PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
NAMED AFTER S. M. KIROV,
ORDER OF THE OCTOBER REVOLUTION AND ORDER OF THE RED BANNER OF LABOR

Volume 236　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1976

ON THE LITHOLOGICAL-PETROGRAPHIC CHARACTERIZATION OF PRECAMBRIAN CONGLOMERATES IN THE UPPER REACHES OF THE VOROGOVKA RIVER (YENISEI RIDGE)

A. D. NYUZHKIN, V. A. GAVRILENKO, A. G. MIROPOV

(Submitted by the Department of Mineral Deposits)

In assessing the prospects for the metal-bearing potential of ancient conglomerates, a whole series of features is taken into account: general geological, mineralogical-geochemical, and also lithological-petrographic. Moreover, the latter play an essential role (3, 5).

The present work is devoted mainly to characterizing the lithological-petrographic composition of coarse-clastic deposits developed at the base of the volcanogenic-terrigenous sequence of the Upper Proterozoic.

The area of investigation is located in the northern part of the Yenisei Ridge, within the northeastern plunge of the Tatarka anticlinorium. The principal structural element of the area is the Vorogovka graben-syncline, formed mainly by rocks of a volcanogenic-sedimentary complex which, in stratigraphic position, most likely corresponds to the Sukhopit Series (1, 2, 3). In the middle part of the complex there occurs a volcanogenic-terrigenous sequence which, by a number of features, is correlated with the lower horizons of the Uderey Formation. Within the sequence three members are distinguished: basal, middle, and upper. In the structure of the lower horizons of the basal member, conglomerates and the gravelites and sandstones that facies-replace them take a substantial part. The middle member is sandy-shaly in composition, and the upper one is volcanogenic-terrigenous.

The basal member has been established in several spatially isolated areas. Its outcrops were observed in the area of Mt. Kovriga, in the upper reaches of the Vorogovka River, and in the headwaters and middle course of the Olenya River. Since the structure and composition of sections of the basal horizons, as well as their relationships with the underlying rocks, are not identical in different parts of the area under consideration, the lithological-petrographic characterization of the conglomerates is in each case preceded by a description of their particular section.

The relationships of the coarse-clastic deposits with the rocks of the underlying sequence are most clearly established in the section on the northwestern slopes of Mt. Kovriga (Fig. 1). The structure of this section is given below (from bottom to top):

1. Greenish-gray microschists. Thickness 8.0 m.

2. Dark-green pyritized metaporphyrites. Thickness 2.5 m.

3. Greenish-gray gravelites with isolated “floating” quartz pebbles up to 1.5 cm in size, pyritized. Thickness 2.5 m.

Fig. 1. Lithological columns of the lower horizons of the basal member (vertical scale 1 : 200). I. Northwestern slope of Mt. Kovriga; II — watershed of the second right tributary of the Vorogovka River and the Olen’ya River (area of elevation 665), III — northwestern slope of elevation 574, left bank of the Olen’ya River (the first seven horizons of the text), IV — watershed of the third right tributary of the Vorogovka River and the Olen’ya River; 1 — schists, 2 — schists with fragments (“eyes”) of effusive quartz, 3 — schists with “pebbles” of the underlying rocks, 4 — sandstones, 5 — sandstones with “floating” pebbles, 6 — gravelly sandstones, 7 — gravelites (gravel conglomerates), 8 — fine-pebble conglomerates, 9 — medium-pebble conglomerates, 10 — coarse-pebble conglomerates, 11 — acid effusives, 12 — metaporphyrites, 13 — diabase porphyrites, 14 — shritization.

1. Sandstones, dark gray with a greenish tint, fine-grained. Thickness 0.8 m.

2. Felsites, pinkish-gray, massive. Thickness 0.2 m.

3. Aleurolitic sandstones, gray, fine-grained. Thickness 1.0 m.

4. Metaporphyrites, greenish-gray, pyritized in the hanging wall of the body. Thickness 0.7 m.

5. Aleurolitic sandstones, greenish-gray, fine-grained. Thickness 0.7 m.

6. Quartz felsites, pinkish-gray, massive. Thickness 2.5 m.

7. Microschists, greenish-gray, interbedded with fine-grained aleurolitic sandstones. Thickness 7.0 m.

8. Gravelites, gray, with individual “floating” pebbles of quartz and rare disseminated pyrite. Thickness 0.9 m.

1. Greenish-gray, fine-grained sandstones. Thickness 1.6 m.

2. Quartz felsite-porphyries. Thickness 0.4 m.

3. Greenish-gray gravelites, compressed, intensely pyritized in the lower parts. Thickness 0.5 m.

4. Gray quartz felsite-porphyries. Thickness 0.3 m.

5. Greenish-gray aleurolite-sandstones. Thickness 0.3 m.

6. Dark-green diabase porphyrites with a uniform dissemination of pyrite. Thickness 10.1 m.

7. Greenish-gray conglomerates, medium-pebble, in places coarse-pebble, weakly pyritized in the lower parts. Thickness 4.2 m.

8. Dark-green diabase porphyrites. Thickness 0.5 m.

9. Fine-grained, greenish-gray sandstones. Thickness 0.4 m.

10. Gray gravelites with an isolated “floating” pebble of quartz. Thickness 1.6 m.

11. Greenish-gray, fine-pebble conglomerates, pyritized. Thickness 0.9 m.

12. Gray, greenish-gray sandstones, passing toward the end of the interval into gravelite-sandstones. Thickness 1.5 m.

13. Gravelites with interbeds of fine-pebble conglomerates. Thickness 1.5 m.

14. Fine-pebble, gray conglomerates. Thickness 0.3 m.

15. Dark-green diabase porphyrites. Thickness 0.5 m.

16. Greenish-gray gravelites. Thickness 1.0 m.

17. Fine-pebble, greenish-gray conglomerates. Thickness 1 m.

18. Green-gray gravelites with an isolated large “floating” pebble 1.5–2 cm in size. Thickness 1.5 m.

19. Medium-pebble, gray conglomerates. Thickness 1.0 m.

20. Even-grained, gray gravelites. Thickness 0.7 m.

21. Fine-pebble, gray conglomerates. Thickness 1.7 m. The total thickness of the rocks of the section is about 65 m. The thickness of the conglomerates and sandstones is 28 m.

A distinctive feature of this section is that here, in essence, a completely gradual transition is established from the volcanogenic-sedimentary green-schist unit with magnetite to the lower coarse-clastic horizons of the overlying unit. The noted character of the transition is expressed in the gradual upward replacement in the section of the green-schist unit of microschists by increasingly coarse-clastic terrigenous rocks. In the lower parts of the transitional zone, among green sericite-chlorite-quartz microschists with biotite, isolated interbeds of aleurolite-schists and sandstones appear. Higher up, in addition to these, thin horizons of gravelite-sandstones and gravelites are observed, and then conglomerates as well. Consequently, the regressive character of the structure of this part of the section is quite clearly revealed here.

Attention is also drawn to the fact that the conglomerates occur stratigraphically above the horizons of acidic and subordinate basic effusives (felsites, felsite-porphyries, quartz porphyries, metaporphyrites), intercalated with green microschists, and, in turn, all these rocks, including the conglomerates, contain concordant bodies of later diabase porphyrites. The latter is confirmed by the increased metamorphism of the conglomerates in the near-contact parts with the porphyrites, their good recrystallization, and also the complete absence of fragments of basic rocks in the conglomerates.

A similar section of coarse-clastic deposits is also found on the Kovriga River, where, above a unit of green microcrystalline schists with magnetite, gradually giving way upward in the section to sandstones and gravelites, there is observed a thirty-meter pack of alternating fine- and medium-pebble conglomerates, gravelites, and subordinate

Fig. 2. Lithological characterization of conglomerates.
1 — quartzites; 2 — schists; 3 — acidic effusives; 4 — sandstones; 5 — feldspars; 6 — gneisses; 7 — quartz–tourmaline rocks; 8 — quartz; 9 — minerals of the heavy [[unclear: tuffogenic?]] fraction; 10 — heavy-mineral concentrate.

the interbeds of sandstones gradually increases and their composition changes. In the clasts, in addition to quartz, feldspar is present in amounts up to 10–15%; the magnetite content decreases. Still higher, separate horizons of gravelly sandstones, gravelites, and then fine-pebble conglomerates are noted. With the appearance of the first horizons of gravelites and conglomerates, a coarse rhythmicity in the structure of the section becomes evident, where each rhythmic packet begins with shales or fine-grained sandstones and is crowned by a conglomerate horizon. In the transition zone from the green-schist sequence to the more coarse-clastic volcanogenic-terrigenous sequence, with a thickness evidently no less than 50–70 m, there is a whole series of conglomerate horizons differing in granulometric composition and in certain other features. The conglomerates of the lower horizons are gravelly and fine-pebble, whereas those of the upper horizons are usually medium-pebble. The latter are better sorted and more uniform in clast composition. The clastic material is better packed and rounded. A fuller characterization of these conglomerates is given in the table (Fig. 2). As in the area of Mount Kovriga, the conglomerates of all horizons are greenish gray, fairly dense, and noticeably compressed. The pebbles are represented predominantly by quartz, quartzites, and subordinate flattened clasts of microshales, sandstones, acidic effusives, and gneisses.

In the composition of the filling mass of the conglomerates, quartz and subordinate grains of feldspar predominate. Heavy-mineral fractions are represented mainly by magnetite and tourmaline, as well as rutile, zircon, and garnet. Moreover, magnetite, rutile, and partly tourmaline, as in the conglomerates of Mount Kovriga, are neogenic constituents of the cement.

The cement is chlorite-sericite-quartz, with individual large flakes of muscovite and, rarely, greenish-brown biotite; it is contact or basal. Micaceous minerals and quartz are in close intergrowth. Regeneration, corrosion, and suture-like joining of clasts are widespread.

Above the conglomerate horizons, with gradual transitions, lie poorly sorted pinkish-gray gravelly sandstones and sandstones enriched in feldspar clasts, which are replaced by light-gray sandstones and dark-gray shales with a lilac tint.

A somewhat different character of the section of the lower horizons of the sequence and of their relations with the underlying formations is observed in the upper reaches (elevation 665 m) of the Olenya River (Fig. 1). Here, above acidic effusives represented by an alternation of quartz porphyries and felsite-porphyries, the following are observed (from bottom to top):

1. Light-green or pinkish-gray quartz-sericite shales enriched in “eyes” of quartz (“eyes” are rounded grains of water-clear quartz up to 1–1.5 mm in size); in the upper part of the section they contain sandstone horizons with “floating” quartz pebbles 0.5–1 cm in size and rare disseminated pyrite. Thickness 25 m.

2. Pinkish-gray, well-sorted, pyritized gravelites. Thickness 2.2 m.

3. Greenish-gray quartz-sericite shales. Thickness 1 m.

4. Light-gray medium-grained sandstones. Thickness 1 m.

5. Greenish-gray gravelites. Thickness 1.2 m.

6. Greenish-gray medium-pebble conglomerates with isolated larger (5–6 cm) pebbles of quartz and acidic effusives, weakly pyritized. Thickness 1.4 m.

7. Gray, fine-grained sandstones, passing upward in the section into pinkish-gray gravelly sandstones and gravelites. Thickness 3.5 m,

1. Pinkish-gray conglomerates, from medium-pebble to coarse-pebble, well rounded, pyritized. Thickness 2 m.

2. Fine-grained pinkish-gray sandstones alternating with greenish-gray quartz–sericite schists. Thickness 2 m.

3. Pyritized pinkish-gray gravelites with individual coarser pebbles (up to 2 cm) of quartz and quartzites. Thickness 2 m.

4. Coarse-pebble conglomerates, weakly pyritized, pink with a greenish tint, well packed (thickness 1 m), passing upward in the section into fine-pebble conglomerates and then into gravelites. Total thickness 2.3 m.

5. Alternation of greenish-gray gravelites, sandstones, and schists. In the lower part of the interval, gravelites with rare disseminated pyrite predominate; higher up, sandstones and schists predominate. Thickness 18 m. The total thickness of the exposed part of the section is about 60 m.

A distinctive feature of this section is the occurrence of terrigenous rocks of the lower horizons of the unit under consideration resting with an erosional unconformity on underlying effusives. According to the studies of F. P. Krendelev (3), the light-green quartz–sericite schists with fused quartz segregations are redeposited products of the destruction of acid effusives. This latter circumstance imparts a definite imprint on the lithological composition of the basal horizons.

In this section, three horizons of conglomerates are noted, occurring at the tops of three regressive packets, each of which begins with sandstones or schists (Fig. 1); moreover, the lower horizon of conglomerates is observed 30 m above the base of the unit, while the upper one marks the beginning of a transgressive rhythm, where coarse-clastic deposits are replaced, quite gradually through gravelites and sandstones, by schists. It is also characteristic that the coarse-pebble conglomerates are confined to the transition zone from the regressive rhythm to the transgressive one.

In terms of the ratio of pebbles to filling material, the conglomerates vary from weakly saturated (the two lower horizons) to strongly saturated (the upper horizon). The latter are characterized by dense packing, good roundness of the clasts, and their sorting. The amount of pebble material ranges from 65–90%. Unlike the conglomerates described above from the area of Mt. Kovriga, here the pebbles include 20–25% pink or sealing-wax-red quartz porphyries, which give the rocks as a whole a pinkish-gray coloration; and in the fine-clastic fraction of the filling material there are grains—“eyes”—of water-clear effusive quartz (up to 70% of the filling mass), as well as pellitized potassium feldspar. Among accessory minerals, tourmaline, rutile, and more rarely zircon are present. Pyrite and pseudomorphs of limonite after it are comparatively widespread here, occurring as scattered disseminations 0.1 to 4 mm in size or as aggregate accumulations. In connection with neogenic quartz and chlorite, magnetite is noted in small amounts.

The cement type is basal in the undersaturated conglomerates, and contact and pore cement in the saturated ones; in composition it is most often sericite–quartz. The predominant mineral of the cement is usually fine-crystalline newly formed quartz, more rarely sericite, associated with acicular newly formed tourmaline and rutile.

The orientation of the newly formed minerals emphasizes the microgranolepidoblastic structure and the fine-fibrous microtexture of the cement.

The coarse-clastic horizons of the basal packet rest with a clear erosional unconformity on the underlying rocks also on the watershed of the third right tributary from above of the Vorogovka River and the Olenya River (northwestern slopes of elevation 574 m). The exposed part of the section here has the following structure (from bottom to top):

1. Light-green, thin-platy microschists. Thickness 49 m.

2. Gravelite-like lilac-gray schistose rocks with numerous pebbles (cobbles) of greenish schists and sandstones. Thickness 6 m.

3. Lilac-gray aleuroschists. Thickness 0.4 m.

4. Lilac-gray fine-grained sandstones. Thickness 1.5 m.

5. Lilac-gray gravelites with interbeds of coarse-grained sandstones. Thickness 5 m.

6. Fine-pebble, intensely pyritized conglomerates of light-gray color with a lilac tint. Thickness 3 m.

7. Lilac-gray sandstones passing into gravelite-sandstones and gravelites. Thickness 8 m.

8. Alternation of lilac-gray gravelites, sandstones, and schists. Total thickness 25 m.

9. Fine-grained lilac sandstones with separate “floating” pebbles of quartz, interbedded with greenish-gray schists. Thickness 16 m.

10. Lilac-gray gravelites, toward the end of the interval passing into fine-grained sandstones. Thickness 8 m.

11. Violet-gray schists interbedded with fine-grained sandstones. Thickness 7 m.

The total thickness of the basal horizons of the member is about 80 m. A noteworthy feature of this section is the nature of the transition from the underlying green-schist sequence, uniform in composition, to the coarse-clastic rocks of the conglomerate member. In the transition zone, on the green microschists, there lies a six-meter horizon of lilac-gray schistose rocks containing, on the schistosity planes, numerous flattened “slickensided” pebbles (cobbles) of sandstones and underlying schists, representing a product of the reworking of sediments of the upper part of the underlying sequence (Fig. 1).

Above the transitional horizon, a regressive series of sediments is observed, consisting of alternating schists, sandstones, and gravelites, more rarely fine-pebble conglomerates. In the structure of the section here, a coarse rhythmicity is also established, with variable thicknesses of individual rhythmic packets and with sandstones and gravelites predominating in their composition.

The conglomerates of this section are predominantly gravel or fine-pebble formations. They are characterized by good packing and sorting of the material. In terms of the ratio of pebbles to cement they are saturated. Their clastic fraction constitutes no less than 85–90% of the rock volume.

In terms of clast composition, the conglomerates and gravelites are distinctly oligomictic. As much as 70–75% of the clasts consist of milky-white, more rarely water-clear and pinkish quartz; light-gray and pinkish quartzites have subordinate importance (20–25%), as do lilac-gray quartz–hematite sandstones (1–3%). The roundness of the clasts is weak—1–2 points. The filling material is not visually distinguishable. Under the microscope, a fine-clastic component of the cementing mass is found, in an amount of 10–15% of it, represented by angular grains of quartz and quartzites, more rarely tourmaline. The cement is exclusively contact or pore cement; in composition it is sericite–quartz with an admixture of hematite, more rarely chlorite.

A characteristic feature of the conglomerates and sandstones of the lower horizons of this section is superimposed carbonatization of the cement. On average, the carbonate content is 3–5%.

Of all the above-described sections of coarse-clastic deposits, the conglomerates developed ... are markedly distinguished in lithological composition.

in the area of the middle course of the Olenya River, 4–5 km above its mouth. The direct relationship of the coarse-clastic rocks with the underlying rocks was not observed. However, on the basis of indirect evidence (observations in rare test pits), it may be considered that the transition here from shales to sandstones and conglomerates is also gradual. A generalized section (Fig. 1) for a series of bedrock exposures developed on the watershed of the third right tributary from the top of the Vorogovka River and the Olenya River is given below (from bottom to top):

1. Unevenly coarse-grained lilac-gray sandstones with “floating” quartz pebbles up to 1.5–2 cm in size. Thickness 2.8 m.

2. Greenish-gray gravelites with a thin (15–20 cm), lens-shaped interbed of medium-pebble conglomerates. Thickness 1.4 m.

3. Ash-gray shales interbedded with fine-grained sandstones. Thickness 0.4 m.

4. Coarse-grained sandstones unevenly enriched in gravel material. Thickness 2.2 m.

5. Conglomerates, predominantly medium-pebble, dark gray with a lilac tint, passing locally into coarse-pebble conglomerates and containing thin lens-shaped interbeds of dense lilac sandstones. Thickness 28 m.

6. Fine-grained dense lilac-gray sandstones with rare “floating” quartz pebbles and indistinct segregations—lenses—of gravel-pebble conglomerates. Thickness 5 m.

7. Pistachio-green chlorite-sericite-quartz shales. Thickness 0.2 m.

8. Medium-pebble dark-gray conglomerates with a lilac tint, passing upward in the section into gravelites and sandstones. Thickness 3.6 m.

9. Interbedding of lilac-gray fine-grained sandstones, shales, and gravelites. The boundaries of the beds are very indistinct, the transitions gradual and difficult to discern. Thickness 5.3 m.

10. Greenish-gray shales with thin, discontinuous horizons of fine-grained sandstones. Thickness 2.2 m.

11. Coarse-pebble conglomerates with individual boulders of quartz and quartzites up to 15–20 cm in size, passing into unevenly clastic gravel-sandstones with interbeds of dark-gray shales. Thickness 4.8 m.

12. Fine-grained dense lilac-gray sandstones. Thickness 2.5 m.

The total thickness of the section is about 34 m.

The conglomerates and gravelites of this section are distinguished by massive structure, without signs of fissility or traces of pressure, and are characterized by poor sorting of material. In clast composition they are distinctly polymictic (Fig. 2). Along with clasts characteristic of the types of conglomerates described above, yellowish-gray and light-gray quartzite-like sandstones (20–25%), dark violet-gray tuffaceous rocks (8–10%), lilac-gray or greenish-gray quartz-chlorite-sericite shales (5–15%), and dark-gray, almost black quartz-hematite shales (5%) are additionally noted here.

The clastic material is poorly rounded (1–2 points). The pebbles are angular and subangular, and for the most part bear traces of deformation. Clasts of shales and quartzite-sandstones have a flattened lenticular form. The filling material (up to 15–20% of the rock volume) is approximately analogous in composition to the coarse-clastic fraction. It should only be noted that it contains a somewhat elevated (up to 10–15%) content of pyroclasts.

The cement is predominantly basal, more rarely contact, and in composition is sericite-quartz with an admixture of ferruginous material. The latter is usually represented by fine, indistinctly crystalline ore dust, more rarely

by plates of hematite occurring in intergrowth with quartz and sericite.

Among the minerals of the heavy-mineral concentrate, tourmaline, zircon, garnet, and limonite pseudomorphs after pyrite were found. Similar conglomerates are widespread in scree deposits on the right bank of the middle course of the Olen’ya River, 4.5 km above its mouth. These screes are evidently products of the destruction of the lower horizons of the sequence under consideration. This statement may be confirmed by the section of deposits of the right bank of the Olen’ya River observed upstream in bedrock exposures. The first bedrock exposure, noted above the screes, is represented by greenish-gray and gray fine-pebble conglomerates with a slight lilac tint, with zones of intense silicification, chloritization, and pyritization. Pyritized conglomerates and sandstones were established here as well as in the screes. Evidently, the bedrock exposure of the silicified and pyritized conglomerates spatially coincides with a tectonic disturbance, since these rocks are noticeably schistose. Along the schistosity planes the development of very fine hematite is observed. The fine-clastic material of the cement is recrystallized. A granoblastic aggregate of quartz forms lens-shaped segregations among the corroded larger fragments. Pyrite is observed in association with newly formed quartz, more rarely with sericite and chlorite.

Stratigraphically above the conglomerates there is a rhythmically bedded series of sediments which, by its position, corresponds to the middle and upper parts of the basal sequence. Moreover, at the beginning of the exposed part of the section the rhythmic beds are represented by gray and lilac-gray gravelly conglomerates and sandstones with subordinate dark-gray shales. The conglomerates and sandstones occur as continuous monolithic beds, persistent along strike, 0.5–4 m thick. Gradually upward in the section, the number and thickness of sandstone beds decrease, and shales become the predominant rocks. The most characteristic feature of this section is the presence of at least 8–9 horizons of pyritized gravelites and, especially, sandstones.

The gravel conglomerates and sandstones are well sorted and densely packed. The comparatively well-rounded clastic material is represented mainly by quartz (75–80%) and microquartzites (15–20%), more rarely by single fragments of other rocks. The cement is contact and pore cement; in composition it is predominantly sericite-quartz, and rarely ferruginous-sericite-quartz. The pyritized rock is distinguished by an increased content of newly formed quartz and sericite, by the local presence of chlorite and ferruginous carbonate, and also by general lightening, evidently due to the conversion of a certain part of oxidized iron compounds into pyrite. The latter is observed both as a uniform dissemination of crystals ranging in size from fractions to 4–5 mm, and as nests of lenticular or rounded form. Aggregate fillings of nests, represented by finely crystalline pyrite, quartz, sericite, and possibly clay minerals, have a specific ash-gray color.

Of interest is the fact of the gradual replacement of gravelites and conglomerates of the basal horizon northeastward by quartz sandstones. Thus, at the mouth of the Olen’ya River and on the left bank of the Vorogovka River, at the base of the basal sequence, conglomerates are completely absent. The lower horizons of the unit here are composed exclusively of oligomictic sericite-quartz or quartz sandstones, which should be considered separately, since here only the coarse-clastic rocks of the basal sequence are in question.

Conclusions

The factual material presented makes it possible to draw the following conclusions concerning the structure, composition, and conditions of formation of the studied horizons of coarse-clastic deposits:

1. In the basin of the upper reaches of the Vorogovka River, at the base of the volcanogenic-terrigenous sequence that we assign to the Uderei suite, there is observed a coarsely rhythmic-bedded series of sediments, represented in some cases (the upper reaches of the Vorogovka River, Mount Kovriga, the headwaters and middle course of the Olenya River) by six to ten rhythmic members with horizons of gravelites and conglomerates with a total thickness of 30–120 m, and in others (the mouth part of the Olenya River) by substantially oligomictic sandstones and sharply subordinate shales.

2. The nature of the relationships of the basal horizons with the underlying deposits varies. Within the western limb of the graben-synclinal structure (the area of Mount Kovriga, the upper reaches of the Vorogovka River), quite distinct gradual transitions are observed through regressive rhythmic members of shales and sandstones to gravelites and conglomerates. At the southeastern closure of the synclinal structure and within its eastern limb, the coarse-clastic formations locally rest with erosion on the underlying shales and acidic effusives.

3. The conglomerates are massive, locally compressed, noticeably laminated rocks which, in terms of grain-size composition, belong chiefly to the fine- and medium-pebble varieties. On the basis of lithological composition and sorting of the clasts, two types of conglomerates are clearly distinguished: oligomictic and polymictic, with widely distributed transitional varieties approaching the oligomictic ones.

A characteristic feature of the polymictic conglomerates, developed, for example, in the middle course of the Olenya River, is poor packing, weak sorting and rounding of the clasts, as well as the predominance of a basal type of cement, as a rule ferruginous–sericite–quartz in composition.

Oligomictic conglomerates are distributed in the middle course of the Olenya River, where upward in the section they gradually replace polymictic ones, and also on the watershed of the third upper right tributary of the Vorogovka River and the Olenya River and in the headwaters of the latter. In clast composition these conglomerates are predominantly quartzose, well packed and sorted, and are often pyritized.

Individual horizons of oligomictic conglomerates and conglomerates approaching them in composition have been noted in the western limb of the structure (the area of Mount Kovriga, the upper reaches of the Vorogovka River). Their distinguishing feature is increased metamorphism, manifested in the development of elements of schistosity, in the appearance of a significant amount of newly formed chlorite and especially magnetite, locally carbonate. The fine-clastic component of the filling material is often completely recrystallized. The locally developed pyritization zones here usually have a crosscutting character with respect to schistosity.

1. Accessory minerals are clearly subdivided into two groups: clastogenic (zircon, cyrtolite, monazite, tourmaline, apatite, garnet, etc.) and newly formed (magnetite, leucoxene, rutile, pyrite, more rarely brannerite, tourmaline, etc.).

2. According to the ratio of the principal iron minerals, which are newly formed minerals (magnetite, pyrite) or a constituent element of the cement (hematite and other iron oxides of indistinctly crystalline habit), the following subtypes may be distinguished among the conglomerates: a) hematitic (middle course of the Olenya River), b) magnetitic (western limb of the syn-

clinal structure), c) pyrite–magnetite (area of Mt. Kovrigi), d) pyrite–hematite (local zones among hematite-bearing conglomerates in the middle reaches of the Olenya River), e) pyritic (individual horizons within the eastern limb of the structure).

1. The formation of the basal member evidently took place under conditions of strongly dissected relief, against the background of developing basic volcanism and continuing tectonic activity in the area; as a result, the character of sedimentation differed in different parts of the area. Thus, in the west of the synclinal structure, conglomerates and the polymictic sandstones replacing them rapidly pass into a monotonous sequence of violet-gray shales. Within the southeastern limb of the syncline, polymictic conglomerates with fragments of similar violet-gray shales are first replaced by horizons of oligomictic sandstones and then by violet-gray shales.

Farther north, at the base of the basal member, conglomerates are completely absent, giving way to fine-clastic varieties. These facts indicate that the formation of the basal horizons did not occur strictly simultaneously. Earlier than the others, horizons of coarse-clastic deposits of the western part of the area were formed; these crown the regressive members in the transition zone from the greenschist sequence with acid effusives to the volcanogenic–terrigenous sequence under consideration, as well as a certain part of the section of the violet-gray shale sequence. After a certain break, polymictic conglomerates with fragments of these shales accumulated in the southeastern part of the territory and, finally, horizons of oligomictic sandstones in the northeastern part of the area.

The formation of the basal horizons of the eastern sectors was preceded by a continental regime, most likely of an insular type, under the conditions of which weathering crusts formed locally.

The presence of horizons of oligomictic conglomerates testifies to a possible repeated reworking of sediments under conditions of a relatively calm tectonic regime.

1. In the degree of regional metamorphism, the conglomerates differ noticeably. The variegated conglomerates of the eastern limb of the structure are relatively weakly metamorphosed. The lilac-gray and violet-gray tones of their coloration are due precisely to the fact that violet-gray shales are present among the fragments, while the cement contains ferruginous material which, in the process of blastesis, has been only partly, and more rarely completely, transformed into hematite. The coarse-clastic rocks of the western part of the area prove to be more metamorphosed. Complete recrystallization of the primary ferruginous-clayey cement, with the formation of a chlorite–sericite–quartz aggregate with magnetite, causes the lightening of these rocks and promotes the appearance of greenish-gray tones of coloration. We see the reason for the increased metamorphism of the deposits of the western limb of the structure in its spatial confinement to the eastern near-axis part of the Tatarka anticlinorium, composed of gneissic and crystalline-schistose sequences of the lower structural stage, intruded by Upper Proterozoic granitoids.

2. Pyritization, sericitization, locally silicification and carbonatization of beds most permeable to solutions (4)—oligomictic sandstones and conglomerates—as well as the formation of cross-cutting mineralized zones in rocks of different composition, owe their origin to processes of regressive metamorphism.

In conclusion, let us note that the lithological–petrographic characterization presented allows us, from this point of view, to speak more definitely about the closeness of some horizons of oligomictic conglomerates and sandstones to the previously distinguished metal-bearing conglomerates of the “sulfide” type of F. P. Krendelev (3, 5).

References

1. G. I. Kirichenko. Geological structure and mineral resources of the Yenisei Ridge. In: Materials on the Geology of Krasnoyarsk Krai, Moscow, Gosgeolizdat, 1960.

2. T. Ya. Kornev. Stratigraphic position, formational affiliation, and composition of the volcanogenic strata of the rift zones of the Yenisei Ridge. In: Stratigraphy, Lithology, and Problems of Metallogeny of the Paleozoic and Late Precambrian of the East of the Altai–Sayan Region, Krasnoyarsk, Book Publishing House, 1968.

3. F. P. Krendelev. Prospects for the search for ancient conglomerates in Siberia. Geology and Geophysics, No. 5, 1965.

4. F. P. Krendelev. Microstylolites and the porosity of ancient conglomerates of the Witwatersrand type. Report at the VII Congress of the Carpatho-Balkan Association in Sofia, 1965.

5. F. P. Krendelev. Classification of ancient metalliferous conglomerates. Proceedings of TPI, vol. 134, 1968.

6. Reference Manual on the Petrography of Sedimentary Rocks, vol. 1, II. Gosnauchtekhizdat. Leningrad, 1958.