PROCEEDINGS
OF THE TOMSK POLYTECHNIC INSTITUTE
OF THE ORDER OF THE OCTOBER REVOLUTION
AND THE ORDER OF THE RED BANNER OF LABOR
named after S. M. Kirov

Vol. 177                                                                                                                                                                                     1971

ON THE MANIFESTATION OF HORIZONTAL ZONATION OF MINERALIZATION IN THE CENTRAL ORE FIELD

V. I. BAZHENOV

(Presented by the scientific seminar of the Departments of Mineralogy, Petrography and Geology
and Prospecting of Mineral Deposits)

As is known, the phenomenon of vertical and horizontal zonation of mineralization was established by W. Emmons as early as the 1920s of the present century. It is manifested as a regular replacement of some mineral associations by others both with depth and in the horizontal plane, with individual zones arranged concentrically around intrusive bodies.

Subsequently, W. Emmons’s views were subjected to criticism by S. S. Smirnov and other researchers. Without dwelling on consideration of the various ideas about vertical and horizontal zonation, it may be noted that, according to modern concepts, the formation of horizontal and vertical zonation may be caused by various factors [5]. Numerous examples of the manifestation of zonation were presented in the proceedings of the International Conference on Problems of Endogenous Ore Formation, held in Prague in 1963.

Work in the Central ore field (Mariinsk taiga) established manifestations of horizontal zonation of mineralization, the description of which is the subject of the present article.

Geological structure of the Central ore field

The Central gold-ore field is one of the oldest gold-ore districts of Western Siberia.

Geologically, it is confined to the northern part of a large granodiorite massif belonging to the Martaiga magmatic complex of Lower Paleozoic age. The petrographic composition of the intrusive rocks is comparatively variegated: granodiorites, quartz diorites, granites, granosyenites. They are connected with one another by gradual transitions. The change of rocks takes place over short distances, and distinguishing their various types during geological mapping does not appear possible. Outside the ore field, in the endocontact of the intrusive massif, more basic rock varieties are encountered: diorites, gabbro, gabbro-diorites, etc., the formation of which T. M. Dembo [3] relates to processes of assimilation of enclosing strata.

The vein rocks associated with the Central granodiorite massif are represented by dikes of spessartites, microdiorites, and diorite porphyrites. Their age is pre-ore [4].

The only type of gold mineralization is gold-bearing quartz veins. By their spatial orientation, among them veins of northeastern and sublatitudinal strike are distinguished. Confined

Fig. 1. Schematic geological structure of the Central ore field. 1 — granodiorites; 2 — spessartites; 3 — diorite porphyrites; 4 — zones of increased fissuring; 5 — quartz veins; 6 — faults; 7 — attitude elements. I — barren zone; II — lead-zinc zone; III — polymetallic zone; IV — antimony zone.

to two systems of zones of increased fissuring, they are concentrated in the form of belts of two directions. Small quartz veins of submeridional strike are encountered rarely (Fig. 1).

The largest and industrially most valuable quartz veins are concentrated at the intersections of zones of increased fracturing of different trends. This accounts for the nodal distribution of quartz veins that is characteristic of the ore field. The largest areas of vein distribution are the following (from south to north):

1. Yubileinyi (Kavkazskaya, Lotereinaya, Altaiskaya, Uralskaya, Petrovskaya, Dmitrievskaya, and other veins); 2. Tsentralnyi—veins: Tsentralnaya, Kazanskaya, Reshayushchaya, Nezavidnaya I, II, III, Belyaevskaya, and others; Oktyabrskii—veins: Sibzolotovskaya II, Tysyachnaya, Veselaya, Millionnaya, Almaznaya, and others.

In addition to these, there are a number of smaller ore nodes and individual quartz veins occurring in zones of increased fracturing outside the ore nodes, as is clearly seen in Fig. 1.

The process of hydrothermal ore formation was complex and was repeatedly interrupted by tectonic movements. In studying the ores, on the basis of the criterion of intersection of mineral associations of different ages, seven successive stages of mineralization are distinguished: barren, quartz-pyrite, quartz-arsenopyrite, quartz-polymetallic, chlorite, a second barren stage, and quartz-carbonate. The barren stage of mineralization consists of dark-gray, in places almost black, fine-crystalline quartz with an admixture of sericite, actinolite, and rutile, which are minerals of a relict mineral association. The mineral association of the quartz-pyrite stage of mineralization is very widespread in all quartz veins of the ore field. It is represented by light-gray or grayish-white quartz with an admixture of pyrite. Its amount may vary. In some cases it forms sparse fine disseminations in vein quartz; in other cases it creates areas of almost monomineralic sulfide ores. The quartz-arsenopyrite stage of mineralization was also widely manifested. It is represented by gray-white quartz with arsenopyrite. The quartz-polymetallic stage was likewise widespread; to one degree or another its mineral associations appeared in all quartz veins of the ore field. They are represented by pyrite, sphalerite, galena, pyrrhotite, chalcopyrite, fahlores, enargite, bismuth, bismuthinite, argentite, silver, gold, calaverite, sylvanite, and other minerals. The chlorite stage of mineralization is widely distributed, but it does not form any large accumulations. It appears in the form of small chlorite veinlets cutting earlier mineral associations.

The second barren stage of mineralization is represented by sulfide-free milky-white quartz. It occurs in many quartz veins of the ore field.

The quartz-carbonate stage of mineralization is the final stage of the hydrothermal process. Its mineral association consists of translucent quartz, calcite, ankerite, epidote, and sometimes a small amount of sulfides: pyrite and sphalerite.

The data presented on the distribution of mineral associations show that within the entire ore field the same mineral associations are distributed. In its different parts the sequence of mineral formation remained the same, and the omission of individual stages of mineralization is not observed. It is also established that the composition of the principal ore-forming minerals as a whole remains one and the same.

The enclosing rocks near the quartz veins undergo hydrothermal alteration, expressed in their beresitization. The thickness of the zones of altered rocks ranges from a few millimeters

up to 4–5 m and depends on the thickness of the zones of crushed rocks. In general, in quartz veins of northeast strike it is higher than in veins of sublatitudinal strike.

Manifestations of Horizontal Zonation of Mineralization

The first manifestations of horizontal zonation within the Central ore field were established by D. A. Timofeevskii. He noted that it is expressed in the uneven distribution of certain minerals, in particular arsenopyrite and molybdenite. Thus, for example, enrichment of the quartz veins with arsenopyrite is clearly established in the northern part of the ore field (Oktyabr’skii and Dorozhnyi areas); in the central part of the ore field, although it is a widespread mineral, it nevertheless does not form large concentrations. In the southern part of the ore field arsenopyrite is unknown. Only on the upper levels of the Kavkazskaya vein were sporadic occurrences of it noted [1].

Molybdenite is distinguished by the opposite character of distribution. It has been encountered exclusively in the quartz veins of the Yubileinyi area. To the north, isolated occurrences of it were noted in the dumps of the Zakoldovannaya mine. In the central and northern parts of the ore field it is unknown.

These conclusions of D. A. Timofeevskii are fully confirmed by our investigations. At the same time, however, as a result of comparative mineralogical and geochemical analysis of quartz veins in different parts of the ore field, we were able to note a number of additional manifestations of horizontal zonation. In distinguishing zones over the area of the ore field, in addition to the mineral composition of the ores, we used the distribution of trace elements in beresites and in certain ore minerals (pyrite, arsenopyrite, sphalerite, galena), as well as paragenetic mineral associations of the ores, morphological features of native gold, and its paragenetic relations with other minerals.

Individual zones within the ore field extend in a sublatitudinal direction, and their sequence is as follows (from south to north): 1—barren, 2—lead-zinc, 3—polymetallic, 4—antimony. The zones were distinguished according to the trace elements most characteristic of a given zone. The disposition of the zones over the area of the ore field is shown in Fig. 1.

Below is a characterization of the individual zones.

Barren zone. It is located in the extreme southern part of the ore field. Within it are the Tumashevskaya, Khrebtovye, and a number of other veins. A characteristic feature of their mineralogical composition is the wide occurrence in them of black sulfide-free quartz of an early generation. Gold mineralization is also weakly expressed. Only rarely are small nests enriched in metal encountered in them. In such cases the gold is coarse. Paragenetically it is associated exclusively with quartz.

The southern boundary of this zone has not been established. It probably extends far to the south, since the gold-bearing quartz veins of the Varvarinskii and Burlevskii areas are distinguished by similar features [2].

Lead-zinc zone. It is situated immediately north of the preceding one. It includes the quartz veins of the Yubileinyi and Sibzolotovskii areas.

In general, the quartz veins of this zone are characterized by an extremely simple mineralogical composition of the ores. The principal ore minerals are pyrite, sphalerite, galena, and gold. Mineralogical study established the absence in the ores of minerals of se—

silver, bismuth, and other late sulfide mineralization, which is also confirmed by spectral analyses of the ores.

Spectral analyses of beresites and pyrite establish that the principal and most characteristic trace elements in this zone are lead and zinc, whose presence is due to mechanical admixture of galena and sphalerite. Elevated copper contents are sometimes established (the Altai vein), which is due to mechanical admixture of chalcopyrite. The remaining trace elements are either entirely absent or sporadically form extremely insignificant concentrations (up to 0.003%). Gold in the ores is usually coarse. It often forms visible segregations. Paragenetically it is closely associated with vein quartz and, in many areas, forms rich concentrations in sulfide-free quartz. On the other hand, enrichment in gold is also observed in parts of quartz veins with a moderate or high sulfide content. But in these cases as well, under the microscope one can see that the gold is paragenetically often associated not with sulfides, but with vein quartz. Thus, a rather weak paragenetic connection of gold with sulfides is established. This circumstance accounts for the weak gold content sometimes observed in this sector in parts of veins with a high content of late sulfides.

A characteristic feature of the quartz veins is also the distribution of gold within them. The analytical expression of the degree of unevenness of mineralization, as is known, is the coefficient of variation. For the quartz veins located within the area of the zone described, this value proves to be the highest and varies from 142 to 402%, averaging 253% over 20 calculations.

Polymetallic zone. It is situated north of the preceding zone and includes the quartz veins of the Central ore cluster, as well as the Erzerum vein, located somewhat north of the latter.

This zone is characterized by the most complex mineralogical composition. The main ore minerals here are pyrite, marcasite, pyrrhotite, chalcopyrite, arsenopyrite, sphalerite, and galena. In addition to them, microscopic studies establish a fairly wide distribution of bismuth, silver, arsenic-antimony, and gold-telluride mineralization. It is represented by native bismuth and silver, argentite, bismuthinite, emplectite, enargite, fahlores, calaverite, sylvanite, and other minerals.

Spectral analyses of beresites, pyrite, arsenopyrite, sphalerite, and galena confirm the data of the mineralogical study of the ores. They show that the quartz veins of the zone described are distinguished by an extremely broad assemblage of trace elements. Among them, the most characteristic are elements of the polymetallic assemblage: lead, zinc, copper, silver, bismuth, arsenic; nickel and antimony are established in small amounts, and sporadically molybdenum, etc.

Gold in the zone described is considerably finer than in the preceding zone. Visible gold occurs here only exceptionally rarely. The bulk of it forms segregations ranging in size from 0.001 mm to 0.1 mm. Paragenetically it is closely associated with sulfides: pyrite, sphalerite, galena, chalcopyrite, and others. Only rarely is an association of gold with vein quartz established, but even in these cases it is situated in the quartz near segregations of sulfides.

The distribution of gold in the veins is more uniform than in the preceding zone. The coefficient of variation ranges from 92% to 239%, averaging 160% over 7 calculations.

Antimony zone. It encompasses the extreme northern part of the ore field. It includes the quartz veins of the Oktyabrsky and Dorozhny areas: Veselaya, Tysyachnaya, Sibzolotovskaya 11, Almaznaya, Millionnaya, Dorozhnaya, and a number of others.

The ores of these veins are characterized by a somewhat simpler composition in comparison with the ores of the polymetallic zone. The principal ore minerals here are pyrite, arsenopyrite, sphalerite, galena, sometimes chalcopyrite, fahlores, etc. Especially characteristic is the high content of arsenopyrite in the veins, which in some areas constitutes the main ore mineral. A. Ya. Bulynnikov [1] noted a fully industrial content of arsenic in these veins.

The content of trace elements in the mineral formations of this zone is characterized first of all by the presence of components of the polymetallic complex (lead, zinc, copper). Along with them, an extremely characteristic impurity in beresites, pyrite, and arsenopyrite is antimony. Its highest concentrations are established in the ores of the Veselaya and Tysyachnaya veins. For example, in all analyzed samples of arsenopyrite from the Veselaya vein its content is about one percent. It should be noted that antimony minerals were not detected even at maximum magnifications in oil immersion.

Gold in the quartz veins of the antimony zone is very fine. The size of individual gold particles in all noted cases did not exceed 0.005 mm. Paragenetically, the gold is very closely associated with sulfides and was observed in mineralogical studies only in close paragenetic association with them, chiefly with arsenopyrite. In association with vein quartz, gold is not encountered here. The nature of the metal distribution in the quartz veins is even more uniform than in the preceding zone. The coefficient of variation for this zone varies from 121 to 162%, averaging, on the basis of 5 counts, 140%.

Thus, from the description of the manifestation of horizontal zonation one may conclude that, in moving from south to north, a regular change is observed in the mineralogical composition of the ores, the complex of trace elements in beresites and in the principal ore-forming minerals, as well as in the morphological features of gold segregations, its paragenetic relations with other minerals, and the character of its distribution in quartz veins.

Incidentally, we note that an attempt to establish, by analogous methods, manifestations of vertical zonation along individual quartz veins of the ore field proved unsuccessful. Therefore we shall not dwell on this further.

Conclusion

At present there is no generally accepted theory of horizontal and vertical zonation. Therefore different authors approach the explanation of this phenomenon in different ways. Ya. Kutina [7] distinguishes two types of zonation: 1—monoascendent and 2—polyascendent. To the first type he assigns zonation created by a continuous influx of solutions. The type of polyascendent zonation includes zonation arising as a result of intermittent influx of solutions. V. I. Smirnov [5] distinguishes six types of zonation: 1—zonation of repeated tectonic ruptures, 2—zonation of tectonic opening, 3—zonation of intra-ore metasomatism.

4—zonality of rock composition, 5—zonality of filtration, 6—zonality of deposition. The first three types of zonality correspond to the concept of polyascendent zonality, and the last three types to J. Kutina’s concept of monoascendent zonality.

A characteristic feature of the manifestation of zonality in the Central ore field is the uniform development of the stages of mineralization in different parts of the ore field, which determines the unity of composition of the principal mineral associations in different sectors of the ore field. Therefore, the first three types of zonality, according to V. I. Smirnov, caused by the successive development of intra-ore tectonics, are entirely unsuitable for explaining the zonality described. The explanation of the phenomenon described should apparently be sought in the zonality of deposition. G. Tischendorf [6] notes that the distribution of trace elements is an important criterion in favor of monoascendent zonality according to J. Kutina (zonality of deposition according to V. I. Smirnov). The explanation of this zonality is connected with a gradual change in physicochemical conditions in space (temperature, pressure, etc.). Their change could alter the rate and direction of chemical reactions, destroy chemical complexes, and this, in turn, could affect the degree of migration of individual metals.

Analysis of the given scheme of zonality in the ore field permits the assumption that the zonality described is a reflection of changes in the physicochemical conditions of ore deposition, caused by the different temperature regime that existed in different parts of the ore field. In this case the highest-temperature conditions apparently existed in the southern part of the ore field (the central part of the intrusive massif), while the stibnite zone, located in the extreme northern part of the ore field, reflects the lower-temperature conditions that existed in the northern part of the intrusive massif. Such a distribution of zones may serve as one of the arguments in favor of a genetic connection between the mineralization and the rocks of the Martaitga complex, to which the Central granodiorite massif also belongs.

REFERENCES

1. A. Ya. Bulynnikov. Arsenic deposits of Western Siberia. Vestn. ZSGRT, No. 1, 1934.

2. A. Ya. Bulynnikov. Gold-ore formations and gold-bearing provinces of the Altai-Sayan mountain system. Tomsk, 1948.

3. T. M. Dembo. Phenomena of anatexis, hybridism, and assimilation in the Caledonian granodiorite intrusion of the northern part of the Kuznetsk Alatau. Sov. geol., No. 51, 1956.

4. N. A. Rozanova. On the relationships of dikes of melanocratic rocks with gold mineralization at the Central deposit (Kuznetsk Alatau). Geol. rudn. m-nii, No. 2, 1959.

5. V. I. Smirnov. Types of hypogene zonality of hydrothermal ore bodies. In: Genetich. probl. rud. Reports of Soviet geologists at the XXI Session of the IGC, Gosgeoltekhizdat, 1960.

6. G. Tischendorf. The use of dispersed elements as a criterion for determining the zonality of endogenous postmagmatic formations. In: Problems of Postmagmatic Ore Formation, vol. 11, Prague, 1965.

7. J. Kutina. A contribution to the classification of zoning in ore veins. — Univ. Carol., Geol., v. 3, N 3, 1957.