Precambrian Volcanogenic-Sedimentary Formations of the Kommunar Mine Area

(Eastern Slope of the Kuznetsk Alatau)

V. E. Nomokonov, L. G. Osipov

(Presented by Professor A. M. Kuzmin)

This work is devoted to the geology of the Kommunar ore field, composed chiefly of volcanogenic-sedimentary rocks of the Riphean. The Vendian, Lower and Middle Cambrian deposits (Fig. 1), widely developed to the ENE of the mine, are younger members of the Riphean-Cambrian geosyncline of the Kuznetsk Alatau and cannot be placed at the same stratigraphic level as the most ancient rocks of the Kommunar district. The volcanogenic-sedimentary strata described below are correlated by us with the metadiabases of the Beloyus series of V. M. Yaroshevich [10], and with the Nyuska (Pyzass) suite of D. I. Musatov [6]. Their Riphean age is reliably demonstrated by the flora Newlandia, contained in the overlying volcanogenic-terrigenous-carbonate Poludennaia suite [8].

In lithological composition, the oldest rocks of the Kommunar ore field are subdivided into two heterogeneous strata: a diabase-porphyrite stratum and a volcanogenic-sedimentary stratum. The first of these corresponds to a spilite-diabase subformation, and the second to a flyschoid subformation of the initial stage in the development of the Riphean-Cambrian eugeosyncline of the Kuznetsk Alatau.

The diabase-porphyrite stratum has a considerable distribution and forms mainly the core of an anticlinal fold, the axial line of which has a nearly meridional strike and passes somewhat west of Podoblachnyi Golets. In addition, its exposures are observed along the Fedorovskii key, where it is brought to the surface along the Saktychul-Fedorovskii fault. Exposures of the stratum extend in a narrow belt southwest of Podzvezdnyi Golets through the Kaliostrovskoe deposit, along the left bank of the Saktychul key, and are traced farther to Olginskii Golets. A considerable part of the diaba-

Fig. 1. Schematic geological map of the Kommunar–Tyurim area, eastern slope of the Kuznetsk Alatau: 1 — Quaternary deposits; 2 — volcanogenic-sedimentary formations of the Lower–Middle Devonian; 3 — andesite porphyrites of the Koshkulak Formation of the Middle Cambrian; 4 — volcanogenic-terrigenous-carbonate deposits of the Lower and Middle Cambrian (Tunguzhul, Kolodzhul, Efremkino, Bezymyannaya, and Karasuk formations); 5 — dolomites of the Tarzhul Formation (Vendian); 6 — volcanogenic-clastic formations of the Kulbyurstug Formation of the Upper Riphean; 7 — volcanogenic-terrigenous-carbonate formations of the Podusinov Formation of the Middle and Upper Riphean; 8 — diabase-porphyrite and 9 — volcanogenic-sedimentary sequences of the Lower (?) and Middle Riphean; 10 — granitoids of the Upper Cambrian; 11 — sites where remains of fossil fauna were collected; 12 — sites where remains of fossil flora were collected; 13 — elements of bedding attitude; 14 — stratigraphic boundaries; 15 — disjunctives.

of the porphyrite sequence in the southern direction is cut off by the Solton granodiorite intrusion (Fig. 2).

In the northern direction, the belt of outcrops of the sequence widens considerably. On the left bank of the Bolshaya Sobaka River, the entire watershed between Klyuch Medvezhii and Klyuch Korovii is composed of rocks of this sequence; farther on it can be traced in the headwaters of Klyuch Medvezhii and the Izekyul River.

Lithologically, the sequence is represented by fairly monotonous rocks. The most widespread are diabases, diabase porphyrites and their tuffs, as well as spilites with spheroidal and pillow jointing. Less common are tuff-lavas and lava breccias. Normally sedimentary formations play a very minor role; they are represented

dark-gray bituminous marbled limestones, siliceous shales, and calcareous sandstones. Rare, thin horizons of keratophyres are also noted.

Horizons and lenses of sedimentary rocks are confined to the upper part of the sequence, which indicates a gradual change in the tectonic regime and an intermittent waning of volcanic activity.

Diabases, in outward appearance, are granular dark greenish-gray and greenish-gray rocks of typically greenstone aspect and massive texture.

Under the microscope they reveal ophitic, pilotaxitic and, very rarely, intersertal structures, obscured to one degree or another by the development of secondary minerals.

They consist of saussuritized and carbonatized laths of plagioclase No. 51–66 (up to 65%), green hornblende \((2v=-64^\circ—68^\circ,\ C:Ng=16^\circ,\ Ng=1.670)\); actinolite \((2v—82—83^\circ,\ C:Ng=12—-16^\circ,\ Ng=1.663)\) in the form of radial-fibrous and tangled-fibrous aggregates (up to 25%). Relicts of pyroxene grains 0.05–0.3 mm in size, referred to augite, are sometimes observed. As a rule, however, the pyroxene is completely replaced by amphiboles, epidote, and chlorite. Zoning is often manifested in the replacement of pyroxene grains: their central parts are composed of epidote, which is surrounded by a rim of green hornblende, which in turn is enveloped by green, almost isotropic chlorite.

Plagioclases are usually replaced by saussurite, carbonate, and epidote.

Accessory and secondary minerals are sphene with leucoxene, apatite, magnetite, and pyrite.

Among the diabases, varieties enriched in quartz (up to 10%) are noted, approaching congadiabases.

Porphyrites show gradual transitions to diabases. These are dark greenish-gray porphyritic rocks, often with an amygdaloidal texture. Porphyritic segregations constitute 5–15% of the rock volume and are usually represented by saussuritized and carbonatized plagioclase No. 48–54; pyroxene is rarely observed, having been entirely replaced by green hornblende and chlorite.

The groundmass is represented by a complex aggregate of secondary minerals (quartz, chlorite, carbonate, albite) and relics of microlites of saussuritized plagioclase.

Accessory minerals are apatite, magnetite, and pyrite. The amygdales have an oval-rounded form, 1–20 mm in size. They are filled with black and white calcite, chlorite, actinolite, and chalcedony.

Spilites are, as a rule, amygdaloidal rocks of dark-gray and greenish-gray color, with pillow or spheroidal jointing.

Under the microscope a fluidal-microlitic structure is observed. Albite microlites are immersed in a felt-like mass consisting of a quartz–carbonate–epidote–chlorite complex, which is a product of recrystallization of glass (?). Amphibole is also noted, often replaced by magnetite and chlorite. The amygdales more often have an oval-rounded and spindle-shaped form; they are filled with calcite, chlorite, chalcedony, actinolite, and epidote.

Lavobreccias and tuffolavas have a sporadic distribution. They consist of fused fragments of lavas, lapilli, bombs, and other products ejected from volcanic apparatuses. This pyroclastic material is cemented by lava of the same composition, in which traces of flow are clearly observed. Epidotization is characteristic.

tuff lavas, which are observed as patches ranging in size from several centimeters to several meters.

As already noted, in the upper part of the diabase-porphyrite sequence there appear thin lenses and horizons (up to 10–15 m) of normally sedimentary rocks—limestones, siliceous shales, sandstones—which are completely analogous to those of the overlying volcanogenic-sedimentary sequence.

The position of the diabase-porphyrite sequence in the section is established quite definitely: it underlies the upper volcanogenic-sedimentary sequence (Fig. 2). This is demonstrated by the drilling of numerous boreholes in the central part of the ore field, which cut the volcanogenic-sedimentary formations and sublayered bodies of gabbro-diorites and, beneath them, expose the diabase-porphyrite sequence. The underlying position of the latter is also confirmed by sections along the Mine Administration–Verkh. Stan road, as well as on the left side of the B. Sobaka River opposite the mouth of Fedorovskii Creek, where the diabase porphyrites are clearly overlain by sedimentary and pyroclastic rocks. The relations of the two sequences have also been exposed by a number of ditches during geological-structural mapping. Thus, north of the Podzvezdnyi bald peak, amygdaloidal porphyrites are in contact with tuff shales of the upper sequence, the contact dipping eastward at an angle of 42°. At the contact there is a small (up to 3 m) zone of interformational schistosity, along which bedding-plane slips probably occurred.

On the western slope of Mount Podoblachnyi, at the head of Braginskii Creek, the diabase-porphyrite sequence is found to dip westward beneath tuffs and tuffaceous sandstones of the volcanogenic-sedimentary sequence. The dip angle is 30°. To the north, the contact of the two sequences is tectonic. At the contact of the two sequences described, there are no traces of any break in sedimentation; usually the porphyrites are overlain by shales or tuffs without visible traces of erosion. The dip of bedding in horizons of sedimentary rocks in the diabase-porphyrite sequence, as well as numerous measurements of current structures, agree with the structure of the upper volcanogenic-sedimentary sequence, which indicates their concordant occurrence. The visible thickness of the diabase-porphyrite sequence in the Kommunar mine area is no less than 1000 m.

The volcanogenic-sedimentary sequence is known in the literature as the Ungur Formation [6, 7]. It is highly heterogeneous in lithological composition. Its characteristic feature is rhythmic structure and the facies discontinuity, along strike, of individual beds and rhythms. Most commonly normal rhythmicity is observed, more rarely reverse rhythmicity.

The usual rhythm consists of the following elements (from bottom to top): tuff conglomerates (sometimes tuff breccias)—tuff gravelites—tuffaceous sandstones—tuffaceous aleurolites—siliceous and tuffogenic shales—marls—limestones. The relative proportions of the various rocks differ, but in general there is some predominance of coarse-clastic and psammitic rocks. Often the normal rhythm passes into a reverse one: upward in the section, limestones are replaced by shales—aleurolites—sandstones—tuff breccias or tuff conglomerates, etc. The normal thickness of the rhythms is very variable both in section and along strike, from several centimeters to several tens of meters. Within macrorhythms, microrhythms are often noted. Very frequently incomplete rhythms occur; these are most often caused by the outpouring of thin flows of porphyritic lavas. Acid effusives and their tuffs are widely distributed within the sequence.

The volcanogenic-sedimentary sequence is cut by an intrusion of gabbro-diorites, the sublayered bodies of which are confined mainly to the contact of the two sequences. Spatially associated with the gabbro-diorite bodies are...

Fig. 2. Geological map of the Kommunar ore field (compiled by L. G. Osipov, A. F. Korobeinikov). Legend:
1 — diabase–porphyrite sequence (greenstone diabases, diabase porphyrites, spilites, tuff and lava breccias);
2 — volcanogenic–sedimentary sequence (tuffs of basic and acid effusives and products of their redeposition) with horizons of keratophyres and quartz porphyries (3), porphyrites (4), tuffaceous and siliceous-argillaceous shales (5), bituminous limestones (6), tuffaceous sandstones (7);
8 — gabbro-diorites and diorite-diabases of the Biya complex of the Upper Proterozoic;
9 — granitoids of the Upper Cambrian–Ordovician (Solgon pluton);
10 — disjunctive faults.

all the principal gold-ore deposits of the ore field are confined. Below we give a brief characterization of the rocks of the sequence.

Keratophyres. It was previously thought that keratophyres, which were combined into a spilite-keratophyre formation, are widely developed in the ore field. However, detailed mapping of the ore-field area, as well as analysis of sections from data of drill holes and underground workings, establishes a comparatively insignificant development of these rocks. All fine-grained and aphyric light-colored rocks were usually included in this category, since without microscopic study it is sometimes impossible to distinguish keratophyres from their tuffs, felsites and felsite-porphyries, as well as some metamorphic and hydrothermally altered rocks. Keratophyres are light greenish-gray, yellowish-gray quartz-feldspar rocks with porphyritic and microporphyritic structure and with a dense lithoid groundmass. The porphyritic segregations are represented by K-feldspar, albite, and sericitized oligoclase-andesine. The groundmass is dense, hornfels-like, microfelsitic, spherulitic, or granoblastic. It consists of albite, brownish K-feldspar, quartz, and secondary minerals—chlorite, sericite, and carbonate; secondary epidote and amphibole are sometimes encountered. The quartz content is uneven; sometimes it appears in significant quantities even in phenocrysts and gives rise to gradual transitions into quartz keratophyres.

Felsites are spatially associated with keratophyres. These are light-gray, sometimes yellowish aphanitic rocks, often with a banded texture. Under the microscope a homogeneous microfelsitic and micropoikilitic structure is visible; more rarely, microporphyritic structures are noted. They consist of alkali feldspar (50–60%) and quartz (40–50%) with an admixture of secondary minerals—flakes of chlorite, sericite, amphibole, and carbonate. They are very limited in distribution.

Porphyrites belong to hornblende-plagioclase varieties and make up discontinuous (up to 50 m) sheets and lenslike bodies among volcanogenic-sedimentary rocks. They have gray and dark-gray colors with barely noticeable porphyritic segregations of plagioclase and hornblende. Under the microscope a porphyritic or blastoporphyritic structure is visible. Phenocrysts 0.3–2 mm in size are represented by andesine No. 35–40 and opacitized hornblende, immersed in a blastomicrogranular groundmass. The latter consists of carbonates, chlorite, plagioclase microlites, and individual quartz grains. Plagioclase is usually replaced by an aggregate of sericite and zoisite, carbonates; hornblende—by magnetite, chlorite, carbonate. Magnetite rims are often observed along the boundaries of hornblende grains.

Tuff-conglomerates and tuff-breccias occupy a dominant position in the section. Some increase in their role in the section is observed in moving southward toward the Solgon area, where they compose up to 80% of the section. They are represented by crystal-lithoclastic, litho-crystal-clastic, and more rarely vitro-crystal-clastic varieties. In most cases the tuffs have a mixed composition and consist of fragments of keratophyres, quartz keratophyres, and porphyrites. Fragments of andesine-labradorite crystals (No. 45–56) are noted, together with a large quantity of fragments of quartz and alkali feldspar. The size of the fragments varies widely, from fractions of a centimeter to 1–2 cm. The crystalloclastic and vitroclastic varieties of tuffs have a more consistent average composition, close to that of plagioclase-hornblende porphyrites. In the tuffs and tuff-breccias there occur numerous fragments of normally sedimentary rocks, from

in which shales and aleuropelites sharply predominate. The cement is fine-fragmental pyroclastic material or volcanic devitrified glass. The tuffs show the most gradual transitions into tuff-conglomerates, tuff-sandstones, and tuff-shales, and from these into normal sandstones and siliceous shales.

Tuff-sandstones and sandstones. Coarse-, medium-, and fine-grained varieties are widespread, with a gradual transition, on the one hand, into gravelites and, on the other, into aleuropelites and tuff-shales. They consist of angular, more or less rounded fragments of effusive rocks, crystals of quartz and feldspar. The cement, as a rule, is of basaltic type: clayey, siliceous-clayey, or ash-clayey. The average ratio of clastic material to cement is 1:2. The cement of the sandstones is everywhere affected by secondary alteration, with the formation of sericite, chlorite, carbonates, and amphibole.

Aleuropelites and tuff-shales are fine-grained layered or shaly rocks forming lens-shaped bodies of varying thickness, from centimeters to tens of meters. They often contain numerous small lenses and interbeds of unreworked pyroclastic material. The tuff-shales are usually thin-bedded and not infrequently include interbeds of vesicular porphyrites. The internal structure of the shales and aleuropelites is very complex as a result of plastic deformations within beds and packets.

The mineral composition of the rocks consists of fragments of quartz and feldspars, clayey material, volcanic ash, and secondary alteration products—carbonates, chlorite, and sericite. Syngenetic pyrite is sometimes abundant, usually confined to the boundary between laminae differing in grain size.

Siliceous and siliceous-carbonaceous shales are much less widespread. They are microgranular, light-gray, bluish- and brownish-gray, or dark-gray rocks with a shaly or banded texture. Under the microscope the bedding is expressed distinctly either by mineral sorting or by grain size. The principal mineral is quartz, observed as angular or weakly rounded grains. Sericite, carbonates, and chlorite also occur. The shales are strongly pyritized. Varieties of shales are noted in which clayey material predominates and in which an admixture of chalcedony is observed.

Limestones occur as lenses, closely interbedded with siliceous shales. Typical varieties are black and dark-gray “sparkling” bituminous types. Only in one case, on the eastern slope of Mt. Podzvezdnyi, were cream-gray crystalline limestones encountered, occurring among shales. The structures of the limestones are fine- or medium-grained, more rarely coarse-grained. Coarsely crystalline varieties, with grains of black calcite up to 3 cm in size, sometimes occur as nests. Under the microscope the rocks exhibit a granoblastic structure and consist of calcite grains 0.08 to 2–3 mm in size, cemented by contact clayey-carbonate material. The cement is often entirely absent, and the limestones are pure marbles.

From the above petrographic characterization of the rocks it is evident that the volcanogenic-sedimentary sequence is composed of pyroclastic products of volcanic activity that are either unreworked or, to some extent, rewashed and sorted. At the same time, a close association of products of acidic and intermediate-basic effusives is observed, with the latter sharply predominating. The visible thickness of the described sequence within the ore field is not less than 800–900 m. In the area of Mt. Ungur, south of the ore field, the volcanogenic-

the sedimentary sequence has a thickness of about 1 km. The total thickness of the volcanogenic-sedimentary formations of the Kommunar mine area, lying definitely below the beds with Newlandia (see Fig. 1), is estimated at no less than 2 km.

The petrochemical features of the volcanogenic rocks of the sequences described and of the subintrusive bodies of the gabbro-diorite-diabase complex were discussed earlier by one of the authors of the present article (5). In terms of chemical composition, geochemical features, and the composition of accessory minerals, a close spatial and genetic relationship is established between the volcanogenic rocks of the formations described and the subintrusive bodies of the gabbro-diorite-diabase complex. This once again proves that the emplacement of the Riphean-Cambrian eugeosyncline of the Kuznetsk Alatau began with the formation of the spilite-keratophyre formation, comprising the spilite-diabase and flyschoid subformations.

REFERENCES

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