TYPES OF GOLD MINERALIZATION IN THE YENISEI RIDGE AND SOME PATTERNS OF THEIR DISTRIBUTION

L. V. Li, V. A. Nelyubov, V. A. Kutsenko (KTU)

The Yenisei gold-bearing province, which belongs to the major metallogenic structures of the Soviet Union, coincides with an ancient folded region known as the Yenisei Ridge and is regarded, in geostructural terms, either as a complexly built meganticlinorium [7], as a major uplift of the Siberian Platform—an anteclise [5], or as a gigantic horst [8]. It covers a vast territory extending in a northern and northwestern direction, in accordance with the strike of the Proterozoic folds, from the basin of the right bank of the Kan River in the south to the basins of the Vorogovka and Chapa rivers in the north, and bounded on the east and west by the deep faults of the adjoining Siberian and West Siberian platforms. The length of the gold-bearing province reaches 600 km, with a width of 20 km in the southern part and 150 and 70 km in the central and northwestern parts.

For the greater part of its extent, the gold-bearing province lies within the Trans-Angara region, characterized by the development of thick metamorphosed Proterozoic sedimentary sequences, intense linear folding, and the widespread occurrence of massifs of granitoid intrusions of various ages. In the view of a number of researchers [1, 2, and others], the Yenisei gold-bearing province is understood to mean only the eastern periphery of this part of the ridge.

In mining and industrial terms, the Yenisei gold-bearing province is a province of placer gold. More than 90% of the gold produced here has been extracted from placers. The presence of numerous, often very rich placer gold deposits raised the task of discovering and studying ore deposits—the primary sources of the gold placers. Considerable progress has been achieved in carrying out this task, especially during the Soviet period of mining-industrial development of the Yenisei Ridge. Over the more than century-long history of geological investigations and gold mining, numerous occurrences of gold ore have been discovered here, now numbering more than two hundred. Of this number of ore occurrences, only several dozen have been studied to some degree and subjected to preliminary evaluation; several deposits have been explored in greater detail, of which only one is currently being exploited. This still insufficient degree of study of the gold-ore occurrences apparently explains why the reserves explored in them are small—several tens of times smaller than the reserves estimated in general—

in placers. There is every reason to suppose that, as a result of further geological prospecting, new industrial deposits will be discovered and the reserves of primary deposits will be sharply increased. Under present conditions, with the placers largely worked out, further successes of the gold-mining industry here are possible only with ever greater development of primary deposits simultaneously with the exploitation of placers. This circumstance makes it necessary to expand work on the study and prospecting of gold-ore occurrences, which should be directed toward identifying deposits not only of the long-known and developed types of gold mineralization, but also of new ones discovered mainly in recent times.

Among the numerous deposits and ore occurrences of gold, the chief mass consists of occurrences of the hydrothermal quartz-vein type. Until comparatively recently the latter was the only known type of gold mineralization. Now occurrences of gold have been established here in magmatic rocks and in metasomatically altered rocks—quartzites and greisens. Metamorphogenic veins of the Alpine type are also gold-bearing.

The discovery of the gold-bearing character of magmatic rocks in the Yenisei Ridge dates to 1950, when P. S. Syrnikov, as a result of sampling, detected elevated gold contents (up to 3.6 g/t) in diabases of the Tokmin complex in the upper reaches of the Olonokon River. Small diabase bodies, linearly elongated in a northwestern direction, are widely developed in this area. Here they are confined to the cores of anticlines composed of deposits of the Kordin Formation, which complicate the northeastern limb of the Panimbin anticlinorium. The length of the diabase massifs reaches up to 6 km, their width from 0.5 to 1 km.

Gold-bearing diabases are dense, massive rocks of greenish-gray or dark-green color, with rare segregations of plagioclase in a cryptocrystalline groundmass. They consist of albitized plagioclase, pyroxene, often replaced by actinolite or uralitic hornblende, amphibole, and have an ophitic or porphyritic texture. Inclusions of ore minerals, chiefly sulfides, are sometimes observed in them. According to the degree of alteration and structural features, the following varieties of rocks are distinguished: 1) altered gabbro-diabases, 2) quartz diabases, and 3) albitized and amphibolized diabases. The altered varieties of diabases are developed in the near-contact parts of the massifs, and inward into the latter they gradually pass into unaltered rocks.

Diabases similar to those characterized are also developed in the upper reaches of the Enashimo River. However, like most of the massifs developed in the interfluve of the Otne and Olonokon, they have almost not been sampled for gold at all. According to N. F. Gavrilov, the gold content in one diabase sample reached up to 7 g/t. Such poor study of the gold-bearing character of diabases does not at present permit an industrial assessment of the type of gold mineralization under consideration. Its further study is important not only in a general geological respect, but may also be of practical interest.

What has been stated applies equally to greisenized granites, whose gold-bearing character was established by us as a result of sampling at two points: in the lower reaches of the Dydan River and in the upper reaches of the Enashimo River in the “Vysoky” area. In the latter, tungsten mineralization is associated with greisenized granites and with quartz veins and veinlets penetrating them, forming a stockwork-type silicification zone. The greisenization of the granites here is expressed at the contact with quartz veins, and these wall-rock altered rocks, developed in the form of:

selvages 0.2–0.6 m wide, merging with one another where the veins converge, form fairly thick zones of greisenization. Near the main granite body there is a series of lenses of greisenized granites up to 1.5 m long. They are separated from the enclosing metamorphosed sedimentary rocks by a tourmaline selvage about 1 cm thick.

The gold content in the greisenized granites reaches up to 3.4 g/t.

An entirely new type for the territory of the Yenisei Ridge is the contact-metasomatic type with superimposed hydrothermal gold mineralization. Ore occurrences of this type have been identified in the upper reaches of the Enashimo River in two areas—the Olimpiadinsky and Oleniy areas, where occurrences of antimony and tungsten mineralization had previously been discovered and explored.

Gold mineralization, spatially coinciding with antimony and tungsten mineralization, is localized in a zone of tectonic disturbance of the interstratal-slip type with normal-strike-slip displacement and is confined to quartz-carbonate skarned rocks, skarns, metasomatic quartzites, hornfelses, and carbonaceous, andalusite, and micaceous schists. The ore zones are a metasomatic deposit with lens-shaped ore bodies 10–15 m long in the Olimpiadinsky area and up to 50–100 m in the Oleniy area, with thicknesses of 0.2–1 m and 1–6 m, respectively. Within these zones there are quartz veins of lenticular form, 10–18 m long and 0.5–2.5 m thick, lying conformably with the enclosing rocks. They are also auriferous. The strike length of the Olimpiadinsky zone is 750 m, that of the Oleniy zone is 880 m; the thickness of the zones is, respectively, 10–17 m and up to 60 m.

The gold-ore zones under consideration are structurally confined to the Enashimo anticline, which complicates the western limb of the Panimbinsky anticlinorium, and are located directly at the exocontact of the granitoid massif of the Posolnensky complex (Oleniy area) or at a short distance from it (Olimpiadinsky area). Such a close spatial relationship of the mineralization with the granitoid massif indicates the possible existence of a genetic connection between them.

The highest gold contents, reaching 8.0 and 18.4 g/t, have been established in metasomatic quartzites carrying sulfide mineralization (pyrite, arsenopyrite, pyrrhotite, chalcopyrite, stibnite). The gold content in skarned rocks, hornfelses, and skarns, which occur, like the quartzites, at the contact of quartz-graphitic and quartz-carbonate schists, does not exceed 0.4 g/t. The gold-bearing character of the schists is usually low. Only in one case were 10 grains of gold found in a crushed sample of micaceous schists.

The metasomatic quartzites sampled by us for gold are contact-altered, skarned, silicified, sometimes weakly hornfelsed rocks of greenish, greenish-gray, and gray color. The texture of the rocks is massive and schistose-banded; the structure is granoblastic, hypidioblastic, and nematogranoblastic. They consist mainly of quartz (up to 50–60%), calcite (up to 35%), actinolite and diopside (up to 15%), zoisite and epidote (up to 15%). Biotite, muscovite, feldspars, andalusite, sphene, zircon, apatite, and sulfides are present in small quantities. Occasionally, typical monomineralic skarns are encountered among these rocks—diopside and actinolite skarns with a small admixture of epidote, zoisite, and calcite. These formations possess the massive and banded texture and granoblastic structure characteristic of skarns. From the data presented, the metasomatic origin of the rocks is quite evident. Metasomatic solutions rose along a fissure zone at the boundary of quartz-carbonate-micaceous and quartz-graphitic schists. In this process the introduction of compon-

...and the replacement processes occurred along the schistosity; this is confirmed by the disposition of actinolite, epidote, zoisite, and, not infrequently, quartz along the schistosity or in the form of parallel chains and lens-like formations.

Gold in all metasomatically altered varieties of rocks is found both in the free state and associated with sulfides of a late stage of mineralization. Visible gold has also been found in polished sections of samples of schists enclosing these rocks. The precipitation of gold from solution, in the main, apparently occurred during the final stage and coincides in time with the deposition of chalcopyrite and antimonite.

The only industrial type of gold mineralization at present on the Yenisei Ridge is the quartz-vein type. Among these, metamorphogenic veins are distinguished; so far they have been studied only in the basin of the Bangaš River. They are developed within the thickness of clayey schists of the Uderey Formation, characterized by a low degree of metamorphism, and occur conformably with the schistosity of the rocks. Veinlets and lenses of quartz usually have small thicknesses (up to the first tens of cm) and almost everywhere show weak gold content. The gold content rarely reaches 2–3 g/t. The concentration of gold in the veins is evidently connected with its accumulation in the process of regional metamorphism of the schists, which, according to V. G. Petrov, are also weakly auriferous.

Hydrothermal quartz-vein gold deposits are morphologically represented by individual large, extended veins or by series of veins which together form vein zones, traceable, not infrequently, for distances of up to the first tens of kilometers. The thickness of the veins reaches up to 15 m. In their position, the ore bodies adapt to the conditions of plicative and fracture tectonics of the metamorphosed Proterozoic rocks. They are localized in zones of crushing and intense cleavage, as well as bedding-parallel separation of rocks, developed predominantly in places of greatest stress in folds of higher orders that complicate the limbs of anticlinoria. The general mineral composition of the veins is fairly uniform. The main mass of the ores consists of quartz, accompanied by carbonates, chlorite, sericite, and other non-ore minerals. Study of the accumulated materials on the quartz of gold-bearing veins makes it possible to distinguish six principal varieties (types): 1—white with a bluish tint, coarse-grained, massive, high-temperature; 2—gray to dark-gray in color, dense, fine- and very fine-grained, quartzite-like; 3—gray, dark-gray, sometimes bluish or smoky, medium- and more rarely fine-grained, semitransparent, fractured, with a glassy to greasy luster; 4—milky-white, grayish-white, sometimes yellowish-white, coarse-grained, usually opaque, weakly granulated, in veins of small thickness fibrous; 5—white and light-gray, opaque and weakly transparent, sometimes intergrown with quartz of the preceding type but separated from it by a sharp contact; 6—gray, whitish-gray, chalcedony-like, with inclusions of a considerable amount of calcite. Analysis of the mutual relations of veins composed of different varieties of quartz showed that, in terms of the time of their formation, the first three types of quartz are earlier formations than the others. On this basis, they are combined into two groups: 1—early generations and 2—late generations.

The ore bodies of the deposits are composed predominantly of one of the varieties of quartz. Thus, the first of the distinguished types is characteristic of the veins of the Vysokog ore occurrence and the upper reaches of the Innokent’evskii Brook. The veins of the Bogunaevskoe and Kuzeevskoe deposits in the Angara-Kansk region, which...

are cut by veins of light-gray, fine-grained, semitransparent quartz of later generations.

The second type of quartz composes the veins of the Aleksandro-Ageevskoe deposit and the Gerfed and Vagansk ore fields. As a special variety, formed as a result of metasomatic replacement of carbonate rocks, it was first distinguished by N. V. Petrovskaya. This same quartz forms rather extensive and thick interbeds in the sedimentary deposits of the Penchenginskaya suite and the lower horizons of the Sukhopitskaya series, which are usually cut by lenticular veins and veinlets of quartz of later generations.

The third type of quartz is very widely developed. It composes the veins of the southwestern part of the Eldorado ore field, the ore bodies of the Pervenets, Proletarskoe, Ayakhta, and other deposits. According to the view of most investigators (B. L. Stepanov, N. N. Gornostaev, T. M. Dembo, and others), it is high-temperature.

The fourth type of quartz is most widely represented at the Sovetskoe, Polyarnaya Zvezda, Poteryaevka, and other deposits. The study of the quartz of the Sovetskoe deposit, carried out by V. A. Bogdanovich, confirmed the views of T. M. Dembo, N. N. Gornostaev, and others concerning the medium-temperature conditions of crystallization of quartz of the type under consideration.

The next type of quartz is characteristic mainly of the Vasil'evskoe deposit and is developed subordinately at the Sovetskoe deposit. Its frequent joint occurrence with quartz of the preceding type does not exclude the possibility that this variety formed as a result of granulation of the latter. Therefore, the separation of the quartz under consideration into a special type is to some extent conditional.

Finally, the last type is known in the basin of the Enashimo River and composes veins developed in sedimentary deposits of the Middle Proterozoic, Vendian, and Lower Cambrian. In small quantities, as the latest generation, it is present in the veins of the Sovetskoe deposit [2].

In analyzing the spatial distribution of veins composed of different types of quartz, a definite relationship is suggested between them and granitoid intrusions of complexes of different ages. The granitoid massifs of the Posol'nenskoe complex are accompanied by quartz veins predominantly of early generations, and those of the Ayakhta complex by veins of late generations. On the basis of these features in the distribution of quartz, it may be supposed that the quartz veins, and consequently the gold mineralization manifested in them, formed at different times. The geochemical characteristics of the composition of intrusions of different ages and the different geological-tectonic conditions of formation of the quartz veins apparently are also causes that determined the unequal fineness of gold in veins accompanying intrusions of different complexes. Gold in veins spatially associated with intrusions of the Posol'nenskoe complex has a fineness no higher than 880–900, whereas in veins accompanying intrusions of the Ayakhta complex the fineness of the gold does not fall below 920–940.

In the spatial distribution of gold occurrences of the quartz-vein type, a distinct belt zonality is indicated. The bulk of them is located on the eastern slope of the Yenisei Ridge, forming a belt elongated in a northern and northwestern direction, extending from the right bank of the Angara River to the basin of the Chaipa River. A small number of ore occurrences is confined to the western slope of the ridge, to its Near-Yenisei part. Between these zones of concentration of gold mineralization lies a belt of lead-zinc mineralization. Comparison of data on geology and magmatism has shown that these zones are confined to structures formed during different stages in the development of the Yenisei folded region. In light of E. T. Shatalo-

[9] these zones may be regarded as two independent belts of gold mineralization.

The first of the belts territorially coincides with the eastern near-axial part of the Tatarka anticlinorium and the northeastern limb of the Panimba anticlinorium, composed predominantly of Middle Proterozoic deposits. Within these anticlinoria, granitoids are widespread, forming numerous large and small massifs. The predominant part of them belongs to the Posolnaya complex; a limited number—to the Ayakhta complex. The manifested gold mineralization is in close spatial association with intrusions of both complexes. Gold occurrences and deposits are situated directly near granitoid massifs or at a comparatively small distance from them. Such a close spatial association of gold occurrences with granite intrusions and the frequent confinement of quartz veins to the exocontacts of the massifs make it possible to assign the latter to the type of hydrothermal veins genetically related to granitoid intrusions.

The second belt is structurally confined to the eastern near-axial part of the Yenisei anticlinorium, composed of Lower and Middle Proterozoic deposits. The known quartz-vein bodies show a close association with granitoid massifs of the Posolnaya complex. The gold mineralization of this belt has as yet been poorly studied. At the same time, the available materials indicate possible significant prospects for the belt.

In conclusion it should be noted that the materials presented are, of course, incomplete, and some conclusions are hypothetical. They also do not exhaust the full diversity of types of gold mineralization developed in the Yenisei Ridge. However, the data already set forth testify quite definitely that the Yenisei gold-bearing province can no longer be regarded as a “classical” province where gold mineralization is manifested only by a single hydrothermal quartz-vein type. The question of the age of the gold mineralization also requires further study. According to the data we have obtained, two epochs of gold mineralization are indicated, and not one, as is currently accepted.

REFERENCES

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