A New Type of Gold Mineralization in the Yenisei Ridge

F. P. Krendelev, A. G. Luchko (Siberian Branch, USSR Academy of Sciences)

Studies by F. N. Shakhov [3], P. S. Bernstein [1], N. V. Petrovskaya [2], and many others have shown that within the Yenisei Ridge one can distinguish five morphological groups of vein or stockwork gold deposits; moreover, the mineralization in all groups is represented by one and the same mineral association: uniform quartz, milky-white or bluish, grayish, coarse-grained, with a minor admixture of carbonates, sericite, chlorite, and, more rarely, albite, rutile, and sulfides. The latter sometimes make up as much as 10–15, and rarely up to 50%, of the volume of the mineral mass. Among the sulfides, pyrite and pyrrhotite predominate; to a lesser extent there are arsenopyrite, galena, sphalerite, and chalcopyrite. The gold is usually fine and invisible, and is distributed extremely unevenly. In some veins, gold tellurides occur. In all cases, ore formation was multistage.

In 1964–68 we had occasion to study the geological structure of the so-called Gerfed ore field, whose ore formations belong to the fifth of the above-listed types.

The geological structure of the Gerfed field is fairly simple, as is clearly seen in the appended sketch map (Fig. 1), and is not described here in detail. We shall mention only that the deposits of the Tei Series form the cores of two brachyfolds cut by the Tatar and Ayakhta intrusions. The contact of the latter cuts all deposits up to the upper parts of the Tungusik Series, which determines the age of this intrusion as post-Tungusik. The contact is active: skarns have formed along the limestones of the Sosnovka Formation and the marbles of the Penchenga Formation. The age of the Tatar intrusion is considered to be the same as that of the Ayakhta intrusion only on the basis of its structural confinement to a single tectonomagmatic zone.

Until 1964 it was believed that the deposits of the Tei Series are gradually replaced by the Sukhopit deposits, and that the contact between them is tectonic and that the “main vein” is located precisely in the contact zone. The difference in the degree of metamorphism of the schist sequences to the west and east of the zone was explained by distance from the Tatar intrusion. Our studies lead to substantially different conclusions regarding the genesis of the “main vein” itself and the mutual relationship of the schist sequences to one another.

The Gerfed (Partizanskoye) deposit was discovered in 1890 and was worked by a small quarry. The structure of the ore body is rather complex. Near the contact or directly in contact with the Penchenga ...

Fig. 1. Diagram of the geological structure of the Terfed ore field.
1 — Evenki Formation \(Cm_3\) — red-colored conglomerates, gravelstones, sandstones, often with calcareous cement; 2 — Lower Cambrian — gray limestones and dolomites, red-colored argillites, marls, and sandstones with Bergeroniaspis; 3 — Oslyanskaya Formation — variegated sandstones, conglomerates, gravelstones, dolomitized argillites; 4 — carbonate–shale deposits of the Dashka Formation; 5 — iron-ore formation, Lower Angara Formation; 6 — carbonate–shale deposits of the Tunguska Series; 7 — variegated limestones of the Solomkova Formation; 8 — deposits of the Sukhoy Pit Series, beginning with the Gorbilok Formation and higher: phyllite-like, aleurolite–clayey and other shales, interbeds of aleurolites, quartzites, calcareous aleurolites, quartzitic sandstones; within the Gorbilok Formation there occur phyllites of chlorite–sericite composition with magnetite; 9 — Kordinskaya Formation — carbonaceous phyllites, locally pyritized, with interbeds of aleurolites and quartzitic sandstones; separate bed-like lenses of basic effusives; at the base, a thin horizon of gravelstones, coarse-grained sandstones, and conglomerates; 10 — Penchenginskaya Formation, dominated by phyllites and quartzitic sandstones; horizons of marbles and carbonaceous crystalline limestones occur; 11 — boundaries of the Tatar–Ayakhta complex; 12 — orthoamphibolites of the Indiglin complex; 13 — halos of the contact influence of granites; 14 — small vein occurrences of gold ore; 15 — occurrences of gold associated with the “main vein”; 16 — occurrences and deposits of iron; 17 — gold-bearing placers; 18 — tectonic disturbances. Meso-Cainozoic deposits are omitted. Black triangles indicate outcrops of oligomictic conglomerates on the Malaya Penchenga [1] and of sulfidized quartzites on the Bolshaya Penchenga [2].

of the suite and the Nikolaevka unit (an analogue of the Korda suite) for almost 12 km there extends a very large so-called “main vein” (see Fig. 1), on the footwall side of which there is a large number of small transverse veins (“leaders,” in South African terminology). The “main vein” is a sheet-like body, in the fine-grained aggregate of quartz of which “small angular ‘shadows’ of fragments of crushed rocks are preserved, in the form of angular accumulations, pelitic ‘dust,’ and flakes of sericite in chlorite. These relicts of a breccia-like texture indicate a metasomatic origin of the quartz-like rocks.” These observations by N. V. Petrovskaya provided grounds for assigning the vein to metasomatic formations. On approaching the main vein, veinlets of coarse-grained quartz from the transverse veins develop into a complex network cutting the metasomatites of the “main vein.”

In the Gerfed ore field there is an entire series of deposits. Along the continuation of the main “vein” to the north, the Nikolaevka, Verkhne-Uderei, and other deposits have been discovered; in geological structure they do not differ in any way from the Gerfed deposit. In addition to bedrock deposits, a large number of fairly rich placers are known for the district.

In the placers of the South Yenisei district, in comparison with other gold-bearing districts, fine gold (less than 2 mm) predominates. The small size of the placer gold undoubtedly reflects the size of the primary inclusions of ore gold, since fine gold is characteristic not only of river placers, where it is well rounded and clearly carried far from bedrock sources, but also of placers of small springs, where the fine gold is almost not rounded at all. The gold of springs is richer than that of valleys, and the fineness of the gold is very high—a minimum of 820, most often 920–940. Let us recall that the gold of the ancient conglomerates of South Africa is also exceptionally fine, and in the placers of the nearest districts it is usually only weakly rounded.

The Gerfed ore field attracted attention because within it there is a certain similarity in geological structure to the areas of conglomerate development in the Witwatersrand. In order for this similarity to be still more complete, it was necessary to discover a stratigraphic unconformity in this area (an oligomictic unit). A stratigraphic unconformity in this area had not previously been noted by anyone. This is explained by the fact that the shale units participating in the structure of the area are extremely uniform throughout the entire section; and certain differences in their mineralogical composition on the hanging and footwall sides of the main “vein” were explained (as noted above) by differences in the degree of their metamorphism. A careful study of material from the mine dumps (there are no outcrops here, and the workings are flooded) of the Nikolaevka deposit showed that, during the driving of adits exposing the contact zone, the dumps received: coarse-grained gravelites with pebbles of shale from the underlying unit and magnetite; variably grained oligomictic sandstones; fine-grained quartz sandstones; and cross-bedded quartz siltstones with traces of ripple marks and currents (Fig. 2).

Cross sections of the deposits (Fig. 3) show that the main “vein” falls within the transition zone from the Korda suite to the Penchenga suite and is situated in a single stratigraphic zone.

Study of the textural features of hand specimens from the “vein” itself proves that relicts of the textures (Fig. 4) of sedimentary rocks are preserved directly within it: oblique bedding, emphasized either by interbeds of siltstone-sized particles (4a), by sulfides (4b),

Fig. 2. Sample from the dump of the Nikolaevskoe deposit
Cross-bedded, gently undulating quartz aleurolites—sample F-306a-65. Photograph, 1/2 natural size

or by interbeds of various-grained sandstones (4в); “shadows” of interbeds of shales, argillites, and other finely scaly rocks (4г); “shadows” of pebbles in the basal parts of the body (the footwall of the vein), revealed owing to the intensive development of sulfides in the cement.

Finally, in borehole 96 of the Nikolaevskoe deposit, in the footwall of the “vein,” we succeeded in finding typical conglomerates with strongly sulfidized cement. These are the first sulfide conglomerates with gold on the ridge.

B. E. Shelekhov kindly made available to us a photograph of the main “vein,” taken in an underground working, in which it is clearly visible that the vein is a complexly constructed bedded body with distinctly expressed oblique bedding (Fig. 5). The quartz veins affecting the bed do not extend beyond the limits of the ore body and are peculiar Alpine veins. The bed is clearly divided into two rhythms, separated by interbeds of shale composition. The picture is somewhat complicated by the fact that all the shales and the “vein” itself are disrupted by a large number of interstratal movements. Very often, typical slickensides are visible on the surfaces of fragments of rocks and ores in the dumps. This is due to the fact that the strike

Fig. 3. Cross section through the main “vein” of the Nikolaevskoe deposit (exploration line 9)

1 — Quaternary loams; loose deposits; 2 — phyllites of the Kordinskaya Formation, with disseminated pyrite, pyrrhotite, and small quartz–carbonate veinlets; 3 — quartz veinlets; 4 — main “vein”; 5 — quartz–biotite–chlorite schists with disseminated chalcopyrite and thin quartz veinlets (Penchenginskaya Formation); 6 — zone of transition from the Penchenginskaya Formation to the Kordinskaya, interbedding of schists, aleurolites, sandstone-like sandstones and gravelites; ripple marks, current marks, and gentle oblique bedding are rarely observed.

The dip of the “vein” coincides with the direction of the tectonic structures and with the direction of the schistosity zones in this area. In some places, especially where bends of the bed are indicated, typical breccia textures arise, which were noted and described by N. V. Petrovskaya.

The transition of the “metasomatites” of the main “vein” into conglomerates is confirmed by direct finds of clastic rocks in the upper reaches of the Malaya Penchenga River. On the diagram (Fig. 1) it is evident that the direction of the “main” vein and of the contact zone coincides with the direction of the tectonic zones. At the northern closure of the brachyfold, the zone of stratigraphic unconformity turns sharply westward. In addition, observations of the character of the rocks on the dumps of ore occurrences of the Gerfedskaya zone indicate a gradual northward burial of the clastic components from south to north. It was precisely for this reason that the presence of conglomerates in the northern framing of the brachysyncline could be assumed. And indeed, in 1968, in the upper reaches of the Malaya Penchenga River, a gently dipping stratigraphic zone of development of coarse-clastic rocks was identified. Here at least three horizons of gravelites and conglomerates of purely quartz composition were found. The pebbles are well rounded, slightly oval—

Fig. 4. Relicts of sedimentary-rock textures in the ores of the Nikolaevskoe deposit.
Oblique bedding, emphasized: a) by interlayers of aleurolitic particle size—Specimen F-312-65; in the middle, an interlayer of marbleized limestone (gray); b) by sulfides—Specimen F-312a-65; c) by layers with grain sizes differing in coarseness. Specimen F-312v-65; d) “shadows” of argillite interlayers—Specimen F-310-65. All specimens reduced by half.

… identical in size. Among the pebbles there occur pebbles of dove-gray quartz, especially characteristic of Precambrian conglomerates. The cement of the conglomerates is strongly limonitized, probably at the expense of leached sulfides. That sulfides were present here is demonstrated by the discovery of sulfide quartzites on the summit of Bolshaya Penchenga.

Analysis of the distribution of gold contents in the “vein” shows that, on the whole, the thickness of the bed and the amount of gold in it are not functionally related (Fig. 6). Within the bed the gold is distributed unevenly and shows a tendency to accumulate toward the footwall side, while at the Nikolaevskoe deposit (Fig. 6) the bed is divided into two rhythms with enrichment at the base of each rhythm.

The ore bodies of the Gerfed field fall within a stratigraphic zone in which there is a sharp change in the overall radioactivity

Fig. 5. Structure of the main “vein.” Nikolaevskoe deposit, mine No. 1, 415 m level, northern drift. Photograph by B. E. Shelekhov.
It is clearly visible that the “vein” is divided into two rhythms, separated by an interlayer of phyllites (shown by the arrow).

rocks, which is usually observed in zones of stratigraphic unconformities, including at the boundary of the Kordinskaya and Penchenginskaya formations. A weak increase in radioactivity within the ore field itself is observed not throughout its entire thickness, but chiefly in the basal, strongly sulfidized part.

The general distribution of ore occurrences of the Gerfed field, and in the Ayakhta area, shows that most of them are located within the distribution of rocks of the Kordinskaya Formation. In other words, a distinct lithological and stratigraphic control is manifested here. Small vein-type deposits are confined to the Uderei Formation. The similarity of the Gerfed field to the Witwatersrand is increased by the presence of a horizon of iron ores in the overlying Lower Angara Formation, just as in South Africa oolitic iron ores are confined to the Pretoria Formation.

One cannot fail to note the extraordinary similarity of the mineralogical composition of typical vein bodies and the main “vein.” Both the veins and the bed are composed mainly of quartz and sericite, with a minor admixture of chlorite. Among the sulfide minerals, pyrite occurs; pyrrhotite is less abundant; chalcopyrite, arsenopyrite, sphalerite, galena, and rutile occur in small amounts; and in barren interbeds—magnetite and ilmenite, sometimes leucoxenized. In some parts of the vein, rather significant admixtures of carbonate of the calcite group and occasionally siderite are observed. In other words, both the veins and the bed are composed of the same minerals as all the other gold deposits of the Yenisei Ridge. In chemical composition, the beds and veins also do not differ from one another or from the ores of deposits of typical sulfide conglomerates (Table 1).

Bed-like bodies and veins are situated at some distance from intrusions and fall within the area of development of metamorphic facies of greenschist alteration of rocks, expressed in the development in schistose rocks of such minerals as quartz, chlorite, and sericite. Biotite cha-

Fig. 6. a — dependence of Au content on the total thickness of ore bodies (according to core-sampling data from columnar drill holes of the Nikolaevskoe deposit). b — change in Au content depending on the distance of the sample from the footwall of the Main “vein” (Nikolaevskoe deposit, according to core-sampling data from columnar drill holes; averaged data for 265 samples)

...is characteristic of the areas closest to the intrusive body; the sericite–quartz association is farthest removed from the intrusive body and is typical of the belt of development of Uderei shales. The Main “vein” marks the zone of disappearance of biotite and chlorite and the appearance of muscovite–sericite, and, with respect to sulfides, the zone where pyrrhotite is replaced by pyrite. This zone coincides with the stratigraphic unconformity. And from the point of view of the intensity

Chemical composition of quartz of the “main” veins and of the enclosing gneisses of the Gerfed ore field

Table 1

I—VII — Samples of quartz from the “main” veins. Samples combined by grouping or by group of boreholes. I — boreholes 88 (p. 24); II — boreholes 86 (p. 25); III — boreholes 86 (p. 26); IV — boreholes 91 (p. 32); V — boreholes 96 (p. 33). VIII—IX — Samples of quartz from a wide vein assigned to the “main” veins, 133, 134, 135. VI — boreholes 93, 85, 130, 140 (p. 37); VIII — adit 1, northern wall (p. 38).

Chemical laboratory of the Angarsk Geological Exploration Expedition. 1964. (All data by B. E. Shelekhova.)

of metamorphism, the area of the Gerfed deposit is not an exception, since all the largest deposits of the type of sulfide conglomerates and vein gold-ore bodies are located within the development of greenschist facies.

Thus, we arrive at the conclusion that the main “vein” is a typical bedded body of siltstones, sandstones, gravelites, and conglomerates, with sulfide gold mineralization superimposed as a result of hydrothermal processes. The main “vein” is an analogue of deposits of the type of sulfide Precambrian conglomerates and shows the greatest similarity to the Piritozu-type sandstones in Brazil.

REFERENCES

1. Bernshtein P. S. Structural factors determining the patterns of distribution and the character of gold mineralization of the Yenisei Ridge. Transactions of the NIGRI-Gold Institute, issue 18, 1951.

2. Petrovskaya N. V. Certain features of intraclastic metamorphism of gold-bearing formations, using the Yenisei Ridge as an example. Transactions of the NIGRI-Gold Institute, issue 21, 1956.

3. Shakhov F. N. Volcanism and metallogeny of the Yenisei Ridge. Bulletin of the West Siberian Geological Administration, issue 5, 1938.