On the Gold Potential of the Kolyvan–Tomsk Folded Zone

B. M. Levashov, P. N. Parshin (TPI)

In a brief communication there is no possibility of dwelling on the particular features of the tectonics, stratigraphy, and lithology of the region, which to one degree or another have been discussed in a number of published works [1, 6, 8]. Here we shall briefly note the structural features of the region described.

The geological structure of the Kolyvan–Tomsk folded zone includes formations from the Lower–Middle (undifferentiated) Devonian up to recent deposits inclusive, forming two major structural stages. The lower structural stage is composed of Paleozoic formations, strongly dislocated during the late Hercynian phase of tectogenesis, with the formation of folds linearly elongated in a NE direction. The principal structural elements of the zone [5] are: the area of the main, or internal, trough of the Hercynian geosyncline, filled chiefly with flyschoid formations of \(D_3\) and \(C_1\); geoanticlinal structures (the Bugotak–Mitrofanovskaya geoanticlinal zone), composed of volcanogenic-sedimentary formations of \(D_{1-2}\); and marginal and transverse troughs of the eastern part of the Kolyvan–Tomsk geosynclinal system, filled with coal-bearing formations of \(C_1—P_1\) (the Tashminsky, Kuznetsk, and Gorlovsky troughs) and usually regarded as independent geostructural units (Fig. 1).

Intrusive formations in the region are very extensively developed in the area of the Novosibirsk Priobye and are represented by granitoids of various composition and by their vein derivatives (aplites, greisens, etc.). Along transverse shear fractures, dike formations are developed, represented by diabases, dolerites, and monzonites [2]. Their relationship with the Novosibirsk granitoid intrusions is unclear; however, in the time of their formation they are evidently often very close to one another. Thus, in the area of the village of Gusinyi Brod a diabase dike approximately 100 m thick is cut by a dike of two-mica plagiogranite 70 cm thick. Granite at the “Borok” quarry is cut by diabase, and at the same time both granite and diabase are intruded by veins of granite-aplite. The granitoids contain rare-metal mineralization. Their emplacement was accompanied by hydrothermal mineralization of the enclosing rocks. This was probably the second (after the Lower–Middle Devonian) stage of hydrothermal mineralization in the Kolyvan–Tomsk folded zone, manifested, probably, chiefly in the area of its main geosynclinal trough (Fig. 1). According to A. M. Kuzmin [3], gold mineralization in quartz veins of sandy-argillaceous shales of \(C_1\) also belongs to this time. Of the same

Fig. 1. Structural diagram of the Kolyvan–Tomsk folded zone.

1. Area of the main Hercynian trough of the Kolyvan–Tomsk geosyncline.
2. Bugotak–Mitrofanovka geoanticlinal zone.
3. Frontal troughs.
4. Salair.
5. Kuznetsk Alatau.
6. Outcrops of Upper Paleozoic granitoid intrusions.
7. Areas of distribution of gold in heavy-mineral concentrates.
8. Identified points of association of gold with polymetallic mineralization: I—Turuntaevo, II—Yashkino, III—Baranovka.
9. Identified points of association of gold with quartz-vein mineralization: IV—Tomsk, V—Motkovo, VI—Petrushikha.

mineralization of this character in the vicinity of the city of Novosibirsk was noted by A. I. Gusev [1].

As a result of regional hydrogeochemical investigations carried out by us within the Kolyvan–Tomsk folded zone, gold was detected at points of polymetallic mineralization [4] (Fig. 1). All these data provided grounds for paying close attention to the identification of gold prospects in the region described. For this purpose, a chemical analysis for gold content, using the method proposed by N. I. Safronov [7], was performed on samples of rocks from the main ore occurrences of the region (see table). These included not only ore occurrences known from previous investigations, but also ore occurrences discovered in the course of the work being conducted. Because, with few exceptions, the majority of ore occurrences are found in outcrops of Paleozoic rocks at the surface, the material collected consists mainly of various morphological types and varieties of hypergenically altered rocks (brown ironstones, rocks enriched to varying degrees with limonite and bleached rocks, material from disintegrated quartz veins, etc.). In studying this material, spectral, electron-microscopic, microchemical, and chemical analyses were used, as well as microscopic studies of transparent and polished thin sections. The most fully studied occurrence of interest is the ore occurrence located near the village of Baranovo, 30 km east of the Yurgа station of the Taiga–Novosibirsk railway. The small amount of exploratory work carried out at this ore occurrence, including the drilling of four boreholes up to 150 m deep each, revealed its prospects for industrial ores.

The rocks hosting the Baranovo ore occurrence are schistose quartz albitophyres and their tuffs, as well as tuffs of mixed composition. The mineralization is localized in steeply dipping zones of crushing and intense schistosity of the rocks and has a disseminated and veinlet-disseminated character of hydrothermal-metasomatic genesis. The main ore minerals are sphalerite, galena, chalcopyrite, and pyrite; arsenopyrite, pyrrhotite, and magnetite are encountered very rarely. The non-ore mineralization is represented mainly by quartz; calcite, barite, and prenite occur, and fluorite is noted in insignificant amounts.

Native gold was discovered during microscopic study of ores from one of the four boreholes that exposed the ore occurrence, at depths of 38.2 and 53.8 m. Spatially it is confined to veinlet ore mineralization developing in accordance with the schistosity of the enclosing rocks. A characteristic feature of native gold in the ores studied is its close association with sulfide minerals—pyrite, sphalerite, chalcopyrite, and galena. At the same time, the character of the associations and the sizes of gold grains with the indicated minerals are different. Whereas in pyrite we observe submicroscopic droplet-like and isometric inclusions of gold ranging in size from 0.004 to 0.01 mm, with sphalerite, chalcopyrite, and galena, in addition to isometric inclusions, gold forms intergrowths in which the gold grains have the most diverse, for the most part irregular, outlines. The grain size of the gold in the intergrowths also increases. Thus, in galena they are predominantly 0.01 mm in size, in chalcopyrite up to 0.04 mm, and in sphalerite up to 0.05 mm. The indicated maximum sizes are mainly those of gold grains in intergrowths with the listed sulfides. Veinlet development of gold in association with chalcopyrite, sphalerite, and galena along cracks in gold-bearing pyrite is often observed. In addition, small irregular segregations of gold in vein quartz enclosing the sulfide minerals occur very rarely. The color

Table

Continuation of the table

and the reflectivity of native gold in the ores studied indicate its low fineness.

The close association of gold with ore minerals and the character of its occurrences described above indicate that gold was deposited during the main stage of ore formation of the Baranov ore occurrence, and from these standpoints it is closest in the Altai–Sayan folded region to the gold of the Leninogorsk group of deposits. The absence of observations of any associations of gold with arsenopyrite and pyrrhotite does not mean that they will not be accompanied by gold in other parts of the ore occurrence, since, of these two sulfides, at least arsenopyrite in the paragenetic sequence of ore-mineral formation stands immediately after the gold-bearing pyrite. In addition, both arsenopyrite and pyrrhotite are sulfides whose close connection with gold is almost constant. It is possible that the indicated sulfides contain gold in the form of dispersed particles, and that it is not detected by ordinary microscopic methods.

Native gold in the primary sulfide ores of the Kolyvan–Tomsk folded zone has been established for the first time. Until the present time, re-

the prospects of the indicated zone only for gold–quartz-type mineralization have been noted. Thus, in Tomsk Oblast, according to available data [3], gold in placers was mined in the system of the Ushaika and Kirgizka rivers. A. A. Kuzmin also reports personal finds of gold in quartz veins on the bank of the Tom River near the Lagerny Garden, and considers the age of the quartz veins to be Upper Paleozoic. Promising for mineralization of this type in the Kolyvan–Tomsk folded zone are the basins of the Sosnovka, Lebyazhya, Iskitim, and Karakan rivers, where there are numerous outcrops of mineralized quartz veins. During work by the Novosibirsk Territorial Geological Administration in the basins of these rivers, gold was discovered during panning.

Thus, in the Kolyvan–Tomsk folded zone there are two types of gold mineralization: quartz-vein and sulfide–polymetallic. The former are apparently the sources of gold in the heavy-mineral concentrates. In our opinion, only the second type of mineralization—i.e., that yielding concentrations chiefly in oxidation zones—can be of practical interest. Considering that polymetallic mineralization is most intensively manifested within the region in volcanogenic formations, prospecting for gold should probably be conducted first of all within the areas of distribution of the latter.

If one takes into account that the volcanogenic formations are developed mainly within the Bugotak–Mitrofanovo geoanticlinal zone (volcanogenic–sedimentary formation \(D_{1-2}\)), then the location of the areas most promising for sulfide mineralization should also be expected within these same limits.

However, in the Meso–Cenozoic history of its development the Bugotak–Mitrofanovo geoanticlinal zone experienced the greatest amplitude of uneven block uplifts in comparison with the other geostructural elements of the Kolyvan–Tomsk folded zone. At the same time, the southern part (the Bugotak horst-anticline) apparently experienced the maximum amplitude of uplift and, accordingly, the maximum amount of erosional truncation. As a result, the optimal depths of sulfide mineralization may also have been brought up to the level of erosional truncation and eroded. Therefore, the northern part of the Bugotak–Mitrofanovo geoanticline and the anticlinal structures in the area of the principal Tertiary geosynclinal trough may be considered more promising for sulfide mineralization (Fig. 1).

REFERENCES

1. Gusev A. I. Geological structure and mineral resources of the city of Novosibirsk. Publ. of the West Siberian Geological Trust, Tomsk, 1934.

2. Ivanov K. V. Some questions of the petrography of dike rocks in the environs of the city of Tomsk. Proceedings of Tomsk State University, 1956, vol. 135.

3. Kuzmin A. M. Upper Paleozoic mineralization in the environs of Tomsk. Geology of Ore Deposits, No. 2, 1961.

4. Levashov B. M. On native gold in primary sulfide ores of the Kolyvan–Tomsk folded zone. Manuscript, TPI.

5. Matveevskaya A. L. On the structure and development of the Kolyvan–Tomsk arc in the area of junction with Salair. Proceedings of the Mining Geological Institute, West Siberian Branch, Academy of Sciences of the USSR, issue 15, 1956.

6. Matveevskaya A. L., Ivanova E. F. Geological structure of the southern part of the West Siberian Lowland in connection with questions of oil and gas potential. Publishing House of the Academy of Sciences of the USSR, Moscow–Leningrad, 1960.

7. Safronov N. I., Polikarpoc̣hkin V. V., Utgof A. A. Spectrogoldmetric surveying as a method for prospecting ore deposits not accompanied by mechanical halos. Ministry of Geology and Mineral Protection of the USSR (VITR). Exchange of Experience, issue 36, 1960.

8. Speranskii B. F. Structures of Paleozoic formations of the Ob–Tom interfluve. Collection on the Geology of Siberia, Publishing House of ZSGU, Tomsk, 1933.