ON THE INHERITANCE OF GOLD MINERALIZATION IN THE BALEY ORE DISTRICT (Eastern Transbaikalia)

V. G. KHOMICH AND S. V. CHEGLYUKOV (BALEIZOLOTO)

In the Baley district there are known ore fields of the gold–silver formation of Early Cretaceous age, Upper Jurassic deposits of the quartz–sulfide and quartz–tourmaline formations, Paleozoic occurrences of gold mineralization of formations of metasomatic quartzites, as well as more ancient gold mineralization of the type of metamorphogenic Alpine-type veins and diaphthorites, associated with the formation of a metamorphic complex widely developed in the district and presumably of Upper Proterozoic age.

For each of the gold-bearing formations established in the district, a complex of features distinguishing it from the others is noteworthy: geological position, composition of the wall rocks, type of ore-hosting fractures, morphology of the ore bodies, relationship with magmatism, genetic complex of near-ore alterations, material composition, fineness of gold, specific saturation, and total metal reserves (see table).

Thus, deposits of the gold–silver formation are localized in terrigenous deposits of the Upper Jurassic–Lower Cretaceous, filling graben-like depressions, and in rocks of the basement—volcanogenic formations of the Shadaron Series \((I_2— I_3Sd)\) and Unda granitoids \((C_2)\). The principal ore-hosting structures, resting on deep faults of the Earth’s crust, are represented by fracture disturbances of pre-ore inception (of the fault–strike-slip type). A paragenetic connection of the mineralization with Jurassic–Cretaceous volcanism is assumed. Studies of recent years [9, 3, 4] have shown that mineral formation took place near the surface under conditions of stepwise pressure drop to 30–40 atm and low temperatures \((150—235^\circ\mathrm{C})\).

Near-ore alterations of the enclosing rocks are expressed in silicification, argillization, sulfidization, carbonatization, and sericitization. Among the ore minerals, a substantial role belongs to complex sulfosalts—compounds of silver, antimony, and copper. Tellurides and selenides are noted. Vein minerals are represented by fine-grained, chalcedony-like quartzes, kaolinite, adularia, etc. Ore textures are metacolloidal, banded, pseudomorphically platy, crustification, and breccia-like. The gold is very silver-rich—its fineness varies from 385 to 730, averaging about 700 [2]. The size of its particles is measured in microns (up to 75%), and to a lesser extent by the first tens of microns. It is distributed in veins in the form of ore shoots. For occurrences of the type under consideration, characteristic

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the highest concentrations of the metal (specific saturation) and the overall gold content. Mineralization of this type is represented by deposits of the Balei ore field.

A large group of deposits and ore occurrences of the quartz–sulfide formation is located among intrusive rocks of the Unda complex ($C_2$), mica schists, gneisses, and phyllites of the metamorphic sequence (Pt?). Their origin is associated with activation in the Mesozoic of an area of completed folding and is controlled by elements of block tectonic structures. Morphologically, the mineralization is represented by systems of relatively extended veins and zones of veinlet-disseminated mineralization, confined to fractures that precede major long-lived ore-controlling faults. A connection [10, 1] is demonstrated between the deposits and a volcanic–plutonic formation represented by granitoids of moderately acidic composition and comagmatic stocks and dikes of porphyries, diorite-porphyries, lamprophyres, hybrid porphyries, granodiorite-porphyries, microdiorites, and sharply porphyritic granites. The formation of gold ores took place at temperatures from 175 to 370°C and higher, and pressures from 420 to 980 atm [4]. Hydrothermal alteration of the host rocks enclosing the mineralization is expressed in K-feldspathization, beresitization, sericitization, and, for some deposits, tourmalinization. Deposits of this formation are divided into two large groups. One of them is characterized by the presence of scheelite in the “foreign” mineral association, the other by molybdenite. N. V. Petrovskaya believes that this difference is due to the geochemical specialization of the processes and of the areas in which these processes manifested themselves [6]. Attention is drawn to the occurrence of scheelite mineralization in the gold–pyrite–arsenopyrite–quartz type of deposits (Shunduinskoye, Fatimovskoye, Kazakovskoye), considerably removed from the stocks of sharply porphyritic granites, and of molybdenite mineralization in the gold–rare-metal–quartz type of deposits (Srednegolgotaiskoye, Chernozipunukhinskoye) in the endo- and exocontact of these intrusions. The areas in which the latter are distributed are characterized by bismuth minerals, sphalerite, galena, chalcopyrite, sulfosalts of lead, zinc, and copper, and tellurides. The vein minerals are represented by medium- and coarse-grained quartz and tourmaline. The gold in deposits of this formation is fine. Particle sizes range from 0.04–0.07 mm to 1.0–2.5 mm. Nuggets weighing up to 100 g are encountered very rarely.

Ore occurrences of the quartz–tourmaline formation are specific to Transbaikalia [7] and are widely manifested in the Balei district, occurring along the southern boundary of the Unda Mesozoic depression among rocks of the Upper Jurassic volcanic–plutonic complex, Carboniferous granitoids, etc. The formation of quartz–tourmaline and tourmaline–sulfide veins took place at the beginning of the Cimmerian metallogenic cycle, associated with post-platform activation of the region. Quartz–tourmaline mineralization in the form of extended veins, often of considerable thickness, and zones of tourmalinization formed at relatively high (280–430°C) temperatures and pressures (750–1130 atm), at depths of about 0.5–1 km. The low gold contents in the ore bodies of the quartz–tourmaline formation did not attract researchers to their study; therefore many questions of geology, details of structure, and interrelations remain unresolved to the present time.

In the Balei district, the presence has been established of gold-bearing formations of Paleozoic age belonging to the formation of metasomatic gold-bearing quartzites [8]. R. M. Faizullin relates the formation of metasomatic quartzites to the establishment of the Unda intrusive body of Carboniferous—

of a different age, in whose granitoids disseminations of native gold have likewise been identified. The vein minerals of the secondary quartzites are represented by quartz, microcline, plagioclase, and micas (biotite, muscovite); the accessory minerals are pyrite, zircon, rutile, and native gold. A sheet-like form is characteristic of the quartzite bodies, which arose through metasomatic replacement of sedimentary rocks and, possibly, acidic effusives. Everywhere the quartzites are uniformly expressed gray and light-gray massive, sometimes banded (schistose) and breccia-like, variably grained rocks. The gold in them is very fine (up to 0.5 mm), high-fineness (900–930). It is distributed extremely unevenly, in various weight proportions, in isolated cases reaching commercial concentrations. The form of the gold particles is veinlike-platy, lump-like, film-like, more rarely spongy and dendrite-like. Monocrystals are sometimes encountered. The gold content of the metasomatic quartzites is low; commercial bodies have not yet been found among them.

Gold-bearing formations of the Alpine-type vein formation are also widespread in the district, developed only among the formations of a metamorphic sequence of presumably Upper Proterozoic age. The veins of the metamorphic sequence are lens-shaped ore bodies, short both along strike and down dip, and highly irregular in thickness. They are controlled by zones of intense schistosity and by extended rupture dislocations concordant with the folding. According to the classification of N. V. Petrovskaya [5], the veins of this formation belong to the low-sulfide type. Formation of the ore bodies took place under conditions of great depths, during prolonged development and repeated recurrence of plastic deformations. The results of chemical analyses show very low gold contents in them, rarely reaching commercial values. Nevertheless, gold placers are known in the district, the probable source of which is considered to be precisely these Alpine-type veins. As in the case of the quartz-tourmaline veins, the latter have been insufficiently studied.

The foregoing shows that in the gold-bearing formations known for the Balei district, alongside differences, there are a number of common features indicating continuity in the development of gold mineralization in the Balei district. Among them one may point to spatial proximity, connection with the same ore-controlling faults, continuity of mineralogical composition, a successive decrease in the depths and initial temperatures of formation, the fineness of the gold, etc.

Analysis of the factual materials shows that between the age, geological position, and conditions of formation of the deposits, on the one hand, and the fineness, concentration, and total amount of gold in them, on the other, there is a definite dependence. In particular, the younger the relative age of the mineralization and the closer it is located to the terminal point of pre-ore migration of the zones of tectonic activation, the smaller the depth and initial temperature of its formation, the lower its average fineness, but the higher its specific saturation and productive ore content.

The data presented testify to a very early ore specialization of the district and to a directed evolution of gold mineralization in space and time.

It seems important to use this directionality for prospecting purposes, since the same character of development is inherent not only in the Balei gold-ore district, but also in certain others, for example, the Shilka-Darasun, Ust-Kara, Mogochinsky districts, etc.

Types of Gold Mineralization of the Baley Ore District

(after M. N. Shmidt, N. V. Petrovskaya, R. M. Faizullin, Yu. V. Lyakhov, M. G. Andreeva, I. M. Adelson, A. V. Shuraev, P. A. Kalashnikov, V. G. Khomich, and others)

References

1. Adelson I. M., Shuraev A. V., Kalashnikov P. A. Some data on the connection between gold and molybdenum mineralization of the Balei district and Mesozoic magmatism. In: Questions of the Geology and Methods of Gold Exploration, issue 1, Chita, 1967.

2. Vorosov A. F., Krasnikov V. I., Lozovsky V. I., et al. Experience in studying the physical properties of certain minerals of the Balei deposit and the specific features of its mineralization. In: Geology and Mineral Resources of Transbaikalia. Materials of the II Scientific Conference, ZabNII, Chita, 1967.

3. Lyakhov Yu. V. Some features of the formation of the Balei gold-ore deposit in light of the results of the study of gas–liquid inclusions. Second scientific conference of the geological sections named after V. A. Obruchev of the Transbaikalian Branch of the Geographical Society of the USSR, Chita, 1965.

4. Lyakhov Yu. V. Results of mineralothermobarometric studies of inclusions in minerals from gold-ore deposits of the Balei district and some aspects of their practical use. Questions of the Geology and Methods of Gold Exploration. Issue 1, materials for the First Scientific-Production Conference on the Geology and Methods of Gold Exploration in the Balei Ore District, Chita, 1967.

5. Petrovskaya N. V. The character of gold-bearing mineral associations and formations of gold ores of the USSR. Reports of Soviet geologists at the XXI Session of the International Geological Congress. Problem 12 — “Genetic Problems of Ores,” Gosgeoltekhizdat, 1960.

6. Petrovskaya N. V. Mineral associations in gold-ore deposits of the Soviet Union. Proceedings of TsNIGRI, issue 76, Moscow, 1967.

7. Smirnov S. S. On the concentration of boron in the northeastern ore province. Doklady of the Academy of Sciences of the USSR, vol. XIV, no. 7, 1944.

8. Faizullin R. M. On the question of the age of gold mineralization in the Balei district of Eastern Transbaikalia. In: Questions of Regional Geology and Metallogeny of Transbaikalia. Issue II, 1966.

9. Khomich V. G., Cheglokov S. V. On the possible depth of formation of epithermal gold mineralization (Balei ore field). In: Questions of the Geology of the Baikal Region and Transbaikalia. Issue 1 (3), Chita, 1966.

10. Cheglokov S. V., Khomich V. G. On the horizontal zonality of the Balei ore cluster. In: Questions of the Geology of the Baikal Region and Transbaikalia. Issue 1 (3), 1966.