TYPES OF GOLD ORE MINERALIZATION IN THE CENTRAL PART OF THE BARGUZIN TAIGA AND THEIR RELATIONSHIP TO MAGMATISM

V. A. GRUSHIN, S. S. KALNICHENKO (TsNIGRI)

The identification of types of gold ore mineralization within the central part of the Barguzin taiga and the clarification of their genetic relationship with intrusive complexes are of great importance in assessing the prospects of the territory and in forecasting ore and placer gold deposits. The work of a group of geologists from TsNIGRI in 1962–1965 was devoted to resolving these questions; it constituted a new stage in the study of the gold potential of the territory of the Barguzin taiga, begun by the TsNIGRI institute as early as 1948–1950 under the direction of S. G. Mirchink and S. D. Sher. The results of the present investigations, set forth below, develop and in part cast new light on the principal questions of gold ore mineralization that were first elaborated by S. D. Sher (1952), who distinguished two main cycles of gold mineralization.

The work of 1962–1965 covered the northern part of the Buryat ASSR, including the upper reaches of the Vitim River and the basins of the Vitimkan, Chika, and Tsipikan rivers.

Structurally, the area under consideration is situated within the central part of the eugeosynclinal belt of the Baikalides. The geological structure of the territory includes deposits of a terrigenous-volcanogenic formation (Suvanikha suite), a carbonate formation (Tilim suite), a carbonate-terrigenous formation (Yaksha suite) of the Upper Proterozoic, and a flyschoid formation of the Lower Cambrian (Tochera suite).

The principal structural elements are the Vitimkan–Tsipikan anticlinorium, which encompasses the western and northwestern parts of the territory, and the Vitim–Amalat synclinorium, confined to its eastern and southeastern parts. These structures have a very complex architecture, characterized by the presence of synclinal downwarps and anticlinal uplifts that are, in turn, complicated by additional folds of higher order.

The predominant trend of the structures is northeastern. In individual areas, however, deviations of their strike to nearly latitudinal and even northwestern are observed. The junction of structures most often occurs along zones of long-lived deep-seated faults, accompanied by numerous disturbances of various orientations.

Within the territory described, intrusive formations are widely developed, among which gabbroids are distinguished.

of the Ikat and granitoids of the Barguzin complexes of the Upper Proterozoic, granitoids of the Lower Paleozoic Vitimkan complex, and a complex of small intrusions and dikes of Mesozoic age.

Upper Proterozoic intrusions have limited development in the region. The granitoids of the Lower Paleozoic Vitimkan complex are much more widespread. Among them, several formations of two phases are conditionally distinguished. In the first phase, the intrusion of large, areally extensive massifs of biotite and biotite–hornblende granites, granodiorites, and quartz diorites took place. The second phase is characterized by the intrusion of small stocks of granitoids and dikes of dioritic and gabbro-dioritic composition. In their distribution, the dikes form clearly expressed linear zones within the Vitim–Amalat synclinorium. A characteristic feature of Paleozoic magmatism is the formation, at the contact with carbonate rocks, of skarns of scapolite–epidote, garnet–pyroxene–scapolite, amphibole–pyroxene, and garnet–epidote composition.

Mesozoic magmatism is represented by small stocks of granites and by dikes of granite porphyries, felsite porphyries, felsites, syenites, diabases, and diabase porphyrites.

These rocks are correlated with the small intrusions and dikes of southern Buryatia, whose absolute age (granite porphyries), according to P. I. Naletov, is 127–140 million years.

The study of gold-ore occurrences, as well as a comparative study of the morphology and fineness of native gold from placers and ore occurrences, has shown that the principal types of gold-ore mineralization within the central part of the Barguzin taiga are: quartz–sulfide veins, veinlet-disseminated occurrences, and magnetite bodies.

Quartz–sulfide veins occur practically everywhere. They form bodies of various extent and thickness: from small veinlets to extended veins ranging in length from the first tens of meters to 1–1.5 km, with thicknesses of 1–5 m. The morphology of the veins and veinlets is diverse. Along with simple ones that fill distinct linear fractures, branching and intersecting forms are observed. For veins developed in the exocontact zone of granitoid massifs and dikes of various composition, more complex forms are typical.

Quartz and carbonate predominate in the composition of the veins. Depending on the amount of ore components, the veins are subdivided into low-sulfide and moderately sulfide varieties. Among the former, veins with pyrite–pyrrhotite and polymetallic mineralization are distinguished.

Pyrite–pyrrhotite veins are most characteristic of the exocontacts of granitoids of the first phase of the Vitimkan intrusive complex and are controlled by fine superposed folding. This folding formed during the emplacement of the intrusions. This is indicated by the bending of the hinges of small folds parallel to the contacts of the massifs and by an increase in the intensity of folding as they are approached. S. D. Sher regarded this folding as superposed, on the basis of the noncoincidence of the axes of small folds and large structures, as well as evidence of the bending, by small folds, of the axial planes of drag folds that accompanied the main folding (S. G. Mirchink, S. D. Sher, 1955).

In the veins under consideration, pyrite and pyrrhotite usually develop together. Monomineral inclusions are comparatively rare. Pyrite forms shapes close to cubic, while pyrrhotite is observed in the form of small veinlets and lenses.

Sampling of these veins shows that they often contain gold.

Low-sulfide quartz–polymetallic veins have a comparatively much smaller distribution. Almost everywhere they are observed in zones of disjunctive disturbances of northwestern and near-latitudinal trends, gravitating toward granitoid massifs of the second phase of the Vitimkan complex. The main ore mineral of these veins is galena. It forms isometric disseminations up to 1 cm across and veinlets in quartz along fractures. Pyrite and pyrrhotite are observed both together with galena, as veinlets and coatings, and separately, as disseminations 2–3 mm in size. Sphalerite is characterized by irregularly shaped segregations, the size of which reaches 8 mm across. Sampling of the quartz–polymetallic occurrences showed a low gold content in them. The veins are spatially confined to the endo- and exocontact zones of granitoid massifs of the second phase of the Vitimkan intrusive complex.

The probable Paleozoic age of the low-sulfide quartz–polymetallic veins is indicated by the higher fineness of the gold developed in them, which is more characteristic of the Paleozoic cycle of gold mineralization than of the Mesozoic.

Moderately sulfide veins in this territory are much less widespread than the type of mineralization already described. Unlike the low-sulfide veins, these formations contain an increased amount of ore components. Depending on the association of ore minerals, the veins are subdivided into quartz–pyrite–arsenopyrite and quartz–polymetallic veins.

The former veins are developed in zones of tectonic disturbances within fields of Paleozoic dikes of basic composition. In this case they are linked with the dikes. This connection is based on the clear spatial coincidence of veins and dikes, the identity of their trend and morphology, their frequent intercalation with one another, and also the high fineness of the gold of these veins, which, as indicated above, is characteristic of the Paleozoic gold-ore cycle.

An example of such formations is provided by the vein bodies of the Verkhne-Khalin ore occurrence. They are confined to a zone of disturbance of northeastern trend, passing through the carbonate–terrigenous deposits of the Yaksha Formation.

The ore bodies are traced for a distance of up to 1,500 m, with thicknesses from 0.2 to 6 m. Branching, pinches, and swellings are very characteristic. The veins have a quartz–carbonate composition, with quartz sharply predominating. The ore minerals are represented by pyrite, arsenopyrite, magnetite, and chalcopyrite. Pyrite is the most widespread. It forms massive aggregates and individual disseminations of various shapes and sizes. Arsenopyrite and chalcopyrite are observed as inclusions in pyrite. In the contact-adjacent parts of the ore bodies, pyrite disseminations, nests of sphalerite, and film-like segregations of magnetite are developed.

The amount of ore minerals is variable. In individual cases it reaches 80% of the entire vein mass. An increase in the content of ore components usually occurs in places of swelling, which are confined to the intersection of veins by small faults and zones of crushing.

The results of sampling carried out during exploration of the deposit showed that gold is distributed unevenly along the veins; its content is low and is directly dependent on the content of ore minerals and, first of all, pyrite.

Moderately sulfide quartz–polymetallic veins are developed mainly in the anticlinorial part of the territory, being confined—

confined to zones of tectonic disturbances of northeastern trend. At the same time, they coincide with zones of development of Mesozoic syenite and diabase dikes. A typical example of these formations is the veins of the Karaftit gold-ore occurrence. It is situated on the limb of an anticlinal fold among the sandy-shaly deposits of the Suvanikha Formation, near their contact with the limestones of the Tilim Formation. Its confinement to the zone of intersection of northeastern- and near-meridional-trending faults is also noted.

The ore occurrence is represented by several crosscutting veins of northeastern strike, reaching a length of 275 m, with thicknesses from 0.2 to 1.2 m. In composition the veins are quartz-carbonate; inclusions of wall rocks are characteristic of them. The ore minerals are represented, in order of abundance, by galena, sphalerite, pyrrhotite, pyrite, and gray ore. Native silver, argentite, and calaverite occur more rarely. The ore minerals develop together, forming branching discontinuous veinlets, irregularly shaped nests, and individual disseminations of polymineral and monomineral composition. Visible gold in these veins is rare. Its average fineness is 750. In polished sections, gold often occurs in association with galena and gray ore. Its content in the veins is uneven and in places very high.

Other moderately sulfide quartz-polymetallic occurrences bear characteristic features of veins of the “Karaftit” type, differing from the latter in size and gold content.

The Mesozoic age of the moderately sulfide quartz-polymetallic veins is indicated by the crosscutting nature of the vein contacts with respect to Mesozoic dikes, the development within the dikes themselves of thin, discontinuous quartz-polymetallic veinlets, and also the character of hydrothermal alteration of the dikes at their contact with the veins.

The genetic connection of the dikes and veins is emphasized by their geochemical affinity, expressed in elevated contents of bismuth, zinc, lead, and copper.

All the described varieties of quartz-sulfide mineralization are accompanied by veinlet-disseminated sulfide mineralization. Study has shown that between them there exists not only a spatial, but also a genetic connection. Everywhere the intensity of veinlet-disseminated sulfides increases sharply on approaching the vein zones. At the same time, zones of sulfide mineralization more or less isolated from quartz veins are encountered extremely rarely.

The composition of the veinlet-disseminated sulfides, as a rule, corresponds to the composition of the ore minerals of the quartz-vein formations that they accompany. The sulfides are observed in the form of rounded, lens-shaped, and isometric segregations, whose sizes vary from fractions to 10–15 mm across. Along with this, but much more rarely, veinlet forms occur; however, the length of the veinlets does not exceed 15–20 mm, with a thickness of 1–3 mm.

In the distribution of sulfide mineralization, the same regularities have been found as in the corresponding types of quartz-sulfide veins. Analysis of rocks with sulfide disseminations shows an uneven and low gold content.

Magnetite bodies are not widely distributed. They are observed only in the eastern part of the district (Troitsky placer mine), where they are confined to zones of crushing in the limestones of the Tilim Formation at their contact with granitoids of the first phase of the Vitimkan intrusive complex. In addition to magnetite, tremolite is widely developed in them, and in con-

—minerals of skarns: garnet, scapolite, pyroxenes, and others, which are in contact with the limestones and sandstones. They form lenticular bodies from the first meters to 30 m long, with thicknesses from several centimeters to 1.5 m. There are a number of occurrences of magnetite with rather dense fine disseminations of visible gold. In addition, gold is present in magnetite in the form of thin crosscutting veinlets together with quartz and carbonate.

Thus, the identified types of gold-ore mineralization are associated with two magmatic stages—Paleozoic and Mesozoic. Genetically associated with the first phase of Paleozoic magmatism, in the form of large massifs of granitoids of the Vitimkan complex, are low-sulfide quartz–pyrite–pyrrhotite veins, the accompanying veinlet-disseminated sulfide mineralization, and magnetite bodies. Associated with the second phase, characterized by the intrusion of comparatively small stocks of granitoids and dikes of diorite and gabbro-diorite composition, are moderately sulfide quartz–pyrite–arsenopyrite and low-sulfide quartz–polymetallic veins, as well as the corresponding disseminated sulfide mineralization.

Finally, the Mesozoic magmatic stage is characterized by small intrusions and dikes of diverse composition, with which a genetic and spatial relationship is observed for moderately sulfide quartz veins with gold–polymetallic mineralization.

REFERENCES

1. Grushin V. A. On the question of the primary sources feeding the gold-bearing placers of the Troitsk area (central part of the Barguzin taiga). Transactions of TsNIGRI, issue 72, 1967.

2. Kalnichenko S. S. Types of primary sources of gold-bearing placers in the headwaters of the Vitim River (central part of the Barguzin taiga). Transactions of TsNIGRI, issue 79, 1968.

3. Mirchink S. G., Sher S. D., with the participation of Grigorieva A. I. Geology, geomorphology, and gold content of the placers of the central part of the Barguzin taiga. ONTI NIGRIzoloto, 1955.

4. Nikolaeva L. A. Morphological features of native gold from a number of placers in the central part of the Barguzin taiga. Transactions of TsNIGRI, issue 63, 1965.

5. Sher S. D. Gold mineralization of the central part of the Barguzin taiga. Transactions of TsNIGRI, issue 19, 1952.