PROCEEDINGS
OF THE TOMSK ORDER OF THE RED BANNER OF LABOR POLYTECHNIC
INSTITUTE named after S. M. KIROV

Volume 185　　　　　　　　　　　　　　　　　　　　　　　　　1970

GRAPHITE IN THE ORES OF THE OLKHOVSKOYE GOLD DEPOSIT

(Eastern Sayan)

D. G. GOGIBERIDZE

(Presented by Professor A. M. Kuzmin)

The Olkhovskoye gold deposit has been known for a long time and has been described more than once in the literature by A. Ya. Bulynnikov (1940), D. A. Timofeevsky (1950), and A. M. Khazagarov (1963). However, strange as it may seem, graphite, which is a characteristic companion of many tectonic fractures and zones of crushing and is often encountered in ores, was almost not mentioned.

Geology of the Deposit

The geological structure of the Olkhovskoye deposit includes limestones of the Chibizhek Formation /Cm₁/, effusive-sedimentary rocks of the Osinovka Formation /Cm₁–₂/, a Lower Paleozoic granodiorite intrusion, and numerous dikes, predominantly of basic composition and up to granite in age. Near the intrusion, the Chibizhek limestones have been transformed into fine- and medium-grained banded, spotted, and massive marbles, while the rocks of the Osinovka Formation have been transformed into various hornfelses. Skarnification is widely developed, extending almost exclusively through the rocks of the Osinovka Formation. Chloritization and sericitization are characteristic of the volcanic rocks; silicification and carbonatization are noted in all rocks.

One of the main features of the occurrence of the granodiorite massif is its almost exclusive confinement to the rocks of the Osinovka Formation, which make up the core of a synclinal fold, and its comparatively rare and shallow penetration into the Chibizhek limestones. Near the intrusive contact, within the belt of metamorphosed rocks, all the industrial mineralization of the deposit is concentrated, the greater part of which, represented by veinlike and columnar sulfide ore bodies, is localized in the limestones. Quartz-sulfide veins and stockworks develop in the hornfelses and granodiorites. Of the total volume of ore minerals, about 90% is accounted for by pyrrhotite and pyrite; the remainder is represented by marcasite and chalcopyrite. In places, arsenopyrite, sphalerite, and magnetite occur in appreciable quantities. The vein minerals are represented mainly by quartz and calcite. Siderite, ankerite, chlorite, and graphite are constantly encountered.

Gold is established with the naked eye only very rarely and only in quartz-sulfide ores. Under the microscope it is observed—

occurs in association with pyrrhotite, pyrite, chalcopyrite, marcasite, tetradymite, and quartz. Usually these are groups of small, 0.001 to 0.03 mm, isometric, oval, and irregularly shaped gold grains, situated at contacts between grains of pyrrhotite, marcasite, pyrite, and quartz, and also along thin fractures cutting these minerals. Gold segregations are generally recorded within “bands” elongated in one direction, where they are observed in association with all the minerals that fall within these “bands.” D. A. Timofeevskii notes the confinement of gold to ore areas enriched with accumulations of graphite.

In terms of time of formation, gold is one of the latest ore minerals and is closely associated with chalcopyrite, whose veinlets distinctly cut the formations of pyrrhotite, pyrite, marcasite, siderite, and quartz.

The principal ore-controlling role at the Olkhovskoye deposit belongs to the “Main” and “Gentle” tectonic zones and to the smaller tectonic zones and fractures associated with them, which form a complex network of rupture disturbances. In their extent, both zones cut all varieties of rocks and, at the same time, are rather clearly drawn toward the contact between the Osinovskaya and Chibizhek formations, being traced, respectively, along the northern and western contacts of the granodiorite massif and dipping toward it. Ore bodies are localized at places where different systems of tectonic fractures are coupled and intersect—fractures having different strike directions, or the same strike but different dip angles.

Features of the Distribution of Graphite

Graphite (more precisely, graphitite) occurs at the deposit in two positions. Most common is its participation in the formation of banded, spotted, and looped textures of marbleized limestones, caused by the development of dark graphitic bands—straight or curved—against the light background of the main mass of the rock. Spotted textures are also associated with the appearance of light spots against the dark background of the rock. The width of the bands ranges from 0.5 to 3 mm in banded and looped textures, and up to several centimeters in spotted textures. Usually bands with diffuse boundaries and a gradual increase in color intensity from the periphery toward the axial line are observed, but very distinct bands—veinlets—also occur, standing out very contrastively against the light background of the carbonate rock. Under the microscope, the confinement of graphite to the boundaries of carbonate grains is established; much more rarely, thin inclusions of it are visible within the grains themselves.

It is difficult to establish any stable regularity in the change of marble textures, including the appearance of large areas of their complete lightening. It is characteristic that in direct contact with granodiorites the lightening of limestones is, as a rule, absent, whereas it is constantly noted along contacts with diorite-porphyrite dikes in the form of a band about 1 cm wide. Not infrequently, the banding of the limestones proves to be concordant with the intrusive contact, sometimes repeating all its irregularities.

Another morphological variety of graphite at the Olkhovskoye deposit consists of veinlets, lenses, smears, and small accumulations, very often accompanying tectonic fractures and zones, regardless of the composition of the rocks that these fractures cut. Especially much graphite occurs along the “Main” and “Gentle” tectonic zones, where it is often associated with chlorite and serves as one of the charac-

diagnostic features of these zones. In the limestones, as well as in the hornfelses, granodiorites, and dike rocks hundreds of meters away from the limestones, graphite develops along fractures in exactly the same way, standing out clearly by its dense black color and shiny slip surfaces. Where a fracture cuts banded limestones, graphite also proves to be present together with the “crosscutting” fracture.

Of interest are the relationships of graphite with the ore bodies. A graphitized tectonic zone running along an ore body splits its adjoining parts into separate slabs or lenses bounded by slip planes with abundant graphite coatings; moreover, traces of slipping are clearly observed both on the coatings and, after their removal, on the ore itself. Lens-shaped pieces of ore are encountered whose surfaces are represented by slickensides and are covered with a layer of graphite up to 1 mm thick.

Within the ore mass, graphite occurs as thin veinlets, small lenses, or irregular accumulations confined to fractures or to boundaries between aggregates of different minerals. Characteristic are thin veinlets of graphite developing along the axial parts of chalcopyrite veinlets that cut pyrrhotite. In one hand specimen the author observed an ore breccia in which the fragments consisted of chalcopyrite with remnants of unreplaced pyrrhotite, and the cement was graphite.

Under the microscope, a close association of graphite with chlorite is established, usually in the form of alternating small lenses.

Graphite-chlorite veinlets and accumulations develop along the margins of pyrrhotite grains, in carbonate veinlets cutting aggregates of pyrrhotite, marcasite, pyrite, and chalcopyrite; in cavities and fractures in the mass of ankerite, with grains and accumulations of pyrite and chalcopyrite also confined to these same cavities (chalcopyrite rims and corrodes pyrite grains and is itself, in turn, rimmed by graphite).

No direct relationships between gold and graphite were observed. Judging from the gold content in graphitized portions of the ores, however, it may be either high or low, though more often high. Yet graphite itself has nothing to do with this. Ores with chalcopyrite and quartz have a high gold content irrespective of the presence or absence of graphite, whereas mineral associations distinguished by a low gold content—for example, pyrrhotite-calcite and pyrite-calcite—do not become richer even under abundant graphitization. In all cases where graphitized ores have an elevated gold content, they contain chalcopyrite.

Conclusions

At the Olkhovskoye deposit there is graphite, formed during the final stage of the hydrothermal phase of mineralization and localized along fractures that earlier served as pathways for the movement of ore-bearing solutions. The source of the carbon-bearing and ore-bearing solutions was apparently the same—the granodiorite intrusion, which assimilated, albeit in small quantity, Chibizhek Suite limestones in its marginal parts.

V. I. Vernadsky (2) pointed to the possibility of graphite formation from certain volatile (or liquid) carbon compounds that penetrated into the enclosing rocks at a not very high temperature and were closely connected with the emanations of syenitic and granitic magmas.

It is not excluded that a certain part of the graphite could have been concentrated in some large fractures even during the process of thermal metamorphism of the bituminous Chibizhek limestones, accompanied by the distillation of bitumens.

References

1. A. Ya. Bulynnikov. Features of the petrology and ore potential of the Olkhovsko-Chibizhek gold-bearing district (Eastern Sayan). Proceedings of the Conference on the Study of Productive Forces, vol. 2, Tomsk, 1940.

2. V. I. Vernadsky. An Essay in Descriptive Mineralogy. Selected Works, vol. 2, Publishing House of the Academy of Sciences of the USSR, 1955.

3. D. A. Timofeevsky. Gold-ore deposits of the Eastern Sayan (Olkhovskoye and Konstantinovskoye). NIGRI-gold, 1950.

4. A. M. Khazagarov. Some features of the localization of gold mineralization in the Olkhovka ore field (Eastern Sayan). Geology of Ore Deposits, No. 3, 1963.