THE POSITION OF GRANITOID FORMATIONS AND THE PLACE OF GOLD MINERALIZATION IN THE EVOLUTIONARY HISTORY OF THE YENISEI RIDGE

L. V. LI, V. M. DATSENKO (KO SNIIGGiMS)

Among the problems of endogenous metallogeny, one of the important ones is the problem of the evolution of magmatism and ore formation in the history of tectono-magmatic development of a mobile region. In the published works of V. I. Smirnov [9], Yu. A. Bilibin [4], D. I. Gorzhevsky and V. N. Kozyrenko [6], and others, general regularities have been noted in the succession of mineralization of various metals in the history of the Earth’s development. At the same time, many particular regularities in the formation of deposits at each stage of the tectono-magmatic cycle have as yet been studied only poorly. In the present article an attempt is made to consider the conditions of formation of granitoid intrusions and the temporal relationships of gold mineralization with granitoid formations.

As is known, the Yenisei Ridge is a polycyclic region whose geosynclinal development ended in the Baikalian folding. The formation of numerous granitoid intrusions and gold deposits is associated with this tectogenesis; gold is the profiling metal determining the metallogenic specialization of the Yenisei folded region. On the basis of available geological materials, two stages may be distinguished in the development of the Baikalian tectono-magmatic cycle: 1—the Riphean (properly geosynclinal) and 2—the Vendian–Lower Cambrian (orogenic). The first of these, in turn, is subdivided into an early and a late stage.

The early stage of the Riphean stage, which, according to radiological data, covers the interval 1400–900 million years, is characterized by subsidence of the region and accumulation of thick (from 4000 to 7000 m) sequences of sedimentary and sedimentary-volcanogenic formations. The greatest downwarping, judging from the thickness of the deposits, was undergone by the East Yenisei zone of deep faults. According to concepts that have been developing recently, at the beginning of this stage two structural-facies zones became clearly individualized in the Riphean geosyncline of the Yenisei Ridge: the western—eugeosynclinal—and the eastern—miogeosynclinal. The boundary between these zones has not yet been reliably established. M. N. Belyankina and E. A. Dolginov [2] draw it along the axial part of the Cis-Yenisei anticlinorium, while V. E. Khain, M. I. Volobuev, and others [15] place this boundary along a zone of submeridional faults extending parallel to the valleys of the Murozhnaya, Lendakha, and Tei rivers.

The eugeosynclinal character of the western zone is established by the widespread development within it of an ophiolitic formation. The beginning of the Riphean was marked here by outpourings of effusives, as evidenced by their presence in the section of the Korda Formation, which is the lower member of the early-geosynclinal sequence of the Sukhopit Series. Judging by the constant participation of effusives in the structure of the remaining part of the section of the Sukhopit Series as well, effusive activity continued throughout the entire early stage of the geosynclinal phase. There are grounds to suppose that the effusives may have had significance for gold content. This is indicated in part by the elevated gold content in them, established by recent sampling in one of the areas. In the eastern miogeosynclinal zone, effusives have limited distribution. The deposits of the Sukhopit Series here are characterized by slate and flyschoid formations, with the siliceous-carbonate formation being of subordinate importance. Moreover, the latter crowns the section of the early-geosynclinal sequence.

At the end of the early stage, in the course of folding, the sedimentary and sedimentary-volcanogenic formations underwent metamorphic transformations predominantly to the greenschist facies, and in individual areas—to high grades of the amphibolite facies. The areas of development of deeply metamorphosed rocks are inseparably associated with the formation there of metamorphogenic (ultrametamorphic) granites, whose absolute age, according to M. I. Volobuev, falls within the interval 950–850 million years. These granitoids are syntectonic with respect to Riphean folding and are grouped by us into a synorogenic formation. Their massifs are concordant with the structure of the enclosing metamorphic sequences and gradually pass into feldspathized gneisses and crystalline schists.

Among the rocks of the formation, several gneiss-like pink and pink-gray porphyroblastic granites with a fine- and medium-grained groundmass are most widely distributed. Magmatic structures are almost never observed in them, and their mineralogical composition also indicates the formation of these rocks in the process of granitization. They consist of quartz (28–37%), potassium feldspar (27–40%), plagioclase (22–30%), biotite (2–3%), secondary minerals (up to 5%), and accessory and ore minerals (about 1%). The structures are porphyroblastic, with areas of poikiloblastic structure. Porphyroblasts up to 2.5 cm in size are represented by potassium feldspar, usually microcline of high triclinicity with an optical axial angle of 76–86° and with a microcline lattice expressed to varying degrees. A complete absence of exsolution perthites is characteristic, although veinlet and spindle-shaped replacement perthites are rather common. The metasomatic character of the microcline is indicated by poikilitic inclusions of plagioclase and quartz preserved in it, as well as by deformations of plagioclase twins surrounding the porphyroblasts. Intergranular (interstitial) development is characteristic of the microcline of the groundmass. Plagioclase is represented by two varieties: albite No. 4–8 and oligoclase No. 16–22. The average composition of plagioclase, determined by recalculating chemical analyses according to the method of E. A. Kuznetsov, is An\(_{16}\). Most grains of oligoclase are enclosed in microcline porphyroblasts; a smaller part, together with albite, occurs in the groundmass. In individual cases the microcline porphyroblasts are surrounded by rims of albite, which is not characteristic of microclines containing relatively undecomposed grains of oligoclase. The latter is usually sericitized; muscovite and epidote sometimes develop along it. Processes of this kind are regarded as the result of potassium metasomatism [1]. In oligoclase

also noted are myrmekitic intergrowths of quartz. The separate crystallization of microcline and albite, and the presence of grains of kyanite and garnet, which are also characteristic of the enclosing metamorphic rocks, in our opinion testify in favor of a metasomatic genesis of the granites.

In addition to the granites and gneiss-granites noted above, the structure of the massifs of the synorogenic formation includes alaskite, hornblende, epidote granites, adamellites, and plagiogranites, connected with them by gradual transitions. The chemical composition of the granites varies within considerable limits and is in a definite dependence on the composition of the rocks enclosing them.

In terms of their association with stages of the geosynclinal process, the formation of the synorogenic granitoid formation in the Yenisei Ridge corresponds to the formation of an association of the plagiogranite type of younger areas. Yu. A. Kuznetsov [8] notes that the absence in particular regions of plagiogranite associations “reflects the special state of the given segment of the crust.” Probably the peculiarity of the Yenisei Ridge consists in the fact that the geosynclinal formations of the Baikalian stage were formed on a “basement” of granitized crystalline rocks of Archean age, where the processes of selective mobilization and metasomatic granitization were facilitated to a considerable degree.

The metallogenic significance of metamorphogenic granites has as yet been only poorly clarified. According to the available materials, magnetite mineralization of the hydrosilicate type [7] shows a connection with them. Also attracting attention are geochemical halos of tungsten surrounding certain massifs. Gold mineralization, however, is not spatially associated with massifs of metamorphogenic granites. It is highly probable that, in the process of metasomatic granitization of metamorphic sequences, gold was removed, together with a number of other components—iron, magnesium, etc.—from the zone of ultrametamorphism into the overlying roof sequences and, chiefly, underwent secondary dispersion there, in connection with the weak development during the epoch of the main phase of folding of favorable ore-localizing structures. Along with this, evidently, there was partial concentration of gold at a considerable distance from the zone of ultrametamorphism in the tectonic structures that formed. There are some grounds for associating the formation of quartz and quartz–feldspar veins with metamorphogenic granites.

The late stage of the geosynclinal stage in the development of the Yenisei Ridge corresponds to the accumulation of terrigenous–carbonate–shale formations (the Tungusik series) under conditions of increasingly differentiated local downwarps, whose separation is connected with inversion of the Central and Priyenisei anticlinoria.

The end of the late geosynclinal stage in the development of the region was marked by intensive uplifts of previously formed positive structures. These uplifts are connected with the formation, in the cores of the Central and Priyenisei anticlinoria, of late-folded (syninversion) granitoids. Among the granitoid massifs, autochthonous and allochthonous ones are distinguished. The latter are timed to the end of the geosynclinal and the beginning of the orogenic stages and intrude the folded structures of the Baikalian cycle. The granitoid intrusions metamorphose the enclosing formations, as a result of which, in the latter, cordierite and andalusite develop, for example. The granites themselves in the endocontact underwent “chilling,” expressed in a gradual decrease in grain size, which serves as an indicator of a sharp temperature gradient between the enclosing rocks and the granites at the moment

formation of massifs. In all likelihood, the granitoids of the formation under consideration, in the course of their development, passed through a stage of magmatic melt. Characteristic features are bands of hybrid rocks in the contact and apical parts of the massifs. Petrographically typical are magmatic microstructures (hypidiomorphic-granular, granitic, more rarely granophyric), as well as the presence of solid solutions of potassium feldspars, the joint crystallization of which is possible only at high temperatures. Crystallization differentiation under these conditions proceeds especially favorably, as a result of which rocks of different sections of an intrusion differ in chemistry. These phenomena, together with the processes of hybridism and assimilation, determined the considerable diversity of the rocks of the formation. The most widespread are biotite and amphibole-biotite granites; subordinate are plagiogranites, amphibole granites, quartz diorites, diorites, alaskites, alkaline granites, nordmarkites, and syenites.

Gold mineralization, widely developed in the Yenisei Ridge, is closely associated in space and time with the granitoids of the late-orogenic formation. Gold-ore fields are often confined to the apical part of the roof of intrusions and frequently to their exocontacts. With respect to granitoid massifs, zoning is manifested in the distribution of different types of gold-ore mineralization, representing the zoning of ore fields according to the classification of V. I. Smirnov [10]. Thus, for example, in the North Yenisei district, in the direction away from the intrusion, a change in the character of mineralization is established in the following sequence: within the body of the granitoid massif—rare-metal–gold-ore, manifested in greisens; in the exocontact aureole—gold-arsenopyrite; and outside the zone affected by the intrusion—gold-pyrite-arsenopyrite-polymetallic. Zoning of ore fields is also indicated in the South Yenisei district.

From the foregoing there follows a sufficiently substantiated conclusion as to the existence of a genetic connection between gold mineralization and late-orogenic granitoids. In connection with this, it should also be noted that the spatial distribution both of granitoid massifs and of gold-ore deposits is determined by zones of deep faults—the East Yenisei and West Yenisei. Hence one further conclusion suggests itself: that the gold-bearing belts in the Yenisei Ridge tend toward zones of maximum downwarping in the Riphean geosyncline.

In analyzing the distribution of gold mineralization, attention is drawn to the regular confinement of all gold-ore deposits of greater industrial importance to fields of development of rocks of a low degree of metamorphism; fields of deeply metamorphosed rocks, as a rule, are characterized by only weak manifestations of mineralization. Such selective localization of mineralization is apparently connected with the differing reactivity of the enclosing formations to the thermal influence of intrusions. As N. G. Sudovikov [12] indicates, in weakly metamorphosed rocks, under the influence of high temperature, mineral formation takes place with the destruction of hydroxyl- and carbonate-bearing minerals and with the release of metamorphogenic solutions. Under these conditions, ore-forming elements have great migrational capacity. Conversely, in deeply metamorphosed rocks—gneisses and crystalline schists—the thermal influence of intrusions is limited, metamorphogenic solutions arise in insignificant quantities, and the migration of ore-forming elements is impeded.

Thus, a close dependence is established between the intensity of mineralization and the metamorphism of the enclosing rocks, which indicates the

development of ore-formation processes under conditions of completed folding and metamorphic transformation of geosynclinal sediments. In time, the metallogenic epoch of gold mineralization coincides with the very end of the late-geosynclinal stage in the development of the Yenisei Ridge. This is confirmed by the localization of gold-ore deposits mainly in zones of intense schistosity and crumpling of rocks, superimposed on folded structures, and by the occurrence of ore bodies conformable with the schistosity (most often).

The accumulation of formations of fore-orogenic troughs is associated with the Vendian–Lower Cambrian (orogenic) stage in the development of the Yenisei Ridge. This stage was preceded by folding, general uplift, and erosion of the territory under consideration. Along the margins of steadily rising areas, troughs were probably laid down at different times, owing to which correlation of the deposits formed in them is difficult. Common to the trough deposits is their occurrence with angular unconformity upon geosynclinal formations. The composition of the orogenic sedimentary complex varied facies-wise. In the Angara region the complex is represented by the predominantly carbonate Taseeva Series; in the northeastern part of the ridge, by the carbonate-terrigenous Chingasan Series; and in the northwestern part, by the carbonate-flysch Vorogovka Series. According to radiological data, the age of the deposits is 600–750 million years.

The emplacement of granitoid intrusions is associated with the activation of tectonic movements noted in the early stage of the phase. The latter are confined to regional faults, bear features of post-orogenic intrusions (with respect to Riphean folding), and are often accompanied by volcanism. The absolute age of the rocks is 590–620 million years.

The predominant rocks of the formation are equigranular, usually medium-grained microcline granites. Alkaline granites have subordinate development, and their dark-colored minerals are represented by alkaline amphiboles. In composition the microcline granites are close to alaskites. Their structure is granitic, more rarely granoblastoid. The observed sharp discordance of the massifs and the chilling of granites in the endocontacts, as well as the mineralogical composition and structures of the granites, indicate their magmatic origin and crystallization from melts. Cordierite, more rarely andalusite, are characteristic of rocks of the contact zones.

Gold mineralization in connection with granitoids of the post-orogenic formation is weakly manifested. It is represented by individual quartz veins with poor gold content, occurring in deposits of the Sukhopit and Tungusik series predominantly conformably with the schistosity of the rocks. Areas of development of these veins have not been explored; the character of the mineralization and the conditions of localization have not been studied.

Summarizing the materials set forth, the following judgment may be expressed. The granitoids of the Yenisei Ridge constitute a single geosynclinal group of formations, the manifestation of each specific formation being associated with a definite tectonic stage in the development of the region. They represent a single evolutionary series of formations—from metasomatic and anatectic to magmatic. The successive replacement of one formation by another is associated with deeper melting of the granitized sequence and with an increase in the mobile phase at each subsequent stage of activation of tectonic processes. I. S. Turkin [13] came to the same conclusion regarding the commonality of the processes of granitoid formation in the Yenisei Ridge on the basis of analysis of rock radioactivity.

In this process of the formation of granitoid intrusions, not entirely

there is a clear causal connection between the epoch of maximum development of gold-ore mineralization and the stage of formation of late-orogenic granitoids. Apparently, one of the main reasons for the intensive formation of gold concentrations is the broad development, during the inversion of the Riphean geosyncline, of structures favorable for the localization of mineralization. During the stage of the main phase of the Baikalian folding, these structures manifested themselves weakly, and therefore the conditions for the formation of gold concentrations were limited.

REFERENCES

1. Barth, G. Theoretical Petrology. Moscow, Foreign Literature Publishing House, 1956.

2. Belyankina, M. N., Dolginov, E. A. On the question of structural-facies zonality of Riphean deposits in the Zaangara part of the Yenisei Ridge. In: Problems of the Geology of Krasnoyarsk Krai. Moscow State University Press, 1964.

3. Bernstein, P. S., Petrovskaya, N. V. The Sovetskoye gold-ore deposit (Yenisei Ridge). Geology of the Principal Gold-Ore Deposits of the USSR, vol. VI, 1954.

4. Bilibin, Yu. A. Metallogeny and types of gold deposits. In: Gold Deposits of the Urals and Kazakhstan. Metallurgizdat, 1947.

5. Volobuev, M. I. Geochronology and geology of granitoids of the Yenisei Ridge. Author’s abstract of Candidate’s dissertation. Moscow State University Press, 1967.

6. Gorzhevsky, D. I., Kozerenko, V. N. The relation of endogenous ore formation to magmatism and metamorphism. Nedra Publishing House, 1965.

7. Komov, I. L. Some regularities in the distribution of iron-ore deposits in the central part of the Yenisei Ridge. Geology of Ore Deposits, no. 4, 1968.

8. Kuznetsov, Yu. A. On the heterogeneity of magmatic rocks (on the example of granites). Geology and Geophysics, 1961, no. 10.

9. Smirnov, V. I. Essays on the Metallogeny of the USSR. Gosgeoltekhizdat, 1963.

10. Smirnov, V. I. The order of endogenous ore zonality. In: Conference. Problems of Postmagmatic Ore Formation, vol. I, Prague, 1963.

11. Sudovikov, N. G. On the question of a possible connection between hydrothermal mineralization and granitization. In: Magmatism and the Associated Useful Minerals. Publishing House of the Academy of Sciences of the USSR, 1955.

12. Sudovikov, N. G. Problems of rapakivi and late-orogenic intrusions. Nauka Publishing House, 1967.

13. Turkin, I. S. Methodology for subdividing granitoid complexes. Proceedings of SNIIGGiMS, issue 44, Novosibirsk, 1967.

14. Fedotyev, K. M. On the formation of hydrothermal solutions containing heavy elements. In: Endogenous Ore Deposits, IGC, XXIII Session, Problem 7, Nauka Publishing House, 1968.

15. Khan, V. E., Volobuev, M. I., Vorobyev, I. V., et al. The main stages of tectonic development of the Yenisei Ridge in the Precambrian and Early Paleozoic. Bulletin of Moscow State University, no. 5, 1967.

16. Huang, F. N. On the origin of granitic magmas and ore deposits. In: Magmatism and the Associated Useful Minerals. Gosgeoltekhizdat, 1960.