REGIONAL ORE-CONTROLLING STRUCTURES OF THE URALS

P. P. Zhelobov (SGI)

It is a well-known fact that magmatogenic deposits on the surface of the Earth’s crust are distributed unevenly, in groups, combining into ore fields, ore zones, etc. The very fact of the local distribution of magmatogenic mineralization makes it possible to raise the question of clarifying the causes of the concentration of deposits within limited areas.

The present article is devoted to characterizing certain regularities in the distribution of gold-bearing territories of the Urals within its regional structures.

The work was carried out by means of a comparative structural analysis of known gold-bearing areas of the Urals, based on materials from geological mapping.

The study of cartographic material in comparison with geophysical data and the distribution of magmatogenic deposits made it possible to reveal certain features of the structure of the Ural geosyncline that substantially influence the distribution of the gold-bearing territories of the Urals.

The work was conducted in close cooperation with the Berezovsky thematic party of the Ural Geological Administration, headed by A. I. Aleksandrov.

After all points of gold mineralization (deposits and ore occurrences) had been plotted on a clean sheet of paper, certain features of the spatial distribution of gold mineralization immediately drew attention. These features appear as definite natural regularities and amount to the following.

Deposits and ore occurrences are arranged in clusters, in groups, with an average distance between points of gold mineralization of 3–5 km. The area occupied by these clusters of gold mineralization usually ranges from 100 to 1000 km². The shape of the areas often proves to be elongated in the meridional direction, but not infrequently is also isometric.

The second feature of the distribution of gold mineralization is manifested in the fact that gold-bearing territories prove to be confined to single, usually submeridional, directions.

For example, such gold-bearing territories as the Susan ore field, the Rezhevsky and Shameisky ore fields, the Maminsky ore cluster, Sinar—

… ore field. This direction is measured by a distance of approximately 200 km.

Another direction is recorded by the distribution of the Ivdel ore zone, the Maslovka ore district, the Larka ore district, the Krasnoturinsk ore field, the Krasnouralsk ore zone, the Nevyansk ore district, and the Berezogorsk ore field. This fairly clearly expressed direction is measured by a distance on the order of 500 km.

Other, no less striking examples could also be cited of the confinement of ore-bearing territories to fairly clearly expressed structural lines.

The third feature of the spatial distribution of gold mineralization in the Urals consists in a tendency toward the latitudinal distribution of gold-ore mineralization.

It turns out that many of the most important gold-bearing territories, while spatially confined to regional zones of submeridional direction, lie within a single latitudinal belt that intersects the Ural structures across their strike.

Thus, for example, such gold-bearing territories as the Nevyansk ore district, the Kunara–Shaidurikhinskoye ore field, the Severo-Konevskoye ore field, and the Rezhevskoye ore field are confined to one latitudinal direction.

A similar example is provided by a latitudinal belt that includes: the Shabrovskoye ore field, the Sysertsky ore district, and the Maminskoye ore field.

Such well-known gold-bearing territories as the Miass–Melentyevskaya ore zone, the Nepryakhinskaya ore zone, the Narybakovskoye ore field, and the Chelyabinsk ore district are also confined to a single latitudinal direction.

It should be noted that the confinement of gold-bearing territories to latitudinal directions cutting the Ural structures is often accompanied by considerable dispersion of gold mineralization along meridional zones, which somewhat masks the very tendency toward the latitudinal distribution of gold mineralization.

The fourth feature of the distribution of gold mineralization that attracts attention is expressed in the confinement of the gold-bearing territories of the Urals to a characteristic bend of its folded structures, known in the literature as the Bashkir (or Ufa) salient.

A characteristic circumstance is that the frontal part of this salient is almost completely devoid of mineralization, whereas its flanks abound in gold-bearing territories.

The ore fields of the Sverdlovsk and Nevyansk districts are confined to the northern flank of the Bashkir salient, while the gold-bearing territories of the Miass and Chelyabinsk districts are spatially connected with its southern flank.

Analysis of the spatial distribution of the gold-bearing territories of the Urals already in itself makes it possible to draw important conclusions of a structural-genetic nature.

The clustered character of the distribution of deposits testifies to the presence—if one may put it this way—of certain centers of mineralization.

P. F. Ivankin [6] was the first to draw attention to the clustered distribution of mineralization; he introduced into the doctrine of ore mineral resources the concept of the “ore bundle.” Using the example of many deposits and ore fields, he demonstrated the plunging of ore columns, ore bodies, and dikes toward definite centers (“roots” of the ore field),

located at depths of several hundred meters from the ground surface.

The localization of endogenous mineralization in limited areas, separated from one another by considerable distances, is connected with specific features of the geological structure of the given segment of the Earth’s crust. However, these features may be manifested in the structure of the lower structural levels and may not always be captured on geological maps, which reflect the geology of the surface of the present-day erosion level, as will be discussed below.

The second characteristic feature of the distribution of the gold-bearing territories of the Urals, manifested in their confinement to single submeridional trends, testifies to the ore-controlling influence of linear structures of regional scale. The great length of ore belts, their frequent confinement to the junctions of major structural-facies zones of the Urals, and their saturation with linearly elongated bodies of ultrabasites—all this indicates the deep-seated emplacement of the structures controlling the distribution of gold mineralization in the Urals. An example is the confinement of the ore belt that includes the Susan, Rezh, Shamei ore fields, the Mamin ore cluster, and the Sinar ore field to the zone of junction of the East Ural uplift with the depression (downwarp) of the same name.

The ore belt indicated by the position of the Ivdel ore zone, the Maslovka and Larya ore clusters, the Krasnoturinsk ore field, the Krasnouralsk ore zone, the Nevyansk ore cluster, and the Berezogorsk ore field is confined to the junction of the Tagil–Magnitogorsk downwarp with the East Ural uplift. A. V. Peive [11] at one time distinguished this zone as the Trans-Ural deep fault. It is now called the Serov–Nevyansk belt of ultrabasite development [9].

An equally pronounced confinement of gold-bearing territories to the junction of the principal structural-facies zones is observed in the example of the Southern Urals, where a number of large gold-bearing territories prove to be spatially connected with the zone of junction of the Magnitogorsk megasynclinorium with the Uraltau megaanticlinorium (the Miass–Melentyevka, Atlyan, Krasnokhtin, Mindyak, and Yuluk ore zones). This zone has received the name of the Miass–Sakmara deep fault [12].

Analysis of the features of the geological structure of the ore belts of the Urals shows their closest connection with belts of ultrabasites. It turns out that gold-bearing territories display a clearer spatial connection with belts of ultrabasic and basic intrusions than with areas of granitoid development. This, at first glance, paradoxical conclusion merely indicates that both the “ore-bearing” intrusions and the ore-bearing solutions paragenetically associated with them rose along the same channels, which were mobile zones of the Earth’s crust.

Diagnosis of the zones of deep faults is often very difficult. Usually three main types of deep-fault zones are distinguished according to the clarity of their manifestation: open, concealed, and buried fault zones.

Open zones of deep faults are well recorded by belts of elongated ultrabasite bodies, more rarely by gabbroic intrusions, and by confinement to the junctions of complexes of different ages. A more difficult task is the identification of concealed zones of deep faults. The methodology for identifying these zones cannot yet be considered developed, but much has already been done in this direction (see the collection Concealed Deep

faults”). In the Urals, concealed zones are usually located along the continuation of exposed zones of deep faults. When comparing geological maps with geophysical materials, one often has to deal with the fact that linear geophysical anomalies are situated along the continuation of the ultrabasic-rock belt. This gives grounds for assuming that the anomalies represent intrusions of ultrabasic rocks that do not reach the surface. In this case the anomaly indicates the position of a concealed deep fault. One often encounters the circumstance that not only chain-like bodies of ultrabasic rocks and linear geophysical anomalies, but also zones of schistosity, hydrothermal alteration of rocks, as well as occurrences of ore manifestations and magmatogenic deposits, fit into a single linear zone. All these features often mark regional mobile zones so clearly that there is no reason to doubt their existence. In a number of cases, linearly elongated granitoid bodies, as well as small intrusions of various composition and age, prove to be associated with these zones.

On this basis we attempted to identify the principal mobile zones of the Urals by analyzing small-scale geological maps and comparing them with geophysical materials and data on the distribution of magmatogenic deposits. The results proved quite unexpected: definite systems of zones of deep faults were revealed (Fig. 1). The most characteristic feature was the parallelism in the strike of the faults*). Thus, for the Northern and Middle Urals a system of meridional faults is characteristic. In the Southern Urals, on its eastern slope, the most highly developed system proved to be faults striking northeast 10°. For the Riphean strata of the Southern Urals, the principal direction of the faults is northeast 30°. The system of the Miass–Kempirsai deep faults has a more complex structure (Fig. 2). Unfortunately, in a short article it is impossible to give a characterization of all those features that served as the basis for distinguishing each zone of deep faulting.

The mobile zones outlined according to the above-named criteria not only show a spatial relationship with magmatogenic deposits, but also shed light on certain features of the deep structure of the Urals. In particular, attention is drawn to the fact that the system of mobile zones of the Bulanash–Bredy trend (northeast 10°) occupies a cross-cutting position relative to the strike of the main structural-facies zones of the Urals. It is also interesting to note the fact that regional mobile zones pass through large granite intrusions without disturbing their mode of occurrence. The latter, as it were, “trace” these zones in separate parts of them in the upper horizons of the earth’s crust.

The third feature of the spatial distribution of the gold-bearing territories of the Urals, manifested in the tendency toward a broad distribution of mineralization, also raises a number of important questions concerning the deep structure of the Urals.

As early as the end of the 1930s, geophysical surveys by D. S. Mikov in the Middle Urals revealed anomalies cutting across the submeridional Uralian structures. It proved impossible to explain the latitudinal strike of these anomalies by features of the structure of the upper structural level of the Urals, exposed on the present-day erosional—

*) It should be said that the tendency toward parallel occurrence of the zones of deep faults of the Urals is considerably masked by the development of various kinds of deep cross faults and feathering structures healed by intrusive bodies.

Fig. 1. Principal systems of mobile zones of the Urals: 1) system of meridional faults, 2) system of faults of the Bulanash–Bredy trend (NE 10°), 3) Miass–Sakmara group of faults, 4) system of faults of the Kyshtym trend (NE 30°), 5) faults in the pre-Riphean basement.

…section, did not seem possible. This gave A. A. Malakhov [7] grounds to express the idea of the development of latitudinal structures buried beneath the Ural geosyncline. Many works were devoted to the further development of these ideas [8, 14, 2, 1, 10].

The conduct of regional geophysical investigations by the thematic party of the Ural Geological Administration (E. M. Anan’eva and others), by staff members of the geophysical faculty of the Sverdlovsk Mining Institute (A. Ya. Yarosh, B. V. Dorofeev), and by the Institute of Geophysics of the Ural Branch of the Academy of Sciences of the USSR (N. I. Khalevin) made it possible to reveal, in the crystalline basement, large structures (the Uvat–Tavda and Ufa structures) that intersect the Ural folded system in a latitudinal direction and outline the position of large blocks with increased rock density. A number of linear anomalies were also detected, revealing individual fault zones.

Fig. 2. Latitudinal belts of intrusions and points of gold-ore mineralization associated with the southern contact of the Uvat–Tavda buried structure: 1) intrusions showing a tendency toward isometric forms, 2) faults established by geological mapping, 3) position of the gravitational step, 4) areas with increased values of gravity, 5) areas with decreased values of gravity, 6) points of gold-ore mineralization.

The establishment of geophysical anomalies cutting the Ural folded structures confronted Ural geologists with the problem of finding some features of the geological structure of the upper structural level in the areas of these anomalies. Consideration of cartographic material from these positions made it possible to reveal, within the Nevyansk–Rezhevsk district, two belts of intrusive bodies characterized by small dimensions and an isometric plan form [8]. The confinement of these intrusions to the southern contact of the Uvat–Tavda structure cannot be regarded as accidental. Gold mineralization also shows a spatial connection with the latitudinal belts of intrusions (Fig. 2).

It is also interesting to note the fact that the boundaries of the Uvat–Tavda and Ufa buried structures coincide with sectors of plunging hinges of large anticlinoria and the development of saddle-shaped synclines. Buried fault zones of the basement of the Urals are recorded

and a number of other features, the analysis of which is the subject of the work of A. G. Bakirov [3].

Gold mineralization also shows a fairly clear connection with the buried structures of the crystalline basement of the Urals. Thus, the gold-bearing territories of the Nevyansk–Rezhevsk district show a spatial connection with the southern boundary of the Uvat–Tavda structure. Spatially connected with the northern boundary of the Ufa buried structure are the Shabry ore field, the Sysert ore cluster, and the Mariinsk ore cluster. The Miass–Melentyevsk ore zone, the Nepryakhino ore field, the Narybakovo ore field, and the Chelyabinsk ore cluster coincide spatially with the southern contact of this structure [5]. At the same time, the gold-bearing territories are somewhat elongated in the meridional direction, which is explained by the controlling influence of Paleozoic zones of deep faults (Fig. 3). Consequently, the areas where Paleozoic deep faults intersect with Riphean buried structures act as an important structural factor favorable for the localization of ore-bearing territories of the Urals. In this we see the reason for the cluster-like distribution of gold mineralization in the Urals, the reason why gold-bearing territories are situated locally, and not along the entire length of the zone of Paleozoic deep faults.

Fig. 3. Character of the influence of buried basement structures on the distribution of certain gold-bearing territories of the Urals: 1) areas with elevated gravity values: 1. Uvat–Tavda buried structure; 2. Ufa buried structure; 2) areas with reduced gravity values; 3) zones of increased tectonic activity; 4) gold-bearing territories.

The ore-controlling influence of buried basement structures makes it possible to proceed to solving the inverse problem: identifying ore-controlling buried structures by the confinement of mineralization to particular directions that cut the Ural submeridional structures. If these directions are confirmed by indirect geological features or by the trend of geophysical anomalies, then the existence of buried fault zones becomes entirely probable. It is characteristic that, among the buried zones, parallelism in their trend is also revealed, allowing three main systems of buried basement faults to be distinguished rather clearly: latitudinal, northwestern, and northeastern faults.

On the whole, a fairly dense network of mobile zones is obtained, the intersection nodes of which act as structures favorable for the localization of ore-bearing territories of the Urals. In areas of intersections and junctions of mobile zones that lack commercial ore bodies, if direct signs of mineralization are present, prospecting work for ore gold should be undertaken.

Fig. 4. Principal types of regional structures favorable for the localization of gold-bearing territories of the Urals. I — areas of intersection of meridional and submeridional Paleozoic deep faults with buried latitudinal faults of the Doryphæan crystalline basement. This position is occupied by the gold-bearing territories indicated in the list below by the numbers: 3, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 24, 25, 26, 27, 29, 32, 33, 36, 41, 42, 43, 45, 46, 47, 48, 52. II — sectors of convergence of parallel-striking deep faults (20, 23, 25, 34, 36, 44, 50, 52). III — areas of intersection of faults of submeridional (Bulanash–Bredy) trend with zones of buried basement faults of north-eastern strike (32, 35, 36, 37, 38, 39, 40). IV — areas of intersection of submeridional deep faults with buried basement faults of north-western strike (34, 35, 45, 46, 53). V — areas of junction of submeridional zones of deep faults with faults of the Miass–Sakmara group (29, 33, 34). VI — sectors situated above buried latitudinal basement faults, irrespective of their intersection by deep faults of another trend (1, 2, 4, 10, 12). VII — sectors situated along the continuation of open zones of deep faults (9, 11, 39, 42). VIII — sectors situated above the area of intersection of buried basement faults of latitudinal and north-western trends (41, 44, 45, 46, 48). IX — sectors situated within open zones of deep faults (22, 35, 54, etc.). X — areas of junction of meridional zones of deep faults with renewed faults of the Kusa (N E 30°) trend (6, 7, 8). XI — sectors of convergence of latitudinal faults of the buried basement (30, 31, 32, etc.). XII — sectors of intersection of deep faults between two parallel zones with a third zone of deep fault (20, 23). XIII — sectors of intersection of meridional and submeridional deep faults (28, 36). XIV — projections of intrusive bodies (or small intrusions) associated with zones of open deep faults (21). XV — projections of intrusive bodies (or small intrusions) associated with buried basement faults (51). XVI — sectors situated above the area of intersection of buried basement faults of north-western and north-eastern trends (35, 41). XVII — areas of intersection of Paleozoic meridional and submeridional zones of deep faults with the boundaries of buried block structures in the Doryphæan crystalline basement, distinguished in the gravity and magnetic fields (15, 16, 17, 18, 19, 24, 25, 26, 27, 29, 30, 32): 1) open zones of deep faults, 2) buried faults of the Doryphæan basement, 3) projections of large granite massifs, 4) areas with increased values of gravity (buried basement blocks), 5) areas with decreased values of gravity, 6) gold-bearing territories.

List of gold-bearing territories

1. Chuval ore field. 2. Martai ore zone. 3. Ivdel ore zone. 4. Kutim ore district. 5. Olkhovka ore zone. 6. Myaslovka ore district. 7. Laryovka ore district. 8. Krasnoturinsk ore district. 9. Mys ore district. 10. Kvarkeno ore zone. 11. Krasnouralsk ore zone. 12. Aikinskaya ore zone. 13. Kharitonovka ore field. 14. Saldinskoe ore field. 15. “Dolgii Mys” ore field. 16. Susan ore field. 17. Nevyansk ore district. 18. Ayat ore district. 19. Rezhevsk ore field. 20. Pervomaisko-Zverevsk ore field. 21. Shameika ore field. 22. Berezovsk ore field. 23. Berezovsk ore district. 25. Shabrovsk ore field. 26. Sysertsk ore district. 27. Maminsk ore district. 28. Sinar ore field. 29. Miass–Melentyevka ore zone. 30. Nepryakhinskaya ore zone. 31. Narybkovo ore field. 32. Chelyabinsk ore district. 33. Atlyanskaya ore zone. 34. Krasokhtinskaya ore zone. 35. Ui ore district. 36. Kochkar ore district. 38. Mindyak ore zone. 40. Avzyanskoe ore field. 42. Balkan ore field. 43. Gumbeika ore field. 44. Ulutau ore zone. 45. Tukan ore zone. 46. Bakr-Uzyak ore field. 47. Bakhtiyar ore field. 48. Mogutov ore field. 49. Sibai ore district. 50. Baimak ore district. 51. Aidyrlinsk ore field. 52. Bredy ore zone. 53. Dzhetygara ore district. 54. East Dzhetygara ore field.

Figure 4 shows the characteristic structural types of combinations of mobile zones and buried structures favorable for the location of gold-bearing territories of the Urals.

It should be said that a similar attempt to identify ore-controlling mobile zones was made by M. P. Russakov and later by G. P. Volarovich for Southern Primorye. As E. A. Radkevich [13] indicates, the idea advanced by these researchers—that mineralization is confined to the nodes of a peculiar network of mobile zones—was confirmed over the following twenty-five years by the discovery of new ore-bearing areas.

The delineation of mobile zones in the Urals is for the most part based on the use of indirect geological criteria. This circumstance inspires a certain distrust of the very methodology for distinguishing mobile zones. However, the results obtained are of undoubted interest and require verification by geological practice.

The question of the mechanism of formation of the mobile zones of the Urals, especially those zones that occupy a secant position relative to the general strike of folded structures, still remains completely undeveloped. One can only suppose an ore-controlling influence of the “principal divisibility” [4] of the Earth’s crust and of regional fracturing, the origin of which may be connected with horizontal movements of matter in waveguides.

The fourth feature of the distribution of gold mineralization in the Urals, manifested in a high concentration of gold-bearing territories on the flanks of the Bashkirian salient, finds its explanation in the very deep fragmentation of the Earth’s crust in these areas. This fragmentation is recorded by the development here of a dense network of intrusive bodies of ultrabasites and gabbroids. In the light of modern geophysical data, Academician A. P. Karpinsky’s hypothesis of the Ufa underground block, which deformed the Urals in its eastward movement, receives confirmation. On magnetometric maps it is possible to identify large basement blocks with a characteristic eastward displacement along latitudinal faults (oral communication by A. Ya. Yarosha). It should be noted that the frontal part of the Bashkirian salient of the Urals is composed of the Ufaley and Sysert metamorphic complexes, which in geophysical characteristics differ sharply from other gneissic fields of the Ural folded zone [1]. As was said above, the frontal part of the salient, unlike its flanks, is devoid of gold mineralization.

We attempted to reflect the quantitative aspect of the influence of tectonic structures on the localization of gold mineralization. From this standpoint, 54 gold-bearing territories were considered. The main ore-controlling role belongs to large rupture disturbances of the Earth’s crust. The most frequent confinement of gold-bearing areas is observed in zones of fault junctions (53.7%). Submeridional zones of first-order deep faults are of substantial importance in the distribution of gold mineralization (42.6%). Forty-eight percent of the gold-bearing areas of the Urals are spatially associated with buried faults of sublatitudinal trend. Gold mineralization displays a close spatial connection with small intrusions (53.7%). In analyzing the character of the spatial connection of gold mineralization with folded structures, it is not difficult to see a tendency of gold-bearing areas toward the limbs of anticlinoria and synclinoria, whose position often corresponds to zones of exposed or concealed deep faults.

Analyzing the character of the location of gold-bearing territories in the regional structures of the Urals, revealing the ore-controlling influence of buried basement structures and exposed and concealed zones of deep-

... faults of great extent, we inevitably arrive at the principal conclusion that it is impossible to understand the regularities in the distribution of gold-bearing areas without studying the features of the geological structure of the lower horizons of the Earth’s crust.

REFERENCES

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