THE ROLE OF MAGNETIC PROSPECTING IN FORECASTING AND EXPLORATION FOR GOLD DEPOSITS IN THE ALTAI MOUNTAINS

F. B. Baksht (ZSGU)

The association of gold mineralization with magmatic rocks of increased basicity is well known. Both quartz-vein and skarn and kolchedan deposits are genetically and spatially associated with moderately acidic granites, granodiorites, and also with diorites and their effusive analogues. This regularity has been established in many regions of the Soviet Union and abroad. Many gold-ore deposits of the Altai also gravitate toward intrusive massifs of melanocratic appearance (Sinyukhinskoye, Lebedskoye, Ulmenskoye, Murzinskoye).

All these rocks, containing increased amounts of iron, possess more or less significant magnetism. As is known, the magnetism of magmatic rocks generally increases with their basicity. This is also true for the Altai Mountains. Here, too, we observe a regular increase in the values of magnetic susceptibility and remanent magnetization in rocks of the magmatic series granite—granodiorite—diorite—gabbro. This is indicated by data from the study of more than 2,000 specimens collected in the area of the Sinyukhinskoye and Ulmenskoye skarn-ore fields.

At the same time, the most magnetic are always small intrusions of the diorite, gabbro, and pyroxenite types and the marginal parts of large granitoid batholiths; magnetic anomalies are also observed everywhere over rocks of the porphyry formation.

The conclusion suggests itself: since magmatic complexes accompanying gold-ore deposits in most cases have increased basicity and magnetism, we must, first of all, direct prospecting toward areas of positive magnetic anomalies¹. This proposition appears to us fundamental in using magnetometry data for forecasting and prospecting for gold-ore deposits. It is precisely this associative connection—“gold—iron—magnetic anomalies”—that explains the facts of coincidence between the areas of greatest distribution of placer and primary gold deposits and areas of an elevated magnetic field. Attention was drawn to this by V. V. Syrovatskii, who studied the regularities of gold mineralization in the Kuznetsk Alatau, Salair, and Gornaya Shoria; D. M. Pecherskii (Northeastern USSR); and E. E. Fotiadi (Siberia and the Far East).

A close spatial connection between gold placers and positive magnetic anomalies is also found in the Gorny Altai (Lebed district, Ulmen–Sinyukha zone, etc.).

Obviously, such a connection should exist not only for placer deposits but also for primary gold deposits; indeed, all the most significant deposits and ore occurrences of gold of the skarn and pyrite types described in the Altai by B. N. Luzin, Yu. I. Tveritinov, Yu. G. Shcherbakov, and others are located within zones of positive magnetic-field values (data of V. I. Serkov, I. V. Shestakov, and the author).

Thus, aeromagnetic survey data can already serve as a basis for forecasting. Together with gravimetric survey materials and the results of structural–formational analysis, they can be used in compiling small-scale gold prospectivity maps, especially for the middle- and high-mountain parts of the Altai, where such important indicators for gold prospecting as placers are absent.

Geophysical materials, considered jointly with the results of geological and geochemical investigations, make it possible to identify more confidently deep-seated faults and the mobile zones accompanying them. In this work, use is made of extended zones of large gradients of the magnetic and gravitational fields, linear narrow chains of magnetic anomalies, elongated zones of magnetic-field minima, etc. Using these criteria, it is possible to trace more reliably over great distances faults of different orders inferred from geological data, to refine their position, and to shed light on many details of the geological-structural plan of the Gorny Altai that until now have been considered problematic. For example, gravimagnetic data confirm the suggestions of G. L. Pospelov and Yu. M. Tveritinov concerning the presence of sublatitudinal structures in the Gorny Altai. The significant role of sublatitudinal structures in the northern part of the Gorny Altai is indicated by the results, described by us earlier, of geophysical investigations of the Sinyukha and Ulmen gold-ore fields, as well as of the area of the upper course of the Biya River.

Thus, magnetometry provides substantial assistance in mapping the major elements of the tectonic structure of the Gorny Altai. It confirms the existence of both near-meridional and sublatitudinal structures (the latter are more ancient). Gold deposits of the contact-metasomatic type are confined to the junction nodes of structures of this kind.

These deposits are located in the exocontacts of large, multiphase granitoid batholiths of complex composition. Such massifs are almost always clearly mapped by magnetic surveys of large and medium scales. Among them are the Sarakoksha, Turochak, Aisky, and a number of other massifs, which are recorded on magnetic maps by elevated (up to 500–800 gammas) magnetic fields. In this case, magnetometry makes it possible not only to outline these massifs but also to subdivide them by composition, as well as to mark the extent of the zones controlling gold mineralization. This is of very great importance under the specific conditions of the northern Gorny Altai, distinguished by very poor exposure.

Thus, for example, with the aid of magnetometry it proved possible to make a positive assessment of the prospects for discovering new skarn gold deposits in the area of the Sinyukha ore field. It was established that the ore-controlling contact of the Sarakoksha massif extends beyond the limits of the ore field much farther than had been assumed from the scanty geological data available. In general, the near-contact zone, toward which numerous gold placers and remnants of skarnized country rocks gravitate, is marked by a single latitudinal belt of very high (up to

several thousand gammas) and a strongly differentiated magnetic field. This is explained by the presence, in contact with the granite massif, of hybrid rocks of basic and intermediate composition.

Another example of the use of magnetometric data in the search for gold in the area under consideration is the Ulmen deposit. As in this entire part of the Altai, exposure here is very poor. Therefore, carrying out geological prospecting and mapping by conventional methods requires large expenditures for mining work. With the aid of magnetometry, however, it proved possible to map the Ulmen diorite massif with an accuracy generally satisfying the requirements of geological mapping at a scale of 1 : 10000. In most cases the discrepancy between geological and geophysical data is 20–40 meters (the Central, Southern, and other sectors). In addition, with the aid of magnetometry it proved possible to determine the dip direction of the contacts of the diorite massif, to obtain much additional information on the tectonics of the ore field, the structure of the massif, etc. On the basis of the geophysical data, taking into account the available geological data, it was possible to compile a schematic geological map of the deposit area and to determine the direction of further prospecting.

As one of the most important criteria in the search for new ore-bearing sectors during large-scale mapping, zones of magnetic-field minima are considered. This is based on data from the study of the magnetic properties of the rocks and ores of the area.

The point is that, whereas gold-ore fields as a whole are associated with magnetite-bearing (that is, magnetic) rocks, individual ore-bearing sectors that have undergone various metasomatic or hydrothermal alterations are often depleted in iron oxides. Both in the skarn deposits studied by us and in other areas, magnetite in the near-ore rocks is replaced by sulfides (pyrite) and ferruginous silicates (garnet, chlorite, epidote, etc.). Sometimes the iron is removed altogether into the enclosing rocks. Processes leading to demagnetization of rocks are also observed in the rocks enclosing pyrite ores. Finally, the loss of magnetism by rocks may be caused by purely physical reasons, governed by crushing processes in ore-hosting tectonic zones.

Demagnetization processes may proceed with varying intensity depending on the specific geological setting, which is especially clearly manifested in secondary alteration of rocks of the vein facies. The magnetism of dikes of various porphyries in the Sinyukha ore field depends on their own composition, on the composition of the enclosing rocks, and on the thickness of these dikes. It is possible that in this case migration of iron also occurs during the formation of the dikes. The concentration of magnetite in the same rocks may vary not only depending on the composition of the enclosing rocks and the dimensions of the bodies, but also on their tectonic position, as, for example, was established for granite massifs in the northeast of the USSR by D. M. Pechersky.

Along with a decrease in the concentration of magnetite (a decrease in magnetism), the opposite process is also frequently observed—the introduction of iron, which leads to an increase in magnetism.

However, in the case considered by us, the first process—the demagnetization of initially magnetic rocks—predominates, and this is currently the principal one for us. The aureole of weakly increased magnetism of marble near the skarn bodies observed at the Sinyukha deposit is measured in only a few meters and cannot change the general pattern of the magnetic field.

Owing to this, during detailed geological-geophysical mapping of ore fields, zones are recommended for priority checking...

local minima of the magnetic field, situated above fields of preferential development of rocks of the sedimentary-metamorphic complex. Although in this way we are unable to distinguish marbles, tuffs, and various metasomatites from one another, nevertheless by this means we considerably narrow the areas for searching for skarn-ore sites (or, in another case, zones of presumed pyrite mineralization).

The possibilities of magnetic prospecting are greatly expanded by combining it with other geophysical and geochemical methods. Especially important here is the role of electrical prospecting in its various modifications. In structural mapping of the Sinyukhinskoye and Ul’menskoye skarn-ore fields we successfully used VES and electrical profiling with alternating and direct current. A large and substantial role in direct prospecting for sulfide ores of skarn and pyrite types is played by electrochemical methods based on the ability of these ores to polarize—the EP and IP methods. In this case the choice of areas for carrying out electrical-prospecting work was always made on the basis of magnetometric data, in accordance with existing ideas about the geological structure of a given site.

Electrical-prospecting anomalies, often coinciding with zones of magnetic-field minima, made it possible to identify a number of points of sulfide mineralization promising for the search for gold-bearing ores of the pyrite and skarn types (the Ul’meno-Sinyukhinskaya zone, the area of the city of Gorno-Altaisk, and others).

Forecasts made from geophysical data are well confirmed by subsequent geological verification. In the Sinyukhinskoye ore field, in each 8 epicenters out of 10 checked by drill holes or pits, previously unknown bodies of skarns or skarn-bearing host rocks were exposed (marbles, tuffs, or metasomatically altered, strongly bleached porphyries). In particular, the supposition was confirmed concerning the prospectivity of the western flank of this deposit, where, as it turned out, the productive marble horizon lies beneath a cover of crystal-lithic tuffs and altered labrador porphyrites.

Sometimes magnetic prospecting, in combination with electrical prospecting and geochemistry, is a means of direct prospecting for gold-ore bodies of various types. These include, first of all, gold-bearing bodies of magnetite skarns, which are rarely encountered at the Sinyukhinskoye deposit but are typical of the Lebedskoye and, apparently, some other deposits. At one of the local and very intense (tens of thousands of gammas) anomalies, gold-bearing magnetite and magnetite-garnet skarns were uncovered. Consequently, such anomalies may indicate the presence of independent ore bodies or serve as a prospecting criterion for discovering new ore sites composed of nonmagnetic skarns.

Nonmagnetic quartz veins occurring among weakly magnetic granites and granodiorites are very well recorded by magnetometry. With the aid of magnetometry, at one of the sites we succeeded in tracing along strike a series of quartz veins for more than 1200 m. Finally, we have positive experience in applying magnetometry in the search for and tracing of gold-bearing pyrite deposits.

From all that has been said above it follows that magnetometry can serve as an effective, mobile, and inexpensive means of searching for and forecasting gold deposits of all types known today in the Gorny Altai.

At the stage of small-scale forecasting we use the connection of gold mineralization with elevated magnetic fields, while in searching for individual ore sites and in large-scale geological-structural—

In [[unclear: word ending in “-nom”]] mapping of ore fields, we are attracted by zones of minima of the magnetic field.

A necessary condition for the maximum effectiveness of using magnetometry for these purposes is, on the one hand, the prompt and competent verification of the anomalies detected and, on the other hand, an in-depth study of the entire body of geological and geophysical information for the area. By combining the efforts of geologists and geophysicists, we will be able to achieve new successes in developing the gold-ore base of the Altai.

1. Author’s note. We believe that cases of reverse magnetization and the negative magnetic anomalies caused by it are relatively rare. ↩