GOLD POTENTIAL OF QUARTZITES OF THE ALTAI–SAYAN REGION

V. A. SARAEV

(Presented by the scientific seminar of the departments of general geology, historical geology, and fossil fuels)

Rocks of essentially quartz composition are formed under a wide range of geological-structural, magmatic, mineralogical-geochemical, and physicochemical conditions. In nature, both magmatic quartzites¹ (for example, nordmarkites) and typical sedimentary rocks are known.

At present, gold potential has been established in many genetic types of quartzites [1—5, 8]. There are no data on the presence of magmatic quartzites in Western Siberia². Silexites (the pegmatite group of quartzites) also remain geologically insufficiently studied. The Pokrovskoe deposit of gold-bearing pegmatites in the Mariinsk taiga [1] may be assigned to the latter. Exploration work has exposed several lens-shaped silexite veins of great thickness. The silexites occur in pinkish-gray alkaline syenites. The quartz veins are noticeably mineralized, with the development of abundant disseminations of galena, yellow sphalerite, and pyrite. The ore bodies contain a noticeable admixture of feldspar; a gradual transition is observed from syenite-aplites into the ore body. The thickness of the silexites along strike is variable. They are localized in the central part of the syenite-aplites.

Among the hydrothermal-metasomatic group of siliceous rocks, the author distinguishes two formations [6]: secondary quartzites and quartzolites. With respect to gold potential, these formations are the most interesting.

Secondary Quartzites

Within the region, two genetic types of secondary quartzites are distinguished [4, 6]: halogen and sulfuric-acid. In the composition of the halogen quartzites the following mineral facies are distinguished: quartz, corundum, topaz, andalusite, diaspore, zunyite, pyrophyllite, and the outermost—sericite. In sulfuric-acid quartzites—quartz, kaolinite, barite, hydromica, and sericite. In the outer zones of secondary quartzites, propylitized rocks develop.

Secondary quartzites are spatially and genetically associated with volcanic regions and are localized in tectonically mobile zones, i.e., where processes of magma differentiation were manifested intensively, where subaerial volcanism took place. In the areas studied they show no confinement to intrusions. Deposits of secondary quartzites of the sulfuric-acid type are confined to zones of disjunctions and are localized in ancient eroded volcanoes of the Salair and Telyas cycles of tectogenesis. Deposits of secondary quartzites of the halogen type show the closest connection with explosion breccias and extrusive rocks of the orthophyre series (for example, in the Kondoma iron-ore district).

Secondary quartzites form after volcanites ranging from basaltic to orthophyric composition, but their main mass is formed at the expense of acid and acid-alkaline volcanites.

Under conditions of the geosynclinal-orogenic regime, secondary quartzites form repeatedly, with the maximum of their development observed in the final stage of Salair volcanism. It is interesting to note that deposits of secondary quartzites are often manifested near skarn zones (the Sheregesh–Shalym–Tashtagol iron-ore field), so that one may speak of a paragenesis of these ore-bearing formations [7].

Secondary quartzites form bodies of varied shape and different dimensions. Most often these are bedded and vein-like bodies with thicknesses from the first decimeters (meters) to several hundred meters. Their strike length ranges from several meters to several hundred meters (rarely the first kilometers). Such bodies are characteristic of quartzites of the sulfuric-acid type. Secondary quartzites of the halogen type are characterized by stock-like bodies up to 700 m in diameter (Mount Kubes near the Sheregesh mine).

The mineral composition of sulfuric-acid quartzites is as follows: quartz, alunite, hydromica, sericite, illite, kaolinite, barite, pyrophyllite; stilpnomelane (?); impurities—chlorite, calcite, dolomite, ankerite, pyrite, tourmaline, rutile, leucoxene, hematite. The mineral composition of halogen quartzites is quartz, topaz, andalusite, corundum, diaspore, zunyite, pyrophyllite, sericite; among the impurities, fluorite, tourmaline, pyrite, rutile, anatase, hematite, and zircon are known.

From the example of the study of secondary quartzites in Gornaya Shoria, the following complex of elements is identified in them: gold-copper-lead-zinc-silver-bismuth-tellurium-tin-arsenic-nickel-cobalt. Ore occurrences of gold, lead, zinc, copper, muscovite-hematite ores, and outcrops of alumina and refractory raw materials (pyrophyllite, kaolinite) are paragenetically associated with secondary quartzites.

The gold content, according to fire-assay data, ranges from traces to the first tens of g/t. In the fields of development of secondary quartzites, signs of the metal were found by heavy-mineral sampling. The gold is completely unworn, bright yellow in color. Grain size is up to 1–3 mm. Gold is localized mainly in monoquartzites and quartz-sericite rocks with sulfide mineralization. Industrial accumulations, however, have not been found.

Quartzolites

Numerous outcrops of quartzolites in the Altai-Sayan region are associated with the intrusion of intrusions of various composition and time of emplacement. Their confinement to zones of disjunctions in carbonat- is also noted.

… and volcanogenic sequences. They develop in various geological settings. The following geological-genetic types may be distinguished: 1) quartzolites in skarn zones; 2) quartzolites in the contact aureoles of granitoid intrusions; 3) quartzolites in tectonic zones at contacts between volcanites and carbonate rocks; 4) quartzolites in tectonic zones among carbonate rocks and, to a lesser extent, silicate rocks.

Especially favorable for the formation of quartzolites were conditions in which Upper Proterozoic and Cambrian strata of substantially carbonate composition were intruded by bodies of the gabbro–granodiorite–plagiogranite series and disrupted by disjunctive structures of various character.

The forms of quartzolite bodies are highly varied: from veins and lenses to stock-like, irregularly shaped deposits elongated along disjunctive structures. The thickness of the bodies ranges from several tens of centimeters to 100 and several hundreds of meters. Their strike length is up to several kilometers.

By structure, the following varieties of quartzolites are distinguished: 1) fine-grained, with preservation of the structural-textural features of the replaced rocks (0.01–0.1 mm); 2) medium-grained, with relicts of the structural-textural features of the original rocks (0.1–1.0 mm); 3) coarse-crystalline, without relicts of the original rocks (>1 mm).

The principal mineral of quartzolites is quartz, whose content ranges from 100 to 75%. Among the secondary minerals known are sericite, feldspars, biotite, amphibole, garnets, pyroxenes, dolomite, calcite, ankerite, barite, graphite, and epidote. Accessory minerals include apatite, leucoxene, zircon, magnetite, hematite, pyrite, and rutile. The silica content varies within the range 85–100%.

Quart zolites, usually weakly auriferous, are considerably developed in all mountain districts of Western Siberia. They have not been systematically studied. As a rule, coarse-crystalline and medium-grained quartzolites are auriferous. Weakly auriferous quartzolites are widely developed in the Olkhovka–Chibizhek district of the Eastern Sayan [1], in the system of the Alambay River in the Salair, in the Suyur-Suyzas, Kamza, and Upper Kondoma districts of Mountain Shoria, near the village of Ust-Isha in the Gorny Altai, in the Alash district of Tuva, etc. A gold deposit associated with quartzolites is known.

The Bogatyr (Gigant) deposit is located in the Nikolki River system, approximately 15 km south of the Berikul deposit [1]. A band of quartzolites up to 40 m thick lies between limestones and diorites. The quartz rocks formed metasomatically at the expense of limestones. In turn, the quartzolites are irregularly silicified and contain disseminations of pyrite, chalcopyrite, etc. The gold content in the total mass of the ore is insignificant, but in places reaches commercial grade.

In the Alash district of Tuva, quartzolites occur between Silurian conglomerates and Cambrian marbles in a zone of intense schistosity. Individual samples show commercial gold content.

In areas where quartzolites are developed, fragments of brown ironstones and tectonic breccias with limonitic cement are often observed. In some gullies, the amount of fragments of brown ironstones reaches 50%. Rich auriferous placers associated with fields of quartzolites and containing large nuggets are also known. The relatively high thermal conductivity and brittleness of quartzolites facilitate the circulation of hydrothermal solutions within them and the appearance of ore mineralization. There are no reliable facts concerning the gold-bearing character of sedimentary formations—siltstones and paraquartzites (quartz sandstones).

Using Gornaya Shoriya as an example, the author attempted to trace the dependence of gold content on the occurrence of the principal gold-bearing formations by individual structural–facies zones (Table 1). The calculations show that the numbers of occurrences of skarns, secondary quartzites, and quartzolites and quartz veins in the mobile (most gold-bearing) zones are approximately equal (in the second and third).

Table 1

Note: 13/76 — in the numerator, the occurrence of the gold-bearing formation within the territory of Gornaya Shoriya; in the denominator, the occurrence of the gold-bearing formation within the area of the given structure.

At present it is impossible to determine the productivity of ore-bearing formations in the region, since many mineral deposits, especially quartzite deposits, have not been evaluated for gold. One thing is certain: quartzites constitute an important group of gold-bearing rocks having both geochemical and industrial significance. The author’s geological observations show that the most favorable for the localization of gold ores are: (a) deposits of secondary quartzites with external zones of propylitization, with sulfide mineralization and a network of tectonic disturbances; (b) deposits of quartzolites in areas of junction between disjunctive structures and intrusive contacts in volcano-plutonic structures, or in ancient conglomerate–aleurolite–sandstone sequences.

The practice of prospecting-and-mapping and exploration work shows that the most gold-bearing in the Altai–Sayan region are quartz veins, stockworks, ferruginous skarns, secondary quartzites, and quartzolites with sulfide mineralization and signs of repeated and intense deformation.

REFERENCES

1. Bulyannikov A. Ya. Gold-ore formations and gold-bearing provinces of the Altai–Sayan mountainous system. Proceedings of TSU, vol. 102, Tomsk, 1948.
2. Vlasov G. M., Vasilevskii M. M. Hydrothermally altered rocks of Central Kamchatka, their ore content and patterns of spatial distribution. “Nedra,” 1964.

1. Nakovnik N. I. Secondary Quartzites of the USSR. “Nedra,” 1968.

2. Saraev V. A. Secondary quartzites of Mountain Shoria. In: Proceedings of the Scientific Conference of Young Scientists of Higher Educational Institutions of Tomsk, vol. 1, Tomsk, 1968.

3. Saraev V. A. Some data on the distribution and quantitative ratios of rocks and gold potential in Mountain Shoria. Izvestiya TPI, vol. 185, Tomsk, 1970.

4. Saraev V. A. On the classification of siliceous rocks. In: New Data on the Geology and Geography of Kuzbass. Novokuznetsk, 1969.

5. Saraev V. A. On the paragenesis of metasomatic formations. Izvestiya TPI, vol. 236, 1972.

6. Shcherbakov Yu. G. Distribution and Conditions of Gold Concentration in Ore Provinces. “Nauka,” 1967.

1. The term “quartzites” is used by the author as a general name for rocks of essentially quartz composition, regardless of genesis. ↩

2. Deposits of quartzites of Rudny Altai are not considered. ↩