GEOPHYSICAL STUDIES IN GOLD PROSPECTING IN THE ALTAI MOUNTAINS

F. B. Bakshit (ZSGU)

Geophysicists first turned their attention to the gold of the Altai Mountains 15 years ago. Systematic study of the gold-bearing districts of the Altai Mountains by geophysical methods has been carried out by the Altai Geophysical Expedition of ZSGU since 1960. The role of geophysical methods in the overall complex of investigations is steadily increasing, especially now, when the problem of searching for concealed deposits is becoming ever more acute. The need for broad introduction of geophysics and geochemistry into the everyday practice of geological prospecting is indicated by the following curious conclusions. Judging from preserved archival materials, all the currently known promising districts both of the Altai and of all Western Siberia were discovered by prospectors even before the Revolution. The area of the Sinyukha deposit is no exception; its gold potential was discovered in 1916. And all significant gold occurrences were discovered by prospectors only through visual finds of ores in outcrops. All this speaks to the comparatively low effectiveness of the classical methods of geological prospecting.

Today the territory of the Altai Mountains is completely covered by regional small-scale geophysical work. Over an area of more than 3,000 sq. km, encompassing almost all the principal gold-bearing areas, investigations at medium and large scales have been carried out. At 11 gold deposits and ore occurrences, detailed work has been carried out, accompanied by specialized petrophysical studies. The experience accumulated by the geophysicists of the Altai makes it possible to draw certain conclusions and recommendations that are not only of local but also of general significance.

Today geophysics is successfully used to solve a very wide range of general problems and specific tasks connected with the study of the entire gold-bearing province as a whole, of individual gold-ore clusters, and of deposits. Let us consider these tasks separately.

1. General problems.

Problems of this kind include, in particular:

the question of the relationship of gold mineralization to intermediate and basic magmatism of the Salair stage of orogenesis;

the thesis concerning the predominance of the near-latitudinal strike of ancient upper-

of Neoproterozoic and Lower–Middle Cambrian structures controlling the distribution of gold mineralization;

the opinion of a number of researchers concerning the pre-Sinian time of inception of the Altai–Sayan geosyncline;

the validity of predicting, in the Altai Mountains, gold deposits belonging to the pyrite, disseminated gold-sulfide (pyrite–pyrrhotite) formation, associated with extended zones of crumpling and schistosity, and several other problems.

Geophysics gives a positive answer to all the questions listed. For solving problems of this type, the information used is obtained chiefly from regional magnetometric, gravimetric, and electrical-prospecting surveys at scales of 1:50,000 and larger. Let us note that our experience has confirmed the advisability of the broad use, for purposes of small-scale study of gold-bearing areas, of surveys by the method of the natural electric field, which clearly records the position of regional zones of deep faults. The latter are spatially and genetically associated with zones of pyritization (with chalcopyrite and pyrrhotite) and graphitization. In their detection and study, geophysics, together with geochemical methods, has the leading role.

1. Particular questions in the study of gold-ore districts and ore fields.

At the stage of detailed investigations, carried out at scales of 1:10,000–1:25,000 and larger, with the aid of geophysics and geochemistry it is possible to solve a whole series of problems constantly confronting geologists studying the almost completely covered territory of the northern part of the Altai Mountains and the south of Mountain Shoria. Among them the following may be named:

structural geological mapping of skarn ore fields and their environs;

study and evaluation of the sulfide content and general metalliferous character of regional zones of pyritization, and detection of local anomalies of high order promising for the search for sulfide ores;

study of the thickness of unconsolidated deposits for the purpose of locating mine workings and selecting the type of such workings;

tracing, along strike, individual structural elements (lithologic boundaries, tectonic disturbances, individual ore zones and ore bodies);

direct prospecting for blind skarn bodies and skarn bodies buried beneath unconsolidated deposits, containing sulfides or magnetite;

study (in section) of eluvial and alluvial gold placers, and study of the oxidation zone of ore deposits;

age correlation of certain magmatic complexes that differ in the character of their magnetism (by the value of the Koenigsberger factor).

Here, first of all, magnetometry is used, followed by electrical prospecting and geochemical methods. Among electrical-prospecting methods, vertical electrical sounding, electrical prospecting by the natural-field method, and induced polarization are applied. The field of application of various modifications of electrical profiling is limited by the abundance of false anomalies caused by the complexity of the geological structure. Among geochemical methods, hydrogeochemistry, heavy-mineral metallometry, and metallometry by primary dispersion halos are effective. Ordinary metallometry by secondary halos, however, is by no means always effective because of the excessively moist landscape.

Examples of successful use of geophysical methods for

Change in the magnetization of rocks in the Earth’s magnetic field

Table

Note: The rock may act synchronously or sequentially, but most often the process proceeds with magnitudes or effects opposite to one another in magnetic sign. Alongside magnetic transformations of the rock substance itself, there are also ferromagnetic minerals present in them—ferrimagnetics, magnetites.

There is practical experience in resolving the questions listed above from work carried out at the Sinyukhinskoye, Lebedskoye, Ulmenskoye, Murzinskoye, and a number of other deposits and occurrences in the Altai Mountains. It is enough to say that one of the two skarn deposits currently being exploited in Altai—the Lebedskoye deposit—was discovered solely with the aid of magnetic surveying (R. A. Pipar, L. N. Pipar), which revealed gold-bearing, magnetite-containing skarn bodies here.

However, although gold-bearing magnetite skarns are not unique in our country, within the total mass of skarn ores they are nevertheless an exception. What is typical and characteristic of the Altai Mountains—as of many (if not all) ore provinces—is the nonmagnetic character of gold ores of the most varied types. The same may be said of other ores as well—copper, polymetallic, and rare-metal ores. As a rule, the altered near-ore magmatogenic rocks of deposits of most gold-ore formations are also nonmagnetic. Many carbonate and carbonaceous-clayey rocks favorable for ore formation, as well as rocks that have undergone various kinds of tectonic effects, are also distinguished by reduced magnetism. It is not difficult to see that the factors that demagnetize rock during ore formation are more numerous and less energy-intensive than the factors leading to an increase in magnetization (see the table). Therefore, negative magnetic anomalies, zones of relative minima of the magnetic field, or simply local areas of quiet field standing out against a high background may be regarded as a universal indirect criterion in the search for deposits of gold, nonferrous and rare metals, and a number of other useful minerals. The exceptions are ores of strictly magmatic (Talnakh-type) and essentially pyrrhotite massive-sulfide formations.

The thesis that local minima of the magnetic field can serve as a prospecting criterion is confirmed both by the practice of investigations in the Sinyukhinskoye and Lebedskoye ore fields of Altai and in many other regions (Central and Eastern Kazakhstan, the Urals, Transbaikalia, Primorye, etc.).

Without dwelling in detail on the methodology of the work, we shall note only the following circumstances, which in our view are important:

1. Geophysical investigations must be carried out comprehensively, using all modern capabilities of geophysics, petrophysics, and geochemistry (including mine and borehole methods).

2. Interpretation of the results of geophysical investigations must be carried out jointly by geologists and geophysicists, on the basis of broad generalizations, using the experience of similar investigations in other ore provinces with a comparable metallogenic profile.

3. To increase the effectiveness of the geological study of Altai, it is necessary to eliminate the lag of geophysics behind geology.

The broad and integrated use of geophysical methods will lead to a sharp increase in the effectiveness of geological investigations in the Altai Mountains.