Some Results of Studying the Regularities in the Distribution of Gold-Ore Fields in the Yenisei Ridge

L. V. Li, V. A. Kushchenko (KO SNIIGGiMS, KGU)

Elucidating the regularities of the distribution and the conditions of localization of mineralization is one of the tasks of metallogeny. At the same time, these questions in the Yenisei Ridge have still been studied insufficiently. In the process of compiling a map of gold content, the authors of the present article carried out a statistical analysis of the distribution of gold-ore fields and deposits. The material set forth below represents a brief summary of the generalizations and studies performed on the geology of gold in the Yenisei Ridge during 1963–1967.

Structural-facies Conditions of Gold Content

One of the characteristic features of the distribution of gold mineralization in the Yenisei Ridge is its zonality, representing the zonality of ore belts according to the classification of V. I. Smirnov [13]. In addition to the main gold-bearing belt, which we have named the Eastern [9], within whose limits the principal mass of gold deposits is concentrated, a second—the Western—belt is outlined, uniting placers and gold ore occurrences of the Near-Yenisei part of the ridge. The latter territorially coincides with the central part of the western structural-facies zone, which, in terms of the geological features of its development, corresponds to the conditions of an eugeosyncline. In the structure of the Yenisei Ridge, only the eastern part of the eugeosyncline is exposed; the most mobile trough zone of the geosyncline, in the opinion of M. N. Belyankina and E. A. Dolginov [1], is situated farther west and is buried beneath the thick sequence of Meso-Cenozoic deposits of the West Siberian Lowland, forming its folded basement. A characteristic feature of the structural-facies zone under consideration is the broad development of basic and acidic effusives, constantly present in sections of the suites of the Sukhopit Series and composing, entirely in some areas, certain suites of the Tungusik Series, as well as the rather wide distribution of small areal massifs and, in terms of body size, ultrabasic and basic rocks that are derivatives of basaltic magma. According to individual sampling data, the effusives assigned by T. Ya. Kornev and V. M. Datsenko to the spilite-keratophyre formation, and the terrigenous rocks occurring among them, carry increased gold content.

The Eastern gold-bearing belt is confined to the inner part of the miogeosynclinal structural-facies zone, within whose limits in

accumulation of thick terrigenous and terrigenous-carbonate sedimentary sequences took place, with a subordinate role of effusives. In this zone the processes of palingenetic granite formation were widely manifested.

In the outer part of the miogeosyncline, corresponding to the Angara–Pit synclinorium, endogenous gold mineralization is weakly manifested. Several fields of gold–quartz mineralization are known here, spatially tending toward the western flank of the synclinorium. The gold-bearing placers confined to this structure are allochthonous formations and are associated with the erosion of primary sources in the inner part of the miogeosynclinal folded zone.

The part of the Eastern structural-facies zone under consideration possesses a number of features that distinguish it from the inner zone of the miogeosyncline. The deposits of some formations of the Sukhoi Pit and Tunguska series here have a reduced but more constant thickness than in the latter; they are characterized by a considerably more persistent facies composition, a coarser composition of the sandy-shaly components of the section, and a greater participation of carbonate sediments in the sections [8]. An important feature of it is the complete amagmatism of the zone.

Thus, gold mineralization in the Yenisei Ridge is manifested under different structural-facies conditions—eugeosynclinal and miogeosynclinal. This determines the specific character of the Yenisei gold-bearing province, distinguishing it from other gold-bearing provinces of the territory of the USSR, which belong either to eugeosynclinal (Ural, Altai-Sayan, Caucasus) or to miogeosynclinal (Lena, Yana-Kolyma, and others) folded zones, and predetermines the potential possibilities for discovering diverse types of gold mineralization within it. The increasingly great variety of types of manifested mineralization revealed as the degree of exploration of the territory becomes more detailed discloses the individualization of the metallogenic aspect of the Yenisei gold-bearing province. In terms of the geological features of the distribution of gold mineralization, it cannot be assigned to either of the two structural-formational types of gold-bearing provinces distinguished by G. P. Volarovich and I. S. Rozhkov [4]. What has been said gives us grounds to distinguish the Yenisei gold-bearing province as typomorphic, characterizing combined eugeosynclinal and miogeosynclinal types of metallogenic structures.

Distribution of Gold-Ore Fields in the Geosynclinal Sequence

As the results of statistical analysis show, gold mineralization is distributed unevenly in the lithologic-stratigraphic complexes of different ages of the geosynclinal sequence. The predominant mass of gold-ore fields is situated in the belt of development of early geosynclinal sediments of the Uderei and Gorbilok formations, which contain, in the eugeosynclinal folded zone, beds of effusives. Moreover, gold mineralization is more intensely manifested among the dark-gray, often nearly black, clayey and aleurite-clayey phyllitized shales of the Uderei Formation, enriched in carbonaceous matter. It is developed somewhat less in the grayish-green, green, clayey and aleurite-clayey quartz-sericite-chlorite shales of the Gorbilok Formation. In all, more than 60% of the deposits and ore occurrences of gold are concentrated in the sediments of these two formations. A considerably smaller number of gold occurrences is localized in the micaceous shales of the Penchengin Formation of the Middle Proterozoic and in the aleurolites, quartzitic sandstones, and quartzites of the Kordin Formation of the Upper Proterozoic, and a very small part of them is unevenly dispersed in fields of development of

terrigenous-carbonate, weakly metamorphosed rocks of the Tunguska series and Middle Proterozoic crystalline schists of the suite of the Kar’pin Range.

Thus, a fairly clearly expressed lithologic-stratigraphic control of mineralization is established. A similar selective localization of gold mineralization, as is known, is characteristic of a number of other gold-bearing provinces. In many of these regions the disseminated gold content of the schists themselves has been established, to which a primary sedimentary origin is ascribed. Convincing arguments in favor of this view are given by R. N. Nifontov [12]. Noting the stratified character of gold mineralization, many researchers express the opinion that gold may have been borrowed by solutions from the enclosing strata. In particular, in recent years this idea has been widely developed by V. A. Buryak [3], who, on the basis of materials from the study of gold mineralization in the Patom Highland, showed the existence of a direct dependence between the intensity of gold mineralization in veins and the composition and gold content of the enclosing strata.

The role of deep-fault structures in the distribution of gold-ore fields

Among the factors influencing the distribution of gold-ore fields, fault structures are of leading importance, and first of all those which, having deep foundations, through their repeated rejuvenation predetermined not only the conditions of sediment accumulation but also caused the stable mobility of the earth’s crust within the zones of their development. In the Yenisei Ridge these include the East Yenisei and West Yenisei deep faults.

The East Yenisei zone of deep faults, traced on the present denudation surface by granitoid massifs of different ages, is linearly elongated in a northern, northwestern direction along the eastern periphery of the inner part of the miogeosynclinal folded zone. The Eastern gold-bearing belt is spatially combined with it. Along this zone passes the boundary between the inner and outer parts of the miogeosynclinal structural-facies zone. According to D. I. Musatov [11], within the Angara-Kan part of the ridge the deep-fault structure under consideration consists of series of closely spaced parallel disjunctives, the planes of which dip eastward at an angle of 60–70°. The total width of the zone here reaches up to 16 km. Judging by the distribution of granitoid massifs, this fault zone has a similar structure also in the Trans-Angara part of the ridge. The width of the zone here, apparently, locally reaches 20–30 km.

From analysis of the distribution of magmatic complexes of different ages, the time of inception of the East Yenisei fault zone is determined as Archean or Lower Proterozoic, and its stabilization most probably occurred in the Vendian. Beginning in Late Proterozoic time, this zone underwent somewhat greater subsidence than the areas adjoining it, and within it thick strata of deposits of the Sukhopit series accumulated. The latter here are distinguished by completeness of section and by a somewhat increased thickness of individual suites. Thus, for example, the thickness of the Uderei suite increases from 900–1000 m in the outer part of the miogeosyncline to 1300–1500 m in the inner part. Less noticeable is the tendency for an increase in thickness in the same direction for the Kordinskaya and Pogoryuiskaya suites.

The West Yenisei zone of deep faults passes along the axial and eastern near-vault part of the Pre-Yenisei anticlinorium. It likewise consists of series of parallel faults, often clearly expressed—

… expressed in the modern morphostructure of the ridge. Their planes dip eastward at an angle of 60–65°. The distribution of ultrabasic intrusions of the Surnikhinsky complex, and of granite-gneiss and granitoid massifs, is subordinate to it.

Forty to sixty kilometers east of it extends the Glushikhinsky deep fault, with a plane dipping eastward at an angle of 60–65°. According to I. V. Vorob’ev, between these zones in Precambrian time there was a geosynclinal trough, which completed its development in Pre-Vendian time. The Western gold-bearing belt coincides with this trough zone and is linearly elongated along it.

Thus, the gold-bearing belts occupy a quite definite position in the tectonic structure of the region. They are confined to the most mobile parts of the Earth’s crust, which represented permeable zones throughout the entire Proterozoic. The latter were also zones of intense granitization and anatexis.

Gold deposits and ore occurrences, in most cases, are located on the hanging wall of deep faults and are controlled both directly by the latter and by ruptures associated with them and preceding them. The predominant mass of deposits shows a close connection with associated and preceding disjunctives.

Of great importance in the distribution of gold mineralization, apparently, is the development of northeast-trending and nearly latitudinal ruptures, which occupy a transverse position with respect to the strike of deep-fault structures. In a number of cases, as, for example, in the Sovetskoye deposit, there is a tendency for gold mineralization to concentrate in areas where northwestern ruptures intersect northeastern or nearly latitudinal ones. A similar regularity is generally observed also in the Erudinskoye ore field, as well as in the ore fields of the South Yenisei district, where, according to N. V. Khakhonina and A. I. Vasil’ev, the deposits are located at sites where submeridional faults intersect sublatitudinal ones.

Structural Features of the Distribution of Gold-Ore Fields

The principal gold-bearing folded structures in the Yenisei Ridge are the Central and Near-Yenisei anticlinoria. The Eastern gold-bearing belt is spatially subordinate to the former, and the Western belt to the latter. In the synclinoria, gold content is manifested very weakly. Within them, only isolated small placers and ore occurrences are developed, characterized by a low concentration of gold in veins.

Analysis of the distribution of gold-ore fields in the anticlinoria has shown that deposits and ore occurrences of gold are localized selectively in their various parts, creating a clearly expressed asymmetry in the gold content of the structures. They are most numerous in the crest part of the Central anticlinorium, which dips gently eastward and is complicated by broad, predominantly low-amplitude, steeply plunging folds, often occupying a transverse position relative to the strike of the regional structure. More than 62% of gold deposits and ore occurrences are concentrated in this part of the anticlinorium. A smaller number (about 30%) of ore fields is located in the eastern, northeastern limb of the Central and Near-Yenisei anticlinoria, characterized by intense linear folding, often to isoclinal, and by the development of small, steeply plunging transverse folds. In the narrow, steeply dipping western and southwestern limbs of the anticlinoria, gold mineralization

or is absent altogether, or is manifested very weakly. The ore occurrences known here are usually situated near the large fault disturbances that cut them. Attention was drawn to this feature of the selective localization of gold mineralization in different elements of anticlinoria by A. K. Meister [10], who related the weak gold content of the western limb of the Tatarka anticlinorium to the absence there of favorable ore-hosting structures. In light of the available materials on the morphology of folded structures, the differing intensity of gold content in the eastern and western parts of the anticlinoria agrees well with the west- and southwest-overturned character of the anticlinoria, the position of their different parts with respect to the planes of dip of deep-seated faults controlling the emplacement of mineralization and granitoid bodies.

In addition to this regularity in the development of mineralization, another dependence of the distribution of gold-ore fields on the structural features of folded structures may be outlined. It is manifested in the regular coincidence of gold-bearing areas with uplifted parts of the vaults of anticlinoria, recorded by outcrops at the surface of deposits of the Tei Series—mainly its Penchenga Formation—and partly by rocks of the Korda Formation of the Sukhopit Series, amid a field of Late Proterozoic formations. While showing a close connection with the uplifted parts of the vaults of anticlinoria, the gold mineralization is localized not in the cores of these uplifts, but in their peripheral parts, composed of deposits of the Gorbilok, Uderei, and other formations. This position of the gold-ore fields in the folded structure was evidently predetermined by the fact that uplift of the vaults of the anticlinoria was accompanied by intense interstratal movements, leading to delamination and the formation of numerous separation cavities, which served as pathways for ore-bearing solutions.

Within regional folded structures, gold-ore fields territorially coincide with anticlinal and synclinal structures that complicate different parts of the anticlinoria. The ore-localizing structures in both cases are zones of intense crumpling and foliation of rocks, developed in the hinge parts and limbs of folds, more rarely zones of crushing and rupture. In anticlinal structures, gold mineralization is often confined to zones of interbed displacement (the Verkhne-Enashiminsky ore field) and to cleavage fractures (the Eldorado ore field).

Relationship of Gold-Ore Fields to Granitoid Massifs

Gold-bearing belts are situated within linear zones of concentration of granitoid massifs. The latter are subdivided into three petrological-age groups (complexes): the Tei, Posolno-Angara, and Ayakhta [5].

Gold-ore fields are spatially associated mainly with massifs of granitoids of the Posolno-Angara complex, and only some of them with Ayakhta granitoids. No spatial connection with gneiss-granites of the Tei complex is shown by the gold-quartz mineralization. Gold-ore deposits are situated at widely varying distances with respect to the granitoid massifs. More than one-third of the deposits and occurrences of gold are located directly in the exocontact zone of granitoid massifs and are removed from them by a distance not exceeding 5 km. A number of deposits are localized in the apical part of the roof of intrusions. Several ore occurrences are also known within the body of a granitoid massif. Predomi-

... the overwhelming number of deposits are located at a distance of more than 5 km outside the zone of near-intrusive alteration of the rocks. Near such deposits there may be undiscovered intrusive bodies, as, for example, has been established by geophysical studies in the area of the ore field of the Sovetskoe deposit and of the Mamonskoe and Petropavlovskoe ore fields. For a large number of deposits situated at a distance of 5–10 km from massifs, there appears to be an undoubted connection with the parts of sheet-like bodies of the intrusion, exposed at the surface, that continue at depth. This is indicated by the location of deposits in the hanging wall of granitoid bodies, by the increased metamorphism of the ore-bearing sequences within the ore fields, and by the steep dip of the ore-localizing structures.

Many researchers, proceeding from the observed spatial association of gold mineralization with granitoid intrusions exposed at the surface, assume the existence of a genetic connection between them. Thus, for example, T. M. Dembo [7] regarded the Kalaminskaya intrusion as the “producer” of the deposits of the Eldorado group. N. V. Petrovskaya [2] linked the formation of the gold-bearing quartz veins of the Sovetskoe deposit with an undiscovered intrusion, the presence of which has now been confirmed by geophysical work. Although the genetic connection of gold mineralization with granitoid intrusions has not been specifically proven in any case, it appears the most probable.

REFERENCES

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