Elements of the Geomorphology of the Nickel-Bearing Weathering Crust of Hyperbasites

A. G. Bakirov

(Presented by Professor A. M. Kuzmin)

In organizing and carrying out prospecting for silicate-nickel deposits associated with the “areal” weathering crust of ultrabasic rocks, geomorphological analysis of the area and determination of the hypsometric limits of mineralization are of great importance. Geologists do not always give this question the necessary attention, although they do note the confinement of the preserved ancient crust to watershed areas of the terrain. In this connection, we would like to share, in the present work, some experience in studying the geomorphological features of the weathering crust of ultrabasic rocks in the Kimpersai area.

The Kimpersai hyperbasite massif is located in Aktyubinsk Oblast. It is confined to the eastern part of the Or–Ilek watershed, which is a continuation of the Guberlin Mountains of the Southern Urals. The watershed is a hilly upland with an average absolute elevation of 420 m. The elevation of individual parts of the study area varies from a maximum of 507 m to a minimum of 310 m. The tributaries of the Or and Ilek rivers originate on the Or–Ilek watershed. The rivers of the western part of the area, such as the Kokpekty and Kuagach, flow into the Ilek; the rivers of the eastern and southern parts of the area—the Tassai, Dzharlybutak, Kyzylkain, Mamyt, and Kairakty—flow into the Or. The rivers of the study area in most cases flow across the strike of the structures, and more rarely along the strike. There is a tendency for some river valleys to be confined to tectonic faults of latitudinal strike (the Kyzylkain, Tagashasai, Mamyt, Shandasha rivers) and meridional strike (the Tassai and Dzharlybutak rivers). The drainage pattern is dendritic, and in places rectangular.

The hyperbasite massif is composed of apoperidotite and apodunite serpentinites. In their peripheral zone there are gabbroids, represented by various gabbros as well as gabbro-amphibolites. The gabbroids also form thick dike-like bodies, which occur among the ultrabasic rocks of the massif in its northern, southwestern, and southern parts.

The enclosing sequences of the hyperbasites are metamorphic rocks of Precambrian, Cambrian, Ordovician, Silurian, and Lower Devonian age. The cover deposits are represented by Upper Cretaceous and Paleocene rocks of marine facies, sandy-clayey deposits of the continental Jurassic, Lower Cretaceous, and Neogene.

Structurally, the area studied belongs to the southern termination of the Ural-Tau anticlinal uplift, complicated by smaller folds of the second order. It is precisely to this uplift that the Kimpersai hyperbasite-gabbroic magmatic complex is confined; it forms an intrusive body of Middle Devonian age. In it, the gabbroids occur above and below the hyperbasites, i.e., they are found on their hanging and footwall sides. In the hyperbasites, broad disjunctive structures of the fault and fault-thrust type are widely developed; as a result, the narrowest part of the massif from the Kyzyldzhar River northward proved to be broken into a number of tectonic blocks, which underwent some displacement relative to one another in post-Jurassic time.

In the area, a weathering crust is observed on ultrabasic and metamorphic rocks, as well as on gabbroids. In the southern part of the massif the crust occupies a considerable area. In the northern part it is smaller and occurs predominantly in the endocontact zone of the hyperbasites with the gabbroids. The weathering crust of the Kimpersai hyperbasite massif has a zonal structure, usual for crusts of the “areal” type with a nontronite profile. In its section, from top to bottom, ochres, nontronites, and leached serpentinites are distinguished. Much less commonly, ochreous and silicified profiles are encountered. According to the generally accepted view, the weathering crust of the Kimpersai massif formed at the end of the Triassic and the beginning of the Lower Jurassic (it is called pre-Jurassic). Subsequently, and especially in Tertiary time, the crust underwent a complex history of transformation and renewal.

In many cases the weathering crust is overlain by variegated deposits, consisting mainly of products of the reworking and redeposition of loose formations of the crust in Jurassic, Cretaceous, and Tertiary time. The variegated rocks include redeposited ochres, kaolin clays, bauxites, brown iron ores, variegated, limestone-bearing, and red-brown sandy clays.

As is known from the literature, within the territory of the massif there are silicate-nickel ore occurrences associated with the weathering crust of serpentinites. Most of them are confined to areas of the crust located in the contact zone of the hyperbasites with the gabbroids. Some of the ore occurrences are associated with the weathering crust of hyperbasites outside this contact zone. The nickel ore consists mainly of nontronites and leached serpentinites, more rarely of ochres and silicified serpentinites. The maximum mineralization is confined to the lower parts of the nontronitized serpentinites.

As early as during geological investigations in the Southern Urals, F. Yu. Levinson-Lessing [3] was the first to note that the modern relief of this area is only a reflection of its pre-Jurassic forms, buried beneath deposits of the Cretaceous and Tertiary periods. That is, the main elements of the modern relief give the impression of forms uncovered and prepared by erosion, rather than newly dissected forms. This idea of F. Yu. Levinson-Lessing on the inheritance in the modern macrorelief of the Southern Urals of large forms of the pre-Jurassic relief was later refined and developed in the work of A. V. Khabakov [5].

D. V. Ponomarev [4], characterizing the distribution of the weathering crust in the southern part of the Or–Ilek watershed, distinguishes an ancient planation surface, its elevated parts, and terrace-like slopes (scarps) of watersheds. Certain questions of the geomorphology of the crust were touched upon in his works by I. I. Ginzburg [1–2], who, in particular, paid attention to elucidating the role of the ancient microrelief in the formation of silicified serpentinites.

On the territory of the Kimpersai hyperbasite massif, three principal types of relief can be distinguished, which has been correspondingly reflected on the schematic geomorphological map compiled by the author of the article (Fig. 1). The types of relief differ both in the form of their manifestation and in origin. The first of them is a gently hilly surface of an ancient peneplain with rare elevations; the second is low-hill terrain; and the third is plains.

Fig. 1. Schematic geomorphological map of the area of the Kimpersai hyperbasite massif.
1—surface of the exhumed ancient peneplain with rare elevations; 2—dissected slopes of the peneplain; 3—gently hilly accumulative plains; 4—disjunctives in ultrabasic rocks; 5—contours of the hyperbasite massif and its satellites; 6—lines of watersheds; 7—silicate-nickel ore occurrences.
G—D—line of the geomorphological profile.

The surface of the ancient peneplain is of denudational origin and represents a largely exhumed and partly dissected surface of ancient, pre-Jurassic relief. To it is confined the crust ...

weathering crust, which is predominantly developed in the southern part of the massif. Remnants of the preserved weathering crust in the form of islets and belts are also present in the northern part of the massif. The average hypsometric elevations of the surface of the Dourian peneplain with the weathering crust are determined by figures of 450–460 m. Against the background of the ancient peneplain, elevations stand out whose heights are 470, 480, 490, and even 509.7 meters. Outcrops of silicified serpentinites are often confined to the elevations mentioned.

The low-hill terrain is of denudation–erosional origin and represents an area of slopes of the dissected Dourian peneplain. It is developed in the northern part of the massif, as well as in the southern part. The hypsometric elevations of the territory occupied by the low-hill terrain range from 350 to 430–440 m. In the northern part of the massif the heights of the low-hill terrain reach 470 m.

Plain relief is developed in individual parts of the massif. Plains are well expressed in the northeastern, eastern, and southwestern parts of the area shown on the map. The plain relief is of accumulative origin. It is due to processes of accumulation of lacustrine-river continental sediments that took place during Jurassic, Cretaceous, and Tertiary time. Beyond the boundaries of the massif and more rarely within its territory, especially in the southern and southeastern parts, the sediments belong to marine facies of the Upper Cretaceous and Paleogene. The hypsometric elevations of plain areas within the contour of distribution of hyperbasites and gabbroids are determined by figures of 420–425 and 440–450 m.

A considerable area is occupied by weathering crust in the southern part of the massif, where its tract, having an irregular form with branching tongues, is elongated mainly in a latitudinal direction. Its tongues have a meridional orientation. Such a plan-view form of the tract of preserved weathering crust in the southern part of the massif is conditioned by the position of the hydrographic network, which has adapted itself to the principal structural trends of the hyperbasite substratum. In the northern and southwestern parts of the massif, belts of weathering crust are confined to contact zones with gabbroids of its western and eastern peripheries, and also with gabbroids composing dike-like bodies.

Let us consider the geomorphological features of the nickel-bearing weathering crust of the area. First of all, let us turn attention to the forms of relief to which the areas of silicate nickel deposits associated with the weathering crust are confined. Such forms are, first, watersheds and plateau-like elevations, often representing plain, gently hilly tracts of terrain, and, second, the slopes of watershed and plateau-like elevations. Moreover, in the second case the most valuable concentrations of nickel are mainly confined to the upper parts of the slopes, to the brows, i.e., to the places of transition from flat and hilly elevations to the slopes proper. Consequently, the areas of distribution of silicate nickel deposits that have the greatest value will, in plan, be located either on the territory of the Dourian peneplain or on its periphery, bordering the latter. This constitutes the first geomorphological feature of the nickel-bearing weathering crust of the Kimpersai hyperbasite massif.

In the spatial arrangement of watershed lines and areas of silicate nickel deposits there is a definite relationship. The former pass through deposit areas or are located at some distance from the latter, as a rule no farther than 1.5 km (Fig. 1). That is, figuratively speaking, the watershed lines are, as it were, threads on which the silicate nickel deposits—

are strung, or to which they are attached by other short threads. This connection follows from the confinement of silicate nickel deposits to the surface of the Doyur peneplain and to the upper parts of its slopes. This is the second geomorphological feature of the nickel-bearing weathering crust of the region.

Since the weathering crust is confined to the Doyur peneplain, it is quite appropriate to pose and resolve the question of the hypsometric limits of silicate-nickel mineralization associated with the “areal” crust, i.e., of the level below which the most valuable ore occurrences will practically be absent in the region. In individual cases, in tectonically complicated areas, the base of the nickel-bearing weathering crust may descend below the hypsometric boundary of mineralization, as, for example, at the Novo-Batamshinsk deposit.

When considering the elevation marks of areas of silicate-nickel deposits in the region, it is possible to establish rather clearly four hypsometric levels¹ on which all the known ore occurrences of the region are located. The elevations of the first level average 420 m, with variations from 400 to 425 m; of the second, 440 m (430–445 m); of the third, 460 m (450–465 m); and of the fourth, 480 m (470–495 m).

Ore occurrences confined to the first level are: Vostochno-Kimpersaiskoe (42)², Novo-Batamshinskoe (40), Promezhutochnoe (39), Novo-Karagachtinskoe (38), Vostochno-Taikettenskoe (11), Taikettenskoe (12), Stepninskoe (18), Sarymurzinskoe (19), Staro-Shirpakainskoe (35), Mamytskoe (36), and Novo-Mamytskoe (37). Ore occurrences of the second level are: Aitpaiskoe (1), Oktyabrskoe (10), Taskudukskoe (27), Shandashinskoe (16), Shilektinskoe (14), Beskudukskoe (20), Shcherbakovskoe (21), Tagashasaiskoe (22), Kyzylkinskoe (24), Kokpektinskoe (25), Dzharlybutakskoe (29), Shuulkudukskoe (28), Shirpakainskoe (34), and Vostochno-Dzharlybutakskoe (33). Deposits of the third level are: Chugaevskoe (44), Kimpersaiskoe (43), Sheganshinskoe (7), Novo-Taikettenskoe (9), and Karaobinskoe (23). The following ore occurrences belong to the fourth level: Buranovskoe (49), Zavodskoe (2), Rozhdestvenskoe (3), Sazdinskoe (4), Novo-Sazdinskoe (5), Kamennyi Kobchik (6), and Blaktaisckoe (26). An intermediate hypsometric position is occupied by ore occurrences lying between the first and second levels: Batamshinskoe (41); between the second and third: Dzhusalinskoe (46), Zapadno-Kimpersaiskoe (8), Novo-Shandashinskoe (17); and, finally, between the third and fourth: Vostochno-Buranovskoe (48) and Zapadno-Dzhusalinskoe (45). The above-mentioned ore occurrences of intermediate position are confined to areas of the base of the weathering crust.

On the geomorphological profiles of the region, terrace-like areas in the relief stand out rather clearly; their surfaces will hereafter be called geomorphological levels. There are four such levels: the first, with an average elevation of 420 m; the second, 440 m; the third, 460 m; and the fourth, 480 m. The most clearly expressed on the profiles is the level with an elevation of 460 m.

The hypsometric levels of areas of silicate-nickel deposits are in some correspondence with the geomorphological levels expressed in the relief and closely connected with the areas of development of the weathering crust. Thus, the third hypsometric level corresponds to the geomorphological level of the surface of the exposed and partly dissected Doyur peneplain, well expressed in the southern part

of the massif and captured in the profile along line G–D (Fig. 2). The second hypsometric level is mainly the level of the upper parts of slopes, i.e., the edges of the donor peneplain, the places of transition to slopes. The greatest number of silicate-nickel deposits within the massif, and in its southern part almost all of them, are precisely confined to the second and third hypsometric levels. As one moves from the north of the massif southward to the Kyzyl-Kain River, the elevation marks of the deposit areas gradually decrease from 490–480 m at the Buranovskoe (49) deposit to 460–450 m at the Chugaevskoe (44), 425–430 m at the Batamshinskoe (41), and 410–420 m at the Novo-Karagatchinskoe (38) deposits.

Fig. 2. Profile along line G–D of the schematic geomorphological map of the Kempirsai hyperbasite massif.

We believe that the occurrence of silicate-nickel ore manifestations at the first hypsometric level is due, on the one hand, to denudation processes and, on the other hand, to tectonic processes. Deposits of this level are characterized by two features. The areas of their distribution bear the greatest traces of erosion. Moreover, among these deposits, more often than among others located at other levels, insignificant ore manifestations are encountered. In this connection we believe that the deposits confined to the first hypsometric level are in most cases associated with areas where the base of the weathering crust is distributed. In these areas, during the continental period of the Late Cretaceous and Tertiary time, the crust underwent erosion and then accumulative leveling through the accumulation of continental sediments of lacustrine-river facies, locally reaching a thickness of 40–60 m (the East Kempirsai (42) deposit).

We suppose that the territory of the massif where the East Kempirsai (42), Promezhutochnoe (39), Batamshinskoe (41), and Novo-Batamshinskoe (40) deposits are located—deposits confined mainly to the first hypsometric level—experienced, during the aforementioned continental period, some subsidence as a result of differential movements of tectonic blocks of hyperbasites. This is indicated, in particular, by the development of small grabens with preserved weathering crust in the areas of the listed deposits.

As indicated earlier, the northern part of the massif, as far south as the latitude of the Kyzylkain River, is divided into a number of tectonic blocks. At the same time a stepped lowering of the latter is observed in a southerly direction, with a total amplitude reaching 70 m. Consequently, there is a regular lowering of the hypsometric elevations of the deposit areas, beginning with the Buranovskoe (49) in the north and ending with the Novo-Karagatchinskoe (38) in the south. The deposits of the northern part of the massif—Rozhdestvenskoe (3), Buranovskoe (49), Novo-Sazdinskoe (5), and others—are confined to the most uplifted tectonic blocks of hyperbasites. This also explains their occurrence at the highest, fourth hypsometric level.

The hypsometric boundary of silicate–nickel mineralization essentially passes along the level of the base of the loose ocher–nontronite stratum, below which the ores, as a rule, are poor. In the ideal case, this boundary is determined by the hypsometric elevation of the deposit area, reduced by a value on the order of 10–20 m. The latter represents the average thickness—somewhat varying for different deposits—of the cover clays and ocher–nontronite products of the weathering crust. Thus, at the Novo-Taiketken (9) and Oktyabrsk (10) deposits the hypsometric boundary, as a rule, passes at an elevation of 430 m. The same may be said of the Chugaevsk (44) and Kimpersai (43) ore occurrences. For the belt of deposits on the northwestern periphery of the massif, including Zavodskoe (2) in the north and Kamennyi Kobchik (6) in the south, the hypsometric boundary is determined at 470 m. The Batamshin (41), East Kimpersai (42), Novo-Batamshin (40), and Promezhutochnoe (39) deposits have a boundary with an elevation of 400 m. The hypsometric boundary of mineralization is fairly clearly defined in the southern part of the massif, including the more northerly Shilektin (14) deposit. This boundary passes mainly along the 430 m contour. In the weathering crust of the southern part of the massif, ore occurrences encountered at elevations below 430 m are, as a rule, insignificant.

At the silicate–nickel deposits of the Kimpersai district, the hypsometric boundary of mineralization in most cases passes along the lowest elevations of their areas. This circumstance is due to the fact that the amplitude of fluctuations in relief elevations within the territory of the ore occurrences most often lies within the above-indicated total average thickness of the cover clays, ochers, and nontronites.

Such are the practical conclusions that follow from consideration of the entire complex of questions concerning the geomorphology of the nickel-bearing crust.

It seems to us that the search for silicate–nickel deposits of the Kimpersai type, or deposits close to them, should be accompanied by a geomorphological analysis of the area of distribution of the nickel-bearing weathering crust, taking tectonic factors into account. This will make it possible to outline the most promising areas and then to carry out prospecting and exploration work purposefully and with the least expenditure.

LITERATURE

1. Ginzburg I. I. et al. Ancient weathering crust on ultrabasic rocks of the Urals, Part 1. Types and morphology of the ancient weathering crust. Transactions of the Institute of Geological Sciences, Academy of Sciences of the USSR, issue 80 (1), 1946.

2. Ginzburg I. I., Savel’ev I. I. Formation of nickel deposits of silicate ores in the Southern Urals. Izvestiya of the Academy of Sciences of the USSR, Geological Series, no. 6, 1939.

3. Levinson-Lessing F. Yu. Geological investigations in the Guberlinsk Mountains. Proceedings of the St. Petersburg Mineralogical Society, ser. II, part 2, 1891.

4. Ponomarev D. V. On the distribution of the weathering crust in the southern part of the Orsk–Alekseevsk watershed. Izvestiya of the Kazakh Branch of the Academy of Sciences of the USSR, Geological Series, no. 8 (26), 1946.

5. Khabakov A. V. Pre-Jurassic relief and the ancient weathering crust in the southern part of the Urals. Proceedings of the State Geological Society, vol. 67, issue 2, 1935.

1. This is the level passing through the mean elevation of the surface of the ore-occurrence area. ↩

2. The numbers denote the numbers of deposits shown on the schematic geomorphological map. ↩