Influence of Neotectonics on the Formation of Placers in an Area of the Eastern Slope of the Kuznetsk Alatau

S. S. Ilyenok (TPI)

The eastern part of the Kuznetsk Alatau borders on the Minusinsk intermontane trough, within which a number of depressions are distinguished. One of the interesting areas of neotectonic manifestation, with which the distinctive structure and composition of gold placers are connected, is the boundary of the indicated ridge with the Tarchinsko-Chebakovsky sector of the North Minusinsk depression. Within the ridge, the valleys of the large rivers Bely and Cherny Iyus and their numerous tributaries contain remnants of fluvial accumulative terraces. Of these, of greatest interest is the clearly expressed high 30-meter terrace in the valley of the Izekiyul River and its tributaries, which possesses rich gold content. However, the main mass of gold is concentrated in the principal valley placer because of the reworking of the terrace deposits.

In the study of placers, the tectonic boundary between the depression and the ridge attracts attention. It consists of a series of subparallel fracture zones and separate weak disjunctives. The distance between individual zones reaches 5–6 km. Along the fractures, displacements with amplitudes from 1.5–6 m to 20–30 m are noticeable. Thus, a series of such displacements gives a stepped relief to the bed of the bedrock. For example, during the mining of the gold placer of the 30-meter terrace of Bolnichny Brook, a left tributary of the Izekiyul River, open workings exposed the bed of the bedrock, forming a series of steep tectonic steps as a result of the development of fault fractures. The amplitude of vertical displacements along the fractures is small, from 0.5–0.5 m to 1.5–2.0 m. It is interesting that the fractures are also traced in the loose lower deposits of the placer, but disappear higher up. The significant enrichment of the placer with gold beneath the tectonic thresholds clearly indicates that the fractures formed simultaneously with the formation of the placers.

In the valley of the same brook, near its confluence with the Izekiyul River, during the driving of an exploratory adit in the same terrace, a fault fracture was again discovered, and the face was stopped because of a sharp projection of bedrock. The fracture has a steep dip to the SW at an angle of 72°, a thickness of 4–6 cm, and a vertical displacement with an amplitude of 1.8 m. It contains tectonic clay with fragments of the enclosing rocks. The fracture is traced both in the bedrock and in the loose

alluvial stratum. In the very upper part of the section the material of the fissure undergoes deluvial displacement, bending toward the slope.

Another interesting area where young tectonics, synchronous with the formation of the placer, is manifested occurs along the valley of the Izekiul River, 4 km below the mouth of the Bolnichnyi creek. Here, for a long time, a rich gold placer was worked, which received the name “Rogozhina Pit.” Its characteristic feature is that the single-bed placer located along the Izekiul River, lying on the bedrock, passes lower down, within the area of the indicated deposit, into four thick beds separated by barren sands 2–4 m thick. If the gold-bearing bed identified by sampling along the Izekiul River had a thickness of 1.5–1.8 m, then the beds of the “Rogozhina Pit” area had swellings up to 3–8 m.

The cause of the formation of such a pit placer was the manifestation of a network of disjunctive fissures with vertical displacements. In this case, the appearance of four gold-bearing beds corresponds to four stages of movements along the fissures, with the appearance of a significant tectonic threshold that served as the cause of the accumulation of gold-bearing alluvium.

In the area of the left tributaries of the Chernyi Iius River above the village of Chebaki, a tectonic zone sharply expressed in the relief is manifested, cutting across the valleys of the Amartachul (Belaya Pilnya), Kolchanovskii, Inzhul, Malyi Chernyi Iius, Poputnaya Ivanovka creeks. At the place where the tectonic zone intersects carbonate rocks, especially in the valley of the last stream, karst phenomena are sharply manifested. Here, between slabs and feather-like projections of limestone, there are deep pockets filled with exceptionally rich gold-bearing alluvium. Workings in the form of pits sunk in individual pockets to a depth of 30–35 m did not reach the end of these pockets, which indicates their considerable depth.

This feature of the rich gold content in the filling of karst formations can be explained only by the fact that their development proceeded parallel with the formation of the placer. The appearance of small karst cavities and depressions in the initial stage of this process was accompanied by their filling with rich gold-bearing alluvium. Further dissolution of the limestone along fissures, possibly accompanied by new movements, led in either case to subsidence of the gold-bearing alluvium and to the appearance in the bedrock of a new trap, in which subsequent accumulation and localization of placer gold took place. Such a process proceeded in parallel and for a long time with the formation of deep pockets filled with alluvium with a small admixture of red-colored clay remaining from the dissolution of the limestone.

All valleys crossing the indicated tectonic zone are divided into two parts. The upper part is well expressed in the relief, and the search for placers here does not present particular difficulty. The second part of the valleys is situated below the tectonic zone and is peculiar. The watersheds here are weakly expressed. The thalwegs of the valleys are buried beneath a thick horizon of alluvium, products brought in by temporary water flows, and deluvial deposits. Prospecting for placers here is possible with the aid of geophysical methods. Thus, for prospecting a placer below the tectonic zone along the Amartachul (Belaya Pilnya) creek, we, together with M. A. Zablotskaya and A. A. Fedorov, applied the method of vertical electrical sounding. It made it possible, from the position of the bedrock, to trace the valley thalweg that controls the position of the gold-bearing placer. It turned out that in the area of the tectonic zone the placer has tectonic thresholds with a displacement amplitude of

to 20–30 m. At the same time, very strong enrichment of the placer with gold was also noted here.

Thus, the synchronous manifestation of young tectonics with the formation of placers in the indicated and adjacent areas leads to an interesting specificity of these placers in individual sectors. Thus, in places of fault tectonic zones separating the Kuznetsk Alatau from the intermontane downwarp, areas of intensive localization of placer gold (the “pit” zone) are developed beneath tectonic rapids and in zones of intensive karst development. Sections of valleys above tectonic zones of the indicated type are located within the range and have well-defined valleys with remnants of gold-bearing accumulative terraces. Other sections, which lie below the tectonic boundary, within the basin, are characterized by buried placers in broad leveled valleys. At the same time, farther downstream, because of frequent shifts of the channel, they show a scattered distribution of gold-bearing deposits and an overall depletion in gold. A sharper decrease in the grain size of the gold is observed and, at the same time, an accumulation of fine and flour-like gold, which should be taken into account when using methods for sampling placers. Even with this peculiarity, they may be promising targets for mechanized mining.