Endogenous Gold-Ore Mineralization in the South of Tomsk Oblast

G. M. Ivanova, S. S. Il’enyuk (TPI)

The rocks of the Paleozoic basement in the south of Tomsk Oblast form the so-called Tomsk uplift. Its eastern part is composed of the Salairides of the northern continuation of the Kuznetsk Alatau, of meridional strike, locally overlain by platform Devonian sediments. In the area of the Ob–Tom watershed, west of the city of Tomsk, are situated the Caledonides of the Pritomsk block. Between the two rigid blocks lie the Hercynides of the Kolyvan–Tomsk folded zone. They are characterized by intense tectonics and form a steep, complex synclinal structure complicated by linear folds of the second order. The zone extends for hundreds of kilometers in a northeastern direction; only near the margin of the Salair Ridge does it change its strike to meridional. Its geological structure includes sandy-clayey deposits of the Upper Devonian and Lower Carboniferous.

Second-order folds are, as a rule, accompanied by fractures and cleavage of the rocks. Later disjunctive disturbances have a northwestern, or sublatitudinal, strike, i.e., perpendicular to the general strike of the rocks. These fractures are filled by dike rocks of monzonite–essexite–diabase composition, with body thicknesses of 3–15 m, up to 30–100 m, and steep vertical dip. The most substantial zone of this direction is a deep-seated fault located in the basin of the Tugoyakovka River and extending, according to geophysical data, for 100–180 km. It has been established that this fault is traced by a large intrusive body extending for 100 km from west to east. The main part of the body is located at a depth of 0.6–0.7 km.

The rocks of the Paleozoic basement in the south of Tomsk Oblast are exposed to a significant degree only along the Tom River and its tributaries. On the watershed areas they are usually covered by a thin mantle of loose deposits, at the base of which lies a weathering crust 5 m to 30–40 m thick.

Three types of endogenous gold-ore mineralization are distinguished in the area: quartz-vein, disseminated and veinlet-disseminated polymetallic with gold, and gold–stibium–mercury.

The quartz-vein type of gold-ore mineralization is widely devel-

is developed in the area. Quartz veins and veinlets, abundantly encountered in outcrops along the Tom, Basandayka, Ushaika, Tugoyakovka, Kamenka, Kirgizka, and other rivers, usually have a small thickness, from several cm to 0.4–0.5 m. They cut the sandy-clayey shales of the Lower Carboniferous, more often being confined to sandstone interbeds. Their strike varies, but is most often NE and NNE. Finds of visible gold have been encountered by a number of researchers and by us in quartz veins on the right side of the valley of the Tom River in the area of the Camp Garden and upstream from it, on the Ushaika River, and also along the Tugoyakovka River in the area of the village of Larino. The presence of gold in quartz veins in the basins of the Kirgizka and Kamenka rivers is also indicated by A. A. Mesyaninov. The results of fire-assay analyses, carried out selectively, showed a noticeable presence of gold and, in a number of samples, also silver.

Heavy-mineral concentrate sampling carried out by us, as well as by geologists of the Tomsk Integrated Expedition, showed the presence of gold in almost all the small rivers—right tributaries of the Tom River: the Basandayka, Bolshaya and Malaya Ushaika, Kirgizka, Kamenka, Tugoyakovka, Sosnovka, and also along the right bank of the Tom River. The largest number of concentrates with gold (up to 70–80%) was encountered in the basin of the Ushaika River in its upper reaches, and also in the lower course, in the sector from the village of Stepanovka to the city of Tomsk; along the Basandayka River in its lower and middle courses; along the Kamenka River in the area of the village of Semiluzhki; along the Tugoyakovka River from the mouth of the Konev key to the mouth of the Kuzmin key, and also in the keys flowing into the Tugoyakovka River (Targanak, Kuzmin, and others). The greatest number of indications of gold in the concentrates was encountered along the Basandayka River (between the villages of Pisarevo and Anikino) and in the area of the village of Larino along the Kuzmin key. Gold is found here most often in the form of unrounded plates up to 0.6–0.8 mm in size.

The results of heavy-mineral concentrate sampling, carried out by the Tomsk Integrated Expedition in the floodplain and terrace deposits of the Shcherbak, Omutnaya, and Kitat rivers, also showed the widespread presence of gold. In particular, 51% of the concentrates from floodplain deposits contain gold, and its amount sometimes reaches high values. More than 80% of the concentrates washed from the terrace deposits of these rivers also contain gold, often with significant concentrations. In Quaternary deposits, gold was encountered in 75% of all washed concentrates.

Thus, the floodplain, terrace, and Quaternary deposits of numerous rivers of the Tom–Yaya watershed in the south of Tomsk Oblast are practically entirely contaminated with fine, dustlike gold. It is noteworthy that the gold in the concentrates is either weakly rounded or not rounded at all, and is usually found together with fresh pyrite, which occurs very often in quartz veins. Hence it is most probable that the gold here was derived precisely from local quartz veins, which are widely developed in the area and cut the Carboniferous deposits. The highest concentration of gold in the veins is observed in sectors of contacts between dikes and the enclosing rocks.

The second type of ore occurrences, in the form of disseminated and veinlet-disseminated polymetallic mineralization with gold, is manifested rather frequently. A number of polymetallic ore occurrences are located in the area: Tugoyakovskoye, Turuntayevskoye, Omutninskoye, Shcherbakskoye, Baranovskoye, and others. In all these ore occurrences the presence of gold is noted [3], and in Turuntayevskoye, according to data of the Tomsk Integrated Expedition, also silver. In particular, during our study of polymetallic mineralization in the Tugoyakovka sector, fine visible gold was encountered in the core of eight boreholes (borehole 134 at a depth ...).

107–108 m, borehole 136 at 16.7–18.5 m, 37.5–40 m, 90.3–92.5 m, borehole 120 at a depth of 104–111 m, borehole 142 at 37–40.8 m, borehole 115 at 103 m, borehole 126 at 145 m, borehole 103 at 64, 3, 69, 6, 90–94.5 m). Spatially, the gold is confined to veinlet ore mineralization developed in quartz or quartz–carbonate veins and veinlets, or in zones of crushing and intense brecciation of the enclosing rocks. A characteristic feature of native gold in such mineralized areas is its close association with sulfide minerals: sphalerite, galena, and chalcopyrite. Gold is present here in the form of small isometric grains with irregular boundaries and ranging in size up to hundredths, sometimes tenths, of a millimeter.

One such ore occurrence, the Tugoyakovskoe, is associated with a deep fault of the same name. The greatest concentration of ore mineralization here is confined mainly to the nodes of intersection of submeridional fracture zones with zones of sublatitudinal strike. We have identified and studied about 7 ore-bearing zones of submeridional strike. It has been established that the mineralization of these younger zones does not remain qualitatively constant. Two types of ore mineralization are distinguished here: a) lead–zinc with gold, b) copper–lead–zinc. The first is confined to zones of disturbance that intersect both the Tugoyakovskaya intrusive belt and the powerful dyke belt situated farther north; the second is encountered in the zone of disturbances cutting the latter.

The third type of gold mineralization belongs to gold–antimony–mercury mineralization, which has been identified mainly in the large Kolarovo–Semiluzhki fault zone of north-northeastern strike. Such mineralization was first discovered in 1947 in the area of the village of Semiluzhki and was explored by the geologist A. A. Mesyaninov. Ore mineralization of the same type was found by us along the Basandaika River. In bedrock outcrops here we found cinnabar. This is the first discovery of cinnabar in bedrock outcrops in the district; previously it had been noted only in heavy-mineral concentrates. The Basandaika ore occurrence, as well as finds of ore occurrences along the Ushaika, make it possible to extend with greater confidence the Kolarovo–Semiluzhki zone, distinguished by M. P. Nagorsky, to the south-southwest. Apparently, we are dealing with an ore zone of enormous scale, more than 35 km long and of considerable thickness. The indicated dimensions refer to the traced part of the zone. Apparently, the zone and ore occurrences within it should be traced farther both in the NE and in the SW directions. The presence of this ore zone is proven by chains of ore outcrops. Thus, in the northern part of the zone the Semiluzhki deposit is known, which has been explored. Farther south, ore occurrences have been discovered along the Malaya and Bolshaya Ushaika rivers, the Basandaika, and near the village of Kolarovo. In the area of the village of Zavarzino radon waters are known. The outlets of the latter form a belt oriented along the indicated fault zone. In the alluvium of all rivers crossing the Kolarovo–Semiluzhki fault zone, cinnabar, antimony glance, and gold have been discovered as a result of the heavy-mineral-concentrate method of prospecting. This fault zone is also rather well expressed in the modern relief, being traced by lake-like expansions of river valleys and by a sharp change in the direction of the streams within the zone.

The localization of ore mineralization, apparently, may be expected at places where the main fault intersects subsidiary tectonic disturbances and dykes having a northwestern strike. It is precisely with such a tectonic node that the ore shoots in the Semiluzhki deposit are associated. The enclosing rocks here are represented by disrup-

with brecciated calcareous-clayey shales. Through the work of A. A. Mesyaninov the mineralized zone has been traced for 150 m, but it apparently extends farther. The area of the outcrops of the ore columns is estimated approximately at 50 m²; the depth of their distribution has not been determined. The antimony content ranges from 0.98% to 21.4%. Three types of ores have been distinguished: 1) quartz ores with stibnite (lenses of crushed quartz containing the ore mineral along cracks); 2) rich breccia stibnite ores, represented by crushed pyritized calcareous-clayey shales cemented by quartz and stibnite; 3) brecciated mineralized rocks with poor disseminated mineralization. The antimony reserves of this deposit have also been roughly calculated. However, it should be noted that in fact this deposit remains poorly studied. The extent of the identified mineralization and its depth have not been determined or traced, and the ores of the deposit also remain unstudied. Finally, it is necessary to note that gold has also been found in the quartz bodies. Thus, A. A. Mesyaninov notes that he encountered gold not only in panned concentrates at the deposit site, but also in drill cores. However, the latter were not subjected to special study.

The Basandaika ore occurrence, first discovered by us, is located in the middle course of the Basandaika River near the village of Anikino. It is represented by quartz veins cutting sandy-clayey shales of the Lower Carboniferous. In the quartz, disseminations of cinnabar and antimonite are observed. Gold was found in the panned concentrates.

Considering the area of the south of Tomsk Oblast as a whole, the following most interesting objects can be distinguished: 1. The Kolarovo-Semiluzhenskaya ore zone, which has a complex type of mineralization (gold, mercury, antimony). In it it is necessary to search for lithological traps in the form of limestone and sandstone horizons, as well as folds, which may be favorable for the localization of mineralization. 2. In the area of distribution of gold-polymetallic mineralization, it is necessary to note the Tugoyakovskoye, Turuntaevskoye, and Baranovskoye ore occurrences. 3. Gold-bearing quartz veins developed in the area are of undoubted interest.

At the same time, it should be noted that the placers of the area have not been studied. In addition to small rivers, a detailed investigation of the gold content of the terraces of the Tom River is necessary. But the most interesting object, in our view, is the coastal-marine and alluvial zircon-ilmenite placers, which have remained completely unstudied for gold; at the same time, in this respect they may prove to be very promising.

LITERATURE

1. Ivanov K. V. Some questions of the petrography of dike rocks in the vicinity of the city of Tomsk. Proceedings of TSU, vol. 135, 1956.

2. Kuzmin A. M. Upper Paleozoic mineralization in the vicinity of the city of Tomsk. Geology of Ore Deposits, No. 2, 1961.

3. Levashov B. M., Parshin P. N. On the gold content of the Kolyvan-Tomsk folded zone. Bulletin of the Tomsk Polytechnic Institute, vol. 134, 1968.