ON THE ROLE OF TRANSVERSE STRUCTURES IN THE FORMATION OF AREAS OF SUPERGENE NICKEL MINERALIZATION IN THE URALS

A. G. Bakirov

(Presented by Professor A. M. Kuzmin)

In the Ural folded region, nickel occurrences are associated with the weathering crust of hyperbasites, which has a regional development. The massifs of ultrabasic rocks of the Urals form meridionally elongated belts. Nickel mineralization is found only in certain areas of the hyperbasite belts and their massifs.

We have been able to establish that, in the localization of points of supergene nickel mineralization in the Urals, an important role was played by certain transverse structures in the form of complicating latitudinal zones of increased fracturing.

These structures are emphasized by dikes of gabbro, gabbro-diabases, diorites, and other rocks similar to them in composition. The dikes mentioned form peculiar latitudinal zones of “magmatic translucence.” In different districts, dike bodies within them are arranged with various orientations: latitudinal, meridional, northwestern, northeastern, and mixed (Fig. 1), characterizing the exposure of the zones. These latitudinal zones of increased fracturing, or more precisely zones of “magmatic translucence” by dike formations, control the distribution of many points of nickel mineralization. The latter may be confined either directly to these zones (the Buruktal, Kairakty, and some other hyperbasite massifs), or to adjacent areas located on the continuation of these zones (the Kimpersai massif and some others).

Fig. 1. Scheme of the arrangement of dikes in latitudinal zones of increased fracturing

Exposure of the zones:

a — latitudinal, b — meridional, v — northeastern, g — northwestern, d — mixed

A similar role is played by dikes and disjunctive structures of latitudinal and sublatitudinal strike. In this case, the nickel mineralization of the weathering crust of hyperbasites is localized not only in disjunctive zones and exocontacts of dikes, with the formation of “pockets” of nickel-bearing crust, but also in areas of the flank continuation of dikes and disjunctive structures. A combination of both of the above cases is also possible. The points of nickel mineralization of the Aiderbak plateau in the Khalilovo district are confined to an area occupying a flank position relative to thick latitudinal dikes of gabbro-diabases (Fig. 2). In the Rezhevsk massif of hyperbasites, in the Pokrovsk sector,

Fig. 2. Geological sketch map of the Aiderbak sector. 1 — effusive-tuffaceous Silurian sequence, 2 — hyperbasites, 3 — gabbro-diabases, 4 — weathering crust of hyperbasites, 5 — sandy-clayey deposits of the continental crust, 6 — mineralization area

there is a thick diabase dike. Nickel mineralization is confined not only to it; it is also traced farther, along the direction of its strike. In this case, isolated mineralization areas trace a sublatitudinal belt that includes the diabase dike and a disjunctive structure (Fig. 3). Traces of increased fracturing

Fig. 3. Geological sketch map of the Pokrovsk sector. 1 — marbled limestones of Devonian–Lower Carboniferous age, 2 — tuffaceous shales of the Lower Carboniferous, 3 — hyperbasites, 4 — diabase dike, 5 — lines of tectonic faults, 6 — mineralization areas. AB — direction of the sublatitudinal belt of mineralization

are manifested in one form or another in many places of nickel mineralization located in a flank position relative to latitudinal dikes, dike zones, and disjunctive zones.

Thus, in all the cases cited above, the isolated latitudinal or sublatitudinal belts of dike development, as well as individual dikes, are an indicator of transverse zones of increased fracturing, sometimes accompanied by disjunctives extending beyond the dikes themselves and favoring the formation and preservation of areas of nickel mineralization in the weathering crust of hyperbasites.

Transverse folds, both synclinal and anticlinal in character, also exerted their influence on the localization of nickel occurrences. In this respect the Akkermanovka (Orenburg oblast) and Tochilnogorsk (Sverdlovsk oblast) areas are indicative (Figs. 4, 5). The first of these is confined to the southeastern part of the so-called Akkermanovka arc of Paleozoic rocks, which is a transverse fold plunging to the east. The second is associated with an anticlinal fold inclined to the west. In both cases the points of nickel mineralization are located near these folds, adjoin them from the southwest. The axial lines of the latter pass almost through them. The areas adjacent to the indicated transverse folds proved to be fractured, with disjunctives, which contributed to the formation and preservation of products of nickel mineralization. At the same time, latitudinal fracturing also influenced their localization, being especially clearly expressed at the Tochilnogorsk nickel occurrence.

Fig. 4. Geological scheme of the Akkermanovka area.
1 — Ordovician tuffaceous shales, 2 — schistose albitophyres and radiolarites of the Devonian, 3 — limestones of the Lower Carboniferous, 4 — hyperbasites, 5 — gabbroids, 6 — clays and sands of the Jurassic, 7 — mineralization area

Fig. 5. Geological scheme of the Tochilnogorsk area.
1 — marbled limestones of the Devonian—Lower Carboniferous, 2 — hyperbasites, 3 — mineralization area

From all the material considered, the following conclusions may be drawn. In many cases, nickel-bearing products of the weathering crust of the hyperbasites of the Urals are localized where meridional structures are complicated by transverse latitudinal or sublatitudinal ones. In this case, the axial lines of inclined transverse folds, the axes of zones of dike development, dikes, and disjunctives serve as indicators, peculiar “arrows,” marking the direction on which, or near which, areas of hypergene nickel mineralization may be encountered.