TECTONIC REGULARITIES IN THE LOCALIZATION OF SILICATE-NICKEL DEPOSITS OF THE KEMPIRSAI DISTRICT

A. G. Bakirov

(Presented by the Department of Mineralogy and Crystallography)

The structure of the parent substratum is of great importance in the formation of deposits associated with the so-called “linear” and contact-karst weathering crusts of ultrabasic rocks. Attention has been drawn to this circumstance in the works of I. I. Ginzburg [5], I. Z. Korin [7], D. G. Ulyanov, G. S. Grishchenko [8], A. A. Glazkovsky [6], and other geologists. In the study, however, of mineral resources confined to the so-called “areal” crust, the role of structures in the localization of mineralization is somewhat underestimated. In this connection, using as an example the deposits of the weathering crust of ultrabasic rocks of the Kempirsai district, we would like to show the major role of tectonic factors and, in particular, of disjunctive structures in the formation, preservation, and spatial distribution of areas of “areal” crust with silicate-nickel mineralization.

The Kempirsai hyperbasite massif is located in Aktobe Oblast. It is elongated in a north-northwesterly direction. Spatially, the massif is confined to the eastern part of the Or–Ilek watershed, which is a continuation of the mountains of the Southern Urals and represents a protrusion of the ancient metamorphic sequence of the Ural-Tau. Orographically, the Or–Ilek watershed is a hilly upland with an average absolute elevation of 420 m.

The geological structure of the district includes ultrabasic rocks, gabbroids, metamorphic formations of the Precambrian, Cambrian, and Ordovician, and then shales and effusives of the Silurian and Devonian (Fig. 1)¹. In places within the district, a pre-Jurassic weathering crust has been preserved. 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.

In the area of investigation, rocks of the hyperbasite-gabbro complex are widely developed. The ultrabasic rocks of the massif are represented by peridotite and dunite serpentinites. In the hyperbasites there occur numerous vein formations, of which the most

¹ The geological-structure scheme shown in the figure was compiled by the author of the article from personal survey data, using materials of Yu. P. Kurazova, N. P. Kheraskov, Kh. S. Rozman, R. S. Segedin, N. I. Leonenok, and the Kempirsai geological-prospecting party.

Fig. 1. Schematic geological structure of the area of the Kempirsai hyperbasite massif: 1—metamorphic sequence enclosing the hyperbasites; 2—ultrabasic rocks; 3—gabbroids; 4—gabbro-diabase dikes; 5—weathering crust of ultrabasic rocks; 6—Meso-Cenozoic sedimentary rocks; 7—disjunctive faults; 8—axis of the Kuagach anticline. The numbers indicate silicate-nickel ore occurrences.

The most extensively developed are the gabbro-diabases. The gabbroids situated in the peripheral zone of the hyperbasites, as well as those forming large bodies within the massif of the latter, include a rather diverse complex of rocks, represented in most cases by hornblende and actinolitized gabbros, as well as gabbro-amphibolites. We regard all gabbroids and vein formations spatially associated with the ultrabasic rocks of the Kempirsai massif as a single complex with the hyperbasites, formed during the Middle Devonian volcanic cycle.

Structurally, the area of the Kempirsai hyperbasite massif belongs to the southern end of the Ural-Tau anticlinorium. The anticlinorium is composed mainly of Proterozoic, Lower Paleozoic, Silurian, and Devonian rocks. Its axis passes through the middle of the district. The anticlinal uplift of the Ural-Tau zone is complicated by smaller second-order folds. These include, in particular, the Kuagach anticline.

The Kempirsai hyperbasite-gabbro intrusion is confined to the axial part of the Ural-Tau anticlinorium. In plan, the massif as a whole is elongated in a northwestern direction. Two parts are clearly distinguished within it: the larger, northern, most elongated and extended part, and the smaller southern part, broadened and, in section, having a form approaching isometric. The boundary between the two parts is marked by a disjunctive, to which the Shandasha and Tagashasai rivers are confined. The western outline of the massif is somewhat curved, which may be explained by the bending of the axis of the Kuagach anticline in the Taiketken area. A small Taiketken satellite of the Kempirsai massif, isometric in plan, is confined to the place of the bend. It should be noted that east of the massif there is another small satellite, the Mamyt satellite, situated precisely on the line of latitudinal strike with the Taiketken one.

The dip angle of the planar banding of the hyperbasites and gabbroids in the western parts of the massif is 50–60°, and in the eastern, central, and northern parts of the massif it is 25–30°. In both cases the dip is westward. The gabbroids occur on the hanging and footwall sides of the hyperbasites, i.e., they underlie the latter and also occur in their roof. In the ultrabasic rocks themselves, large folded structures are observed. They are marked by planar banding and chromite bodies, which in most cases have western and eastern dips in different parts of the massif, with an almost meridional strike. In the cores of the synclines of these structures, gabbroids localized in the roof of the hyperbasites occur. Smaller folded forms can also be observed in outcrops of ultrabasic rocks, as well as of gabbroids. Their amplitude is measured in centimeters, meters, and tens of meters. The dip angle of the limbs of folds in the hyperbasites in most cases varies within the range 25–60°. Taken together, all this gives us grounds to suppose that the hyperbasites and gabbros form a folded intrusive deposit.

In addition to plicative structures, disjunctive structures are also expressed in the district. The most significant of them is the meridional Borodinovka fault, which can be traced in the eastern part of the district. Less clearly expressed is the Shandasha-Tagashasai disjunctive of the fault type. In the metamorphic sequence of the extreme northern part of the district, a meridionally elongated fault is expressed, extending far beyond the limits of the district. The listed disjunctives are of pre-Lower Devonian age. The Borodinovka fault was repeatedly renewed later as well, in particular in post-Paleogene time.

Disjunctive dislocations are also widely manifested within the massif of ultrabasic rocks. For example, latitudinal normal-fault and normal-strike-slip disturbances break the massif into six tectonic blocks (Fig. 1). The latter are marked by bends of the eastern contour of the hyperbasites, by the absence of gabbro in a number of peripheral parts of the massif, and by latitudinal disjunctives emphasized by dikes of gabbro-diabases. The length of the latter reaches 3 km, and their thickness is 15–20 m.

The blocks of the massif themselves are not monolithic. They, in turn, are cut by disjunctives of latitudinal strike—normal faults and normal-strike-slip faults—into smaller tectonic blocks. By blocks we mean elongated areas of hyperbasites bounded on both sides by tectonic disturbances of normal-strike-slip character, to which dikes of gabbro-diabases are confined. The width of the blocks mentioned ranges from 100 to 300 m. In the erosion section the latter have the form of bands of varying width. Such dissection is fairly well marked in the northern part of the third block¹ and in the central—

Fig. 2. Diagram of the distribution of tectonically weakened zones and ore fields in the Kempirsai hyperbasite massif: 1 — contours of hyperbasites; 2 — ore fields; 3 — tectonically weakened zones: I — Shaganshko-Dzhusadinskaya; II — Taiketkeno-Batamshinskaya; III — Shilektino-Kyzylkainskaya.

... part of the fourth. The fractures of the massif are most likely a reflection of block movements of its basement.

In the tectonic blocks of the massif, weakened zones of latitudinal extent are expressed rather clearly. They are the most fragmented parts of the blocks, characterized by the broad development within them of gabbro-diabase dikes confined to the numerous latitudinal faults of the intrusion. There are three such tectonically weakened zones in the massif (Fig. 2). The first of them—the Sheganinsko-Dzhusalinskaya zone—2.5 km wide, is located in the northern part of the massif; the second—the Tayketkeno-Batamshinskaya zone—9.5 km wide—is in the central part; and the third—the Shilektino-Kyzylkainskaya zone, 10 km wide—is in the southern part of the massif. Gabbro-diabase dikes also occur in the intervals between the zones mentioned, but there they are rarer. The Tayketkeno-Batamshinskaya tectonically weakened zone is precisely confined to the bend of the axis of the Kuagach anticline. In the hyperbasites there are upthrusts and small thrust faults of NW and NE strike, which impart a scaly structure to individual parts of the massif, especially those situated in its southern part.

Latitudinal normal faults and normal-strike-slip faults, in our opinion, are Middle Devonian. This is indicated by the gabbro-diabase dikes confined to the aforementioned disturbances. The dikes are genetically related to the Middle Devonian hyperbasites. An Upper Paleozoic age may be assumed for the upthrusts and thrust faults, which most likely arose during the Variscan cycle of tectogenesis, when significant crumpling of the hyperbasite-gabbro intrusive body took place. This is confirmed in other nearby areas. The latitudinal structures were renewed later as well, in particular in Tertiary time, since the disturbances cut the pre-Jurassic weathering crust, as well as Upper Cretaceous and Paleocene deposits.

Among the ultramafic rocks of the massif, massive bodies of banded and gneiss-like gabbroids are observed, represented mainly by gabbro-amphibolites, hornblende and actinolitized gabbros. In erosional section these bodies have the form of discontinuous belts elongated in the meridional direction or close to it. Four belts of gabbroids are manifested rather distinctly: Buranovskaya, Shcherbakovskaya, Dzharlybutakskaya, and Kyzylkainskaya [4]. The lengths of the belts range from 3 to 15 km. The width of the erosional section of the gabbroid bodies within the belts is not constant and most often varies from 50 to 350 m, rarely reaching 2 km (the Shcherbakovskaya belt). In many cases they are broken into tectonic blocks, somewhat displaced relative to one another along disjunctives that in most cases have a latitudinal strike. Such a block structure is especially clearly manifested in the Buranovskaya belt. We regard these gabbroid bodies as peculiar remnants of deep synclinal troughs of the gabbro roof, complicated by disjunctives.

In the district, within the area occupied by hyperbasites, there are sectors of “blanket” weathering crust to which silicate-nickel concentrations are confined. The most significant of them are located in the contact zone of hyperbasites with gabbroids. The structure of the parent substrate of ultramafic rocks largely determines the character of the lower boundary of the weathering crust. The presence in the crust of deep pockets and troughs filled with ochres and nontronites and confined to the most fractured sectors of ultramafic rocks is well known. Such sectors are often the contact zones of hyperbasites with gabbro-amphibolites, gabbro-diabases, and other rocks. This can be...

can be found in the works of I. I. Ginzburg [5], I. Z. Korin [7], and other geologists.

A. N. Aleshkov [1] was the first to draw attention to the possible role of the tectonic factor in the formation of silicate-nickel deposits within the Kempirsai massif; he approached the solution of this question in a somewhat distinctive way. He traced within the massif tectonic lines of “Alpine faults,” along which, in his opinion, hydrotherms penetrated that participated in the formation of silicate nickel deposits. A. N. Aleshkov’s attempt to connect the formation of deposits with the tectonics of the massif—an attempt, incidentally, not carried through to the end—served, in essence, as the basis for his proof of the hypogene origin of silicate nickel ores. These views, as untenable, were decisively rejected by other investigators of the weathering crust and nickel deposits.

In 1953, in our work [2], considerations were set forth to the effect that the most valuable silicate nickel deposits associated with the areal weathering crust of the hyperbasites of the Kempirsai massif are confined to the nodes of intersection of its principal structural directions, marked by dikes and fractured zones. The outline of the area of distribution of weathering-crust products with an ore-grade nickel content, i.e., the outline of the ore body in plan, we called the ore contour; and the line characterizing the direction of extension (elongation) of the ore contour, the axis of the ore contour. Single-axis and double-axis ore contours were distinguished. The conclusion was drawn that areas of the weathering crust with single-axis contours are confined to tectonically weakened zones connected mainly with one structural direction in the massif, whereas areas with double-axis contours are confined to zones of intersection of two structural directions. In the present article we attempt to develop further, and in a broader way, the views set forth above.

A considerable part of the silicate-nickel ore occurrences is situated in areas of the eastern and western contact zones of the northern part of the massif. In this connection one important detail may be noted: the linear arrangement of a number of deposits relative to one another, that is, the occurrence, at the ends of certain lines elongated in the latitudinal direction, simultaneously of deposits of the western contact on one side and of the eastern contact on the other, as well as of the Buranovskaya belt. An analogous picture is also seen in the south. Only here the latitudinal lines pass through deposits situated in the middle of the erosional section of the southern part of the massif.

Some deposits are grouped into ore fields, representing separate groups of closely situated ore occurrences located at the ends of belts of latitudinal or nearly latitudinal strike. The Rozhdestvenskoye (3¹) and Sazdinskoye (4) ore occurrences form one ore field. Opposite it is a second ore field, including the Buranovskoye (49) and Vostochno-Buranovskoye (48) deposits. The ore field situated on the western periphery of the massif and including the Kamennyi Kobchik (6) and Sheganshinskoye (7) ore occurrences corresponds to a grouping of deposits in the eastern part of the massif—another ore field, including the Zapadno-Dzhusalinskoye (45), the southern part of the Novo-Buranovskoye (47), and the Chugaevskoye (44) deposits.

In the west, the Taiketskoye ore field is rather clearly distinguished, and opposite it, in the east, the Kempirsai-Batamshinskoye. In the first—

¹ The figures denote the numbers of silicate-nickel ore occurrences shown on the scheme of tectonic structure.

among them are such ore occurrences as Novo-Taiketskoye (9), Oktyabrskoye (10), Taiketskoye (12), Zapadno-Kempirsaiskoye (8), Vostochno-Taiketskoye (11), and Tassaiskoye (13). The Kempirsai–Batamsha field includes the Vostochno-Kempirsaiskoye (42), Batamshinskoye (41), Novo-Batamshinskoye (40), and Promezhutochnoye (39) deposits. East of the Taiketen and Kempirsai–Batamsha ore fields, on the territory of the small Mamyt satellite of hyperbasites, are the Novo-Mamytskoye (37) and Mamytskoye (36) ore occurrences. In the south the Shilektin–Shcherbakov ore field is clearly expressed; it includes the Shilektinskoye (14), Beskudukskoye (20), and Shcherbakovskoye (21) deposits, as well as the sector of borehole 50 (15).

We have drawn attention to the presence, within the massif, of three latitudinal tectonically weakened zones saturated with gabbro-diabase dikes. These include, for example, the Shengashinsko-Dzhusaly, Taiketen–Batamsha, and Shilektin–Kyzylkain zones. If we now look at the ore fields, we shall see that all of them are situated precisely on the western and eastern margins of the above-mentioned zones. We shall hereafter call such a location of two ore occurrences or ore fields at the ends of latitudinal lines, belts, or zones connecting the western and eastern periphery of the massif the latitudinal correspondence of ore occurrences, and also of ore fields.

Consequently, the places of localization of ore fields are confined to the nodes where the latitudinal, tectonically weakened zones of the massif intersect the endocontact zone of the hyperbasites, which is also weakened and fissured. In these nodes there formed sectors of the most strongly disintegrated and tectonically “loosened” ultrabasic rocks, on which a thick weathering crust with deep roots developed, bearing considerable concentrations of nickel. The most ore-bearing sectors, in turn, were localized in the endocontact zone of the hyperbasites on the peripheral parts of the ends of narrower structures that intersect the massif in a latitudinal or nearly latitudinal direction, as mentioned above. Such structures were most often narrower zones, disintegrated by a series of small, closely spaced disjunctives, or else tectonically weakened zones with barely perceptible manifestations of disjunctive tectonics; more rarely they were individual faults and strike-slip faults. Hence also the latitudinal correspondence of ore fields and deposits.

Ore occurrences are found not only at the ends of lines of latitudinal strike, but also on the lines themselves, which, for example, is well expressed in the southern part of the massif. Thus, in the areas of the deposits one can observe belts of serpentinization and fragmentation of serpentinites extending from west to east. An elongation of the ore bodies in the same direction is also noted. All this, taken together, testifies to the great role of latitudinal disjunctive structures in the formation of silicate-nickel deposits on the territory of the Kempirsai hyperbasite massif.

In the distribution of deposits and ore fields their latitudinal correspondence is not always observed. This circumstance is explained by the geomorphological features of the weathering-crust region and is connected with the erosion of ore-bearing sectors of the crust by rivers and their tributaries.

On the scheme (Fig. 1) the contour of the hyperbasite contact is shown, plotted from the data of mapping, prospecting and exploration work, and magnetometry. The contour line of the eastern part of the massif is uneven and locally forms distinctly or weakly expressed bay-like “gulfs” projecting into the gabbro-amphibolites. It is precisely to these “gulfs” that the silicate-nickel ore occurrences are confined: Karagachti-

...skoye (38), Promezhutochnoye (30), Batamshinskoye (41), East Kempirsai (42), and the northeastern part of the ore body of the Novo-Buranovskoye (47) deposit. The irregularities of the eastern contour of the ultrabasic rocks are of disjunctive origin and are associated with faults and strike-slip faults of latitudinal or nearly latitudinal strike, which we mentioned above. They caused the displacement of individual blocks of the hyperbasite massif relative to one another in post-Jurassic time, as a result of which grabens were formed, most often asymmetrical, in which the ore-bearing weathering crust was preserved from erosion.

In this connection we would like to draw attention to certain details of the geological structure of the central ore body of the East Kempirsai (42) deposit—precisely the one confined to the graben expressed in plan as a “bay” in the eastern contour of the massif. On the basis of data obtained by us during large-scale mapping of the quarry of the East Kempirsai deposit, a diagram was compiled of the geological structure of the area of its central ore body, on which the rocks and elements of their structure are shown (Fig. 3).

Hyperbasites and gabbroids at the deposit are folded into gentle folds. The planar banding of the above-mentioned rocks in most cases has a northwestern strike and a southwestern dip at an angle of 15 to 35°. The diagram rather clearly shows an uneven boundary between serpentinites and gabbroids, with asymmetrical bay-shaped “gulfs.” It is noteworthy that the northern lines of the contours of the bay-shaped “gulfs” coincide with the tectonic lines of individual faults or series of small closely spaced disjunctives of fault and fault–strike-slip character, of latitudinal and nearly latitudinal strike, most often confined to the limbs of gentle folds. This circumstance gives grounds for assuming that, at the East Kempirsai deposit, the bay-shaped “gulfs” at the boundary of serpentinites and gabbroids are mainly of disjunctive origin. The age of the disjunctives is most likely Tertiary, since the disturbances cut the pre-Jurassic weathering crust. Cases are observed in which disjunctive fractures are filled with chalcedony, the formation of which we associate with the process of Tertiary silicification of Eocene–Oligocene age [3].

Fig. 3. Diagram of the geological structure of the area of the central ore body of the East Kempirsai deposit: 1—hyperbasites and their weathering crust; 2—gabbroids and their weathering crust; 3—gabbro-diabase dikes; 4—disjunctives; 5—contours of the mapped area.

As we see, in the area of the East Kempirsai deposit, to a certain degree and as if in miniature, the tectonic plan of the eastern periphery of the hyperbasite massif is expressed. In both cases we observe grabens. They determined the “bay-shaped” character of the contours in the ero-

in the erosional section and contributed to the preservation of areas of weathering crust with significant concentrations of nickel.

Latitudinal disjunctives within the territory of the Kempirsai massif exerted a twofold influence on the formation and localization of the ore-bearing crust. First, they conditioned the tectonic preparation of the parent substrate. This led to the formation of a sheetlike crust with an uneven surface at its base, complicated by deep “pockets,” “wells,” and “troughs” in the most fractured areas of the hyperbasites. Second, the latitudinal faults and fault-displacements caused the formation of grabens, in which individual blocks of nickel-bearing crust were preserved from erosion.

Both of these influences manifested themselves unambiguously, owing to the inheritance of disjunctive movements; that is, the tectonic preparation of the substrate and the formation of grabens took place in the same areas. This circumstance promoted the localization and preservation of the richest nickel-bearing ore crust, which was especially clearly expressed in the eastern endocontact zone of the hyperbasites.

We noted above the presence within the massif of discontinuous bodies of gabbroids, which in the erosional section form the Buranovskaya, Shcherbakovskaya, Dzharlybutakskaya, and Kyzylkainskaya belts. Silicate-nickel ore occurrences, localized in areas of the weathering crust of serpentinites in the contact zone with the aforementioned gabbroids, are spatially associated with the first three of these. These deposits, in turn, form ore belts, which are elongated areas of distribution of groups of closely spaced deposits. We gave them the same names as the gabbroid belts. The Buranovskaya ore belt includes the ore occurrences: Buranovskoye (49), Novo-Buranovskoye (47), Zapadno-Dzhusalinskoye (45), Chugayevskoye (44), and Kempirsaiskoye (43). The Shcherbakovskaya belt includes Beskudukskoye (30), Shcherbakovskoye (21), Tagashaganskoye (22), and Karaobinskoye (23). The Shuulkudukskoye (28), Dzharlybutakskoye (29), and Yuzhno-Dzharlybutakskoye (30) ore occurrences form the Dzharlybutakskaya ore belt. No significant silicate-nickel deposits are directly associated with the Kyzylkainskaya gabbroid belt. But parallel to it and to the south are located the Shirakainskoye (34), Vostochno-Dzharlybutakskoye (33), Dzhaingizagachskoye (32), and Staro-Shirakainskoye (35) ore occurrences.

Ore belts do not differ in principle from ore fields. In both cases we see areas encompassing groups of closely spaced ore occurrences of the same genetic type and form of manifestation. We shall only note the somewhat greater elongation, in most cases, of ore belts as compared with fields.

We have distinguished the two above-mentioned categories of deposits depending on the character of the structures to which they are confined. Thus, ore fields owe their formation to latitudinal disjunctive structures. Ore belts, however, are associated with meridional structures of a substantially folded character.

Some deposits—for example Buranovskoye (49), Shcherbakovskoye (21), Beskudukskoye (20), and others—simultaneously belong to an ore field and to an ore belt. This is explained by the fact that they are confined to areas of intersection of the latitudinal and meridional structures of the massif. Incidentally, in the ore occurrences located in the central parts of the erosional section of the southern half of the massif, one can see traces not only of latitudinal but also of meridional tectonic disturbances, only on a smaller scale. Consequently, these ore occurrences are also confined to nodes of intersection of the principal structural directions of the hyperbasites, as was noted above.

It seems to us that, when investigating the “areal” weathering crust of hyperbasites of the Kempirsai type, it is necessary to study carefully the tectonics of ultrabasic massifs in connection with the general structural features of the district. This will make it possible to identify more fully the regularities in the spatial distribution of silicate–nickel deposits and will allow prospecting and exploration for nickel to be carried out in a more purposeful and effective manner.

REFERENCES

1. Aleshkov A. N. On the question of the age of the weathering crust in the Southern Urals. Priroda, No. 1, 1941.

2. Bakirov A. G. On certain structural features of areas of the weathering crust of serpentinites of the Kempirsai massif. Izv. Tomsk Polytechnic Institute, vol. 74, issue 1, 1953.

3. Bakirov A. G. On the processes of silicification and the genesis of the so-called silicified serpentinites in the weathering crust of ultrabasic rocks. Tr. Tomsk State University, vol. 124, 1953.

4. Bakirov A. G. On peculiar plicative-disjunctive structures of gabbroids in hyperbasites. Izv. Tomsk Polytechnic Institute, vol. 121, 1963.

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

6. Glazkovskii A. A. Nickel. Evaluation of deposits during prospecting and exploration, 1949.

7. Korin I. Z. Formation of weathering deposits as a function of geological structure. Izv. Academy of Sciences of the USSR, Geological Series, No. 1, 1950.

8. Ulyanov D. G., Gritsaenko G. S., Krutov G. A. et al. Deposits of silicate–nickel ores of the Orsk–Khalilovo district. Tr. VIMS, No. 118, 1937.

1. The blocks are numbered in the direction from north to south. ↩