Influence of Fracturing on the Distribution Pattern of Dike and Postmagmatic Formations in the Area of the Kommunar Gold-Ore Cluster (Khakassia)

A. F. Korobeinikov

(Presented by Prof. A. M. Kuzmin)

In 1960–1961 the author took part in prospecting work in the area of the Solgon sector, adjoining from the south the ore field of Kommunar, where we discovered gold ore occurrences of the stockwork type. This stockwork mineralization is located 5–6 km from the known Kommunar deposits. The new factual material obtained forms the basis of the present article.

Within the Solgon sector (Fig. 1) rocks of two different sequences, presumably of Middle Cambrian age, are exposed: a sedimentary-effusive, essentially schistose sequence, and a diabase-porphyrite sequence. Intrusive formations are rather diverse and numerous. The oldest among them are small stock-like, lens-shaped, and subconcordant bodies of uralitized diorites, gabbro-diorites, and diorite-diabases of the gabbro-diabase intrusive complex [14]. In the western part of the area, rocks of the large Solgon granodiorite massif are exposed (an intrusion of quartz monzonites according to A. Ya. Bulynnikov [2]), represented mainly by granodiorites, hornblende and pyroxene diorites, and even pyroxenites (in the marginal part). A somewhat special type is represented by olivine gabbro-diabases and gabbros, which may be the youngest.

The vein series of intrusive formations, creating a sublatitudinal dike belt, is very diverse. The dikes fall into two age groups (in the order of their emplacement): 1) fine-grained diorites, diorite porphyrites, malchites, quartz albitites, quartz and syenite porphyries, aplites, diorite-pegmatites, and 2) gabbro-diabases and porphyrites. Both groups of dikes are genetically connected with the granodiorite intrusion.

In the near-contact zone of the granodiorite intrusive body, the enclosing rocks have been subjected to processes of contact metamorphism and metasomatism, with the formation of hornfelses, metasomatites, and skarned rocks. The intensive development within the sector of processes of contact metamorphism, extending 700–1000 m beyond the limits of the granodiorite-

Fig. 1. Schematic geological-structural plan of the Solgonsky area (compiled by A. F. Korobeinikov): 1 — sedimentary-effusive sequence: aleurolite schists, interbeds and lenses of acidic effusives, their tuffs, limestones; 2 — diabase-porphyrite sequence: greenstone diabases, porphyrites, tuffs and tuff breccias, interbeds and lenses of limestones and siliceous shales; 3 — gabbro-diabase intrusive complex: uralitized gabbro-diabases, gabbro-diorites, diorite-diabases, and porphyritic diorites; 4 — Martaiga intrusive complex (after N. A. Fogelman, 1956): pyroxene and hornblende diorites, granodiorites (first phase of intrusion), granite-aplites and syenites (second phase of intrusion); 5 — olivine gabbro-diabases and gabbro-diorites; 6 — vein facies: fine-grained diorites, malchites, albitites, quartz and syenite porphyries, diorite porphyrites; 7 — aplites, diorite-pegmatites; 8 — gabbro-diabases and porphyrites; 9 — ore zones; 10 — skarn zones; 11 — skarn-magnetite lenses; 12 — quartz-gold-ore veinlets and veins; 13 — schistosity and trachytoid texture; 14 — disjunctive faults; 15 — zones of schistosity; 16 — fracture diagrams; 17 — direction of striation on slickensides and their dip angles.

of the pluton, the presence of separate exposures of it at a distance from the main massif, the behavior of the line of the eastern contour, and the direction of banding and trachytoid structure—all this makes it possible to assume a gentle attitude of the eastern contact of the intrusive body. This is all the more so because primary tectonics above all reflects the position of the plane of contact between the pluton and the enclosing rocks [10]. The gentle contacts of the named intrusive body undoubtedly exerted a substantial influence both on a certain regional character of the metasomatic processes and on the distribution of gold mineralization within the site and the district as a whole.

The rocks of both sequences compose the eastern limb of a large anticline, gradually plunging northward; their strike is submeridional (az. 345—15°)^1, and their dip is at an angle of 43—58° to the east and west. The more plastic formations of the sedimentary-effusive sequence form additional folds of the second, third, and higher orders. The prevailing direction of the axes of the additional folds is northeastern (az. 5—15°). The amplitudes of the spread of their limbs amount to the first hundreds of meters. Along the boundary between the sedimentary-effusive and diabase-porphyrite sequences, a large disjunctive disturbance of near-meridional direction (az. 340—350°) is traced, with a steep dip to the west (60—70°). It is accompanied by a series of subparallel and feathering disjunctives (Fig. 1). The latter have a dominant southwestern strike (az. 230—250°) with a steep dip mainly to the SW at an angle of 55—70°.

All these disturbances are characterized by small displacements in the NW direction and, most likely, are reverse strike-slip faults. The disjunctives are represented by zones from 0.5 to 3 m thick of intensely fractured, sometimes crushed and mylonitized rocks, in places impregnated with iron hydroxides and containing small lenses of quartz. The tectonic zone described evidently played a very significant role in the localization of both metasomatic and gold-ore occurrences. Thus, this major disturbance, probably of a sheared character, represents a series of subparallel ruptures passing along the contact of two heterogeneous sequences. This disjunctive suture is possibly one of the branches of the Syya fault [14], which extends along the valley of the Bolshaya Syya River in a submeridional direction. The fault has been traced for a distance of more than 45 km and reaches a width of 1—2 km.

The character of fracturing of the rocks and of the vein filling of the fractures was studied by us along lines transverse to the dominant strike. More than 3,550 measurements were made at 22 points (Fig. 1), the size of the sites at which did not exceed 3—5 sq. m. The number of measurements at each station was 135—391. As statistical processing of the fractures shows, the optimum number of measurements at each point is about 150. A number of measurements exceeding 200 generally has no substantial effect on the detection of maxima of fracturing, but only somewhat refines the character of the behavior of each maximum (concentrated, diffuse, and girdle structure).

Before proceeding to characterize the fracturing of the various rocks, we shall present the accepted systematics of fractures.

Among the entire diversity of fracture systems developed in the rocks of the site, two main genetic types may be distinguished: primary and secondary. To the first type we assign fractures both endogenous [6, 3] and tectonic, caused by processes of epeirogenesis. The second group includes all fractures formed in the rocks under the influence of orogenic movements, including shear fractures and

^1 The attitudes of the rocks are given with allowance for magnetic declination.

tension fractures that feather out from major faults. According to their orientation relative to the attitude elements of the rocks, all systems are classified as 1) longitudinal, 2) transverse, and 3) diagonal tension and shear fractures.

In studying the fracturing of the rocks of the sedimentary–effusive sequence (Fig. 1), it is established that two systems of longitudinal fractures are predominant: az. dip. \(88—113^\circ \angle 50—75^\circ\) and \(241—325^\circ \angle 50—60^\circ\). In the terminology of A. A. Trofimov [13] and A. A. Belitsky [1], the first system is longitudinal normal-secant, while the second corresponds to longitudinal oblique-secant (cleavage after A. V. Pyak [12]). Less widespread are two transverse systems of tension fractures: az. dip. \(161—193^\circ \angle 55—70^\circ\) and \(348^\circ \angle 70—80^\circ\). Longitudinal oblique-secant fracturing usually has a greater scatter in attitude elements than transverse fracturing. The most clearly expressed systems evidently formed during the period of maximum deformation, whereas earlier-formed endogenous fractures were either used by new deformations (inheritance [8]) or were lost among the predominant newly arisen tectonic fractures, complicating the latter and creating a large fan of scatter for each system. In this, an undoubted role was played by factors of local reorientation of stresses under unchanged regional deformation, owing to a reduction of some components of stress during the period when the first series of fractures arose [16].

In the diabases and diorite-diabases, to which the gold mineralization of the area is spatially confined (Fig. 1), a system of transverse tension fractures of sublatitudinal strike with steep dip to the south and four systems of shear fractures are clearly manifested. Two of them are longitudinal systems and two are transverse to the strike of the rocks. The most highly developed is the system of transverse tension fractures (az. dip. \(150—200^\circ \angle 50—85^\circ\)); of lesser importance are two longitudinal shear systems (az. dip. \(243—256^\circ \angle 60—80^\circ\) and \(71—115^\circ \angle 60—65^\circ\)). The system of transverse shear fractures (az. dip. \(320—330^\circ \angle 45—50^\circ\)) is the least clearly expressed. It is interesting that the fracture diagrams constructed for the rocks of the hanging wall of the Solgonsky fault give maxima with a large fan of scatter of the fracture attitude elements, or have a belt-like structure (in the terminology of V. A. Nevsky [9]). Evidently, this is explained by the superposition of feathering shear and tension fractures on the previously existing fracturing of the rocks. Such a combination of fractures creates varying degrees of rock blockiness, from fine (from 3–5 to 10–15 cm) in porphyrites to medium (30–60 cm) in diorite-diabases. In almost all the indicated systems, fractures with slickensides are encountered (Fig. 2). However, their predominant number falls on the systems of transverse and diagonal shear fractures. It has been established that fractures with slickensides of near-meridional strike (az. dip. \(103^\circ \angle 62^\circ\)) are cut by sublatitudinal ones (az. dip. \(153^\circ \angle 80^\circ\)). Judging by the striation of the slickensides (az. dip. \(240—250^\circ \angle 25—30^\circ\)), it is possible

Fig. 2. Point diagram of the orientation of fractures with slickensides; fracture planes (a) are displaced along fractures (b)

suggest that the dominant tectonic movements took place along systems of transverse fractures. Obviously, these displacements were caused by repeated movements along the Solgon fault. Tectonic movements along longitudinal fracture systems occurred more rarely and, probably, are associated with the stage of formation of a major disjunctive seam, passing approximately along the contact of two heterogeneous rock sequences. We do not discuss here the primary (diagenetic) fractures of the rocks described, since they are completely obscured by superimposed tectonic fracturing.

Linearly elongated and lens-shaped bodies of the gabbro–diorite intrusive complex, subordinate to the folding (Fig. 1), possess two conjugate systems of transverse extension fractures (dip azimuths \(148—161^\circ \angle 50—75^\circ\) and \(303—320^\circ \angle 55—70^\circ\)) and two longitudinal and diagonal systems of shear fractures (dip azimuths \(210—260^\circ \angle 55—62^\circ\) and \(48—110^\circ \angle 37—60^\circ\)). In contrast to the enclosing rocks, the small intrusive bodies have isometric, concentrated maxima on circular diagrams. The smaller the gabbro–diorite and diorite bodies, the clearer and more concentrated are the maxima of the fracture systems. This is in full agreement with L. A. Schreiner’s proposition [15] that an increase in strength with a decrease in the linear dimensions of the cross section is a general regularity.

Along shear fractures of NNW strike, dipping to the NE at angles of \(62—72^\circ\), tectonic movements of normal-fault–strike-slip character took place, caused by displacements along a disjunctive feature that is traceable along the body of porphyritic diorites (Fig. 1). This is indicated by the inclination of striations on slickensides in NE directions at an angle of \(35—45^\circ\) to the horizontal. In the footwall of the Solgon disturbance we encountered olivine gabbro-diabases (Fig. 1), differing from the rocks of the gabbro-diabase intrusive complex by better crystallization and freshness. Obviously, they are of younger age. In them, transverse systems of shear fractures are developed (dip azimuth \(161^\circ \angle 40—60^\circ\)) and extension fractures (dip azimuth \(160^\circ \angle 70—90^\circ\)). Two longitudinal systems of the shear type are less widespread (dip azimuths \(260^\circ \angle 70^\circ\) and \(87^\circ \angle 85^\circ\)).

In large dikes of diorite porphyrites of sublatitudinal strike, two systems of shear fractures and two of extension fractures are developed. The shear fractures are longitudinal, and the extension fractures transverse, relative to the strike of the dike bodies, and form, respectively, two conjugate systems with dip angles of \(50—70^\circ\). Fracturing in the large dike bodies generally coincides with that of the enclosing rocks.

The Solgon intrusive massif, in its marginal part, reveals linear and plane-parallel textures. In the rocks of the granodiorite series, located closer to the central part of the massif, banding and trachytoid structure are vertical and strike at azimuths \(15—20^\circ\). Diorites of the contact zone possess banding with dip azimuths \(170^\circ \angle 55^\circ\) and \(120^\circ \angle 60—80^\circ\). If one takes into account that banding and trachytoid structure are elements of prototectonics [5], then the emplacement of the diorites, and then of the granodiorites and granite-aplites, took place under different tectonic conditions; moreover, in a number of outcrops we found acute-angled xenoliths of melanocratic diorites among leucogranodiorites and granite-aplites. Obviously, here we are dealing with different phases of a single intrusion.

In the formations of the Solgon massif, 5 or 6 fracture systems are clearly manifested (Figs. 1, 3). In contrast to the rocks of the continent, in the intrusive body described two conjugate systems of submeridional fractures with steep dips to the west and east are developed. They bear all the features of extension fractures. Sometimes four such systems appear, two of which occupy a subordinate position, complicating the first two. In the ag—

the aggregate their maxima fall within a single belt (along strike). Apparently, the latter two systems, which often coincide with banding and trachytoid structure, are primary or prototectonic [10, 5], while the former are secondary (tectonic) [11].

For the rocks of the massif, a belt-like structure of the maxima in the fracture diagrams along strike is very characteristic. It is interesting that fractures with steep dips (70–90°) have a belt-like arrangement of maxima, whereas gently dipping systems (30–50°) are isolated into separate concentrated maxima. The determining factor in the scatter of fractures in diorites and granodiorites was probably the superposition of tectonic fractures on prototectonic ones. From the circular

Fig. 3. Composite diagram of maxima of fracturing, dikes, and hydrothermal veinlets: 1—maxima of fracture systems (the size of the dots depends on the number of fractures; 22 partial diagrams based on 3558 measurements); 2—dikes of fine-grained diorites; 3—dikes of diorite porphyries; 4—dikes of diorite pegmatites; 5—dikes of aplites; 6—dikes of gabbro-diabases and porphyrites; 7—dikes of quartz albitites; 8—dikes of malchites; 9—quartz–feldspar veinlets; 10—quartz–ore veinlets.

fracture diagrams it is established that the most highly developed systems are steeply dipping ones, and the less developed are gently dipping ones (Fig. 3).

In the near-contact part of the pluton, among the diorites at the contact with a large dike of diorite porphyries, elliptical jointing has been established. Its axis of elongation is directed approximately along the eastern contact of the intrusive body. This jointing was possibly caused by the thermal effect during the intrusion of the diorites into the rocks of the continent, since the fractures as it were bend around the contact of the massif, following its configuration. In addition,

among the intrusive rocks, gently dipping sheet jointing is manifested almost everywhere. This prototectonic fracturing is most developed in the central part of the intrusion, and its formation took place under the influence of both external and internal forces, owing to phenomena of great internal stress and considerable mobility during the period of pluton formation. In general, the fracture systems in the granodiorites and leucocratic diorites situated closer to the central part of the massif differ sharply, in their attitude elements and in the character of the vein material filling them, from those of the continent. At the same time, the diorites of the near-contact zone possess fracture systems of the same type as those in the enclosing rocks (az. dip 310—330°, angle 50—80°).

In fractures of sublatitudinal strike, with gentle (40—45°) dips to the south and north, slickensides are often observed. Judging from their morphology and the strike of the grooves, one may assume slight displacements in the latitudinal and NE directions with gentle angles. Most likely, these movements are strike-slips or normal-strike-slips.

As shown by the data from the study of fracturing and the orientation of dikes in the area (Figs. 3, 4), it is established that dike formations of different composition and age tend toward definite fracture systems: 1) transverse tectonic tension fractures, 2) longitudinal fractures of a disjunctive character, and 3) prototectonic (sheet) fractures. Systems of transverse, extended tension fractures of sublatitudinal strike (Fig. 1), with steep dips to the south and north, distributed both in the rocks of the continent and in the marginal part of the granodiorite massif, controlled dikes of different composition and age, except for aplites and diorite pegmatites (Fig. 1). Dikes of submeridional trend are less widespread and, as a rule, tend toward the zone of the Solgon fault. They were controlled by subsidiary fractures of a disjunctive character, as indicated by their numerous tectonic contacts (Fig. 1). Along these fractures, repeated movements occurred even after the formation of dikes within them. Thus, in the area of gold-ore occurrences (Fig. 1), dikes of diorites and porphyrites have been encountered that have been transformed into breccia and tectonic sand. Veins of aplites and diorite pegmatites are somewhat separated from the rest of the dike group and are spatially confined to the Solgon intrusion or its exocontact part; they were controlled by prototectonic fractures of the sheet type and, more rarely, by tectonic tension fractures. Aplite dikes are, as a rule, thin (up to 0.5—1 m), with lengths up to the first hundred meters. Within the essentially shale sequence, dike formations have not been established by us. Apparently this is explained by the fact that in the rocks of this sequence, owing to their physicomechanical properties, large tension fractures are not formed, while shear fractures of a disjunctive character during the period of dike formation proved to be closed. The intrusion of magmatic melt most likely occurred under conditions of prevailing extension, against a general background of periodic alternation of tectonic movements of opposite direction.

Skarn formations are confined mainly to areas with intense fracturing of the longitudinal type. Skarn-magnetite lenses, as a rule, tend toward the contacts of limestones with effusives, where either disjunctive disturbances or narrow zones of increased fracturing are observed. Owing to these factors of structural control, skarns and skarnoids extend as a chain along the Solgon disturbance, occurring in the rocks of the hanging wall (Fig. 1). In places of intense development of shear fractures subsidiary to the main fault, the usual lenticular form of skarns assumes irregular outlines. Swellings of skarn bodies are usually located in places of co—

stresses of two principal systems of tectonic fractures: sublatitudinal and submeridional. Thus, fracture tectonics, along with lithological control, was one of the decisive factors in the placement and structure of the skarn bodies. During subsequent deformations in the skarns, as the most brittle formations, systems of shear fractures arose (dip azimuth 300–320° ∠ 30–50°), which were later used by hydrothermal solutions. In addition to skarns, metasomatic, quartz-scapolite, quartz-feldspar, and quartz-feldspar-actinolite veinlets are widespread in the area. These formations, often of hornfels structure, developed mainly in diabase porphyrites and diabases, are, as a rule, confined to gently dipping (15–45°) transverse and, less commonly, longitudinal systems of shear fractures. The thickness of individual veinlets does not exceed 1–5 cm. The main veinlets are accompanied by a series of subparallel and obliquely intersecting thin apophyses and bear all the features of metasomatic replacement. The noted formations within the area create several sublatitudinal zones 10–15 m thick, located in the rocks of the hanging wall of the fault. In processes of metasomatism, the intensity of percolation is of great importance; it acquires its greatest value in tectonically weakened zones, and here phenomena of fracture permeability and porosity undoubtedly played a role [4, 7]. The density of open macro- and microfractures was greatest in the more brittle rocks of the effusive sequence, favorably situated relative to the Solgon intrusion and the principal controlling fault.

The hydrothermal manifestations of the area are very diverse and numerous: quartz-feldspar, quartz, quartz–gold-ore, and quartz-carbonate veinlets. Their greatest concentration is noted in diorite-diabases and rocks of the effusive sequence of the hanging wall of the Solgon fault. Analysis of summary point diagrams of the orientation of hydrothermal veinlets (Fig. 5) and of the fracturing of the various rocks of the area reveals the confinement of gold-bearing quartz veinlets to transverse systems of tensile fractures and, more rarely, shear fractures (dip azimuth 143–180° ∠ 50–80°). Quartz-feldspar and actinolite veinlets tend mainly toward transverse systems of shear fractures dipping gently to the north and south. They are occasionally confined to longitudinal shear systems (Fig. 3). Quartz-carbonate veinlets, in turn, formed in fracture systems with dip azimuths of 180–215° ∠ 20–37° and 310–330° ∠ 60–80°, rarely 340–350° ∠ 80°. Fractures of sublatitudinal strike enclosing quartz–gold-ore associations bear all the features of tensile fractures: they die out rapidly along strike and dip, are generally irregular in shape, with rough walls, and often overlap en echelon. The textural features of the veinlets indicate the filling of open transverse fractures by quartz-ore material: the quartz is often comb-like, drusy, without signs of replacement. Diorite-diabases and porphyrites, as more brittle formations, during deformation produced systems of slightly open tensile and shear fractures, which is why they proved more permeable to hydrothermal solutions. The localization of gold mineralization, in addition to lithological factors, depends on the position of fractures relative to ore-conducting channels and on the degree of their density and openness during the period when ore-bearing solutions entered. In rocks of the sedimentary-effusive sequence, hydrothermal formations are rare. Small barren or weakly gold-bearing quartz veinlets and lens-like or lenticular segregations are controlled by all fracture systems. However, the largest of them coincide with longitudinal fracturing (schistosity).

In studying the orientation of quartz veinlets among bodies of uralitized gabbro-diorites and gabbro-diabases, it was established that most of them are confined to four systems of shear fractures (az. dip \(270—290^\circ \angle 15—55^\circ\), \(350—20^\circ \angle 20—50^\circ\), and \(50—80^\circ \angle 50—70^\circ\)). The thickest veinlets (up to \(15—20\ \text{cm}\)) of weakly gold-bearing quartz tend toward longitudinal systems (az. dip \(270—295^\circ \angle 60—70^\circ\)). In general, these formations are weakly gold-bearing.

Among the rocks of the Solgon intrusion, hydrothermal formations are rare and are represented by quartz, rarely weakly gold-bearing veins and veinlets. Most of them tend toward sublatitudinal gently dipping fractures. Sometimes the veins are observed to be confined to the contacts of dikes of fine-grained diorites of sublatitudinal strike with gentle (\(20—25^\circ\)) dip to the south. Thus, a quartz vein located in the near-contact part of the Solgon massif (Fig. 1) locally cuts a dike diagonally and is almost everywhere accompanied by a zone of quartz veinlet apophyses, especially on the side of the hanging wall. During the hydrothermal process, tectonic movements occurred repeatedly. This is indicated by the facts of crushing of the vein material, cemented by quartz of later generations, and by the bending of actinolite fibers growing in fractures perpendicular to the walls of amphibole veinlets.

Fig. 4. Summary point diagram of dike orientations:
1 — fine-grained diorites;
2 — diorite porphyrites;
3 — aplites and diorite-pegmatites;
4 — porphyrites;
5 — gabbro-diabases;
6 — quartz albitites and syenite-porphyries;
7 — malchites.

Fig. 5. Summary point diagram of orientations of hydrothermal veinlets:
1 — quartz–feldspar;
2 — quartz–gold-ore;
3 — quartz–carbonate;
4 — Solgon disjunctive disturbance.

The factual material presented makes it possible to draw the following general conclusions:

1. Among the entire variety of fracture systems, two main types are distinguished: primary and secondary, with the latter predominating. Rocks differing in physicomechanical properties possess their own specific fracture systems. In the sedimentary-effusive sequence, the domi-

the prevailing ones are two longitudinal shear systems, in diabases, porphyrites, and linearly elongated small intrusive bodies of any composition—two transverse extension systems and two longitudinal shear systems. The fracturing of rocks of small intrusions, subordinate to folding, does not differ from that of the continent and reflects the manifestation of the prevailing regional deformation [11]. The rocks of the Solgon massif, differing in composition, possess their own elements of the attitude of banding and trachytoid texture. In the massif there occur both primary and secondary fractures, arranged in 6 systems: two longitudinal with respect to the banding and trachytoid texture, and four longitudinal and obliquely cutting ones. The primary fractures of the intrusive body controlled dikes of aplites and diorite-pegmatites.

1. Disjunctive disturbances of the shear type are, as a rule, confined to contacts of various strata and beds. The principal causes of their formation were the coincidence of the contact of heterogeneous rocks with the main shear plane. The inception of such disjunctives took place under conditions of maximum compression and began already in the folding phase. The Solgon fault, situated within the sphere of influence of the granodiorite intrusion, is evidently the controlling one for skarnization and gold mineralization of the area. This is quite definitely indicated by the character of the distribution of postmagmatic formations: zones of silica-alkaline metasomatism, skarnization, and hydrothermal manifestations are often encountered together and are arranged in a belt along the disturbance, in the rocks of its hanging wall (Fig. 1).

2. Dike formations were controlled by two types of fractures: tectonic transverse extensional fractures and diagonal shear fractures of a disjunctive character. During the period of emplacement of dikes of different composition and age, the direction of the principal deformation generally remained unchanged, but extension was rhythmically replaced by compression; this is indicated by the confinement of late porphyrite dikes to fractures diagonally cutting the veins of diorite porphyrites while preserving their general strike.

3. The inception of the Solgon disjunctive suture was accompanied by intense fracturing. The first to arise were longitudinal fractures of the shear type, which subsequently were used by skarnizing emanations. Then, during the hydrothermal stage, previously formed feathering extension and shear fractures formed and reopened. Ore deposition took place mainly in transverse extension fractures. Tectonic fractures that arose before the inception of the controlling disturbance participated in the distribution of ore matter only if they fell within the sphere of influence of the principal ore-conducting channels. Open fractures and the microfractures associated with them were very important structural elements in the distribution of ores in zones of movement of solutions, as well as in the emplacement of gold in individual parts of ore bodies. The most favorable places for gold mineralization were the junctions of transverse extension and shear fractures, where ore columns and nests formed. In accordance with the complex system of tectonic fractures, stockwork ores arose. Their formation occurred under complex stress conditions, with locally varying compression and extension against a background of general extension.

A distinct recurrence is established in the directions of fractures of different ages. Thus, the same transverse tectonic fractures often contain dikes of different composition and age, quartz-scapolite, quartz–feldspar–actinolite metasomatic formations, quartz–gold-ore, and later quartz–carbonate veinlets. This regularity is probably due to a certain anisotro-

lines of physicomechanical properties of the deforming medium, which became especially pronounced after the formation of the first, largest dikes.

The example of the Solgon ore occurrences once again emphasizes the connection of gold mineralization with major tectonic faults of submeridional strike and with the fracturing associated with them. Therefore, the presence of major pre-dike disturbances and zones of feathering fractures near the granodiorite massif can evidently serve as an important prospecting criterion for gold mineralization. This regularity also applies to the Kommunar ore field.

LITERATURE

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