THE PLACE OF GOLD MINERALIZATION IN THE MAGMATIC PROCESS OF THE TRANS-ANGARA PART OF THE YENISEI RIDGE

I. S. TURKIN (SNIIGGiMS)

Many years of prospecting, exploration, and exploitation of gold-ore deposits in the north of the Yenisei Ridge have revealed the principal features of their structure, spatial position, and genesis. Now that the main gold-ore deposits of the ridge have practically been worked out or are being worked out, it is very important, using the accumulated experience, to make a forecast for the search for new gold deposits here.

The present communication examines only one of the genetic questions of gold mineralization—namely, its place in the unified magmatic process of the Precambrian epoch.

Analysis of the character of the evolution of magmatism and ore formation (petro-metallogenic evolution), on a regional scale for the entire Trans-Angara region of the Yenisei Ridge, has made it possible to establish the genetic connection of the leading (gold, rare-metal, and polymetallic) mineralization with the corresponding type of magmatism, and also the place of gold in the general metallogenic sequence.

A. K. Meister [1910], T. M. Dembo [1941], and other researchers pointed to a genetic connection between gold mineralization in the Yenisei Ridge and granitoids. However, it was subsequently found that the overwhelming majority of gold deposits, regardless of their spatial position relative to granitoid intrusions, preserve a striking uniformity of their mineral composition and structural features. This enabled P. S. Bernstein (1951, 1954), N. V. Petrovskaya (1951, 1954), and a number of other researchers to refine the views of A. K. Meister and T. M. Dembo.

Recognizing the paragenetic connection of gold mineralization with granitoids, P. S. Bernstein, N. V. Petrovskaya, M. B. Borodaevskaya, and others believe that the ore-bearing gold-containing solutions are deep-seated and are connected with the formation of intrusions of granite batholiths only paragenetically.

Of all the diversity of granitoids of the Yenisei Ridge, M. N. Borodaevskaya assigns to gold-ore only weakly differentiated granite batholiths that do not contain a dike series of magmatic rocks.

Detailed studies of the magmatism of the Yenisei Ridge have shown that it occupies a broad spatial-temporal range. Many of the magmatic formations (from gabbro-diabases to traps) are poten-

may potentially be gold-bearing, and some magmatic rocks (including granites) have the same features of genetic or paragenetic kinship with gold mineralization as do the granites classified by M. B. Borodaevskaya as gold-ore-bearing.

Establishing the genetic and paragenetic relationship of gold mineralization with a definite type of magmatism can be accomplished only by analyzing the entire petrometallogenic evolution of magmatism and mineralization.

On the basis of our studies, the character of the petrometallogenic evolution of magmatism and mineralization for the Trans-Angara part of the Yenisei Ridge appears as follows (Table 1).

Table 1

Character of the petrometallogenic evolution of magmatism and mineralization of the Trans-Angara region of the Yenisei Ridge

Without going into the details of Table 1, we shall dwell only on that part of it which points to gold mineralization.

The first thing to which attention should be drawn is the association of gold mineralization with a complex of differentiated granitoids, the composition of which varies within broad limits (from alaskites to diorites), though tending only toward their more basic varieties. The second is the broad time range of formation of gold mineralization (Middle–Upper Proterozoic), with an increase in its role in the most highly differentiated granitoids of the Tatar-Ayakhta complex.

It should be emphasized that gold is associated with those magmatic rocks with which the sulfides of lead, zinc, copper, as well as antimony, arsenic, and partly mercury, occur.

This explains the predominant paragenesis of gold mineralization with sulfide mineralization.

Considering the areas of distribution of granitoid rocks and gold mineralization, we find an explanation for the question raised by N. V. Petrovskaya concerning the presence of barren fields within a single belt of gold mineralization,

extending along the axis of the central anticlinorium of the ridge from the Angara River to the Podkamennaya Tunguska River.

Naturally, the areas of distribution of granitoids of the Teya and partly the Nogatino complexes, as well as the leucocratic varieties of rocks of the Tatar–Ayakhta complexes, do not bear gold mineralization.

Among the differentiated granitoids of the Tatar–Ayakhta, partly Nogatino, and Noiba complexes, only granitoids of increased basicity—granodiorites, diorites, diorite-diabases—have a genetic connection with gold mineralization. It is precisely with these varieties of granitoids that the areas of distribution of gold mineralization are spatially associated. It is curious that massifs of differentiated granites almost always contain, at their endo- and exocontacts, hybrid amphibolites and amphibolitized hornfelses (the eastern gold-bearing belt of the Central anticlinorium), as well as biotite-amphibole schists.

However, where gold mineralization is spatially removed from granitoid massifs, it is located predominantly in phyllites and phyllitized schists, for the most part of carbonaceous-graphitic composition, and is represented by the quartz–gold-ore formation.

It is this form of gold mineralization that is represented by the largest deposits (Sovetskoye, Partizanskoye, Gerfed, Polyarnaya Zvezda, and others).

The depth of occurrence of the gold-ore veins reaches 300 m (Sovetskoye), and possibly much more.

A few words about the structural features of gold-ore deposits. As a rule, gold-ore deposits are situated in anticlinal (monoclinal) structures, gravitating toward their axial parts (Sovetskoye, Vagan, Kvartsevaya Gora), and very rarely in the axial parts of synclines (Ayakhta).

Gold-ore veins occur predominantly in cleavage fractures or at the junction of the latter with longitudinal concordant fractures. In this case the form of the veins is usually branching and irregular.

Ore bodies of complex form and vein zones arise in zones of intense interstratal displacements (at the moment of ore formation), accompanied by cleavage extension fractures.

A few words about the genesis of gold mineralization. Observations show that spatially the gold mineralization is connected with granitoids of increased basicity of the Nogatino and Tatar–Ayakhta complexes, but may be removed from them, being located only in phyllites and phyllitic schists with an admixture of carbonaceous-graphitic material (Sovetskoye deposit). No example has been noted of a spatial connection between gold mineralization and the leucocratic varieties of granites or with basic and ultrabasic intrusions of the Tokma, Surnikha, and other complexes. Moreover, the well-known Ayakhta gold deposit, which lies within the granitoid massif of the same name, is situated in a remnant of hornfelsed schists among granodiorites and diorites.

The age of the gold mineralization (according to geological data) corresponds approximately to (or is somewhat younger than) the age of the granitoids of the Tatar–Ayakhta complex.

And, finally, the accessory mineral composition of the quartz–gold-ore veins and of the quartz veins occurring in the granitoids of the Tatar–Ayakhta complex is similar in many respects (sphene, rutile, tourmaline, etc.).

All this and a number of other features indicate a paragenetic connection of the gold mineralization with granitoids of increased basicity of the Tatar–Ayakhta and, to a lesser extent, the Nogatino and Noiba complexes.

References

Bernshtein P. S. and Petrovskaya N. V. The Sovetskoye gold-ore deposit (Yenisei Ridge). Geology of the Most Important Gold-Ore Deposits of the USSR, vol. VI. NIGRIzoloto Publishing House, Moscow, 1954.

Bernshtein P. S. Structural factors determining the regularities of distribution and the character of gold mineralization of the Yenisei Ridge. Transactions of the Institute NIGRIzoloto, OBTI, Moscow, 1951.

Borodayevskaya M. B. Some petrogenetic features of gold-bearing magmatic formations and the types of gold-ore deposits of the USSR associated with them. In: Magmatism and the Relation of Mineral Resources to It, Proceedings of the 2nd All-Union Petrographic Conference, Moscow, 1960.

Dembo T. M. Petrology of the deposits of the Eldorado group in the Northern Yenisei taiga. Transactions of NIGRIzoloto, issue 14, 1941.

Meister A. K. Rocks and conditions of gold content in the southern part of the Yenisei District. Geological Investigations of the Gold-Bearing Regions of Siberia. Yenisei Gold-Bearing District, issue IX, 1910.

Petrovskaya N. V. Mineralogical prospecting criteria under the conditions of the Yenisei Ridge. Transactions of the Institute NIGRIzoloto, OBTI, Moscow, 1951.