STRUCTURAL FEATURES OF THE ORE FIELDS OF THE ZNAMENITINSKY MINING DISTRICT IN KHAKASSIA

S. S. Il’enok

In the history of exploration and exploitation of the ore deposits of the Znamenitinsky mining district in the Kuznetsk Alatau (Khakassia), there have been many geological puzzles. Thus, the pinching-out of one of the ore bodies of the Znamenitaya vein along strike was at first understood as the pinching-out of the deposit, until it was established that these bodies appear to the left along the course to the east and replace one another in an en echelon manner. Later, the transition from veins of belt-like structure with geodes and zonal quartz, characteristic of deposits at shallow depths, to saddle-shaped veins, as well as other features, seemed strange. Meanwhile, the difficulties in deciphering the structure led to the fact that the Znamenitaya vein, which had been discovered in 1902, remained in the exploration stage until 1929. Hence the significance of serious attention to the analysis of the history of ore structures of the district, of their appearance in different environments, and in particular to the question of determining the direction of stress for deciphering the regularity of fracture formation, becomes clear.

The district is composed of an effusive-sedimentary sequence, among which porphyrites, diabases, amygdaloids, rarer quartz keratophyres, thin lenses of black limestones, sandstones, and conglomerates predominate. The rocks generally have a northeastern, nearly meridional strike and are gathered into large folds. Their age is tentatively assigned to the upper Proterozoic. Within the indicated sequence there are two intrusive massifs. One of them forms the Spassky golets and, plunging northward, forms a number of small satellites within the folded structure of the enclosing sequence, which crop out in the area of the Spasskoye deposit. The intrusive is extremely variable in the composition and texture of its rocks. Its main mass is composed of quartz pyroxene–hornblende diorite. The northern satellites of the intrusion have a sharply hybrid character.

Another intrusive massif crops out in the central part of the district, forming the high Belsky golets. The intrusion is concordant in character and is located at the boundary of the porphyrites and the overlying sandy-limestone sequence. At the same time, many rocks of this massif were formed as a result of active reworking of the latter. Both massifs are associated with the intrusive cycle of the upper Proterozoic.

The most interesting ore occurrences are the Spasskoye deposit, spatially confined to the first intrusion, and the Znamenitinskoye deposit, spatially connected, in turn, with the second intrusion. The ore bodies are represented by quartz veins situated in zones of crushing and fracturing of the porphyrite sequence. The veins contain admixtures of ankerite, calcite, and also various ore minerals.

The study of the ore structures showed that their development depends to a considerable extent on the heterogeneity of the environment. In this, the leading factors

for the district is the position of the site in relation to the competent intrusion, as well as the direction of stress.

In this connection, three types of ore structures are distinguished, which may pass into one another and are genetically closely interrelated.

The first type of structures is observed in the area of the enclosing rocks at the site of the frontal abutment of stress in relation to the intrusion. Here the first phase of stress manifestation apparently led to the development of normal cleavage cracks, along which early emanations began to rise. The latter, altering the rocks with a transformation of the mineral composition and the appearance of mica, substantially promoted the transition of fractured effusives into sericite–chlorite schists. The schistose rocks are gathered into series of small folds with amplitudes of 13–15 m, 5–7 m, 0.8–1.5 m and, finally, down to fine plication. A good example of this type of structure is the vein field 5–8 of the Spasskoye deposit (Fig. 1).

Fig. 1

Layout scheme of veins 5–8 of the Spasskoye deposit. 1—quartz diorites; 2—porphyrites; 3—chlorite–sericite schists; 4—quartz veins.

Fig. 2

Typical form of saddle-shaped infilling from the area of vein 5–8 of the Spasskoye deposit: 1—chlorite–sericite schist; 2—quartz with ore minerals.

The ore bodies in this area have the form of saddle-shaped veins up to 2–3 m thick in the crest of the fold (Fig. 2). In the limbs of the fold the vein has, at best, a stringer texture, often passing into a series of abundant veinlets along the schistosity of the rocks. Sometimes steeper fractures cut the schistosity in the limbs (Fig. 3), and then the vein resembles the form of an “S.”

This “S”-shaped form of the ore shoots represents a form of transition from the structures of saddle-shaped veins to veins filling rupture fractures, for the body of folded schists, being consolidated by early portions of quartz, reacts to the further manifestation of stress by brittle deformations. As a rule, these fractures are filled with later quartz. In view of the predominance of compression processes in the area, this type of fracture is not developed. But in the direction toward the northeastern boundary of the ore area, rupture dislocations predominate; the zone of schists becomes narrow; folds, if encountered, are in the form of plication. The ore bodies acquire the form of linear quartz veins.

It must be said that the saddle-shaped veins themselves in this deposit are the richest and often contain rich vein ores. In the ores, although banding is also observed, arising from the rhythmic flaking-off of fractures, what is typical is brecciation of the ores and an abundance of generations of quartz. The type of fracture filling belongs to that in which the opening of fractures proceeds more slowly than their filling. Massive ores are characteristic, with geodes completely absent.

The second type of structures appears in the section of transition from the frontal buttress to the section where the contact line coincides with the direction of stress. Here structures are observed consisting of intersections of fracturing of two directions—

Fig. 3

“S”-shaped form of ore filling from the section of veins 5–8 of the Spasskoye deposit. 1—chlorite-sericite schist; 2—quartz with ore minerals.

—directions: the first type, described above, which in the region is characterized by the development of zones of compression of northeastern strike, and fractures of latitudinal strike, arising as shear fractures. This type of structure is also manifested at the Spasskoye deposit, but in miniature, owing to the small size of the intrusive body and the comparatively smooth transition from one direction of contact to another.

This type of structure is represented especially vividly in the Znamenitinskoye deposit, which consists of the conjugate quartz veins Znamenitaya and Pozdnyaya (Fig. 4). The first has a latitudinal strike, and the second a northeastern strike, close to meridional. In the upper horizons the intersection has the form of a branching, with the formation of a structure resembling a “horse’s tail.” In this case the southern wing of the Pozdnyaya vein branches. Such a structure is connected with the formation of additional tension fractures at the junction of both tectonic zones. Their arcuate character is well explained by the elasticity of the material. With depth, toward

at the 7th working level, veins of both directions merge at almost a right angle.

The ore textures are stringer-like, banded, and more rarely breccia-like. Linear ruptures predominate, rather than simple crushing, as in ores of the first field of structures. Vein formation took place in several stages. Each of them began with cracking of the vein and the subsequent filling of cracks and pores. This mechanism of the process for fissure veins has been studied sufficiently well. The veins grew both in thickness and in length in several episodes. In terms of the ratio between the rate of filling of the space of the fracture and the rate of its opening, which is well revealed by study of the textural and structural features of each mineral stage of deposition, both of its main types are present; that is, there are deposits of stages of mineral formation when the opening of cracks proceeds more slowly than their filling, and also, conversely, when filling is slower than opening. The latter circumstance accounts for the formation of geodes, which, however, are rare.

Fig. 4

Scheme of the formation of ore structures of types II and III in the Znamenitinsky deposit area, according to the pattern of fractures at the 7th working level. 1—quartz monzonites, albitic diorites, and albitites; 2—porphyrites; 3—quartz veins.

The third type of ore structures is manifested in areas where the direction of the contact of the intrusive body with the enclosing mass approaches the direction of stress. As an example one may cite the northern contacts of the district intrusions (Fig. 4). Owing to the influence of the intrusive body as a rigid buttress, torsional structures develop. In this case, fractures capable of gaping are formed. The fractures have an en echelon structure, which is one of the forms of feather fractures.

The emergence of weakened zones led to the appearance of magma, which here produced abundant small acmolith-like bodies, chiefly,

composed of albitic diorites and albitites. The replacement of en echelon veins occurs in intrusive bodies, although often in the latter only a bend or branching of the vein is manifested.

It is interesting that the successive opening of a fracture inevitably takes place with differential movement of its hanging wall relative to the inert masses of the footwall adjoining the competent intrusion. At the point where the fracture dies out, elastic deformations remain, which are released by the formation of a narrow local zone of crumpling or as a system of small feathery fractures. Early stages of such structural wedging-out are also observed in connection with the successive growth of a vein. They coincide with the boundaries of deposits of early mineral stages.

Typical of the ore bodies are banded textures, geodes, and zonal and comb quartz.

In the area of the Spasskoye deposit, the structures are accompanied by the settling of ore material throughout the mass of fractures, with its deposition in quartz geodes.

Turning to the ellipsoid of deformation, we can see that under the action of stress in a homogeneous medium (at a distance from the competent intrusion), apparently, two principal systems of fractures will normally develop. Upon transition to an area bordering a competent intrusion, other conditions are created. When the contact of the intrusion is almost normal to the direction of stress, the rock is crushed (Fig. 5), and its recrystallization and movement of material occur with the formation of folded schlieren. All this vividly recalls the sea, where gentle waves, like waves of stress, on reaching a rocky shore, owing to compression pass into steep waves of small amplitudes and break up into series of secondary waves. An oblique direction of the contact is favorable for the development of one system of fracturing and, conversely, unfavorable for the other (Fig. 6). In this respect the contact is the guide of these lines. Therefore, in passing from the first to the second area we observe that saddle-shaped veins, through the development of forms of an intermediate type—namely, structures of an “S”-shaped appearance—pass into linear fissure veins. Such a transition is especially well expressed where the contact of the intrusive body bends smoothly.

Fig. 5. Development of stresses in the area of a frontal buttress. 1—quartz monzonites; 2—porphyrites.

Fig. 6. Favorable direction for the development of fractures—1—and unfavorable for fractures—2—in the area of an oblique contact line relative to the direction of stress. 1—quartz monzonites; 2—porphyrites.

The shortening of space that arises during folding in the area of a frontal buttress inevitably leads to the appearance of torsional stresses in the enclosing mass, where the intrusive contact is oriented approximately parallel to the direction of stress. As a result, a second latitudinal system of fractures appears, arising in zones of extension. Thus, tangential forces cause the appearance of radial stresses. The intersection of these two systems—the northeastern and the latitudinal—is the most favorable for the localization of mineralization. Such is the close genetic interdependence of the types of structures discussed above.

The Znamenitinsky northeastern ore zone has a length of about 7 km, but is not filled with mineral deposits throughout its entire extent. It contains only separate quartz veins, spaced 400–1500 m apart.

Undoubtedly, the zone was constantly under conditions of compression, which was an unfavorable factor for ore deposition. Under these conditions, even local bends of the intrusive contact could have been the cause of the formation

Fig. 7

Basic structural principle of the formation of the Yakovlevskaya vein in the area of a bend in the contact. 1—quartz monzonites; 2—porphyries.

of a weakened fracture field and subsequent ore deposition. Such, for example, is the basic principle of formation of the complex Yakovlevskaya quartz vein (Fig. 7); therefore, bends of the contact under known conditions are a guiding prospecting indicator.

The established dependence of the position of the above-mentioned types of ore structures on the competent intrusive massif makes it possible to combine heterogeneous structures into a single genetic scheme. This, in turn, provides valuable criteria for evaluating ore occurrences during exploration and exploitation, and is also a good prospecting indicator.