A. G. BAKIROV

ON SOME FEATURES OF THE STRUCTURE OF SECTIONS OF THE WEATHERING CRUST OF SERPENTINITES OF THE KEMPIRSAI MASSIF

In the study of deposits of useful minerals, it is of great importance to establish the dependence of the spatial distribution of ore bodies on the structure of the parent rocks enclosing the deposit, and on the structure of the district.

Knowledge of the structural features of ore bodies contributes greatly to the identification of the genesis of deposits, the sequence of mineralization, as well as to the correct evaluation of deposits and the planning of prospecting and exploration work.

In most cases, the structure of ore fields of deposits of endogenous origin is readily established and mapped.

The structural features of exogenous deposits of the weathering crust of the so-called linear type, confined to major tectonic zones, can be determined without particular difficulty. The structure is also revealed quite clearly in cases where, within the areas of distribution of the weathering crust, there are well-pronounced sections of parent rocks with developed fracturing, to which the thickest crust is confined.

It is considerably more difficult to determine the structure of exogenous deposits of the weathering crust of the so-called areal type, confined to a parent substrate that lacks clearly expressed tectonic zones. Moreover, geologists dealing with weathering-crust deposits of the above-mentioned type often underestimate the importance of studying the structures of these deposits.

In studying useful minerals confined to the weathering crust¹ of serpentinites of the Kempirsai hyperbasite massif, we had occasion to encounter features of the structure of sections of the crust and the dependence of their spatial position on the structure of the parent substrate; it is to this that we wish to draw attention.

The Kempirsai hyperbasite massif is located on the territory of the Novorossiisk and Stepnoi districts of Aktyubinsk Oblast, Kazakh SSR.

The pluton is confined to the Or–Ilek watershed—the southern continuation of the Guberlya Mountains of the Southern Urals. The watershed itself is a hilly projection of the ancient metamorphic sequence of the Ural-Tau.

The pluton is elongated in the NNW direction. Its area is approximately 1100 sq. km.

The hyperbasite massif is composed of apoperidotitic and apodunitic serpentinites; gabbroids occur along its periphery.

The sequences enclosing the pluton are metamorphic rocks of Precambrian and Cambrian age, schists and effusives of the Silurian and Devonian. The cover sequence is Upper Cretaceous—

... Cretaceous and Paleogene rocks of marine facies and sandy-clayey deposits of the continental Paleogene and Neogene.

The massif forms a body that dips steeply to the southwest and, in its mode of occurrence, approaches a sheet-like interformational intrusion.

Within it, four principal structural trends have been identified: northwestern, northeastern, meridional, and then—latitudinal or close to it (see the rose diagram, Fig. 1). These trends are marked by dikes and rare fractured zones.

In the area of the Kempirsai hyperbasite massif, a whole series of preserved areas of weathering crust is known, composed of ochres, nontronites, and leached serpentinites, in which a complex of various secondary minerals occurs, formed during the weathering of ultrabasic rocks.

Fig. 1. Rose diagram of the strike of the principal structures of the Kempirsai hyperbasite massif.

The association of the areas of weathering crust with the thickest loose sequence of weathering products—ochres and nontronites—with the endocontact zone of hyperbasites with gabbroids is rather clearly expressed. This is a characteristic feature of the spatial distribution of the above-mentioned areas on the territory of the massif.

In the areas of the crust, loose ochre-nontronite weathering products form sheet-like deposits, the lower surface of which is complicated by pockets, wells, and troughs and has a rather intricate form.

Upon careful examination of maps and sections of the areas of crust, it is not difficult to notice that the weathering products in these areas are not distributed randomly in space, but are adjusted to the structure of the parent substrate—their areas of distribution have a strictly definite orientation. Erosional truncations of areas of the crust, with nearly horizontal surfaces, have in plan the form of elongated ellipsoidal or irregular ovals, which we shall hereafter call ore contours.^1

On the geological maps of many preserved areas of weathering crust, one or several trends are drawn fairly distinctly—

^1 The ore contours shown in the figures (Figs. 2–5) mainly include the areas of distribution of loose products of the weathering crust, which have the greatest practical value.

... of the elongation (extension) of ore outlines. For convenience, we shall call the line that characterizes the direction of elongation (extension) of an ore outline shown on a geological map the axis of the ore outline¹. Where one line can be drawn, we have uniaxial outlines (Figs. 2, 3); where two can be drawn—biaxial ones (Figs. 4, 5).

In the strike of the axes of the ore outlines, four principal directions stand out quite clearly: latitudinal, northwestern, northeastern, and northern.

Fig. 2

Fig. 3

Fig. 4

Fig. 5

When comparing the rose diagrams of the strike of the structural directions of the pluton and of the axes of the ore outlines² (Figs. 1, 6), we see a great similarity between them, a fact to which no one has hitherto paid attention.

Thus, the following two characteristic features are revealed quite clearly in the spatial distribution of the preserved areas of the weathering crust within the territory of the Kempirsai hyperbasite massif.

1. The confinement of areas of the crust having the thickest ochre–nontronite sequence to the endocontact zone of the hyperbasites.

1. The adjustment, in the distribution of loose weathering products in areas of the crust, to the structure of the parent substrate, which found its expression in the similarity between the strike of the main structural trends of the pluton and the axes of the ore contours.

All this indicates that the spatial distribution and preservation of the weathering crust (more precisely, of the roots of the weathering crust) was determined entirely by the tectonics of the massif, by the presence of tectonically weakened zones, along which the processes of ancient weathering developed most deeply.

Within the massif, the most tectonically weakened was the contact zone of the hyperbasites with gabbro, where at the present time we also encounter the thickest ochre–nontronite sequences of the weathering crust. Within this zone, the configuration and orientation of the areas of the crust were conditioned by the structure of the parent substrate of the zone.

Fig. 6. Rose diagram of the strike of the axes of ore contours in areas of the weathering crust of the Kempirsai hyperbasite massif.

Areas of the weathering crust with single-axis ore contours were confined to tectonically weakened zones associated with one structural trend of the pluton, while areas with two-axis contours were confined to zones associated with two structural trends. The latter are, in essence, nodes of intersection of the main structural trends of the pluton, to which areas with thick nontronites, having the greatest practical value, are quite often confined.

Knowledge of the dependence of the spatial distribution of ochre–nontronite formations in areas of the weathering crust of serpentinites on the structure of the parent substrate is of great importance, since it makes it possible to orient prospecting and exploration work correctly toward mineral resources associated with the weathering crust.

Therefore, in studying areas of the weathering crust of ultrabasic rocks, proper attention must also be paid to their structural features.

LITERATURE

1. Ginzburg I. I. et al. Ancient weathering crust on ultrabasic rocks of the Urals. Part II. Transactions of the Institute of Geological Sciences (No. 2), issue 81, 1947.

2. Zinovkin A. D. On the tectonics of the Kempirsai pluton. Soviet Geology, 12, 1940.

3. Korin I. Z. Formation of weathering deposits as a function of geological structure. Proceedings of the USSR Academy of Sciences, geological series, 1, 1950.

1. The axes of the ore outlines in the figures are shown by two straight parallel lines. ↩↩

2. The rose diagrams show the strike directions of the axes of the ore outlines of almost all areas of the weathering crust of the massif, whereas the figures show only some of them. ↩