On the Prospects for Prospecting for Pyrite Deposits in the Altai Mountains

F. B. BAKSHT, Yu. I. TVERITINOV, V. M. FISAK, G. F. KHAFIZOV (ZSGU)

Indications of pyrite mineralization in the Altai Mountains have long been noted. These include zones of disseminated pyritization, intensive bleaching of rocks, and “iron hats,” known in areas where Cambrian volcanogenic-terrigenous sequences are developed. Many investigators have associated placer gold mineralization with the erosion of these zones. Geologically and metallogenically, the Altai Mountains are in many respects similar to the adjacent regions of Tuva, Salair, and Rudny Altai, where deposits of the copper-pyrite type are known. The possibility of identifying analogous deposits in the Altai Mountains is supported by data from geological and geophysical investigations carried out in recent years in the northern part of this region.

In 1963–1966, areal geophysical prospecting for sulfide ores by the natural electric field method was conducted at a number of gold-bearing sites in Northern Altai. The prospecting was carried out in accordance with the following sequence of stages. The first stage consisted of reconnaissance work at scales of 1 : 200,000–1 : 50,000. The second stage was the detailed study of the anomalous zones identified, at scales of 1 : 10,000–1 : 25,000. The third stage was a comprehensive study of the anomalies along calculated profiles. Along with the usual methods (self-potential electrical prospecting and magnetometry), IP electrical prospecting, VES, and sometimes gravity prospecting and metallometry were used. The fourth stage involved, at anomalous sites, detailed heavy-mineral/metallometric work, opening the epicenters of anomalies by trenches, and drilling the anomalies with core boreholes.

As a result of the work carried out, intensive anomalies of the natural electric field (up to 400–700 mV) were for the first time identified and traced for many kilometers in the Altai Mountains, and the sulfide nature of some of them was established. Such are the anomalies of the Ulmen site, the Siya gold-bearing zone, and a number of anomalies along the western margin of the Katun anticlinorium (Maiminskaya, Ust-Yubinskaya, and others).

The anomalies are characterized by a linear form in plan and by very large gradients of the observed fields, reaching 10 or more millivolts per meter. The parameters of the anomalies practically do not change with time. All SP anomalies coincide with induced-polarization anomalies amounting to 15–17%. The minima of the natural field...

almost always accompanied by zones of a low magnetic field and by areas of increased electrical conductivity (according to EEP data).

The anomalous zones show a spatial connection with zones of deep faults of the Salair stage of emplacement. In a number of cases the lines of deep faults are clearly traced by chains of local magnetic anomalies caused by intrusive bodies of pyroxenites, gabbro, and diorites elongated along the strike of the faults. Deep faults along the boundaries of differently named structures are commonly marked by gravity steps.

In the anomalous zones there are rocks of the spilite-diabase, andesite, hyperbasite, and gabbro-diorite-diabase magmatic formations, characteristic of the early stages in the development of mobile zones and of all pyrite-bearing provinces of the world.

Anomalies of the natural field in the areas studied are localized exclusively within Lower–Middle Cambrian volcanogenic-terrigenous sequences, tending toward zones of schistosity, boudinage, and contacts of rocks having different mechanical properties. Most often these are contacts of sandstones and strongly schistose black siltstones, sometimes of schists and porphyrites. The dip of the contacts is usually steep. The anomalous zones are persistent along strike. Usually they have the form of narrow linear zones measuring 0.1–0.3 × 2–3 km and more, measured along the isoline of −100 mV.

In the surface horizons the anomalies are represented by weakly weathered rocks, often ferruginized. Sometimes rather strong bleaching and limonitization of the rocks are manifested. With depth, the bleached rocks are replaced by dark-gray and black schists, cataclastic sandstones, and intermediate-basic tuffs impregnated with sulfides: pyrite, pyrrhotite, marcasite, and chalcopyrite. Metallometry reveals low-intensity aureoles of lead in the anomalous areas.

Several generations of sulfides are established.* An early generation of pyrite and chalcopyrite is observed in sandstones in the form of syngenetic dissemination, cataclastic grains, and rotated clastic segregations in which a relict banded texture is preserved. The time of crushing and schistosity of the rocks corresponds to pyrite II—mainly millstone pyrite, developing along schistosity. Pyrite has a metacolloidal texture; sooty varieties predominate, making up as much as 50% of the rock and giving it a black color. In the schistose rocks, lens-shaped segregations of quartz with limonite and pyrite are often encountered; they represent fragments of quartz-sulfide veins torn apart and stretched along the schistosity. The schistosity is clearly cut by transverse calcite veinlets, with which pyrrhotite and chalcopyrite are associated. Pyrrhotite, in turn, is sometimes replaced by pyrite III, containing relics of pyrrhotite and chalcopyrite.

Near-ore alterations are expressed in silicification, sericitization, and chloritization of the rocks.

Chemical-analytical studies of the sulfidized rocks have not yet revealed in them industrial concentrations of copper, gold, silver, or other components. Moreover, detailed panned-concentrate and metallometric work in the Siya zone established that its gold content is not connected with erosion of sulfidized rocks recorded by EP anomalies. In the anomalous zones not a single geochemical—

* Description of the trenches was carried out by T. N. Sobol and A. L. Beskhodarnova. The polished sections were described by M. K. Ershova.

copper halo. Thus, for the time being it is not possible to speak with complete certainty about the prospects for the indicated metals in the detected EP anomalies. At the same time, there is no doubt that the sulfide occurrences exposed in the anomalous areas belong to the category of ore occurrences of the pyrite type. The composition of trace elements established in them is characteristic of “supra-ore” halos of pyrite deposits. According to the data of N. A. Belyaevsky, N. V. Petrovsky, and others (1963), pyritized rocks with the noted composition of admixtures are usually located above deposits of copper-pyrite ores and at distances from them of up to 300–400 m. It follows from this that EP anomalies and the sulfidization representing them in the territory studied may be used as indicators of pyrite deposits not exposed at the present-day surface. The large scale of the sulfidization occurrences serves as a prerequisite for the possible presence of large pyrite bodies in the region.

The question considered above was the assessment of the prospects for pyrite mineralization mainly in connection with EP anomalies. But in the same areas, zones of intense bleaching and “iron caps,” which do not produce anomalies in the natural electric field, are also widely developed. In these cases, material of the “cap” type extends to depths of several tens of meters from the surface, inaccessible to exploration pits. It is natural to suppose that in this case a greater intensity of oxidation (as compared with the anomalous areas) corresponds to a greater saturation of the rocks with sulfides. Indeed, in the brown iron ores of the described type of ore occurrences, relicts of leached sulfur pyrite are sometimes found (in particular, in the Ulmen area), containing 0.07–0.42% copper, “traces”—0.6 g/t gold, 0.03–0.1% silver, 0.01–0.03% bismuth, 0.01–1% zinc, 0.01–0.3% arsenic, etc. Placer gold content in the areas is definitely connected with the erosion of some “iron caps.” Sometimes copper geochemical halos are spatially associated with them. Thus, in comparison with the anomalous zones, these ore occurrences prove to be more interesting. They can probably be used as direct indicators of pyrite deposits that have an outcrop at the surface but have undergone deep oxidation in the near-surface horizons. Unfortunately, it is precisely these, the secondary ore occurrences of the pyrite type, that until now have not been subjected to serious assessment.

What has been set forth makes it possible to raise the question of the probable prospects of Northern Altai for deposits of the gold–copper-pyrite type and, in this connection, of the advisability of expanding geological-geophysical work primarily in those areas where signs of pyrite mineralization already exist (the Ulmen-Siinskaya zone, the lower course of the Katun River, and others).

It should be emphasized that it is necessary to carry out areal surveys in promising areas by the natural-field method, as the most effective means of detecting and tracing zones of sulfidization. It is also necessary to perform a complex of geological-geophysical investigations in areas where “iron caps” are developed but are not expressed in the natural field.

REFERENCES

Belyaevsky N. A., Petrovsky N. V., Solovov A. P., Khrushchov N. A., Shatalov E. T. Complexes of methods used in metallogenic study of covered territories and in prospecting for mineralization that does not crop out at the surface. In: Problems of the Study and Methods of Prospecting for Concealed Mineralization. Gosgeoltekhizdat, Moscow, 1963.