On the Issue of Predicting Primary Gold Deposits Based on Magnetic Survey Results

L. Ya. Erofeev

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

Magnetic surveys are now widely used for studying both the ore fields of gold deposits and areas promising for gold mineralization. In the latter case, the work is usually carried out at a scale of 1:10,000 and smaller. The results of these investigations are widely used in geological mapping, the ultimate aim of which is to determine the areas most promising for gold mineralization. In most cases these surveys provide useful information on the lithological structure of the areas studied, make it possible to determine the location of the most significant ruptural disturbances, and the dikes of strongly magnetic rocks. All this is sufficiently well known.

In the present work we shall dwell on the possibility of using the results of magnetic surveys for predicting and for the direct identification of zones of possible mineralization of certain primary gold deposits, a possibility which, as it seems to us, is not yet being properly realized at present.

The essence of this possibility lies in the fact that, for many primary gold deposits of the vein type, a characteristic feature in their structure is noted: for example, the presence of two dominant, mutually intersecting tectonic directions, or of one distinctly expressed direction. Investigations carried out on the structure of the magnetic fields of a number of vein-type gold-ore deposits, and their comparison with the geological structure, have convincingly shown that the structural features of the deposits are almost always reflected in practice in a noticeable way in the magnetic field.

It is quite obvious from this that, with the aid of magnetic measurements, in addition to the tasks indicated above, one can also solve problems connected with studying the structural features of promising areas and problems of searching for individual ore clusters within known deposits.

In solving these problems, special requirements must be imposed on the method of conducting the magnetic survey. In this case the survey must be carried out at a scale that should be determined from those regularities in the relation of the magnetic field to the structural features of the deposits that occur in the given district. At the same time it should be borne in mind that, for each deposit, one can indicate such a survey scale, any substantial deviation from

deviation of which in one direction or another will adversely affect the results of the study. According to our data, the scale of the survey when solving these problems at bedrock gold deposits of the vein type varies within wide limits and, basically, ranges from \(1:100\) to \(1:10000\). For example, to study the structural features of the granodiorite massifs of the Mariinskaya taiga, in which gold deposits of the Tsentralnoye and Komsomolskoye types occur, the survey may be carried out at a scale of \(1:1000\)—\(1:2000\), while under the conditions that exist in the Darasun area (Transbaikalia), measurements may be performed at a scale of \(1:2000\)—\(1:5000\).

In those cases where, for economic reasons, it does not seem possible to carry out measurements over the entire area under study at the required scale, measurements may be conducted on separate sites. The dimensions of the sites and the network of their placement within the area under study are determined in each specific case, depending on the dimensions of the ore clusters and the density of structural features reflected in the magnetic field. According to our data, for example, in searching for ore clusters within the Aprelkovo gold-ore deposit (Transbaikalia) and the Tsentralnoye deposit (Mar-taiga), the survey may be conducted on separate sites measuring \(20 \times 20\)—\(50 \times 50\) m, spaced \(100\)—\(200\) m apart. To solve these problems, as indeed in all cases of magnetic surveying, it is preferable to use a square grid. However, in many cases, when the area under study is characterized by the development of structures that are relatively stable along strike, a rectangular grid may be used. However, when a microsurvey is employed, the grid, as a rule, should be square.

There are also a number of specific features in the methodology for interpreting and presenting the research materials.

The survey results in this case, as is customary for areal measurements, should be presented in the form of an isoline plan. The isoline interval must be chosen as small as possible. This value must undoubtedly be determined with account taken of the accuracy of the measurements and the character of the variation in field intensity. The latter should be evaluated statistically, for example, by means of the autocorrelation function.

Qualitative interpretation of isoline plans from the positions indicated is, for the most part, difficult, which, incidentally, is also the main reason for the absence of an assessment of the prospects of areas studied by magnetic surveying in structural terms. To obtain obvious regularities in this case, one should use a quantitative method of statistical characterization of the directionality of isolines. For this purpose, the area subject to processing by this method, if it is large, may be divided into squares whose side length is determined by the obvious characteristic features in the isoline plan. For each such square or separate survey site, a rose diagram of the directionality of the isolines is constructed. During processing, isolines whose strike is clearly associated with the boundaries of individual rock massifs should, as far as possible, be excluded from statistical consideration. In the case where microsurvey sites are processed, the regional background must be removed. When the processed squares are large, in addition to the rose diagram of the directionality of the isolines, it is very useful to construct rose diagrams of the directionality of the isolines of the local relief and diagrams of the directionality of the boundaries of individual rock massifs.

After such processing, the study sites or squares are combined into groups according to selected characteristics, and for each group

a general diagram is constructed. Thus a map of features is constructed, according to which the area is subdivided by structural characteristics.

At present, in some gold-ore provinces, considerable areas directly adjacent to known ore fields have already been covered by magnetic surveying. This material undoubtedly needs to be reinterpreted from the positions set forth above, which may provide very useful information both on the structural characteristics of ore fields and on the structure of adjacent areas.

In analyzing this material it is necessary to take into account that the scale of 1:10,000, which was adopted for the most part in studying the surroundings of gold-ore deposits, is the maximum permissible for some types of ore fields, while for the majority it may prove to be much too small. Therefore, obtaining a negative result from these surveys in the detection, for example, of fault-intersection sites is not proof of their absence in the area under study. This is due to the fact that the distance between measurement profiles at such a scale is most often commensurate only with the largest changes in field intensity, which characterize the largest structural features of the area being studied.

Our observance of the above recommendations, for example at the Tsentralnoye, Darasunskoye, and Aprelkovskoye deposits, produced a positive effect in the study of the structure of the ore fields of these deposits.