ON THE ROLE OF DEEP FAULTS IN THE DISTRIBUTION OF PRECAMBRIAN GOLD MINERALIZATION IN THE SOUTHERN SIBERIAN PLATFORM

V. A. Buryak

(Institute of the Earth’s Crust, Siberian Branch, Academy of Sciences of the USSR)

In the Sayan–Baikal folded region, which borders the Siberian Platform from the south, gold is very widely distributed and is a characteristic typomorphic metal. At least two main epochs of gold ore formation are distinguished—the Precambrian and the Caledonian. The intensity of manifestation of these gold-ore formations of different ages is sharply uneven and is determined by a combination of various factors.

Precambrian gold-ore occurrences in the territory under consideration are represented by gold-bearing quartz veins with an insignificant quantity of sulfides, and by zones of poor sulfide dissemination in intensely dislocated sandy-shale strata. The spatial distribution of these types of mineralization on a regional scale is determined by a zone of deep faults (a marginal suture) separating the Siberian Platform from the geosynclinal formations of the Sayan–Baikal folded region that frame it (Fig. 1).

These deep faults are characterized by considerable length (hundreds to thousands of kilometers), linear elongation parallel to the geometric forms of the platform basement, great depth of penetration (they control the distribution of ultrabasic and basic intrusions), reverse-fault–strike-slip movements of different amplitudes, and an en echelon arrangement of smaller ruptures. According to geophysical data, deep faults are well expressed by anomalous magnetic fields, the axes of which are usually parallel to the strike of the faults. The inner (“central”) parts of fault zones are emphasized by transitions of gravity values and by the most intensely manifested processes of ultrametamorphism and palingenic magmatism. Tectonic movements along the faults occurred repeatedly both in the Proterozoic and in later times, up to the Cenozoic, determining the plan and character of development of folded and smaller rupture structures, magmatic formations and, most importantly, mineral formations.

The degree of regionally expressed metamorphic transformation of the rocks is also determined by their spatial position with respect to this zone of deep faults. In general, somewhat schematically, we distinguish the following four different-facies metamorphic zones (belts), situated at different distances from the fault zone: 1) a central zone (belt), spatially coinciding with the most mobile and permeable for heat flow

part of a fault zone; it is characterized by intensely expressed processes of metamorphism and palingenic magmatism; 2) a zone (belt) of manifestation of regional-metamorphic transformations of the epidote–amphibolite facies; 3) a zone (belt) of development of the greenschist facies of metamorphism; 4) a marginal peripheral zone embracing diagenized and epigenized, practically unmetamorphosed sedimentary sequences.

Fig. 1. Schematic arrangement of metamorphic facies and endogenous Precambrian mineralization; 4 — unmetamorphosed rocks with lead–zinc telethermal mineralization; part of the zone of deep faults, characterized by intensely expressed processes of metamorphism (amphibolite and granulite facies) and palingenic magmatism with rare-metal and mica-bearing pegmatites; 2 — zone of development of the epidote–amphibolite facies of metamorphism with high-temperature pneumatolytic–hydrothermal rare-metal mineralization; 3 — zone of development of the greenschist facies of metamorphism with gold and gold–polymetallic mineralization; 4 — unmetamorphosed rocks with lead–zinc telethermal mineralization; 5 — complexly metamorphosed metamorphic formations not subdivided by facies; 6 — mica-bearing pegmatites; 7 — rare-metal pegmatites; 8 — rare-metal high-temperature hydrothermal and pneumatolytic–hydrothermal mineralization; 9 — gold and gold–polymetallic mineralization; 10 — lead–zinc telethermal mineralization. Deep faults: 1 — Biryusinsky, 2 — Main Sayansky, 3 — Primorsky, 4 — Davan zone.

These different-facies metamorphic zones of the Upper Proterozoic (?) stage of metamorphism have a linearly elongated, regional distribution parallel to the deep fault; they are broadly conformable with regional folded structures and are not associated with individual intrusive massifs or local folded and rupture structures. The transitions between adjacent different-facies zones are gradual; the mineral associations for rocks of the same composition within the zones are stable. All this makes it possible to conclude that these metamorphic zones constitute a single group or series of facies of a single, very long-developing progressive stage of metamorphism.

Naturally, as a result of the emplacement of younger (Paleozoic and Mesozoic) intrusions, leading to the formation of contact hornfelses and the development of superimposed zones of dynamothermal and regressive metamorphism, the noted metamorphic zonation becomes considerably more complicated and distorted. In this case, reconstruction of the primary (Precambrian) metamorphic zonation that controlled the distribution of Precambrian mineralization may present considerable difficulties.

Precambrian gold mineralization forms elevated concentrations only within the third zone, in rocks of the greenschist facies of metamorphism, i.e., at a quite definite distance from deep faults. In the first zone (in the central belt), mica-bearing and ceramic pegmatites with rare-earth mineralization are developed. In the second zone, high-temperature pneumatolytic-hydrothermal mineralization, including rare-metal mineralization, has been manifested. And, finally, at the greatest distance from the zone of deep faults—in rocks practically unaffected by metamorphic processes—only polymetallic mineralization of the telethermal type is observed in industrial concentrations; if gold is present here at all, it occurs in extremely insignificant amounts.

In other words, the spatial distribution of Precambrian mineralization, including gold mineralization, with respect to the zone of deep faults is determined by the mobility (ionic densities) of the elements and by the values of the isobaric potentials ($\Delta Z$) of their compounds (Table 1). The most mobile and least energy-intensive elements (Hg, Zn, Pb, Cu, Au, As, Sb, etc.), forming mineral compounds (HgS, PbS, ZnS, FeS$_2$, CuFeS$_2$, native gold, etc.) characterized by small values of $\Delta Z$, are concentrated at the greatest distance from the fault zone. As one approaches the fault zone and as the degree of metamorphism of the enclosing strata increases, less mobile elements (Mo, Sn, Be, Nb, Ta, Yb, etc.) are localized, forming more energy-intensive compounds with elevated values of $\Delta Z$ (MoS$_2$, SnO$_2$, beryl, columbite, tantalite, etc.). It is quite clear that gold, as a metal characterized by high mobility (low ionic density), can produce elevated concentrations only in comparatively low-temperature zones of metamorphism, i.e., where there is a substantial decrease in the temperature of the mineralizing solutions.

Consequently, in the general series of Precambrian ore formations, gold occupies a quite definite spatial and geochemical position, tending toward the outer zones of ancient metallogenic belts.

It is characteristic that, within the greenschist facies of metamorphism, the distribution of gold is in turn determined by a whole series of factors, among which the principal role is played by ruptural and plicative tectonics, as well as by the lithology and chemical composition of the enclosing rocks. Regionally, elevated concentrations of gold are confined to places of regional bends in the strike of zones of deep faults and of the folded structures bordering them, with the convexity facing toward the platform (Lena gold-bearing district, Yenisei Ridge, North Baikal gold-bearing district—the basins of the Tya, Neryunda, and Verkhnelensk rivers, etc.). Moreover, the more sharply expressed the bend (Lena, Yenisei Ridge), the more intensely gold mineralization is manifested. Where the strike of the fault zone and folded structures is calm and sustained, mineralization is usually undeveloped or else is manifested extremely weakly.

Favorable host rocks for the localization of elevated concentrations of gold are substantially siliceous sand-

Comparative characteristics of different ore-mineralization zones

Table 1

Note: The values of \(\Delta Z\) are taken from F. A. Letnikov [3]: for the first zone, according to the series CaO—Nb\(_2\)O\(_5\); for the second, according to the series SnO\(_2\)—MoS\(_2\); for the third, according to the series FeO—Au\(_2\)O\(_3\); and for the fourth, according to the series ZnS—Sb\(_2\)O\(_3\); the values of ionic densities are taken from Yu. G. Shcherbakov [4]: for the first zone, according to the series Ta—Nb; for the second, according to the series Sn—Mo; for the third, according to the series Fe—Au; and for the fourth, according to the series Zn—Sb.

sandy-shale, especially shale strata. Carbonate rocks are unfavorable for the localization of mineralization in them.

The investigations carried out show that this influence of the composition of the host rocks on the development of mineralization is apparently due to two causes: 1) the increased primary-sedimentary concentration of gold and sulfides in sandy-shale strata and 2) the most intensive development in these rocks, because of their sharply reduced basicity, of acid leaching (silicification), which ensures intensive leaching of gold and other metals from the rocks¹ and their concentrated associated deposition. Conversely, the almost complete absence of gold in carbonate rocks is due to the absence or very weak development in them of an initial (sedimentary or volcanogenic-sedimentary) concentration of gold and other ore elements, as well as to the high basicity of these rocks, which hinders the development of acid leaching and subsequent deposition of metals.

It must be noted that the observed dependence of mineralization on the degree of metamorphism of the host rocks, caused by varying distance from deep faults, is clearly expressed and, in combination with other prospecting criteria, can be successfully used in the practice of geological investigations. There is a regional metallogenic zonality, expressed in the presence of a single, successively developing “column” of ore zones (belts), characterized by a definite mineralization and by the degree of progressive metamorphism of the host rocks. Evidently, these heterofacial ore belts arise in a regular manner and develop in time and space in close connection with the formation of a long-lived zone of deep faults and the development of processes of regional metamorphism.

Upper Proterozoic magmatic rocks, synchronous in time with the processes of metamorphism and mineralization, are developed only within the first zone. These are typical ultrametamorphic formations. A direct genetic connection between gold and other types of mineralization and them is not established. Perhaps in this case one may speak of a paragenetic connection between magmatism and mineralization: the primary cause of the formation both of magmatic bodies and of mineralization is the processes of metamorphism (ultrametamorphism), which in turn are conditioned by the presence of a zone of deep faults.

Intrusions of Paleozoic granitoids developed in the various zones are clearly later in relation to the processes of folding, metamorphism, and the mineralization under consideration.

The impression is created that metamorphic, including ultrametamorphic, transformations of rocks (palingenesis), as well as the formation of gold and other types of endogenous Precambrian mineralization, were conditioned by the influx, along the zone of deep faults, of metamorphosing (“through-magmatic,” according to D. S. Korzhinskii) solutions. At the same time, gold and many other ore components were presumably supplied mainly not by introduction along the zone of deep faults from deep horizons of the Earth’s interior, but as a result of metamorphic mobilization from the entire thickness of the metamorphosed rocks.

This, in particular, is indicated by the close interrelation of the processes of metamorphism and mineralization, the fairly clear lithostratigraphic control in the development of mineralization, and the dependence of its composition and intensity on the primary-sedimentary mineralization of the host rocks,

as well as a regular change in the composition of trace elements in volcanogenic-sedimentary sequences as the degree of their metamorphism increases, etc. [1, 2].

In conclusion, it should be noted that the data presented on the zonality of the distribution of mineralization relative to the zone of deep faults and on its connection with metamorphic processes make it possible, in our opinion, to approach more correctly the solution of a whole series of questions not only concerning gold but also other types of mineralization and, in particular, the elucidation of the genesis of the so-called telethermal polymetallic deposits, widely developed in the Baikal region and, possibly, in the Angara region. Obviously, they cannot be regarded as sedimentary-metamorphic or as typical hydrothermal postmagmatic formations genetically connected with particular magmatic massifs. Their formation, like that of Precambrian gold-ore formations, may have occurred in close connection with the development of zones of deep faults and metamorphic processes.

REFERENCES

1. Buryak V. A. On the influence of processes of regional metamorphism on the development of gold-sulfide mineralization in the central part of the Lena gold-bearing district. In: Physicochemical Conditions of Magmatism and Metasomatism, Proceedings of the 3rd All-Union Petrographic Conference. Nauka Publishing House, 1964.

2. Buryak V. A. Dependence of mineralization of ancient Precambrian formations on regional metamorphic zonality. Doklady AN SSSR, vol. 163, no. 5, 1965.

3. Letnikov F. A. Isobaric Potentials of Mineral Formation (Chemical Affinity) and Their Application in Geochemistry. Nedra Publishing House, 1965.

4. Shcherbakov Yu. G. Regularities in the concentration of ore elements in granitoids and deposits. In: Problems of Ore Genesis, reports of Soviet geologists, XXII Session of the International Geological Congress. Nedra Publishing House, 1964.

1. Naturally, in the present case what is meant is the leaching of gold not only from the wall rocks, but also from the entire metamorphosed sequence. Nor is an additional introduction of gold along the zone of deep faults from deeper horizons of the Earth, possibly from its subcrustal parts, denied. ↩