SOME STRUCTURAL PATTERNS IN THE LOCALIZATION OF SUPERGENE NICKEL DEPOSITS OF THE URALS

A. G. Bakirov

(Presented by Prof. A. M. Kuzmin)

In the Urals, supergene nickel deposits associated with the weathering crust of ultrabasic rocks are well known. According to the generally accepted view, the crust formed at the end of the Triassic and the beginning of the Lower Jurassic; subsequently it underwent transformation and renewal in Cretaceous and Tertiary time. Deposits of three types are distinguished: areal, fissure, or linear, and, finally, contact-karst.

A considerable literature is devoted to the weathering crust of ultrabasic rocks and to the deposits of supergene nickel ores of the Urals. The geochemistry and mineralogy of the crust have been studied most fully. This is due above all to I. I. Ginzburg and his many students. Somewhat less well covered are questions of the localization of supergene nickel deposits as a function of the structural features of the parent substrate.

Questions of the tectonics of the areas of distribution of the weathering crust and nickel ore occurrences are considered in the works of I. I. Ginzburg [4], A. A. Glazkovsky [5], I. Z. Korin [7], M. A. Karasik [6], I. I. Edelshtein [8], A. G. Bakirov [2], the collective of geologists of the Ufaley area, and other researchers. In these works the influence of the structural features of the substrate on the localization of mineralization is considered either in general terms or as applied to individual deposits. Attention is drawn to fractured and tectonically weakened zones of hyperbasites, which favored the formation and preservation of the nickel-bearing weathering crust. I. Z. Korin [7] points to the confinement of rich supergene nickel occurrences of the contact-karst crust to areas where disjunctive disturbances of different strikes intersect. I. I. Edelshtein [8] treats the question in the same way with respect to the district of the Eastern group of deposits described by him. N. I. Arkhangelsky [1] notes the localization of nickel-bearing portions of the weathering crust of the Urals hyperbasites at the junctions of anticlinal and synclinal structures. At the same time he attaches great importance to comparatively shallow meridional grabens, in which, in his opinion, the ore-bearing products of the crust were preserved from erosion. All the conclusions of the above-

the studies by the aforementioned researchers are correct and valuable, since they indicate to practicing geologists the lines of search for the most important, in an industrial sense, deposits associated with weathering crust.

We believe that far from all possibilities have been used for revealing both regional and local regularities in the formation and preservation of hypergene nickel ore occurrences. In this connection, we would like to share our experience in this field. As a result of many years of work, we have come to the conclusion that within the Ural fold belt there are tectonically weakened areas and concealed faults of latitudinal strike, which conditioned its transverse subdivision into latitudinal structures¹ of various categories: provinces, regions, zones, and belts [3]. At the same time, we draw attention to the ore-controlling significance of the axes and junctions of the structures distinguished.

In this connection, analysis of the position of nickel-bearing intrusions in the latitudinal structural plan of the Ural fold belt, and of the influence of exposed and concealed disruptions of latitudinal strike on the localization of hypergene nickel deposits, is of considerable interest. In relation to the structures of the Ural fold belt distinguished by us, its nickel-bearing intrusions may be subdivided into intrazonal and interzonal ones. The former include hyperbasite massifs confined to zones of “inter-granite straits”²—the Aidyrlya, Rezhevskoy, and other massifs. Examples of the latter are the Kimpersay, Khalilovo, Shevchenko, Ufaley, and other massifs, which cut across a number of zones. The most nickel-bearing³ portions of intrazonal massifs are those adjacent to the axes of the distinguished zones. In interzonal hyperbasite massifs, the regions of near-axial belts and places where latitudinal zones join are characterized by greater nickel content.

Hyperbasites confined to areas of latitudinal bending of meridional folded structures (the Ufaley and Vostochny regions) are characterized by rich nickel concentrations. Nodes of intersection of tectonically weakened zones, disjunctives, and also folded structures of latitudinal, northwestern, and northeastern strike with meridional structures are favorable for the formation of ore concentrations (deposits west of Rezh, then those of the Vostochnaya group, Tochilnaya Gora, etc.).

A characteristic feature is the development of latitudinal dyke fields. In them, individual vein bodies may have different orientations (the Buruktal, Kairakty, and other massifs).

The influence of latitudinal tectonics on the localization of the most ore-bearing parts of the weathering crust is also manifested in the territories of individual massifs. Here, first of all, the fragmentation of the latter by latitudinal and sublatitudinal disjunctives into a series of tectonic blocks is noted. This feature is especially clearly expressed in the Ufaley and Kimpersay districts. Hyperbasite massifs that have undergone such fragmentation are characterized by latitudinal-flexural bends of their contours and by numerous dykes extending from west to east.

Latitudinal structures are less clearly expressed in the territory of the Shevchenko hyperbasite massif. They appeared in the form of indistinct zones

of increased fracturing superimposed on disturbed areas of northeastern strike. The entire set of manifestations of fracture tectonics determined the localization of ore concentrations within the massif and the peculiar boomerang-like plan shape of the areas of the weathering crust richest in nickel.

Traces of the influence of latitudinal tectonics on the formation of nickel ore occurrences can be seen in the Rezhevskaya group of deposits (recall, for example, the latitudinal diabase dike at the Pokrovskoye deposit), then in the area of the Khalilovo massif and in several other places. Let us also point to the development of nickel-bearing weathering crust in parts of hyperbasite massifs that tend toward their northern or southern boundaries, which run along lines of latitudinal strike (Kulikovsky, Travnikovsky, and other massifs).

In addition to disjunctive disturbances, folded structures in the strata enclosing the hyperbasites also influenced the localization of ore occurrences. The form in which these structures are expressed in the massifs is as follows: expansions, “inflations” of their individual parts, and latitudinal-flexural bends of the boundaries of the hyperbasites. Ore concentrations in the areas of the Gulinsky, Kamennodolsky, and Alapaevsk serpentinite massifs are associated precisely with such places.

The localization of nickel mineralization in the weathering crust of hyperbasites was influenced by transverse structures, varied in form and scale of expression. These include latitudinal bends, flexures, folds, disjunctive structures, and others. Zones of increased fracturing of latitudinal and, less commonly, northwestern and northeastern strike, often marked by dikes, were of great importance. Hence the varied character of the tectonic nodes in which the hypergene nickel deposits of the Urals are localized, regardless of their type. All this should be taken into account in compiling metallogenic maps, in the prognostic assessment of massifs, and in carrying out prospecting and exploration work for hypergene nickel.

LITERATURE

1. N. I. Arkhangel’skii. The role of tectonics in the localization of sedimentary deposits of Mesozoic age on the eastern slope of the Urals. Collection “Patterns in the Distribution of Deposits in Platform Covers,” Publishing House of the Ukrainian SSR, 1960.

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

3. A. G. Bakirov. The relationship of ore formations of the Urals with its latitudinal structures. Doklady AN SSSR, vol. 148, No. 5, 1963.

4. I. I. Ginzburg 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.

5. A. A. Glazkovskii. Nickel. Evaluation of deposits during prospecting and exploration, issue 4, 1949.

6. M. A. Karasik. Types of nickel deposits of the Rezhevskaya district (Middle Urals). Doklady AN SSSR, vol. 52, No. 8, 1946.

7. I. Z. Korin. Formation of weathering deposits as a function of geological structure. Izv. AN SSSR, Geological Series, No. 1, 1950.

8. I. I. Edel’shtein. The role of disjunctive disturbances in the formation of hypergene deposits of nickel and cobalt. Exploration and Protection of Mineral Resources, No. 2, 1962.

1. We understand them in the sense of latitudinal belts that differ from one another in certain features of geological structure. ↩

2. These are areas of transverse sagging of the folded structures of the Urals, separating from one another regions where large granite massifs developed. ↩

3. When we speak of nickel content, we mean hypergene nickel content associated with the weathering crust of hyperbasites. ↩