Geological Features of Precambrian Conglomerates in the Northeastern Part of the Yenisei Ridge in Relation to the Problem of Their Metal Potential

A. D. Nozhkin (TPI)

The interest of geologists in deposits of the ancient conglomerate type is well known; in essence, these deposits constitute the main basis for the development of the gold industry in capitalist countries. Such deposits have been discovered in Africa, Brazil, Canada, Australia, and Finland. Their common features are a Proterozoic age, proximity to platform structures, and the confinement of mineralization to oligomictic rock varieties \[1, 5, 6, 7\].

The position of provinces of metal-bearing conglomerates in the folded margins of platforms and on their slopes makes it possible to distinguish large regions promising for the discovery of deposits of this type. In Siberia, one such region is the Yenisei Ridge \[5, 6\], the northeastern part of which has been the subject of the author’s geological investigations for a number of years.

In the present case, the northeastern part of the ridge is understood as the territory of the interfluve of the Tei—Uvolga—Chapa—Vorogovka rivers, extending in a northwestern direction for a distance of about 130–150 km. The outline of this area spatially coincides with a zone of deep faulting confined to the junction area of two major Precambrian structures: the Tatar anticlinorium and the Korda-Lebyazhin synclinorium.

In accordance with the general scheme of the tectonic development of the Yenisei Ridge, three structural stages can be clearly distinguished in the structure of this territory, formed as a result of the manifestation of different phases of the Baikalian cycle of tectogenesis \[14\]. The lower stage is represented by metamorphic formations of the Tei series, developed in the near-axial part of the Tatar anticlinorium, and also in the area of the Noiba rigid uplift. The middle stage is composed of sedimentary-metamorphosed and volcanogenic sequences of the Sukhopit series, especially widely distributed within the Vorogovka graben-syncline and the Chapa syncline. Finally, the upper structural stage is formed by sedimentary rocks of the Chingasan (Taseeva) series of the Vendian complex, which make up the Uvolga graben.

Conglomerate formations and the quartz sandstones that laterally replace them are known here on at least three stratigraphic levels: at the base of the Sukhopit series—in the Korda (after the Korda River)

Formation, in the lower part of the Uderei (Olenya River, according to F. P. Krendelev) Formation and at the base of the Chingasan Series—in the Lopatin Formation.

The conglomerates of the Kordino Formation are developed in the lower part of a terrigenous, coarse-rhythmic-bedded sequence that lies unconformably on the marbles and crystalline schists of the Penchenga Formation of the Tei Series. This sequence is fairly widespread in the area; however, coarse-clastic rocks have so far been recorded only in the southwestern limb of the Vorogovka graben-syncline (headwaters of the Vorogovka River). Southeast of the territory described, conglomerates and gravelites within the Kordino Formation occur considerably more often. Geologists of the Krasnoyarsk Geological Administration, in the course of areal surveys, identified them in the upper reaches of the Panimba, Kalami, and Dydana rivers, in the estuarine parts of the Vangash and Eruda rivers, in the middle course of the Enashimo River, along the Vostrotininsky River (a tributary of the Tei River), on the Noiba River, and in other places. It may be supposed that the Kordino Formation regionally rests with erosion on the underlying formations.

The conglomerates of the Vorogovka River are usually fine-pebble, distinctly schistose (compressed), and pinkish-gray or greenish-gray in color. The clastic material is represented by well-rounded pebbles of quartz and microquartzites, and by sharply subordinate flattened pebbles of schists and amphibolites. The cement is quartz–chlorite–sericite, sometimes with an admixture of biotite, and also with pyrite, observed as fine disseminations or threadlike veinlets. Among the conglomerates and especially the gravelites and sandstones there occur varieties enriched in feldspar, a very characteristic component of the clastic rocks of the Kordino Formation in the southeastern districts of the ridge. Above the conglomerates and sandstones lie dark-gray hornfels-like, biotite-bearing schists and siltstones, interbedded with sandstones and quartzites and containing individual horizons of altered basic effusives.

The thickness of the Kordino Formation in the area has not been precisely established, but it is evidently no less than 400–500 m.

Upsection, the dark-gray schists and quartzites of the Kordino Formation are gradually replaced by green quartz–chlorite–sericite schists and acidic effusives of the Gorbilok Formation. The schists are characterized by elevated contents of tourmaline, pyrite, and especially magnetite, essentially a typomorphic mineral for this formation. Among the effusives, sheet bodies of lavas, tuff-lavas, and tuffs of quartz porphyries predominate; they are very intensely schistose and sericitized and are locally cut by subvolcanic intrusive bodies of the same composition. The thickness of the formation is about 500–600 m.

Above follows a sequence of variegated terrigenous and volcanogenic-terrigenous rocks, among which horizons of conglomerates and gravelites, as well as oligo-mictic sandstones facies-replacing them, are widespread chiefly in the lower part of the section.

R. P. Krendelev described them [5, 6] as part of the Sukhopit Series under the name of the Olenya River Formation, at the base of which he established and studied in detail a weathering crust [6].

Within the Vorogovka graben-syncline, at the base of the sequence, there is a bed of coarse-clastic variegated deposits represented by conglomerates, gravelites, and sandstones. The conglomerates are medium- and fine-pebble and well sorted. Quartz sharply predominates in the pebble composition (70–90%); quartzites, schists, and acidic and basic effusives are present in variable amounts. Moreover, the quartz pebbles are well rounded, whereas pebbles of other composition often retain angular forms. The cement is fine-grained, essentially ...

predominantly quartz–sericite material, and in the green-colored varieties, quartz–chlorite–sericite material with pyrite. The red-colored conglomerates and gravelites contain in the cement, in addition, hematitic ore dust. The conglomerates are an integral part of a coarse rhythmically bedded series of sediments, usually represented by oligomictic, often pyritized gravelites and sandstones, as well as aleurolites and shales with an admixture of tuffaceous material of basic composition. In this case the lower parts of the rhythmic packets are more often colored gray and greenish-gray, and the upper parts dark gray and gray-violet.

The variegated formations are overlain by fine-clastic deposits, also rhythmically bedded, characterized by a predominance within the rhythmic packets first of violet-gray and lilac, and then of black phyllite-like shales. Among them occur horizons of tuffs of basic composition, sheet-like bodies of plagioclase porphyrites and diabases up to 30–40 m thick. The total thickness of the unit is about 600–700 m.

A section of this unit similar in structure and rock composition is exposed on the Chapa River, upstream from the mouth of the Devyatka River. In the lower parts of the unit (Nizhnyaya Veduga River) coarse-clastic sediments are observed; higher up follow rhythmically bedded, essentially shaly rocks containing interbedded bodies of basic effusives.

We are inclined to regard this part of the Precambrian section as a stratigraphic analogue of the Uderei Suite. Indeed, the conglomerates and the volcanogenic-sedimentary formations overlying them lie stratigraphically above the horizons of feldspathic gravelites characteristic of the Korda Suite of the northern part of the ridge, and of green microcrystalline shales with magnetite, typical of the Gorbilok Suite of the Transangara region; and they are overlain by a fairly monotonous unit of dark-gray shales with massive beds of quartzitic sandstones, most characteristic of the Pogoryui Suite of the central and northern parts of the region. The latter is distinguished in the area by many investigators [2, 13]. As for certain specific features of the unit that we assign to the Uderei Suite (the variegated appearance of the rocks, the broad development, along with phyllite-like shales, of oligomictic sandstones and conglomerates, and also the presence of volcanogenic formations), they are due to the spatial confinement of this area to a zone of deep faulting at the boundary of two structural-facies zones of the ridge: the inner and the outer. In the section of the inner (eugeosynclinal) zone, as is known [2], products of volcanic activity are widely developed. Geological mapping carried out in recent years has established volcanogenic formations in the Yenisei-adjacent zone of the ridge within the section of the Sukhopit Series at different stratigraphic levels, including in the Uderei Suite.

Finally, the third stratigraphic level of areal development of conglomerates is confined to the base of the Chingasan Series. The rocks of this series lie with a profound erosional break on older formations. Terrigenous deposits—conglomerates, gravelites, and sandstones—are developed in the section of the lower red-colored unit, which belongs to the Lopatin Suite and is known in the literature as the basal red-colored formation of the Late Precambrian.

Conglomerates usually occur in the lower horizons of the suite as interbeds up to 20 m thick; more rarely (Mount Dimgan) they are its predominant variety. The conglomerates are mostly fine-pebble, and contain a significant (up to 20–40%) admixture of gravelly and sandy material. Composition of clasts: poorly rounded, rounded

quartz pebbles (up to 60%), flat pebbles of phyllitic schists (15–20%), semi-rounded fragments of gneisses and granites (10–15%), as well as quartzites and amphibolites. The cement is sandy-silty with an admixture of clay minerals, sericite, muscovite, and hematite. The composition of the gravelites and sandstones, both in section and along the strike of the suite, is highly variable, but in general it is established that in its lower and middle horizons their polymictic varieties predominate, whereas in the upper parts of the suite oligomictic, essentially quartzose rocks are also often observed.

The thickness of the suite in the central part of the Uvolzh graben is about 700–800 m.

The Lopatin suite is conformably overlain by carbonate and phylloid deposits of the Chividin suite, above which lie the variegated sandstones and limestones of the Nemchan suite—the youngest stratified formations of the region.

Thus, the Precambrian conglomerates in the area studied have been established in terrigenous formations that differ in composition, conditions, and time of formation and correspond to the stratigraphic level of the Kordin, Uderei, and Lopatin suites.

The conglomerates and sandstones of all three formations are, to one degree or another, regionally metamorphosed. In the lower formations, metamorphism is manifested in blastesis and in the transformation of fine-clastic rocks into schists. In them, newly formed quartz, chlorite, sericite, biotite, tourmaline, and magnetite appear. In the rocks of the upper formation, only partial recrystallization of the ferruginous material and the clay component of the cement takes place. Phenomena of regeneration and corrosion are not often observed [8].

Against the general background of regional metamorphism of the rocks, zones of locally manifested alterations associated with the development of processes of regressive metamorphism stand out rather clearly. Such zones are usually controlled by disjunctive structures, being spatially confined to the boundaries of grabens (the Uvolzh graben) and graben-synclines (the Vorogov graben-syncline), to zones of distribution of young intrusions, etc. For the latest stage of alteration, clearly expressed in the upper formation of conglomerates and sandstones, the following sequence is characteristic: 1) alkaline metasomatism, 2) fluorite mineralization, 3) chloritization, carbonatization, in places epidotization and albitization, 4) sericitization, pyritization, sometimes silicification and hematitization. The processes of alkaline metasomatism are manifested in potassium feldspathization and albitization, as a result of which zones of feldspathized rocks develop in the formations of the upper structural level, while in the more competent metamorphic sequences of the lower structural level vein-like feldspathic bodies are formed, often cut by later veinlets of fluorite. In general, the mineral complexes of the early stages of the stage under consideration are developed locally. Much more widely manifested are the processes of chloritization, carbonatization, and the latest phenomena of sericitization, silicification, and pyritization. They have the character of extended zones and develop chiefly in the formations of the two upper structural levels, often conformably with the bedding of the rocks, especially along such easily permeable horizons as conglomerates and sandstones, in the footwall of gently dipping horizons of effusives or dense schists (the Vorogov graben-syncline), or are controlled by schistosity (the Uvolzh graben).

In the lower formations of conglomerates and sandstones, in addition to the alterations noted, products of earlier processes are widely developed,

which led, in particular, to the formation of quartz, quartz-sulfide, and carbonate veins and, not infrequently, to intensive decomposition of the effusives.

In general, the locally metamorphosed rocks in the sections under consideration of the terrigenous sequences occupy a quite definite spatial position. In addition, they differ in color, since they have a gray, and more often a greenish-gray color; in mineralogical composition: they contain appreciably more such newly formed minerals as quartz, carbonate, albite, chlorite, and especially sericite and pyrite, which, compared with diagenetic pyrite, contains a noticeable admixture of As, Ni, Co (the Uvolga graben) [9]. There are, naturally, substantial differences also in the chemical composition of the unaltered and altered terrigenous rocks. The latter have higher contents of Fe²⁺, K, Al, Mn, Ni, Co. Zn, As, Bi, Sr, Mo, TR, and Y are present in appreciable amounts in them. The altered rocks are also characterized by increased background radioactivity.

The gold content proper of the terrigenous coarse-clastic formations has not yet been investigated. However, the presence of gold in individual concentrates from the alluvium of streams draining these sequences, as well as its increased content in some samples collected during incidental investigations by F. P. Krendelev and A. M. Molokoedov, make it possible, with greater justification, to carry out specialized work for gold here. To a greater extent, the potential gold content of the conglomerates and sandstones, especially of the Sukhopit Series, is indicated by a certain similarity of some of their geological features to those of classical deposits of the world [6, 7], as well as by the spatial position of the conglomerate formations on the northern continuation of a rather large zone of distribution of placers and gold-ore mineralization. This zone extends from the settlement of Motygino through Ayakhta, Eldorado, Sovrudnik, and farther north to the Vorogovka River. And if contrast (vein) mineralization develops in the phyllites and schists of the Gorbilok and Uderei suites, it is entirely reasonable to think that it may be disseminated (in the form of inclusions) in the cement of coarse-clastic oligomictic horizons.

It is important to emphasize that, in the noted zone, the formations of Precambrian conglomerates prove to be the most intensely locally (hydrothermally) metamorphosed. This is explained by the fact that, at least in the north of the ridge, this zone spatially coincides with the above-mentioned deep fault. Bodies of intrusions of different ages are situated in the fault zone: gneiss-granites, Ayakhta (Nogatin and Noibin) granites, plagioclase and diabase porphyries and diabases, quartz and nepheline syenites, and, finally, kimberlites and olivine pyroxenites [3, 4, 10, 11, 12]. The manifestation of magmatism so varied in composition caused the broad development of products of hydrothermal metamorphism and of a distinctive ore mineralization, for which a gold–rare-metal trend is established quite definitely [3, 4, 12]. Naturally, the mineralization caused by metamorphism from the side of the indicated intrusions may be of different types and ages. But at the same time it should be remembered that, for example, essentially all of the metamorphism of the lower two formations of conglomerates and oligomictic sandstones may be due to the gneiss-granite intrusion, whereas the metamorphism of the upper formation is caused by the manifestation of intrusions of the syenite granitoid series and, to an insignificant degree, by trap volcanism.

As early as 1938, F. N. Shakhov [15] pointed to a possible connection of gold mineralization with manifestations of magmatism younger than the Tataro-Ayakhta intrusions. In recent years, in the works of A. D. Nozh-

...Kina and F. P. Krendelev [3, 4] emphasized the idea of a possible connection between gold, as well as rare-metal mineralization on the ridge, and post-Proterozoic rocks of increased alkalinity.

The material presented allows us to think that, in the geological assessment of the metal potential of formations of Precambrian conglomerates, special attention should evidently be given to zones of locally altered, greenish, pyritized oligomictic rocks.

REFERENCES

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