Some Structural and Petrochemical Features of Paleozoic Magmatism in the Urik–Kitoi Gold-Ore District of the Eastern Sayan

V. A. Globa (KazIMS)

The magmatism of the Eastern Sayan is regarded as a constituent element of the Caledonian orogeny (Obruchev, 1949; Dodin, 1958; Suloev, 1960; Krasilnikov, 1963, and others). In the development of magmatism, a general tendency is noted toward a successive increase in the alkalinity regime from early to later formations (Lur’e, 1952). In explaining such an evolution, the time factor is especially emphasized (Dodin, 1963). Magmatic activity is explained either by the aggressive influence of the adjacent Tuvinian geosyncline (Zonenshain, Kudryavtsev, 1960), or by the successive activation of the internal tectonic structures of the Eastern Sayan (Mossakovskii, 1963; Khrenov, 1961; Mitrofanov et al., 1964; Globa, 1963; Gordienko, 1964, and others).

Within the district, Lower Paleozoic effusives are widespread in the headwaters of the Oka, where the name Sarchoi suite has become established for them. The rocks are represented by quartz porphyries, plagioporphyrites, andesites, and their tuffs. Volcanogenic formations everywhere lie with a sharp angular unconformity upon the Mongosha limestones of the Upper Proterozoic. They correlate well with the Lower Cambrian porphyrites of Tuva.

The spatial position of rocks of the hyperbasite formation is controlled by internal deep-seated faults. The massifs are grouped into an extended linear belt. It is traced along the northern peripheral part of the Kitoi–Sarchoi marginal trough in the form of two clearly expressed branches. Both branches encircle the Gargan block of the Archean and, joining in the headwaters of the Kitoi, form the largest in Siberia, the Ospin hyperbasite pluton (Fig. 1). In analyzing the structural position of the hyperbasites, their maximum confinement to the junction of the Kitoi–Kholba system of faults with the Main Sayan suture is noted. The massifs show a preferential tendency toward tectonic contacts of the most contrasting Precambrian structures—the Gargan–Butogol uplift and the Ilchir synclinorium. In a westerly direction, toward the internal zone of the Caledonides, the saturation of sublatitudinal deep-seated faults with ultrabasic magma decreases, and gabbroids acquire the predominant influence.

The hyperbasite formation is represented by ultrabasic and partly basic rocks. The former predominate in the eastern part

of the trough (Kitoy), the second in the western (Sarkhoy). A petrochemical study of the rocks of the hyperbasite formation indicates weak differentiation of the initial magma, its extremely high magnesiality, and insignificant contents of alumina, alkalis, lime, and titanium. By the features listed, they are similar to the Tuvan ones.

Granitoids in the territory of the Eastern Sayan are widely distributed and show considerable diversity. The most ancient—Precambrian—form the Sayan plutonic belt, whose spatial position is determined by the Main Sayan fault. The granitoids make up monogenic batholith-like massifs, chiefly of palingenic facies, among which the early ones are accompanied by mica-bearing pegmatites and the late ones by rare-metal pegmatites (Smirnov, Buldakov, 1962).

The Lower Paleozoic granitoids, owing to the specific geoanticlinal regime of the Eastern Sayan, predominate in rocks of the rigid substrate of the Baikalides, and only some of them are situated in Cambrian volcanogenic-sedimentary deposits. In all cases, their active contact with Lower Cambrian hyperbasites is established. The rocks are represented by two branches—sodium and potassium (Mitrofanov, Avdontsev, 1964). Derivatives of the first branch are localized on the periphery of Early Caledonian downwarps in close association with subshear faults, predominantly in the zone of convergence with the marginal suture; those of the second—in activated sectors of geoanticlinal zones (Fig. 1).

Fig. 1. Scheme of the distribution of magmatism in the Caledonian structure of the southeastern part of the Eastern Sayan. (Compiled by the author using materials of V. P. Arsent’ev, F. K. Volkolakov, and others.)

1. Zones of stable uplifts of the Caledonides: I—Tunka geoanticline; II—Oka geoanticline; 2. Kitoy-Sarkhoy near-fault downwarp (III). 3. Oka (Gargan) block of the Archean. 4. Disjunctives. IV—Main Sayan fault; V—Kitoy fault; VI—Oka fault; VII—Kandat fault.

Magmatic formations: 5—Precambrian granitoids; 6—Cambrian volcanogenic deposits; 7—hyperbasites and basites of Cambrian age; 8—Lower Paleozoic granitoids of the sodium branch; 9—Lower Paleozoic granitoids of the potassium branch; 10—Middle Paleozoic effusive-intrusive series of rocks.

The granitoids of the sodium branch are confined to the Kitoi–Sarkhoi system of deep faults, chiefly to its outer branch—the Kholbin branch—which borders the Gargan block of the Archean from the north. The intrusions are recorded as an elongated belt consisting of a number of scattered massifs with a total area of more than one thousand sq. km. They compose fissure and interformational bodies whose roots descend into magma-conducting channels. The rocks are characterized by a varied composition—from gabbro to plagiogranites, with quartz diorites predominating in distribution. The early representatives are characterized by an association of plagioclase and hornblende, and the late ones by plagioclase and quartz. Petrochemical studies establish a calc-alkaline composition for the rocks of the early series and a substantially sodic one for the late representatives. The rocks differ from the average types of R. Daly in their smaller amount of alkalis and higher content of lime and silica.

From the example of the Kholbin intrusive it is evident that the formation of the sodium branch of granitoids in the course of crystallization differentiation proceeded along the line of successive enrichment of magmatic products in alkalis, with a corresponding depletion in lime. In the diagram compiled by the method of G. S. Momdzhy (1965), these changes are expressed by hyperbolic (for alkalis) and parabolic (for lime) curves (Fig. 2). Points (H, M) reflect an inversely proportional relationship between the content of alkali metals and magnesium, recalculated to their atomic percentages in the four-component system \((\mathrm{MgO}+\mathrm{CaO}+\mathrm{Na_2O}+\mathrm{K_2O}=100\%)\). Points (C, M) record the ratio between calcium and magnesium in percent, recalculated according to their atomic amount in the four-component system. It follows from the diagram that the values H and C of quartz diorite do not fit these curves, apparently in connection with local hybridism. The critical moment of crystallization differentiation in the magmatic chamber corresponds to the maximum value of C on the CM curve, after which the stage of alkali activation begins. This turning point corresponds to diorite.

The formation of granitoids of the potassium branch, similar in age, took place under a stable regime in second-order geoanticlinal zones (Fig. 1). These are most often interformational, discordant, batholith-like massifs with areas from the first hundreds to one thousand sq. km. The largest of them—the Nilovsky, Munku-Sardyk, and partly Sarkhoi massifs—occur in a Precambrian substrate. The granitoids formed in three phases. Among the rocks, derivatives of the late phase occupy the dominant place. The processes of crystallization differentiation are somewhat obscured by local assimilation. In the course of crystallization, as lime decreases, the content of alkalis and silica increases; basic plagioclases are replaced by acidic ones in association with potassium feldspar and quartz. However, even the most alkaline varieties have no excess of alkalis. Therefore their end members are represented not by syenites but by leucocratic potassium granites. The latter, according to G. D. Afanas’ev (1963), correspond to the low-temperature eutectic association (quartz–K-feldspar–albite) of the final stage of crystallization of the magmatic melt.

The formation of the rocks, using the Munku-Sardyk pluton as an example (Fig. 2), differs from the preceding cases in the sharply limited range of the magnesian component (abscissa axis) with a large range of the alkali and lime components (ordinate axis). Here the early stage of crystallization was weakly manifested. The main crystallization coincides with the stage of alkali activation and, as it were, corresponds to the final stages of formation of the granitoids of the Kholbin pluton. Its final ...

Fig. 2. Crystallization differentiation of granitoids of the Kholba (1), Tannuola (2), and Munku-Sardyk (3) massifs.
Abscissa axis—atomic quantity of magnesium in %; ordinate axis—atomic quantity, in %, of alkalies and lime; dots—total alkali content in % (4); crosses—amount of lime in % (5). \(a\)—gabbro; \(b\)—diorite; \(v\)—quartz diorite; \(g\)—granodiorite; \(d\)—granite (indices 1, 2, 3 indicate the assignment of rocks to the corresponding massifs).

The products are represented by subalkaline leucocratic granites, close in composition to the eutectics of the three-component system \((\mathrm{SiO_2}—\mathrm{K_2O}—\mathrm{Na_2O})\). According to Yu. M. Sheinmann (1963), such magmas are formed with a significant role of regenerated products owing to selective melting, first of all, of the roof rocks.

For comparison of the evolution of coeval granitoid magmas of the Eastern Sayan and Tuva, some studies on the Tannuola complex are cited (Pinus, 1961, 1963). Comparison shows (Fig. 2) a very similar dynamics in the formation of the Tannuola and Kholba granitoids. Such similarity, according to G. S. Momdzhi, testifies to their genetic commonality. This is also confirmed by the petrographic and formational affinity of the granitoids.

The diagram considered, however, is inconvenient in that the total amount of alkalies obscures the individual behavior of the sodium and potassium series during the formation of granitoid complexes. The picture of magmatic crystallization in the three examples considered above is revealed more clearly by separate analysis of the oxides of sodium and potassium. As is seen from the diagram (Fig. 3), the alkalies in the process of multiphase formation of the Kholba pluton behave in a differentiated manner. The potassium component (the KM curve) at first increases smoothly and, having reached a maximum in granodiorite, then stabilizes. The ratio of potassium oxide to magnesium in the final rock—plagiogranite,

\[ K_k=\frac{K}{M} \]

does not exceed unity. The sodium component exhibits a stable tendency to increase, reaching its greatest value in plagiogranite,

\[ K_{Na}=\frac{Na}{M}=\frac{40}{11}=3.6. \]

The ratio of these two coefficients,

\[ \frac{KNa}{K_k}=3.6, \]

emphasizes the dominance of the sodium potential in the final member of the Kholba granitoids. In the same sequence, but more smoothly—which, possibly, is explained by the larger number of chemical analyses—the formation of rocks of the Tannuola complex proceeded. The maximum value of the potassium potential in them corresponds to granodiorite, and that of the sodium potential—to plagiogranite. Thus, for the Kholba intrusive body an increas-

...sodium activity, which reaches a maximum in plagiogranites—the final products of crystallization of the gabbro-plagiogranite melt. It is interesting that the noted activity is inherited by epimagmatic solutions, accompanied by sodium metasomatism. It is characteristic of a number of magmatic provinces of the Union (Afanas'ev, 1960). The activity of potassium, on the contrary, in the final stages of crystallization sharply weakens, as if suppressed by the sodium cation against the background of its increased concentration.

The opposite picture takes place in the formation of the potassic granitoids of the Munku-Sardyk pluton. The activity of potassium in this series of rocks consistently increases along a curve close to a hyperbola, reaching the greatest concentration in the microcline granite that completes the formation of the massif. The concentration of sodium at first increases, with a maximum value in biotite granite, and then drops sharply, changing in general along a parabolic curve (Fig. 3). The ratio $\frac{KNa}{K}$ in the final differentiates—microcline granites—is only 0.5, which is seven times less than in the plagiogranites of the Kholbin pluton. In connection with the increased activity of potassium, postmagmatic potassium metasomatism is noted.

Middle Paleozoic magmatism manifested itself in a distinctive volcano-plutonic form. Volcanic products accumulated in near-fault troughs, and intrusive ones—in suture disjunctives of the Oka-Kandat fault system. The Main Sayan Fault also took part in the localization of Devonian magma (Fig. 1). For Middle Paleozoic magmatism, a close spatial-age and genetic commonality of volcanic and plutonic processes is established (Gordienko, 1964). In the formation of the rocks, volcanic, subvolcanic, and plutonic facies of a single comagmatic series are distinguished. Granitoid magma is characterized by increased alkalinity, considerable ferruginousness of femic minerals, poverty in magnesium, and saturation with lime. The kinship between granitoids and alkaline rocks is substantiated by their comagmatic connection and spatial commonality, closeness in time of formation, and a unified chemistry of the mafic minerals. The formation of the extreme members of the granitoid series—alkaline and non-feldspathoidal syenites—represents a particular case of crystallization of granosyenite magma. As the magma crystallized, its residual portions...

Fig. 3. Crystallization differentiation of the granitoids of the Kholbin (1), Tannu-Ola (2), and Munku-Sardyk (3) massifs. The abscissa is the atomic amount of magnesium in %; the ordinate is the atomic amount of potassium (dots), sodium (crosses), in %; $a$—gabbro, $b$—diorite, $v$—quartz diorite, $g$—granodiorite, $d$—granite (indices 1, 2, 3 show the belonging of the rocks to the corresponding massifs).

became increasingly undersaturated with silica. Against the background of significant desilication, the average alkali content changes from 8.8 to 12%. It is characteristic that the process proceeded with a successive increase in sodium activity and a progressive fall in potassium potential. According to V. P. Kostyuk and T. Yu. Bazarova (1966), the entire mechanism of formation of alkaline rocks is reduced to the redistribution of alkalies as a result of deep assimilation of calcium and a change in activity between potassium and sodium. However, the subordinate development of alkaline rocks among the extensive fields of Middle Paleozoic granitoids indicates a local manifestation of this process. Much more often, crystallization differentiation falls into the normal series, ending with the formation of feldspar granitoids accompanied by a vein series of rocks of variegated composition. For the most part, the Devonian granitoids have a leucocratic appearance. They are characterized by processes of albitization, perthitization, quartzification, and quartz–feldspar metasomatism (Shmotov, Gordienko, Globa, 1964).

Fig. 4. Distribution of alkalies and silica in granitoids of the Middle Paleozoic complex of the Eastern Sayan. The abscissa axis is silica content in %. The ordinate axis is alkali content: total (solid line), potassium (dots), sodium (dashed line), in %. 1—granites; 2—syenites; 3—alkaline syenites; 4—nepheline syenites.

The spatio-temporal directionality in the development of the Caledonian tectono-magmatic structures predetermined the corresponding regional zonality of the mineral complexes associated with them. Individual massifs of the hyperbasite belt are accompanied by chrysotile-asbestos mineralization, an example of which is the largest Ilchir deposit. Granitoids of the sodium branch are accompanied by an extensive group of deposits and ore occurrences of gold in the Urik–Kitoi interfluve (Globa, 1963). Molybdenum mineralization is associated with granitoids of the potassium series (the Nokhor skarn molybdenum deposit, etc.), and gold in small concentrations (upper reaches of the Oka River). Specific rare-earth mineralization and aluminous raw material are connected with the Middle Paleozoic complex.

Metallogenic zoning of the Eastern Sayan makes it possible to distinguish a number of structural-metallogenic zones, among which the spatially convergent chrysotile-asbestos and gold-ore zones are of particular interest. One of the most important features of gold mineralization

is its close spatial association with sodium granitoids under a common controlling structure—the Kitoi-Sarkhoi deep fault. The ore fields (deposits) are grouped predominantly in intra-block faults. In the distribution of gold mineralization, horizontal and, to a lesser extent, vertical zoning is manifested. Moderately sulfide gold-polymetallic mineralization is localized in the apical parts and endocontacts of intrusions; low-sulfide quartz-pyrite ores occur in exocontacts at a distance of 2–5 km from the nearest massifs; low-sulfide gold-silver mineralization shows no visible connection with intrusions. Along with spatial proximity, a temporal convergence is noted between gold mineralization and rocks of the Kholba intrusive complex, especially its late differentiates—dikes and small intrusions.

Geochemical studies of sodium granitoids and gold-ore mineralization establish an affinity between them in tin, silver, beryllium, zinc, yttrium, and elements of the iron group—cobalt, nickel, chromium, manganese, titanium, and vanadium. In the intrusive rocks, tungsten, arsenic, bismuth, cadmium, lithium, and barium have not been found. These trace elements are absent in the gold ores of the district. Particularly noticeable in them is the absence of arsenic, which, for example, in the gold-bearing ores and granitoids of the Yenisei Ridge is regarded as typomorphic.

The gold-ore deposits of the Urik-Kitoi district form a single genetic series of hydrothermal formations in close association with the Early Caledonian gabbro-plagiogranite formation.

REFERENCES

1. Afanas’ev G. D. Some regularities in the development of magmatism in folded regions of the USSR. In: Reports of Soviet Geologists at the XXI Session of the International Congress. Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.

2. Afanas’ev G. D. On the problem of granites. In: Problems of Magma and the Genesis of Igneous Rocks. Moscow, Publishing House of the Academy of Sciences of the USSR, 1963.

3. Globa V. A. Principal features of the geology and gold content of one of the districts of the Eastern Sayan. In Proceedings of the East Siberian Geological Institute, Siberian Branch, Academy of Sciences of the USSR, issue 13, Irkutsk, 1963.

4. Gordienko I. V. On the connection of volcanism with intrusive activity in the zone of convergence of the Oka and Main Sayan faults. In Reports of the Academy of Sciences of the USSR, vol. 195, no. 3, 1964.

5. Dodin A. L. Eastern Sayan. In: Geological Structure of the USSR, vol. 2, Moscow, 1958.

6. Dodin A. L. Intrusive complexes of the Eastern Sayan. Abstracts of reports, 3rd All-Union Petrographic Conference, 1963.

7. Zonenshain L. P., Kudryavtsev G. A. The position of the principal granitoid intrusions in the tectonic structure of the Sayano-Tuva mountain region. In: Caledonian Orogeny. Moscow, Publishing House of the Academy of Sciences of the USSR, 1960.

8. Kostyuk V. P., Bazarova T. Yu. Petrology of alkaline rocks of the eastern part of the Eastern Sayan. Moscow, “Nauka,” 1966.

9. Krasil’nikov B. N. On Precambrian marginal troughs of Siberia. Geology and Geophysics, no. 5, 1963.

10. Lur’e M. L. Caledonian volcanism of the Eastern Sayan and Khamar-Daban. In Proceedings of VSEGEI, Leningrad, 1952.

11. Mitrofanov F. P., Avdon’tsev N. A. The East Sayan mobile area in the Lower Paleozoic. In: Magmatic Complexes. Precambrian of the Eastern Sayan. Proceedings of the Laboratory of Precambrian Geology, Academy of Sciences of the USSR, issue 18, Leningrad, 1964.

12. Molodzh G. S. Chemical composition of rocks as a source of information on the processes of formation of intrusive massifs. Soviet Geology, 1965, no. 10.

13. Mossakovsky A. A. Tectonic development of the Minusinsk depressions and their mountainous framing in the Precambrian and Paleozoic. Moscow, Gosgeoltekhizdat, 1963.

14. Obruchev V. A. Tectonics of the western part of the Sayano-Baikal Caledonian folded region. In Reports of the Academy of Sciences of the USSR, no. 5, vol. 68, 1969.

15. Pinus G. V. The Tannuola intrusive complex. Proceedings of the Siberian Branch of the Academy of Sciences of the USSR, issue VI, Novosibirsk, 1961.

16. Pinus G. V. Lower Cambrian volcanism of Tuva. Proceedings of the Siberian Branch of the Academy of Sciences of the USSR, issue VII, Novosibirsk, 1961.

1. Ginus, G. V. On the question of the genesis of the magma forming the rocks of the gabbro–plagiogranite magmatic formation (on the example of Tuva). In: Problems of Magma and the Genesis of Igneous Rocks. Moscow, Publishing House of the Academy of Sciences of the USSR, 1963.

2. Smirnov, A. D.; Buldakov, V. V. Intrusive Complexes of the Eastern Sayan. Moscow, Publishing House of the Academy of Sciences of the USSR, 1962.

3. Suloev, A. I. Magmatism of the Eastern Sayan. Soviet Geology, 1960, No. 6.

4. Khrenov, P. M. Caledonian magmatic rocks of the Sayan–Baikal mountain region and problems of their metallogeny. In: Problems of Tectonics. Moscow, Gosgeoltekhizdat, 1961.

5. Sheinman, Yu. M. On the tectonic conditions of magma formation. In: Problems of Magma and the Genesis of Igneous Rocks. Moscow, Publishing House of the Academy of Sciences of the USSR, 1963.

6. Shmotov, A. P.; Gordienko, I. V.; Globa, V. A. On certain features of metamorphism within the Oka deep fault (Eastern Sayan). Izvestiya of the Academy of Sciences of the USSR, Geological Series, No. 11, 1964.