Structural Characteristics of the Lebed Skarn-Gold Deposit

N. F. Stolbova (TPI)

Much attention is being given to the study of skarn deposits and their structural features. The latter help us view a deposit in the course of its development, in its relation to the structure of the district and to its tectonic history. The structural features of the Lebed deposit, characterized below, are in this respect of undoubted interest.

The area of the deposit belongs to the so-called Kondom–Lebed mobile zone. Its submeridional structures determine the general features of the structure of the site and of the district. Cambrian rocks (the lower structural stage) are gathered into tight submeridional, as well as northwestern and northeastern folds, complicated by faults. The Lebed intrusion, which cuts these rocks, forms a series of conformable, discordant elongate-type massifs formed under hypabyssal conditions. The Maiskii massif, with which the deposit is directly connected, is small in size and elongated in a submeridional direction. Judging from geophysical and geological data, it is connected with the Lebed and Magalak massifs beneath a small thickness of enclosing rocks. Along its eastern contact there is a known skarn zone extending for seven kilometers, with a thickness of 50–200 meters, and consisting of separately situated elongated skarn bodies.

Industrial concentrations of gold have been found in the northern part of the zone. The results of large-scale geological and micromagnetic surveys, supplemented by the study of fracturing and the analysis of summary diagrams of 40,000 fractures, make it possible to draw certain conclusions about the structure of the deposit.

The enclosing rocks (Fig. 1), which make up the eastern half of the area, belong to the lower subformation of the Kayanchinskaya Suite of Lower Cambrian age. They are represented not only by andesite-basalt porphyrites, but also by volcaniclastic gravelites, sandstones, and siltstones. In the medium-clastic varieties, thin lenses of marble occur. From the spatial position and orientation of bedding in the volcaniclastic sandstones and siltstones, their north-northeastern strike is established. Rare measurements of the dip of bedding and certain features of fracturing in the volcaniclastic rocks reflect their monoclinal occurrence.

The western half of the area is composed of rocks of the Maiskii massif (Fig. 1): quartz diorites, granodiorites, syenites. Gabbro-diorites occurring among the enclosing rocks (eastern part of the area) pre—

Fig. 1. Geological-structural scheme of the deposit (compiled by D. G. Berlimble, N. F. Stolbova)

1 — pyroxene and plagioclase porphyrites; 2 — volcaniclastic conglomerates and gravelites; 3 — volcaniclastic sandstones and aleurolites; 4 — gabbro-diorites; 5 — quartz diorites; 6 — granodiorites; 7 — syenites; 8 — amphibole and gneiss-like rocks; 9 — skarns; 10 — tectonic disturbances of varying thickness: a) 5–10 m, b) 10–20 m; 11 — elements of attitude: a) bedding, b) gneissosity; 12 — geological boundaries.

are the most convex near-contact parts of the massif, which here is located at a shallow depth below the land surface.

Along the contact between the massif and the enclosing rocks there runs a submeridional fault, dipping at an angle of 80–90° to the west and east. The width of the tectonic zone is 5–20 meters. Within it the rocks are represented by cataclasites, mylonites, and ultramylonites. A very rare occurrence of postmagmatic products in them is characteristic, which apparently indicates the prolonged nature of compression conditions along the fault. Within the intrusion the existence of three more large faults is inferred: a northwestern, steeply dipping one—in the northern part; a northeastern one, with the displacer dipping to the southeast at angles of 65–70°—in the central part; and a sublatitudinal one—in the southern part, dipping northward at an angle of 80°. All of them have mylonitization zones 5–10 m wide, clearly expressed in the relief by a sharp lowering and waterlogging; in addition, they are well traced by certain maxima on jointing diagrams.

Fault tectonics led to the formation within the intrusive body of blocks that underwent normal-fault–strike-slip displacements. The recorded relative amplitude of displacement of two of them is 15–20 m. In general, the central block proves to be more downthrown than the northern ones.

The present-day erosional surface passes at the level of the near-contact part of the intrusion. This is demonstrated by the fact that, on passing to higher hypsometric levels, medium-grained granodiorites are replaced by fine-grained quartz diorites. In the relatively downthrown blocks, moreover, transitions into melanocratic hybridized and gneiss-like rocks are noted. Here there are frequent areas with abundant schlieren and xenoliths, as well as hornfelsed and skarnified relics of the enclosing rocks.

The study of the structural features of the intrusive body made it possible to distinguish elements of prototectonics: the planar orientation of dark-colored components and primary fractures. In melanocratic and hybrid rocks, pyroxenes and amphiboles clearly reflect the planar orientation of the long axes of the crystals. Their orientation is close to horizontal; schlieren and xenoliths are also subordinate to it. The primary jointing of intrusive rocks can be judged by comparing summary diagrams for the intrusive and enclosing rocks. The sharp difference in the orientation of maxima and the appearance in the intrusive rocks of a distinct maximum of gentle jointing dipping to the WNW at angles of 10–25° attract attention. Such an orientation of gentle jointing corresponds to the gentle occurrence of planar-linear structures in the intrusive rocks and indicates that the jointing belongs to bedding. The latter, taking into account the diffuse nature of the maximum, may indicate that the contact was gently inclined and uneven. The unevenness of the roof intrusive contact is also indicated by the position of large xenoliths of the enclosing rocks at different hypsometric levels. In the longitudinal section of the central block, according to the position of relics of the enclosing rocks and the planar orientation of minerals in the magmatic rocks, a saddle-shaped configuration of the intrusion may be assumed.

Overall, characterizing the structural position of the intrusive body, it may be emphasized that it is represented at the deposit by its apical part, has an uneven, saddle-shaped configuration of the surface, and is conformable.

Only post-skarn dikes are known at the deposit. They are few in number, but uniform in composition—these are andesite porphy-

rites and trachyandesite porphyries. They have a small thickness (20–60 cm), a subparallel orientation, a northwestern strike changing to submeridional, and a steep dip to the northeast or vertical. It is noteworthy that the dikes abut against the submeridional fault; in relation to it they are arranged at angles of 45°, i.e., they are pinnate (Fig. 2).

Metamorphic and postmagmatic formations are represented by hornfelses, high-temperature bimetasomatic and infiltration skarns, partly recrystallized, and also by superposed hydrothermal products. The latter include microcline formations, gold-bismuth-telluride mineralization, and quartz-carbonate-sulfide mineralization.

The listed metamorphic and hydrothermal formations are characterized by a distinctive mineral composition and spatial separateness.

Of interest is the structural position of the bimetasomatic skarns. They extend along the eastern contact of the intrusive, forming a trough-like zone coinciding with the strike of the enclosing rocks. The width of the zone is 50–200 meters. The outlines of the skarn-zone body are controlled by the form of the intrusive contact.

Fig. 2. Scheme of the distribution of rocks in the skarn zone:

1 — skarns of the first stage; 2 — skarns of the second stage; 3 — magnetite rocks; 4 — carbonate-sulfide veins; 5 — dikes.

Of special interest is the tectonic control of the skarns. Thus, the skarns of the second stage of Rudnaya Mountain were formed after a tectonic disturbance, which partly crushed the bimetasomatic skarns and laid out channels among them for the infiltration of solutions. Structural elements in the infiltration skarns—such as, for example, the orientation of breccia-like rocks, monomineralic bodies and veinlets, drusy coarse-crystalline formations among fine-grained ones, and also the direction of banded textures—indicate the NE direction of fracturing used by the solutions.

The position of the magnetite bodies is also noteworthy. In the area described there are four of them; each is characterized in plan by a trough-like, flame-like form, spatial separateness, and the distinctive character of the surrounding rocks. The bodies are arranged en echelon, with a plunge toward the submeridional fault, against which they abut at angles of 40–50° (in plan). Their orientation is northwestern near the disjunctive; with distance from it, it changes to submeridional. It is assumed that, along with other factors, pinnate fracturing played a significant role in the formation of the magnetites.

Later microcline and gold-bismuth-telluride mine-

mineralization are spatially closely associated with infiltration skarns and tend toward the inner parts of the magnetite mound-like bodies.

The form of the gold-ore bodies is ellipsoidal; their orientation is generally submeridional. The form and orientation of the southern body, on which exploration and mining work is being completed, are the most indicative. The form of this body is ellipsoidal and curved. Its orientation changes from submeridional to northwestern as it approaches the disjunctive structure. At the same time, its direction along the median axis also changes from steep to northeastern (75–80°).

The study of fracturing shows its similarity in the gold-ore bodies (the first and the second) and its difference from the fracturing in the non-gold-ore bodies. The principal difference is that northwestern fracturing is almost absent in the non-gold-bearing skarns. It follows from this that, for gold mineralization, northwestern fracturing and the most weakened zones of this direction were of great importance.

The results of generalizing data on the orientation of hydrothermal veinlets of the later stages reflect the fact that the most productive and long-lived fractures prove to be those of N–S orientation, with angles of 50–80° to the SE and NW, i.e., feathering fractures of the central fault.

For the second ore body, it is noteworthy that a four-meter vein of the youngest carbonate-sulfide mineralization (the remaining numerous veinlets are no more than a centimeter thick and are oriented in different directions) cuts across it in the middle. At the same time, its direction coincides with the orientation of all the above-listed formations. The dikes are oriented in the same way (Fig. 2). All this indicates the existence of a weakened zone and its constant use by subsequent portions of solutions and magma.

Taking all the foregoing into account, the following conclusions may be drawn. The Lebed skarn zone is closely connected with the roof contact of the intrusive massif. For the early, essentially bimetasomatic skarns, lithological control is decisive. Nevertheless, the spatial confinement of the zone to a submeridional disjunctive structure may indicate a certain role of this structure in the process of skarn formation. The disjunctive structure played an important role at lower temperatures, when the surrounding rocks became more brittle and more susceptible to mechanical deformation. During repeatedly recurring tectonic movements it was closed and therefore remained completely inaccessible to the penetration of solutions. At the same time, the feathering fractures, periodically opening slightly, served as supply channels for postmagmatic solutions and thus played a significant role in the localization of mineralization.