On the Basandayka Antimony–Mercury Ore Occurrence in Tomsk Oblast

G. M. Ivanova

(Presented by the scientific seminar of the Department of Petrography)

As a result of field observations connected with the author’s investigations of endogenous ore formations in the south of Tomsk Oblast, in the summer of 1967, together with geological technician S. E. Ivanova, we succeeded in discovering the Basandayka antimony–mercury ore occurrence.

This ore occurrence is situated along the Basandayka River, 1.5 km upstream from the village of Anikino. The ore occurrence is represented by quartz veins in which ore minerals of antimony and mercury—antimonite and cinnabar—were encountered. The ore occurrence was discovered at a place where both slopes, composed of bedrock, rise rather steeply from the water’s edge; the floodplain at this place becomes rather narrow, and then widens considerably. Here the river forms loop-shaped meanders. The outcrop of bedrock at the base of the slope is 20 m and 14 m long, and 5–7 m high (Fig. 1).

Exposed are mainly clayey shales interbedded with sandstone, belonging to the Visean stage of the Lower Carboniferous. Their strike is northeastern, 20°; the dip is to the northwest at an angle of up to 87°. In outcrop 18, numerous quartz veins and veinlets, with thicknesses from 1–2 mm to 5–8 cm, are encountered rather frequently across the strike of the rocks. The bedrock outcrop at observation point 19 is represented by intensely weathered clayey shales and sandstones, often altered to the state of clay. It was not possible to measure their bedding elements here. Among these rocks quartz veins are also encountered, but they are intensely destroyed; one of them has a thickness of up to 25 cm, and its dip appears to be southeastern at an angle of 70–75°.

The clayey shales of the enclosing sequence have a dark-gray color and a shaly texture. Under the microscope it is found that the main mass is a weakly polarizing aggregate consisting of the finest grains of quartz, chlorite, and carbonaceous matter, immersed in clayey material; at the same time, small flakes of chlorite, quartz grains, and carbonaceous-clayey matter are usually oriented along the schistosity.

Microscopically, the sandstones are represented by gray or dark-gray layered rocks with a fine-grained structure. Under the microscope they reveal a fine-grained clastic structure and relatively poor sorting of the clastic material. The clastic material is represented by quartz, feldspar (plagioclase and potassium feldspar), and fragments of such rocks as microquartzites, acidic effusives with a microfelsitic structure, and plagioclase porphyries. Their cement is siliceous, of a regeneration character.

The quartz composing the veins and veinlets in the outcrop is usually porous—

Fig. 1. I. Plan of the location of outcrops 18 and 19 along the bank of the Basandayka River (1 km below the village of Klyuchi). II. Section of outcrop 18. III. Section of outcrop 19. 1 — soil-vegetation layer; 2 — deluvium; 3 — weathering crust of clayey shales and sandstones; 4 — clayey shale; 5 — sandstone; 6 — bedded sandstone; 7 — weathered quartz vein with visible antimonite; 8 — quartz veins; 9 — sampling sites (channel and hand specimens).

…ty, cavernous with epidote, impregnated with iron hydroxides. In addition to brown and yellow ochres, represented mainly by limonite and hematite, red ochres are present here. In the quartz vein of outcrop 19, visible microscopic nest-like disseminations of antimonite and fine disseminations of cinnabar were encountered.

Antimonite is represented by a nest of small acicular, radiating crystals of grayish-bluish color with metallic luster and perfect cleavage. The size of the nest is 1.5 cm; the length of an individual crystal reaches 0.8–1 cm (Fig. 2).

In addition, as a result of weathering, antimonite is often destroyed and leached out, so that only relict cavities remain here, having the form of needles and filled with bright-yellow, locally reddish ocher, as well as brown coatings of iron hydroxides.

Fig. 2. Prismatic radiating crystals of antimonite. Outcrop 18.

(Fig. 3). Examination of such specimens in reflected light showed the presence here of fine relict disseminations of antimonite. Antimonite was also encountered in one more specimen in the form of a small nest 0.5 cm across, also composed of an aggregate of fine radiating crystals. This mineral is rather brittle, with a dark-red streak. In reflected light the mineral shows a white color and a fairly strong luster. The reliability of the mineral determination was checked by etching it with concentrated KOH, as a result of which an orange-yellow precipitate formed.

Cinnabar was found in one specimen as a fine dissemination, likewise of nest-like character, together with antimonite. The color of the mineral is bright red, with a strong adamantine luster. In polished section it is seen that the mineral forms small isometric grains with slightly serrated edges, in accumulations forming a mosaic texture (Fig. 4). Along with the indicated ore minerals, pyrite and marcasite were also found in polished sections. It should be noted that gold is encountered rather often in panned concentrates from the Basandayka River near this ore occurrence.

In addition to the Basandayka ore occurrence, a large outcrop of ore of the indicated type had previously been known to a number of researchers of the Tomsk Polytechnic Institute and geologists of the Tomsk Geological Expedition in the area of the village of Semiluzhki, where exploration work using drilling was carried out in 1947 by geologist A. Mesyaninov. He determined

Fig. 3. Prismatic crystals of antimonite in quartz. Outcrop 19.

Fig. 4. Dissemination of cinnabar (black) and antimonite in quartz. Occurrence 18.

the latitudinal strike of the ore zone, but the ore outcrops in this direction were lost. An analysis of A. Mesyaninov’s materials, carried out by geologist M. N. Nagorskii of the Tomsk Integrated Expedition, showed that A. Mesyaninov’s interpretation was incorrect. M. N. Nagorskii was the first to determine the northeastern strike of this ore-bearing zone, giving it the name Kolarovo–Semiluzhki. New finds of ore outcrops discovered by us make it possible to extend this zone more confidently to the southwest, where signs of antimony mineralization are also noted in the area of the village of Kolarovo. The existence of this zone is also confirmed by the presence within it of the Zavarzino radon-water deposit. Although the Zavarzino waters are associated with faults of northwestern strike, in general they form a belt along the Kolarovo–Semiluzhki fault zone. According to our data and the data of geologists of the Tomsk Integrated Expedition, as well as of some staff members of TPI, cinnabar, antimonite (stibnite), and gold have been found by the heavy-mineral method of prospecting in the alluvium of the small rivers B. and M. Ushaika, Basandaika, Kirgizka, Kamenka, and others, which cross the Kolarovo–Semiluzhki fault zone. In addition to antimony–mercury mineralization, gold is also present in the area of the Kolarovo–Semiluzhki tectonic zone, occurring not only in heavy-mineral concentrates (A. M. Kuzmin, 1919) but also in drill cores. Thus, according to A. Mesyaninov, gold was encountered in the cores of 7 boreholes on the Kirgizka River. We found gold not only in heavy-mineral concentrates, but also in the cores of 8 boreholes located here together with polymetallic mineralization (middle course of the Tugoyakovka River).

The Kolarovo–Semiluzhki fault zone is also fairly well expressed in the modern relief, being traced by lake-like widenings of river valleys. All these data allow the conclusion that, in the area, there is apparently an ore zone of enormous length, reaching on the order of 35 km.