ON THE NEED TO TRACE THE BALEI–DARASUN FAULT WITHIN THE BORSHCHEVOCHNY RIDGE

L. A. ZASHCHINSKY, I. P. NOVIKOV

(Presented by Prof. Dr. D. S. Mikov)

In 1958–1959 N. A. Fogelman was the first to put forward the supposition that the Balei and Darasun deposits are confined to a single zone of a long-lived deep fault (Fogelman, 1963). The basis for tracing this fault was provided, in addition to her own route observations, by data from the interpretation of aerial photographs (V. S. Zorin) and small-scale geophysical investigations (A. A. Solodov).

Although the characterization of the aforementioned zone as long-lived is, to a known extent, confirmed by the diversity in composition and age of the effusive and dike complexes and mineralization controlled by it, the indications of the deep-seated character of the fault in this zone are purely hypothetical. Indeed, the depth of the methods used by the above-mentioned investigators, as is known, does not exceed the first hundreds of meters.

In the course of tracing the fault, numerous tectonic dislocations analogous to those developed west and east of the Balei–Darasun line—where gold occurrences are also widespread—may have been assigned to its zone without sufficient justification.

It is difficult to overestimate the prospecting significance of N. A. Fogelman’s structural constructions.

In her concept, further searches for gold mineralization, with the aim of expanding the raw-material base of this region, should be concentrated in the belt of the Balei–Darasun fault, as well as in zones of other analogous structures of northwestern strike.

In light of N. A. Fogelman’s ideas, the prospects for discovering mineralization of the Balei–Taseevo type within the Arbagar depression—whose structural position is analogous to the Undino–Daina depression—appear very important.

Apparently, the development of N. A. Fogelman’s views will inevitably lead to a tendency to distinguish analogous zones in order to explain the facts of the broad development of gold-ore mineralization east and west of the Balei–Darasun line.

Such a tendency may be of great importance from a prospecting standpoint. On the other hand, constructions of this kind are highly significant from a theoretical point of view, since they assume entirely special properties of the Mohorovičić and Conrad boundaries within a very limited segment of the Earth’s crust. Indeed, in developing such

views, it is not difficult to arrive at the hypothesis that, in the relief of these boundaries along the parallel of the Mongol–Okhotsk suture, from the mouth of the Onon River to the meridian of the town of Nerchinsk, there is manifested a whole series of deep faults of meridional or northeastern orientation.

Only deep geological (especially seismic) investigations can provide a basis for further studies of the structural control of ore mineralization in the Balei–Darasun district. At the very least, the characterization of the Balei–Darasun fault as a deep-seated one is in urgent need of confirmation by DSS data. This applies equally to such important structures of the region as the Mongol–Okhotsk suture itself (Gorzheevsky, 1961, 1966) and the Tura–Ili fault (Fogelman, 1965). However, if the configurational features of the latter structures are clear, the Balei–Darasun fault still needs to be traced in the field of development of the Borshchevochny granitoid massif.

Apparently, in 3–5 years the question of organizing DSS in this area, for solving a broad range of problems of regional geology and prospecting, will acquire primary importance. Such work, directed toward the study of the Balei–Darasun fault, will have to be carried out along latitudinal profiles both in the area of the Mesozoic folded structures near Balei and north of the Mongol–Okhotsk suture in the Darasun area.

If north of the suture the fault has been well traced by the above-mentioned investigators, then in the field of development of the Borshchevochny granites its tracing has been reduced merely to interpolations between observations made at Balei and Aprelkovo. The latter is connected with objective difficulties in tracing the fault. Geomorphological data under conditions of alpine-type relief, characteristic of the Borshchevochny Ridge, provide very limited material for conclusions; the results of magnetic and gravity surveys are also greatly complicated by the influence of relief. Moreover, the latter can hardly be interpreted with confidence without DSS data, i.e., in principle they do not solve the problem prior to the conduct of deep seismic investigations. Deep investigations, however, are too expensive to be begun without first exhausting the possibilities of tracing the fault by other geological and geophysical methods.

In our view, an attempt should be made to trace the fault by a complex of methods, the basis of which should be electrical prospecting, and, first of all, its modifications possessing the greatest depth of investigation. One should also try to solve the problem by deep geochemical methods: helium and hydrogeochemical surveys and lithogeochemical surveying over primary dispersion halos. The cost of such investigations will not be too considerable.

We believe that tracing the fault (if it actually exists in this area) in the belt from the town of Balei to the Aprelkovo mine, 10–15 km wide, can be accomplished over the course of two field seasons at a cost of up to 50 thousand rubles.

In carrying out such work, in our opinion, not only the Chita Geological Administration, responsible for conducting deep geological investigations in Transbaikalia, is interested, but also the “Zabaikalzoloto” trust, since the matter concerns determining the position of the principal ore-controlling structure with which the raw-material base of its most important enterprises is connected. In addition, the area of the proposed investigations directly adjoins two large ore fields and may prove promising in terms of discovering new points of mineralization.

In February 1967, in a report at the production geological conference of the Balei Section of the Transbaikal Branch of the Geographical Society of the USSR and the “Baleizoloto” combine, we drew the attention of production organizations to the need to carry out such work. At present, part of our recommendations is being implemented.

REFERENCES

1. D. I. Gorzhevsky, E. M. Laz’ko. The Mongol–Okhotsk deep fault. Doklady of the Academy of Sciences of the USSR, vol. 137, no. 5, 1961.

2. D. I. Gorzhevsky, N. A. Fogelman. Tectonic and metallogenic regionalization of activation zones, based on the example of Transbaikalia. Doklady of the Academy of Sciences of the USSR, vol. 166, no. 1, 1966.

3. N. A. Fogelman. Some features of the geology and metallogeny of the Balei–Darasun gold-bearing district. Transactions of TsNIGRI, issue 41, 1962.

4. N. A. Fogelman. Geological conditions of formation of the Tura–Ilinsk gold-bearing district of central Transbaikalia. Transactions of TsNIGRI, issue 63, 1965.