPOSIT IN THE MARIINSK TAIGA

L. Ya. EROFEEV

(Presented by Professor D. S. Mikov)

The gold ore deposit is located within a large intrusive massif composed mainly of granodiorites, among which biotite and biotite-hornblende varieties are distinguished.

The vein rocks are represented by veins of aplite and pegmatite and by dikes of spessartite. In addition, ore-bearing and barren quartz veins are widely developed at the deposit.

The fracture tectonics of the ore field has not been studied sufficiently. At present, six systems of fractures are distinguished; fractures of northeastern and latitudinal strike are the most highly developed.

Disjunctive disturbances are widespread over the area of the ore field and significantly complicate the geological structure. They have various spatial orientations, and on this basis they are divided into three groups.

Loose deposits are developed everywhere. Their thickness is 3–12 m. No regular relationship is observed between the thickness of the loose deposits and the relief of the area.

The physical properties of the rocks and ores at the deposit have not been studied.

In various places of this deposit, in different years, geophysical methods were tested, mainly for purposes of geological mapping. Within the complex of these investigations, magnetic measurements were also used, both areal and along individual profiles. All these investigations, including magnetometric ones, proved ineffective.

In 1963 an experimental micromagnetic survey was carried out at this deposit in order to determine the possibility of applying it for mapping and studying the structure of the ore field.

The survey was carried out on a 2 × 2 m grid on sites measuring 20 × 20 m, located randomly throughout the ore field. Measurements were made according to the scheme CP–5–6 working points—CP. The control point was selected in the middle of the site. The root-mean-square error of the survey was 1.5 gammas. In all, 117 sites were worked. Some of them were located in known geological conditions, while others were exposed by trenches.

As a result of comparing the magnetic fields with the geological structure of the sites, it turned out that magnetic surveying can solve a number of tasks in studying the geological structure of the ore field. For example,

one can quite reliably carry out searches for, and tracing of, major faults. Fault zones, as a rule, are marked in magnetic fields by reduced relative field values, while the walls of faults are marked by increased gradients. The difference in magnetic-field intensity between normal granodiorites and rocks in the fault zone reaches 100–500 gammas. Such major faults have a width of 3–30 m and a sufficiently great extent.

Figure 1 shows an isodynamic map of the vertical component of the magnetic field for one of the sites (area 400 sq. m, observation grid \(2 \times 2\) m), situated on the wall of a tectonic fault. Trench 4115, driven for verification purposes, exposed weathered medium-grained granodiorites of biotite–hornblende composition, cut by jointing cracks in various directions, and very crushed, kaolinized, epidotized, and chloritized granodiorites. In the fault zone, the granodiorites have in places been ground down to clays.

Fig. 1. Isodynamic map: 1 — observation points; 2 — disrupted, kaolinized, and epidotized granodiorite; 3 — normal granodiorite; 4 — tectonic clay

From the isodynamic map, the contact of the crushed, kaolinized rocks with the normal granodiorites is confidently outlined, and its strike is clearly determined. The anomaly caused by this disturbance amounts to 400 gammas within the site. Characteristic of the magnetic field is a uniform decrease in its intensity after the band of sharp gradient from east to west. This is probably due to the gentle dip of the contact toward the undisturbed rocks.

Spessartite dikes and aplite veins at this deposit, as a rule, are accompanied by zones of crushing in the near-contact parts.

The thickness of these zones varies: from several tens of centimeters to 2–3 m. Cases are also noted of strong pinching of the veins, up to their complete disappearance. These zones of crushing in the near-contact parts, together with the dikes, as our investigations have shown, possess a somewhat reduced magnetization in comparison with the enclosing granodiorites and, owing to this, can also be mapped by means of magnetic surveying.

The site (Fig. 2) was opened by a trench. The near-contact zone here is represented by granodiorites that have been kneaded to the state of clays. Their thickness is about 3 m. Along the face of the trench two dikes of spessartite were noted. The dike

Fig. 2. Contour-line plan: 1 — granodiorite; 2 — spessartite; 3 — tectonic clay; 4 — observation points

located in the left wall is monolithic and stands out distinctly among the debris of weathered granodiorites. The second dike is strongly destroyed (to rubble) and kaolinized.

The increase in the magnetic-field intensity on both sides from the middle of the site occurs almost identically. In the middle of the site a rather distinct zone of lowered field values is indicated.

Comparing the contour-line plan with the geological situation, one can be convinced that, in their magnetic properties, the dikes are very close to the crushed rocks and have a reduced magnetization in comparison with the enclosing granodiorites. Otherwise the monolithic dike located in the left wall of the trench and emerging almost at the present-day surface would have noticeably distorted the field in the left part of the site.

In our opinion, the uniform increase of the field on both sides is caused by normal fractured granodiorites, which are in contact with the dikes; while the entire area occupied by the dikes and by the zone of crushing between them is marked by a decrease in field intensity as a single whole.

Spessartite dikes and aplite veins have small thicknesses, amounting, together with the crushing zone, to no more than 3–5 m. The anomalous effect caused by this formation reaches 50–150 gamma.

In the area described, dikes are often located along the margins of zones 50–100 m wide, distinguished by reduced values of the magnetic field. The pattern of the magnetic field here is analogous to the field of the tectonic fault described above (Fig. 1), with the only difference that the anomalous effect in this case is somewhat smaller.

As an example, let us present one of the sites (Fig. 3), exposed by ditches.

The isodynamic plan of this site is divided by a band of increased gradient into two parts—into a region of positive and negative values of the field relative to an arbitrary zero. Moreover, both parts have a fairly quiet field.

Fig. 3. Isodynamic plan: 1—spessartite; 2—observation points; 3—granodiorite; 4—tectonic clay

When the data from the mine working were compared with the magnetic field, it turned out that the dike is located in the band of increased gradient of the magnetic field. The rocks composing the regions of reduced and elevated values, however, show no visible difference, although in the region of reduced values a zone of meter-scale thickness of granodiorites altered to clays was noted, which was not reflected on the isodynamic plan.

Such a pattern of the magnetic field can be caused only by the contact of two rocks differing in magnetic properties. It is probable that the zone of reduced intensity values corresponds to intensely fractured rocks, especially since in this part there is a thick tectonic fracture filled with clay. It is also possible that this contact is a boundary separating granodiorites of different composition.

The spessartite dike itself, like the fracture, was not detected. Evidently, it has magnetic properties close to those of the rocks composing the left-hand part of the site.

Quartz veins of the ore field in the zone of the weathering crust are usually represented either by monolithic quartz broken by joint fractures, or by crushed, pyritized, and kaolinized quartz. The veins are often accompanied by zones of granodiorites altered to clay. Sometimes, along the strike, a quartz vein disappears, and along its continuation there remains only a tectonic fracture filled with clay or with berezitized granodiorites.

For example, the tectonic fault described above (Fig. 1) is nothing other than the continuation of a quartz vein called “Severnaya,” which is currently being mined.

Figure 4 gives a schematic plan of the micromagnetic survey of 15 sites on a \(2 \times 2\) m grid. The fields of the sites have been reduced to one CP.

Fig. 4. Schematic diagram of the magnetic field: 1 — area of lowered field-intensity values; 2 — area of increased gradient; 3 — rose diagram of directions of field-intensity isolines; 4 — area of detailed survey; 5 — trenches

The general structure of the magnetic field presents a rather complex picture. The magnitude of the magnetic field varies over the site within an interval of 500 gamma, with a noticeable decrease in intensity from the southeastern part of the area toward the northwestern part. Moreover, this decrease is very uneven, i.e., zones of sharp gradients with changing strike are outlined. In the northwestern part of the area, against the background of a calm field, a belt of the lowest values is outlined.

Zone 1 extends across the entire area as a belt 40–60 m wide. The boundary of the zone is marked on both sides by a narrow (3–6 m) belt of increased field gradient. The change in intensity in this belt reaches 200 gamma.

In the middle part, the zone of reduced values has an offshoot of almost latitudinal direction (zone 2).

Similar offshoots, but of lesser intensity, are observed in the southeastern part of the area (zones 3, 4). The eastern and northeastern parts of the area have a calmer and somewhat elevated field. Within it, zones of low field values, bounded by a northeastern trend, are outlined rather clearly.

It is also characteristic of the field of the entire site that most isolines of the magnetic field have, in the main, a northeastern direction. This feature is emphasized still more clearly by roses of directions of the isolines, constructed for each area. To clarify the geological setting that determined the character of the magnetic field of the site, 6 trenches were driven.

Trench 4126 passed through very weathered granodiorites of biotite composition, medium-grained structure, and massive texture. Along the face of the trench, four zones of beresites with quartz veinlets are noted. The right flank of the area of reduced field values was exposed by trench 4127. Rocks analogous to those of trench 4126 were found in the trench.

In both trenches, the grus of weathered granodiorites along the entire face is outwardly identical, although the trenches expose areas of normal field, pass through belts of sharp gradient, and enter the area of reduced values.

Zones of beresites and beresitized granodiorites are localized in the belt of increased gradient and in the zone of reduced field values.

A very noteworthy fact is that the dip direction of the beresites or beresitized granodiorites in the weathering zone agrees with the direction of dip of the field intensity. This regularity is observed not only in this site in all trenches, but also in all cases across the ore field where comparison of the magnetic field with the geological structure has been carried out.

The zone of reduced field values, as already noted, probably corresponds to rocks differing in composition or in degree of fracturing from the rocks of the peripheral areas. The contact between these formations is rather sharp. The beresites and beresitized granodiorites in the weathering zone do not affect the character of the magnetic field.

Thus, the belts of increased gradient correspond to the contact of intensely fractured rocks with rocks of normal fracturing, or to the contact of rocks differing in mineralogical composition.

The belt of increased gradient that bounds zone 1 of reduced values on the eastern side divides in the middle part. This division occurs because a spessartite dike leaves the zone of reduced field values, which, together with intensely altered (to clay) granodiorites, was exposed by trench 4125 in zone 2.

Zones 3 and 4 (Fig. 4) are analogous in the character of the magnetic field to the main zone of reduced values. Unlike it, they only have smaller areas and are limited in extent.

The right flank of zone 3 was exposed by trenches 4129 and 4130. In the first trench, a thin beresite vein was found and small tectonic fractures were noted; in the second, only tectonic fractures.

Obviously, this anomaly is caused by an element that accompanies the main structure and is subsidiary with respect to it. This is clearly visible both from the plan of the isolines and from the data of the mine workings. Zone 4 probably reflects the same kind of structural element.

In contrast to the four zones described, which on the isoline plan are clearly delineated by belts of increased gradient, the zones designated

number 5 on the isoline plan; they are distinguished by relatively smooth decreases in the field and are small in size.

The portions of these zones have not been exposed by mine workings, but it may be confidently assumed that they are caused by rocks analogous to the rocks of zone 1. Possibly these are intensely fractured portions of the rocks.

The strike of these zones coincides with the strike direction of the main zone.

As was already noted, the direction of the field isolines for each site and for the whole area is, in general, the same northeastern direction.

Such an orientation reflects the strike direction of the principal geological formations of this area—the zone of altered rocks, the zone of beresites and beresitized granodiorites, the zones of faults, the direction of dikes, etc.

Thus, the magnetic field at the deposit has a fairly complex structure, closely connected with the geological structure of the area.

Characteristic of the field structure is the presence of zones of reduced values elongated in various directions, which by width can be divided into 2 groups. The first group includes zones 2–15 m wide, and the second 50–200 m wide. Their boundaries are sharply outlined by rather narrow bands of increased gradients.

The study of the magnetic field at this deposit provides valuable material for investigating the structure of the ore field.

Most tectonic disturbances in the granodiorite massif (regardless of what fills them) are mapped with sufficient confidence by magnetic surveying.

Also clearly distinguished are areas of rocks in the form of bands, along the margins of which dikes, quartz veins, beresites and beresitized granodiorites, etc., are localized; however, when identifying fracture zones with different fillings, it does not appear possible to determine by magnetic surveying what they are filled with—dikes, quartz veins, granodiorites ground to clay, etc.

When mapping bands distinguished by reduced field values, it is necessary to pay special attention to the contacts of these rocks with normal granodiorites, i.e., on magnetic plans, to zones of increased field gradient, since these areas are often the sites of localization of the vein rocks of the deposit.