GEOMORPHOLOGICAL FEATURES OF THE DISTRIBUTION OF NICKEL-BEARING WEATHERING CRUSTS IN THE URALS

A. G. Bakirov

(Presented by Professor A. M. Kuzmin)

In the opinion of many geologists, shared by the author, the Triassic–Jurassic nickel-bearing weathering crusts of the hyperbasites of the Urals are confined to the pre-Jurassic denudation planation surface, which in post-Triassic time underwent greater or lesser transformation. In the present-day erosional section, only fragments of the aforementioned surface are exposed. During the Mesozoic and Cenozoic the nickel-bearing weathering crusts mentioned were repeatedly subjected to erosion and renewed, being brought into the zone of hypergene reworking. They also experienced periods of stabilization after being overlain by deposits of the Jurassic, Cretaceous, Paleogene, and partly Neogene. In different regions of the Urals these processes had their own specific features.

Mesozoic and Paleogene weathering crusts are most widespread within the development of the following genetic types of relief, territorially separated in certain geomorphological regions. These are the residual mountains of the eastern slope, the Trans-Ural peneplain, and the continental, marine, socle, and accumulative plains [4]. Areas of distribution of nickel-bearing weathering crusts of hyperbasites are most often characterized by undulating-hilly and plain relief, often complicated by separate elevations within which silicified serpentinites are developed. Modern mountainous and mountain-hilly relief is unfavorable for the preservation of ancient crusts [5].

According to I. I. Ginzburg, Jurassic and Lower Cretaceous depressions situated near massifs of ultrabasic rocks are a factor promoting the localization of nickel-bearing weathering crusts. V. A. Gutsaki [6], A. D. Naumov [10], and other geologists express approximately the same view. In this connection the so-called stabilizing role of Mesozoic, sometimes Paleogene and very rarely Neogene, depressions is often emphasized; these depressions favored the preservation of weathering crusts in areas adjacent to them and occupying a marginal position relative to the depressions.

The idea of the connection between exogenous deposits and depressions received its fullest and most complete expression in the works of A. P. Sigov [13, 14]. In his opinion, the localization in the Urals of Meso-Cenozoic deposits of sedimentary origin, as well as of weathering crusts, is controlled by erosion-structural depressions having mainly a meridional extent. These depressions were initiated as early as the Triassic and Jurassic and essentially represented valleys of Mesozoic

rivers. Most often, the places of their localization were predetermined by tectonics. They were subsequently developed in the process of erosional activity. The depressions mentioned continued to develop later as well, in particular in the Oligocene, as is evidenced by the confinement to them, in a number of areas, of the valleys of the Upper Oligocene river network. In the opinion of A. P. Sigov [14], ore occurrences associated with the weathering crusts of the Urals are precisely confined to the depressions mentioned and to their margins.

Let us recall that, beginning in the Oligocene–Miocene, the Urals began to experience meridional neotectonic uplifts, which proceeded most intensively in the western part of the Urals, where their amplitudes reached 500 m. These uplifts led to the formation of dissected relief that did not favor the preservation of products of rock weathering. Therefore it is no accident that nickel-bearing crusts are present in the central and eastern districts of the Urals, and among them in those areas where neotectonic uplifts were expressed relatively weakly. The amplitudes of the latter most often vary within the range of 150–200 m.

I. I. Ginzburg [5] relates the wide development of ochres in nickel-bearing areas of the Irgiz–Tobol watershed to the uplift of the latter in the Miocene or Pliocene. V. A. Gutskik [6] indicates that ochres are nickel-bearing in the case of their short-term formation in places of intensive neotectonic uplift. With prolonged formation of the weathering crust, however, the horizons of nontronites and nontronitized serpentinites prove to be the most nickel-bearing, while naturally alloyed iron ores form in place of the ochre horizon.

It is known that in the Neogene, in a number of areas of the Urals, swell-like transverse uplifts appeared, caused by Alpine tectogenesis. N. A. Preobrazhenskii [12] indicates that at least two phases of Tertiary tectogenesis (swell formation) are outlined in the Southern Urals—the Middle Miocene and the Upper Pliocene, separated by an epoch of significant epeirogenic subsidence of the region. M. V. Orlov established in the Southern Urals 12 zones of uplifts and subsidences, arranged transversely (in a latitudinal and sublatitudinal direction) relative to the mountain structure and cutting the Paleozoic structures (cited after the work of N. V. Bashenina [4]). N. A. Preobrazhenskii [12] distinguishes four transverse swells: the first of them is confined to the interfluve of the Ural and Sakmara rivers; the second, third, and fourth pass respectively through the latitudes of Meleuz, Sterlitamak, and Ufa. The most important and best known of them is the swell of the Obshchii Syrt. A number of large transverse swells in the Urals were distinguished by B. P. Krotov [8]. Nickel-bearing weathering crusts are most often confined to the junctions of the transverse uplifts and downwarps mentioned. Their tendency toward the flanks of the youngest structures of meridional orientation is also noted.

In different areas of the Urals, the exposed Mesozoic weathering crust of hyperbasites occurs at elevations from 160 to 600 m. Such a range of fluctuations in the hypsometric marks of the crust is largely due to the unequal uplifts of areas of the pre-Ochre peneplain, associated with block dislocations of the Paleozoic basement [14]. In most cases, areas of development of ore-bearing crusts have elevations within the range of 310–410 m.

In the geomorphological analysis of the localization of nickel-bearing weathering crusts, attention should be paid to determining the hypsometric boundary of mineralization. What is meant is identifying the level below which ore-bearing formations of the weathering crust of the mantle, mantle-fracture, and mantle-contact subtypes are absent. The author [3] carried out investigations to identify such a level in the Kempirsai district. Later, analogous attempts were made by other researchers.

The links between areas of development of nickel-bearing weathering crusts and certain forms of the modern relief, and the interrelations of the latter with the ancient relief of the epoch of formation of the weathering crust, deserve attention. These questions have important theoretical and practical significance.

As early as the time of geological investigations in the Southern Urals, F. Yu. Levinson-Lessing [9] was the first to note that the modern relief of this region is only a reflection of its pre-Ordovician forms, buried beneath deposits of the Cretaceous and Tertiary periods and subsequently exposed and exhumed by erosion. This idea of F. Yu. Levinson-Lessing concerning the inheritance, in the modern relief of the Southern Urals, of major forms of the pre-Ordovician relief was later refined and developed in the work of A. V. Khabakov [15].

In this connection, the confinement of nickel-bearing weathering crusts of hyperbasites to watershed spaces and to the upper parts of the slopes of watersheds of the modern relief is not accidental; within these areas, isolated portions of the exposed pre-Ordovician planation surface crop out. This feature in the localization of the above-mentioned crusts was noted in one form or another by B. P. Krotov [7], A. G. Bakirov [3], D. V. Ponomarev [11], A. D. Naumov [10], and some other geologists. At the same time, the above-listed investigators limited their conclusions only to the areas of the Middle or Southern Urals described by them.

In the author’s opinion, the watershed localization of most nickel-bearing areas of the weathering crust of hyperbasites relative to modern streams should be raised from the level of a particular feature of individual regions of the Urals to the rank of a regularity of all-Ural scale, manifested as a dominant tendency and explained by the textomorphic character of modern orographic elements and their inherited development. As an example, one may cite the weathering crusts of the Kempirsai, Buryktal, and Shevchenko districts, confined respectively to the watersheds of the Ori and Irgiz, Tobol and Irgiz, and Ural and Tobol rivers, which most often have a meridional trend. Watershed localization is also characteristic of other areas of development of nickel-bearing crusts.

The author [3], using the Kempirsai district as an example, showed the confinement of areas of nickel mineralization of the crust to a latitudinal zone about 3–4 km wide, embracing parts of watershed spaces. In this case, the aforementioned areas are located both on watershed lines and at a distance of 1–2 km from these lines, in places on slopes adjoining the watersheds.

Small streams of latitudinal or northeastern trend, sometimes occurring near the outcrops of nickel-bearing weathering crusts (the Buryktal, Kairakty, Akkermanovka, and other districts), somewhat complicate the form of manifestation of the noted regularity. Most often these streams are young, having arisen during a period of major redistribution of the hydrographic network in the Miocene. The latter was due to latitudinal trough formation associated with Alpine tectogenesis.

The above-mentioned regularity is also manifested in the localization of nickel-bearing karst aleurites. Thus, the Ufalei district of development of the latter is confined to the main watershed of the Urals. The Gulyin, Petrovsky, Polevskoy, and other areas tend toward the watersheds of modern streams. An exception is formed by the karst aleurites of the Aidyrlinsky district, localized in the valley of the Suunduk River.

In the author’s work [2], a definition of the subsurface microrelief of the weathering crust was formulated. It was defined as the surface separating

…a loose ocher–nontronite thickness of the crust from leached and silicified serpentinites. In the process of erosion, the ochers and nontronites are washed away. The silicified serpentinites are prepared out in the form of positive forms of the present-day microrelief. The latter adapts itself to the underground microrelief of the crust and inherits it.

In this connection, in the author’s opinion, the statements of some geologists that silicified serpentinites are supposedly confined to elevated areas of the relief, while ochers and nontronites are confined to lowered ones, sound somewhat unfortunate. Causes and effects should be interchanged, and one should say that elevated parts of the terrain are confined to outcrops of silicified serpentinites, while lowered ones are confined to sites of development of nontronites and ochers.

Small negative forms of microrelief may be seen in areas of development both of weathering crusts of hyperbasites and of karst aleurites. By general admission, the cause of the formation of such forms lies in the settling of loose formations of the crust and in subsidence phenomena accompanying the formation of karst cavities.

In the Cretaceous period, and especially in the Cenozoic—in the Oligocene, Miocene, and Pliocene—small rivers of the watersheds, temporary watercourses, and sometimes lakes and swamps were often confined to such negative forms of microrelief. As a consequence, there occurred the erosion of nickel-bearing crusts and karst aleurites, the formation of small depressions, and their subsequent filling with accumulations of deluvium, as well as with proluvial and alluvial deposits (valley alluvium)—the Kempirsai, Ufalei, and other districts. In this connection, the work of G. S. Anan'ev, E. G. Anan'eva, and others [1] deserves attention. In it, the authors describe the wide distribution of buried logs located within the Ural–Tobol interfluve, which represents a classic example of an ancient peneplain. These logs are filled with red-colored Neogene deposits.

Not only various red-colored deposits, clays, sands, and pebble beds, and sometimes lignite-bearing sediments overlying the nickel-bearing weathering crust, but also well-sorted quartz sands owe their formation to the river network of the Oligocene and Miocene watersheds and their lakes. The latter are confined to areas of development of some nickel-bearing weathering crusts of hyperbasites and karst aleurites (the Buruktal, Kempirsai, Rezh, and other districts).

Credit should be given to A. P. Sigov [14] for the relationship he revealed of various Mesozoic and Cenozoic sedimentary mineral deposits of continental origin (placers of valuable minerals, quartz sands, clays, bauxites, iron ores, and others) to erosion-structural depressions having a predominantly meridional orientation. Some areas of nickel mineralization of the karst type, observed in the Aidyrlya and places in the Lipovka and Rezh districts, are also associated with them. Formally, the position of many other nickel-bearing occurrences of the weathering crust of hyperbasites relative to erosion-structural depressions may be regarded as marginal, in connection with the location of the former on watersheds adjacent to the depressions.

In the author’s opinion, it is more appropriate to emphasize not so much the marginal position of nickel-bearing weathering crusts and karst aleurites in relation to the aforementioned ancient depressions as their watershed position relative to modern small rivers. This thesis acquires special importance when the question of the location of one or another erosion-structural depression and its relation to forms of the modern relief is of a debatable character.

At the same time, the above-mentioned depressions should be distinguished, as large tectonomorphic negative forms of the ancient relief—consisting mainly of Mesozoic river valleys—from small, ephemeral Oligocene, Miocene, and Pliocene river valleys and ravines confined to interfluve areas. The streams of the interfluves tended willingly toward negative forms of the microrelief of the weathering crust of hyperbasites and toward karstified areas of limestones and marbles in the zone of their contact with masses of ultrabasic rocks.

The small rivers of the interfluve areas were undoubtedly connected with erosional-structural depressions, which served as their base level of erosion. But the former and the latter are qualitatively different objects when one is speaking of their paleogeographic features and of the sites of hypergene nickel mineralization associated with them. All this is of great importance in forecasting and prospecting for the latter.

LITERATURE

1. G. S. Anan’ev, E. G. Anan’eva, V. A. Gorshkov, E. N. Stefanovich. Paleogeographic conditions of development of the ancient erosion network of the Ural–Tobol interfluve. Vestnik MGU. Geographical series, No. 6, 1964.

2. A. G. Bakirov. On the genesis of the microrelief of the weathering crust of ultrabasic rocks of the Kempirsai massif. Izv. TPI, vol. 74, issue 1, 1953.

3. A. G. Bakirov. Elements of geomorphology of nickel-bearing weathering crust. Izv. TPI, vol. 120, 1961.

4. P. V. Bashenina. Origin of the Relief of the Southern Urals. OGIZ-Geografgiz, 1948.

5. I. I. Ginzburg. Ancient weathering crust on ultrabasic rocks of the Urals. Part II. Geochemistry and geology of the ancient weathering crust in the Urals. Tr. Inst. geol. nauk AN SSSR, issue 81, 1942.

6. V. A. Gutsak. Weathering crust of the Orsk Trans-Urals. Uch. zap. Sarat. gosud. universiteta, issue 67, 1961.

7. B. P. Krotov. Geological grounds for initiating searches for deposits of the Anatol’sk type. Dokl. Akad. nauk SSSR, vol. 40, No. 6, 1943.

8. B. P. Krotov. Iron-nickel ores of the Urals. Tr. Inst. geol. nauk AN SSSR, issue 100, 1949.

9. F. Yu. Levinson-Lessing. Geological investigations in the Gubernlin Mountains. Notes of the St. Petersburg Mineralogical Society, ser. II, part 2, 1891.

10. A. D. Naumov. Development of the relief and paleogeographic conditions of formation of exogenous mineral deposits in the Orsk Trans-Urals. Questions of Geology of the Southern Urals and the Volga Region, issue 2, 1964.

11. D. V. Ponomarev. Weathering crust of the Shevchenko massif of ultrabasic rocks. In: Weathering Crust, issue 5, 1963.

12. N. A. Preobrazhenskii. On the history of epeirogenic oscillations of the Southern Urals and the Cis-Urals. In: Materials on the Geomorphology of the Urals, issue 1, Gosgeolizdat, 1948.

13. A. P. Sigov. On the age and origin of longitudinal depressions of the Urals. Geographical Collection I. Geomorphology and Paleogeography. Publishing House of the Academy of Sciences of the USSR, 1952.

14. A. P. Sigov. Questions of the metallogeny of the weathering crust of the Urals in geomorphological interpretation. In: Weathering Crust, Publishing House of the Academy of Sciences of the USSR, issue 5, 1962.

15. A. V. Khabakov. Jurassic relief and ancient weathering crust in the southern part of the Urals. Izv. Gosud. geolog. obshch-va, vol. 67, issue 2, 1935.