New Gold-Ore Occurrences in the Kommunar Gold-Bearing District

(Eastern Slope of the Kuznetsk Alatau)

A. F. Korobeinikov

(Presented by Professor A. M. Kuzmin)

In 1960 we discovered gold-ore occurrences in the Solgon area (Fig. 1), which adjoins the Kommunar ore field on the southern side. The ore occurrences identified are situated in the near-contact zone of a large granodiorite massif. In this work, primary attention is given to the structure of the area, since structural control of mineralization is one of the decisive factors in the distribution of gold mineralization within the area.

The Solgon area forms part of a large anticline that gradually plunges northward. The rocks of the area, represented by two heterogeneous Middle Cambrian sequences—sedimentary-effusive, substantially schistose, and diabase-porphyrite (Fig. 1)—make up the eastern limb of this anticline. The strike of the rocks is submeridional; dips are at angles of 43–58° to the east and west. The more plastic rocks of the sedimentary-effusive sequence form subsidiary folds of the second, third, and higher orders, thereby complicating the general monoclinal attitude of the strata. The prevailing direction of the axes of the subsidiary folds is north-northeast (az. 0–10°), in accordance with the general strike of the rocks of the district. The amplitudes of the limbs of small folds are on the order of the first hundreds of meters. The rocks of the diabase-porphyrite sequence generally occur monoclinally, with dip angles within 40–55° to the west (?). The rocks of both sequences are cut by small intrusive bodies of diorite-diabases, gabbro-diorites, and gabbro-diabases, which, as a rule, have an attitude conformable with the enclosing rocks. In the area, dikes of diorite-porphyrites, microdiorites, and porphyrites are significantly widespread.

All rocks of the district have been subjected to processes of intense contact metamorphism, with the formation of contact hornfelses and zones of albitized and skarned rocks. The intensive development, within the described area, of contact-metamorphic processes extending 600–800 m from the contact of the Solgon granodiorite intrusion, and the presence of individual small outcrops of it far from the contact of the main massif, make it possible to suppose that the rocks of the area are underlain by an ore-bearing granodiorite intrusion. The gentle contacts of the parent intrusion,

undoubtedly influenced both a certain regional character of the metasomatic processes and the distribution of mineralization within the area.

Along the contact of the diabase and essentially schistose sequences, a large fault of north–northwestern trend (340–350°), with a steep dip (60°) to the west, can be traced. This fault is accompanied by a series of subparallel and branching disjunctive structures. The branching disjunctive structures have a predominant southwestern strike (az. 230–250°), dipping mainly to the south at an angle of 55–70°. All these branching faults are apparently characterized by small displacements and most likely are reverse strike-slip faults.

Fig. 1. Geological-structural sketch map of the Solgon area
(compiled by A. F. Korobeinikov).

1 — sedimentary-effusive sequence — schists, sandstones, acidic effusives and their tuffs; 2 — keratophyres, albitophyres; 3 — interbeds and lenses of limestones; 4 — diabase-porphyrite sequence; 5 — diorite-gabbros; 6 — gabbro-diabases and gabbro-diorites; 7 — granodiorites, diorites; 8 — dikes of diorite-porphyrites; 9 — dikes of porphyrites; 10 — dikes of microdiorites; 11 — zones of skarnization; 12 — zones of albitization; 13 — tectonic faults; 14 — zones of stockwork mineralization; 15 — diagrams of rock fracturing (after the method of V. V. Sukhoruchkin).

The disjunctive structures are represented by zones from 0.5 to 3 m thick, composed of intensely schistose, crushed, and mylonitized rocks, sometimes impregnated with iron oxides and hydroxides and containing veinlets of quartz. Evidently, these tectonic faults, which formed a large weakened zone, served as conduits for ore-bearing solutions.

Thus, the large fault, probably of shear character, represents a series of subparallel disjunctive structures passing along the contact of two heterogeneous sequences (Fig. 1). The large fault described may be one of the branches of the Syya fault (after N. A. Fogelman, 1956), which is traced along the valley

of the Bolshoi Syin River in a meridional direction. The Syisky fault has been traced for a distance of more than 45 km and reaches 1–2 km in width.

The Solgon disturbance, in turn, is accompanied by numerous feathering tension and shear fractures, which develop preferentially in the competent rocks of the diabase-porphyrite sequence and diorite-diabases. Tectonic fracturing, caused both by folding processes and chiefly by the emplacement of a major disjunctive seam, is most intensely developed in the rocks of the hanging wall of the disturbance. In gabbro-diorites located far from the disturbance, mainly only two systems of shear fractures and one system of tension fractures are expressed (Fig. 1). At the same time, in the rocks of the hanging wall of the disturbance, 4–5 systems of tectonic fractures are developed. In the porphyrites and diabases, the dominant fractures are shear and tension fractures of sublatitudinal strike, dipping south at angles of 40–75°, and a system of shear fractures of meridional strike, dipping east at angles of 40–80°. In the keratophyres and albitophyres, shear fractures of submeridional strike already predominate (az. 350°), dipping NE at angles of 40–70°. Tension and shear fractures of sublatitudinal strike, dipping south at angles of 80–90 and 45°, are considerably less widespread. Slates and limestones are broken by numerous tectonic fractures, among which three shear systems are distinguished (strike az. 15°, SW, angle 75°; strike az. 335°, NE, angle 75°; strike az. 330°, SE, angle 40°) and a system of tension fractures (strike az. 80°, SW, angle 70°). Cleavage coincides with the first system of shear fractures. In gabbro-diabases and diorites, the most clearly expressed is a system of tension fractures of sublatitudinal direction (strike az. 90°, south, angle 75–85°) and two systems of shear fractures (strike az. 80°, SW, angle 55°, and 335°, SE, angle 60–80°). Evidently, the first system of tension fractures coincides with the system of cross fractures Q according to Cloos. In the diorite-diabases, to which the gold mineralization of the area is chiefly spatially confined, five principal fracture systems are developed:

1. Fractures of ESE strike (az. 75–90°), dipping south at angles of 65–87°, are the most distinctly expressed. These fractures rapidly pinch out along strike and dip; sometimes they overlap one another en echelon; the fracture walls are, as a rule, rough. The main mass of gold-bearing quartz veinlets, and sometimes dikes of microdiorites and porphyrites, is confined to them (Fig. 1). Most of the indicated fractures are probably feathering tension fractures of the main disturbance. Some of them may belong to the cross fractures Q according to Cloos.

2. A dense network of fractures of the same direction, but dipping at a gentler angle of 45–55°. These are shear fractures. They are more rectilinear and persistent, sometimes with slickensides inclined to the SE at an angle of 25–30°, i.e., displacements of a strike-slip character sometimes occurred along them.

3. Fractures of NE strike (20°), dipping SE at an angle of 40–50°, are also of shear character.

4. Fractures of the same strike are somewhat more weakly expressed, but dip NW at an angle of 45°. These are also shear fractures. Rare quartz-ore and, chiefly, barren quartz-albite veinlets are confined to both fracture systems.

5. Fractures of NW strike (az. 300°), dipping SW at angles of 30–50°, are the least widespread. These are shear fractures, subpa-

parallel to the main fault. Quartz-carbonate and quartz-feldspar barren veinlets tend toward them.

Thus, in the diorite-diabases four systems of shear fractures and one system of extension fractures are clearly manifested. In the extension fractures, small quartz–gold-ore veinlets 5–10 cm thick formed, while in shear fractures of sublatitudinal strike, thin and very thin quartz-ore veinlets formed. The most favorable sites for ore deposition were the junctions of extension and shear fractures, where ore shoots were deformed. The most probable dip of the ore shoots is to the south at an angle of 50–65°.

On the basis of the foregoing, it may be assumed that the principal deformation stress in the area was oriented in the latitudinal direction, if one accepts that the extension fractures are oriented normal to the long axis of the deformation ellipsoid. Evidently, at the maximum of brittle deformations, intense fracturing and a large disjunctive fault were formed. In the Solton area the contact of rocks of different competence coincides with the main shear plane, which was one of the principal reasons for the formation of a large disjunctive structure.

From diagrams of fracturing in the different rocks of the area it is established that diorite-diabases, gabbro-diorites, and diabases, as more brittle formations, have a smaller shear angle relative to the axis of maximum compression and therefore a greater tendency to open. In the present case the bodies of igneous rocks and effusives played the role of competent blocks and served in places as sites of fracture localization. The fracture tectonics of the igneous rocks of the area is connected with regional deformation, as has been noted by many investigators for other regions as well [1, 2].

The fracture tectonics of the intrusive bodies of the area is characterized by the regularity of the fracture systems and by their persistence over a considerable distance. In the Solton area, in accordance with the complex system of fractures developed in the diorite-diabases, stockwork or stockwork-like ores were formed. Ore formation is directly dependent on the density of quartz veins. The fracture structure of the area, along with the quantity and routes of circulation of solutions, was one of the decisive factors in the distribution and structure of the gold-ore bodies.

Within the area, three age-different stages of hydrothermal mineralization are clearly distinguished, separated by phases of tectonic movements, crushing of previously formed minerals, and the opening of new fractures: 1) quartz-albite, 2) quartz–gold-ore, with which the formation of white fine-grained quartz, actinolite, pyrite, more rarely prehnite and gold is associated, 3) quartz-ankerite-calcite, associated with the final stage of hydrothermal activity; here, along with quartz, ankerite, and calcite, diabantite and pistacite occur. The hydrothermal formations were considered by us in greater detail in a previous article [2].

Tectonic movements also continued after the final formation of the ore occurrences. This is indicated by numerous displacements, slickensides, and separate zones of crushing observed in diorite-diabases and porphyrites penetrated by ore veinlets. In addition to the tectonic fractures noted, joints are manifested among the effusive and intrusive formations. However, they do not play any significant role in the localization of gold mineralization. Only feldspar and quartz-feldspar barren veinlets develop in them. Only near ore-bearing tectonic zones are they sometimes filled with quartz-ore material. Thus, fracture tectonics prepared both the pathways for circulation of hydro-

thermal solutions and the sites of localization of the gold-ore occurrences of the area.

As for the dike formations, which are rather widely developed in the Solgon area (Fig. 1), in relation to the gold mineralization they are pre-ore. The following facts support this: 1. The dikes, like the majority of the gold-ore veinlets, fill tension fractures in diabases and diorite-diabases. 2. Quartz-ore veinlets are often situated along the dikes; moreover, on approaching a dike, they change their strike from cross-cutting to subparallel, as if spreading out along the contacts of the dikes. In these places the dikes, although weakly, are affected by processes of hydrothermal metamorphism: partial silicification, chloritization and amphibolitization of pyroxenes, saussuritization, epidotization, and carbonatization of plagioclases. Evidently, the dikes here played the role of peculiar barriers limiting the spread of hydrothermal mineralization. 3. Nowhere were we able to establish facts of an obvious cutting of ore veinlets by dikes or xenoliths of silicified rocks (ores) in the dike bodies. Probably, the emplacement of the dikes preceded the hydrothermal processes. Most likely, the magmatic source both for the dike formations and for the subsequent hydrothermal ore manifestations was a single one.

The confinement of the ore deposits of the Kommunar ore field to zones of faults of submeridional strike was noted by D. I. Kalinnikov and N. A. Fogelman [4].

The association of gold-ore deposits with submeridional faults is also noted for the Central deposit and a number of other deposits in the Kuznetsk Alatau [3].

The example of the Solgon ore occurrences once again emphasizes the connection of gold mineralization with large tectonic zones of submeridional strike. Therefore, the presence of major dislocations near the parent Solgon intrusion can evidently serve as an important prospecting criterion. Within the described area, the northern part of the area, adjacent to the Solgon dislocation, is highly promising for ore gold; there, according to data from heavy-mineral sampling, individual promising sites are identified.

REFERENCES

1. MacKinstry G. I. Structure of hydrothermal ore deposits. In: Problems of Ore Deposits, no. 1, 1959.

2. Pospelov G. L. Materials for the tectonics of intrusions. Proceedings of the Tomsk Polytechnic Institute, vol. 62, issue 1, 1944.

3. Rozanova N. A. Structural conditions of localization of gold mineralization at the Central deposit (Kuznetsk Alatau). Geology of Ore Deposits, no. 2, 1960.

4. Fogelman N. A., Shablovskii A. E. Conditions of localization of stockwork deposits within the Kommunar ore field on the eastern slope of the Kuznetsk Alatau. Transactions of NIGRIzoloto, no. 21, 1956.