Nickel-Bearing Weathering Crusts of the Urals and Their Classification

A. G. Bakirov

(Presented by Professor A. M. Kuzmin)

The doctrine of the weathering crust has been well developed by Soviet scholars. Definitions of the crust have also been formulated. Thus, B. B. Polynov [10] writes that “the weathering crust is that upper part of the lithosphere which is composed of loose products of the disintegration of igneous and metamorphic rocks” (p. 24). In the opinion of I. I. Ginzburg and K. K. Nikitin [5], “The weathering crust consists of rocks altered as a result of the action upon them of atmospheric agents, soil and ground waters, ground air, the vital activity of organisms and the products of decomposition of the latter, including both those that have preserved and those that have not preserved the structure of the parent rocks” (p. 151). B. B. Polynov [10], in essence, assigns to the weathering crust the upper part of the lithosphere, composed of eluvial formations and sedimentary rocks that have not undergone metamorphism. That is, in his opinion, the formation of the crust is due to processes of weathering and sedimentation.

I. I. Ginzburg [3] criticizes B. B. Polynov for too broad an understanding of the weathering crust and believes that the crust should include mainly eluvial products and formations close to them in genesis.

In the author’s opinion, the definition of the weathering crust given by B. B. Polynov is acceptable for understanding it in a broad planetary sense, when eluvial and sedimentary formations are jointly opposed to primary magmatic and metamorphic rocks, and when eluvial and sedimentary processes are considered in the unity and interrelation necessary for geochemical and theoretical-cognitive conclusions on a large scale. In this understanding of the weathering crust, geochemists, geomorphologists, geographers, and soil scientists often need it. If, however, researchers have to study comparatively small territories and to contrast qualitatively different eluvial and sedimentary formations with one another, then the definition of the crust given by B. B. Polynov is no longer suitable. Another formulation is required, one that embraces phenomena of limited scale and other characteristics. Hence the necessity for two definitions of the weathering crust—in a broad and in a narrow sense.

In the author’s opinion, the weathering crust is a zonal continental formation composed of mineral accumulations of essentially eluvial origin, formed under conditions of the Earth’s su-

surface as a result of the prolonged and often repeated decomposition of rocks of different genesis that are within the sphere of influence of gases associated with the atmosphere, of surface and ground waters, of organisms, and of the products of their vital activity. The material composition of the rocks of the weathering crust is determined mainly by those minerals that arose as new formations, including infiltration ones¹, owing to the essentially vertical movement of dissolved substances and their subsequent deposition in the crust profile, with a greater or lesser role played by residual mineral components and by the removal of many elements beyond the limits of the crustal area. Characteristic features are the development of stage minerals in the crust and the process of subsidence of its loose formations. The zonal structure of the crust is due to the attenuation of weathering processes with depth and to the vertical migration of elements.

The above definition of the weathering crust, in its narrow sense, is based on the well-known formulations of I. I. Ginzburg [3], V. P. Petrov [9], and other researchers. At the same time, the author has drawn attention to the role of the vertical migration of elements in the formation of the crust profile and to the subsidence of the loose formations of the latter, which are most highly developed in the weathering crust of ultrabasic rocks.

Many weathering crusts of hyperbasites and karst fillings developed in carbonate strata in the zone of contact with ultrabasic rocks are nickel-bearing.

At present, the following types of weathering crusts are distinguished in educational and specialized literature: areal, linear, and contact-karst. The first of these are characterized by broad areal development; the second, by their confinement to disjunctives; and the third, to zones of contact between hyperbasites and carbonate rocks. The foundations of such a subdivision were laid by D. G. Ulyanov and were given final form by I. I. Ginzburg. This classification to some extent, and in a somewhat generalized manner, reflects the actual state of affairs, the objective reality. It has been used, and continues to be used, at the present time.

It should be said that the above-mentioned classification was developed in the 1930s and essentially characterizes the initial stage in the study of nickel-bearing crusts. At present our knowledge of crusts has broadened. The need has arisen to detail the existing classification. It is appropriate to recall that A. A. Glazkovsky [6], I. I. Ginzburg [4], and other geologists, in their works, pointed to the existence of mutual transitions between different types of crusts and to the need to distinguish intermediate varieties.

Recently I. I. Ginzburg and K. K. Nikitin [5] proposed the following classification of weathering crusts according to morphology. They distinguish areal, linear, and mixed (combined) weathering crusts. Among the linear crusts they distinguish: linear-fracture, linear-contact, and contact-karst. Among the mixed crusts they distinguish: fracture-areal, contact-areal, karst-areal, and others.

The desire to make the classification more detailed and to distinguish transitional members in it should be welcomed in every possible way. This corresponds to the increased level of our knowledge about the crust and to the demands of practice. At the same time, we consider it necessary to introduce certain refinements into the above morphological classifications.

In the author's opinion, the widely used term “contact-karst crust,” applied to nickel-bearing karst fillings, is not entirely correct. Of course, the latter have certain features in common with mineral formations that arise during the hypergene alteration of hyperbasites, since in the formation of both, weathering processes exerted their influence and infiltration was manifested. But this is a similarity in secondary features. In the main respect—in their genetic nature—they differ sharply. The nickel-bearing weathering crusts of hyperbasites are eluvial formations, whereas karst fillings are distinctive sedimentary rocks. To call the latter contact-karst crusts means introducing a large element of convention into the systematics of hypergene nickel-bearing mineral complexes.

In the classification cited above by I. I. Ginzburg and K. K. Nikitina [5], in our opinion, the term “mixed crust” is somewhat unfortunate. It would be more correct to speak of intermediate or transitional crusts characterized by a mixed or alternating composition of the mineral formations of different crusts.

In the classifications, in our opinion, the terms “areal” and “linear” are unfortunate. These terms have more of a geometric meaning; they do not sufficiently reflect the content and impart a non-geological character to the classification itself. It would be more correct to speak not of areal and linear crusts, but of blanket and fissure crusts, as corresponding most closely to a morphological classification and more fully reflecting their essence²).

When a “linear” weathering crust is mentioned, what is meant is a crust confined to fissured zones and disjunctives of some single direction. In reality, however, the situation is far from this. Our investigations have shown that the nickel-bearing weathering crusts of the Urals that are most interesting industrially, including the so-called “linear” ones, are confined to nodes of intersection of fissured zones. The same regularities are also found in other regions of the Soviet Union. In this connection, to call fissure weathering crusts “linear” means underestimating transverse structures, which play an important role in the localization of nickel-bearing parts of the crust [1].

We believe that there is every reason to distinguish two main types of hypergene nickel-bearing mineral complexes—the weathering crust of hyperbasites and, further, karst fillings³) (inflow, after G. F. Lungerzauzen, 8). We consider it possible to subdivide each of the above types into three principal morphological subtypes—blanket, contact, and fissure—and three intermediate ones—blanket-contact, blanket-fissure, and contact-fissure—which may be combined with one another.

The characteristics of the substrate, then the geological position, material composition, and thickness of the hypergene nickel-bearing formations, the degree of dissection of the lower boundary of their bodies—all this may serve as criteria

²) It should be said that some geologists are not satisfied with the term “linear crust,” and they speak of “linear-fissure” crust.

³) They are also often called karst aleurites. This term is widespread in the practice of nickel geologists. It is used to characterize, in general, all karst fillings without distinction as to the particle size of the clastic material entering into their composition. In this case, what is meant are karst aleurites situated within the sphere of geochemical influence of the weathering crust of hyperbasites, from which nickel enters the inflow.

for the classificational subdivision of both weathering crusts of hyperbasites and karst aleurites. Both have their own specificity. At the same time, certain common features also appear in their identical subtypes.

Thus, the crusts and influvium of the blanket subtype, especially in surface karst forms, are characterized by the development of clayey products: nontronites, montmorillonites, and beidellites in the crust of hyperbasites, and halloysites and, more rarely, montmorillonites and nontronites in the influvium. Fissure and contact-fissure crusts are notable for the development of ochres and siliceous formations. The latter develop along carbonate rocks and leached serpentinites. In intermediate blanket-fissure crusts, an alternation is observed of areas of clayey and ochreous composition with local manifestations of silicification.

Karst aleurites of the blanket, fissure, and contact subtypes are rare and usually compose small areas (Rezhevskoy and Lipovskiy districts). The contact-fissure varieties of nickel-bearing influvium are the most developed (Ufaleiskiy and other districts). The largest areas of the weathering crust of Ural hyperbasites belong to the blanket-fissure and blanket-contact subtypes². In terms of nickel concentration, however, in most cases the productive crusts are those of the contact-fissure and blanket-fissure subtypes (Serovskiy, Buryktalskiy, Kempirsaiskiy, and other districts).

It should be said that, at hypergene deposits, the close proximity of areas characterized by different subtypes of weathering crust is not uncommon. Mutual transitions between them are also observed. Thus, a blanket crust of hyperbasites may adjoin a fissure crust and pass into it (Shevchenkovskiy hyperbasite massif); transitions of a contact-fissure crust of hyperbasites into influvium of the same subtype are not rare (the area of V. Ufaley), etc.

We have attempted to systematize the nickel-bearing weathering crusts of the Urals on the basis of the existing classifications of D. G. Ul’yanov, I. I. Ginzburg, and K. K. Nikitin. In our classification, greater importance is assigned to contact and contact-fissure weathering crusts of hyperbasites and to karst aleurites. Karst fillings have been separated into an independent group, which is of important methodological significance. This requires the use of specific methods of investigation in their study.

The proposed classification can be developed and detailed.

LITERATURE

1. A. G. Bakirov. Some structural regularities in the localization of hypergene nickel deposits of the Urals. Izv. TPI, vol. 127, issue 1, 1964.

2. I. I. Ginzburg. Geochemistry and geology of the ancient weathering crust in the Urals. Tr. Inst. Geol. Nauk AN SSSR, issue 31, 1947.

3. I. I. Ginzburg. Types of ancient weathering crusts, forms of their occurrence, and classification. In the collection Weathering Crust, issue 6, Publishing House of the Academy of Sciences of the USSR, 1963.

4. I. I. Ginzburg, A. A. Kain, et al. Types and morphology of the ancient weathering crust. Tr. Inst. Geol. Nauk AN SSSR, issue 80, 1946.

1. I. I. Ginzburg, K. K. Nikitin. Terms and concepts relating to the weathering crust. Review of geological concepts and terms as applied to metallogeny. Publishing House of the Academy of Sciences of the USSR, Moscow, 1963.

2. A. A. Glazovsky. Nickel. Evaluation of deposits during prospecting and exploration. Issue 4, Gosgeoltekhizdat, Moscow, 1949.

3. B. P. Krotov. Classification, nomenclature, and characteristics of exogenous deposits. Proceedings of the Conference on the Genesis of Iron, Manganese, and Aluminum Ores. Publishing House of the Academy of Sciences of the USSR, Moscow, 1937.

4. G. F. Lungersgauzen. Influxium—a special genetic type of continental formations. Reports of the Academy of Sciences of the USSR, vol. 171, no. 3, 1966.

5. V. P. Petrov. Fundamentals of the Study of Ancient Weathering Crusts. Nedra, Moscow, 1967.

6. B. B. Polynov. Weathering Crust. Part 1. Publishing House of the Academy of Sciences of the USSR, Moscow, 1934.

1. Let us recall that B. P. Krotov [7], in his classification of exogenous deposits, assigned the nickel deposits of the hyperbasite weathering crust to the category of cementation deposits. I. I. Ginzburg distinguishes an infiltration weathering crust. ↩

2. In the present erosional section, blanket weathering crusts of hyperbasites are encountered rarely, fragmentarily, and occupy small areas. They are often mentioned with a certain degree of conditionality. The crust to which the Aiderbak deposit is confined is assigned by us to the blanket-fissure subtype. ↩